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The Horne Mine: Geology, History, Influence on Genetic Models,and a Comparison to the Kidd Creek Mine
HAROLD L. GIBSON
Mineral Exploration Research Centre, Department of Earth Sciences, Laurentian UniversitySudbury, Ontario, Canada, P3E 2C6
DAVID J. KERRDepartment of Geological Sciences and Geological Engineering, Queens University
Kingston, Ontario, Canada, K7L 3N6
SERGIO CATTALANIINCO Technical Services Limited
Copper Cliff, Ontario, Canada, P0M 1N0
Received June 1, 1999; accepted January 10, 2001.
Abstract The Horne mine was truly a world class Cu and Au deposit. Between 1927 and 1989,it produced some 260 t of Au and 1.13 Mt of Cu from 53.7 Mt of ore that averaged 2.22% Cu, 6.1 g/t
Au and 13 g/t Ag. The total value of Au and Cu production from the Horne deposit at metal pricesof US$300/oz Au and US$1.00/lb Cu is an outstanding US$5.2 billion. The Horne mine was also acompany builder. After optioning the property from Ed Horne (Tremoy Syndicate) in 1922, theThomson-Chadbourne Syndicate discovered the deposit in 1923 and quickly grew to becomeNoranda, one of the worlds premier mining companies. This discovery fuelled exploration and,along with subsequent discoveries in the Val dOr-Cadillac camps, led to the economic develop-ment of northwestern Quebec. The Horne deposit influenced and continues to influence geneticmodels for volcanogenic massive sulfide (VMS) deposits. Early observations at Horne contributedto an epigenetic replacement theory for VMS deposits. The most recent genetic model for the Horne,invoking sub-seafloor sulfide replacement of silicified and sericitized volcaniclastic host rocks withina graben, has subsequently been proposed for another giant VMS deposit, the Kidd Creek mine. TheHorne and Kidd Creek deposits show many similarities, such as localization within synvolcanicgrabens, long-lived hydrothermal activity uninterrupted by volcanism, sub-seafloor replacement sul-fides, stacked sulfide lenses, zone refining, silicified footwall rocks characterized by high positive18O values, and association with FIII rhyolites. Notable differences between the two deposits
include the lack of andesitic, basaltic or komatiitic flows at Horne, different inferred water depths,high Au content at Horne versus negligible Au, but sub-economic to economic concentrations of Sn,In and Cd at Kidd Creek. 2001 Canadian Institute of Mining, Metallurgy and Petroleum. Allrights reserved.
Part 1 The Horne Deposit
The world-class Horne volcanogenic massive sulfide
(VMS) deposit, located within the Archean Noranda Vol-canic Complex (NVC), produced some 260 t of Au and 1.13Mt of Cu from 53.7 Mt of ore that graded 2.22% Cu, 6.1 g/tAu and 13 g/t Ag (Table 1). Between 1927 and 1989, theHorne mine produced more Au and Cu than any otherArchean deposit in Quebec. The total value of the Hornemine production at todays metal prices is a staggeringUS$5.2 billion.
The first part of this paper is a summary of theexploration, discovery and production history of the
Horne mine and its impact on mineral exploration, theCanadian mining industry and the economic develop-ment of northern Canada. This summary is followed bya description of the geology, alteration, and mineraliza-tion at the Horne and how observations at the Hornehave influenced current genetic models for VMS
deposits. In Part 2, the Kidd Creek deposit is brieflydescribed and compared to the Horne deposit. Geneticprocesses and physical characteristics common to bothdeposits and their implication for exploration are thendiscussed in Part 3. This paper is dedicated to the lateW.L. Bankie Bancroft, former chief geologist at theHorne (Fig. 1). Bankies knowledge of the Horne mine,his detailed notes, records, and samples, as well as hisinfectious enthusiasm for its geology has benefited allresearchers.
Explor. Mining Geol.,Vol. 9, No. 2, pp. 91111, 2000 2001 Canadian Institute of Mining, Metallurgy and Petroleum.
All rights reserved. Printed in Canada.0964-1823/00 $17.00 + .00
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Without doubt, the discovery of the Horne mine can beattributed to the dogged determination and abilities of oneman Edmond Horne (Fig. 2). The discovery of the Horneis best chronicled in the book entitled Noranda by Roberts(1956) and is only briefly summarized herein as well as inTable 2. It is important to remember that prospecting innorthern Quebec during these early years was incredibly dif-ficult with no existing access except canoe routes, and inmost respects, was more challenging and costly than explo-ration in the remote areas of today.
Edmond Horne was an experienced prospector. Born in
Nova Scotia in 1869, he prospected and worked in gold
mines there, as well as in Colorado, British Columbia,Labrador, and the Cobalt, Kirkland Lake and Porcupinemining camps of Ontario. Horne first turned his eyes uponnorthwest Quebec in 1911. From Kirkland Lake, Hornemade three arduous trips to Lake Osisko (known today asLac Tremoy) in Quebec via the Ottawa and Kinojevis riversin 1911, 1914, and 1917. During these early visits, Horneprospected the area around Lake Osisko, noting mineralizedrhyolite outcrops. However, his samples did not yield sig-nificant Au values. Undaunted by the low assay results, andwith faith in the favorable geology surrounding LakeOsisko, Horne managed to raise $225 from 10 individualsand the Tremoy Lake Prospecting Syndicate was formed.
With this grubstake, Horne ventured again to Lake Osiskoand staked 70 acres, not realizing at that time that his firstclaim would ultimately contain about 95% of all the oreexploited from the Horne mine.
The 1921 field season proved to be a turning point. Sur-face sampling near what later became the No. 2 and No. 1shaft zones yielded Au values of 5.1 to 8.6 g/t and 25.7 to54.9 g/t, respectively (gold, at this time, was worth $20.67an ounce). These encouraging values led to the staking of anadditional 160 acres and, as Hornes exploits were begin-ning to attract attention, the Tremoy Lake Prospecting Syn-dicate staked an additional 400 acres in 1922. During the1922 field season, prospecting resulted in the discovery of
mineralization, which returned values of 17.1 g/t Au over0.61 m. This discovery later became known as the A ore-body.
Many companies and individuals were closely monitor-ing Hornes exploration results in far gone Quebec; how-ever, it was the Thomson-Chadbourne Syndicate that wouldsuccessfully acquire the Tremoy Lake Prospecting Syndi-cates claims in August of 1922. The Thomson-ChadbourneSyndicate had been formed in 1921 by two American min-ing engineers, S.C. Thomson and H.W. Chadbourne, toexplore in northern Ontario, particularly in the Kirkland
92 Explor. Mining Geol., Vol. 9, No. 2, 2000
Fig. 1. Bankie Bancroft (far left) and other members of the Horne mine exploration and pro-duction staff, notably Peter Price and Gord Suffel. Photo circa the 1940s (from B. Bancroftsnotebooks).
