Intrusive and basement rock sources of lead in hydrothermal systems of the Taupo Volcanic Zone, New Zealand

0016-7037/92/55.00 + 00

Gwchimica et Cosmochimica Acla Vol. 56. pp. 2821-2829 Copyright 0 1992 Pergamon Pres Ltd. Printed in U.S.A.

Intrusive and basement rock sources of lead in hydrothermal of the Taupo Volcanic Zone, New Zealand

systems

JEFFREYW. HEDENQUIST’and BRIAN L. GULSON~ ‘Mineral Resources Department, Geological Survey of Japan, l-l-3 Higashi, Tsukuba 305, Japan 2Division of Exploration Geoscience, CSIRO, PO Box 136, North Ryde, NSW 2 113, Australia (Received April 30, 199 1; accepted in revised form April 13, 1992)

Abstract-The Taupo Volcanic Zone is a complex volcanic-tectonic region over 250 km long, corresponding to the volcanic arc of the Taupo-Hikurangi subduction system. Volcanism is dominantly rhyolitic, with minor amounts of andesite, dacite, and basalt. The volcanics overlie a downfaulted basement of Mesozoic metasediment ( Waipapa and Torlesse terranes). Over twenty major geothermal systems are active, related to the young volcanism. Samples of sulfides (galena, sphalerite, chalcopyrite, and pyrite) from five of the geothermal systems were collected from drill core and their Pb-isotopic compositions measured. Representative fresh samples from all volcanic and basement rock types of the region were also analyzed and show a range of values by which they may be distinguished. Sulfides from the Broadlands and Waiotapu geothermal systems are homogeneous in their Pb-isotopic compositions, with values similar to the average for the fresh rhyolites and andesites. The results indicate that the source of Pb (and possibly other metals) in the Broadlands and Waiotapu hydrothermal fluids is related to the parent magma of the volcanics. A magmatic fluid could be the source, consistent with independent geochemical and isotopic evidence for other magmatic fluid-derived components at Broadlands, and to a lesser extent at Waiotapu, which are both anomalously mineralized in precious and base metals. In contrast, pyrite Pb from the unmineralized Kawerau geothermal system has isotopic compositions which vary over a range similar to that of the underlying Waipapa metasedimentary basement, present at depths of only 1000 m. This observation suggests that the Pb at Kawerau was mainly leached from the basement rocks, with a component of isotopically homogenous, magma-related Pb not present in the hydrothermal fluids. Isotopic compositions of Pb from the unmineralized Wairakei and Mokai systems are intermediate between the Broadlands and Kawerau examples. These data are interpreted to result from leaching of Pb from both the basement and volcanic rocks, consistent with the thick (at least 3 km) sequence of volcanics in the center of the zone, where Wairakei and Mokai are located. These results suggest that epithermal gold prospects may be distinguished on the basis of their Pbisotopic signature; those exhibiting a homogeneous and magmatic signature have a higher potential to contain ore, all else being equal, because of the potential for higher, magmaticderived metal concentrations in the hydrothermal fluid. This suggestion appeals to be applicable only to dilute hydrothermal systems, perhaps because the low salinity precludes leaching as an effective process to solubilize metals. Isotopic studies have commonly been used to help constrain the sources of fluid components in hydrothermal systems (e.g., CRAIG, 1963; O’NEIL and SILBERMAN, 1974; TAYLOR, 1971; DOE et al., 1979; MACFARLANEand PETERSEN, 1990). These and other studies of active and extinct systems at shallow levels (geothermal systems and epithermal ore deposits, respectively) indicate that most water in the convecting cell is meteoric in origin. However, there is increasing recognition, particularly from geothermal systems, for a magmatic source of some of the components, especially the gases. The study of active hydrothermal systems removes the possibility of overprinting from later hydrothermal or metamorphic events that often disturbs the original isotopic signatures (if a component left any isotopic signature in the first place). Furthermore, a wealth of geochemical information can be obtained only from the active system, where physical measurements to 2 or 3 km depth are made directly, and samples of the (potentially ore) fluids may be collected. In this paper, we consider the Pb isotope trends, both from sulfide mineralization and altered whole rocks, the constraints on the source of the hydrothermal Pb, and the distinctions

INTRODUCTION WE CHOSE THEGEOTHERMALsystems of the Taupo Volcanic Zone, New Zealand, to identify the source(s) of hydrothermal Pb using Pb isotopes. Samples of sulfides and fresh and altered host rock were collected from drill cores of five extensively studied geothermal systems: Broadlands, Waiotapu, Kawerau, Wairakei, and Mokai (Fig. 1). Samples of unaltered volcanics (basalt, andesite, dacite, and rhyolite) and basement metasediment (Waipapa and Torlesse termne greywacke) were also collected to establish the framework neceSSiiIy to examine the possible sources of the hydrothermal Pb. The unaltered samples form the basis of the companion paper on petrogenesis (GRAHAM et al., 1992 ) . The geothermal systems are active analogues to epithermal precious- and base-metal ore deposits of the Circum Pacific (HENLEY and ELLIS, 1983; HEDENQUIST, 1986, 1987). The results should have widespread and general application to understanding the hydrology of hydrothermal systems and in determining the source of Pb (and possibly other metals) in epithermal ore deposits in other similar terranes.

