Current Research (2009) Newfoundland and Labrador Department of Natural Resources Geological Survey, Report 09-1, pages 13-42 TECTONOSTRATIGRAPHIC ARCHITECTURE AND VMS MINERALIZATION OFTHE SOUTHERN TULKS VOLCANIC BELT: NEW INSIGHTS FROM U–Pb GEOCHRONOLOGY AND LITHOGEOCHEMISTRY J.G. Hinchey and V. McNicoll 1 Mineral Deposits Section 1 Geological Survey of Canada, 601 Booth Street, Ottawa, ON, K1A 0E8 ABSTRACT To clarify their tectonostratigraphic affinity within the Victoria Lake supergroup and to better understand these mineral- izing environments, U–Pb geochronology, trace-element lithogeochemistry and Sm/Nd isotopic geochemistry were applied to the host rocks of the Tulks East, Tulks Hill and Boomerang volcanogenic massive sulphide (VMS) deposits, in central New- foundland. A subvolcanic porphyry from the Tulks Hill deposit, dated previously at 498 +6/-4 Ma, provides a minimum age for the nearby Tulks Hill and Tulks East deposits. Two, new U–Pb zircon ages were obtained from the felsic tuff that hosts mineral- ization at the Boomerang deposit and from a felsic dyke interpreted to be broadly synvolcanic. The combined TIMS and SHRIMP data for these two samples indicate an identical U–Pb age of 491 ± 3 Ma. This date is younger than the 498 +6/-4 Ma age from Tulks Hill, although the errors do overlap at their older and younger limits, respectively. Inheritance patterns in the Boomerang samples suggest the presence of older crustal material having Cambrian (514–510 Ma) ages akin to those reported from the Tally Pond group, an older sequence within the Victoria Lake supergroup. The new results suggest that VMS mineralization in the Tulks area and at Boomerang may represent temporally discrete events, despite some apparent similar- ities. The age determined for the Boomerang deposit is closer to (but not identical with) a U–Pb date of 487 ± 3 Ma, obtained some 30 km to the southwest of the Boomerang deposit, from a unit termed the Pats Pond group. This suggests that the younger sequence of rocks may be regionally extensive, as proposed by other workers, and implies that it may have potential elsewhere for VMS mineralization similar to the Boomerang deposit. The comparison of lithogeochemical patterns from the Tulks East, Tulks Hill and Boomerang deposit areas, with pub- lished data from the Pats Pond group, is complicated by the effects of hydrothermal alteration near the VMS mineralizing environments. Nevertheless, examination of immobile trace-element signatures suggests that these sequences cannot be dis- tinguished on the basis of their geochemistry. The volcanic and pyroclastic rocks are all broadly arc-related, and show a mix- ture of calc-alkaline and tholeiitic signatures that perhaps record the construction and later rifting of individual arc sequences. However, Nd isotope signatures from felsic rocks in the Boomerang area and the Pats Pond group are higher (gNd of +4 to +5.5) than those from the Tulks Hill and Tulks East areas (gNd of about +3). Although not a straight-forward corre- lation, as the volcanic rocks that host the Tulks East and Tulks Hill deposits also locally contain higher gNd values of between +4 to +5, the latter observation may also be supportive of a link between the Boomerang area and the Pats Pond group. Collectively, the new results, when taken with the earlier results, suggest that the tract of rocks known as the Tulks Vol- canic Belt includes rocks of more than one age, but of generally similar geochemistry and tectonic setting. There is an imme- diate need for more geochronological studies and age determinations of other VMS deposits in this area, to ascertain how they might fit into a revised view of this complex but economically important package of rocks. INTRODUCTION The Island of Newfoundland boasts numerous vol- canogenic massive sulphide (VMS) deposits within Cambri- an–Ordovician volcanic arcs and back-arc basins of the Dunnage Zone. The Dunnage Zone is divided by a major suture zone, the Red Indian Line (RIL), into a western ter- rane representing the Laurentian margin of the ancient Iape- tus Ocean and an eastern terrane representing the Gond- wanan margin of Iapetus (Williams et al., 1988; Figure 1). 13
Transcript
Current Research (2009) Newfoundland and Labrador Department of Natural ResourcesGeological Survey, Report 09-1, pages 13-42
TECTONOSTRATIGRAPHIC ARCHITECTURE AND VMS MINERALIZATION OF THE SOUTHERN TULKS VOLCANIC BELT: NEW
INSIGHTS FROM U–Pb GEOCHRONOLOGY AND LITHOGEOCHEMISTRY
J.G. Hinchey and V. McNicoll1
Mineral Deposits Section1 Geological Survey of Canada, 601 Booth Street, Ottawa, ON, K1A 0E8
ABSTRACT
To clarify their tectonostratigraphic affinity within the Victoria Lake supergroup and to better understand these mineral-izing environments, U–Pb geochronology, trace-element lithogeochemistry and Sm/Nd isotopic geochemistry were applied tothe host rocks of the Tulks East, Tulks Hill and Boomerang volcanogenic massive sulphide (VMS) deposits, in central New-foundland.
A subvolcanic porphyry from the Tulks Hill deposit, dated previously at 498 +6/-4 Ma, provides a minimum age for thenearby Tulks Hill and Tulks East deposits. Two, new U–Pb zircon ages were obtained from the felsic tuff that hosts mineral-ization at the Boomerang deposit and from a felsic dyke interpreted to be broadly synvolcanic. The combined TIMS andSHRIMP data for these two samples indicate an identical U–Pb age of 491 ± 3 Ma. This date is younger than the 498 +6/-4Ma age from Tulks Hill, although the errors do overlap at their older and younger limits, respectively. Inheritance patterns inthe Boomerang samples suggest the presence of older crustal material having Cambrian (514–510 Ma) ages akin to thosereported from the Tally Pond group, an older sequence within the Victoria Lake supergroup. The new results suggest that VMSmineralization in the Tulks area and at Boomerang may represent temporally discrete events, despite some apparent similar-ities. The age determined for the Boomerang deposit is closer to (but not identical with) a U–Pb date of 487 ± 3 Ma, obtainedsome 30 km to the southwest of the Boomerang deposit, from a unit termed the Pats Pond group. This suggests that theyounger sequence of rocks may be regionally extensive, as proposed by other workers, and implies that it may have potentialelsewhere for VMS mineralization similar to the Boomerang deposit.
The comparison of lithogeochemical patterns from the Tulks East, Tulks Hill and Boomerang deposit areas, with pub-lished data from the Pats Pond group, is complicated by the effects of hydrothermal alteration near the VMS mineralizingenvironments. Nevertheless, examination of immobile trace-element signatures suggests that these sequences cannot be dis-tinguished on the basis of their geochemistry. The volcanic and pyroclastic rocks are all broadly arc-related, and show a mix-ture of calc-alkaline and tholeiitic signatures that perhaps record the construction and later rifting of individual arcsequences. However, Nd isotope signatures from felsic rocks in the Boomerang area and the Pats Pond group are higher (gNdof +4 to +5.5) than those from the Tulks Hill and Tulks East areas (gNd of about +3). Although not a straight-forward corre-lation, as the volcanic rocks that host the Tulks East and Tulks Hill deposits also locally contain higher gNd values of between+4 to +5, the latter observation may also be supportive of a link between the Boomerang area and the Pats Pond group.
Collectively, the new results, when taken with the earlier results, suggest that the tract of rocks known as the Tulks Vol-canic Belt includes rocks of more than one age, but of generally similar geochemistry and tectonic setting. There is an imme-diate need for more geochronological studies and age determinations of other VMS deposits in this area, to ascertain howthey might fit into a revised view of this complex but economically important package of rocks.
INTRODUCTION
The Island of Newfoundland boasts numerous vol-canogenic massive sulphide (VMS) deposits within Cambri-an–Ordovician volcanic arcs and back-arc basins of the
Dunnage Zone. The Dunnage Zone is divided by a majorsuture zone, the Red Indian Line (RIL), into a western ter-rane representing the Laurentian margin of the ancient Iape-tus Ocean and an eastern terrane representing the Gond-wanan margin of Iapetus (Williams et al., 1988; Figure 1).
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14
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J.G. HINCHEY AND V. McNICOLL
The Victoria Lake supergroup (VLSG), located east of theRIL in the Exploits Subzone of the Dunnage Zone, is animportant exploration target for VMS deposits. The impor-tance of the VLSG is highlighted by the opening of the DuckPond mine in 2007 with proven, and probable, reserves of4.1 Mt containing 3.3% Cu, 5.7% Zn, 62 g/t Ag and 0.85 g/tAu (Aur Resources, press release, May 2007). The 2004 dis-covery of the Boomerang deposit in the Tulks Volcanic Belt(TVB) of the VLSG, containing an indicated resource of1.36 Mt, grading 7.09% Zn, 3.00% Pb, 0.51% Cu, 110.43 g/tAg, and 1.66 g/t Au (Messina Minerals, press release, June2007), has also generated interest in this part of the studyarea.
The TVB is bimodal, and is dominated by felsic vol-canic rocks containing interbedded mafic volcanic, pyro-clastic, volcaniclastic and sedimentary rocks (Evans andKean, 2002). The age of the TVB was originally constrainedby a U–Pb age of 498 +6/-4 Ma on a subvolcanic porphyrynear the Tulks Hill VMS deposit (Evans et al., 1990). Morerecent mapping and compilation by the Geological Surveyof Canada, coupled with geochronology and lithogeochem-istry (van Staal et al., 2005; Zagorevski et al., 2007a) indi-cated that the TVB is composite, and also includessequences of volcanic and sedimentary rocks.
This paper presents and discusses new data from thesouthern part of the TVB including U–Pb geochronologicaldata obtained from host rocks to the Boomerang VMSdeposit. This deposit sits within the originally defined 'TulksVolcanic Belt', but was placed in the newly defined PatsPond group, (ca. 488 Ma; Zagorevski et al., 2007a; vanStaal et al., 2005).
REGIONAL GEOLOGY AND METALLOGENIC FRAMEWORK
The Dunnage Zone of the Newfoundland Appalachians(Figure 1) represents the vestiges of Cambro-Ordoviciancontinental and intra-oceanic arcs, back-arc basins, andophiolites that formed in the Iapetus Ocean (Kean et al.,1981; Swinden, 1990; Williams, 1995). The zone is bisectedby an extensive fault system (the Red Indian Line, RIL) intoa western peri-Laurentian segment (Notre Dame and Dash-woods subzones), and an eastern peri-Gondwanan segment(Exploits Subzone). The two main subzones of the DunnageZone are differentiated on stratigraphic, structural, faunal,and isotopic characteristics (Williams et al., 1988). The RILseparates the Buchans Group, and locally the Red IndianLake group (Rogers et al., 2005), which formed on the Lau-rentian side of the Iapetus Ocean, from the VLSG, thatformed on the Gondwanan side of Iapetus. The deformationassociated with final closure of the Iapetus Ocean culminat-ed during the Silurian (dated directly as syn- to post- 432 ±
1.4 Ma; Zagorevski et al., 2007b), at which time, thrustingand folding juxtaposed these initially geographically distinctvolcanic belts.