Fig. 2. Photograph of Ed Horne (fromB. Bancrofts notebooks).
Table 1. Horne mine production summary
Horne mine production, 19271976 8 942 470 oz Au>17 330 000 oz Ag1 133 830 t Cu
Remnor production, 19851989 102 170 oz AuEstimated total gold production 9 044 640 oz AuEstimated resource remaining 2 900 000 t containing 425 000
oz Au and 13 000 t Cu
Table 2. Horne mine exploration history
Discovery is the result of prospecting by Edmund Horne during fourprospecting seasons (1911, 1914, 1917, 1920).
1920 Tremoy Lake Prospecting Syndicate established.1920 Horne and Miller staked 70 acres; first claim staked in 1920
contained about 95% of all exploitable ore.1921 Sampling finally yielded significant gold values. Additional
160 acres staked.1922 400 additional acres staked. Discovered A orebody at surface
(0.5 opt Au over 2 ft).1922 Thompson-Chadbourne Syndicate acquired the Horne claims.1922 Establishment of the Northern Canada Mines Limited (i.e.,
Noranda) with J.Y. Murdoch as first president.1923 Noranda optioned Horne claims.1923 Second drill hole (October) was discovery hole; 131 ft averag-
ing 0.29 opt Au, 0.94 opt Ag, 5.61% Cu.1925 Production decision based on reserves of 611 500 tons grading
0.27 opt Au and 5.66% Cu.
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March of that year. In 1923, Noranda initially focussed onthe Powell and then the Chadbourne claims because theywere considered more prospective than the Horne claimsand were 100% held by Noranda. While the first shaft innorthwestern Quebec was being sunk on the Chadbourneclaims, a single drill rig was moved from the Powell claimsto the Horne claims to test known surface showings on aproperty that was costing the syndicate substantial optionpayments. In late August of 1923, the second drill hole (col-lared in massive sulfide) intersected 40 m averaging 9.94 g/tAu, 32.2 g/t Ag and 5.61% Cu. This was the Horne mine dis-covery hole. The sulfide lens intersected would eventuallybe known as the D orebody. Not only had gold been discov-ered but, to everyones surprise, copper as well. Thus wasborne Noranda, one of Canadas premier integrated miningcompanies.
Following the 1923 discovery, the Horne property wasaggressively explored (Table 3). By the end of 1924, a totalof 554 000 t of proven and indicated ore grading 9.25 g/t Auand 5.66% Cu had been delineated (Roberts, 1956). Basedon these reserves, a positive development decision wasmade, and the sinking of the No. 1 and No. 2 shafts was ini-tiated in 1925, followed by the sinking of the No. 3 shaft in1926. Plans for a smelter were drawn up. Construction of asmelter was an economic gamble, given the relatively lim-ited ore reserves at this stage; however, it paid off hand-somely in the years to come when the Horne deposit proved
to be much larger than initially projected. Commercial pro-duction at the Horne mine started in late 1927 less than
Lake area. After difficult negotiations with the thirteen indi-vidual partners of the Tremoy Lake Prospecting Syndicate,a deal was struck whereby the Thomson-Chadbourne Syn-dicate optioned a 90% interest in the Lake Osisko property
for $320,000. The deal required $5000 payable at the begin-ning of January 1923, followed by $5000 every six monthsuntil January 15, 1928, with $265,000 payable on the finaloption date. The remaining 10% interest eventually took theform of shares in Noranda, the Canadian operating companyincorporated in December of 1922 to finance the Thomson-Chadbourne Syndicates exploration and development of theLake Osisko property. The syndicates lawyer, J.Y. Mur-doch, became Norandas first president, a position that heheld until 1956.
The Thomson-Chadbourne Syndicate acquired newground known as the Powell and Chadbourne claims in thesummer and fall of 1922. Rumours of Au-bearing quartz
veins on the Powell claims led to a staking rush in the sum-mer of 1923, with some 86 000 acres already staked by
The Horne Mine: Geology, History, Influence on Genetic Models H.L. GIBSON ET AL. 93
Table 3. Horne mine production history
1923 Discovery.1925 Sinking of the No. 1 and No. 2 shafts began.1926 Sinking of the No. 3 Shaft began; smelter planned.1927 Production started 752 oz Au, 250 tons Cu in 1927.
Cu smelter treated 9743 tons with a value of $14,103.Reserves 1 087 200 t grading 9.02 g/t Au, 6.73% Cu.
Noranda came into production.1976 Production ceased final production estimated at53 706 990 t grading 6.1 g/t Au, 13 g/t Ag, 2.2% Cu.
198689 Remnor production phase mined 604 000 t grading 5.8 g/tAu.Resource remaining 2 900 000 t grading 5.0 g/t Au, 0.45%Cu.No. 5 Zone >150 Mt of massive to semi-massive sulfidegrading 0.14% Cu, 0.3 g/t Au, 0.7% Zn.
Fig. 3a. Annual Horne mine gold production. Diamonds representproduced ounces of gold (taken from annual reports of NorandaInc.), whereas, squares represent contained ounces of gold (calcu-lated from the average grade of the tonnes mined). Gold producedfrom the Horne mine is reported separately in annual reports onlyup to 1968; thereafter, the total annual gold production has beenestimated from the average grade of total Horne mine tonnestreated by the Horne metallurgical complex. The isolated square in1989 represents the total gold derived from the Remnor productionphase.
Fig. 3b. Annual Horne mine copper production. Diamonds rep-resent produced tonnes of copper (taken from annual reports ofNoranda Inc.), whereas, squares represent contained tonnes ofcopper (calculated from the average grade of the tonnes mined).Copper produced from the Horne mine is reported separately inannual reports only up to 1968; thereafter, the total annual cop-per production has been estimated from the average grade oftotal Horne mine tonnes treated by the Horne metallurgicalcomplex.
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four and a half years after the discovery drill hole and inthat first year, the mine yielded 752 oz of Au and 227 t of
Cu. The smelter came on stream in December 1927 andprocessed 9743 t of ore by the year-end. With the Hornemine in production, Noranda had its first cash flow andreserves stood at 1 087 200 t grading 9.02 g/t Au and 6.73%Cu (Roberts, 1956). Mine workings would eventuallyextend to a depth of 2440 m (8000 ft), although nearly all theproduction came from the Upper H and Lower H orebodiesin the upper 950 m of the mine.
The Horne mine remained in continuous productionuntil 1976, although there were sharp reductions in pro-duction due to a labor shortage following World War IIand a strike in 1953. Figures 3a and 3b illustrate the Hornemines annual production of Au and Cu. The discrepancy
between total ounces or tonnes produced (diamonds, fromsmelter production records) and contained ounces ortonnes (squares, based on the average grade of the tonnesmined each year) is a result of inaccurate grade estima-tion, unforeseen mining dilution and, of course, metallur-gical recovery. The largest difference between actual pro-duction and contained metal occurred over the periodfrom 1930 to 1940.
