Punctuated Evolution of a Large Epithermal Province: The ... · These deposits have produced...

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IntroductionEPITHERMAL Au-Ag deposits form from hydrothermal sys-tems that most commonly develop in active volcanic regions(e.g., Simmons et al., 2005). Epithermal provinces occur inremnant and active volcanic arcs worldwide, including theGreat Basin of the western United States (e.g., John, 2001;

Saunders et al., 2008), the Mexico silver belt (e.g., Albinson etal., 2001), Japan (e.g., Hishikari: Izawa et al., 1990), the Patag-onia province of Argentina (e.g., Sillitoe, 2008), and the Hau-raki goldfield of New Zealand (e.g., Christie et al., 2007). Workin these, and in other epithermal provinces has focused mainlyon descriptions of individual deposits, and yet few integratedstudies have documented the temporal evolution of volcanicsystems and the relationships between tectonics, volcanism,hydrothermal activity, and the formation of epithermal golddeposits. Here, we document the regional chronology of theHauraki goldfield, providing new 40Ar/39Ar dates of hydro -

Punctuated Evolution of a Large Epithermal Province: The Hauraki Goldfield, New Zealand*

JEFFREY L. MAUK,1,† CHRIS M. HALL,2 JOHN T. CHESLEY,3 AND FERNANDO BARRA3,*1 School of Environment, University of Auckland, Private Bag 92019, Auckland Mail Centre, Auckland 1142, New Zealand

2 Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-10053 Department of Geosciences, University of Arizona, Tucson, Arizona 85721

AbstractThe Hauraki goldfield in the Coromandel volcanic zone contains approximately 50 adularia-sericite epither-

mal Au-Ag deposits in a 200-km-long by 40-km-wide north-south−trending belt. These deposits have producedapproximately 320,000 kg Au and 1.5 Mkg Ag and formed from Miocene to Pliocene subaerial hydrothermalsystems. The goldfield has been divided into three provinces (northern, eastern, and southern), based on thehost rocks and geologic setting of the deposits (Christie et al., 2007).

In the northern province of the goldfield, adularia from Paritu yields a single 40Ar/39Ar plateau date of 16.32± 0.13 Ma, and adularia from Opitonui yields a preferred 40Ar/39Ar age of 13.15 ± 0.03 Ma. Two Re-Os datesof molybdenite from porphyry-style mineralization at Ohio Creek overlap within error and yield dates of 11.87± 0.06 and 11.97± 0.08 Ma; geologic relationships suggest that this is the likely age of mineralization in thenearby Thames epithermal deposits.

In the eastern province, adularia from the Ohui deposit gives a preferred 40Ar/39Ar age of 8.29 ± 0.25 Ma,adularia from the Broken Hills deposit gives a preferred 40Ar/39Ar age of 7.12 ± 0.02 Ma, and adularia from theWharekirauponga prospect yields a preferred 40Ar/39Ar age of 6.32 ± 0.12 Ma.

In the southern province, adularia from quartz veins at the Maratoto deposit provide a preferred 40Ar/39Arage of 6.41 ± 0.04 Ma, and adularia from a quartz vein at the Sovereign deposit yields a preferred 40Ar/39Ar ageof 6.70 ± 0.16 Ma. Two dates from vein adularia at the world-class Martha deposit overlap within error, and weinterpret a preferred age for the deposit of 6.16 ± 0.06 Ma. Two samples of molybdenite from veins in theMartha deposit yield discrete Re-Os dates of 6.37 ± 0.03 and 6.51 ± 0.03 Ma. Adularia from one quartz veinfrom the Favona deposit yields a 40Ar/39Ar date of 6.05 ± 0.04 Ma. Host rock and vein adularia from theKarangahake deposit yield 40Ar/39Ar plateau dates that range from 6.90 ± 0.20 to 5.71 ± 0.13 Ma, which mayreflect more than one stage of mineralization or protracted fluid flow. Adularia from veins at the Waiorongo-mai deposit yields a preferred 40Ar/39Ar age of 5.71 ± 0.03 Ma, and adularia from a vein at the Eliza deposityields a preferred age of 4.47 ± 0.06 Ma. The southernmost deposit in the Hauraki goldfield, Muirs Reef, hasadularia in quartz veins that yield 40Ar/39Ar plateaus dates of 2.12 ± 0.11 to 1.78 ± 0.16 Ma.

Combined with previous work, these results indicate that mineralization in the Hauraki goldfield rangesfrom 16.3 Ma in the north to 2 Ma in the south, and clusters into two distinct groups that correlate with loca-tion, volcanic stratigraphy, and mineralization style. The first group, from ~16.3 to ~10.8 Ma contains epither-mal veins, including bonanza-style veins, and porphyry-style mineralization that formed in the northernprovince in an arc that was dominated by andesitic volcanism. The second period of mineralization occurs pri-marily from 6.9 to 6.0 Ma in the eastern and southern provinces, when precious metals were deposited intothicker colloform-crustiform banded veins that formed in extensional settings in an arc that was erupting bimodal andesite-rhyolite compositions. Therefore, even though volcanism in the Coromandel volcanic zonewas active from 18 to 2 Ma, Au-Ag mineralization was focused into two discrete periods of this arc formation,and the style of mineralization changed through time, coinciding with a change in style of volcanism. In addi-tion, while Hauraki goldfield mineralization discontinuously lasted more than 11 m.y., greater than 80 percentof the known gold endowment was deposited in a relatively brief 0.9 Ma window between 6.0 and 6.9 Ma.These changes through time likely reflect, at least in part, reorganization of the Miocene Northland andColville volcanic arcs in the New Zealand region of the southwest Pacific.

† Corresponding author: e-mail, [email protected]*A digital supplement, App. Table A1, is available at **Present address: Instituto de Geologia Economica Aplicada, Universi-

dad de Concepción, Concepción, Chile.

©2011 Society of Economic Geologists, Inc.Economic Geology, v. 106, pp. 921–943

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thermal adularia and Re-Os dates of molybdenite from veins.These methods provide precise and accurate dates, and this isthe first published application of Re-Os dating in NewZealand. We combine these new and previously publisheddata to (1) document the north to south advance of epithermalmineralization in the goldfield, (2) help constrain the durationof hydrothermal mineralization in the Waihi district, and (3)evaluate working hypotheses for the integrated tectonic, vol-canic, and hydrothermal evolution of this important epither-mal province. Available data demonstrate that although min-eralization occurred discontinuously from about 16.3 to 2 Ma,most (~80%) of the known gold endowment was deposited ina relatively brief 0.9 Ma window between 7 and 6 Ma.

Regional Geology

In the last 25 m.y., the North Island of New Zealand hasundergone a considerable shift in the locus of volcanism,from the north- to northwest-trending Northland arc be-tween 25 and 15 Ma, to the Coromandel volcanic zone be-tween 18 and 2 Ma, and then to the currently active north-east-striking Tonga-Kermadec-Taupo Volcanic Zone arc (Fig.1; Adams et al., 1994; Ballance, 1999; Hayward et al., 2001;Carter et al., 2003; Briggs et al., 2005). Although there is wideconsensus on this overall evolution, important details remainunclear, including the timing of the significant clockwise ro-tation of the arc from north-northwest to northeast trending.

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Ker

mad

ec R

idge

Ker

mad

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Col

ville

Rid

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Hav

re T

roug

hSouth Fiji Basin

Thr

ee K

ing

s R

idg

e

TVZ

CV

Z

NVA

40°

35°

175°

180°

175°

W

30° S

0 100 200

kilometres

N

1124

FIG. 1. Bathymetric and shaded digital elevation maps showing the location of volcanic arcs in the North Island of NewZealand. Bathymetric map of New Zealand’s undersea topography from National Institute of Water and Atmospheric Re-search, New Zealand (CANZ, 1996). Abbreviations: CVZ = Coromandel volcanic zone, TVZ = Taupo Volcanic Zone, NVA =Northland volcanic arc, 1124 = Ocean Drilling Program (ODP) Leg 181 deep-ocean site 1124 from Carter et al. (2004).

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The Hauraki goldfield contains approximately 50 adularia-sericite epithermal Au-Ag deposits that occur in a north-south−trending belt in the Coromandel volcanic zone, which followsthe length of the Coromandel Peninsula (Fig. 2; Christie etal., 2007). Most orebodies consist of single to multiple quartzveins, and some deposits also had significant production from

stockworks and/or hydrothermal breccias. Total productionfrom 1862 through 2009 was 335,000 kg Au and 1.6 million kgAg (Christie et al., 2007, and Newmont Waihi productionrecords).

Jurassic graywacke and argillite of the Manaia Hill Groupforms the basement of the region and is locally intruded by

PUNCTUATED EVOLUTION OF THE HAURAKI GOLDFIELD 923

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0 25 km

36.5°

37.5°

37° S

N

Paritu

Waiomu

Ohio Creek

ThamesRe-Os 12.0 - 11.9 Mamo

Waiorongomai

Neavesville

Golden Cross

Favona

Kuaotunu

Opitonui

Ar-Ar 16.3 Ma

Broken Hills

WharekiraupongaMaratoto

Komata

Karangahake

Sovereign

a

K-Ar 10.8 Mas

K-Ar 11 Mai

Ar-Ar 6.4 Maa

Ar-Ar 6.1 Maa

Ar-Ar 6.7 Maa

ElizaAr-Ar 4.5 Maa Muirs Reef

Ar-Ar 2.1-1.8 Maa

Ar-Ar 6.9? - 6.1? Maa

Ar-Ar 5.7 Maa

Ar-Ar 6.1 Maa

MarthaAr-Ar 6.2 Maa

Ar-Ar 7.0 Maa

Ar-Ar 6.3 Maa

Ar-Ar 6.9 Maa

OhuiAr-Ar 8.3 Maa

Ar-Ar 7.1 Maa

K-Ar 14.1 Maa

Ar-Ar 13.2 Maa

175.5°D

Coromandel

Porphyry deposits

Epithermal deposits

Very small 20 t

Bonanza Au-Ag

36.5°

175.5° 176°

37.5°

37° S

C

Hauraki R

ift

Alluvial sediments

Quaternary TVZ volcanic& volcaniclastic rocksL. Miocene - PlioceneWhitianga Group rhyolite

Pliocene Kaimai Subgroupandesite & daciteL. Miocene Omahine Subgroupandesite

Miocene - Pliocene WaiwawaSubgroup andesite & dacite

E. Miocene KuaotunuSubgroup andesite & dacite

LegendE. Miocene dioriteintrusionsJurassic greywacke basement

Epithermal Au-Ag deposit

Porphyry Cu deposit

Fault

Inferred calderaboundaries

B

Northern

Eastern

Southern

AB, C, & D

Hauraki R

ift

FIG. 2. A. Location of the Hauraki goldfield in the North Island of New Zealand. B. Location of the northern, eastern,and southern provinces of the Hauraki goldfield (from Christie et al., 2007). C. Geology of the Hauraki goldfield (fromChristie et al., 2007). D. Location of mineral deposits in the Hauraki goldfield, vein orientations, past Au production, andages of deposits (modified from Christie et al., 2007). The bar symbol shows the main strike of veins at each deposit. The K-Ar dates are from Skinner (1986), Hunt and Roddick (1992, 1993), Sinusat (1992), Adams et al. (1994), and Brathwaite andChristie (1996), whereas the 40Ar/39Ar and Re-Os dates are from Mauk and Hall (2004), and this publication. Subscripts showthe mineral that was dated: a = adularia, i = illite, mo = molybdenite, s = sericite.