Mapping by the Geological Survey of Newfoundlandand Labrador (GSNL) in the 1970s and 1980s (e.g., Kean,1977; Kean et al., 1981; Evans and Kean, 2002 and refer-ences therein) indicated that the TVB (see Figures 1 and 2)represents the remnants of one of several bimodal Cambrianto Ordovician volcanic-arc sequences. Together with adja-cent volcanic and sedimentary belts of variable tectonicaffinities, it belongs to the VLSG (Evans and Kean, 2002);subdivided into the TVB (ca. 498 Ma), the Long Lake Vol-canic/Volcaniclastic Belt (ca. 505 Ma), and the Tally PondVolcanic Belt (ca. 515 Ma). In addition to the Cambro-Ordovician volcanic and volcaniclastic rocks of the VLSG,there are also large areas of late Precambrian (565–563 Ma)plutonic rocks (Evans et al., 1990), which represent inliersof old basement, most likely of the crustal block, Ganderia(e.g., van Staal et al., 1998). Previous lithogeochemicalstudies, based largely on subordinate mafic volcanic rocks,indicate that the VLSG is composed of distinct geochemicalgroupings representing different tectonic environments, e.g.,active arc, arc-rift, back-arc, and mature arc (see Swinden etal., 1989; Evans and Kean, 2002).
Evans and Kean (2002) divide the VLSG into the north-ern and southern terrains, separated by the Rogerson LakeConglomerate. The TVB, part of the northern terrain, isbounded to the north by the RIL and the sedimentary andvolcaniclastic rocks of the Harbour Round Belt (e.g., RedIndian Lake group of Rogers et al., 2005), and to the southby a geophysical anomaly, in the form of a regionally exten-sive magnetic high, separating the TVB from the Long LakeBelt.
The most common rock types of the southern TVB con-sist of light grey to white, quartz ± feldspar porphyritic fel-sic-intermediate pyroclastic rocks, massive rhyolite, and fel-sic-intermediate ash tuffs through to tuffs and lapilli tuffs,locally bimodal breccias, and minor subvolcanic porphyries.Mafic volcanic rocks are subordinate and are dominated bytuffs, lapilli tuffs, breccias, sills, and locally pillow lavas.Black shales, argillites and greywackes are also locallyabundant.
The TVB has been subjected to lower to middle, green-schist-facies metamorphism and moderate to strong defor-mation. The presence of well-developed, bedding-parallel,regional foliations defined by alignment of chlorite andsericite commonly obliterates primary textures in the rocks.The stratigraphy typically strikes northeast and dips steeplyto the northwest, and the belt is transected by shear zonesand faults.
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All rocks, within the southern part of the TVB, wereconsidered to be of similar age based on a 498 +6/-4 MaU–Pb date, of a subvolcanic porphyry, located close to theTulks Hill VMS deposit (Evans et al., 1990). This age wasrecently re-interpreted by using a weighted average of the207Pb/206Pb ages, versus the original linear regression tech-nique, resulting in a slightly younger age of 496.5 ± 1 Ma(G.R. Dunning, personal communication, 2008).
Recent mapping and geochronological studies by theGeological Survey of Canada (GSC) (Rogers et al., 2005;van Staal et al., 2005) resulted in the interpretation of thesouthern TVB as a series of generally westward-youngingtectonostratigraphic units including the Tulks group (ca. 498Ma), the Pats Pond group (ca. 487 Ma), the SutherlandsPond group (ca. 462–457 Ma; Zagorevski et al., 2008, Dun-ning et al., 1987), and the Wigwam Brook group (ca. 453Ma; van Staal et al., 2005; Zagorevski et al., 2007a). TheWigwam Brook group was dated from a sample of quartzand feldspar phyric tuff immediately south of Pats Pond, butthe age of the Pats Pond group was obtained from a sampleof bimodal breccia collected in the Burgeo Highway area,approximately 30–35 km southwest of the Boomerangdeposit, in a package of rocks physically disconnected fromthose that host the Boomerang deposit.
PATS POND GROUP
As the Pats Pond group is used throughout this reportfor comparative purposes with the host rocks to VMSdeposits, a brief description of the characteristic rock typesis given. As described by Zagorevski et al. (2007a), the PatsPond group is dominated by intermediate quartz-phyric andmafic tuffs. The stratigraphically lowest unit within thegroup consists of calc-alkaline pillow basalt, overlain byfeldspar ± quartz-phyric ash, crystal and lapilli tuffs. Theseare stratigraphically overlain mainly by quartz-phyricandesitic tuffs, and the stratigraphically highest portion ofthe group is dominated by basaltic to andesitic tuff, lapillituff, and rhyolitic tuff. From the base to the top, Zagorevski(2007a) subdivided the group into six informal geochemicalsubunits, PP1 through to PP6 (see pages 33-36).
VOLCANOGENIC MASSIVE SULPHIDE DEPOSITS
LITHOLOGICAL AND STRATIGRAPHIC SETTING
Massive sulphide deposits in the southern TVB arecharacteristically associated with felsic-intermediate vol-canic rocks (locally quartz ± feldspar, ash-crystal tuffs andrhyolitic flows) hosted within sequences of volcaniclasticand sedimentary rocks containing lesser amounts of maficvolcanic rocks. Abundant bimodal sills are associated with
volcaniclastic sedimentary rocks (argillite/wacke) within themineralized sequences and appear to have been emplacedwhen the latter were still unconsolidated. This suggests apossible arc-rift or back-arc basin tectonic environment formassive sulphide formation.
From north to south, the three main known VMSdeposits in the southern TVB are the Tulks East, the TulksHill, and the Boomerang deposits. Mineralization in allthree deposits is associated with intense sericite–silica–pyrite, and locally chloritic, alteration and is interpreted tohave formed through sub-seafloor replacement processesrather than by exhalative processes (e.g., Squires et al.,2005a,b and Hinchey, 2007).
The Tulks East and Tulks Hill deposits occur within theJacks Pond formation of the Tulks group (van Staal et al.,2005 and Lissenberg et al., 2005), whereas the Boomerangdeposit occurs within the younger Pats Pond group(Zagorevski et al., 2007a and van Staal et al., 2005). Thesethree deposits are evaluated and discussed in terms of theirhost-rock-types and relationships, geochronology, and litho-geochemical characteristics. For comparative purposes, thelithogeochemistry and geochronology of the Pats Pondgroup of Zagorevski et al. (2007a) will also be examined.
TULKS EAST DEPOSIT
The Tulks East deposit is hosted by a series ofsericite–silica–pyrite and, locally, chlorite–carbonate-altered felsic tuffs and lapilli tuffs, quartz-phyric rhyoliticflows and local basaltic sills. Both the hanging wall andfootwall of the deposit have undergone hydrothermal alter-ation and contain stringer-style sulphide mineralization,within an alteration envelope, extending approximately1600 m along strike, 200 m across strike, and at least 400 mdown (McKenzie et al., 1993; Noranda, 1998). The presenceof intense hydrothermal alteration and sulphide stringers, inboth the hanging wall and footwall stratigraphy, are indica-tive of a replacement process for mineralization (Hinchey,2007). Footwall stratigraphy consists mainly of felsic-inter-mediate ash, crystal (quartz ± feldspar), lapilli tuffs and rhy-olitic flows (Plate 1), with minor conglomerate, mafic tuffs,and intermediate to mafic amygdaloidal sills. Mineralizationis found toward the top of this stratigraphic package which,in turn, is overlain by a thick sequence of intercalatedgraphitic argillite, hosting the Tulks East fault, and mafic tointermediate sills and dykes. Hanging-wall rocks, strati-graphically above the graphitic argillite, are dominantlymafic to intermediate sills and lesser amounts of quartz-phyric felsic volcanic rocks.
Petrographically, the quartz-phyric rhyolites displaywell-preserved textures and contain partially resorbed and
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embayed quartz crystals within a fine-grained, recrystallizedquartz matrix (Plate 1). The matrix is often partiallyreplaced by sericite. The resorption textures suggest that thequartz phenocrysts, having variably preserved crystals, wereunstable in the melt prior to its solidification, being perhapsrelated to increased temperatures caused by mafic magmainflux. In contrast, the felsic tuffaceous rocks contain abun-dant broken quartz crystals in fine-grained, quartz–sericitematrix.
To date, the Tulks East deposit represents the largestaccumulation of sulphide mineralization in the TVB and is
distributed in three massive sulphide lenses (termed the A,B, and C zones) totalling about 5.6 Mt (Barbour and Thur-low, 1982). The A-Zone lens is the largest accumulation ofsulphide containing about 4.5 Mt of massive sulphide (~ 2%base metals (Zn+Cu+Pb)), but the smaller B-Zone (approx-imately 0.23 Mt) has higher grades (approximately 8.7% Zn,0.66% Cu, 1.26% Pb, 58.7 g/t Ag, and 0.14 g/t Au; Barbourand Thurlow, 1982). The C-Zone contains approximately 1Mt of lower grade, pyritic massive sulphide. The close spa-tial proximity and overall geological similarities betweenthe Tulks East and Tulks Hill deposits have led to theassumption that both are of the same age.
17
Plate 1. Macroscopic and microscopic features of the host felsic rocks from the various VMS deposits. (A) Massive andhomogenous quartz-phyric rhyolite from the Tulks East deposit. (A') Quartz crystals within a fine-grained recrystallized quartzmatrix from a rhyolitic facies at Tulks East deposit. Note the delicate resorption textures occurring as embayments within thequartz crystal, indicative that the quartz phenocrysts were unstable in the melt prior to its solidification. (B) Quartz-phyricrhyolite from the Tulks Hill deposit. (B') Rounded and resorbed quartz phenocrysts within a fine-grained recrystallized quartzand sericite matrix from the Tulks Hill deposit. (C) Quartz- and feldspar-phyric, crystal volcaniclastic tuff from theBoomerang deposit. (C') Quartz and feldspar crystals surrounded by a sericitic matrix from the Boomerang deposit.
CURRENT RESEARCH, REPORT 09-1
TULKS HILL DEPOSIT
The Tulks Hill deposit occurs at a similar stratigraphichorizon as the Tulks East deposit, and is hosted by quartz-phyric rhyolite (Plate 1) and altered felsic-intermediate vol-canic rocks, dominated by blue quartz ± feldspar-phyric(locally lapilli-rich) tuff. In addition to the felsic-intermedi-ate tuffs and rhyolite, the stratigraphy also contains maficsills, black argillite and shale, and intermediate volcanicrocks, very similar to those observed at the Tulks Eastdeposit.
