Magmatic Contribution to Low Sulfidation Epithermal Deposits Simmons1995

Chapter 20

MAGMATIC CONTRIBUTIONS TO LOW.SULFIDATION EPITHERMAL DEPOSITS Stuart F. Simmons Geothermal Institute, The University of Auckland, Private Bag 92019,Auckland, I,{ewZealand

INrRonucrron Low-sulfidation epithermaldepositsform at 100 m depth) at Fresnillo and Antamok-Acupan (Table 4). Inclusion fluids hosted by quartz, calcite, and sulfides were releasedunder vacuum by thermal decrepitation or crushing10- to 20-g monomineralicsamples; their isotope compositionswere measuredon a Nier-type double focussing mass spectrometer. '7 at AntamokThe R/Ra values, between 6 to Acupan and between I and 2 at Fresnillo (Fig. 12), indicate a component of mantle He, presumablytransportedto shallow crustal levels by ascendingmagmas(Simmons1986;Simmons et al. 1988). The Fresnilloresultsseemlow for arc-related fluids, and they may result from three different lHe processes:1) preferentialoutwarddiffusionof "He; over 2) in-.situradiogenicaccumulationof -He *H"; or 3) radiogenic accumulation of associatedwith long magma residencetimes and crustal contamination.Of these, the last seems most likely, given the relativelythick continental crust through which Fresnillo magmasmigrated (see Simmonset al. 1988). The helium-isotope

O a

A Fresnillo O Baguio

(g

E CE

tAtA a t m o . p h " r ihc " l i u r l

6 He concentration (x 10 ) cc STP/g Figure 12. Helium-isotopecompositions(R/Ra) versus heliumamountfor inclusion fluidscontained in quartz,calcite,andsulfidesfrom Fresnillo,andquartz (Simmons1986;Simmonset from Antamok-Acupan a/.1988).

ratios of geothermalemanationsassociatedwith arc volcanismand similarly thick crustrangefrom 1.30to 2.16 R/Ra in the SouthernVolcanicZone. andl.44to 6.47 R/Ra in CentralVolcanic Zone of the Andes, thus supporting this interpretation (Hilton et al. 1993). For Fresnillo, the results further indicate relatively uniform compositious throughtime irrespectiveof mineral host or fluidinclusion salinities, confirming that processes governinghelium input are decoupledfrom those affecting other fluid components (including metals), consistent with observationsof TVZ geothermal fluids and data from other (Simmonset al. 1987). hydrothermaldeposits Note that investigatorswishing to pursueHeisotope analysesof epithermal materials should ensurethat their samplesare shielded from the effects of cosmic radiation,especiallyat higher elevations of about 1500 m asl or more, as 'H" "un be produced by nuclear cosmogenic reactions involving spallation or neutron by'Li (e.g ,Kurz 1986). Sucheffects absorption are interpretedfor surfacevein-quartz samplesat 2200 m asl from the Fresnillodistrict,which have anomalousvaluesexceeding100 R/Ra (Simmons 1986; Simmons et al. 1986). The observed exponential decreasein R/Ra with depth (from

r S.F.Simmons

I l6 R/Ra at the surfaceto 65 R/Ra at l.l m depth) along with calculations of the cosmic-ray attenuationlength in rocks,however,indicatethat these radiation effects are unlikely to penetrate depths greater than about l0 m at Fresnillo (Simmons1986;Simmonsel al. 1986).

Fresnillo

2000 N, /He

N2-Ar-He Ratios Problemsassociatedwith the analysisof gas speciesin f'luid inclusionsare relatedto lossof H2 and H2S (through diffusion and post-extraction reaction;see Graney & Kesler this volume) and these artefactsalter the redox state calculatedfor er publisheddata(e.g.,Roedder1984;Hedenquist al. 1992). However, such problems should not of N, Ar and He, thoughthere affect measurement are few available measurementson epithermal materials.Norman & Musgrave (1994) reported data from three epithermaldeposits,includingthe SantoNiflo vein, Fresnillo(Fig. l3). Gaseswere for measuredby a quadrupolemass spectrometer decrepithermal vacuum by under fluids released tation or by crushingof 0.1 to 5 g of inclusionbearing material.The two smaller depositsfrom New Mexico (not\shown) both have gas trends "basaltic" signature.The indicating a possible Fresnillo data (Benton 1991)form a broad linear pattern that roughly overlaps with the mixing envelope having andesitic and meteoric endmembers. These results are consistent with salinity, and stable- and helium-isotope data, which support the existence of magmatic contributions in the Fresnillo fluids. Unfortunately,data from coexistingfluid inclusionsand stable isotopes are unavailable to assess covariationsor furtherconstrainthis interpretation'