Au and Cu production from 1927 to 1944 (18 years)almost equaled the production of the subsequent 32 years(1945 to 1976), partly because of increased mining depths.Bearing in mind that the total metal production for the
period 1969 to 1976 is calculated, not measured, the Hornemine yielded a total production of 1 133 830 t of Cu,>17 330 000 oz of Ag and an outstanding 8 942 470 oz ofAu from 53 706 990 t of ore until mine closure in 1976. TheAg production figure is a minimum since production recordsonly cover the period from 1927 to 1959. A later phase of
production (the Remnor project of 1986-89) exploitedgold-only, flux ore from the upper levels of the Horne mine.The Remnor project added an additional 102 170 oz of Au(calculated ounces) production from 604 000 t of ore. Rem-nor increased the total Au production from the Horne mineto 9 044 640 oz.
Economic and Social Significance
The production and reserve statistics in Tables 1 and 3indicate that the Horne mine was an incredibly rich andprofitable deposit. The mine was clearly a company-
maker and provided Noranda with the capital, technicalexperience, and confidence required for its continued suc-cess and growth into one the worlds premier integratedmining companies. Also, it had a tremendous impact onexploration in northwestern Quebec, and led directly to sub-sequent discoveries of gold and base metal deposits in theregion, including Waite-Amulet (1925), Quemont (1944),Millenbach (1966) and Ansil (1980) (Fig. 4; Chartrand andCattalani, 1990). Norandas success at Horne provided theeconomic impetus and infrastructure (railways, roads, labor,etc.) which fuelled the development and settlement of north-western Quebec.
94 Explor. Mining Geol., Vol. 9, No. 2, 2000
Fig. 4. Exploration and discovery history of NW Quebec (afterLulin, 1990).
Fig. 5. Noranda Volcanic Complex showing the location of VMSdeposits, Mine Sequence and Horne stratigraphy, the subvolcanicFlavrian-Powell plutons and major structural elements includingthe Horne Creek Fault (after Kerr and Gibson, 1993; Richard,1998).
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and 10) with stratabound, massive to disseminated Cu-Au-Ag (-Zn) sulfide mineralization known to occur at six strati-graphic levels. The large Upper H and Lower H bodies occurnear the top of the middle formation. Much of thestringer/disseminated sulfide mineralization at the Horne
sidence during Cycle IV volcanism, mainly along the HorneCreek fault, was probably on the order of 1 km to 2 km (Gib-son, 1990).
Geology of the Horne Mine
The Horne mine lies within the Horne Block, an east-west-trending wedge of dominantly rhyolitic volcanicrocks that is bound to the north and south by major sub-ver-tical faults (the Horne Creek and Andesite faults). Thesefaults define the southern margin of the Noranda Cauldron(Figs. 5 and 7). The north-facing volcanic stratigraphywithin the Horne Block is sub-vertical and strikes east-southeasterly. Attempts to stratigraphically or geochrono-logically correlate the Horne rhyolites with volcanic unitsoutside the Horne Block have been largely unsuccessful.
Although massive to semi-massive sulfides extend fromsurface to a depth of >2650 m, nearly all production camefrom the Upper H and Lower H orebodies (Fig. 8) located
within the upper 950 m of the workings. These two orebod-ies have limited strike lengths (max. 600 m) and a sub-ver-tical plunge, and thus fall entirely within Ed Hornes origi-nal claim.
The Horne volcanic sequence is approximately 900 mthick and consists of rhyolitic lobe-hyaloclastite lava flows,related autoclastic breccias, volcaniclastic rocks includingprimary pyroclastic and syn-eruptive redeposited, mono-lithic to heterolithic volcanic tuffs and lapilli tuffs (largelymass flow and debris flow deposits), and minor dacitic lavaflows and cryptodomes. Kerr and Gibson (1993) informallydivided the Horne sequence into three formations (Figs. 9
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Fig. 7. Geological plan of the Horne mine on the 200 ft level, showing Upper H massive sulfide (UH) and G orebody. Mine coordinatesin feet (after Kerr and Gibson, 1993).
Fig. 8. Vertical, north-south cross-section through the Horne mineand Upper H (UH) and Lower H (LH) massive sulfide lenses. SeeFigure 7 for legend and location of section (after Kerr and Gibson,1993).
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ever, it is chiefly composed of pyrite (with quartz andsericite) and averages only 0.7% Zn, 0.3 g/t Au and 0.14%Cu. A limited tonnage of gold ore was developed in two,
higher-grade shoots oriented parallel to the long (down-dip)dimension of the No. 5 Zone (Fig. 11).Post-mineral deformation brought about a geometrical
complexity that hampered proper reconstruction of theHorne stratigraphy for many years. Faulting, particularly anortheasterly-trending set of curviplanar splays off theAndesite Fault, sliced the Upper H massive sulfide lens intosmall bodies that were each assigned a different alphanu-meric name by mine geologists. Furthermore, four suites ofintrusive rocks are known to crosscut the sulfide mineraliza-tion. The oldest intrusions are small-volume, porphyriticcryptodomes of intermediate composition that breached thewestern side of the Upper H orebody and caused significant
remobilization of sulfides on a local scale. Next to intrudewas an extensive swarm of dioritic (metadiabase) dikesand sills. These mafic dikes are particularly prevalent on theupper mine levels, and early researchers considered them tohave had great genetic significance with respect to formationof the sulfide ores. Narrow, post-metamorphic, quartz syen-ite dikes of Late Archean age and two wide diabase dikes ofProterozoic age constitute the last two intrusive phases tocut the Horne orebodies. Some of the most visibly spectac-ular concentrations of gold and telluride minerals at theHorne occurred with quartz veins associated with quartzsyenite dikes. These auriferous quartz veins (now inter-
mine is contained within a broad, discordant alteration pipe,which extends at least 450 m stratigraphically below theUpper H body. The small, sub-economic G Zone massive
sulfide lens occurs atop a series of rhyolite flows in theuppermost formation of the Horne volcanic pile. Most of theRemnor production came from a sericite-dominated alter-ation pipe below the G Zone.
The Upper H and Lower H orebodies are composed ofpyrite and pyrrhotite, with lesser amounts of chalcopyrite,magnetite and, locally, sphalerite, plus trace amounts ofnative gold (electrum) and Au-Ag telluride minerals (Price,1933). The two sulfide bodies are bowl-shaped in mine planbut elongate in the vertical dimension (Fig. 8). Their elon-gation is attributable to a graben-like, paleo-geographic(synvolcanic) depositional setting rather than to post-min-eral tectonic stretching.
