Wt Percent NaCl Equiv Research Articles

Hydrothermal Alteration and Veins at the Epithermal Au-Ag Deposits and Prospects of the Waitekauri Area, Hauraki Goldfield, New Zealand

The Waitekauri area of the Hauraki goldfield, New Zealand, contains several adularia-sericite epithermal Au-Ag deposits and prospects. From west to east, the area contains the Sovereign, Jubilee, Scimitar, Scotia, Teutonic, and Jasper Creek deposits and prospects, which are hosted by andesitic and dacitic flows, breccias, and localized pyroclastic and air fall deposits. Drill core reveals spatial and temporal zonation of alteration and vein minerals along a 3-km-long composite cross section through the area. Most host rocks are intensely altered, with 100 percent of the igneous minerals replaced by hydrothermal minerals, although the alteration intensity becomes more variable and weaker toward the east. Alteration minerals include quartz, adularia, albite, chlorite, pyrite, illite, interstratified illite-smectite, smectite, calcite, hematite, and minor epidote. Many of these minerals have zoned distributions; adularia is widespread at Sovereign, but is restricted to shallow levels at both Scotia and Jasper Creek. Albite occurs in a discrete zone below adularia at Scotia, and minor epidote is restricted to Sovereign and Jubilee. Illite occurs throughout Sovereign and Jubilee and at the western margin of Scotia and Scimitar, where it grades eastward into interstratified illite-smectite and smectite at Teutonic and Jasper Creek. Veins are typically less than 10 cm wide, but have diverse mineralogy with zoned distributions. Quartz veins dominate at Sovereign and Jubilee, whereas calcite veins are more abundant at Scotia, Scimitar, and Jasper Creek. Laumontite occurs at Scotia and locally at Scimitar, whereas veins of clinoptilolite and mordenite ± calcite occur at Jasper Creek and stilbite veins occur at Teutonic. Fluid inclusions in quartz and calcite homogenized between 132° and 310°C and trapped a dilute solution with an apparent salinity of less than 2.6 wt percent NaCl equiv. Homogenization temperatures are highest at Sovereign (avg. 241°C), Jubilee (avg 239°C), and Scimitar (avg 236°C), lower at Scotia (avg 204°C) and lowest at Teutonic (avg 168 °C) and Jasper Creek (avg 162°C). Estimated positions of the paleowater table above Sovereign, Jubilee, Scimitar, Scotia, Jasper Creekg and Teutonic relative to present elevations was at least 690, 750, 575, 450, 225, and 150 m above sea level, respectively; the deposits and prospects, therefore, span a 600-m vertical interval. Individual deposits and prospects have undergoen at least 35 to more than 455 m of erosion with the greatest erosion to the west. Alteration intensity, alteration and vein mineral distributions, and fluid inclusion microthermometry are interpreted to indicate that Sovereign and Jubilee formed at relatively high temperatures, whereas Teutonic and Jasper Creek formed at relatively cooler temperatures. Several hydrologic reconstructions are possible, including (1) a single hydrothermal system with an inclined water table and significant lateral outflow to the east, or 2) a single low-relief hydrothermal system with a flat-lying water table that has subsequently been displaced by postmineral faults or tilted approximately 10° to the east. Regardless of the preferred reconstruction, the Sovereign and Jubilee deposits appear to have formed in the main zone of fluid upflow, whereas the Teutonic and Jasper Creek prospects appear to have formed toward the margin. Moreover, the greatest erosion has occurred at the Jubilee and Sovereign deposits (~300–400 m), and these may represent the roots of a more extensive vein network that has largely been eroded.

Physicochemical Evolution of a Porphyry-Breccia System: A Laser Ablation ICP-MS Study of Fluid Inclusions in the Mount Leyshon Au Deposit, Queensland, Australia

The evolution of magmatism, brecciation, alteration, and mineralization is described for the Mount Leyshon intrusive complex, Queensland, Australia, using P-T-V-X analysis and major to trace element analysis of single fluid inclusions using laser ablation ICP-MS. The ~290 Ma Mount Leyshon intrusive complex is a subvolcanic porphyry-breccia system mined for ~97 metric tons (t) Au (70 Mt at ~1.4 g/t Au) between 1986 and 2000. The Mount Leyshon intrusive complex is characterized by several generations of genetically related breccias and magmatic intrusions. Both alteration and mineralization can be traced to specific intrusive events. The earliest phase of brecciation was followed by widespread biotite-magnetite alteration (stage 1). Fluid inclusions associated with stage 1 were trapped at >350°C and >145 bars and were anomalously K and Fe rich. This stage was followed by the emplacement of rhyolite to dacite stocks and coeval breccia pipes (stage 2). Fluids exsolved from these magmas simultaneously exsolved Na-K-Fe-Mn-rich brine (~60 wt % NaCl equiv) and dilute vapor at ~500 bars (~2-km paleodepth) and >600°C. Pressures likely oscillated between lithostatic and sublithostatic conditions in response to the transient rupture and sealing of intrusions and wall rocks. Hydrothermal fluids infiltrating breccia pore space produced base metal mineralization (stage 3). Stage 3 fluid inclusions contain 2 to 10 wt percent NaCl equiv and were trapped at 300° to 400°C below 500 bars. Stage 3 fluids were depleted in K, Fe, and Mn, and enriched in Ca, Sr, and Ba relative to stage 2 brines. This trend is consistent with the transition from potassic alteration in stage 2 to feldspar-destructive phyllic alteration in stage 3. Cu, Zn, and Pb were conservative elements above 350°C and precipitated nonselectively as sulfides below 350°C in a bulk ratio very similar to that measured in fluid inclusions. Subsequent emplacement of andesite to rhyodacite dikes (stage 4) was marked by Au and additional base metal sulfide mineralization in veins and breccia cavities. Stage 4 fluids are chemically similar to stage 3 but contain several mol percent CO2, perhaps suggesting a more mafic, CO2-charged magma source relative to earlier intrusive pulses. In addition to fluid cooling, the destabilization of Au bisulfide complexes attending phase separation of H2O-CO2-NaCl fluids below 400°C and 400 bars was the likely cause for Au precipitation. Mount Leyshon appears to indicate that CO2 can play a crucial role in the transport of gold-bearing solutions from a magmatic source at depth into much shallower hydrothermal environments.