2821

2822

J. W. Hedenquist and B. L. Gulson inate recent volcanic deposits, with an estimated volume of about White Island A

torus, Taupe, Okataina, and Mangakino (youngest to oldest); they are characterized by peripheral rhyolite domes flanked by extensive ignimbrite plateaux. Dacites (as domes), andesites, and high-alumina basalts (GAMBLEet al., 1990) comprise the remaining volume of extrusive rocks. Andesites occur at both ends of the zone, principally in the southernmost Tongariro Volcanic Center, though they are also present beneath the surface in the Rotokawa, Kawerau, and Waiotapu geothermal systems (Fig. I ), with over loo0 m of andesites resting on basement at Rotokawa. Various petrogenetic models have been suggested for the volcanic rocks of the Taupo Volcanic Zone; these are reviewed in the companion paper (GRAHAMet al., 1992). The principal conclusions of that paper are as follows. ( I ) The voluminous rhyolitic ignimbrites and lavas have similar Pb isotope compositions to the andesites, particularly in x”Pb/mPb vs. 206Pb/204Pb,suggesting a close genetic relationship. One model involves partial melting of early formed andesite plus minor crustal contamination; simple partial melting of the metasediments is not tenable on isotopic grounds. (2) Recent andesites in the Tongariro area are the products of contamination of basaltic parents by the Mesozoic metasediments. (3) The basalts range from very primitive high-alumina types to clearly contaminated derivatives, while the dacites are related either to the rhyolites (Type I) or andesites (Type II).

Lake Rotorua

‘XOTHERMAL 0

Dekeated

AREAS

by drllng

0 Dekwated by geophysics o n

Q Ruapehu

0

Other thermal areas VOLCANOES
59

systems. The two potential sources of Pb to the hydrothermal systems are the local crust (i.e., leaching of the metasedimentary basement and/or volcanics) and primary magma (i.e., contribution from a magmatic fluid). ELLIS and MAHON ( 1964, 1967) suggested crustal leaching was the source of most solutes, while more recent studies support a magmatic source for some components of the hydrothermal systems (GIGGENBACH, 1986, 1989a,b; among the geothermal

HEDENQUIST, 1986; HEDENQUIST et al., 1990). SE-I-MNG AND PETROCENESIS TAUPO VOLCANIC ZONE

GEOTHERMAL SYSTEMS OF THE TAUPO VOLCANIC ZONE

7.5 Km

FIG. 1. Location map of the geothermal systems and volcanoes in the Taupo Volcanic Zone. Three of the systems studied, Broadlands, Waiotapu and Kawerau, lie near the eastern rift margin, while Wairakei and Mokai lie nearer the center.

TECTONIC

12,000km’ or 97.8% ofthe total eruptives(COLE, 1979). Eruptions have occurred principally in six main centers, Kapenga, Maroa, Ro-

OF THE

The Taupo Volcanic Zone is a complex volcano-tectonic region

over 250 km long by 50 km wide, flanked to the east and west by mountain ranges up to 2000 m in altitude (COLE,1979, 1984). This zone is the volcanic portion of the Taupo-Hikurangi arc-trench or subduction system (COLE and LEWIS, 1981 ), consisting of a line of andesitedaeite volcanoes along the eastern margin and extensive rhyolite-ignimbrite volcanic centers to the west. Subvolcanic basement comprises Mesozoic turbid&e sequences (greywacke-argillite) more than 30 km thick (&YNERS, 1980). Outcrops ofthese metasediments occur outside the zone, with the characteristics of the two terranes, Waipapa and Torlesse, discussed in the companion paper (GRAHAM et al., 1992). The boundary between the eastern, axial facies Torlesse terrane and the western, marginal facies Waipapa terrane is thought to lie beneath the Taupo Volcanic Zone, though the nature and exact location of the boundary is unknown. The basement is penetrated by drilling to depths of 1000 to 2500 m in geothermal systems along the eastern rift margin of the zone. In contrast, drilling to 2800 m depth in the center of the zone at Mokai has not reached basement (Fig. 1 ), indicating at least nearly 3 km of volcanic fill here. Pleistocene to Recent rhyolitic pyroclastic depositsand lavas dom-

The geothermal systems of the Taupo Volcanic Zone (Fig. 1) have been extensively studied over the last five decades in the course of their exploration, assessment, and development as an energy source, making them among the best understood in the world. Half of the twenty major systems have had more than one geothermal well drilled, with the Wairakei, Broadlands-Ohaaki, and Kawerau systems extensively drilled and developed. Summaries of more recent geological, geophysical, and geochemical results are provided by ELLIS and MAHON (1977), HENLEY et al. (1984, 1986), HEDENQU~ST ( 1986), and ALLIS ( 1990). Recent detailed studies have examined the geochemical structure of the systems and considered the physical processes and mineral-fluid reactions, including those occurring Mow drilled depths, which control the chemical composition of the fluids and determine the sources of the hydrothermal components ( GIGGENBACH,

1980, 1981, 1984, 1986, 1988, 1989a,b; CHRISTENSON,1987; HEDENQUISTand BROWNE, 1989; HEDENQUIST, 1990; HED-

mQum et al., 1990;

LINKER et al., 1990). Other recent studies have demonstrated the equivalence of the geothermal systems to the epithermal environment in which many shallow, precious-, and base-metal ore deposits formed (HENLEY and ELLIS, 1983; HENLEY, 1985; HEDENQUIST and HENLEY, 1985a,b; BROWN, 1986; HENLEY et al.,