The Lower and Upper H orebodies are stratabound, butnon-stratiform, deposits that are co-eval and co-stratigraphicbut formed in two separate hydrothermal vent sites on theseafloor. Upper H extended from surface to a mine depth of395 m, whereas, the Lower H extended from mine depths of365 m to 945 m. Together, the H orebodies yielded over 90%of the Horne mine production. Today, both orebodies areessentially mined out. A third, sub-economic, massive tosemi-massive sulfide body the No. 5 Zone strati-graphically overlies the Lower H orebody and extends to amine depth of at least 2650 m (Fig. 11). The tabular andstratiform No. 5 Zone contains in excess of 150 Mt; how-
The Horne Mine: Geology, History, Influence on Genetic Models H.L. GIBSON ET AL. 97
Fig. 9. Simplified and idealized stratigraphic sections through the Horne and Kidd Creek deposits.
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preted as a local remobilization phenomenon), together withthe location of the Horne mine between second-order splaysoff the Cadillac-Larder Lake Break (the Horne Creek andAndesite Faults), helped foster a popular belief in the 1980sthat Au at the Horne deposit was introduced by a post-vol-canic, epigenetic overprint event. However, the syngeneticAu model is now generally accepted (Kerr and Mason,1990; MacLean and Hoy, 1991; Barrett et al., 1991; Catta-lani et al., 1993).
The felsic stratigraphy hosting the Horne deposit isdominated by low-potassium rhyolite (
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and breccia, and typically replaces plagioclase phenocrystsand other grains. An early formed chlorite phase is alsocommon in altered rhyolites in the quartz-sericite zonewhere it is replaced by sericite in highly altered rocks. TheFe/(Fe+Mg) ratios of this chlorite resemble those in the bulkrock (0.5) suggesting minimal addition or removal of Fe orMg during its formation (MacLean and Hoy, 1991). Epidote,pyrite and leucoxene are minor components of altered pla-gioclase alteration and also occur as open-space filling min-erals. Calcite occurs primarily as veinlets, with or withoutquartz, and as disseminations within the breccia matrix andtuff. Disseminated calcite is interpreted to be a product of anearly, synvolcanic alteration.
Mass change calculations (MacLean and Hoy, 1991;Barrett et al., 1991; Cattalani et al., 1993) indicate that thequartz-sericite zone is enriched in Si, K, Rb, and Ba anddepleted in Na, Ca, and Sr (Fig. 13). Calculated changes inMg and Fe are commonly small. The extent of secondary
sericite formation is limited by the amount of available Al inthe precursor rock (MacLean and Kranidiotis, 1987) and,therefore, is controlled by the replacement of plagioclase; Kis supplied by the altering fluid (MacLean and Hoy, 1991).Calcite occurs along the periphery of the sericite alteration
in rhyolite breccia, and as a cherty component of tuffaceousunits. Sericite is most abundant in the matrix of rhyolite tuff
The Horne Mine: Geology, History, Influence on Genetic Models H.L. GIBSON ET AL. 99
Fig. 11. Longitudinal, vertical cross-section through the Upper and
Lower H orebodies and the No. 5 Zone. View looking north.Isopach maps of Upper and Lower H are presented on the right(after Kerr and Mason, 1990).
Fig. 12. Chondrite-normalized spider diagram depicting the FIIIaand FIIIb affinity of the Horne and Kidd Creek footwall rhyolites,respectively. The Horne and Kidd Creek rhyolite data each repre-sent an average of six least-altered samples (after Kerr and Gibson,1993; Prior et al., 1999). Gd values for the Horne samples were cal-culated by linear interpretation between Sm and Tb. Normalizationvalues after Nakamura (1974).
Fig. 13. Calculated mass gains for (a) SiO2 and K2O, and (b) Baand K (after Barrett et al., 1991).
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zones and complements the significant Ca and Sr depletion,which characterizes more proximal alteration assemblages.
At Horne, the vast amounts of Si and K added to theoverall rock mass (at the expense of Ca and Na) dwarfs theenrichment in the ore suite (Cu, Au, Zn, Fe and S; Fig. 13).Even the amount of Ba added to the system was greater thanthe amount of Cu. Ba occurs as a trace constituent in sec-
ondary sericite rather than in barite (MacLean and Hoy,1991; Fig. 13). The Horne footwall alteration (sericitizationand silicification) is characterized by large additions of Siand K and consequent mass gain, along with high positive18O values. On the other hand, the smaller intracauldronVMS deposits are characterized by chloritic alterationaccompanied by significant mass loss and low 18O values(Gibson and Kerr, 1993). These intracauldron deposits typi-cally boast a sericitized envelope around a prominent, chlo-ritized core where hydrothermal temperatures are inferred tohave been higher (Knuckey et al., 1982); at Horne, the cor-responding chlorite cores are more restricted in size and ver-
tical extent but the enveloping sericite zone is broader. Thus,it is essentially the proportion of alteration sericite to chlo-rite that differs between the lava-hosted intracauldron VMSdeposits and the Horne deposits that are hosted in coarsevolcaniclastic rocks. The higher proportion of sericite alter-ation to chlorite alteration is interpreted to reflect a pro-nounced increase in the degree of dilution and cooling ofupwelling hydrothermal fluids by down-drawn seawater inthe volcaniclastic-dominated sub-seafloor environment atHorne (Gibson and Kerr, 1993; Gibson et al., 1999). Thelarge Kidd Creek VMS deposit at Timmins, Ontario, is alsocharacterized by an extensive zone of footwall and hanging-wall sericitization and silicification and by localized zonesof discordant footwall chlorite alteration (Campbell et al.,1984; Huston et al., 1995).
Intense chloritization associated with chalcopyrite,pyrrhotite, magnetite, and locally Au is limited to relativelysmall volumes of rhyolite in the immediate stratigraphic
footwall to, and along the western and eastern contacts of,the H orebodies. The chlorite zone is characterized byquartz-chlorite-leucoxene or chlorite-leucoxene quartz,with sericite and epidote present locally. Evidence ofreplacement of sericite by chlorite and depletion of quartz iscommon. The chlorite in these zones is distinctly more Fe-rich (Fe/Fe+Mg = 0.65 to 0.85) than chlorite of rocks in thequartz-sericite zone (MacLean and Hoy, 1991; Barrett et al.,1991; Cattalani et al., 1993). Chlorite geothermometry usingthe calibrations of Cathelineau and Nieva (1985) and Krani-diotis and MacLean (1987) has yielded calculated tempera-tures of 250C to 310C in the chlorite zone and 230C to275C in the quartz-sericite zone. Rocks from the chlorite
zone have lost mass or volume mainly due to silica removal.Chloritized zones also show significant depletions in Si, Na,Ca, K, and related trace elements such as Ba, Rb, and Sr,whereas, they have been enriched in Fe and, to a lesserextent, Mg (MacLean and Hoy, 1991; Cattalani et al., 1993).