The Genesis of Distal Zinc Skarns: Evidence from the Mochito Deposit, Honduras

The Mochito deposit, located in Honduras, is a distal Zn(-Pb-Ag) skarn in which economic mineralization (sphalerite and subordinate, argentiferous galena) is found in mantos and chimneys dominated by garnet and pyroxene that replaced limestone and a mixed limestone-siliciclastic unit. Except for a few variably altered, unmineralized diabase dikes, evidence of igneous activity is conspicuously absent. The nearest felsic igneous rocks, volcanic rock units, crop out ~13 km from the deposit. Early skarn and skarn proximal to faults consist dominantly of garnet that evolved from grandite, with a composition of ~Ad55 to andradite (≥Ad90), whereas later skarn, or skarn distal to faults, is mainly made up of pyroxene (Hd70). Magnetite and pyrrhotite locally form between the garnet and pyroxene skarns. Analyses of the whole-rock chemistry show that formation of grandite skarn involved large additions of all major elements (except Ca) and most trace elements, whereas formation of andradite skarn (from grandite skarn) involved losses of most of these elements; the notable exceptions are Ca and Fe, which were added. Mass changes in pyroxene skarn were not evaluated because of the heterogeneous nature of the precursor. Primary fluid inclusions in grandite and associated low-iron sphalerite are interpreted to have been trapped at ~370°C and a pressure of 500 bar. These inclusions have a mean salinity of 14 wt percent NaCl equiv. By contrast, primary inclusions in pyroxene and associated high-iron sphalerite were trapped at ~400°C and have a mean salinity of 5 wt percent NaCl equiv. Fluid inclusions could not be observed in andradite. Small proportions of CO2, CH4, and N2 were detected by gas chromatographic analyses of the fluids released by crushing small samples of the host mineral; CO2 was the most abundant of these gases. Based on LA-ICPMS analyses of individual fluid inclusions, Na and Ca were the principal metals in the fluids (median concentrations of 3.2 wt %), followed by K (0.9 wt %) and Mn (0.3 wt %). Median concentrations of ore metals Zn, Pb, and Ag were 6,000, 900, and 50 ppm, respectively. Analyses of phase equilibria and related thermodynamic calculations indicate that the log f O2 during the grandite and pyroxene skarn stages was >–30.3 and <–30.2, respectively. The pH during the grandite stage was ~5.0. In the absence of reliable data on bulk fluid chemistry, a pH for the pyroxene stage could not be estimated. Based on the calculated Si content of the fluid and the mass addition of Si during grandite skarn formation, the fluid/rock ratio was between 500:1 and 1,000:1. Evaluation of the solubility of sphalerite, galena, and argentite based on the physicochemical characteristics of the putative ore fluid indicate that the ore metals were transported dominantly as chloride complexes and deposited in response to an increase in pH. We propose a model in which relatively oxidizing hydrothermal fluids, exsolving from magma at a depth of >4 km, interacted with graphitic limestones in the Mochito graben during an episode of mid-Tertiary intraplate extension. These fluids rose through faults created by the extension and were cooled by the overlying sedimentary succession. Early skarn formed in an environment of high-fluid flux proximal to the faults and was dominated by grandite because of the oxidizing nature of the fluids. With continued heating of the rocks by subsequent pulses of fluid, temperature increased and the locus of interaction expanded into unaltered limestone distal to the faults, where lower fluid/rock ratios and the presence of graphite promoted buffering of the fluid to lower f O2 and formation of pyroxene skarn. Ore mineral deposition (dominantly sphalerite), which began during or after the formation of grandite skarn, reached its maximum after hydrothermal activity was focused in the lower fluid/rock ratio regime of pyroxene skarn formation, occurring in response to the sharp drop in pH that accompanied neutralization of the fluid by limestone.

Fluid Inclusion Constraints on the Genesis of Gold in the Darasun District (Eastern Transbaikalia), Russia

Fluid Inclusion Constraints on the Genesis of Gold in the Darasun District (Eastern Transbaikalia), Russia

A Review of Fluid Inclusion Constraints on Mineralization in the Irish Ore Field and Implications for the Genesis of Sediment-Hosted Zn-Pb Deposits

Many fluid inclusion studies have been carried out in the Irish Midlands basin ore field (Lower Carboniferous) since the earliest work by Ed Roedder in the late 1960s. Results show that, in the ore deposits, the total range in fluid salinity is 4 to 28 wt percent NaCl equiv but with the majority billing in the moderate-salinity range between 8 and 19 wt percent. This variability is interpreted in terms of mixing between moderate-salinity ore fluids and low-temperature brines during ore formation. The most northerly ore deposits of Navan and Abbey-town are, distinct in containing fluids of both lower and higher salinity than is typical of the Waulsortian-hosted deposits farther south (Tynagh, Silvermines, Lisheen, and Galmoy). Subeconomic prospects tend to display a narrower range, in salinity mostly at the lower end of the range observed in the ore deposits. In some prospects, and cm the margins of some ore deposits, evidence for dilution is observed, interpreted to reflect mixing between hydrothermal fluids and unmodified seawater. This process is inferred to be unfavorable for mineralization.Homogenization temperatures, a reasonable proxy for true trapping temperatures in the ore field, range from 70 degrees to 280 degrees C but with the majority falling between 130 degrees and 240 degrees C. There is no evidence for systematic stretching or leakage of inclusions related to the postentrapment heating implied by elevated thermal maturity indicators. The highest temperatures are observed in the Waulsortian-hosted systems, with peak temperatures of -280 degrees C supported by local, high-grade Cu and Ni mineralization. in the Navan and Abbeytown deposits, lower temperature fluids appear to have been more prevalent. The subeconomic prospects formed over essentially the same temperature range as the ore deposits (90 degrees-270 degrees C), with the exception of the morphologically and texturally distinct Mississippi Valley-type (MVT) systems in the region (e.g. Kinnitty, Harberton Bridge) that formed at lower temperatures (50 degrees-100 degrees C).Similar hydrothermal fluids to those recorded in both deposits and prospects are widely observed in dolomite (and sometimes calcite) cements within Courceyan-Arundian-age rocks, indicating that hydrothermal fluid activity occurred over an extremely large area (>30,000 km(2)) and probably over an extended time period. There is a broad regional division in fluid properties, suggesting that the northwestern and southeastern provinces, separated by the trace of the Iapetus suture zone, may represent partly decoupled, large-scale flow regimes. Up to three, low-temperature brine types are also recorded by cements in the host-rock sequence, indicating that a complex range of evaporation and fluid-rock interaction processes were ongoing in the shallow basin succession during the period of hydrothermal activity.Halogen data show that fluids involved in mineralization were originally seawater-derived brines, produced by evaporation to varying degrees. Relatively high temperature, basement-interacted hydrothermal fluids were derived from partially evaporated seawater (molar Cl/Br = 559-825). Their compositions can be explained by dolomitization in the Carboniferous succession prior to circulation to depth; alkali exchange, reduction, and metal-leaching from the lower Paleozoic basement; and mixing with low-temperature brines that locally penetrated the upper parts of the basement rock package. Fertile ore fluids appear to be characterized by higher delta O-18 (+7 to +9 parts per thousand), lower delta D (-25 to -45 parts per thousand) and much higher metal contents than otherwise similar fluids sampled in basement-hosted feeder veins distal to deposits. This may reflect highly efficient metal scavenging in deeper and/or higher temperature reaction zones that underlie the principal deposits. In the ore deposits, these fluids mixed with Br-enriched bittern brines (Cl/Br similar to 290) produced by evaporation of Carboniferous seawater past halite saturation. It is inferred that bittern brine generation occurred in the shallow marine shelf regions in the footwalls to the synsedimentary fault systems that controlled the localization of mineralization. These brines then migrated into hanging-wall depressions where they ponded within permeable sediments and became enriched in H2S via bacteriogenic sulfate reduction. The coincidence of structurally controlled, high-temperature reaction zones, brine-producing footwalls, and hanging-wall traps, with bacterial blooms above upwelling plumes of hydrothermal fluids, can be interpreted as a self-organizing system that locally converged on ore-forming conditions. Understanding the first-order structural control of the ore systems will therefore be critical for predicting new deposits.The Irish ore field presents arguably the best database available on the thermal and chemical characteristics of hydrothermal fluids involved in sediment-hosted ore genesis. The system shares much of the variety and complexity observed in other intracratonic basin-hosted Zn-Pb(Ba) ore districts. This includes the coexistence of contrasting styles of mineralization that are typically observed in the more distal and platform-marginal parts of the basinal environment. The thermal and chemical fluid heterogeneity observed is typical of modern intracratonic basin systems and should be expected in large paleohydrothermal systems where recharge of surface-derived fluids is involved.