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Late Miocene Coromandel Group dioritic intrusives (Skin-ner, 1972). The Manaia Hill Group is overlain by Miocene toPliocene Coromandel Group andesite and dacite; this is sub-divided into the Kuaotunu Subgroup, which formed in thenorthern part of the goldfield approximately 18 to 11 Ma, andthe Waiwawa Subgroup, which formed in the southern andeastern part of the goldfield approximately 10 to 5.6 Ma (Ed-brooke, 2001, and references therein). The southern goldfieldalso contains the 8.1 to 6.6 Ma Omahine Subgroup and the 5.6to 3.8 Ma Kaimai Subgroup. Late Miocene to Pliocene (ca.11-1.5 Ma) Whitianga Group rhyolites overlie and interfinger

with Coromandel Group rocks, and the volumetrically minorMercury Basalts formed from 9.1 to 7.8 Ma and from 6.0 to4.2 Ma (Skinner, 1986; Adams et al., 1994; Fig. 3). Lithologyexerts a strong control on the location of orebodies; greaterthan 95 percent of Hauraki goldfield production was derivedfrom veins hosted by andesite and dacite of the CoromandelGroup, even though other units form nearly 40 percent of therocks in the goldfield (Brathwaite et al., 2001a; Christie et al.,2007).

The Hauraki goldfield contains NNW- and NNE-strikingfaults and fractures with steep to moderate dips, which range

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Tephra thickness (m)cumulative:ODP 1124

Taupo Volcanic

Zone

Corom

andel Volcanic Z

one

0 2 4 6 8 12 1410

Northern

Southern

Eastern

Au production by province

100

1,000

10,000

100,000

1,000,000

log Au production (kg)

15

0

5

10

0

5

10

0

5

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Kua

otun

u S

ubgr

oup

Wai

waw

a S

ubgr

oup

Mer

cury

Bas

alts

Whi

tiang

a G

roup

(rh

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Coromandel Group (andesite)

Par

ituP

luto

nics

Om

ahin

eS

ubgr

oup

Kai

mai

Sub

grou

pVolcanic rocks of the Coromandel Volcanic Zone

Age

(M

a)

FIG. 3. Generalized stratigraphy of the Coromandel volcanic zone, showing volcanic units referred to in this text. Modi-fied from Edbrooke (2001). Cumulative tephra thickness from drill core in Ocean Drilling Program (ODP) Leg 181 deep-ocean site 1124. Redrawn from Carter et al. (2003). Histogram showing log Au production (kg) for the Hauraki goldfield vs.date of the deposits. The deposit ages are placed into 1 Ma groupings, and the ages are based on data from this paper andother sources cited herein.

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from regional scale to strike lengths of a few hundred meters(Spörli et al., 2006). The north-northwest−striking faults areequally downthrown to the east and west, but most north-northeast−striking faults are downthrown to the south (Skin-ner, 1986). This displacement has exposed the Jurassic base-ment in the northern portion of the Coromandel Peninsula,and exposes younger volcanic rocks to the south (Fig. 2C).

The currently active NNW-trending Hauraki Rift formsthe western margin of the Coromandel volcanic zone. Thebounding faults on the rift have offsets of up to 4 km, andthe rift is filled with a sequence of sediments and volcanicflows (Hochstein et al., 1986; Hochstein and Ballance,1993). The initiation of the Hauraki rift is poorly con-strained and may have been around 7 Ma (Hochstein andBallance, 1993). However, sedimentologic evidence indi-cates that there was no significant graben between the Hau-raki goldfield and the west coast of New Zealand in the latePliocene (ca 3−2 Ma), and the more recent volcanic recordindicates that the main subsidence in the southern Haurakirift occurred between 2.1 and 1.2 Ma (Briggs et al., 2005;Hayward et al., 2006).

Local GeologyWe used 40Ar/39Ar and Re-Os geochronology to determine

the dates of hydrothermal minerals from the Paritu, Opitonui,Ohio Creek, Ohui, Broken Hills, Wharekirauponga, Mara-toto, Sovereign, Karangahake, Martha, Favona, Waiorongo-mai, Eliza, and Muirs Reef deposits, and this section de-scribes the local geology of these deposits. Combined withour previous work at Neavesville, Golden Cross, and Komata(Mauk and Hall, 2004), we now have 40Ar/39Ar and Re-Osdates from deposits that have contributed more than 90 per-cent of the Au production of the Hauraki goldfield.

Porphyry Cu-style mineralization at Paritu in the northernCoromandel Peninsula is hosted by a small diorite to gran-odiorite pluton that intrudes the basement Manaia HillGroup and early to middle Miocene andesites. The porphyryCu- style mineralization occurs around Ongohi Stream andconsists of chalcopyrite with magnetite and pyrite in hy-drothermally altered quartz diorite (Skinner, 1976; Brath-waite and Pirajno, 1993).

The Opitonui gold-silver epithermal deposit produced 512kg Au and 457 kg Ag from electrum-bearing quartz veins thatoccur in Kuaotunu Group andesite. Mineralization occurs innorth- and east-trending quartz veins (Christie et al., 2007).

The Ohio Creek porphyry Cu-Au-Mo prospect was drilledbetween 1978 and 1981, which outlined mineralization with0.1 to 0.2 percent Cu, 0.2 to 0.4 ppm Au, and ≤0.01 percentMo (Merchant, 1986). The porphyry prospect shows potassic,phyllic, and advanced argillic alteration, and spatial relation-ships indicate that mineralization at Ohio Creek is geneticallyrelated to epithermal mineralization in the nearby Thamesdistrict (Brathwaite et al., 2001b). The Thames district wasthe second largest Au producer in the Haruaki Goldfield,with production of 44,847 kg Au and 21,780 kg Ag from bo-nanza-style veins (Christie et al., 2007).

The Ohui deposit was worked between 1893 and 1910, withrecorded production of 5 kg Au and 3 kg Ag (Christie et al.,2007). Mineralization occurs in Coromandel Group andesiteand Whitianga Group rhyolite, although quartz veins are

generally thicker in the andesite (Brathwaite et al., 2001a).The Ohui deposit lies at the northern end of the Karanga-hake-Ohui structural trend, a north-northeast−trending struc-tural corridor that contains several epithermal deposits andprospects (Rabone, 1991); veins at Ohui strike both north-northeast and northwest (Brathwaite et al., 2001a; Fitzgeraldet al., 2006). Colloform-banded vein textures and local sinterin the area suggest that the veins are high level; sulfide and oreminerals include pyrite, electrum, Ag-Se sulfides, native Ag,proustite, pyrargyrite, chalcopyrite, and marcasite (Fitzgeraldet al., 2006).

The Broken Hills epithermal deposit produced 737 kg Auand 964 kg Ag from quartz veins and breccia pipes that cutrhyolite of the Whitianga Group. This production makes Bro-ken Hills the second largest rhyolite-hosted Au producer inthe Hauraki goldfield (Christie et al., 2007). Kinematic indi-cators show that quartz veins formed in an extensional envi-ronment, and ore shoots commonly strike north-south,whereas barren sections of the veins more commonly strikenorth-northwest (Nortje et al., 2006; Rabone, 2006a). Veinwidths and grades can change by several orders of magnitudeover 1 to 2 m along strike of the veins, and ore minerals in-clude electrum, acanthite, aguilarite, and naumannite (Moore,1979; Rabone, 2006a).

The Wharekirauponga epithermal gold-silver prospect ishosted by Whitianga Group flow-banded rhyolite and rhy-olitic tuff, and an andesite dike; the prospect has virtually nohistoric production. The rhyolites occur in a north-northeast–trending graben, and mineralization occurs in sheeted tostockwork quartz veins and as disseminated mineralization.Most veins strike north-northwest or north-northeast, and oreminerals include electrum, chalcopyrite, and arsenopyrite(Rabone et al., 1989; Brathwaite et al., 2001a; Christie et al.,2006).

The Maratoto epithermal deposit produced 138 kg Au and544 kg Ag from northeast-trending quartz veins with localizedfan-shaped bonanza mineralization. Ore minerals includepyrite, marcasite, sphalerite, galena, chalcopyrite, acanthite,hessite, and electrum. The deposit occurs in highly alteredandesites of the Waipupu Formation of the CoromandelGroup (Main, 1979; Christie et al., 2007).

The Sovereign (formerly known as Maoriland) Au-Ag de-posit in the southern Waitekauri valley produced 141 kg Auand 89 kg Ag; it occurs in intensely altered andesite, dacitebreccias, and pyroclastic flows of the Waipupu Formationthat show significant K metasomatism. Quartz veins follow anorth-northeast-striking normal fault in a highly silicifieddacitic tuff breccia (Haworth and Briggs, 2006; Grodzicki etal., 2007; Booden et al., 2011; Simpson and Mauk, 2011).