Although traditionally considered to be highly immo-bile during water-rock interaction, REE mobility inintensely hydrothermally altered volcanic rocks hostingVMS deposits was demonstrated by Campbell et al. (1984),MacLean (1988), and Barrett et al. (1991). At Horne, theLREEs exhibit the largest degree of mobility, whereas, theHREE are less so. REE losses are greatest in the chloritezone and REE gains are greatest in the silica-sericite zone.
Data from Kidd Creek (Campbell et al., 1984) and a studyof the Phelps Dodge VMS deposits (MacLean, 1988) indi-cate that REE are typically leached from high-temperature,Fe-chlorite alteration zones and re-precipitated in periph-eral, lower-temperature, silica-sericite zones.
Sulfur isotopic results from samples collected at Horneare similar to those obtained from most other studies of Pre-cambrian massive sulfides, with reported 34S compositions
100 Explor. Mining Geol., Vol. 9, No. 2, 2000
Fig. 14. Plot of measured 18O values against (a) calculated 18O,and (b) normative quartz (after MacLean and Hoy, 1991).
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tion within a larger sub-seafloor hydrothermal system.
Another possible explanation for the anomalously high,18O values of silicified and sericitized rhyolites at Hornerelates to their strong pervasive silicification. As shown byMacLean and Hoy (1991), alteration mineralogy directlyaffects bulk-rock oxygen isotopic composition (Figs. 13 and14; MacLean and Hoy, 1991). For example, high positive18O values are expected in silicified zones, whereas, signif-icantly lower 18O values are expected in chloritized zones.The higher-than-expected 18O values that characterizesericite-quartz-altered volcanic rocks at Horne are inter-preted to result primarily from an early, widespread, low-temperature silicification of the host rhyolites. The excellentcorrelation between bulk rock 18O values and wt% SiO2(which is directly related to silica addition or leaching)reported by Barrett and MacLean (1991) support this inter-pretation. An alternative model would require the presence
O-enriched fluid. This was first suggested by Beatyand Taylor (1988) to explain the high overall 18O values atKidd Creek.
Evolution of Genetic Models
The Horne mine was discovered more than 40 yearsbefore the advent of a modern understanding of the syn-genetic nature of VMS deposits. Early workers, such as Price(1933) and Suffel (1935), noted a spatial association of mas-sive sulfide ore with faults and shear zones and, particularlynear surface, an elongation of the orebodies parallel to these
northeasterly and northwesterly structures. These geologistsalso correctly recognized the replacement textures within thesulfide ores, most importantly, the replacement of pyrite-sphalerite by chalcopyrite-pyrrhotite-magnetite, a processthat they interpreted to be the result of rising fluid tempera-tures. This sulfide paragenetic sequence, down-played in the1970s during the syngenetic conversion, was rediscoveredby Eldridge et al. (1983) in their study of the Kuroko VMSdeposits of Japan, and by numerous researchers studyingblack smoker sulfide deposits on the modern seafloor (seereferences in Lydon, 1984, 1988). Study of modern seafloorsulfide deposits has constrained the timing, chemical andmechanical aspects of the progressive sulfide replacement
(now referred to as zone refining) and the parageneticsequence first recognized by early researchers at Horne andelsewhere is now recognized as typical of almost all VMSdeposits, regardless of geologic age.
Gold mineralization was once thought to represent theyoungest paragenetic stage at Horne. Early genetic modelsof the 1920s and 1930s invoked fault-controlled, epigenetichydrothermal replacement of favorable host rocks (i.e., rhy-olites). Mafic volcanic rocks were considered less chemi-cally susceptible to replacement, and mafic dikes were inter-preted to have acted as physical barriers to hydrothermalfluids and were, therefore, important ore controls. This epi-
of ore sulfide minerals in the range of magmatic sulfur val-ues (0 to 2 per mil; Hoy, 1988; MacLean and Hoy, 1991;Cattalani et al., 1993). Temperature estimates are inconsis-tent, therefore mineral deposition is inferred to haveoccurred under disequilibrium conditions from fluids thatwere isotopically homogeneous with respect to sulfur(MacLean and Hoy, 1991). Carbon isotopic data from cal-cites (13C = -4.5 to -2.0 per mil) suggest fluid isotopic com-positions of 13C = -11.5 per mil, assuming T =300C50C and neutral to acidic fluids (MacLean and Hoy,1991). These isotopic characteristics are compatible withseawater compositions.
Bulk rock 18O values range from 3.9 to 11.6 per mil,with a majority of samples between 7 and 11 per mil (Fig.14; MacLean and Hoy, 1991). Quartz-sericite-altered rhyo-lite ranges in 18O values from 6.6 to 11.6 per mil, whereas,chloritized rhyolite exhibits 18O values that range from 3.9to 4.4 per mil (MacLean and Hoy, 1991). Whole rock 18O
changes generally reflect equilibrium isotope fractionationin the order 18Ochlorite
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genetic theory strongly influenced worldwide VMS geneticmodelling for the next four decades.
During this time, scientific debate at the Horne minecentered around the age of the sulfide mineralization withrespect to the Proterozoic diabase dikes, which were gener-ally sulfide-free but which rarely hosted high-grade chal-copyrite veins perpendicular to their contacts. Price (1933)and Suffel (1935) interpreted the ores to be post-diabase,whereas, Peale (1930) considered the ores to be pre-diabase.Cooke et al. (1931) and Price (1948) proposed that the oreswere penecontemporaneous with diabase intrusion.
By 1948, three years after the nearby Quemont deposit
was discovered, most of the production from the Horne wascoming from the Lower H orebody. Price (1948) reaffirmedthe prevailing theory that the sulfide-gold ores were epige-netic and controlled by an upward-branching network ofNW (concordant) and NE-trending faults. Petrographicobservations still suggested that the gold mineralization wasparagenetically later than the base metal sulfide mineraliza-tion. Price (1948) also cited the angular discordance of thecontacts of the Upper H and Lower H orebodies as addi-tional evidence in favor of an epigenetic replacement modelfor the deposit.
Our understanding of the formation of VMS depositswas radically changed in the mid- to late-1960s. At the Horne
and similar deposits in the Abitibi, researchers such asRoscoe (1965) and Gilmour (1965) proposed that massivesulfide bodies were actually fumarolic deposits, whichformed at the same time as their host rocks. This vol-canogenic theory was universally accepted only after yearsof debate within the scientific community. At Horne,Mookherjee and Suffel (1968) and Darling and Suffel (1969)clearly established that the massive sulfide mineralization ofthe Upper H and Lower H bodies not only pre-dated the Pro-terozoic diabase dikes but also the dikes of metadiabase (latevolcanic diorite). Sinclair (1970, 1971) and Fisher (1970)documented the synvolcanic nature of the No. 5 Zone.