The Bingham Canyon Porphyry Cu-Mo-Au Deposit. III. Zoned Copper-Gold Ore Deposition by Magmatic Vapor Expansion

Fluid inclusion microthermometry and laser-ablation ICPMS microanalysis are combined with geological and textural observations to reconstruct the spatial and temporal evolution of magmatic fluids that formed the subvolcanic porphyry Cu-Au(-Mo) ore deposit at Bingham Canyon, Utah. The Bingham Canyon orebody is exposed over ~1.6 km vertically and has the shape of an inverted cup with distinct metal zoning. Fluid inclusions in the barren but highly veined and potassically altered deep center of the system have intermediate density (~0.6 g cm−3) and a salinity of ~7 wt percent NaCl equiv. They have subequal concentrations of Na, K, Fe, and Cu and contain minor CO2. The intermediate-density fluids were trapped as a single phase, mostly at >500°C and >800 bars. The Au-Cu-rich center near the top of the orebody contains low-density vapor inclusions (~0.2 g cm−3) coexisting with brine inclusions containing ~45 wt percent NaCl equiv. The vertical transition of different inclusion types indicates phase separation of the single-phase input fluid upon volume expansion associated with a pressure drop to 200 ± 100 bars. Mass-balance calculation based on all analyzed inclusion components indicates that the mass of the vapor phase exceeded that of the brine by ~9/1. The vapor contained Cu as its dominant cation (~1.5 wt %) and contributed about 95 percent of the total amount of copper transported to the base of the orebody. Bornite, chalcopyrite, and native gold were precipitated in a narrow temperature interval from 430° to 350°C, into secondary pore space created by local redissolution of vein quartz as a result of retrograde quartz solubility in the vapor-dominated fluid system. Intermediate-density fluid inclusions in the deepest parts of the peripheral copper ore zone have identical density and composition, including similar gold contents, as those in the deep center. Microthermometry and statistical estimation of phase proportions in the inclusions show that the vapor in the peripheral Cu-rich but Au-poor ore zone remained denser, and the separating brine was less saline (~36 wt % NaCl equiv), compared to vapor and brine in the central Au-Cu ore zone. This indicates that the peripheral fluids experienced a lower degree of phase separation, due to slightly higher fluid pressure at equivalent temperature, compared to more strongly expanding fluids in the center of the system. The systematic zoning of Au/Cu within the ore shell, despite compositionally similar input fluids, is interpreted to have resulted from slightly different pressure-temperature-density evolution paths of magmatic fluids. Copper was selectively precipitated in the peripheral ore zone, in contrast to complete coprecipitation of Au and Cu in the central upflow zone of the vapor plume. The formation of particularly rich Cu-Au ore in the center of the upward-expanding fluid plume is consistent with published experimental data, showing that the solubility of metals in hydrous vapor decreases sharply with falling pressure, due to destabilization of the hydration shell around metal complexes in expanding vapor. This interpretation supports the classic vapor plume model for porphyry copper ore formation but additionally emphasizes the role of sulfur-bearing complexes as a key chemical control on magmatic-hydrothermal metal transport and the deposition of Cu and Au in porphyry ores. Our interpretation of selective Cu ± Au precipitation as a function of vapor density can explain the more general observation that most gold-rich porphyry copper deposits are formed in shallow sub-volcanic environments, whereas deeper seated porphyry Cu-(Mo) deposits are generally gold poor.

Evolution of Magmatic Vapor to Gold-Rich Epithermal Liquid: The Porphyry to Epithermal Transition at Nevados de Famatina, Northwest Argentina

Fluid inclusions in the subeconomic porphyry Cu-Mo-Au system at Nevados de Famatina and closely associated high-sulfidation epithermal Cu-Au-(Ag-As-Sb-Te) veins at La Mejicana, northwest Argentina, were studied to reconstruct the evolution of hydrothermal fluids from their deep magmatic source to the shallow epithermal environment. Field geology, vein petrography, fluid inclusion microthermometry, and single-inclusion microanalysis by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) were combined to determine the evolution of pressure, temperature, and ore metal concentrations in the fluids. Cathodoluminescence imaging complements transmitted light petrography to constrain the successive stages of quartz formation and the entrapment sequence of the fluid inclusion populations. Aqueous liquid inclusions of ~5 wt percent NaCl equiv salinity trapped in quartz-sericite-pyrite (QSP) veins between 360° and 325°C contain unusually high concentrations of Cu (400–5,000 ppm), As (~200 ppm), Sb and Te (both up to 100 ppm), and Au (several ppm) along with other ore-forming elements. These veins are paragenetically transitional between subeconomic porphyry copper mineralization exposed in the valley floor, and high-sulfidation epithermal veins with high Au grades, which are preserved along a high ridge adjacent to the porphyry stock. Low-density vapor and hypersaline liquid (i.e., brine) inclusions were trapped in early-formed quartz of the porphyry stockwork veins, between 450° and >600°C. The brine contains lower Cu and Au concentrations than coexisting vapor inclusions and texturally even earlier inclusions of intermediate density that are trapped in phenocrysts and some stockwork veins; the latter may be equivalent to a parental fluid. The low- and intermediate-density fluids compositionally overlap in salinity with the aqueous liquids recorded by the later transitional quartz-sericite-pyrite veins. Fluid inclusions and geologic time relations indicate that the transitional quartz-sericite-pyrite veins were the channelways for metal-rich aqueous liquids that generated the high-sulfidation epithermal Cu-Au deposit. These mineralizing liquids are interpreted to have evolved by continuous density increase (“contraction”) from a low-salinity, S-rich magmatic fluid of low to intermediate density. This vaporlike fluid was produced by exsolution from a magma that existed at greater depth during the late stages of cooling of the magmatic-hydrothermal complex, and probably underwent minor brine separation prior to reaching the present level of exposure. Epithermal mineral precipitation occurred upon dilution of the low-salinity magmatic fluid with meteoric water, which entered the hydrothermal system as it was cooled and successively eroded during continued magmatic fluid ascent.

Geology and Fluid Characteristics of the Mina Velha and Mandiocal Orebodies and Implications for the Genesis of the Orogenic Chega Tudo Gold Deposit, Gurupi Belt, Brazil