The quartz-phyric rhyolites display well-preserved tex-tures and the quartz occurs, both as rounded to teardrop-shaped crystals, in a matrix dominated by fine-grainedrecrystallized quartz and sericite (Plate 1), to partiallyresorbed quartz crystals as observed at Tulks East. As atTulks East, the felsic tuffaceous rocks contain abundant bro-ken quartz crystals in fine-grained, quartz-sericite matrix.
Prominent sericite, chlorite, pyrite and silica alterationis observed within the host felsic quartz-eye tuff and quartz-phyric rhyolite in proximity to the sulphide lenses. Thesefeatures, along with alteration and related stringer mineral-ization present, in both the hanging wall and footwall, areindicative of a replacement process for mineralization (Keanand Evans, 1986). Alteration associated with the sulphidelenses has been observed over a 2000-m-long zone (McKen-zie et al., 1993).
The deposit consists of four tabular massive sulphidelenses (T1 to T4) collectively containing 720 000 tonnes ofmassive sulphide, grading 5.6% Zn, 1.3% Cu, 2.0% Pb, 41g/t silver and 0.4 g/t gold (Jambor and Barbour, 1987). Lens-es T1, T2 and T3 outcrop on surface and are marked byheavily gossaned areas, whereas the T4 lense occurs atdepth. Isoclinal folding in the area suggests that some of thelenses may represent structural repetitions of the same hori-zons (Moreton, 1984; Saunders, 1999). In addition to pyriteand base metals, significant magnetite also occurs within theT1 and T2 lenses and serves to discriminate the ore fromother massive-sulphide bodies in the area. The significance,if any, of the magnetite is unknown.
BOOMERANG DEPOSIT AND RELATED ZONES
The Boomerang, Domino and Hurricane VMS depositsare located in the southern portion of the TVB in the vicini-ty of Pats Pond, approximately 17.5 km southwest of thesouthern tip of Red Indian Lake (Figures 1 and 2). For thepurposes of this report, all of the massive sulphide lenseswill be grouped together under the name 'Boomerang' tosimplify discussion.
The Boomerang deposit stratigraphy consists of a seriesof felsic-intermediate volcanic rocks includingquartz±feldspar phyric ash-crystal tuffs (Plate 1), lapillituffs, coarse-grained volcaniclastic rocks (conglomerate andbreccia), sedimentary rocks (black argillite, siltstone, chertand black shales), felsic, intermediate and amygdaloidalmafic sills, and intermediate dykes. The rocks are dominat-ed by volcaniclastic material, with a general lack of massive,coherent rhyolite, which contrasts with the local abundanceof the latter at the Tulks Hill and Tulks East deposits.
Based on observed inter-fingering and 'soft-sediment'intrusive textures, the bimodal sills are considered to be syn-chronous with the volcanic and sedimentary rocks in thestratigraphy, an observation substantiated by geochronology(see below, pages 20-24). Well-defined fining-upward sedi-mentary sequences (e.g., turbiditic sequences) are common-ly observed in drillcore, and along with the bimodal sillsmay indicate an arc-rift type environment. All rock types,with the exception of some of the late sills, are overprintedby strong northwest-dipping foliations. The observation thatsome sills are foliated whereas others are massive with littleto no foliation suggests possible different ages for varioussills, or competency contrasts.
The deposit's stratigraphy is divided into a hanging-wallsequence, a mineralized horizon, and a footwall sequence(see Squires et al., 2006; Dearin, 2006). The hanging wallsequence consists of undifferentiated, locally fining-upward, felsic-intermediate volcanic rocks dominated byquartz±feldspar crystal-ash tuffs, fine-grained sedimentaryrocks (black shales/argillite/greywacke/chert), volcaniclas-tic conglomerate/breccia, and bimodal sills. Footwall rocksconsist of strongly sericitized felsic-intermediate volcanicrocks (dominated by fine-grained, pyroclastic crystal-ashtuffs) that commonly contain base-metal stringer sulphides.The rocks are extremely sericitized and display an intensefoliation and a local crenulation cleavage. The mineralizedhorizon of the Boomerang deposit consists of stronglyaltered, fine-grained pyroclastic felsic-intermediate volcanicrocks (ash-crystal tuffs) and sedimentary rocks (blackshales, chert and argillite) that are intimately associated withmassive sulphide mineralization. The intercalation of vol-canic, pyroclastic and sedimentary rocks provided afavourable environment for the formation of the Boomerangmassive sulphide lense via replacement (see Hinchey, 2007;Squires, 2008; Squires et al., 2005).
The dominant quartz–feldspar–phyric, felsic-intermedi-ate tuff consists of abundant, variably preserved, quartz andfeldspar crystals in an intermediate fine-grained ground-mass, commonly altered to sericite (Plate 1). All textures atthe Boomerang deposit are indicative of volcaniclastic
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19
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CURRENT RESEARCH, REPORT 09-1
facies compared to the more coherent facies observed in theTulks East and Tulks Hill deposits.
Hydrothermal alteration, predominantly sericite–sili-ca–pyrite along with local chlorite and carbonate, andstringer base-metal sulphides, occur in both the hangingwall and footwall of the massive sulphide lense, althoughthe alteration is much more intense in the footwall rocks.The deposit displays metal zonations with a Cu- and Zn-enriched bottom and a gold-enriched top. A recent NI43-101compliant mineral resource estimate for the Boomeranglense returned an indicated resource of 1.36 Mt grading7.09% Zn, 3.00% Pb, 0.51% Cu, 110.43 g/t Ag, and 1.66 g/tAu (Messina Minerals Inc., press release, June 21, 2007).An additional 0.7 Mt of inferred resources is estimated forthe Boomerang and Domino lenses. In contrast to the medi-um- to coarse-grained, massive sulphides associated withthe Tulks Hill and Tulks East deposits, the sulphides in theBoomerang deposit are dominated by fine- to medium-grained, banded and wispy, sphalerite–galena–chalcopy-rite–pyrite intergrowths.
U–Pb GEOCHRONOLOGY
Geochronological studies were initiated in an attempt tofurther characterize the stratigraphy of the southern TVBand constrain the timing of VMS mineralization. The databelow summarize results of U–Pb TIMS and SHRIMPgeochronology from the host rocks of the Boomerangdeposit. The U–Pb geochronology analysis was done at theGeochronology Laboratory, of the Geological Survey ofCanada, as part of a collaborative study with the GSC.
ANALYTICAL METHODS
The geochronology samples were collected from drill-core. Heavy mineral concentrates were prepared by standardcrushing, grinding, Wilfley table, and heavy liquid separa-tion techniques. Mineral separates were sorted by magneticsusceptibility using a FrantzTM isodynamic separator.
The U–Pb isotope dilution-thermal ionization massspectrometer (ID-TIMS) analytical methods utilized in thisstudy are outlined in Parrish et al. (1987). Multigrain zirconfractions for TIMS analyses comprised between 10 to 25grains (see Table 1) and were very strongly air abraded fol-lowing the method of Krogh (1982). Details of zircon mor-phology and quality are summarized in Table 1. Treatmentof analytical errors follows Roddick (1987), with regressionanalysis modified after York (1969). The U–Pb TIMS ana-lytical results are presented in Table 1, where errors on theages are reported at the 2F level, and displayed in a concor-dia plot (Figure 3a).
20
Tabl
e 1.
U–P
b TI
MS
anal
ytic
al d
ata
Isot
opic
Rat
ios6
Age
s (M
a)8
Frac
t.1D
escr
iptio
n2W
t.U
Pb
320
6 Pb4
Pb5
208 Pb
207 Pb
±1SE
206 Pb
±1SE
Cor
r.720
7 Pb±1
SE20
6 Pb±2
SE20
7 Pb±2
SE20
7 Pb±2
SE%
ugpp
mpp
m20
4 Pb
pg20
6 Pb23
5 UA
bs23
8 UA
bsC
oeff
.20
6 PbA
bs23
8 U23
5 U20
6 PbD
isc
JHC
-06-
239
(z91
27):
Ash
/lapi
lli tu
ff, B
oom
eran
g de
posi
tA
1 (Z
;21
) C
o,C
lr,fI
n,fF
r,Eu,
El, D
ia30
363
106
630.
210.
7038
20.
0132
00.
0816
40.
0004
20.
700
0.06
253
0.00
097
505.
95.
054
1.1
15.7
692.
165
.128
.0A
2 (Z
;20)
C
o,C
lr,fI
n,fF
r,Eu,
El, D
ia40
575
407
290.
190.
6288
10.
0027
80.
0794
60.
0001
20.
698
0.05
739
0.00
020
492.
91.
549
5.4
3.5
506.
715
.42.
8A
3 (Z
;21)
C
o,C
lr,fI
n,fF
r,Eu,
El, D
ia41
817
372
470.
180.
6319
90.
0031
20.
0798
90.
0001
30.
709
0.05
738
0.00
023
495.
51.
649
7.3
3.9
506.
017
.32.
2B
1 (Z
;19)
B
r,Clr,
fIn,
fFr,E
u,Pr
,Dia
3359
540
025
0.19
0.62
987
0.00
285
0.07
949
0.00
013
0.70
00.
0574
70.
0002
149
3.1
1.5
496.
03.
650
9.6
15.7
3.4
B2
(Z;2
5)
Co,
Clr,
fIn,
fFr,E
u,Pr
,Dia
4083
725
370
0.18
0.63
532
0.00
430
0.07
980
0.00
017
0.70
20.
0577
40.
0003
249
4.9
2.0
499.
45.
352
0.0
23.9
5.0
C1
(Z;1
0)
Br,C
lr,fI
n,fF
r,Eu,
St,D
ia33
645
673
160.
180.
6320
90.
0017
90.
0793
20.
0001
10.
686
0.05
779
0.00
012
492.
11.
349
7.4
2.2
521.
99.
45.
9C
2 (Z
;15)
C
o.C
lr,fI
n,fF
r,Eu,
St,D
ia60
998
266
120.
170.
6641
30.
0069
00.
0799
30.
0001
70.
530
0.06
026
0.00
057
495.
72.
051
7.1
8.4
612.
940
.219
.9
Not
es:
1 Z=zi
rcon
. Num
ber i
n br
acke
t ref
ers
to th
e nu
mbe
r of g
rain
s in
the
anal
ysis
.2 Fr
actio
n de
scrip
tions
: Br=
Ligh
t Bro
wn,
Co=
Col
ourle
ss, C
lr=C
lear
, fFr
=Few
Fra
ctur
es, f
In=F
ew In
clus
ions
, Eu=
Euhe
dral
, El=
Elon
gate
, Pr=
Pris
mat
ic, S
t=St
ubby
Pris
m, D
ia=D
iam
agne
tic.