1986; HEDENQUIST, 1986, 1987). This equivalence was one of the principal reasons for initiating a Pb isotope study to determine the source(s) of the Pb in the hydrothermal sys-

tems. Geology and Stratigrnphy of the Geothermal Systems The five geothermal systems for which we have Pb isotope data on sulfides are Broadlands, Waiotapu, Kawerau, Wairakei, and Mokai. The first three lie near the eastern margin of the Taupo Volcanic Zone (Fig. 1 ), while the other two are nearer the center of the zone. The greywacke basement

Sources of Pb in hydrothermal systems

is irregularly offset by a series of normal faults along the eastem margin of the zone, as indicated by drilling, with the resulting graben filled mainly by rhyolitic volcanics. The Torlesse metasediments occur progressively deeper from east to west across the Broadlands system, varying from 1000 m depth in the east to 2200 m depth only 2.5 km further west ( HEDENQUIST,1990). In contrast, the relatively shallow drilling at Waiotapu, to maximum depths of 1100 m, did not reach basement ( HEDENQUISTand BROWNE,1989 ) . Further north, at Kawerau, Waipapa metasedimentary basement ranges from 600 to 1300 m in depth, east to west across the system (CHRISTENSON,1987). Drilling at Wairakei and Mokai to maximum depths of 2300 and 2800 m, respectively, did not reach basement, indicating at least nearly 3 km of volcanic fill near the center of the Taupo Volcanic Zone. The basement in this area may be Torlesse or Waipapa terrane or both. The stratigraphy in these five geothermal systems is similar, with variably welded ash flow tuffs (ignimbrites) and their erosional equivalents dominating the volcanic sequence. The volcanic units are largely flat-lying, though can be offset by the north northeast-trending normal faults which are common in the Taupo Volcanic Zone. Lava flows of rhyolite, dacite, and andesite are common in wells, particularly in the Kawerau system; basaltic units have not been encountered. Volcanic domes, ranging in age from <200 ka to as young as 3 ka, occur on the margins (or within a few km) of each geothermal system (rhyolitic at Broadlands and Mokai, rhyolitic and dacitic at Waiotapu and Wairakei, and andesitic and rhyolitic at Kawerau ) . Geochemistry of the Geothermal Systems The chemical composition of the deep geothermal fluids in the Broadlands, Waiotapu, Kawerau, Waimkei, and Mokai systems is broadly similar and is reviewed for each system in detail by HEDENQUIST( 1990), HEDENQUISTand BROWNE (1989), CHRISTENSON(1987), HENLEY (1990), and HEDENQUISTet al. ( 1990). The deep fluid in each case ascends by convection to the base of the drilled system, with maximum measured temperatures ranging from 300 to 335°C. This fluid has a near-neutral pH and is comprised of alkali chlorides and dissolved gases, the dominant gas being CO1 followed by subordinate H2S (HENLEYet al., 1984). Chloride concentration in the 300°C fluid varies by a factor of only 3 (from 800 to 2400 mg/ kg), with Kawerau the lowest and Wairakei and Mokai the highest. In contrast, the total gas content in this high-temperature fluid varies by over an order of magnitude, from about 2.5 wt% at Broadlands and Kawerau to about 0.1 wt% at Wairakei and Mokai. The fluid generally starts boiling once it has ascended to about 2000 or 1500 m depth, at a temperature of about 3OO’C; it continues to boil to the surface or until it mixes with cooler waters on the margin of the system. These two processes, boiling and dilution, are the principal physical processes occurring in the systems ( GIGGENBACHand STEWART, 1982). The diluent is often steam-heated groundwater, as at Broadlands and Waiotapu (CO*-rich due to the gas content of the condensing steam), or a combination of cold and steamheated groundwaters, as at Kawerau and Wairakei. Dilution

2823

solely by cold groundwater occurs only in the Mokai geothermal system (HENLEY and PLUM, 1985; HEDENQUISTet al., 1990). Alteration minerals are dominated by quartz, chlorite, adulatia, albite, illite, mixed-layer illite-smectites, calcite, calcsilicates, and sulfides, mainly pyrite. Within individual systems, temperature is the principal factor controlling the distribution of alteration minerals ( BROWNE,1978). Kaolinite, cristobalite, alunite, and native sulfur are the minerals formed in oxidized and acid condensate zones near the surface. The mineralization of the geothermal systems is reviewed in HENLEYet al. ( 1986). Both Waiotapu and Broadlands have hot springs which actively precipitate Au and Ag-rich arsenic and antimony sulfides. Gold concentrations at the surface range up to 85 mg/kg ( WEISSBERG,1969). Base-metal sulfides are common in both systems at depth, particularly at Broadlands, where drill core contains up to several wt% Pb and Zn (EWERSand KEAYS, 1977). In contrast, the preciousand base-metal mineralization noted to date at Kawerau is very local and low grade (CHRISTENSON,1987; HENLEY et al., 1986), while mineralization at Wairakei and Mokai has not been reported (HENLEY et al., 1986). Evidence for Sources of Fluid Components in the Geothermal Systems HEDENQUIST( 1986) summarized the information available at the time constraining the sources of hydrothermal fluid components. The similarity of the deuterium content of most geothermal waters and local precipitation led CRAIG ( 1963) to conclude that these fluids are dominated by meteoric waters which have been heated and have acquired their solutes during deep convection. The oxygen isotope shift is due to interaction of the meteoric waters with the relatively heavy 6 “0 fresh volcanic rocks and sediments. Although generally true for most New Zealand systems, HEDENQUIST (1986) noted that the deep fluids in the Broadlands and Waiotapu systems are enriched in deuterium by 6 to 8L over local meteoric values, with 3 to 6% shift in 6 “0. One explanation for this deuterium shift is the admixture of a small component of magmatic water of a composition similar to the magmatic discharges (800°C) of New Zealand volcanoes. If true, then the lack of deuterium shift for systems such as Kawerau, Wairakei, and Mokai would be consistent with a much smaller magmatic water component; their 6 “0 values are only shifted 1 to 2%0from the meteoric line, wholly due to water-rock interaction. The 3He/4He ratios of geothermal discharges ( TORGERSON et al., 1982) indicate a deep-seated, magmatic source of the helium. The R/Rir values of 7.1 to 7.7 for Broadlands and Waiotapu and 4.2 for Kawerau are consistent with the general trends of the nonreactive trace gases Nz , Ar, and He; Wairakei values range from 6.2 to 8.2, higher than expected based on the non-reactive gas signature. GIGGENBACH( 1986) noted a clear trend in the nonreactive gases, from air-saturated groundwater to magmatic compositions (the latter defined by Nz-rich discharges of New Zealand &c-alkaline volcanoes). The nonreactive gas results indicate that Wairakei and Mokai fluids essentially preserve air-saturated meteoric values (for the relations among these gases), while Broadlands has