The Chega Tudo gold deposit is located in the Gurupi belt of northern Brazil. Gold mineralization in the Mina Velha (upper level) and Mandiocal (intermediate and lower levels) orebodies is hosted in highly strained Paleoproterozoic (2148‐2160 Ma) felsic to intermediate metavolcanic rocks and schists that record greenschist facies metamorphism. The orebodies are broadly concordant with the regional northwest-oriented foliation, and mineralization comprises thick quartz vein sets and small quartz-carbonate-sulfide veinlets and enclosing hydrothermally altered host rocks. The hydrothermal mineral assemblage is postpeak metamorphism, syn- to late tectonic, and includes quartz, calcite, chlorite, white mica, pyrite, subordinate chalcopyrite, and traces of sphalerite and galena. Free-milling gold occurs in sulfide-poor veins in the upper and lower zones of the deposit, whereas refractory gold was deposited in fractures of pyrite and quartz in the more gold-enriched intermediate (±lower) level. Fluid inclusion and stable isotope (O, H, C, and S) data indicate that the two styles of gold deposition were produced by a combination of fluid immiscibility, fluid-rock reactions (sulfidation, carbonatization), and probably mixing, occurring in two discrete stages within a single mineralizing event associated with fluid flow within and around an active shear zone under fluctuating pressure conditions. The refractory gold was deposited at 340° to 370°C from a CO2-CH4-H2O‐NaCl fluid having salinity of 5.8 ± 2.7 wt percent NaCl equiv, variable CO2 contents (typically 12‐22 mol %), and up to 6 mol percent CH4. The δ18O and δD values of this fluid are 7.9 to 9.4 per mil and ‐29 to ‐37 per mil, respectively, indicating a metamorphic origin. Carbon isotopes show contrasting values in fluid inclusion CO2 (δ13C = ‐24.1‰) and calcite (δ13C = of ‐3.8‰). The strongly negative value of fluid inclusion CO2 is interpreted to be an organic signature acquired at the site of deposition by reaction of the deeply sourced ore-forming fluid with carbonaceous schists present in the deepest part of the deposit. The higher value for calcite probably records a crustal source. Oxygen fugacities calculated for the whole range of T-P-XCO2 conditions yielded log fO 2 between ‐28.7 and ‐30.5, indicating reduced conditions for the fluid, which is in agreement with the mineral paragenesis, fluid inclusion compositions, and sulfur isotope values. The second fluid, responsible for the deposition of the free-milling gold in veins and probably part of the gold of the lower zone of the deposit, had a lower temperature (330°‐340°C). This CO2-H2O‐NaCl fluid had a salinity of 1.6 to 2.5 wt percent NaCl equiv and contained 11 to 13 mol percent CO2. The δ18O and δD values of this fluid are 5.3 to 7.2 per mil and ‐12 to ‐30 per mil, respectively, also indicating a metamorphic source. The δ13C value of fluid inclusion CO2 is ‐6.9 per mil, likely representing a mantle-derived carbon source. Log fO 2 values between ‐30.5 and ‐31.5 also indicate slightly more reduced conditions for this fluid. Both fluids had near-neutral pH, between 5 and 6.2, and δ34S values of H2S between ‐0.2 and ‐1.0 per mil, interpreted to reflect derivation of sulfur from magmatic sulfides.

Hydrothermal Breccias and Veins at the Kelian Gold Mine, Kalimantan, Indonesia: Genesis of a Large Epithermal Gold Deposit

Mineralized hydrothermal breccias and veins formed during and after the waning stages of maar-diatreme-related volcanic activity at Kelian, Kalimantan, Indonesia. Subsurface phreatic explosions occurred around the margins of the diatreme breccia complex, focusing high-temperature fluid flow and generating several large, mineralized hydrothermal breccia bodies. Tectonic, phreatomagmatic, and hydraulic processes also contributed to hydrothermal brecciation. Explosive phreatic brecciation was followed by in situ hydraulic brecciation, and then by minor veining as the system returned to steady-state geothermal conditions. Brecciation and mineralization mostly occurred 400 to 700 m below the paleowater table. The Kelian deposit contained more than 240 t Au prior to mining. Precious metals occur with sulfide minerals as disseminations, in sheeted and conjugate veins and as breccia cement. There is a progression from pyrite-dominated (stage 1) to base metal sulfide-dominated (stages 2 and 3) to sulfosalt-dominated hydrothermal breccias and veins (stage 4). In terms of gangue minerals, the system evolved from illite-quartz to adularia- and/or quartz- and/or illite-dominated and then carbonate-dominated. Boiling produced abundant bladed carbonate during stages 3 and 4. Overall, carbonate and base metal sulfide minerals are abundant, whereas quartz is comparatively minor. Free gold is most abundant in stages 3 and 4 but also occurs in stages 1 and 2. Native gold occurs principally as inclusions in pyrite, sphalerite, galena, arsenopyrite, quartz, bladed carbonate, and sulfosalts. Hydrothermal alteration assemblages are zoned about contacts, faults, breccias and veins, and consist of secondary quartz, illite, pyrite, chlorite, and various carbonate minerals. Kelian preserves evidence of a magmatic component to the mineralizing fluids. Fluid inclusion analyses revealed the presence of saline fluid inclusions, in particular from stage 3. Isothermal mixing of low-salinity (∼0 to 2 wt % NaCl equiv) with moderate-salinity fluids (10 to 25 wt % NaCl equiv), rather than boiling, resulted in the range of salinity and homogenization temperatures of fluid inclusions in sphalerite, carbonate and quartz deposited during stage 3. Salinities of ∼4 to 6 wt percent NaCl equiv for inclusions in adularia and quartz from stages 2 and 4 and in rhodochrosite and proustite-pyrargyrite from stage 4 may also reflect a component of mixing of low-salinity water with a moderately saline fluid. Sulfur isotope values from Kelian sulfides (δ 34S = -1.4 to 5.5 per mil) are consistent with a magmatic sulfur source. Both boiling and fluid mixing contributed to high-grade Au mineralization at Kelian. Rapid formation (hundreds of years or less) is inferred for the individual hydrothermal breccia bodies. © 2008 Society of Economic Geologists, Inc.

Postorogenic Origin of the Stratiform Cu Mineralization at Lufukwe, Lufilian Foreland, Democratic Republic of Congo

The Central African Copperbelt, which stretches across the border between Zambia and the Democratic Republic of Congo, is one of the largest sediment-hosted stratiform Cu-Co provinces in the world. The triangular-shaped Lufilian foreland is located to the northeast of the Copperbelt. Recently sediment-hosted stratiform copper occurrences have been explored in the foreland, including the Kinkumbi prospect in the Lufukwe anticline. Although the stratiform copper mineralization in the Copperbelt is mainly concentrated in the Roan Group sedimentary rocks, where it is commonly associated with cobalt, the Lufilian foreland copper mineralization occurs in sedimentary rock units of the Nguba and Kundelungu Groups where it is mainly associated with silver. This paper examines the metallogenesis of the stratiform copper mineralization at Lufukwe. The Lufukwe anticline is situated in the eastern part of the Lufilian foreland and is composed of Neoproterozoic sedimentary rocks belonging to the Katanga Supergroup. The Kinkumbi prospect, a sediment-hosted stratiform copper-silver occurrence, is located in the northern part of the western flank of the anticline. This part of the anticline is characterized by disseminated copper-silver mineralization hosted in the lower 10 to 15 m of the Monwezi Sandstone (Nguba Group). A comparison between the location of high copper grades in surface samples and boreholes and the location of structural lineaments visible on ASTER images indicates that the mineralization is spatially related to northeast-southwest to east-northeast‐west-southwest strike-slip faults. These faults are nearly perpendicular to the strike of the host rock and postdate both the Lufilian folding and deposition of the entire Katanga Supergroup. The Monwezi Sandstone was subjected to strong compaction and silica cementation (authigenic quartz overgrowths), followed by intense feldspar dissolution, which resulted in a well-developed secondary porosity represented by dissolution cavities. Copper sulfides are mainly concentrated in these cavities and partially replace the detrital grains. The copper mineralization is both hypogene (chalcopyrite, bornite, and chalcocite) and supergene (digenite, covellite, and minor native copper), with malachite and chrysocolla as the main oxidation products. Point counting and grain size measurements demonstrate that the sandstone horizons with high copper content (>1.25% Cu) are those with a detrital grain size larger than 175 µm, more than 35 percent altered feldspars and little or no fine-grained matrix. Microthermometry of fluid inclusions indicates that the authigenic quartz overgrowths precipitated from a moderate-temperature (80°‐130°C), high-salinity (18.8‐23.4 wt % CaCl2 equiv) H2O-NaCl-CaCl2 fluid. The hypogene copper-silver mineralization was deposited from a hot (120°‐180°C) and low- to moderate-salinity (1.9‐7.7 wt % NaCl equiv) H2O-NaCl fluid with a general trend of increasing homogenization temperatures with increasing salinities. The interpretation of the available structural, stratigraphic, petrographic, and fluid inclusion microthermometric data constrain the timing of the mineralization to a time after the Lufilian folding and deposition of the entire Katanga Supergroup. The data presented support a postorogenic fluid-mixing model in which the mineralization is related to the mixing of a copper-rich mineralizing fluid with a temperature ≥180°C and salinity ≥7.7 wt percent NaCl equiv, likely migrating upward along northeast-southwest‐ to east-northeast‐west-southwest‐oriented strike-slip faults, with a colder, low-salinity, reducing fluid in the Monwezi Sandstone. The location and distance between the northeast-southwest to east-northeast‐west-southwest faults strongly influenced the spatial distribution of the copper mineralization in the anticline, and the variability in grain size and composition of the Monwezi Sandstone caused the preferential lateral migration of the mineralizing fluid through the lower 10 to 15 m. Copper precipitation was possibly induced by reduction by preexisting noncopper sulfides (pyrite and arsenopyrite) and hydrocarbons and by the drop in fluid temperature and salinity. Based on this research the areas with dominant northeast-southwest to east-northeast‐west-southwest structural lineaments are considered favorable sites for further copper exploration in the Lufilian foreland, especially where these lineaments cut previously deformed Katanga sediments. Coarse-grained sandstones with no fine matrix that underwent intense feldspar dissolution are the most promising host rocks for the late disseminated stratiform copper mineralization in the Lufilian foreland. Airborne gamma-ray spectrometric surveys could be used as a powerful exploration tool for targeting ore-related, K-depleted zones in these sandstones.