3 Rad
ioge
nic
Pb4 M
easu
red
ratio
, cor
rect
ed fo
r spi
ke a
nd fr
actio
natio
n5 To
tal c
omm
on P
b in
ana
lysi
s co
rrec
ted
for f
ract
iona
tion
and
spik
e6 C
orre
cted
for b
lank
Pb
and
U a
nd c
omm
on P
b, e
rror
s qu
oted
are
1 s
igm
a ab
solu
te; p
roce
dura
l bla
nk v
alue
s fo
r thi
s st
udy
rang
ed fr
om <
0.1-
0.1
pg
for U
and
1-3
pg
for P
b; P
b bl
ank
isot
opic
co
mpo
sitio
n is
bas
ed o
n th
e an
alys
is o
f pro
cedu
ral b
lank
s; c
orre
ctio
ns fo
r com
mon
Pb
wer
e m
ade
usin
g St
acey
-Kra
mer
s co
mpo
sitio
ns7 C
orre
latio
n co
effic
ient
8 Cor
rect
ed fo
r bla
nk a
nd c
omm
on P
b, e
rror
s qu
oted
are
2 s
igm
a in
Ma
J.G. HINCHEY AND V. McNICOLL
The Sensitive High Resolution Ion MicroProbe(SHRIMP II) analyses were conducted using analytical anddata reduction procedures described by Stern (1997) andStern and Amelin (2003). Zircons from the samples andfragments of the GSC laboratory zircon standard (z6266 zir-con, with 238U/206Pb age = 559 Ma) were cast in an epoxygrain mount (mount IP419), polished with diamond com-pound to reveal the grain centres, and photographed in trans-mitted light. The mount was evaporatively coated with 10nm of high purity Au, and the internal features of the zirconswere characterized with backscattered electrons (BSE) uti-lizing a scanning electron microscope (SEM). Analyseswere conducted using an O! primary beam projected ontothe zircons with an elliptical spot size of 25 Fm (in thelongest dimension). The count rates of ten isotopes of Zr+,U+, Th+, and Pb+ in zircon were sequentially measured usinga single electron multiplier. Off-line data processing wasaccomplished using customized in-house software. TheSHRIMP analytical data is presented in Table 2. CommonPb-corrected ratios and ages are reported with 1F analyticalerror, which incorporate an external uncertainty of 1.1% incalibrating the standard zircon (see Stern and Amelin, 2003).The 238U/206Pb ages for the analyses have been corrected forcommon Pb using both the 204- and 207-methods (Stern,1997), but there is no significant difference in the results.
The data are plotted in concordia diagrams with errorsat the 2F level (Figures 3b and 3c), using Isoplot v. 3.0(Ludwig, 2003) to generate the plots. A Concordia age (Lud-wig, 1998) is calculated for some of the samples presented.A Concordia age incorporates errors on the decay constantsand includes both an evaluation of concordance and an eval-uation of equivalence of the data. The calculated Concordiaage and errors quoted in the text are at 2F with decay con-stant errors included.
RESULTS
Sample JHC-06-239 (z9127)
A sample of intermediate ash to lapilli tuff was collect-ed from diamond-drill core, Hole GA-05-016 (interval345.7–360.1 m). The sample sits directly above the massivesulphide zone at the Boomerang deposit. The sulphide in thedeposit replaces this same rock type (Squires et al., 2005;Hinchey, 2007) suggesting that the rock was not completelyconsolidated during the time of the mineralizing processes,and, as such, the age is interpreted as being syn-mineraliza-tion.
The sample yielded abundant zircon of fairly goodquality, and only minor fractures and inclusions were pres-ent in almost all of the grains; zircon morphology rangesfrom stubby prismatic to elongate. Multigrain zircon frac-tions were analyzed by ID-TIMS. These data, which were
21
Figure 3. a) U–Pb concordia diagram for the ash/lapilli tufffrom the Boomerang deposit; analyzed using ID-TIMS tech-niques, b) U–Pb concordia diagram for the ash/lapilli tufffrom the Boomerang deposit; analyzed using SHRIMP IItechniques. A cumulative probability plot of the data is inset,c) U–Pb concordia diagram for the felsic dyke from theBoomerang deposit; analyzed using SHRIMP II techniques.A cumulative probability plot of the data is inset.
CURRENT RESEARCH, REPORT 09-1
analyzed in four different U–Pb chemistry batches, containa significant amount of common lead (12–70 pg, Table 1),which is related to the inclusions present in most of the zir-con, as opposed to procedural lead blanks. Some of the zir-con analyses are quite discordant (20–28%) and containinherited components (C2 and A1, not plotted); however,some of the fractions are nearly concordant (Figure 3a;Table 1). A weighted average of the 238U/206Pb ages of themost concordant analyses is calculated to be 493.0 ± 2.4 Ma(MSWD=1.8). There is some scatter of the data, which ismost likely a result of minor inheritance in these analyses.
Representative zircons from the sample were placed ona grain mount for imaging with a backscatter detector on ascanning electron microscope. Many of the zircons are inter-preted as magmatic having well-defined oscillatory zoning(Plate 2a). Other grains appear to contain inherited cores andshow good core-rim relationships.
The SHRIMP data for this sample define a cluster over-lapping concordia (Figure 3b; Table 2). A Concordia age iscalculated to be 490.6 ± 2.9 Ma (MSWD of concordanceand equivalence = 0.72; probability = 0.95; n=22). This cal-
22
Plate 2. Representative back-scattered electron images of zircons from: a) felsic ash – lapilli tuff from the Boomerang depositand b) felsic dyke from the Boomerang deposit.
J.G. HINCHEY AND V. McNICOLL
23
Tabl
e 2.
U/P
b SH
RIM
Pan
alyt
ical
dat
aA
ges
(Ma)
Spot
UT
hT
hPb
*20
4 Pb20
4 Pb±
204 Pb
208 Pb
± 20
8 Pb20
7 Pb±20
7 Pb20
6 Pb±20
6 PbC
orr
207 Pb
±207 Pb
206 Pb
±206 Pb
207 Pb
±207 Pb
nam
e(p
pm)
(ppm
)U
(ppm
)(p
pb)
206 Pb
206 Pb
f(20
6)20
420
6 Pb20
6 Pb23
5 U23
5 U23
8 U23
8 UC
oeff
206 Pb
206 Pb
238 U
238 U
206 Pb
206 Pb
JHC
-06-
239
(z91
27):
Ash
/lapi
lli tu
ff, B
oom
eran
g de
posi
t91
27-1
.110
843
0.41
09
20.
0002
480.
0002
690.
0043
0.12
550.
0117
0.58
510.
0515
0.07
960.
0013
0.29
80.
0533
0.00
4549
47
341
204
9127
-2.1
105
440.
435
91
0.00
0101
0.00
0225
0.00
180.
1311
0.01
300.
6021
0.04
410.
0794
0.00
120.
327
0.05
500.
0038
492
741
316
491
27-3
.110
946
0.43
89
00.
0000
100.
0000
100.
0002
0.14
090.
0065
0.64
360.
0200
0.08
000.
0010
0.52
40.
0584
0.00
1649
66
544
5991
27-4
.114
287
0.63
612
30.
0003
030.
0002
200.
0052
0.18
500.
0122
0.60
780.
0454
0.07
940.
0012
0.31
30.
0555
0.00
4049
27
434
168
9127
-6.1
6435
0.56
05
10.
0002
960.
0004
290.
0051
0.16
370.
0180
0.55
120.
0808
0.07
880.
0014
0.24
10.
0507
0.00
7348
98
227
301
9127
-7.1
134
590.
456
110
0.00
0010
0.00
0010
0.00
020.
1609
0.00
580.
6229
0.01
750.
0793
0.00
100.
566
0.05
700.
0013
492
649
152
9127
-8.1
5523
0.42
14
00.
0000
100.
0000
100.
0002
0.14
200.
0084
0.63
260.
0573
0.07
850.
0017
0.35
40.
0585
0.00
5048
710
548
198
9127
-9.1
8849
0.57
67
20.
0003
410.
0002
480.
0059
0.15
190.
0181
0.61
130.
0489
0.07
900.
0013
0.32
20.
0562
0.00
4349
08
458
179
9127
-10.
112
257
0.48
49
40.
0004
940.
0002
540.
0086
0.13
130.
0110
0.54
260.
0478
0.07
660.
0011
0.27
80.
0514
0.00
4447
66
258
209
9127
-11.
170
270.
395
53
0.00
0612
0.00
0344
0.01
060.
1108
0.01
970.
5747
0.06
240.
0767
0.00
120.
269
0.05
440.
0057
476
738
725
691
27-1
2.1
178
880.
508
150
0.00
0010
0.00
0010
0.00
020.
1639
0.00
500.
6329
0.02
320.
0800
0.00
110.
470
0.05
740.
0019
496
650
773
9127
-13.
198
590.
626
87
0.00
1009
0.00
0352
0.01
750.
1630
0.01
620.
4974
0.06
380.
0788
0.00
110.
231
0.04
580.
0058
489
70
091
27-1
4.1
177
110
0.64
215
00.
0000
150.
0002
860.
0003
0.19
500.
0120
0.64
360.
0529
0.07
910.
0012
0.30
10.
0590
0.00
4749
07
569
182
9127
-15.
117
011
60.
705
150
0.00
0010
0.00
0010
0.00
020.
2074
0.00
660.
6532
0.02
630.
0804
0.00
110.
444
0.05
900.
0021
498
656
681
9127
-18.
173
420.
597
60
0.00
0010
0.00
0010
0.00
020.
2046
0.01
610.
6479
0.02
320.
0786
0.00
100.
481
0.05
980.
0019
488
659
570
9127
-19.
151
220.
442
40
0.00
0010
0.00
0010
0.00
020.
1473
0.01
040.
6605
0.02
960.
0798
0.00
130.
472
0.06
000.
0024
495
860
489
9127
-20.
195
350.
378
81
0.00
0118
0.00
0362
0.00
210.
1231
0.01
590.
6373
0.07
380.
0796
0.00
120.
247
0.05
810.
0066
494
753
226
991
27-2
1.1
102
440.
450
80
0.00
0050
0.00
0340
0.00
090.
1408
0.01
410.
6196
0.06
310.
0795
0.00
140.
294
0.05
650.
0056
493
847
423
391
27-2
2.1
162
104
0.66
014
30.
0002
250.
0002
780.
0039
0.19
760.
0122
0.60
050.
0531
0.08
010.
0013
0.31
00.
0544
0.00
4649
78
387
202
9127
-23.
112
951
0.41
111
30.
0002
860.
0002
520.
0050
0.12
720.
0110
0.65
840.
0591
0.08
110.
0012
0.28
40.
0589
0.00
5150
37
563
201
9127
-63.
114
656
0.39
412
00.