Very little ore deposit research was undertaken at theHorne during the 1970s and 1980s when the volcanogenicmodel was being consolidated and refined by an extraordi-nary explosion of field-based studies at massive sulfidedeposits around the world. Because of this research vacuum,geologists attempting to explain the high gold grade of theHorne orebodies relied upon historical observations andinterpretations, which had an epigenetic bias. This led to atwo-pronged genetic model wherein gold mineralizationwas epigenetic, and overprinted pre-existing syngeneticmassive sulfides. The Horne and two other gold-rich mas-sive sulfide deposits (Quemont, Delbridge) occur along the
Horne Creek Fault at the southern margin of the NorandaCauldron, i.e., closer to the Kirkland Lake-Cadillac Break, asignificant structure that localized lode Au deposits fromKirkland Lake to Val DOr. Perhaps more than any otherfactors, this spatial coincidence of the Horne deposit withthe Horne Creek Fault, a structure that controlled the forma-tion of the nearby Donalda Au deposit (Riverin et al., 1990),together with the presence of late syenite intrusions atHorne, contributed to the popularity of the epigenetic over-print model. The epigenetic model was disputed in theearly 1990s by Kerr and Mason (1990), Barrett et al. (1991),Cattalani et al. (1993) and Kerr and Gibson (1993) who pre-sented a variety of field and petrographic evidence that indi-
cated a synvolcanic origin for both the gold and copper min-eralization at Horne. Ongoing geochronologic studies alsosupport this syngenetic model.
The Current Horne Deposit Model
The Upper H and the Lower H orebodies and the No. 5Zone are interpreted to have formed in a paleo-graben on themargin of a submarine rhyolitic edifice dominated by lavaflows and related volcaniclastic deposits (Fig. 15; Kerr andMason, 1990; Barrett et al., 1991; Cattalani et al., 1993;
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Fig. 15. Reconstructed plan view and cross-section of the Horne paleo-graben (after Kerr and Mason, 1990).
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positive 18O values, which are unusual for VMS depositsand were first recognized by Beaty and Taylor (1988) atKidd Creek, are interpreted to be a product of an initial,widespread, low-temperature silicification of the footwallrocks. Chlorite geothermometry suggests that the earlysericite-quartz-(Mg-chlorite) alteration (and silicification)occurred at temperatures of 230C to 275C (MacLean andHoy, 1991; Cattalani et al., 1993). This early sericite-quartz(Mg-chlorite) alteration was accompanied by the replace-ment and cementation of the rhyolitic volcaniclastic brec-cias by pyritemarcasite-sphalerite. Precipitation of most ofthe sulfide in the sub-seafloor environment was an efficientmetal containment mechanism that contributed significantlyto the ultimate large size of the Horne deposit. The apparentlongevity of the Horne hydrothermal system is also cited asa factor that favored the formation of a large sulfide mass.
Sustained, lower-temperature fluid flow led to wide-spread alteration of the rhyolite breccias and sub-surface
precipitation of pyrite, quartz, sericite and Mg-chloritewithin breccia pore spaces. This encouraged a progressiveself-sealing of the footwall rocks within the paleo-graben.Self-sealing of the seafloor vent field inhibited the coolingand dilution of discharging hydrothermal fluid by entrainedseawater. This resulted in more focused upflow and allowedthe sub-seafloor, ore-depositional environment to achieveand sustain higher fluid temperatures (250C to 310C).Discordant zones of Fe-chlorite alteration and Cu-Au min-eralization which cross-cut early sericite-quartz-(Mg-chlo-rite) alteration represent this later, higher-temperature stage(Kerr and Mason, 1990; MacLean and Hoy, 1991; Barrett etal., 1991). These same fluids were responsible for the
replacement of lower temperature Fe and Zn sulfides by Au-and Cu-bearing assemblages dominated by pyrrhotite,pyrite, chalcopyrite and magnetite (Barrett et al., 1991; Kerrand Gibson, 1993). This progressive zone refinement ismost obviously reflected in the lower (footwall) parts of theUpper and Lower H deposits (Cattalani et al., 1993),although the two orebodies have actually each undergoneextensive zone refining (Fig. 16). Nearly all early sulfideassemblages in the Upper and Lower H Au-Cu deposits havebeen recrystallized and undergone partial to completereplacement by later, higher-temperature sulfide assem-blages (Kerr and Mason, 1990). In contrast, the large, strat-iform (clastic) No. 5 Zone can be thought of as a pyrite-
dominated, unrefined, low-grade Zn-Au deposit.Faults and shear zones, which early workers believed to
be key ore controls, are now interpreted as post-sulfide min-eralization structures that were preferentially developed inthe most strongly altered and mineralized (i.e., least compe-tent) host rocks. The apparent post-sulfide timing of the goldmineralization, which may be observed at a microscopicscale but which lacks definition at the stope scale, is readilyexplained by the local, tectonic remobilization of gold fromthe sulfide bodies in conjunction with shearing and emplace-ment of later intrusions. Tourigny et al. (1993) and Larocqueet al. (1993) documented similar small-scale, tectonic remo-
Kerr and Gibson, 1993). Unlike most intracauldron VMS
deposits that are interpreted to have formed by exhalationand precipitation of sulfides at and near the seafloor, theHorne deposit is interpreted to have formed primarilybeneath the seafloor by replacement of permeable rhyoliticfragmental rocks that in-filled the paleo-graben (Kerr andMason, 1990; Gibson and Kerr, 1990; Kerr and Gibson,1993). This interpretation explains the angular discordanceof the western, southern and eastern contacts of the Upper Hand Lower H bodies which Price noted in 1948, and is con-sistent with (a) field evidence of wholesale replacement ofrhyolitic units, (b) sulfide and silicate mineral textures andparagenesis (c) hydrothermal fluid parameters outlined byMacLean and Hoy (1991), and (d) inferred physiochemical
conditions of mineralization.The initial hydrothermal discharge was unfocused and
occurred along much of the >2.5 km length of the paleo-graben. Venting occurred at numerous sites rooted withinthe permeable breccias that floored the subsidence structure,but was particularly focused where graben-axial faults inter-sected transverse faults. Ascending hydrothermal fluids(evolved seawater) underwent sub-surface cooling due tomixing with down-drawn seawater. This resulted in the for-mation of a pervasive sericite-quartz-(Mg-chlorite) alter-ation assemblage in which the K, Si, and Mg were derivedprimarily from seawater (MacLean and Hoy, 1991). High,
The Horne Mine: Geology, History, Influence on Genetic Models H.L. GIBSON ET AL. 103
Fig. 16. Cu, Au and sulfide mineral zoning within the Lower Horebody. po = pyrrhotite, py = pyrite and cp = chalcopyrite (afterKerr and Mason, 1990).
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bilization of gold at the deformed Bousquet 2 and MobrunVMS deposits.