Fluid Inclusion Evidence for Magmatic-Hydrothermal Fluid Evolution in the Porphyry Copper-Molybdenum Deposit at Butte, Montana

The porphyry Cu-Mo deposit in Butte, Montana, formed where magmatic hydrothermal fluids, introduced with injections of porphyrytic dikes, fractured and permeated the Butte Quartz Monzonite. These fluids formed a stockwork of quartz and quartz-sulfide veinlets with a variety of styles of potassic and sericitic alteration envelopes. The distribution of vein and alteration types and the distribution of fluid inclusions in these veins record the progressive pressure, temperature, and compositional evolution of the hydrothermal fluids that formed this world-class deposit. Deep drilling and 1,300 m of offset along the Continental fault provide a vertical view of almost 3 km through the Butte deposit. Deep veins within and below the highest Mo grades are quartz dominated with thin K-feldspar or, less commonly, biotitic alteration rims. Fluid inclusions in deep veins trapped a single phase aqueous fluid containing 2 to 5 wt percent NaCl equiv and 2 to 8 mol percent CO2 at temperatures between 575° and 650°C and pressures between 200 and 250 MPa, corresponding to depths between 6 and 9 km. Although Cu grades are low in this region, abundant chalcopyrite daughter minerals in fluid inclusions indicate that the fluids were Cu rich. Fluids that formed these veins transported Cu from the magma below, upward into the region of Cu mineralization with only minor Cu precipitation. Over a kilometer above the bulk of deep quartz and quartz-molybdenite veins, the highest Cu grades are in and around chalcopyrite-bearing quartz-sulfide veins with biotitic alteration (early dark micaceous veins), and their upward, equivalent magnetite-chalcopyrite-pyrite-quartz veins with wide K-feldspar, green sericite, and chlorite alteration (pale-green sericitic veins). These veins contain more evidence for brine-vapor unmixing than any other vein type. The upward progression of early dark micaceous veins to pale-green sericitic veins formed where low salinity, CO2-bearing fluids, similar to those trapped in deep quartz veins, ascended, de-pressurized, sometimes unmixed, and cooled from ~650°C at 90 MPa to ~475°C at ~50 MPa. As low salinity, CO2-bearing, aqueous fluids, similar in composition to fluids trapped in deep quartz veins, cooled at shallow depths, they formed late pyrite-quartz veins with sericitic alteration. These veins formed from fluid cooling at temperatures between 370° and 450°C at transiently hydrostatic pressures between 40 and 70 MPa, corresponding to depths of 4 to 7 km. Most pyrite-quartz veins formed at pressures and temperatures above the H2O-NaCl-CO2 solvus, but evidence for brine-vapor unmixing is also present. Pyrite-quartz veins formed at progressively greater depths as the hydrothermal system cooled, overprinting much previous mineralization. Late Cu-Pb-Zn-Ag-As-rich Main stage veins formed from dilute fluids containing <3 wt percent NaCl equiv and <2 mol percent CO2. These fluids were trapped between 230° and 400°C under hydrostatic pressures between 20 and 60 MPa and depths of 2 to 6 km. No evidence of boiling is observed in Main stage veins. Fluid inclusion phase relationships indicate that the Butte porphyry Cu-Mo deposit formed at 5 to 9 km depth, greater than any other porphyry-type deposit. At Butte, the similarity in bulk composition of fluids trapped in early quartz-rich veins with potassic alteration and late pyrite-quartz veins with sericitic alteration implies that an underlying magma continually provided low salinity, CO2-bearing fluids of relatively constant composition during the entire life of the hydrothermal system. We hypothesize that rather than resulting from changes in fluid chemistry due to magma crystallization, the entire suite of vein and alteration types and the ore metal distribution reflect the path of cooling, depressurization, and wall-rock interaction of a parental magmatic-derived fluid of relatively constant initial composition. Fluid inclusions, vein and alteration relations, and ore metal distribution indicate that Cu and Mo were introduced into the hydrothermal system by the same fluids, but that the mechanisms of precipitation of these metals were decoupled. Early dark micaceous and, to a greater extent, pale-green sericitic, veins have wide alteration envelopes and contain more evidence for fluid unmixing than any other vein type, which suggests that chalcopyrite precipitation was driven by a combination of fluid unmixing, fluid-rock reaction, and fluid cooling between 650° and 475°C. Most molybdenite mineralization, however, is in quartz-dominated veins with little or no alteration that are dominated by low salinity inclusions. These veins formed in response to pressure decrease rather than cooling. After chalcopyrite and molybdenite precipitation, low salinity fluids cooled, usually at temperatures and pressures above the H2O-NaCl-CO2 solvus, to produce significant acid and voluminous sericitic alteration accompanied by pyrite-quartz vein formation that overprints much of the deposit and contains anomalous but noneconomic Cu.

The Cu Stockwork and Massive Sulfide Ore of the Feitais Volcanic-Hosted Massive Sulfide Deposit, Aljustrel, Iberian Pyrite Belt, Portugal: A Mineralogical, Fluid Inclusion, and Isotopic Investigation