The Karangahake epithermal gold-silver deposit was thethird largest Au producer in the Hauraki goldfield, with his-toric production of 29,425 kg Au and 97,290 kg Ag; recent ex-ploration has identified a resource of 5,700 kg Au and 23,000kg Ag (Stevens and Boswell, 2006). Mineralization occurs pri-marily in the Maria and Welcome and/or Crown veins, whichare quartz veins that contain pyrite, chalcopyrite, electrum,acanthite, and local sphalerite and galena (Brathwaite, 1989;Mauk et al., 2006a). Karangahake produced Au ore over ananomalously large vertical extent of greater than 700 m. Pro-ductive veins strike north to north-northeast, fill extensional


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fractures, and occur in andesite flows and flow breccias of theWaipupu Formation of the Coromandel Group. Veins con-tinue upward into the overlying Whitianga Group rhyolite,where they break up into a stockwork that is too low grade tobe economic (Brathwaite, 1989; Smith et al., 2003; Stevensand Boswell, 2006).

Through 2009, the Waihi (Martha) deposit produced210,944 kg Au and 1,299,893 kg Ag (Lorrance Torckler, writ.commun., 2010); it has produced nearly 70 percent of the Auin the Hauraki goldfield (Christie et al., 2007). From 1883through 1952, the deposit was worked as an undergroundmine that exploited several veins along a strike length ofgreater than 1.5 km and from a vertical extent of up to 575 m(Brathwaite and Faure, 2002). Since 1988, it has been workedas an open-pit mine with a planned length of 840 m, width of575 m, and depth of 250 m, which exploits remnants of veinstargeted by earlier mining, stope fill, vein breccias, and theabundant veins and veinlets that occur between the mainveins (Brathwaite et al., 2006). The deposit shows distinctmineral zonation, with pyrite, chalcopyrite, acanthite, elec-trum, and minor sphalerite and galena at shallow to interme-diate levels, and pyrite, sphalerite, and galena at depth. Veinsoccur in andesite and quartz andesite of the Waipupu For-mation of the Coromandel Group (Brathwaite and Christie,1996).

The Favona deposit is a recently discovered blind epither-mal deposit that occurs approximately 2 km east of theMartha mine. When mining began in 2006, the deposit hadreserves of 1.1 Mt at 10 ppm Au and 36 ppm Ag (Torckler etal., 2006). The deposit occurs in altered andesite of theWaipupu Formation of the Coromandel Group (Brathwaiteand Christie, 1996; Simpson and Mauk, 2007). The ore con-sists of veins and vein breccias that strike north-northeast andformed in an extensional environment (Mortimer, 2009). Oreminerals include pyrite, chalcopyrite, acanthite, tetrahedrite,and electrum, as well as uncommon naummanite (Ag2Se) andaguilerite (Ag4SeS); sphalerite and galena are more abundantat depth (Mauk et al., 2006b).

The Waiorongomai deposit produced 885 kg Au and 1,259kg Ag from a series of quartz veins that strike northeast; thesesplay off the north-striking, 5-km-long barren Big Buck Reef(Wellman, 1954; Bates, 1989; Christie et al., 2007). The veinsoccur in andesite of the Waipupu Formation of the Coro-mandel Group. Mineralized veins contain pyrite, chalcopy-rite, galena, spahlerite, and electrum, and limited data sug-gest that base metal sulfides increase with depth (Bates, 1989;Christie et al., 2007).

The Eliza deposit contains a 1-m-wide quartz vein thatstrikes north-northeast, but this vein was not economic andrecorded production was only 5 oz Au (Houghton and Cuth-bertson, 1989).

From 1912 to 1922, the Muirs Reef mine exploited anorth-northeast−striking, west-dipping colloform bandedvein that typically ranges from 0.5 to 2 m thick. The vein oc-curs in predominantly andesitic host rocks that containminor rhyolite units (Rabone, 2006b). It was mined along astrike length of approximately 300 m and downdip for ap-proximately 100 m and yielded 1,299 kg Au and 385 kg Ag(Christie et al., 2007). The deposit is the southernmost in theHauraki goldfield.

Materials and MethodsWe analyzed adularia and molybdenite from 15 deposits in

the Hauraki goldfield (Fig. 4; App; online digital supplementTable A1). Our primary intent is to provide a set of dates for ep-ithermal deposits in the Hauraki goldfield on a regional scale,so we have analyzed a limited number of samples from a rela-tively large number of deposits.

Petrographic studies show that illite commonly replacesadularia in host rocks, whereas adularia in veins generally lacksillite (Fig. 4). Therefore, in most places we determined 40Ar/39Ardates using adularia from veins (App.). The paragenetic se-quence of vein minerals in various deposits has been summa-rized by other workers, and in the Hauraki goldfield, veins thatcontain adularia are commonly banded and contain electrumand various sulfide minerals as trace to minor phases inter-grown with these banded veins (e.g., Simpson et al., 2001;Brathwaite and Faure, 2002; Begbie et al., 2007; Christie etal., 2007; Simpson and Mauk, 2007, 2011; Fig. 4). Therefore,the 40Ar/39Ar dates of vein adularia that we report here repre-sent the age of mineralization of various deposits.

We analyzed adularia from veins and/or host rocks at 14 de-posits in the Hauraki goldfield at the University of Michigan(Table 1; Apps. 1, 2). Some vein samples, such as those fromBroken Hills, contain coarse-grained adularia crystals thatwere directly separated by handpicking. Adularia separateswere obtained from other samples by heavy liquid separationfollowed by handpicking or by staining with sodium cobaltini-trite followed by handpicking. We quantified the amount ofadularia in these separates by comparing X-ray diffraction(XRD) profiles of the separates to XRD profiles from a seriesof two-component mixtures with known concentrations ofquartz and adularia. X-ray diffraction patterns were collectedusing a Philips PW 1050/25 diffractometer. Due to the fine-grained nature of the adularia and its common intergrowthswith quartz, we could not obtain pure adularia separates forsome samples. However, quartz was the only impurity thatwas identified by XRD analyses, and because it contains nopotassium in its lattice, we conclude that it is not a source ofcontamination, only a dilutant. All samples have AucklandUniversity (AU) numbers and are lodged in the collection ofthe Geology Department at the University of Auckland.

Samples for 40Ar/39Ar analysis from Martha and Favonawere irradiated for 6 h in location L67 of the Phoenix-FordMemorial Reactor at the University of Michigan, and sam-ples from other deposits were irradiated in location 5C at theMcMaster Nuclear Reactor, which is located on the campusof McMaster University, Hamilton, Ontario, Canada. Adu-laria was wrapped in pure aluminum foil and loaded intofused silica tubing for neutron irradiation. Samples werestep-heated using the defocused beam from a 5-W argon-ioncontinuous laser, and analyses were carried out using a VG1200S mass spectrometer equipped with a Daly detector op-erating in analog mode. The dates quoted are relative to anage of 27.99 Ma for standard biotite FCT-3, which in turn isrelative to an age of 520.4 Ma for standard hornblendeMMhb-1 (Hall and Farrell, 1995). The Ca interference cor-rections were 39Ar/37Ar = 1397, 36Ar/39Ar = 0.421, and the Kcorrection was 40Ar/39Ar = 0.019 for the Phoenix-Ford irradi-ation. The interference correction factors for the McMaster

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irradiation were 1449, 0.425, and 0.037 for 39Ar/37ArCa,36Ar/39ArCa, and 40Ar/39ArK, respectively. The J values werecalculated by interpolating a cosine function that was fittedthrough values measured from standards as a function of po-sition within the irradiation can. Individual sample J valueerror estimates include measurement uncertainties fromanalyses of standards plus scatter of the standard valuesabout the fitted J function. Mass discrimination was moni-tored daily, although the Baur-Signer source of the VG1200Sdoes not exhibit rapid changes in mass discrimination. Overthe past 3 yrs, the atmospheric 40Ar/36Ar ratio using the Fara-day detector has been steady at ~301 and the ratio on theDaly detector has been ~290. All analyses were performed

using the Daly detector and quoted isotope ratios and agesare corrected to for mass discrimination assuming an atmos-pheric 40Ar/36Ar ratio of 295.5.

Plateau dates were calculated as error weighted averagesfor adjacent gas fractions. A minimum of 50 percent of the39Ar was released within at least three successive steps toform a plateau, and the scatter about the error weighted av-erage has a mean squared weighted deviate (MSWD) of nomore than 2.0. If the MSWD is >1, the plateau date error isscaled up by multiplying by MSWD. The total gas date is cal-culated by adding up all the gas volumes and calculating adate from the derived isotope ratios. This date should beequivalent to a conventional K-Ar date.


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FIG. 4. Representative photographs of samples analyzed in this study. A. Molybdenite in quartz vein from the Marthamine. Sample AU 59725. B. Adularia from quartz vein at Paritu. Sample AU 56763. C. Banded quartz vein with adularia andbase metal sulfides from the Waiorongomai deposit. The adularia has been stained yellow. Sample AU 33973. D. Coarse-grained adularia from the Broken Hills deposit. Sample AU 56756. E. Transmitted light photomicrograph showing adulariagrains in a quartz vein from the Waiorongomai deposit. Sample AU 33870. F. Transmitted light photomicrograph showingporphyritic andesite host rock from the Karangahake deposit. Adularia has completely replaced plagioclase phenocrysts, andillite has partially replaced the adularia. Sample AU 57488.

https://sina-pub.ir

928 MAUK ET AL.

0361-0128/98/000/000-00 $6.00 928

TAB

LE

1. 4

0 Ar/

39A

r A

ge D

eter

min

atio

ns o

f Adu

lari

a fr

om th

e Pa

ritu

, Opi

tonu

i, B

roke

n H

ills,

Wha

reki

raup

onga

, Mar

atot

o, S

over

eign

, Wai

hi, F

avon

a, K

aran

gaha

ke, a

nd W

aior

ongo

mai

Dep

osits

(a

ll er

rors

are

±2σ

)

AU

no.

CM

Hal

l no.

Loc

atio

nPl

atea

uM

SWD

F39

(%

)Is

ochr

on40

/36

Inte

rcep

tn

MSW

DTo

tal g

asC

omm

ents

5675

6M

C07

-S16

aB

roke

n H

ills

7.15

± 0

.05

0.94

100%

7.14

± 0

.06

296.

2 ±

1.6

130.

957.