J. W. Wedenquistand B. L. Gulson

2824

a signature similar to that of the magmatic endmember; Waiotapu and Kawerau are intermediate. When considering C&, the major gas ~arn~nen~ it is clear that systems with a meteoric gas signature have a low total gas content, whereas the magmatic gas signature ( high IV2) is associated with the high-gas systems. GI~ENBACH f 1986) interpreted this reiationship to indicate a constant flow ~~~u~tion ) meteoric gas component to circulating hydrothermaf waters, whereas

some systems acquire a high gas content (and distinctive com~itio~ai signature) from a magmatic input at the base ofthe convection eefl, GIGCENBACH{ 1989at,b ) refined these relations by considering chloride and gases, plus Li, Rb, and Cs; he attempted to quantify the size of the rn~~rnati~ component to the various systems and also took into consideration the eff&ctof mine& fluid reactions on these relationships. He confirmed his pre-

Table 1: Lead isatope parameters for sulfide mineral separates from New Zealand geothermal systems.

BRUADMNDS BR7-2690 BR7-2690 BR?-26sQ BRl-2770 BR7-2770 BR7-2P60 BR7-~~ BRIS-7361 BRl6i-1DOIJ 3R16-12% BR16-1250 BRt&2690 BRI6-2690 BR t6-270f3 BRfS-27frO BRIh-326U BR16-4440 BR16-4440 BR16.4440 BR16-4440 BRI?-2004 BR42 w&box

GnA GIIB pv GtU% GnB hr Sph SDh Sph Py Spb Q SPb PY Sph cp GIlA CnA GtlB EnB Spb pU

a”m6

18.808 18.827 18.836 18.82% 18.833 18.838

815 815 815 839 839 897

as293 O&292 0.8297 0.8297 0.8293

897

OS8292 IS.818

15.599

38.632 38.681

2239

O.&?P6

i8.818

376 376 815 815 818

O.S290 0.82Po O.IEzpI 0.8295 0.8292 0.8291 0.8294 0.8300 O&P2 0.8293 0.8291 0.828P

18.823 18.814 18.835 18.841 18_817 18.826 f8.838 18.819 18.832 18.830 18.826 18.830

0.82PI

18.820

15.614 15.619 19.621 15.627 f.5.621 lS.mS IS.611 15.627 15.604 15.597 IS.617 15.628 15.603 IS.609 t5.624 mm 15.61S 15.617 15.608 15.609 15.6#

238

ixa?o

712

0.8292

18.820 18.81s

IS.602 15.601

38.641 38.624

575 332

18.840

0.8308 0.8319 0.8294 0.8307 0,&304 O&IO6 0.8035 0.8313 0.8334 0.8325

IS.633 15.624 IS.612 LS”S99 15.616 15.614 lS.&X 15.614 15.613 15.608 IS.620 15.634

38.7228

18.7130 18.791 18.752 18.829 18.797 18.793 18.799 18.801 18.775 18.744 18.7%-l

38.676 38.550 38.S94 38.683 38.667 38.648 38.662 38.661 38.607 38.587 38.6%

22 13 21 21 216 95 69 263 258 176 180 21

407 1155 7% 423

NE89 0.8291 0.8291 0.8292

18.829 18.844 18.858 18.818

15&m 15.624 15.635 IS.604

38.654 38.71 t 38.827 38.647

53 291 307 22

260 1039 1660 850

Q83w 0&2P4 O&?% tL%wQ

18.828 18.847 18.857 18.781

818 989 1348 1348 1348 1348 607 ejects

57 122 2f2 273 450 569 107 514 514 1059 1297 16I4

a.8298 0.8315

38.685

29300

38.699 38.720 38.7#1 38.637 38.660 38.717 38.642 38.620 38.681i 38.718 38.643 38.648 38.705 38.701 38.492 38.684 38.663 38.667 38.632

2J3u f 1,500

WAJRAKEl WK12 wK207 wK21s wK224

py Py

MO&if MK2 MK2 MK.2 MK6

Py

Py=F’yrite, Sph=Sphaferitc, Gn=Galena, Cp=&hakopyrite A, B=RcfRat analysis from sqamte spiit of sample a, b=f)rrpiiwe rn~§~rern~~t from the same sampk