The Variscan Feitais volcanic-hosted massive sulfide deposit in the Aljustrel district of the Iberian Pyrite Belt consists of 55 million metric tons of Zn-Pb-Cu massive sulfide overlying a Cu-rich stockwork. The massive ore is overlain by up to 30 m of feldspar-phyric, rhyolitic volcaniclastic rock and locally by a jasper and/or chert layer up to 15 m thick. The massive sulfide orebody consists dominantly of pyrite, sphalerite, galena, chalcopyrite, tetrahedrite-tennantite, arsenopyrite, and bournonite, together with minor quartz, chlorite, sericite, carbonate, and barite. The orebody is up to 100 m thick and is underlain by a tabular alteration zone of chloritedominated, locally silicified, felsic volcanic rock, the upper 30 to 60 m of which contains chalcopyrite-quartzchlorite-sericite-carbonate‐bearing stockwork vein(let)s that prior to deformation were at a shallow angle to the base of the massive orebody. Chloritized footwall rocks extend up to 20 m below the Cu stockwork zone and are underlain by up to 50 m of quartz-sericite-pyrite‐altered rhyolitic rock. The stockwork veins also contain pyrite, tetrahedrite-tennantite, sphalerite, and arsenopyrite. Pyrite, both in stockwork and massive ore, locally displays partly recrystallized framboidal, reniform, and cellular textures. Two generations of quartz, Q1 and Q2, and carbonate in the stockwork veins contain primary (in growth zones) and pseudosecondary fluid inclusions, with homogenization temperatures of 270° to 315°C and salinities of 2.2 to 8.1 wt percent NaCl equiv. The δ34S(CDT) values of massive and stockwork ores range from ‐15.4 to +4.7 (mean, ‐2.8) and ‐11.2 to +11.9 (mean, ‐0.4) per mil, respectively, the lowest values from colloformtextured pyrite. With no evidence of oxidation of sulfide sulfur during mineralization, the most negative values indicate an origin by biogenic reduction of seawater sulfate. The δ13C(PDB) values for carbonates, ‐7.5 to ‐13.7 and +9.3 to ‐14.3 per mil in massive and stockwork ore, respectively, indicate an origin mostly by oxidation of methane derived from organic matter in underlying sedimentary rocks and possibly a contribution of magmatic carbon. There are no significant lateral or vertical variations in S isotope values in sulfides or C-O isotope values in carbonates, either in massive or stockwork ore. The δ18O(SMOW) values for quartz in stockwork and massive sulfide are 11.6 to 13.9 and 16.7 to 17.9 per mil, respectively. Coexisting, and texturally contemporaneous, carbonate and quartz in stockwork veins are not in isotopic equilibrium, indicating that the C-O isotope values may have been reset. The δ18O values of fluid calculated to be in equilibrium with quartz at fluid inclusion homogenization temperatures are 4.2 to 5.2 per mil. Barite from the hanging wall and massive ore yields δ34S values (21.9‐27.9‰) equal to or slightly higher than those of coeval seawater; 87Sr/86Sr ratios (0.708438‐ 0.709063) are slightly more radiogenic than those of coeval seawater (0.7080‐0.7085), and much more radiogenic than those of coeval volcanic rocks (0.703304‐0.706642), probably representing mixtures between seawater Sr and radiogenic Sr in fluids sourced in the crustal pile. Deposition of the massive sulfide on the sea floor is suggested by its stratiform nature, the stronger alteration of footwall relative to hanging-wall rocks, the stockwork system terminating sharply at the base of the massive sulfide, the presence of sedimentary-like textures in the massive sulfide, the absence of replacement fronts, and the presence of framboidal and other sea-floor depositional textures indicative of fluid quenching. The sheetlike form, lack of rubble mounds and chimneys, scarcity of barite, reduced mineral assemblage, and metal zoning distinguish Feitais from Kuroko-type deposits. It shares most of the characteristics of those Iberian Pyrite Belt deposits for which a brine-pool origin has been proposed based on fluid inclusion data,

Formation of the Tharsis Massive Sulfide Deposit, Iberian Pyrite Belt: Geological, Lithogeochemical, and Stable Isotope Evidence for Deposition in a Brine Pool

Formation of the Tharsis Massive Sulfide Deposit, Iberian Pyrite Belt: Geological, Lithogeochemical, and Stable Isotope Evidence for Deposition in a Brine Pool

Fluid Evolution and Origins of Iron Oxide Cu-Au Prospects in the Olympic Dam District, Gawler Craton, South Australia

Geologic observations suggest two stages of hydrothermal activity at a number of presently subeconomic iron oxide copper-gold systems in the Olympic Dam district, eastern Gawler craton. They contain high-, and moderate- to low-temperature Fe oxide-rich hydrothermal alteration. The mineral assemblages include magnetite-calc-silicate-alkali feldspar ± Fe-Cu sulfides and hematite-sericite-chlorite-carbonate ± Fe-Cu sulfides ± U, REE minerals. In all documented prospects, the minerals of the hematitic assemblages replace the minerals of the magnetite-rich assemblages. The bulk of the subeconomic Cu-Au mineralization is associated with the hematitic alteration assemblages. Microanalysis by proton ion probe (PIXE) of hypersaline fluid inclusions in magnetite-rich assemblages, however, demonstrates that significant amounts of copper (>500 ppm) were transported by the early-stage high-temperature (>400°C) fluids responsible for the magnetite-rich alteration. These brine inclusions contain multiple solid phases (liquid + vapor + multiple solids) including chalcopyrite in some cases. In comparison, inclusions of the hematitic stage are relatively simple liquid + vapor types, with homogenization temperatures of 200° to 300°C and containing 1 to 8 wt percent NaCl equiv. The Br/Cl ratios of the magnetite-forming fluids measured by PIXE lie beyond the range of typical magmatic and/or mantle values, allowing for the possibility that the fluids originated as brines from a sedimentary basin or the crystalline basement. Sulfur isotope compositions of chalcopyrite and pyrite demonstrate that sulfur in both alteration assemblages was derived either from cooling magmas and/or crystalline igneous rocks carried by relatively oxidized fluids (∑SO42− ≈ ∑ H2S, δ34Ssulfides from −5 to +2‰) or from crustal sedimentary rocks (δ34Ssulfides from +5 to +10‰). Oxygen and hydrogen isotope compositions of waters calculated for minerals of the magnetite-rich assemblage have δ18O values of +7.7 to +12.8 per mil and δD values of −15 to −21 per mil. The only available δ18O and δDfluid values for the hematitic assemblage are +4.7 and −9 per mil, respectively. The isotopic compositions of both fluids, coupled with the available literature data, can be explained in terms of fluid reequilibration with felsic Gawler Range Volcanics or other felsic igneous rocks in the region and with metasedimentary rocks of the Wallaroo Group at low water-to-rock ratios prior to their arrival at the mineralization sites. The lack of significant copper mineralization associated with magnetite-forming fluids that carried copper suggests that there was no effective mechanism of saturation of copper minerals or the quantity of these fluids was not sufficient to produce appreciable copper mineralization. Association of the copper-gold mineralization with the hematitic alteration in the subeconomic prospects can be explained by a two-stage model in which preexisting hydrothermal magnetite with minor associated copper-gold mineralization was flushed by late-stage oxidized brines that had extensively reacted with sedimentary or metamorphic rocks. The reduction of these brines, driven by conversion of magnetite to hematite, resulted in precipitation of copper and gold. The oxidized brines may have contributed additional copper and gold to the system in addition to upgrading preexisting subeconomic Cu-Au mineralization. When compared to published models for the Olympic Dam deposit, the new data for fluids in subeconomic Fe-oxide Cu-Au prospects of the Olympic Dam district indicate the diversity of origins of iron oxide-copper-gold systems, even within the same geologic region.