20 ±

0.1

4R

hom

bohe

dral

adu

lari

a cr

ysta

ls

from

vei

n56

756

MC

07-S

16b

Bro

ken

Hill

s7.

12 ±

0.0

61.

1610

0%7.

12 ±

0.0

629

6.3

± 2.

613

1.22

7.15

± 0

.10

Dup

licat

e of

abo

ve56

757

MC

07-S

17a

Bro

ken

Hill

s7.

12 ±

0.0

31.

7898

.7%

7.12

± 0

.04

Rho

mbo

hedr

al a

dula

ria

crys

tals

fr

om v

ein

5675

7M

C07

-S17

aB

roke

n H

ills

7.08

± 0

.10

346.

8 ±

148

91.

84Pl

atea

u po

ints

5675

7M

C07

-S17

bB

roke

n H

ills

7.12

± 0

.03

0.47

100%

7.12

± 0

.03

295.

1 ±

5.8

130.

517.

12 ±

0.0

5D

uplic

ate

of a

bove

5972

2M

C24

-m3a

Eliz

a4.

49 ±

0.1

20.

3485

%4.

49 ±

0.1

230

4.3

± 9.

813

0.71

4.64

± 0

.23

Adu

lari

a fr

om q

uart

z ve

in59

722

MC

24-m

3aE

liza

4.49

± 0

.20

288.

8 ±

181

40.

51Pl

atea

u po

ints

5972

2M

C24

-m3b

Eliz

a4.

46 ±

0.0

70.

598

%4.

47 ±

0.0

727

7.3

± 12

130.

684.

33 ±

0.1

3D

uplic

ate

of a

bove

5972

2M

C24

-m3b

Eliz

a4.

47 ±

0.0

827

9.7

± 71

70.

51Pl

atea

u po

ints

5500

7M

I91-

G26

aF

avon

a6.

01 ±

0.1

40.

9659

.8%

6.76

± 0

.13

Adu

lari

a fr

om q

uart

z ve

in55

007

MI9

1-G

26b

Fav

ona

6.07

± 0

.10

1.52

60.9

%6.

52 ±

0.0

9D

uplic

ate

of a

bove

5972

0M

C24

-m6a

Jubi

lee/

Scot

ia6.

26 ±

0.0

71.

4710

0%6.

26 ±

0.0

829

6.2

± 8.

013

1.60

6.32

± 0

.10

1- to

2-m

m-w

ide

adul

aria

vei

nlet

5972

0M

C24

-m6b

Jubi

lee/

Scot

ia6.

14 ±

0.0

61.

7010

0%6.

13 ±

0.0

629

7.7

± 12

131.

816.

14 ±

0.0

7D

uplic

ate

of a

bove

4356

6M

C07

-S28

aK

aran

gaha

ke-

6.44

± 0

.07

Adu

lari

a fr

om c

omb

quar

tz v

ein

4356

6M

C07

-S28

bK

aran

gaha

ke6.

40 ±

0.0

81.

6184

.1%

6.29

± 0

.14

302.

4 ±

8.6

131.

866.

47 ±

0.0

9D

uplic

ate

of a

bove

4356

6M

C07

-S28

bK

aran

gaha

ke6.

28 ±

0.3

330

4.7

± 24

81.

70Pl

atea

u po

ints

4358

2M

C07

-S27

aK

aran

gaha

ke6.

90 ±

0.2

01.

1776

.9%

6.60

± 0

.23

Whi

te b

luis

h, b

recc

iate

d qu

artz

ve

in43

582

MC

07-S

27a

Kar

anga

hake

6.54

± 0

.37

312.

6 ±

159

0.50

Plat

eau

poin

ts43

582

MC

07-S

27b

Kar

anga

hake

6.12

± 0

.23

1.13

59.4

%6.

38 ±

0.2

1D

uplic

ate

of a

bove

4358

2M

C07

-S27

bK

aran

gaha

ke6.

15 ±

0.2

729

2.5

± 10

91.

23Pl

atea

u po

ints

5748

8M

C07

-S26

aK

aran

gaha

ke5.

75 ±

0.1

01.

5683

.4%

5.59

± 0

.10

Hos

t-ro

ck a

dula

ria

5748

8M

C07

-S26

aK

aran

gaha

ke6.

16 ±

0.4

128

0.3

± 15

91.

17Pl

atea

u po

ints

5748

8M

C07

-S26

bK

aran

gaha

ke5.

71 ±

0.1

30.

7672

.8%

5.61

± 0

.13

Dup

licat

e of

abo

ve57

488

MC

07-S

26b

Kar

anga

hake

5.81

± 0

.19

292.

1 ±

58

0.58

Plat

eau

poin

ts18

165

MC

07-S

14a

Mar

atot

o6.

29 ±

0.0

80.

5555

.3%

6.22

± 0

.08

306.

4 ±

1613

1.31

6.21

± 0

.08

Adu

lari

a in

qua

rtz

vein

1816

5M

C07

-S14

aM

arat

oto

6.22

± 0

.12

318.

0 ±

424

0.16

Plat

eau

poin

ts18

165

MC

07-S

14b

Mar

atot

o6.

35 ±

0.1

01.

1793

.7%

6.32

± 0

.12

301.

0 ±

1313

1.43

6.32

± 0

.12

Dup

licat

e of

abo

ve18

165

MC

07-S

14b

Mar

atot

o6.

35 ±

0.1

629

5.3

± 38

111.

29Pl

atea

u po

ints

1817

2M

C07

-S13

aM

arat

oto

6.44

± 0

.05

1.04

83.7

%6.

47 ±

0.0

528

7.7

± 1.

413

0.83

6.08

± 0

.09

Coa

rse

adul

aria

in c

omb

quar

tz v

ein

1817

2M

C07

-S13

aM

arat

oto

6.50

± 0

.11

277.

3 ±

2910

1.01

Plat

eau

poin

ts18

172

MC

07-S

13b

Mar

atot

o6.

60 ±

0.0

70.

9163

.6%

6.49

± 0

.07

Dup

licat

e of

abo

ve18

172

MC

07-S

13b

Mar

atot

o6.

66 ±

0.1

528

4.2

± 27

60.

98Pl

atea

u po

ints

1817

6M

C07

-S15

aM

arat

oto

6.37

± 0

.07

1.14

94.9

%6.

36 ±

0.0

924

9.5

± 10

131.

706.

34 ±

0.0

9C

oars

e ad

ular

ia in

terg

row

n w

ith

com

b qu

artz

in v

ein

1817

6M

C07

-S15

aM

arat

oto

6.32

± 0

.12

312.

1 ±

3411

1.13

Plat

eau

poin

ts18

176

MC

07-S

15b

Mar

atot

o6.

43 ±

0.0

81.

2487

.2%

6.41

± 0

.10

299.

2 ±

7.2

131.

776.

42 ±

0.1

1D

uplic

ate

of a

bove

1817

6M

C07

-S15

bM

arat

oto

6.47

± 0

.06

285.

5 ±

7210

1.39

Plat

eau

poin

ts46

851

MI9

1-G

21a

Mar

tha

5.86

± 0

.31

296.

0 ±

3.8

141.

956.

00 ±

0.3

9A

dula

ria

from

vei

n br

ecci

a46

851

MI9

1-G

21b

Mar

tha

6.43

±0.

30D

uplic

ate

of a

bove

4687

6M

I91-

G19

aM

arth

a6.

17 ±

0.1

90.

5987

.1%

6.03

± 0

.25

307.

1 ±

1214

0.90

6.38

± 0

.24

Hos

t-ro

ck a

dula

ria

4687

6M

I91-

G19

aM

arth

a6.

18 ±

0.1

9D

uplic

ate

of a

bove

5500

8M

I91-

G20

bM

arth

a6.

14 ±

0.0

41.

1799

.8%

6.10

± 0

.04

299.

4 ±

2.8

81.

206.

17 ±

0.0

5C

oars

e ad

ular

ia c

ryst

als

in v

ein

5500

8M

I91-

G20

cM

arth

a6.

20 ±

0.0

71.

2292

.0%

6.18

± 0

.08

297.

3 ±

7.8

81.

706.

23 ±

0.0

8D

uplic

ate

of a

bove

5500

9M

I91-

G25

aM

arth

a6.

05 ±

0.1

20.

8470

.9%

6.32

± 0

.19

Hos

t-ro

ck a

dula

ria

5500

9M

I91-

G25

bM

arth

a6.

09 ±

0.1

20.

8686

.2%

5.97

± 0

.14

325.

5 ±

158

0.94

6.28

± 0

.14

Dup

licat

e of

abo

ve55

010

MI9

1-G

27a

Mar

tha

6.19

± 0

.05

1.20

99.3

%6.

15 ±

0.0

529

7.3

± 2.

68

1.35

6.26

± 0

.08

Coa

rse

adul

aria

cry

stal

s in

hos

t roc

k

https://sina-pub.ir


0361-0128/98/000/000-00 $6.00 929

5501

0M

I91-

G27

bM

arth

a5.

96 ±

0.0

50.

6769

.0%

6.13

± 0

.09

Dup

licat

e of

abo

ve59

723

MC

24-m

4aM

uirs

Ree

f 2.

12 ±

0.1

11.

693

.5%

2.38

± 0

.22

Adu

lari

a fr

om q

uart

z ve

in59

723

MC

24-m

4bM

uirs

Ree

f 1.

78 ±

0.1

61.

5396

.1%

2.30

± 0

.26

Dup

licat

e of

abo

ve59

723

MC

24-m

4cM

uirs

Ree

f n/

a2.

15 ±

0.1

2D

uplic

ate

of a

bove

5971

9M

C24

-m5a

Ohu

i8.

25 ±

0.5

51.

9395

.9%

8.18

± 0

.62

Adu

lari

a fr

om q

uart

z ve

in59

719

MC

24-m

5aO

hui

8.31

± 0

.20

-6.2

± 8

67

0.06

Plat

eau

poin

ts59

719

MC

24-m

5bO

hui

8.42

± 0

.50

0.94

80%

8.53

± 1

.18

599.

2 ±

1731

131.

278.

88 ±

0.7

9D

uplic

ate

of a

bove

5971

9M

C24

-m5b

Ohu

i8.

47 ±

0.2

743

.7 ±

143

50.

13Pl

atea

u po

ints

5971

9M

C24

-m5c

Ohu

i8.