38.716 38.746 38.778 38.633

16 64 2x

2825

Sources of Pb in hydrothermal systems ( GIGGENBACH,198 1) that the principal anions and the gases in the geothermal systems originated primarily from a magmatic brine, whereas the major cations were derived from neutralization reactions during water-rock interaction. ELLISand MAHON (1964, 1967) demonstrated that it is possible to derive many of the constituents of the geothermal fluid, including the chloride, from hydrothermal reaction with greywacke and the volcanics. This derivation is consistent with GIGGENBACH’S ( 198 1) assertion of wallrock leaching as the source of major cations, but disagrees on the chloride source. Recently, HEDENQUISTet al. ( 1990) provided 36C1evidence against either the silicic volcanics or the greywacke basement as a leaching source for the geothermal chloride. Rather, the results from the Mokai geothermal system suggest a more primitive source (implying lower U and Th concentrations) to account for the observed 36C1concentration through nuclear spallation. The carbon isotopic compositions of CO2 and CH4 and the sulfur isotopic composition of sulfur (as sulfide) in the geothermal systems are consistent with a variety of sources of these components, with no unambiguous single explanation (summarized by HEDENQUIST, 1986 ) . However, in the case of Broadlands, the range of CO* 6 13C is very narrow, from -6 to -7%0, in contrast to the lighter values and a larger range for CO2 at Wairakei, -3 to -6% (LYON and HULSTON, 1984). These isotopic values indicate a more homogenous, magmatic source for the Broadlands carbon. In summary, the deuterium shift from local meteoric values for some of the geothermal systems, the chemical and isotopic composition of some of the gases, and the chemical and isotopic composition of some dissolved components indicates a distinct, though variable, magmatic component in the geothermal systems of the Taupo Volcanic Zone. The Broadlands system has the most consistent “magmatic” signatures of any system, with Waiotapu and Kawerau being intermediate. To the west, the low gas systems of Wairakei and Mokai show little if any evidence for a magmatic contribution. This apparent variation may relate to the proximity of systems to the “andesite arc” along the eastern margin of the Taupo Volcanic Zone ( GIGGENBACH, 1986; HENLEY, 1990)) with those closest to the present magmatic activity having the largest magmatic signatures. vious suggestion

SOURCES OF HYDROTHERMAL LEAD BASED ON ITS ISOTOPIC COMPOSITION The isotopic compositions of Pb in galena, sphalerite, chalcopyrite, and pyrite from the systems studied are listed in Table 1 (see GRAHAM et al., 1992, for analytical details) and are plotted on Fig. 2, together with the fields for different fresh rocks (data from GRAHAM et al., 1992). The isotopic compositions of fresh rhyolite have a very narrow range (*06Pb/204Pb = 18.81 to 18.85), despite having been sampled over a 75 km strike length of the Taupo Volcanic Zone (GRAHAM et al., 1992). Samples collected from potentially altered geothermal drill core form a slightly larger rhyolite field (zo6Pb/204Pb = 18.78 to 18.85). However, with the exception of one sample, their Pb concentrations range from 13 to 21 mg/kg, similar to that of the surface rhyolites ( 11 to 18 mg/kg; Table 2 of GRAHAM et al., 1992), indicating

207pb 204Pb

15.65

. Kawarau ir Mokai 15.55 1 1 I I I I I I I I I 1 I I ItI Wairakei I 1 I I 16.60

16.70

16.60

16.90

266Pb/

l!

0

20‘%b

PIG. 2. Pb-isotope ratio plot of the composition of fresh rocks of the Taupo Volcanic Zone (from GRAHAMet al., 1992, who discuss analytical details). Fields are drawn to encompass the fresh samples of Waipapa and Torlesse terrane metasediments and rhyolites. The fresh (surface) rhyolite samples are contained in the field of short dashes, whereas the total rhyolite field (including samples from geothermal drill core which are potentially altered) is slightly larger and offset to lower 207Pb/204Pband 2”r’Pb/204Pbratios (within the solid line). The fresh andesite field is slightly smaller than and is completely contained by the rhyolite field (Fig. 5 of GRAHAM et al., 1992). The data for the Broadlandsand Waiotapu sulfides are plotted individually, with an ellipse (dotted) drawn around all samples; these samples are wholly contained within the fresh rhyolite field. The heterogeneous sulfide compositions of the Kawerau system are also shown, spanning the field of Waipapa terrane me&sediments. The compositional range of samples from Wairakei and Mokai (stars) is centered on but exceeds the narrow field of Broadlands and Waiotapu samples; in contrast, Wairakei and Mokai samples are not as heterogeneous in Pb-isotopic composition as those from Kawerau. The composition of altered Torlesse terrane greywacke from Broadlands BR 39 is shown as a cross. The straight dashed line is the Pb evolution curve for Pbrich massive sulfide deposits of GUMMING and RICHARDS ( 1975).

there has not been a large hydrothermal addition of Pb (though the Pb could have been exchanged). Most sulfide samples are from volcanic host rocks, with only two samples from Broadlands and three from Kawerau coming from the greywacke basement. The fresh andesites (not plotted) have a slightly smaller variation (206Pb/204Pb = 18.78 to 18.83) than that of the total rhyolite field, with the andesite compositional range similar to that of the rhyolites collected from drill core ( GRAHAM et al., 1992). Although most of the andesites were collected near Ruapehu, three samples are from White Island and other areas further northeast, spanning a distance of 240 km. The compositional fields of Torlesse and Waipapa terrane metasediments are large and clearly distinguished from each other; the Waipapa field overlaps partially with the rhyolite field (Fig. 2). The (rare) basalts have less radiogenic Pb isotope values ( 2wPb/204Pb mainly 18.74 to 18.77, though two samples have ratios of 18.80; GRAHAM et al., 1992). The Pb-isotopic values for sulfides from Broadlands and Waiotapu define a tightly constrained ellipse (Fig. 2 ) , which includes all data + 2 standard deviations about the mean. In contrast, all the data for Kawerau pyrites (Fig. 2), except for two samples, lie outside the ellipse and also exhibit a significant heterogeneity compared with the sulfides from

2826

J. W. Hedenquist and B. L. Gulson

Broadlands and Waiotapu. The pyrite samples from Wairakei and Mokai are similar in Pb-isotopic composition to the Broadlands and Waiotapu samples, though they have a larger range of values, with most samples plotting on the margin of, or outside, the ellipse of Broadlands samples.