The Nature and Origin of Gold Deposits of the Tarcoola Goldfield and Implications for the Central Gawler Gold Province, South Australia

The Tarcoola goldfield in central South Australia is hosted by the Paleoproterozoic Paxton Granite and Tarcoola Formation metasedimentary rocks. Both of these units are intruded by narrow dikes of the Lady Jane Diorite, which is considered to be part of the Gawler Range-Hiltaba volcano-plutonic event. Crosscutting relationships and 40Ar/39Ar dating of alteration sericite and primary hornblende have demonstrated that veining, alteration, and mineralization occurred synchronously with intrusion of the diorite dikes at ~1580 Ma. Mineralization at the Perseverance deposit in the Tarcoola goldfield is hosted by quartz veins in both granite and the lower parts of the Tarcoola Formation. Alteration intensity and mineralization are related to fracture density. Very high gold grades (up to 270 ppm) are recorded in carbonaceous siltstones of the Tarcoola Formation. Primary fluid inclusions from mineralized quartz veins at Perseverance are dominated by two-phase aqueous inclusions and three-phase H2O-CO2 (l) - CO2 (v) ± CH4 inclusions. Both inclusion types have salinities of <10 wt percent NaCl equiv, with evidence of other salts being present. Both types homogenize between 150° and ~340°C. They occur together in single quartz grains, which suggests the coexistence of aqueous and carbonic fluids. At the Perseverance deposit the maximum depth of fluid inclusion formation is estimated as 10 km. Processes resulting from pressure fluctuations during brittle fracturing causing phase separation are suggested to be the dominant gold precipitation mechanism, although definitive fluid inclusion evidence for either phase separation or fluid mixing is lacking. Wall-rock sulfidation may have also contributed to gold deposition. Pb isotope compositions of galena from gold-bearing mineralized veins at Perseverance are similar to K-feldspar and whole-rock samples of the Paxton Granite. This is consistent with a common source of Pb and possibly other ore components for both the granite and galena, but not all potential reservoirs were tested. Variably altered and gold mineralized samples that had Paxton Granite precursors show a loss of Nd and a trend from eNd(1580Ma) values of about −6.2 in slightly altered samples toward more positive eNd values of about −3.3 in high-grade gold samples. Of the local lithologic units, the Lady Jane Diorite has eNd(1580Ma) values of about +0.1 and is the most probable source of the more primitive Nd isotope components in mineralized samples. Mineralization in the Tarcoola goldfield has similarities to both orogenic and intrusion-related gold styles. The demonstrated coeval timing of diorite dikes and mineralization at Tarcoola, together with the Nd isotope constraints on metal sources, and the presence of similar mafic dikes in several other known gold prospects in the central Gawler gold province, suggests that the diorite was an essential part of the gold mineralization event throughout this province.

Characteristics and Origin of the Oak Dam East Breccia-Hosted, Iron Oxide Cu-U-(Au) Deposit: Olympic Dam Region, Gawler Craton, South Australia

Oak Dam East is a large (~560 Mt of 41–56 wt % Fe), Cu-Au-U-bearing, massive Fe oxide deposit in the northeastern Gawler craton, South Australia. It is one of several unmined deposits in the same province as the giant Olympic Dam deposit, and it lies beneath more than 500 m of younger cover rocks. The Oak Dam East breccia complex formed in a near-surface environment and was characterized by an early history of skarnlike albite-calc-silicate ± magnetite formation (from hypersaline, 400°–500°C fluids). Large-scale brecciation was an early feature, and the resulting complex was progressively filled in and replaced by (1) hematite and goethite (hematite stage I), (2) magnetite-apatite-quartz ± actinolite (magnetite stage), (3) hematite-goethite-quartz-chalcedony-pyrite-chlorite-monazite (hematite stage II), and (4) chalcopyrite-pitchblende-illite-hydromus-covite-florencite-carrolite (Cu-U-(Au) mineralization stage). Hematite stage II assemblages are volumetrically most abundant and were formed from fluids of 1 to10 wt percent NaCl equiv that were boiling at 170° to 190°C and had δ18O values more negative than −3.7 to +1.7 per mil, interpreted as meteoric water. The upper 200 m of the breccia complex contains spectacular colloform hematite void fillings, interpreted to have replaced hydrothermal goethite. The Cu-U mineralized zones resemble Olympic Dam ore in terms of geometries, being grossly subhorizontal, 10 to 70 m thick, and crosscutting with respect to the host breccias. The mineralization is zoned vertically from pyrite (δ34S = −6 to 0‰) at depth to shallower chalcopyrite (δ34S = −14 to −7‰). High-grade pitchblende occurs centrally within the chalcopyrite zone (e.g., 10 m of 0.46% Cu and 3,700 ppm U3O8), straddling a boundary between K mica + illite alteration and underlying Fe-Mg chlorite alteration. The deposit is assigned to the 1570 to 1595 Ma iron oxide Cu-Au-U event in the eastern Gawler craton on the basis of its similarity to other Cu-Au-U deposits in the province. An LA-ICPMS Pb isotope age for monazite in hematite stage II was calculated to be 1455 ± 20 Ma, and possibly as young as 1370 ± 54 Ma, to explain all isotopic data. This is an age of isotopic resetting. Our preferred model is that Cu-U mineralization developed within a stratified hydrothermal brine complex analogous to the Salton Sea geothermal system. Copper, Fe, and S in the deep brine was sourced from an evolving deep reservoir, with local compositional controls on sulfur isotopes including cooling, boiling, and replacement of previous pyrite. Chalcopyrite (±U) veins and disseminations formed as a result of mixing with overlying, steam-heated, U-rich, saline ground water. Mixing-induced cooling and dilution only occurred episodically, following hydrofracturing events that ephemerally linked the brine layers within the substantially cemented upper complex. By contrast, high-grade UO2 formation occurred as deep upflow waned, via reaction of the upper brine with hematite stage II Fe-Mg chlorite along an interface that descended with time. This resulted in an increase of U grades through time caused by leaching and reprecipitation.

Epithermal Au-Ag and Related Deposits of the Hauraki Goldfield, Coromandel Volcanic Zone, New Zealand

Epithermal Au-Ag vein deposits of the Hauraki goldfield in the Coromandel volcanic zone, New Zealand, produced 320,000 kg Au and 1.5 million kg Ag between 1862 and 2006 from a total land area of approximately 2,900 km2. Approximately 69 percent of the production was from the world-class Martha deposit at Waihi. Some 97 percent of total past gold production was from deposits hosted in Miocene andesite and dacite of the Coromandel Group, although this unit represents only 61 percent of the rocks exposed in the goldfield. The deposits consist of quartz ± calcite vein systems, 0.5 to 4.5 km long, within alteration zones up to 20 km2 in size. They were mined over vertical intervals of 170 to 330 m but up to 575 m at Martha and 700 m at Karangahake. Electrum and acanthite are the main ore minerals, although pyrite, sphalerite, galena, chalcopyrite, sulfosalt minerals, and Te- and Se-bearing minerals are present in some deposits. Stibnite and cinnabar occur locally as late minerals, and some deposits are overprinted by late massive calcite. Fluid inclusion ice-melting temperature (Tm) measurements indicate that fluids associated with base metal and precious metal deposition had apparent salinities of between 0.5 and 2 wt percent NaCl equiv with a maximum of 6.1 wt percent NaCl equiv. When fitted to hydrostatic boiling point with depth curves, the homogenization temperature (Th) data define paleodepths spanning a vertical interval from near the water table to a depth of approximately 1,300 m. The presence of vapor-rich fluid inclusions, and occurrences of lattice textures and vein adularia, indicate that periods of boiling coincided with deposition of some epithermal ores. Sulfur and carbon isotope values of vein minerals are consistent with a magmatic origin for sulfur and carbon. Oxygen isotope values of vein quartz and calcite are consistent with derivation from evolved meteoric water, with possibly a small magmatic water component in some samples. The epithermal deposits can be broadly subdivided into northern, southern, and eastern groups based on their host rocks, age, variability of vein strike, Au/Ag ratios, occurrence or relative abundance of key minerals such as adularia, calcite, and arsenopyrite, the grain size of vein quartz, and differences in vein textures. Early (ca. 14–10 Ma), steeply dipping planar veins occur in the oldest andesite and dacite in the northern portion of the goldfield, but some of these are bonanza-grade veins that contained significant gold resources. Larger veins, typically with well-developed crustiform textures, formed in andesite and dacite in the southern gold-field at 7 to 6 Ma, and these contributed most of the Au and Ag that has been recovered from the goldfield. In contrast, rhyolite in the eastern goldfield hosts smaller, less well developed veins plus stockworks that have produced the least amount of Au and Ag. Epithermal mineral deposits in the Hauraki goldfield evolved through time, reflecting changes in volcanic activity, and more speculatively, tectonic and structural history and setting.