24 ±

0.3

50.

1792

.2%

8.24

± 0

.34

277.

5 ±

126

130.

628.

35 ±

0.4

9D

uplic

ate

of a

bove

5971

9M

C24

-m5c

Ohu

i8.

52 ±

2.2

9-5

58.0

± 6

654

60.

11Pl

atea

u po

ints

5676

0M

C07

-S22

aO

pito

nui

13.1

7 ±

0.07

0.65

97.6

%13

.25

± 0.

0728

6.8

± 3.

213

0.65

13.0

2 ±

0.10

Adu

lari

a fr

om q

uart

z ve

in56

760

MC

07-S

22a

Opi

tonu

i13

.24

± 0.

1228

7.3

± 11

100.

46Pl

atea

u po

ints

5676

0M

C07

-S22

bO

pito

nui

13.1

4 ±

0.08

0.67

76%

12.9

6 ±

0.09

Dup

licat

e of

abo

ve56

760

MC

07-S

22b

Opi

tonu

i13

.01

± 0.

9632

0.5

± 18

04

0.96

Plat

eau

poin

ts56

761

MC

07-S

23a

Opi

tonu

i13

.12

± 0.

070.

4810

0%13

.13

± 0.

0729

2.9

± 4.

413

0.39

13.0

9 ±

0.10

Adu

lari

a fr

om q

uart

z ve

in56

761

MC

07-S

23b

Opi

tonu

i13

.20

± 0.

111.

7797

.9%

274.

8 ±

24.4

13.1

0 ±

0.07

Dup

licat

e of

abo

ve56

761

MC

07-S

23b

Opi

tonu

i13

.01

± 0.

9632

0.5

± 18

04

0.96

Plat

eau

poin

ts56

762

MC

07-S

24a

Opi

tonu

i12

.82

± 0.

091.

2198

.1%

12.8

2 ±

0.10

295.

5 ±

4.8

131.

6112

.77

± 0.

12D

uplic

ate

of a

bove

5676

2M

C07

-S24

aO

pito

nui

12.8

4 ±

0.09

289.

4 ±

5.4

110.

82Pl

atea

u po

ints

5676

2M

C07

-S24

bO

pito

nui

13.1

4 ±

0.08

1.26

86.2

%13

.15

± 0.

0928

6.7

± 2.

613

1.53

12.7

9 ±

0.11

Adu

lari

a fr

om q

uart

z ve

in56

762

MC

07-S

24b

Opi

tonu

i13

.21

± 0.

1426

1.0

± 59

81.

24Pl

atea

u po

ints

5676

3M

C07

-S25

aPa

ritu

16.3

2 ±

0.13

1.69

55.3

%16

.19

± 0.

13A

dula

ria

from

qua

rtz

vein

5676

3M

C07

-S25

aPa

ritu

16.5

1 ±

0.31

284.

0 ±

175

1.41

Plat

eau

poin

ts56

763

MC

07-S

25b

Pari

tu-

16.3

6 ±

0.21

Dup

licat

e of

abo

ve56

758

MC

07-S

20a

Sove

reig

n6.

81 ±

0.0

60.

9896

.4%

6.70

± 0

.07

297.

4 ±

0.6

131.

077.

06 ±

0.1

1H

ost-

rock

adu

lari

a56

758

MC

07-S

20a

Sove

reig

n6.

87 ±

0.2

029

4.4

± 3.

410

1.05

Plat

eau

poin

ts56

758

MC

07-S

20b

Sove

reig

n-

6.87

± 0

.15

Dup

licat

e of

abo

ve56

759

MC

07-S

21a

Sove

reig

n6.

56 ±

0.0

80.

9362

.1%

6.30

± 0

.11

Hos

t-ro

ck a

dula

ria

5675

9M

C07

-S21

aSo

vere

ign

6.57

± 0

.09

295.

4 ±

1.4

71.

12Pl

atea

u po

ints

5675

9M

C07

-S21

bSo

vere

ign

6.55

± 0

.04

0.68

68.6

%6.

47 ±

0.0

4D

uplic

ate

of a

bove

5675

9M

C07

-S21

bSo

vere

ign

6.53

± 0

.06

298.

8 ±

7.2

70.

65Pl

atea

u po

ints

5972

1M

C24

-m7a

Sove

reig

n6.

69 ±

0.1

70.

8210

0%6.

69 ±

0.1

829

7.4

± 12

130.

896.

70 ±

0.3

2A

dula

ria

from

qua

rtz

vein

5972

1M

C24

-m7b

Sove

reig

n6.

74 ±

0.3

80.

7194

.4%

6.58

± 1

.02

Dup

licat

e of

abo

ve59

721

MC

24-m

7bSo

vere

ign

6.97

± 0

.50

214.

8 ±

149

30.

72Pl

atea

u po

ints

5972

1M

C24

-m7c

Sove

reig

nn/

a6.

37 ±

0.1

9D

uplic

ate

of a

bove

3387

0M

C07

-S18

aW

aior

ongo

mai

5.74

± 0

.04

0.81

99.3

%5.

82 ±

0.0

6C

oars

e ad

ular

ia in

qua

rtz

vein

3387

0M

C07

-S18

aW

aior

ongo

mai

5.75

± 0

.04

292.

5 ±

3.8

110.

62Pl

atea

u po

ints

3387

0M

C07

-S18

bW

aior

ongo

mai

5.68

± 0

.04

1.44

99.5

%5.

68 ±

0.0

429

6.8

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For the Re-Os dates of samples from Ohio Creek andMartha, we used the methods of Barra et al. (2003), and theanalytical work was undertaken at the University of Arizona.Approximately 0.02 to 0.1 g of pure molybdenite was hand-picked and loaded into a Carius tube. Spikes of 185Re and 190Oswere added, along with 8 mL of reverse aqua regia (threeparts of HNO3, 16 N, and one part of HCl, 10 N). About 2 to3 mL of hydrogen peroxide (30%) was added to ensure com-plete oxidation of the sample and spike equilibration. Thetube was heated to 240°C for approximately 8 h, and the so-lution was later subjected to a two-stage distillation processfor osmium separation (Nägler and Frei, 1997). Osmium waslater purified using a microdistillation technique described byBirck et al. (1997) and loaded on platinum filaments withBa(OH)2 to enhance ionization. After osmium separation, theremaining acid solution was dried, and the residue was dis-solved in 0.1 N HNO3. Rhenium was extracted and purifiedthrough a two-stage column using AG1-X8 (100–200 mesh)resin and loaded on nickel filaments with BaNO3.

Samples were analyzed by negative thermal ion mass spec-trometry using methods discussed in Barra et al. (2003). Un-certainties were calculated using error propagation, takinginto consideration errors from spike calibration, the uncer-tainty in the rhenium decay constant (0.31%), and analyticalerrors. All rhenium and osmium in molybdenite samples weremeasured with Faraday collectors. Blank corrections were in-significant for natural Os. In some cases, replicate analyses ofsingle molybdenite samples were performed; in other cases,samples were analyzed only once because the date obtainedwas identical to other samples of the same deposit or theamount of sample was limited.

ResultsTable 1 shows 40Ar/39Ar dates of adularia from the Paritu,

Opitonui, Ohui, Broken Hills, Wharekirauponga, Maratoto,Sovereign, Jubilee/Scotia, Karangahake, Martha, Favona,Waiorongomai, Eliza, and Muirs Reef deposits (App. 2). Errors within the samples are 2σ, and all samples fall beneath

the mean MSWD of 2. The samples were run twice, andTable 1 also shows the duplicate analyses.

Table 2 shows preferred ages for orebodies of the Haurakigoldfield, and the basis for their determinations. In this man-uscript, we use accepted nomenclature where “date” refers tothe date derived from a single determination, and “age” is theactual time when the deposit formed, which may be a singledate or an average of several dates (cf. Richards and Noble,1998).

Adularia from a tourmaline-bearing quartz vein at Parituyields a single 40Ar/39Ar plateau date of 16.32 ± 0.13 Ma (2σ)from steps 9 through 13 (55% of gas), and total gas dates of16.19 ± 0.13 and 16.36 ± 0.21 Ma (Table 1; Fig. 5A). Steps 9through 13 define an isochron with a date of 16.51 ± 0.31 Ma,an MSWD of 1.41, with no indication of excess argon, andoverlaps within error of the plateau date. Thus, we use theplateau date as the preferred age of this sample (Table 2).Previous K-Ar results for the Paritu diorite pluton provideddates of 16.0 to 17.1 Ma (Richards et al., 1966). When read-justed using Steiger and Jäger (1977) decay constants, thesedates range from 16.4 to 17.6 Ma (Adams et al., 1994). Thisrange, combined with the single date from our work, is con-sistent with mineralization occurring soon after emplacementof the pluton, but additional geochronology on the plutonicrocks and veins has great potential to provide much tighterconstraints on the timing of plutonic activity, mineralization,and uplift in this area.