0.634.

Interpretation

The Broadlands and Waiotapu samples lie within the field of Pb isotope values for samples of fresh rhyolitic domes and ignimbrite flows from around the Taupo Volcanic Zone (Fig. 2) though they are also similar to the values of fresh andesite. The rhyolites from geothermal drill core (some of which are potentially altered) also plot within the field of fresh rhyolites, most within the sulfide ellipse (GRAHAM et al., 1992). We suggest these results indicate a close genetic relationship between the parent magmas of the rhyolitic domes and ignimbrite flows, and the hydrothermal Pb. Thus, the Broadlands and Waiotapu geothermal fluids must derive the majority of their Pb through interaction with rhyolitic (or possibly andesitic) material (rock or magma) or a related magmatic fluid. In conjunction with the other lines of evidence for a magmatic source of several of the components in the geothermal systems, particularly for the Broadlands system, we suggest the most likely source for the isotopically homogenous Pb is a magmatic fluid related to a parent magma of silicic (rhyolitic) composition; it is unlikely that the meteoric convection penetrates the magma itself. Although there is also a background component of Pb to the hydrothermal system leached from the various host rocks, this is masked by a much larger contribution by the magmatic fluid (similar to the argument for the Nz-Ar-He systematics). An alternative hypothesis is for the geothermal fluids to circulate only through rocks of rhyolitic (or andesitic) composition. Although the depth to greywacke basement in the middle of the Taupo Volcanic Zone is not established, it varies from 1000 to 2300 m deep at Broadlands (summarized by HEDENQUIST,1990). It is not possible that the geothermal system circulates only through the upper 1000 to 2000 m, as deep drilling indicates convection also occurs within the basement (though there are also most probably intrusives of rhyolitic composition in the basement). It is unlikely that the majority of the Pb in solution would be derived by leaching at shallow levels above the basement, particularly once fluids are cooling. One sample of altered Torlesse terrane greywacke (whole-rock analysis A279 in Table 2 of GRAHAM et al., 1992) from Broadlands well BR 39 has a Pb isotopic composition lying within the ellipse for Broadlands sulfides (Fig. 2) and within the field of fresh rhyolite. Therefore, the Pb isotopes of this altered greywacke now have a rhyolitic composition, indicating that the Pb originated from a deeper source. We do not have any information on the amount of deep, solidified talc-alkaline intrusives in the basement, another potential leaching source of Pb. Mineralization at Kawerau is much less abundant than at Broadlands and Waiotapu; sulfides other than pyrite are very scarce (CHRISTENSON,1987 ). Furthermore, the Pb concentration in the Kawerau pyrites ( 13 to 263 mg/kg) averages at least an order of magnitude less than the Pb concentration

0.626

‘r EVo&andsi w ;Torle*se

0.0

0.02

0.04

0.06

l/Pb

0.06

0.1

FIG. 3. Pb 207/206 vs. I /Pb (mg/ kg) plot illustrating the homogeneous Pb compositions of sulfides from the Broadlands and Waiotapu geothermal systems. The homogeneity of isotopic composition for the Broadlands and Waiotapu Pb contrasts with the scatter of data for Pb from pyrite of the Kawerau system and their similarity with the Waipapa terrane metasediments which underlie the Kawerau system, consistent with the lower concentrations of Pb in the Kawerau samples being derived from a heterogenous source. Pb-isotopic data from Wairakei and Mokai are also less homogenous, and Pb concentrations in pyrite much lower, than for Broadlands and Waiotapu samples. The data for the rocks are from GRAHAM et al. ( 1992). See Fig. 2 for explanation of symbols. in Broadlands pyrite (minimum 2960 mg/ kg; Table 1 ), where galena and sphalerite are also commonly observed in drill core. In contrast to the Broadlands and Waiotapu samples, the range of Pb-isotopic values for the Kawerau system coincides with the scatter of Pb compositions for the Waipapa terrane metasediments (Fig. 3), the principal basement rocks underlying Kawerau. This suggests that the Pb in solution was derived dominantly from leaching of the basement rather than from a magma-related source, although there is a shift of some isotopic values towards Pb isotope compositions of Broadlands sulfides. The clear Pb isotope distinction of Kawerau from the Broadlands and Waiotapu systems supports the hypothesis that the majority of the Pb in the latter systems is derived from a magmatic source. If the Pb with a rhyolitic signature in the Broadlands system was derived by leaching of a compositionally equivalent batholith at depth, it would be difficult then to explain the distinction at Kawerau, where we consider the Pb is derived by leaching of the basement metasediments. The results for Wairakei and Mokai are intermediate between the Broadlands and Kawerau examples (Fig. 2), with Pb isotope values having a larger variation than the Broadlands ellipse, but with most samples plotting within the larger fresh rhyolite field. The me&sedimentary basement lies below 2300 and 2800 m at Wairakei and Mokai, respectively, the maximum drilled depths. Therefore, one explanation could be that Pb is derived by leaching of the much deeper basement (both Torlesse and Waipapa terrane), with this heterogeneous composition then affected by further leaching of Pb from 2.5 to 3 or more km of rhyolitic volcanics. Support for this hypothesis comes from ( 1) one sample each from Wairakei and Mokai, which plot in the compositional field of Torlesse terrane (Fig. 2)) (2) mineralization at Wairakei and Mokai,