The final factor that likely contributed to the large sizeof the Horne deposit is its location along a major synvol-canic fault (the Horne Creek Fault) that defines the south-ern structural boundary of the Noranda Cauldron (Figs. 5and 6). This fault provided the long-lived, structurally con-trolled permeability required for sustained hydrothermaldischarge in the mine area and the formation of this giantdeposit. The Horne deposit also benefited from its locationoutside of the cauldron where hydrothermal discharge wasnot interrupted or suffocated by repeated, voluminous vol-canic eruptions of the sort that buried hydrothermal ventsand plagued development of the contemporaneous intra-cauldron deposits.
Thus, the extraordinary size of the Horne depositreflects its location in an area of high heat flow and well-developed cross-stratal permeability that favored sus-tained and long-lived discharge (Gibson and Kerr, 1993).The permeable nature of the footwall volcaniclastic suc-cession enhanced development of a large VMS depositby subseafloor sulfide precipitation and replacement andthe location of the Horne deposit within a paleo-grabenlikely contributed to its preservation (Fig. 15). Syn-depositional subsidence can be an important factor inboth the formation and the preservation of giant VMSdeposits.
Part 2 Comparison of the Horne
Deposit with the Kidd Creek Deposit
It is appropriate to compare the giant Horne and KiddCreek deposits since they represent the sort of VMS explo-ration targets that are highly sought by the mining industryworldwide. Combined past production and reserves at KiddCreek stand at 150 Mt grading 2.4% Cu, 6.5% Zn, 0.23% Pband 90 g/t Ag. Although the Horne and Kidd Creek depositsoccur in different geologic settings, they share some com-mon characteristics that suggest that similar geologicalprocesses and depositional conditions may have contributedto their formation. A comparison of these two famous
deposits may, therefore, provide information on how, whyand where giant VMS deposits form.
Despite its larger size, one might argue that the industryimpact of the Kidd Creek deposit was less pronounced thanthat of the Horne as its wealth did not accrue to a singlecompany and, consequently, Kidd Creek was not a com-pany builder like the Horne. The discovery of the KiddCreek deposit did little to open up northeastern Ontario asit was discovered nearby an established gold mining camp.Nevertheless, the economic and social impact of the KiddCreek mine on northern Ontario and, indeed, on Canada hasbeen impressive. Furthermore, Kidd Creeks current contri-
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Fig. 17. Geologic plan of the Kidd Creek Deposit (2800 Level; after Prior et al., 1999).
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been referred to as epiclastic deposits by Barrie et al. (1999),they are not products of mechanical or chemical weatheringand are best referred to as volcaniclastic rocks because theyrepresent primary, syn-eruptive, locally-transported depositsderived from autobrecciation and collapse of underlyinglava domes and pre-existing massive sulfide deposits. For-eign or accidental fragments and thin (
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sistent with the VMS-norm wherein an early formedpyrite-sphalerite assemblage is replaced by chalcopyrite andpyrrhotite. The results of zone refining are particularly pro-nounced at Kidd Creek, where chalcopyrite-rich ore hasbeen replaced locally by bornite mineralization (Hanningtonet al., 1999).
Significant proportions of the North, Central, and Southorebodies developed within the Mixed Fragmental unit bysub-seafloor replacement of the volcaniclastic breccias andby subsequent zone-refining (Barrie et al., 1999; Hanning-ton et al., 1999). Evidence of sub-seafloor replacementincludes: (a) the post-depositional replacement of rhyoliticfragments and matrix by pyrite and sphalerite, (b) thepreservation of vestiges of massive pyrite and sphaleritewithin the chalcopyrite-pyrrhotite ore, and (c) discordantcontacts between massive sulfides and bedding. The exis-tence of massive pyrite and sphalerite clasts within theMixed Fragmental unit indicates that brecciation and trans-
port of pre-existing sulfide deposits occurred prior to themain, sub-seafloor sulfide deposition event(s).
Hydrothermal Alteration and Isotopic Studies
Koopman et al. (1999) recognized four types of alter-ation at Kidd Creek, namely, (1) silicification (+SiO2), (2)sericitization (+K2O, SiO2, MgO, -Na2O, -CaO), (3) chlo-ritization (+FeO), and (4) talc-carbonate alteration. Silicifi-cation was early and is the most widespread and dominantalteration type. Koopman et al. (1999) describe four texturalvarieties of silicification that are semiconformable with
respect to both the footwall and hangingwall rhyolites yetshow a spatial association with the footwall chalcopyritestringer mineralization. Sericitization forms a broad, semi-conformable alteration zone that is preferentially developedwithin the footwall of the massive sulfide deposits but whichalso extends into the overlying QP Rhyolite. Fe-chloritiza-tion occurs along the margins of footwall chalcopyritestringer zones, which likely represent the main fossil con-duits for ascending hydrothermal fluids.
Oxygen isotope data of Beaty and Taylor (1988) andHuston et al. (1995) show that the altered Footwall Rhyolitehas distinctively high 18O values that range from +10 to +16per mil, consistent with the pervasive but variably intense
silicification of this unit. Huston et al. (1995) also traced thishigh 18O zone along strike and into the hangingwall QPRhyolite and also recognized a sub-zone of comparativelylow 18O (+10 to +13 per mil) in the immediate footwall tothe deposits, which could be attributed to isotopic exchangeduring late-stage, high-temperature alteration.
The Kidd Creek mine sequence is interpreted to haveformed within a linear, fault-controlled paleo-topographic
depression that initially developed in a flat-lying komatiiticlava plain (Gibson and Kerr, 1993; Bleeker, 1999). Barrie(1999) interpreted the komatiites as channelized flows con-tained within a graben that remained tectonically active dur-ing subsequent felsic volcanism. The Footwall Rhyolite is aproduct of voluminous eruptions that issued primarily fromtwo graben-parallel fissures (Prior, 1996). The resulting rhy-olite lava dome/cryptodome complex was some 2.5 kmlong, 300 m high, and 600 m wide. Breccias that flank, over-lie and occur within the lava dome complex were derivedthrough autobrecciation and mass wasting of the domes.
The Mixed Fragmental volcaniclastic unit representsthe transported equivalent of autoclastic breccias and earlysulfide deposits (Prior, 1996). The remarkable absence ofultramafic clasts within the volcaniclastic unit suggests thatthe graben walls had minimal topographic relief during ini-tial erosion and re-sedimentation of the volcanic and sulfidedebris. Thus, the rate of volcanic construction of the rhy-
olitic edifice largely kept pace with the rate of subsidence.Massive sulfide deposits of the North, Central, andSouth orebodies formed partly at the seafloor and partly inthe underlying volcaniclastic breccia. The QP Rhyolite waserupted along the length of the graben above the orebodies(Prior et al., 1999). Later, voluminous outpourings of basaltburied the rhyolitic edifice, filled the graben and spread outover the komatiitic lava plain and flanking rhyolite deposits.Thick gabbro sills that were emplaced into the rhyolite-basalt sequence represented subvolcanic feeders to thesebasalt flows (Barrie et al., 1999). The concentration of maficsills in the immediate mine environs implies that the KiddCreek rhyolitic center was also a center for subsequent
basaltic eruptions. Lastly, the presence of sericitized and/orsilicified rhyolite clasts in the lowermost breccia unit of theFootwall Rhyolite implies the existence of an earlyhydrothermal event that preceded formation of the KiddCreek VMS deposit. Thus, the massive sulfide orebodiesrepresent the culminating products of a long-lived, episodichydrothermal system (conservatively 5.5 Ma 3.8 Maaccording to Bleeker et al., 1999).