0000
310.
0001
820.
0005
0.11
890.
0131
0.62
010.
0393
0.07
890.
0012
0.35
60.
0570
0.00
3448
97
493
137
9127
-65.
119
513
00.
685
173
0.00
0262
0.00
0337
0.00
460.
2050
0.01
470.
6094
0.06
230.
0790
0.00
130.
286
0.05
590.
0055
490
844
923
691
27-2
5.1
104
650.
645
101
0.00
0186
0.00
0275
0.00
320.
1890
0.01
250.
6578
0.06
350.
0856
0.00
160.
315
0.05
580.
0052
529
1044
322
0
JHC
-06-
240
(z91
28):
Fel
sic
dyke
, Boo
mer
ang
depo
sit
9128
-8.1
225
121
0.55
719
00.
0000
100.
0000
100.
0002
0.17
080.
0046
0.62
610.
0155
0.07
980.
0010
0.58
80.
0569
0.00
1249
56
489
4591
28-9
.122
212
30.
572
180
0.00
0010
0.00
0010
0.00
020.
1759
0.00
670.
6307
0.02
050.
0772
0.00
090.
475
0.05
920.
0017
480
557
564
9128
-2.1
229
108
0.48
719
00.
0000
100.
0000
100.
0002
0.15
830.
0099
0.63
810.
0216
0.07
760.
0010
0.48
10.
0596
0.00
1848
26
590
6691
28-3
.112
554
0.44
510
30.
0003
590.
0002
680.
0062
0.12
420.
0113
0.55
380.
0490
0.07
750.
0011
0.28
50.
0519
0.00
4448
17
279
208
9128
-4.1
162
710.
455
130
0.00
0010
0.00
0010
0.00
020.
1388
0.00
510.
6169
0.01
780.
0794
0.00
110.
566
0.05
640.
0014
492
646
754
9128
-5.1
137
630.
474
110
0.00
0010
0.00
0010
0.00
020.
1514
0.00
600.
6279
0.02
060.
0796
0.00
100.
499
0.05
720.
0016
494
650
064
9128
-6.1
262
160
0.63
223
10.
0000
630.
0001
630.
0011
0.19
890.
0077
0.63
890.
0387
0.08
040.
0010
0.32
40.
0576
0.00
3349
96
515
132
9128
-10.
118
110
30.
590
152
0.00
0197
0.00
0392
0.00
340.
1747
0.01
650.
6083
0.07
620.
0798
0.00
120.
246
0.05
530.
0068
495
742
329
991
28-1
1.1
290
139
0.49
424
00.
0000
100.
0000
100.
0002
0.15
270.
0041
0.61
710.
0151
0.07
870.
0010
0.63
70.
0569
0.00
1148
86
487
4391
28-1
2.1
131
660.
520
110
0.00
0019
0.00
0288
0.00
030.
1579
0.01
360.
6447
0.05
530.
0806
0.00
120.
289
0.05
810.
0048
499
753
219
291
28-1
3.1
156
680.
447
122
0.00
0184
0.00
0176
0.00
320.
1227
0.01
020.
5667
0.04
920.
0788
0.00
120.
297
0.05
220.
0044
489
729
420
391
28-1
4.1
136
690.
524
111
0.00
0120
0.00
0195
0.00
210.
1533
0.01
490.
6205
0.03
910.
0786
0.00
120.
360
0.05
730.
0034
488
750
213
691
28-1
5.1
161
740.
472
130
0.00
0010
0.00
0010
0.00
020.
1505
0.01
220.
6552
0.01
850.
0795
0.00
100.
539
0.05
980.
0014
493
659
653
9128
-17.
112
356
0.47
010
00.
0000
100.
0000
100.
0002
0.15
020.
0081
0.62
850.
0235
0.08
010.
0012
0.49
80.
0569
0.00
1949
77
487
7491
28-7
.129
815
30.
529
242
0.00
0095
0.00
0077
0.00
170.
1587
0.00
480.
6019
0.02
020.
0773
0.00
110.
519
0.05
650.
0016
480
647
065
9128
-19.
119
011
80.
640
161
0.00
0055
0.00
0187
0.00
100.
2020
0.00
890.
6537
0.04
020.
0798
0.00
110.
334
0.05
940.
0035
495
658
113
291
28-3
.217
879
0.45
714
70.
0005
500.
0002
790.
0095
0.13
410.
0126
0.56
930.
0525
0.07
860.
0013
0.29
40.
0525
0.00
4748
88
308
216
9128
-4.2
408
216
0.54
635
80.
0002
900.
0003
090.
0050
0.15
880.
0126
0.62
700.
0567
0.08
140.
0011
0.27
60.
0559
0.00
4950
47
449
208
9128
-6.2
247
191
0.79
821
90.
0005
350.
0003
520.
0093
0.22
350.
0180
0.57
800.
0631
0.07
780.
0015
0.29
30.
0539
0.00
5748
39
367
256
9128
-8.2
217
143
0.68
219
10.
0000
530.
0001
700.
0009
0.21
440.
0094
0.66
710.
0380
0.07
990.
0010
0.34
70.
0606
0.00
3349
66
623
120
9128
-16.
196
862
50.
667
870
0.00
0006
0.00
0018
0.00
010.
2068
0.00
240.
6572
0.01
080.
0823
0.00
100.
798
0.05
790.
0006
510
652
722
9128
-18.
112
0410
150.
871
113
10.
0000
100.
0000
100.
0002
0.25
890.
0056
0.64
950.
0126
0.08
300.
0009
0.67
60.
0568
0.00
0851
46
482
32
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CURRENT RESEARCH, REPORT 09-1
culation utilized all of the SHRIMP analyses except one thatis highlighted in red on Figure 3b. This older analysis at ca.530 Ma (Table 2) is interpreted to be from an inherited zir-con. The date of 491 ± 3 Ma is taken to be the crystallizationage of the zircon in the tuff and constrains the timing of min-eralization. Although the sulphides have replaced this hostrock, the interval between deposition and mineralization isnot considered significant (e.g., see Hinchey, 2007).
Sample JHC-06-240 (z9128)
A sample of a felsic dyke was collected from diamond-drill core, Hole GA-05-079 (interval 247.1–264.0 m),directly above the sulphide zone at the Boomerang deposit.The sample yielded a very small amount of zircon; notenough material for ID-TIMS analysis, so the zirconretrieved was placed on a grain mount and analyzed usingthe SHRIMP. Backscatter SEM images reveal oscillatory-zoned grains that appear magmatic and also grains with pos-sible inherited cores (Plate 2b).
The SHRIMP data define a cluster of data overlappingconcordia (Figure 3c; Table 2). A Concordia age, utilizingall of the SHRIMP analyses except 2 outlined below, is cal-culated to be 490.7 ± 2.9 Ma (MSWD of concordance andequivalence = 0.96; probability = 0.54; n=20). This date of491 ± 3 Ma is interpreted to be the crystallization age of thefelsic dyke and it suggests that the dyke is synchronous withthe tuffaceous rocks represented by sample JHC-06-239(z9127).
Two of the analyses are slightly older with ages of ca.510 and 514 Ma (Table 2; highlighted in red on Figure 3c).These zircons are interpreted to be entirely inherited grainsin the rock. These inheritance ages are the same as the crys-tallization ages of volcanic rocks previously dated at theDuck Pond deposit (McNicoll et al., 2008). As the zirconyield was poor in the sample from this study, there is thepossibility that all of the zircon in the felsic dyke isxenocrystic, i.e., inherited from the rocks that the dyke hasintruded.
GEOCHEMISTRY
A representative suite of all volcanic and volcaniclasticrock types from the TVB was analyzed for major and traceelements using Inductively Coupled Plasma – EmissionSpectrometry (ICP-ES) methods outlined in Finch (1998). Asmaller subset of samples was analyzed at Acme AnalyticalLaboratories for trace elements and rare-earth elements(REE) by four-acid digestion, Inductively Coupled Plasma –Mass Spectrometry (ICP-MS) techniques. Samples wereseparated into groups based upon their association with thespecific VMS deposits described above and, where possible,
were further divided into hanging wall and footwall cate-gories to aid in identifying the affects of alteration. Selectedsamples were also analyzed for Sm/Nd isotopic composi-tions (Table 3) by Thermal Ionization Mass Spectrometry(TIMS) techniques at Memorial University. The completegeochemical database derived from this project will bereleased later, when the project is completed. Selectedchemical ratios for various rock types from each deposit arelisted in Table 3 to aid in discussion and comparison.
The samples analyzed in this study cover all rock typesand include both altered and unaltered samples. It is verydifficult to obtain unaltered material for geochemical analy-ses, because outcrop is limited, and exploration is naturallyfocused around areas of mineralization. Most of the samplesrepresent drillcore.
ELEMENT MOBILITY CONSIDERATIONS
It is important to account for the effects of elementmobility, especially if using lithogeochemistry to makeinferences about tectonic settings or primary rock composi-tions. As the major rock types within the TVB comprise fel-sic-intermediate volcanic rocks, it is necessary to firstunderstand the effects that the replacement of primary min-erals (predominantly feldspar), and volcanic glass by sec-ondary hydrothermal minerals, have upon chemical elementsystematics. The most common hydrothermal alterationprocess affecting the felsic-intermediate volcaniclastic rockshosting VMS deposits of the southern TVB is replacementof primary feldspar by sericite, effectively resulting in a gainof K from hydrothermal fluids and a loss of Na and Ca fromthe rock. Additional replacement of feldspars and sericite bychlorite results in an addition of Mg ± Fe to the rocks. Basedon the degree of alteration of the samples within this study,it is assumed that the Na, K, Ca, Mg, Fe and SiO2 have actedas mobile elements. Although other major oxides such asAl2O3 and TiO2 are commonly assumed to be immobileunder most alteration conditions (e.g., Barrett and MacLean,1999), the presence of intense alteration, including locallyintense carbonate alteration proximal to massive sulphidehorizons, suggests that these elements should be also usedwith caution. For further discussions on the potential mobil-ity of these elements, as well as other high-field-strength-elements (HFSEs), see Pandarinath et al. (2008), Jiang et al.(2005), Finlow-Bates and Stumpfl (1982), and Hynes(1980). The low-field-strength-elements (e.g., Ba, Rb, Cs,Sr) are considered to be mobile under the alteration condi-tions in this study, and are not used to discriminate betweenrock types. The REEs (with the exception of Eu (e.g., Sver-jensky, 1984; Whitford et al., 1988)) are generally consid-ered to be immobile except under extreme hydrothermalalteration conditions, when the light REEs may becomemobile (Campbell et al., 1984; MacLean and Barrett, 1993).