Sources

2827

of Pb in hydrothermal systems

which is unknown, similar to Kawerau, in contrast to the abundant precious- and base-metal mineralization at Broadlands and Waiotapu, and (3) the low concentrations of Pb in pyrite from Wairakei and Mokai (Table 1 and Fig. 3), implying a low concentration of Pb in the hydrothermal fluids, as might be expected if the Pb was derived largely through inefficient leaching of host rock by these dilute chloride solutions. Our hypothesis is that Pb in sulfide (precipitated from hydrothermal fluid) at Kawerau is derived largely from leaching of basement; at Wairakei and Mokai it is derived mainly from a mixture of basement and volcanic leaching, whereas at Broadlands and Waiotapu the majority of Pb is derived from a deep, magma-related source. This is supported only qualitatively by the gas data, which indicate a lesser though still detectable magmatic component for Kawerau in comparison with Broadlands. Both Waiotapu and Kawerau have similar degrees of “magmatic gas signature,” though the two samples of Waiotapu Pb are comparable with Broadlands in isotopic composition. The gas compositions of Wairakei and Mokai indicate the least “magmatic” contribution. Thus, although the Pb-isotopic composition of sulfides indicates that Kawerau is the “least magmatic,” the Nz-Ar-He gas composition suggests a partial magmatic contribution to these gases. This distinction may indicate that the gases and metals are decoupled by processes which favor the concentration of one or the other, even if high concentrations of both metals and gases are related to magmatic contributions. This suggestion of decoupled component source is similar to the case for chloride and gas concentrations in the Taupo systems, which do not vary sympathetically (GIGGENBACH, 1986, 1989a; HENLEY, 1990). In contrast, the 6D and 6180 shift from local meteoric values of the Broadlands and Waiotapu hydrothermal fluids suggests a magmatic component, while at Kawerau and particularly Wairakei and Mokai the small shift from local meteoric water compositions in only 6”‘O suggests water-rock interaction is solely responsible, consistent with the Pb-isotope interpretation. The low concentration of Pb in pyrite from Kawerau, Wairakei, and Mokai, in comparison with Broadlands and Waiotapu, implies comparatively lower Pb in solution. This may be due to the inability of these dilute chloride fluids to effectively scavenge Pb from the rocks through which they circulate, consistent with the lack of mineralization at Kawemu, Wairakei, and Mokai. Measured concentrations of trace metals in New Zealand geothermal systems are uniformly low, with 1.O and 0.8 pgf kg Pb in Wairakei and Broadlands well discharges, respectively, in contrast to orders of magnitude higher concentrations in more saline systems (ELLIS and MAHON, 1977). However, samples from the New Zealand systems were collected from the weir box, after the fluid had boiled to 1OO’C. In this situation, metals such as Au, Ag, Pb, and Zn are deposited before sampling, making the published analyses incorrect for the deep fluids (BROWN, 1986). Therefore, it is not possible to confidently compare measured trace metal concentrations between the systems, and we must rely on an indirect method such as the Pb content of pyrite. In summary, the homogenous Pb-isotopic composition in the Broadlands and Waiotapu systems is consistent with a

magmatic source related to the parent magma of the rhyolitic (and/or andesitic) rocks of the Taupo Volcanic Zone. Where the magmatic fluid component to the overall hydrothermal system is smaller, as at Kawerau, the background Pb in solution, derived mainly from leaching of the basement greywacke, results in a more heterogeneous isotopic composition for the hydrothermal Pb. Where there is no evidence for magmatic components to the hydrothermal fluid, as at Wairakei and Mokai, and the volcanic sequence is relatively thick (at least 3 km), it appears that Pb may be leached from both basement and volcanic rocks. GENERAL IMPLICATIONS This study has implications for mineralization of epithermal deposits that are extinct analogues of the Taupo Volcanic Zone systems. The first is that the source of Pb (and possibly the source of other metals in solution) is directly related to the magmatic heat engine which is driving the hydrothermal system. However, this magmatic source can be variable, probably over the life of the system, and certainly between systems, even for those in the same district which outwardly (in terms of geology and alteration) appear similar. If a magmatic contribution is necessary to achieve potentially oreforming concentrations of metals in solution, then the systems with the most potential for ore may have a magmatic signature to the metals. The magmatic gas contribution is also important, for gases such as HIS are critical ligands in the transport of many metals in the epithermal environment, particularly Au (SEWARD, 1973). On the basis of our study, it appears that the Pb-isotopic signature can be used to help define the magmatic component of a meteoric convection cell, in conjunction with other constraints. The more significant the magmatic (and metal) contribution to the hydrothermal system, the more homogenous the Pb isotope signature may become. This idea is worth testing on some extinct systems for which the ore potential is well known (both on economic deposits as well as altered systems without ore), and which also have other detailed geochemical studies available to assist in interpretation. In conjunction with other information on the potential of an epithermal prospect, the Pb isotopes may distinguish between prospects where there is a homogenous Pb isotope signature (i.e., a high Pb concentration in the fluid, whether or not contributed from a magmatic source, implying a high potential), and those with a heterogenous Pb composition (i.e., due to a background, wallrock-leached concentration of Pb in the original fluid, and hence a lower potential for ore mineralization). This idea was originally suggested by DOE and STACEY ( 1974), who also considered the relationship between Pbisotopic signature and prospect potential, deposit size, source of Pb, etc., based on more general data. A conclusion similar to ours was reached for base-metal veins in Peru, where an homogenous Pb-isotopic signature of ores is equivalent to that of associated andesite plutons (MACFARLANEand PETERSEN,1990 ) . However, we stress that our conclusions are not universally applicable. For instance, in the huge Mississippi-valley type (MVT) deposits, there are large variations in Pb-isotopic composition, inherited from leaching of various