Three samples of adularia from quartz veins from Opitonuiyield six 40Ar/39Ar plateau dates that range from 12.82 ± 0.09to 13.17 ± 0.07 Ma. Five plateaus give exceptionally flat spec-tra that overlap within error, although one replicate plateauyielded a slightly younger date, presumably due to minor Arloss. An error weighted average of the five plateaus that over-lap is 13.15 ± .03 Ma, and the MSWD of the average is 0.25;we use this as the preferred age of the deposit (Tables 1, 2;Fig. 5B). The samples yielded nine isochrons with MSWDvalues less than 2.0, and seven of these yield dates that over-lap with the five plateau dates above, and three of the six total

930 MAUK ET AL.

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TABLE 2. Summary of Preferred Ages for the Hauraki Goldfield

Location Province Preferred age (Ma) Comments Reference

Broken Hills Eastern 7.12 ± 0.02 Average of four 40Ar/39Ar plateaus from adularia from two quartz veins This studyEliza Southern 4.47 ± 0.06 Average of two 40Ar/39Ar plateaus from adularia from one quartz vein This studyFavona Southern 6.05 ± 0.08 Average of two 40Ar/39Ar plateaus from adularia from one quartz vein This studyGolden Cross Southern 6.98 ± 0.11 Average of four 40Ar/39Ar plateaus from adularia from two quartz veins Mauk and Hall (2004)Karangahake Southern 6.9 to 6.1 Three 40Ar/39Ar dates of adularia from two quartz veins This studyKomata Southern 6.06 ± 0.06 Average of two 40Ar/39Ar plateaus from coarse adularia from one vein Mauk and Hall (2004)Kuaotunu Northern 14.1 ± 0.2 K-Ar adularia Skinner (1986)Maratoto Southern 6.41 ± 0.04 Average of four 40Ar/39Ar plateaus from adularia from three quartz veins This studyMartha Southern 6.16 ± 0.06 Average of two 40Ar/39Ar plateaus from adularia from one quartz vein This studyMuirs Reef Southern 2.1 to 1.8 40Ar/39Ar plateau dates of adularia from one quartz vein This studyNeavesville Eastern 6.88 ± 0.04 Average of four 40Ar/39Ar plateaus from coarse adularia from two veins Mauk and Hall (2004)Ohio Creek Northern 12.0 to 11.9 Re-Os molybdenite This studyOhui Eastern 8.29 ± 0.25 Average of three 40Ar/39Ar plateaus from adularia from one quartz vein This studyOpitonui Northern 13.15 ± .03 Average of five 40Ar/39Ar plateaus from adularia from three quartz veins This studyParitu Northern 16.32 ± 0.13 Single 40Ar/39Ar plateau date of adularia from one quartz vein This studySovereign Southern 6.70 ± 0.16 Average of two 40Ar/39Ar plateaus from adularia from one quartz vein This studyWaiorongomai Southern 5.71 ± 0.03 Average of four 40Ar/39Ar plateaus from adularia from two veins This studyWaiomu Northern 10.8 ± 0.1 K-Ar sericite Skinner (1986)Wharekirauponga Eastern ~6.3 Average of two 40Ar/39Ar plateaus from adularia from one quartz vein This study

Note: all averages listed above are error weighted

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gas dates also overlap with these plateau dates (Table 1). Oneplateau, two isochrons, and three total gas dates cluster around12.8 Ma and overlap within error, but we prefer the older ageof 13.15 ± 0.03 Ma because the plateaus used to derive thatage are exceptionally flat and very reproducible (Fig. 5B).Host rocks from the Kuaotunu Subgroup have whole-rockK-Ar dates of 11.4 Ma, and unaltered plagioclase from theMahinapua Andesite yielded a date of 13.8 ± 0.3 Ma (Skinner,1986). Given the 13.15 Ma age of mineralization at Opitonui

and the 13.8 Ma date for the plagioclase separate, it is likelythat the 11.4 Ma whole-rock date is too young.

One sample of molybdenite from 337.9-m depth in DDH 6at Ohio Creek yielded total Re and 187Os concentrations of1,191 to 1,334 ppm and 149 to 165 ppb, respectively (Table2). The two Re-Os dates overlap within error and yield datesof 11.87 ± 0.06 and 11.97 ± 0.08 Ma (2σ). These samples areslightly older than the K-Ar date of 11.2 ± 0.3 Ma determinedon hypogene alunite from altered rocks of the advanced


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02468

101214161820

Totalg as age = 16.19 0.13 MaPlateau age = 16.32 0.13 MaMSWD = 1.69f 39 = 55.3%

02468101214161820

Total gas age = 13.02 0.09 MaPlateau age = 13.17 0.06 MaMSWD = 0.65f 39 = 97.6%

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0123456789


0123456789


02468

1012


0123456789

Total gas age = 6.70 0.32 MaPlateau age = 6.69 0.18 MaMSWD = 0.82f 39 = 100%

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0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.90123456789


0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.00123456789


Age

(Ma)

Fraction of 39Ar Released

(A) (B)

(C) (D)

(E) (F)

(G) (H)

(I) (J)

(K)(L)

Paritu sample AU 56763 Opitonui sample AU 56760

Ohui sample AU 59719 Broken Hills sample AU 56757

Wharekirauponga sample AU 56755 Maratoto sample AU 18176

Sovereign sample AU 59721 Favona sample AU 55007

Karangahake sample AU 43582 Waiorongomai sample AU 33870

Eliza sample AU 59722 Muirs Reef sample AU 59723

FIG. 5. Representative age spectra for samples of adularia from quartz veins. All error estimates are 2σ and error boxesare 1σ. The double arrows show the extent of the plateaus and the steps included in the plateaus, and f39 is the fraction of39Ar that was released. A. Paritu sample AU 56763 (MC07-S25a). B. Opitonui sample AU 56760 (MC07-S22a). C. Ohui sam-ple AU 59719 (MC24-m5c). D. Broken Hills sample AU 56757 (MC07-S17a). E. Wharekirauponga sample AU 56755(MC07-S29b). F. Maratoto sample AU 18176. (MC07-S15a). G. Sovereign sample AU 59721 (MC24-m7a). H. Favonasample AU 55007. (MI91-G26b). I. Karangahake sample AU 43582. (MC07-S27a). J. Waiorongomai sample AU 33870.(MC07-S18b). K. Eliza sample AU 59722 (MC24-m3b). L. Muirs Reef sample AU 59723 (MC24-m4a).

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argillic cap at Ohio Creek (Hunt and Roddick, 1992) and sig-nificantly older than the 40Ar/39Ar date of 10.71 ± 1.35 Ma ofa relatively unaltered hornblende-augitie dacitic porphyrydike (Hunt and Roddick, 1993). Both molybdenite sampleshave high Re and 187Os concentrations and overlap withinerror. The discrepancy between the hypogene alunite andmolybdenite dates may be explained by (1) slightly protractedor episodic mineralization, (2) Re and Os has a higher closuretemperature in molybdenite than K-Ar in alunite, so the for-mer method may yield the age of mineralization whereas thelatter might yield the cooling age, or (3) that the K-Ar date isslightly disturbed. Based on the comparative studies of K-Arand 40Ar/39Ar dates presented in this paper, we prefer (3), butthe other two possibilities cannot be ruled out.

Hydrothermal mineralization at Thames is constrained byK-Ar dates of 11.6 ± 0.3 to 10.9 ± 0.2 Ma from illite selvagesof auriferous quartz veins in the eastern part of the Thamesdistrict (Sinsuat, 1992). However, the K-Ar dates of the illiteare particularly subject to analytical uncertainties, and there-fore we interpret the likely age of mineralization at Thames asbetween 12 and 11 Ma. Within this interval, it is likely closerto 12 Ma, based on our new Re-Os results and also on the ge-ologic data that suggests a genetic relationship between theOhio Creek and Thames mineralization (e.g., Brathwaite etal., 2001b).

One sample of adularia from a quartz vein at the Phoenixworkings in the Ohui deposit yielded three 40Ar/39Ar plateaudates that overlap within error and range from 8.24 ± 0.35 to8.42 ± 0.50 Ma. An error weighted average of the threeplateaus that overlap is 8.29 ± 0.25 Ma, and the MSWD ofthe average is 0.19; we use this as the preferred age of thedeposit (Tables 1, 2; Fig. 5C). The samples yielded sixisochrons with MSWD values less than 2.0 and three totalgas dates that overlap with the plateau dates, although threeof the isochrons have 2σ errors that exceed 1 Ma (Table 1).Mineralization occurs in both Coromandel Group andesiteand Whitianga Group rhyolite, but the absolute ages of thehost rocks at Ohui are not well-constrained, and thereforethe timing between mineralization and volcanism remainsunclear.

Two samples of rhombohedral adularia crystals from theNight Reef at the Broken Hills deposit yield four exception-ally flat 40Ar/39Ar plateaus that range from 7.12 ± 0.03 to 7.15± 0.05 Ma (Table 1; Fig. 5D). These results overlap withinerror to give an average of 7.12 ± 0.02 Ma, which is our pre-ferred age of the deposit; the MSWD of the average is 0.46(Table 2). The samples yielded four isochrons with MSWDvalues less than 2.0 and four total gas dates, all of which over-lap with the plateau dates and our preferred age (Table 1).Obsidian from the Whitianga Group host rocks has fissiontrack and K-Ar dates that indicate that volcanism in the re-gion occurred between 7.2 and 7.8 Ma (Skinner, 1986) andsuggest that Broken Hills mineralization occurred shortlyafter rhyolite volcanism.

Adularia from one sample of a colloform quartz vein in theWharekirauponga prospect yields 40Ar/39Ar plateaus of 6.24 ±0.08 and 6.37 ± 0.07 Ma (Table 1; Fig. 5E). The dates overlapwithin error to give an average of 6.32 ± 0.12 Ma, but theMSWD of the average is 5.51, and the combined age fails thechi-squared statistical test of Wendt and Carl (1991), so our

preferred age of the Wharekirauponga prospect is approxi-mately 6.3 Ma (Table 2). The sample also yielded threeisochrons with MSWD values less than 2.0 and four total gasdates, all of which overlap with the plateau dates and our pre-ferred age (Table 1). Veins at the Wharekirauponga prospectoccur in the Maratoto Rhyolite, which elsewhere in the re-gion has K-Ar dates between 7 to 6 Ma (Brathwaite andChristie, 1996). Postmineralization Whakamoehau Andesitenear the prospect has a K-Ar date of 6.61 ± 0.12 Ma (Brath-waite and Christie, 1996), which is older than our preferredage of mineralization. Therefore, more work is required to re-fine the relationships between volcanism and mineralizationin the Wharekirauponga area.

Six 40Ar/39Ar plateau dates of three adularia samples fromquartz veins at the Maratoto deposit range from 6.29 ± 0.08to 6.60 ± 0.07 Ma (Table 1; Fig. 5F). Four of these valuesoverlap within error, and the average of these values is 6.41± 0.04 Ma, which we interpret as the preferred age for thedeposit; the MSWD of the average is 1.44 (Table 2). Thesample also yielded eight isochrons with MSWD values lessthan 2.0 that overlap with the above plateau dates and ourpreferred age, and four total gas dates also overlap withthese results (Table 1). One plateau and two isochrons yielddates of approximately 6.6 Ma, which are older than ourpreferred age of 6.41 Ma. Although this may indicate pro-tracted mineralization, this seems unlikely for this sample,which is a duplicate analysis of a single clot of coarse-grained adularia in a coarse-grained quartz vein. Because wewere not analyzing adularia from different bands within thevein, or different vein samples, this discrepancy is difficultto explain. Nonetheless, our preferred age of 6.4 Ma is sim-ilar to previous K-Ar dates of adularia from veins of 6.2 ± 0.1to 5.2 ± 0.1 Ma (Skinner, 1986), and our results suggest thatthe deposit formed at the waning stages of deposition of thehost 7.9 to 6.3 Ma Waipupu Formation (Brathwaite andChristie, 1996).