24
J.G. HINCHEY AND V. McNICOLL
The coherent behaviour of the heavy REEs in the samplessuggest that they were immobile, whereas there are slightshifts in light REE concentrations (especially La) indicatingsome mobility. The HFSEs, e.g., Zr, Hf, Nb, Ta, Y, Th, areimmobile in almost all cases (e.g., Barrett and MacLean,1999; Lentz, 1999). The coherent behaviour of these ele-ments in the samples suggests that they remained essential-ly immobile during alteration. The samples analyzed in thisstudy include both volcanic and volcaniclastic rocks.Whereas element immobility can be proven in strictlycoherent volcanic facies in the belt, other factors couldaffect the lithogeochemistry of tuffaceous rocks.
The geochemical data from the three main deposits inthe southern TVB, as well as that of the Pats Pond group ofZagorevski et al. (2007a) and Rogers (2004), are plotted onan alteration box plot in Figure 4 (Large et al., 2001). Thisplot uses two common alteration indexes viz., the Hashimo-to Alteration Index (AI; Ishikawa et al., 1976) and the chlo-rite–carbonate–pyrite index (CCPI; Large et al., 2001; seeTable 3 for formulae). High AI values represent sericite andchlorite alteration products from the breakdown of plagio-clase feldspars and volcanic glass; whereas high CCPI val-ues represent chlorite, Fe–Mg carbonates and pyrite alter-ation typically associated with VMS deposits. Most of thefelsic-intermediate rock types that host the VMS deposits atthe Tulks Hill, Tulks East and Boomerang deposits are dis-placed to the right of the 'least altered box', with relativelyhigh AI and CCPI, suggesting that they have been widelyaffected by hydrothermal activity. For this reason, emphasisis placed on relatively immobile REE and HFSE in the fol-lowing discussion. In contrast, the data from the Pats Pondgroup (Zagorevski, 2007a) do not show strong alterationeffects as the focus of that study was to sample for geo-chemical correlations and tectonic discrimination, withaltered rocks being avoided.
Tulks East Deposit
The host rocks to the Tulks East deposit are bimodal.The felsic-intermediate (andesitic composition) host rocksare subdivided into crystal tuffs, lapilli tuffs, and rhyolites.Quartz-phyric crystal tuffs dominate both the hanging walland footwall sequences, and samples from both areas haverelatively low Zr/TiO2 (283 and 272) and Nb/Y ratios (0.07and 0.03), suggestive of a subalkaline affinity (Table 3; Fig-ure 5). Felsic lapilli tuffs from the hanging wall and footwallhave very similar Nb/Y ratios with slightly lower Zr/TiO2
ratios (201 and 189 respectively), whereas rhyolite from thehanging wall and footwall have similar Zr/TiO2 but morevariable Nb/Y ratios. Quartz–feldspar-phyric crystal tuffsfrom the hanging wall have a distinctly higher Zr/TiO2 ratio(809) but with similar Nb/Y ratios. However, the average isskewed by one sample with a high Zr/TiO2 ratio. The HFSE
(e.g., Zr, Hf, Y, Nb and Ta) contents of all the felsic-inter-mediate rocks are low to moderate, characterizing them asvolcanic-arc to ocean-ridge-type rocks on the commonlyused HFSE diagrams (Figure 6). Primitive-mantle-normal-ized plots for the felsic-intermediate rocks (Figures 7 and 8),are characterized by weak to moderate LREE enrichments(with the exception of the footwall felsic tuffs that have aslight depletion), as shown by the LaN/SmN ratios in Table 3,slight depletions in MREE causing the slightly concaveupward patterns (refer to the GdN/LuN ratios in Table 3 andFigures 7 and 8), slight to moderate overall REE fractiona-tions, strongly negative Nb and Ti anomalies, variablystrong negative Eu anomalies, and slightly positive Zr andHf anomalies. Most of the felsic-intermediate rocks havevery low Zr/Y and La/Yb ratios, similar to published valuesfor tholeiitic rocks (Barrett and MacLean, 1999; Table 3).The Sm/Nd isotopic analyses of two samples of felsic tuffyielded gNd (498 Ma) values of +2.90 and +3.10 (Table 4).
The mafic volcanic rocks from Tulks East are charac-terized by relatively flat, extended trace-element plots, withvalues three to four times that of primitive mantle concen-trations (Figure 9). Samples display a strong negative Nbanomaly, a slight positive Eu anomaly, and slightly negativeZr and Hf anomalies. The one sample of an andesitic sill hasa unique chemistry with a downward concave, extendedtrace-element profile, similar to that associated with ocean-island or back-arc-basin basalt. On a Ti-V discriminationdiagram (after Shervais, 1982), the mafic volcanics plot pre-dominantly in the island-arc tholeiite field, whereas the onesample of an andesitic sill plots in the alkaline field (Figure10), and on a Th-Zr-Nb plot (after Wood, 1980) the maficvolcanics plot predominantly in the arc-basalt field with theone sample of the andesitic sill plotting at the N-MORB toE-MORB boundary.
Tulks Hill Deposit
The host rocks to the Tulks Hill deposit are alsobimodal. Felsic host rocks are dominated by quartz-phyricrhyolite, with minor amounts of quartz-phyric felsic tuff,quartz–feldspar-phyric tuff and lapilli tuff. Rhyolite fromthe hanging wall and footwall have relatively high averageZr/TiO2 ratios (928 and 972) and low to moderate Nb/Yaverage ratios (0.06 and 0.08), suggestive of a subalkalineaffinity (Table 3; Figure 5). However, there appears to be atleast two subgroups of rhyolite, viz., a group having rela-tively low HFSE and REE and a group with relatively high-er HFSE and REE (see Figures 5, 6, and 8; not subdividedfor the purposes of Table 3). Using this chemical differenti-ation, the average Zr/TiO2 of the enriched group is 1420 ±100, significantly higher than the other rhyolite. As there isno way to visually or petrographically distinguish theserocks, they have been combined in one group for compari-
25
CURRENT RESEARCH, REPORT 09-1
26
Table 3. Summary of some key major- and trace-element ratios for the felsic rocks associated with the VMS deposits in thesouthern TVB and the Pats Pond group
TULKS EAST STRATIGRAPHYFel. Lapilli Tuff
Fel. Tuff (HW) Fel.Tuff (FW) HW FW Qtz-fld Tuff (HW) Rhyolite (HW) Rhyolite (FW)Average Average AverageAverage Average Average Average
n = 4, 1 n = 5, 3 n = 1, 1 n = 2, 1 n = 3, 2 n = 2, 2 n = 1, 1
TULKS HILL STRATIGRAPHYFel. Tuff (HW) Fel.Tuff (FW) Fel. Lapilli Tuff (HW) Qtz-fld Tuff (HW) Rhyolite (HW) Rhyolite (FW)Average Average Average Average Average Average
n = 2, 0 n = 2, 2 n = 1, 0 n = 1, 1 n = 12, 5 n = 3, 1
AI= Hashimoto index = 100*[(MgO+K2O)/(MgO+K2O+Na2O+CaO)] (Ishikawa et al., 1976)CCPI = chlorite-carbonate-pyrite index = 100*[(MgO+FeO*0/(MgO+FeO*+K2O+Na2O)] (Large et al., 2001)Na/K = Na2O/K2Osamples normalized to primitive mantle (values after Sun and McDonough, 1989)Eu/Eu* = Eupm/(Gdpm*Smpm)0.5, Nb/Nb* = 0.5*Nbpm/(Thpm+Lapm), Zr/Zr* = 0.5*Zrpm/(Gdpm + Smpm), Ti/Ti* = 0.5*Tipm/(Gdpm + Smpm)pm = primitive mantle normalizedn= number of samples analyzed for (major-element analysis, trace-element and REE alalysis)
CURRENT RESEARCH, REPORT 09-1
son purposes. Felsic tuffs, from the hanging wall and foot-wall, have much lower Zr/TiO2 averages (84 and 460) com-pared to the rhyolite, and have similar Nb/Y ratios. Althoughnot analyzed for REEs, a sample of a quartz–feldspar-phyrictuff from the hanging wall of the deposit also has very highZr/TiO2 (1512), similar to the subset of rhyolite describedabove. The HFSE (e.g., Zr, Hf, Y, Nb, Ta) contents of all thefelsic-intermediate rocks are low to moderate, characteriz-ing them as volcanic-arc to ocean-ridge-type rocks on com-monly used HFSE diagrams (Figure 6). Primitive-mantle-normalized plots for the felsic-intermediate rocks are char-acterized by moderate to strong LREE enrichments (see theLaN/SmN ratios in Table 3 and Figures 7 and 8), slight deple-tions in MREE causing the slightly concave upward patterns(refer to the GdN/LuN rations in Table 3 and Figures 7 and 8),moderate overall REE fractionations, strongly negative Nb
and Ti anomalies, slightly negative Eu anomalies, andapproximately flat Zr and Hf. Most all of the felsic-interme-diate rocks have very low Zr/Y and La/Yb ratios, similar topublished values for tholeiitic rocks (Barrett and MacLean,1999; Table 3). The Sm/Nd isotopic composition analysis oftwo samples of felsic tuff yielded gNd (498 Ma) values of+2.99 and +3.02 (Table 4).
The mafic volcanics from Tulks Hill are characterizedby weak to moderately fractionated extended trace-elementplots, with values four to ten times that of primitive mantleconcentrations (Figure 9). Samples display a strong negativeNb anomaly, a strong negative Ti anomaly in two of the foursamples, a slightly positive Eu anomaly, and negative Zr andHf anomalies. As such, the samples appear as two groupswith calc-alkaline basalt to island-arc tholeiite signatures.
28
Figure 4. Alteration box plots of Large et al. (2001), with vectors for various alteration minerals and alteration versus dia-genetic fields. CCP index = chlorite–carbonate–pyrite index. A) = Tulks East deposit, B) = Tulks Hill deposit, C) =Boomerang deposit, D) Pats Pond group. AI= Hashimoto index = 100*[(MgO+K2O)/(MgO+K2O+Na2O+CaO)] (Ishikawaet al., 1976), CCPI = chlorite–carbonate–pyrite index = 100*[(MgO+FeO)/(MgO+FeO+K2O+Na2O)] (Large et al., 2001).
J.G. HINCHEY AND V. McNICOLL
29
Figure 5. Nb/Y versus Zr/TiO2 rock type classification diagram (Winchester and Floyd, 1977). A) Tulks East deposit, B) TulksHill deposit, C) Boomerang deposit, D) Pats Pond group. Pats Pond group chemistry from Rogers (2004) and Zagorevski etal. (2007a).
CURRENT RESEARCH, REPORT 09-1
On a Ti-V discrimination diagram, the two mafic volcanicsamples, with the island-arc tholeiite signature, plot in thesame field, whereas those with calc-alkaline basalt signa-tures contain very little V and plot toward the bottom of the
diagram (Figure 10), and on a Th-Zr-Nb plot (after Wood,1980) all of the mafic volcanic samples plot in the arc-basaltfield.