1000

100 Air

looo He

2oo Ar

of nitrogen,argon Figure13.Relativeconcentrations and heliumin gasesfrom Fresnilloinclusionfluids 1994). (Benton1991,Norman & Musgrave

one component's origin can be checked for internal consistenciesby comparing it to other componentsthat behavein a similar manner(e.g., oxygen and hydrogenisotopescan be comparedto chloride,and helium isotopescan be comparedto COz, N2, and Ar). ln epithermal deposits, temporal constraints are relative as determined from the mineralogicrecord, with millimeter to centimeter-scale parageneses restricted to distancesof a few hundredmetersor less;hence, interpretation of spatial variations in the compositionsof paleo-fluidsat a fixed point in time acrossa depositis extremelydifficult. Data quality is also restrictedby the errors inherent to analyses and interpretationsof minerals and inclusionfluids. Thus, active geothermalsystems provide a scale of comparison for spatial and DISCUSSION temporal relations and a framework for interpretation not available from study of lowIn this paperI haveattemptedto documentthe deposits. sulfidation main geochemical evidence which indicates With these caveats,helium isotopesand N2magmatic contributions to low-sulfidation epiAr-He ratios can provide the most diagnostic thermal environments. The strength of this evidenceof magmaticcontributionsto epithermal evidenceis bolsteredby studiesof activesystems' deposits;the few availabledata indicatethat these where fluid compositions from geothermal componentsare promisingtracersof fluid origins systems in different stagesof evolution can be and deserve much further investigation. In comparedon a regionaland global scale.Besides contrast,enrichmentin both oxygen and hydrogen this temporal constraint,the capacityto analyze isotopes relative to meteoric water, and high of of chloride,can providepermissive concentrations all fluid componentsmeansthat interpretations

410

Low-suffidationEpithermalDeposits evidence of magmatic contributions(Table 1). Currently,Fresnillo is the only depositfor which all of thesetechniqueshave been applied,though there is a gap in the continuity of samples investigatedfrom this deposit, and the isotope studiesare reconnaissance in scale. Although there is evidence of magmatic contributions,it shouldbe clearthat the dominant source of water entering most low-sulfidation epithermalenvironmentsis meteoric,but this is not the issue here. Instead, a number of researchers have summarily discounted the possibility of magmatic contributionsin magmarelated ore-forming hl,drothermal systems, arguingthat water-rock interactionis sufficientto explain enrichmentsboth for oxygen and hydrogen isotopes,and the origins of othercomponents, includingmetals(e.g.,Taylor 1973 Campbeller al. 1984; Seal & Rye 1992),notwithstandingthe fact that a few percentmagmatic water could also accountfor the same isotopic enrichments(e.g., O'Neil & Silberman 1974 Sawkinset al. 1979). The problem then relates to the framework of interpretation, and I believe this involves appreciation: I ) of the nature of magmatic components, 2) that magmatic components potentiallycontributeto ore formation,and 3) that magmatic contributionscan reach shallow epithermalenvironments. The nature of magmatic componentsis best understood from examination of degassing volcanoes and study of porphyry ore deposits (".9., Hedenquist & Lowenstern 1994). These componentsare mostly volatile and includewater, carbondioxide, chlorine (as HCI), sulfur (as SO2 and H2S), and base and preciousmetals, all of which are observedin low-sulfidationepithermal environments,with chlorine and sulfur being important for metal transport. The signatures which record the appearance of magmatic componentsin the epithermal environment are mostly restricted to those in Table l. Other potential tracers, such as the isotopic compositionsof carbon,sulfur, and lead,are commonly ambiguousdue to effectsrelatingto redox stateor crustalcontamination,and are difficult to interpret (seeHedenquist& Lowenstern1994). That magmaticfluids can reachand influence low-sulfidation epithermal environments is