One sample of adularia from a quartz vein at the Sovereigndeposit yielded 40Ar/39Ar plateau, isochron, and total gas datesof approximately 6.7 Ma. All dates overlap within error, andthese dates also overlap the plateau, isochron, and total gasdates of host-rock adularia from the Sovereign deposit (Table1; Fig. 5G). We therefore used the average of the two veinadularia plateau dates, which is 6.70 ± 0.16 Ma with anMSWD of 0.07, as the preferred age for the Sovereign de-posit (Table 2). The Sovereign deposit is hosted by the 7.9 to6.3 Ma Waipupu Formation (Brathwaite and Christie, 1996),but uncertainties in the absolute ages of the host rocks at Sov-ereign preclude any rigorous interpretations of the relation-ship between volcanism and mineralization.

A small quartz-adularia vein near the Sovereign deposit, inthe area between the Jubilee and Scotia deposits, yieldedplateau, isochron, and total gas dates that overlap within error(AU 59720; Table 1). The plateau dates barely overlap at the2σ level to yield an average of 6.19 ± 0.12 Ma, but the MSWDis 6.35, and the combined age fails the chi-squared statisticaltest of Wendt and Carl (1991), so we interpret the age of thisvein as approximately 6.2 Ma. This age does not overlap withour preferred age of the Sovereign deposit, and therefore, weinterpret this vein as representing fracture-controlled fluidflow adjacent to the Sovereign deposit at a later time.

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Three samples of adularia from host rocks at the Marthadeposit yielded five 40Ar/39Ar plateau dates that range from5.96 ± 0.05 to 6.19 ± 0.05 Ma, and a grab sample of adulariafrom a quartz vein at Martha yielded two 40Ar/39Ar plateaudates of 6.14 ± 0.04 and 6.20 ± 0.07 (Table 1; Fig. 6). We con-sider the dates from the adularia within the quartz vein to bethe most robust, because adularia from host rocks commonlyshows some replacement by illite, whereas adularia fromveins is unaltered. The alteration of host-rock adularia likelycontributes to the larger errors and more irregular spectra ob-tained from host-rock adularia (Fig. 6A); in contrast spectrafrom vein adularia are relatively flat and have significantlysmaller errors (Fig. 6B). The two dates from vein adularia atMartha overlap within error, produce an error weighted meantotal gas date of 6.19 ± 0.05 Ma, an error weighted meanplateau date of 6.16 ± 0.06 Ma, and an error weightedisochron date of 6.12 ± 0.06 Ma. All three dates overlapwithin error, and we use the average of the plateau dates tointerpret a preferred age for this vein sample of 6.16 ± 0.06Ma; we also use this as the preferred age of the Martha de-posit (Table 2).

Samples of molybdenite from two other veins in the deeplevels of the Martha deposit have total Re and 187Os concen-trations of 136 and 299 ppm and 9.0 and 20 ppb, respectively,and Re-Os dates of 6.37 ± 0.03 and 6.51 ± 0.03 Ma. Thesedates do not overlap with each other, and we interpret eachdate to be the age of each molybdenite-bearing vein (Table3). The 40Ar/39Ar plateau dates of vein adularia and the Re-Osmolybdenite dates are similar to a previously reported K-Ardate of 6.58 ± 0.54 Ma, but significantly younger than other

K-Ar dates of 6.92 ± 0.30 and 7.23 ± 0.38 Ma determined onadularia from strongly altered andesite at the Martha deposit(Brathwaite and Christie, 1996). These K-Ar dates overlapwith or exceed the 6.12 to 6.43 Ma total gas dates of adulariain this study (Table 1), which are equivalent to K-Ar dates.These older dates may reflect excess argon in these hy-drothermal samples, which may occur in fluid inclusions inthe adularia or other minerals that were included in the sam-ples. Results from our step-heated spectra support this, be-cause they commonly show older ages and a spike in the Cl/Kvalues at the beginning of the run, indicating that there is arelatively large release of Cl at the beginning of the run (Fig.6). The Cl likely comes from the abundant fluid inclusions inthese samples.

Adularia from one quartz vein from the Moonlight portionof the Favona deposit yields 40Ar/39Ar plateaus of 6.01 ± 0.14and 6.07 ± 0.10 Ma (Table 1; Fig. 5H). The dates overlapwithin error giving our preferred age for the Favona depositof 6.05 ± 0.08 Ma. This preferred age for the Favona depositoverlaps within error with our preferred age of 6.16 ± 0.06Ma for the Martha deposit, indicating that these depositsformed more or less synchronously at 6.1 Ma. This supportsprevious interpretations that both deposits formed within thesame hydrothermal system, as indicated by geologic, fluid in-clusion, mineralogical, and geophysical constraints (e.g.,Brathwaite and Faure, 2002; Simpson and Mauk, 2007; Mor-rell et al., 2011).

Host-rock adularia from the Karangahake deposit yields40Ar/39Ar plateau dates of 5.71 ± 0.13 and 5.75 ± 0.10 Ma(Table 1; Fig. 5I). Two samples of adularia from different


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Age

in M

a

Total gas age = 6.38 0.24 MaPlateau age = 6.17 0.19 MaMSWD = 0.59, f39 = 87.1%

0.000

0.002

0.004

0.006

0.008

Cl/K

0.0

0.2

0.4

0.6

0.8

1.0

Ca/

K

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0


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Age

in M

a

Total gas age = 6.23 0.08 MaPlateau age = 6.21 0.07 MaMSWD = 1.55f 39 = 100%

0.000

0.002

0.004

0.006

0.008

Cl/K

0.00

0.02

0.04

0.06

0.08

0.10

Ca/

K

(A) (B)

Martha host rock adularia: sample AU46876 Martha quartz vein: sample AU55008

FIG. 6. Age spectra from two adularia samples from the Martha deposit. All error estimates are 2σ and error boxes are1σ. A. Host-rock adularia from sample AU46876 (MI91-G19a). B. Adularia from quartz vein sample AU55008 (MI91-G20c).

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quartz veins in the deposit yield three 40Ar/39Ar plateau datesranging from 6.12 ± 0.23 to 6.90 ± 0.20 Ma (Table 1; Fig. 5I).Although the dates from the host-rock adularia overlap withinerror, petrographic analysis shows that late illite replaces thishost-rock adularia, and therefore these plateau dates arelikely disturbed. Dates from the quartz vein adularia do notoverlap, and the host-rock adularia dates are up to 1.2 Mayounger than the dates of adularia from the veins. Therefore,we cannot interpret a single preferred age for mineralizationat Karangahake. Previous K-Ar results from Karangahake alsoreflect this complexity. Adularia from altered andesite and as-sociated mineralization at Karangahake yielded K-Ar dates of4.8 ± 0.1 and 6.0 ± 0.1 Ma (Skinner, 1986). The 6 to 7 MaMaratoto Rhyolite of the Whitianga Group forms a cap over7.9 to 6.3 Ma Waipupu Formation andesite at MountKarangahake (Brathwaite, 1989; Brathwaite and Christie,1996). Stockwork quartz veins cut the rhyolite cap, so some ofthe mineralization must be younger than the Maratoto Rhyo-lite. We recognize three possible explanations for these com-plex and apparently conflicting results: (1) the Karangahakearea may have undergone more than one phase of mineral-ization, (2) hydrothermal alteration of the host rocks mayhave continued for a long period of time after the veins wedated had formed, or (3) the Karangahake deposit formed atrelatively deep levels at higher temperatures than most de-posits in the Hauraki goldfield (Brathwaite, 1989; Christie etal., 2007), and therefore some of the dates may be coolingages rather than crystallization ages. Of these, we consider (1)and (2) to be most likely, but (3) cannot be ruled out.

Adularia from the Waiorongomai deposit was extractedfrom a quartz vein and from a quartz-base metal sulfide vein,and yielded 40Ar/39Ar plateaus that range from 5.68 ± 0.04 to5.74 ± 0.04 Ma (Fig. 5J). The results overlap within error(Table 1), and the average of the four plateaus is 5.71 ± 0.03Ma, which we use as our preferred age of mineralization atthe Waiorongomai deposit; the MSWD of the average is 1.95(Table 2). The samples also yielded six isochrons with MSWDvalues less than 2.0 that overlap with our preferred age, andthree of the four total gas dates also overlap with the plateaudates and our preferred age (Table 1). Andesitic volcanicrocks have yielded K-Ar dates of 6.65 Ma from an area nearthe Waiorongomai deposit, which is consistent with the age ofthe Waipupu Andesite (Brathwaite and Christie, 1996).

One sample of adularia from a quartz vein at the Eliza de-posit yielded two 40Ar/39Ar plateau dates of 4.49 ± 0.12 and4.46 ± 0.07 Ma, which overlap within error. An errorweighted average of these plateaus is 4.47 ± 0.06 Ma, whichis our preferred age of the Eliza deposit, and the MSWD ofthis average is 0.12 (Table 1; Fig. 5K). The samples yieldedfour isochrons with MSWD values less than 2.0 and two totalgas dates that overlap with the plateau dates (Table 1).

One sample of adularia from a quartz vein at the MuirsReef deposit yielded two 40Ar/39Ar plateau dates and threetotal gas dates that range from 1.78 to 2.38 Ma (Table 1; Fig.5L). The older plateau date overlaps the three total gas dateswithin error, but the younger plateau date does not overlapthe other dates. Therefore, rather than interpreting a singlepreferred age for the Muirs Reef deposit, we conclude that itformed between 2.1 and 1.8 Ma (Table 2).