30
Figure 6. Yb versus Ta (Pearce et al., 1984) discrimination diagrams for the host felsic volcanic rocks of the three main VMSdeposits in the southern Tulks Volcanic Belt and the Pats Pond group. A) Tulks East deposit, B) Tulks Hill deposit, C)Boomerang deposit, D) Pats Pond group. Pats Pond group chemistry from Rogers (2004) and Zagorevski et al. (2007a).
J.G. HINCHEY AND V. McNICOLL
Boomerang Deposit
Host rocks to the Boomerang deposit appear to form acontinuum from mafic through to felsic-intermediate com-
positions. However, the andesitic sills illustrated in Figure 5have different chemical characteristics and may not begenetically associated with the host rocks to the deposit. Ifthese are excluded, the data have a bimodal distribution. The
31
Figure 7. Primitive-mantle-normalized trace-element plots for the southern Tulks Volcanic Belt and Pats Pond group felsictuff rocks. Note that in 7A to 7C, these rocks host the massive sulphide deposits. A). Tulks East deposit, B) Tulks Hill deposit,C) Boomerang deposit, D) Pats Pond 4 (PP4), E) Pats Pond 6 (PP6). Pats Pond group chemistry from Rogers (2004) andZagorevski et al. (2007a). Primitive mantle values from Sun and McDonough (1989).
CURRENT RESEARCH, REPORT 09-1
felsic-intermediate rocks have much lower Zr/TiO2 ratiosthan those associated with the Tulks Hill deposit, and moreclosely resemble data from Tulks East (Figure 5). Felsic-intermediate host rocks are dominated by quartz-phyric tuff
that have average Zr/TiO2 ratios of 152 and 156, and aver-age Nb/Y ratios of 0.07 and 0.11 in the hanging wall andfootwall, respectively. Quartz–feldspar-phyric tuffs in thehanging wall and footwall have very similar ratios, whereas
32
Figure 8. Primitive-mantle-normalized trace-element plots for other felsic to intermediate rocks from the southern Tulks Vol-canic Belt and Pats Pond group. A). Tulks East deposit, B) Tulks Hill deposit, C) Boomerang deposit, D) Boomerang deposit,E) Pats Pond 3 (PP3). Pats Pond group chemistry from Rogers (2004) and Zagorevski et al. (2007a). Primitive mantle valuesfrom Sun and McDonough (1989).
J.G. HINCHEY AND V. McNICOLL
lapilli tuffs in the hanging wall, ash-rich tuffs, and felsic sillshave higher Zr/TiO2 ratios of 326, 489 and 233 respectively(Table 3). The HFSE (Zr, Hf, Y, Nb, Ta) contents of all thefelsic-intermediate rocks are low to moderate, characteriz-ing them as volcanic-arc to ocean-ridge type rocks on com-monly used HFSE diagrams (Figure 6). Primitive-mantle-normalized plots for the felsic-intermediate rocks are char-acterized by weak to moderate LREE enrichments (refer tothe LaN/SmN ratios in Table 3 and Figures 7 and 8), flatMREE patterns that have only very minor local depletions,weak to moderate overall REE fractionation, weak to mod-erate negative Nb and Ti anomalies, slightly negative Euanomalies, and prominent positive Zr and Hf anomalies forthe felsic-intermediate tuffs, lapilli tuffs, and some felsicsills (Figures 7 and 8). Most of the felsic-intermediate rockshave very low Zr/Y and La/Yb ratios, similar to publishedvalues for tholeiitic rocks (Barrett and MacLean, 1999;Table 3). The Sm/Nd isotopic composition analysis of foursamples of felsic-intermediate tuff from the Boomerang andnearby Domino deposits yielded gNd (491 Ma) values from+4.07 to +5.27 (Table 4).
The mafic volcanic rocks from Boomerang are charac-terized by weak LREE-enriched to flat extended trace-ele-ment plots, with values four to ten times that of primitivemantle concentrations. These characteristics are very similarto the profile for N-MORB (Figure 9). Samples display amoderately positive Th anomaly and a moderately negativeNb anomaly. As such, the samples have island-arc tholeiitesignatures. In contrast, samples of the andesitic sills havemuch higher concentrations of HFSE and REE, displayhigher degrees of REE fractionation, and have variablymoderate to strong negative Nb and Ti anomalies. These
features are suggestive of transitional calc-alkaline basalt toisland-arc tholeiite signatures (Figure 9). On a Ti-V dis-crimination diagram, the mafic volcanic samples plot pre-dominantly within the island-arc tholeiite field but are tran-sitional into the MORB field, whereas the andesitic sills pre-dominantly plot within the alkaline field (Figure 10), and ona Th-Zr-Nb plot (after Wood, 1980) the mafic volcanics plotpredominantly in the arc-basalt field with one sample of theandesitic sill plotting in the N-MORB field.
Pats Pond Group
As mentioned earlier, the Pats Pond group, as definedby Zagorevski et al. (2007a), has been subdivided into sixstratigraphic subunits. For the purposes of this report, thechemistry will be discussed based on rock type, consistentwith the previous descriptions of deposit geochemistry.
Felsic-intermediate rocks of the Pats Pond group arehosted within the PP4 and PP6 subunits. Subunit PP4 con-sists of quartz ± feldspar, felsic-intermediate tuffs, and ischaracterized by relatively low Zr/TiO2 (139) and Nb/Y(0.05) ratios, placing the samples in the subalkalineandesitic field (Figure 5). The HFSE (Zr, Hf, Y, Nb, Ta) con-tents of all the felsic-intermediate rocks are low to moderate,characterizing them as volcanic-arc rocks on commonlyused HFSE diagrams (Figure 6). Primitive-mantle-normal-ized plots for subunit PP4 are characterized by moderateLREE enrichments (refer to the LaN/SmN ratios in Table 3and Figures 7 and 8), moderate depletion of MREE, weakoverall REE fractionations, moderate to strong negative Nband Ti anomalies, slightly negative Eu anomalies, and neg-ligible Zr and Hf anomalies (Figure 7). Subunit PP4 has low
33
Table 4. Sm/Nd isotopic data
ModeleNd Age
Sample Deposit or Unit Sm Nd 147Sm/144Nd 143/144 Age 143/144 CHUR T(DM)(measured) (measured) (Ma) initial (T)c ***DePaolo
** T DM = Nd depleted mantle model age - calculated using the Goldstein 143/144 0.513163; and 147/144 of 0.2137calulated using present day chondritic uniform reservoir with 143Nd/144Nd = 0.512638 & 147Sm/144Nd = 0.1967 ***TDM Depaolo - calculated using the two stage evolution of Depaolo, 1981(143/144) is adjusted from the deviation to JNdi-1 Standard (0.512115), mean measured value of the standard gives 0.512135 +- 9 (2sigma StdDev)Pats Pond data (PP6, PP4, and PP1) from Zagorevski et al., 2007a.
CURRENT RESEARCH, REPORT 09-1
34
Figure 9. Primitive-mantle-normalized trace-element plots for mafic to intermediate volcanics of the southern Tulks VolcanicBelt and Pats Pond group rocks. A) Tulks East deposit basalt and andesite, B) Tulks Hill deposit basaltic sills, C) Boomerangdeposit basaltic sills, D) Boomerang deposit andesitic sills, E) Pats Pond group (PP1) basalt, F) PP2 mafic tuff and PP5intermediate tuff. Pats Pond group chemistry from Rogers (2004) and Zagorevski et al. (2007a). Shown for comparison areN-MORB, E-MORB and OIB. Primitive mantle values, and N-MORB, E-MORB and OIB values from Sun and McDonough(1989). N-MORB = mid-ocean-ridge basalt, E-MORB = enriched mid-ocean-ridge basalt, OIB = ocean-island basalt.
J.G. HINCHEY AND V. McNICOLL
35
Figure 10. Ti-V (Shervais, 1982) discrimination plot for mafic rocks of the southern Tulks Volcanic Belt and Pats Pond group.A) Tulks East deposit, B) Tulks Hill deposit, C) Boomerang deposit, D) Pats Pond group. Pats Pond group chemistry fromRogers (2004) and Zagorevski et al. (2007a). BON = boninite, IAT = island-arc tholeiite, MORB = mid-ocean-ridge basalt,BABB = back-arc-basin basalt.
CURRENT RESEARCH, REPORT 09-1
Zr/Y and La/Yb ratios, similar to published values fortholeiitic rocks (Barrett and MacLean, 1999; Table 3). TwoSm/Nd isotopic composition analysis yielded gNd (487 Ma)values of +4.7 and +5.5 (Table 4).
Subunit PP6 consists of high-silica trondhjemitic rhyo-lite. Compared to subunit PP4, it displays much higherZr/TiO2 ratios with similar Nb/Y (Figure 5). The HFSE (Zr,Hf, Y, Nb, Ta) contents of these rocks are low to moderate,plotting on the boundary of volcanic-arc and ocean-ridge-type rocks on commonly used HFSE diagrams (Figure 6).Primitive-mantle-normalized plots for subunit PP6 are char-acterized by moderate LREE enrichments (refer to theLaN/SmN ratios in Table 3 and Figure 7), moderate depletionof MREE, weak overall REE fractionations, moderate tostrong negative Nb and Ti anomalies, negative Eu anom-alies, and prominent positive Zr and Hf anomalies (Figure7). A Sm/Nd isotopic composition analysis yielded gNd (487Ma) of +3.86 (Table 4).
Intermediate andesitic tuff comprises subunit PP3 and asample from this unit was used to date the Pats Pond group(Zagorevski et al., 2007a). These rocks consist of feldspar ±quartz, felsic-intermediate tuffs and have very similar chem-ical characteristics to subunit PP4 (see Figures 7 and 8).
Mafic to intermediate volcanic rocks of the Pats Pondgroup consist of subunits PP1, PP2 and PP5 of Zagorevskiet al. (2007a). Subunit PP1 consists of transitional calc-alka-line basaltic andesite to island-arc tholeiite where the maficvolcanic rocks have strong, enriched LREE's, strong Thenrichment, prominent negative Nb anomalies, slight nega-tive Ti anomalies, and negative Zr and Hf anomalies (Figure9). Andesitic sills have much higher concentrations of HFSEand REE, display higher degrees of REE fractionation, andhave variably moderate to strong negative Nb and Ti anom-alies; suggestive of transitional calc-alkaline basalt toisland-arc tholeiite signatures. On a Ti-V discrimination dia-gram, the mafic volcanic samples plot predominantly with-in the island-arc tholeiite field (Figure 10), and on a Th-Zr-Nb plot (after Wood, 1980) the mafic volcanics all plot inthe arc-basalt field. The Sm/Nd isotopic composition analy-sis of two samples of subunit PP1 yielded gNd (487 Ma) val-ues from +0.33 to -0.53 (Table 4).