Part 3 Summary of the Horne-KiddCreek Comparison
1. The formation of the Horne and Kidd Creek VMSdeposits reflect a strong element of geomorphic and synvol-canic structural control. Both deposits are interpreted tohave formed within an extensional environment character-ized by the development of elongate, synvolcanic grabens.Wyman et al. (1999) further infer a proto-arc geodynamicsetting for Kidd Creek. The massive sulfide orebodies arehosted by locally transported rhyolite autoclastic brecciaand mass flow deposits occurring along the carapace andflanks of underlying rhyolite ridges. The distribution of the
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8. High 18O values characterize silicification at boththe Horne (+6 to +11.6 per mil) and Kidd Creek (+9 to +15per mil) deposits. Huston et al. (1995) interpreted 18Oenrichment (>13 per mil) in the hangingwall rhyolite atKidd Creek to be the result of low temperature silicifica-tion (
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4. The Horne deposit is typified by high gold grades andproduced more than 9 million ounces of gold, whereas, theKidd Creek ores, with an anomalously low gold content, areenriched in Sn, Sb, As, Ag (lower temperature Zn-Pb ore)and Bi, Se and In (higher temperature Cu-rich ore; Han-nington et al., 1999). Horne would have been economicstrictly as a gold mine, whereas, gold is not even recoveredat Kidd Creek. Gold enrichment cannot be simply related toa higher degree of zone refining since Kidd Creek, whichboasts higher base metal grades and a bornite zone, is morerefined than the Horne. More likely, the higher gold contentof the Horne ores reflect deposition from hydrothermal flu-ids that were inherently gold-rich, possibly due to involve-ment of magmatic hydrothermal fluids and/or depositionunder shallower water conditions. In shallow water VMSsystems, boiling of hydrothermal fluids and consequentphase separation can be an effective mechanism for concen-trating gold; unequivocal evidence of boiling or a shallow
water environment has not yet been documented at Horne.
The Horne deposit by any criterion including size,grade, or economic and social significance is a world classore deposit. Firstly, the Horne deposit gave birth to NorandaMines Ltd., which subsequently grew into Canadas largestdiversified resource company. Secondly, the Horne mine andsmelter opened up northwestern Quebec for both settlementand exploration in the early part of the 20th century and con-tributed, in a broader sense, to the economic growth of Que-
bec and Canada. Thirdly, the Horne deposit, throughout itshistory, has given rise to new ideas regarding the genesis ofancient VMS deposits and has thus profoundly affected min-eral exploration strategies. Research conducted in theNoranda region as a whole and at the Horne mine has beenfundamental to our understanding of concepts, such as: (a)the importance of synvolcanic structures in controlling theformation and preservation of VMS deposits; (b) the natureof sub-seafloor sulfide replacement processes and develop-ment of the zone-refining model; (c) the interplay of volcan-ism, sedimentation, and hydrothermal activity in space andtime and the economic importance of long-lived hydrother-mal systems; (d) the identification of volcanic settings and
rhyolite petrochemical compositions that are favorable forVMS deposits; and (e) the exploration significance of exten-sive footwall silicification and high 18O values.
The following list of empirical and genetic explorationcriteria is derived from characteristics and processes com-mon to both the Horne and Kidd Creek deposits.1. Spilitized, bimodal, tholeiitic (komatiitic) volcanic suc-cessions containing proximal rhyolitic volcaniclastic andpyroclastic rocks. Key indicators of proximity includedome-cryptodome volcaniclastic complexes, large volumesof monolithological breccias, abundant dikes and sills andabrupt lateral facies changes.
2. Proximity to long-lived synvolcanic structures. Evidenceof such structures includes: a) a high density of subvolcanicdikes and sills; b) major structural dislocations marked byunit truncations; c) abrupt lateral facies changes, and/orchanges in strike/dip of primary layering; and d) post-min-eral structures that could nevertheless represent reactivatedsynvolcanic faults. Synvolcanic structures commonly boundand control the location of subvolcanic intrusions and mul-tiple dome-cryptodome complexes. Long-lived structuresare necessary for the sustained hydrothermal fluid dischargethat is key to the formation of giant VMS deposits.3. An elongate, tectonically active, paleotopographic depres-sion (e.g., graben) which can both focus hydrothermal activ-ity and enhance the preservation potential of the resultingsulfide bodies.4. Sulfide accumulation primarily by sub-seafloor cementa-tion and replacement rather than exhalation.5. Evidence of a long-lived hydrothermal system, including
the stratigraphic stacking of VMS-style mineralization char-acterized by high base metal content.6. Widespread silicification and/or quartz-sericite alterationcharacterized by high 18O values, especially in combinationwith Zn enrichment. Early silicification can create a thermaland hydrologic cap rock that favors both thermal maturationand containment/focusing of ascending hydrothermal fluids.7. Evidence of high heat flow, including the presence of asubvolcanic intrusion, FIII rhyolites, ultramafic flows andhighly evolved and diverse mafic volcanic rocks.8. Intercalated wackes, argillites, and/or carbonaceousargillites that overlie fragmental volcanic successions. Firstly,clastic or chemoclastic sedimentation is evidence of a hiatus in
volcanic activity, and, secondly, an impermeable veneer ofsediments can act as a thermal and hydrologic insulator, i.e., acap rock, which can help to physically contain sulfide precipi-tation and to promote sub-seafloor replacement of footwallrocks and pre-existing sulfides. These sediments (particularlygraphitic argillites) can be enriched in base and precious met-als (especially Ag) in the vicinity of a VMS deposit.
Much of our knowledge of the Horne deposit is a resultof earlier work by Peter Price, Gordon Suffel and, in partic-
ular, W.L. Bankie Bancroft to whom this paper is dedi-cated. Special thanks are also extended to the mine geologystaff of the late 1980s, including Paul Cregheur, Guy Laperle,Chris Kloeren, and Gaetan Lavalliere. Discussions with ourresearch colleagues at the Horne and Kidd Creek mines,most notably Tim Barrett, Wally MacLean, Larry Hoy, BobMason, Dave Watkinson, Glen Prior, Dave Richardson, RonCook, Mark Hannington, Wouter Bleeker, Tucker Barrie andJim Franklin, are greatly appreciated. Reviews by CatharineFarrow and Noelle Shriver and by Mining and ExplorationGeology reviewers Frank Chartrand, Alan Galley and CeciliaJenkins improved the manuscript.
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