2828

J. W. Hedenquist and B. L. Gulson

sources ( CROCETTI et al., 1988). This may be due to the brine fluids responsible for MVT mineralization, which will be more efficient in leaching Pb than the dilute (generally less than 1 wt% NaCl) fluids responsible for epithermal precious metal mineralization. And in the Salton Sea, hot brines are very Pb-rich, having leached sediments with only average concentrations of Pb (DOE et al., 1966), resulting in homogenous Pb-isotopic compositions of sulfides. DOE et al. ( 1979) also note that base-metal mineralization in the Creede epithermal deposit, formed from saline fluids (up to 10 wt% NaCl), has a homogenous, though nonmagmatic, Pb-isotopic signature. Therefore, the application of our results appears to be restricted to environments where fluids are very dilute (i.e., most epithermal gold deposits; HEDENQUISTand HENLEY, 1985b). In this situation, a magmatic source may be necessary to provide potentially ore-forming concentrations of metals to the otherwise dilute hydrothermal fluid. CONCLUSIONS The rocks of the Taupo Volcanic Zone, particularly the basement greywackes and the rhyolites, have Pb isotope signatures distinct from each other. The hydrothermal Pb of sulfides in the Broadlands and Waiotapu geothermal systems is isotopically homogeneous and is consistent with the rhyolites (and/or andesites) as the source of the Pb. In contrast, the hydrothermal Pb of pyrite from the Kawerau geothermal system is isotopically heterogeneous, matching the heterogeneity shown by the underlying Waipapa terrane metasediments. This observation is consistent with the Pb in this system being derived from the nonvolcanic basement. An intermediate range of Pb-isotopic composition of sulfides from the Wairakei and Mokai geothermal systems may be due to leaching of Pb from both the basement and overlying volcanics, consistent with the much thicker volcanic sequence here than in the other systems. It is notable that the Broadlands and Waiotapu systems are highly mineralized locally in precious and base metals, whereas only pyrite with much lower Pb concentrations is found at Kawerau, Wairakei, and Mokai, implying that the isotopically homogenous source of Pb contributes to mineralization of a system. The hydrothermal Pb in the Broadlands and Waiotapu systems could possibly be derived from leaching of deep silicic intrusives or even the crystallizing roof of a pluton. However, we favor an alternative explanation. In conjunction with independent evidence indicating a magmatic source for other components in the fluids of these two geothermal systems, we suggest that the Pb (and perhaps other metals) in solution originate from an admixture of a small amount of magmatic fluid related to deep silicic (and/or andesitic) magmas, which are the parents of the voluminous rhyolitic pyroclastic and lava flows (and andesite lavas). In contrast, the basement metasediment source indicated for the hydrothermal Pb in the Kawerau system suggests leaching by circulating fluids. An intermediate Pb-isotopic composition of sulfides from Wairakei and Mokai, along with the greater thickness of volcanics, is consistent with the Pb being leached from both basement and volcanic rocks. Although some of the Pb in the Broadlands and Waiotapu systems may derive from leaching of basement rocks, a much larger Pb contribution

from a magmatic source (with a distinct isotopic composition) camouflages the smaller sedimentary component. Our study suggests that significant mineralization in the epithermal environment may be greatly favored by (even contingent on?) a magmatic contribution of metals to the dilute, dominantly meteoric hydrothermal fluids. If true, the epithermal prospects with greater potential for ore-grade mineralization should have a more homogenous (and magmatic) Pb isotope signature than other prospects which did not have a sizable magmatic contribution of metal to the hydrothermal system. Although these latter systems may appear outwardly identical to the former in terms of alteration mineralogy and other geochemical characteristics (except for size of metal anomalies), they should have more heterogeneous Pb isotope signatures, indicative of a basement leaching source of their (low) metal content. In reviewing the literature, it appears that this model is only applicable to dilute hydrothermal systems (favoring epithermal gold mineralization), where the salinity is not sufficient to allow metals to be effectively leached from basement rocks. Acknowledgments-This

study was initiated while JWH was with DSIR Chemistry, Wairakei, and we acknowledge their support of this project. We thank Patrick Browne, Judy Dean, Bruce Houghton, Ted Lloyd, Stuart Simmons, and Peter Wood for assistance in collecting some of the sampies, Bruce Christenson for providing the pyrite separates from Kawerau, and Michael Korsch for keeping the mass spectrometers operating at peak efficiency and for software development. We appreciate constructive comments on an earlier version of the manuscript by Stanley Church, Bruce Doe, Ian Graham, Ken Ludwig, Andrew Macfarlane, Akira Sasaki, and Noel White.

Editorial handling; K. R. Ludwig

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