probably better documented than mosr geo_ scientists realize, with a much clearer magmatic connection existing for high-sulfidation epithermal environments(Arribas this volume). At one extremeare the eruptionsof magma through geothermal systems, which in recent history includethe 1886eruptionof Mt. Tarawerain New Zealand (Simmons el a/. 1993), the 19?6-1977 eruption at Krafla in Iceland (with the first recordeddischargeof magma from a geothermal well: Larsenet al. 1979),andthe l99l eruptionof Mt. Pinatubo,Philippines,formerly a geothermal prospectof the PhilipineNationalOil Company (with two pre-eruptionexplorationwells: Delfin et al. 1992). At the other extreme is the evidencefor steady influx of magmatic components(helium, nitrogen, chlorine, and water) into geothermal systemsand epithermaldeposits.Theseextremes also representend-memberson a time-scaleof influence,one nearly instantaneous, from hoursto days, and the other continuous,over hundredsto tens of thousandsof years. At time-scales in betweenare the pulses of fluid that reflect the magmaticinputs inferredfor Fresnillo,Hishikari, and Comstock. These sharp changes in fluid compositionshave not been recognizedin active systems,though wells have only been monitored for a maximum of about35 years(e.g.,Wairakei). Fluid pulsesare also known from the mineralogic record of some active geothermalsystems; for example,618O.ut.it" valuesat Kawerauindicatethe former field-wide presenceof a carbon dioxide and 6r8o-enrichedthermal fluid (up to 5 "/un comparedto currentvaluesof -3.75 ''ln,,)of likely magmaticorigin (Christenson1989). Thus both transient and persistent influxes of maglnatic contributionsare possible. Only at Fresnillo, for which high-salinity brines of rnagmaticorigin are interpreted,can a causeand effect relationshipbetween magmatic inputsand mineralizationbe considered.For other deposits,such as Hishikari and Comstock. the availability of metal-transporting ligands in mineralizingfluids cannotbe assessed by current analyticaltechniquesand, therefore,any genetic link betweenmagmaticinputs and mineralization is inferred only by spatial associationbetween precious-metaloccurrenceand isotopicallv(6180

S.F. Simmons

and 6D) enriched gangue minerals. Even in the 'lVZ. where relatively high concentrations of "gassy" precious metals are being fluxed in magmatic containing lluids geothermal contributions, the cause and effect relation is ambiguous as the source of aqueous sulfur, assuming it accounts for the aqueous Au and Ag, cannot be traced. The ultimate source of metals in low-sulfidation epithermal deposits thus remains poorly understood. ACKNOWLEDGMENTS I thank J. W. Hedenquist, J. Margolis, R. Sherlock, and J. F. H. Thompson fbr their perceptive comments and criticisms of an earlier version of this paper, and thank Louise Cotterall, who drafted some of the Figures. REFERENCES AHMAD, M., SOLOMON, M. & WALSHE, J. L. (1987): Mineralogicaland geochemicalstudiesof the Emperor gold telluride deposit, Fiji' Econ Geol.82.345-370. A N D E R S O N .W . B . & E A T O N , P . C ' ( 1 9 9 0 ) : G o l d mineralizationat the Emperor mine, Vatukoula, Fiji. .t. Geochem.Explor. 36, 267-296. BENTON, L.B. (199l): Compositionand Source of the Hydrothermal Fluids of the Santo Nino Vein, Fresnillo, Mexico, as DetLrminedfrom'-S'loS', Stable Isotope and Gas Analysis M.S' thesis, New Mexico Tech. Soccoro,New Mexico, USA' "Andesiticwater":a phantom BLATTNER, P. (1993): of isotopic evolution of water-silicatesystems' E a r t h P l a n e t .S c i .L e t t . 1 2 05 l l - 5 1 8 . BLATTNER, P. & LASSEY, K. R' (1989): Stable isotopeexchangefronts,Damk6hlernumbers,and fluid to rock ratios.Chem.Geol. 78 381-392' B R O W N , K . L . ( 1 9 8 6 ) :G o l d d e p o s i t i o nf r o m g e o t h e r mal dischargesin New Zealand.Econ. Geol Sl 9'79-983. B R O W N E , P . R . L .( 1 9 6 9 ) : S u l f i d em i n e r a l i s a t i oinn a Broadlandsgeothermaldrill hole' Taupo Volcanic Zone, New Zealand. Econ. Geol- 64 156-159'

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