DiscussionEpithermal and porphyry deposits in the Hauraki goldfield

show ages that progressively young southward and clusterinto two groups that are distinct in time (Fig. 7). The north-ern province of the goldfield contains deposits that are olderthan 11 Ma, and the southern and eastern provinces containdeposits that are younger than 8.3 Ma (Mauk and Hall, 2004;Ward et al., 2005; Christie et al., 2007). Deposits in differentprovinces show some significant differences, and there is alsoa change in volcanic activity, and more speculatively, struc-tural style that coincides with the 11 to 8.3 Ma hiatus betweendeposits in the northern versus those in the southern andeastern provinces. This section begins with a discussion of thepossible duration of hydrothermal mineralization in the Waihidistrict and the implications of our data for metallogeny. Wethen synthesize available data about mineralization, volcan-ism, and structural setting of the deposits and conclude byevaluating hypotheses for the tectonic and volcanologic evo-lution of the Hauraki goldfield and its links with epithermalmineralization.

Duration of hydrothermal mineralization in the Waihi district

The Waihi district contains both the Martha mine and therecently discovered Favona deposit (Brathwaite et al., 2006;Mauk et al., 2006b, Torckler et al., 2006; Simpson and Mauk,2007). Both deposits sit within a single magnetic quiet zone

934 MAUK ET AL.

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TABLE 3. Re-Os Data for Molybdenite from the Ohio Creek and Waihi Deposits (all errors are ±2σ)

Weight Total 187Re 187Os Age Error AU no. Northing Easting Location Comments (g) Re (ppm) (ppm) (ppb) (Ma) (2σ)

59724 6420700 2762700 Waihi Waihi extension: 1916: molyb- 0.113 135.6 84.9 9.01 6.37 0.03denite veins from deep levels of the Waihi deposit

59725 6420700 2762700 Waihi Molybdenite from the deep levels 0.021 298.5 186.9 20.3 6.51 0.03of the Waihi deposit

59726 6452400 2737200 Ohio Creek DDH 6- 337.9 m; molybdenite 0.067 1333.9 835.0 165.2 11.87 0.06from porphyry depost near Thames

59726 Ohio Creek DDH 6- 337.9 m; molybdenite 0.055 1190.7 745.4 148.6 11.97 0.08from porphyry depost near Thames

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that was produced by destruction of magnetite in the host vol-canic rocks during hydrothermal alteration (Morrell et al.,2011). Previous workers agree that geological and geophysicalevidence is consistent with both deposits forming from thesame hydrothermal system (e.g., Brathwaite and Faure, 2002;Torckler et al., 2006; Simpson and Mauk, 2007; Morrell et al.,2011). Our preferred ages of the Martha and Favona depositsoverlap within error (6.16 ± 0.06 and 6.05 ± 0.08 Ma, respec-tively) and support this hypothesis.

The Re-Os dates of 6.37 ± 0.03 and 6.51 ± 0.03 Ma ofmolybdenite-bearing veins from deeper levels of the Marthadeposit do not overlap within error and are older than ourpreferred 40Ar/39Ar age of vein adularia from shallower levelsof the deposit. Therefore, available data indicate that hydro -thermal systems were active in the Waihi district from 6.5 to6.1 Ma, with a span of approximately 390,000 to 460,000years. However it is also possible that the molybdenite andgold veins are the product of shorter lived but separate hydro -thermal systems that overlap spatially. Additional work is

required to establish whether the molybdenite veins, which arevery rare, formed from the same type of hydrothermal systemas the epithermal orebodies at Martha and Favona, or whetherthey formed from a different type of earlier hydrothermal ac-tivity. Nonetheless, the overall longevity of hydrothermal ac-tivity in the Waihi district is clearly indicated by our dates.

Elsewhere, constraints on the lifespans of the hydrothermalsystems that formed epithermal deposits are relatively un-common. Some of these suggest that mineralization occurredduring a long time interval, whereas others provide evidencefor mineralization forming during a brief time window. Forexample, the Hosen vein at Hishikari, Japan, formed during aperiod of 260,000 years, and the hydrothermal system thatproduced the mineralization had a lifetime of approximately600,000 years, lasting between 1.25 and 0.64 Ma (Sanematsuet al., 2006, and references therein). There is still geothermalactivity at Hishikari, which suggests that the hydrothermalsystem may have been active, in some form, for 1.25 Ma. TheLadolam gold deposit on Lihir Island, Papua New Guinea,formed approximately 0.6 to 0.5 Ma (Carman, 2003). Thearea remains an intense center of geothermal activity, with avariety of geothermal manifestations in the open pit and anestimated heat flow of 50 to 70 mW (Simmons and Brown,2006). In contrast, Henry et al. (1997) provided 40Ar/39Argeochronology from the Round Mountain deposit in Nevada,which indicates that this 16 Moz orebody may have formed inas little as 0.05 Ma. Similarly, Leavitt et al. (2004) obtaineddates from several veins at Midas in Nevada, and all dates es-sentially overlapped within error, indicating that this depositalso formed in a short time period.

Future work in the Hauraki goldfield and elsewhere shouldendeavor to address two important questions: what is the totalduration of hydrothermal activity that leads to epithermalorebodies, and does mineralization in individual depositsform from discrete, episodic events that are separated by sig-nificant time lags or does mineralization form more or lesscontinuously through time? We note, for example, that mostvein textures suggest the former, whereas most geochemicalmodels assume the latter.

Progression of ages, metallogenic trends, and Au deposition through time

The geochronology of mineral deposits in the northernHauraki goldfield remains incomplete, in part because oflocal absence of suitable material such as vein adularia ormolybdenite for dating. Nonetheless, the available dataclearly show southward-younging of mineralization, whichranges from 16.3 Ma in the north to 12 Ma in the south nearThames (Fig. 2; Table 3). The oldest mineralization is in thenorthern part of the goldfield at Paritu, which is constrainedby our single 40Ar/39Ar plateau date of 16.3 Ma. Adularia fromthe Kuaotunu deposit yielded a K-Ar date of 14.1 ± 0.2 Ma(Skinner, 1986), which is significantly older than our pre-ferred age of 13.1 Ma for adularia from the nearby Opitonuideposit. Sericite from Waiomu has a K-Ar date of 10.8 ± 0.1Ma (Skinner, 1986), and our results lead us to prefer a 12 Maage for mineralization at Ohio Creek and Thames.

There is a temporal and spatial hiatus of mineralization be-tween the northern province and the southern and easternprovinces of the Hauraki goldfield, even though volcanic


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0

37°S

37.5°

176°

175.5°

36.5°

N

Paritu

Waiomu

Ohio Creek

Thames

11.9 Ma

6.9-6.1 Ma

6.4 Ma

10.8 Ma

Waiorongomai

Neavesville

Golden Cross

Kuaotunu

Opitonui

5.7 Ma

14.1 Ma

6.9 Ma

16.3 Ma

Broken Hills7.1 Ma

WharekiraupongaMaratoto

Karangahake6.7 MaSovereign

Petote Stream12.6 Ma

13.2 Ma

7.0 Ma

Ohui8.3 Ma

Porphyry Cu deposit

Epithermal deposit

Epithermal depositshowing extension direction

25 km

Komata6.1 Ma Martha

Favona

6.3 Ma

6.2 Ma6.1 Ma

FIG. 7. Map of the Hauraki goldfield, showing the ages of epithermal andporphyry deposits, and also the direction of extension, where constrainedfrom kinematic indicators. Data sources in text.

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activity during this period appears to be more continuous(Fig. 2; Table 3). The data suite from the southern and east-ern goldfield is more robust, with the oldest ages from theOhui (8.3 Ma), Broken Hills (7.1 Ma), Neavesville (6.9 Ma),Golden Cross (6.9 Ma), and Sovereign (6.7 Ma) deposits.Mineralization continued at Maratoto (6.4 Ma), Whareki-rauponga (6.3 Ma), Komata (6.1 Ma), Martha and Favona(6.1 to 6.0 Ma), and then at the Waiorongomai (5.7 Ma), Eliza(4.5 Ma), and Muirs Reef (2.1 to 1.8 Ma) deposits (Mauk andHall, 2004; this paper).

In general, the ages of mineralization young from north tosouth, but some important variations exist. For example, un-altered volcanic rocks in the Hauraki goldfield typically showshort-wavelength high-amplitude magnetic anomalies butareas underlain by altered volcanic rocks whose magnetite hasbeen destroyed by hydrothermal alteration show magneticquiet zones (Locke et al., 2006; Morrell et al., 2011). A singlelarge magnetic quiet zone in the southern goldfield envelopsthe Maratoto (6.4 Ma), Golden Cross (6.9 Ma), Komata (6.1Ma), and Sovereign (6.7 Ma) deposits (Morrell et al., 2011).However, the variable ages of these deposits and their loca-tions suggest that these deposits formed from several differ-ent hydrothermal systems whose alteration halos overlapped,and not from a single hydrothermal system that migratedthrough time. Therefore, although deposits in the Haurakigoldfield generally young from north to south, on the localscale there can be significant variability that presumably re-flects the complex evolution and spatial overlap of the parenthydrothermal systems.

Although more high-quality data would greatly enhanceour knowledge of mineralization in the Hauraki goldfield, theavailable dates constrain almost 90 percent of the Au recov-ered from the goldfield (Fig. 3). Deposits from differentprovinces in the goldfield have different ages, metal endow-ments, and mineralization styles. The northern province of

the Hauraki goldfield has produced 59,137 kg Au from mas-sive to crustiform veins in the Kuaotunu Subgroup of theCoromandel Group and includes the bonanza-style veins atCoromandel and Thames, as well as minor porphyry-stylemineralization (Figs. 2, 3; Christie et al., 2007). The easternprovince of the goldfield contains stockwork veins, dissemi-nated mineralization, and veins that occur in rhyolite of theWhitianga Group but has produced only 1,607 kg Au. In con-trast, the southern province has produced 307,640 kg Aufrom colloform to crustiform veins and local stockwork veinsthat occur predominantly in andesite of the Waiwawa Sub-group of the Coromandel Group (Figs. 2, 3; Christie et al.,2007). Therefore, even though gold mineralization in theHauraki goldfield began around 16.3 Ma and continuedepisodically to approximately 1.8 Ma, greater than 80 percentof the Au that has been recovered from the goldfield was de-posited in the southern province of the goldfield in the rela-tively short 0.9 Ma window between 6.9 and 6.0 Ma.

Volcanic and structural evolution

The North Island of New Zealand and the adjacent Pacificregion have undergone significant tectonic reorganizati

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