Subunit PP2 consists of calc-alkaline basalt and mafictuffs. This subunit displays very similar chemistry to thePP1 rocks, with the exception that it has overall lower abun-dances of HFSE and REE and it lacks the negative Ti anom-aly (Figure 9). Subunit PP5 consists of an intermediate tomafic tuff and has chemical characteristics very similar tothe mafic volcanic rocks of the Boomerang deposit. Thisunit has fairly flat to weak depleted LREE, with overall flatextended trace-element patterns, and values two to threetimes that of primitive mantle concentrations. The chemical
profile parallels that for N-MORB; although with lower ele-ment concentrations. The unit displays a moderately posi-tive Th anomaly and a moderately negative Nb anomaly, andas such has an island-arc tholeiite signature.
DISCUSSION
This study was begun to attempt to decipher lithologi-cal packages that host VMS mineralization in the southernTulks Volcanic Belt, in hopes of aiding exploration. TheU–Pb geochronological data and geochemical data present-ed in this paper lead to five main conclusions. First, the newU–Pb results show that the volcanic rocks included withinthe TVB actually include felsic rock sequences of at leasttwo ages, i.e., 498 +6/-4 Ma and 491 ± 3 Ma. As a first-orderconclusion, this supports the proposal that the southernTulks Belt is composed of westward-younging volcanicrocks (e.g., van Staal et al., 2005; Zagorevski et al., 2007a).However, the ages from Tulks Hill and Boomerang depositsoverlap in error at their respective lower and upper limits, sothe results do not preclude an interpretation, in which thereis a continuum of volcanism over several million years,rather than having discrete sequences. Second, the agesfrom both Boomerang and Tulks Hill deposits constrain thetiming of VMS mineralization and suggest that there wasmore than one mineralization event in this area. This is par-ticularly true if we assume the age on the porphyry at TulksHill represents a minimum age for mineralization. Third,that the age from the felsic sill at the Boomerang deposit isidentical to the host sequence confirms that bimodal volcan-ism was synchronous with mineralization, as previouslyproposed by Hinchey (2006) based on textural relationships.Fourth, zircon inheritance from the Boomerang deposit sam-ples suggest that, in part, these rocks developed on a sub-strate of older (Cambrian) rocks possibly equivalent to otherparts of the VLSG, such as the Tally Pond group (e.g.,McNicoll et al., 2008; cf. Zagorevski et al., 2007a). Finally,the variations in the felsic and mafic rock types and theirgeochemical and isotopic signatures, suggest that thesesequences probably developed in a complex environment ofepisodic arc development and rifting. These five points arediscussed in more detail below.
AGES OF VOLCANISM AND MINERALIZATION
The identical ages of 491 ± 3 Ma obtained for the felsictuff that hosts the massive-sulphide mineralization atBoomerang, and a crosscutting felsic dyke, closely constrainthe timing of mineralization. The age is younger than the498 +6/-4 Ma age obtained on a subvolcanic porphyry at theTulks Hill deposit (Evans et al., 1990), although their errorenvelopes do overlap at their respective older and youngerlimits. If the revised age of 496.5 ± 1 Ma for Tulks Hill isused (G.R. Dunning, personal communication, 2008), thereis no overlap between these ages.
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J.G. HINCHEY AND V. McNICOLL
The age of 491 ± 3 Ma obtained for the Boomerangdeposit is slightly older than the 487 ± 3 Ma age reported forthe Pats Pond group (Zagorevski et al., 2007a). However,these ages overlap extensively at their older and youngerlimits, respectively.
There is no simple interpretation of these data, but thecloser correspondence of the ages from the Boomerangdeposit and the Pats Pond group suggests that a link betweenthese sequences is more likely than a link betweenBoomerang and the Tulks deposits. The alternative interpre-tation is that all of these subdivisions of the VLSG representnothing more than a long-lived period of volcanism and sed-imentation that extended from ca. 498 to ca. 487 Ma, a peri-od of some 11 Ma.
The identical ages obtained for the felsic dyke and thevolcaniclastic felsic host tuff at the Boomerang deposit haveimplications for the environment of VMS mineralization.The presence of such synchronous sills, both felsic andmafic, is characteristic of high-temperature extensionalregimes such as those found in rifted-arc environments. Thehigh heat flow in such settings leads to the formation ofhydrothermal convection cells and VMS-style mineraliza-tion at or below the seafloor (e.g., Franklin et al., 2005; Gal-ley et al., 2007). A rifted-arc environment is also favouredby the variable chemical signatures of the felsic and maficrocks at Boomerang, as discussed below.
ZIRCON INHERITANCE PATTERNS
The SHRIMP data presented herein indicate zirconinheritance of ca. 510 and 514 Ma from the felsic sill sam-ple and inheritance of ca. 530 Ma from the felsic tuff sam-ple. The inheritance ages from the felsic sill are similar tothe crystallization ages of volcanic rocks previously dated atthe Duck Pond deposit, within the Tally Pond group (McNi-coll et al., 2008, and references therein). These data suggestthat the host rocks to the Boomerang deposit formed on asubstrate represented by these older rocks, rather than beingjuxtaposed at a later time. The ca. 530-Ma inherited zirconfrom the felsic tuff could represent an older volcanic source.Zagorevski (2007a) documented inheritance of ca. 560 Mafrom the Pats Pond group, and Squires and Moore (2004)and McNicoll et al. (2008) documented inheritance of ca.565 Ma and 573 Ma zircons within felsic rocks of the TallyPond group, suggesting that these rocks were built on a Pre-cambrian substrate. The development of the VLSG thusappears to have been a sequential process of arc magmatism,in which arcs were built on older continental basement,rather than in an ensimatic environment (cf. Rogers et al.,2006 and Zagorevski et al., 2007a).
LITHOGEOCHEMICAL PATTERNS
The detailed geochemistry provided in this report pro-vides information on the tectonic settings of felsic and maficrocks associated with VMS mineral deposits and also allowscomparison of these host sequences.
The relatively low HFSE and REE concentrations of therocks, coupled with the ubiquitous, yet variably developed,negative Nb and Ti anomalies on primitive-mantle-normal-ized, extended trace-element plots for the felsic and inter-mediate rocks are diagnostic of formation in an arc environ-ment (e.g., Pearce and Peate, 1995). In light of the zirconinheritance, it could be argued that the negative Nb and Tianomalies in the felsic rocks could be due to re-melting ofolder crustal source material with arc parentage. However,the synchronous mafic rocks of calc-alkaline and island-arctholeiitic affinity suggests overall development in an arcenvironment.
A comparison of the felsic-intermediate host rocks fromthe four areas illustrates some subtle but potentially impor-tant differences. First, rhyolites from the Tulks Hill deposithave elevated Zr/TiO2 ratios compared to the rocks fromother areas. However, in this case, the variation in theZr/TiO2 ratios is predominantly related to lower TiO2 ratherthan variation in HFSE concentrations, and this pattern ismost likely indicative of greater fractionation in the TulksHill deposit rhyolites. It should be noted that the felsic tuffsfrom the Tulks Hill deposit have Zr/TiO2 ratios similar tothose of felsic tuffs and rhyolites from other areas.
Additional chemical variations in the felsic-intermedi-ate volcanic rocks occur in the degree of LREE enrichmentand the extent of the negative Nb and Ti anomalies, whichappear to be correlated. Tulks Hill deposit samples appear tohave greater LREE enrichment and larger and more pro-nounced negative Nb and Ti anomalies compared to theBoomerang deposit samples, whereas the Tulks East depositsamples have similar LREE enrichment and similar, butslightly larger, overall Nb and Ti anomalies. The Boomerangfelsic-intermediate rocks have extended trace-element pat-terns similar to the PP4 grouping from the Pats Pond group,and positive Zr and Hf anomalies similar to the PP6 group-ing. The mafic volcanic rocks throughout the belt vary fromtransitional calc-alkaline basalts to island-arc tholeiites atTulks Hill, Boomerang and in the Pats Pond group. Thisconclusion is dependant upon the assumption that theandesitic sills at the Boomerang deposit are the same age asthe other volcanic rocks. In contrast, the mafic volcanicrocks at Tulks East deposit and true mafic sills at theBoomerang deposit are characterized by island-arc tholeiitic
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CURRENT RESEARCH, REPORT 09-1
signatures. The mixtures of calc-alkaline and tholeiiticsequences are best explained by the progressive rifting ofpredominantly calc-alkaline arcs (cf. Zagorevski et al.,2007a). The variations in the chemistry of felsic and inter-mediate rocks may also reflect this general process.
The last chemical variation that is observed between thefour areas is differences in gNd of the felsic rocks. TheBoomerang deposit and Pats Pond group felsic-intermediatesamples have higher gNd (+4 to +5.5) compared to thosefrom the Tulks Hill and Tulks East deposits (gNd of around+3). Although there is no easy interpretation of these data,the apparent bimodal grouping suggests that the Boomerangarea is more similar to the Pats Pond group than either theTulks Hill or Tulks East deposits. However, it should benoted that felsic-intermediate rocks similar to those that hostthe Tulks Hill and Tulks East deposits locally contain eNdsignatures of around +4 to +5.0 (e.g., Rogers, 2004;Hinchey, J. unpublished data) adding to the ambiguity ofsuch data.
CONCLUSIONS
New U–Pb zircon data, isotopic data, and geochemicaldata from volcanic rocks in the southern TVB of the VLSGare not simple to interpret. In conjunction with previousdata, the results suggest that VMS mineralization at theBoomerang deposit is resolvably younger than that at theTulks Hill deposit. However, the ages are closer to thoseobtained elsewhere in that part of the unit defined as the PatsPond group. Subtle lithogeochemical variations, and highergNd signatures for felsic-intermediate samples from theBoomerang deposit and the Pats Pond group also appear tosupport this correlation. Inherited zircon of ca. 530–510 Masuggests that the host rocks at Boomerang were depositedupon a substrate of older rocks including material of similarage to the Tally Pond group and, perhaps, also Precambrianbasement.
Follow-up research, including additional lithogeochem-ical, isotopic and geochronological studies, has been initiat-ed to characterize the host rocks to VMS deposits in thenorthern part of the TVB. Results will hopefully aid in fur-ther unravelling the complexities in the TVB.
ACKNOWLEDGMENTS
The geochronological work summarized in this articlewas conducted at the GSC geochronology laboratory. Thelaboratory staff are thanked for their assistance with sampleprocessing and analysis. Staff of Messina Minerals Inc. andProminex Resources Corp. are thanked for granting accessto company drillcores and data, and for many informativediscussions. This report was reviewed by Andy Kerr, Law-
son Dickson and Alex Zagorevski, who are thanked formany useful comments that helped improve the scientificmerit of this paper.
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