Eocene Gold Ore Formation at Muteh, Sanandaj-Sirjan Tectonic Zone, Western Iran: A Result of Late-Stage Extension and Exhumation of Metamorphic Basement Rocks within the Zagros Orogen

A multidisciplinary study including field geology, microstructure analysis, 40Ar/39Ar dating, and fluid inclusion microthermometry and Raman spectrometry has been carried out on the Muteh gold deposit located in a greenschist to amphibolite facies metamorphic rock complex of the Sanandaj-Sirjan tectonic zone, Zagros orogen, Iran. The Muteh gold deposit has been previously interpreted as genetically related to Precambrian granitic intrusions, as an exhalative hot-spring deposit related to Paleozoic rhyolitic-acidic tuffs, and to local metamorphic processes. Host rocks of the gold deposit are predominantly schist and gneiss, and subsidiary amphibolite and quartzite, and are intruded by leucogranites. In the vicinity of the gold deposit, the metamorphic and granitic rocks display a subhorizontal mylonitic foliation, containing a northeast-oriented stretching lineation. Field investigations in one of the producing open pits show that the gold orebodies are controlled by northwest-oriented normal faults and joints, dipping to the northeast and the southwest, and crosscutting the ductile fabric of the host rocks. Both the ductile fabric of the host rocks and the gold ore-controlling brittle structures are interpreted to have formed within a single, continuous extensional event, which started with ductile deformation and gradually changed into brittle deformation. Hydrothermal alteration associated with ore formation consists of quartz, muscovite, pyrite, dolomite-ankerite, and albite, which crosscuts the ductile fabric and overprints the metamorphic minerals of the host rocks. Pyrite is the dominant opaque mineral and is the major phase associated with gold. Chalcopyrite, marcasite, bismuth, galena, sphalerite, and pyrrhotite are subsidiary to rare phases. 40Ar/39Ar incremental-heating and in situ laser-ablation age data reveal a coherent sequence of cooling and hydrothermal events in the metamorphic complex hosting the Muteh gold deposit. Muscovite samples from the alteration zone and from one quartz vein from the orebodies yield 40Ar/39Ar ages between 55.7 and 38.5 Ma and show that gold mineralization is the youngest among the different dated events. This Eocene age is consistent with the young structural setting of the gold orebodies revealing an emplacement along northwestoriented normal faults, which can be correlated with Tertiary extensional tectonic events reported by previous field investigations. The data also indicate that gold ore formation is coeval with magmatism in the adjacent Tertiary Urumieh-Dokhtar magmatic belt and with intrusive activity within the Sanandaj-Sirjan zone, with a 40Ar/39Ar age of 54 Ma near Muteh according to our study. The remaining 40Ar/39Ar data reveal a Cretaceous to early Tertiary metamorphic, magmatic, and deformation history of the complex hosting the Muteh gold deposit. The fluid inclusion study showed that distinct fluids were present during the regional geologic evolution predating ore formation and later Eocene gold ore formation. Early regional fluids consist of an H2O-CO2-NaCl fluid with subsidiary N2 and CH4 that underwent local unmixing. The salinities of the liquid-rich inclusions resulting from unmixing range between 2.2 and 17.5 wt percent NaCl equiv, and those of the CO2-rich, vaporrich inclusions range between 2.8 and 7.2 wt percent NaCl equiv. The high salinity of the regional liquid-rich fluids is attributed to metamorphism of evaporite-bearing sedimentary rocks, magmatic intrusions, retrograde hydration reactions, metamorphism of impure marbles, and migmatization. The fluid inclusions spatially associated with the gold veins postdating regional metamorphism and ductile deformation of the host rocks contain a low-salinity aqueous fluid with a dilute, low-density CO2 component, and maximum salinities between 2.6 and 5.8 wt percent NaCl equiv. Gold deposition at Muteh occurred during or after late stages of Eocene brittle extension as a result of exhumation of the host metamorphic complex, possibly during magmatic activity in the Sanandaj-Sirjam zone. It is most likely linked to the introduction of a distinct low-salinity, low CO2 content aqueous fluid during crustal extension. These fluids are younger than, and therefore unrelated to, the highly saline and CO2-rich, regional metamorphic-magmatic fluids. Our results refute previous models advocating Precambrian to Paleozoic ages of the Muteh gold deposit.

Geology of the Sari Gunay Epithermal Gold Deposit, Northwest Iran

The Sari Gunay epithermal gold deposit is located within a mildly alkaline latitic to trachytic volcanic complex in central-northwest Iran. Intrusive and volcanic rocks that host the deposit have been dated at between 11.7 and 11.0 Ma (with one younger sample at 8.0 Ma; 40Ar/39Ar dating of igneous biotite and hornblende), whereas sericitic alteration associated with an early stage of hydrothermal activity occurred between ~10.8 and ~10.3 Ma (the best age estimate is 10.7 Ma obtained by 40Ar/39Ar dating of sericite). The main hydrothermal system was structurally localized by a diatreme breccia complex centered on the Sari Gunay hill. Earliest hydrothermal activity, only encountered at depth (>300 m below surface), produced weak potassic alteration and quartz-sulfide-magnetite veining with low grades of Cu and Au mineralization, reminiscent of shallow-level (≤ 1.5-km depth) porphyry gold systems. Paragenetically later quartz-tourmaline breccias and veins followed the south-southwest–north-northeast trend of the diatreme breccias and produced strong sericitic alteration and silicification in igneous clasts and wall rocks. Fluid inclusions indicate temperatures of 246° to 360°C and salinities of 34.4 to 46.1 wt percent NaCl equiv in hypersaline inclusions coexisting with low-density, CO2-bearing vapor-phase inclusions. These fluids do not appear to have deposited gold, but the breccias provided further structural permeability for flow of later mineralizing epithermal fluids. Invisible gold occurs in solid solution in fine-grained arsenical pyrite and minor arsenopyrite, deposited in the early stages of quartz-adularia-pyrite-stibnite veins. Late-stage minerals in these veins include realgar, orpiment, cinnabar, and extremely rare wire gold. Liquid-rich fluid inclusions in these veins have an average homogenization temperature (≈ trapping temperature) of 199° ± 24°C, salinities of 3.6 ± 1.1 wt percent NaCl equiv, and coexist with low-density CO2-bearing vapor-phase inclusions, suggesting low-pressure conditions. Phase separation may have been a contributory cause of gold deposition, but precipitation of sulfides in response to rapid cooling of the system may have been the main control on deposition of pyrite-hosted gold. The Sari Gunay gold deposit and its host volcanic system form part of the Takab belt of structurally controlled, middle Miocene, mildly alkaline volcano-plutonic complexes. The sediment-hosted Zarshuran gold deposit, located ~190 km to the north-northwest along the belt, is also related to middle Miocene magmatism. The Takab belt is marginally younger than, and displaced to the southwest of, the extensive Eocene-Miocene Urumieh-Dokhtar calk-alkaline volcanic arc, which hosts several large porphyry Cu deposits (e.g., Sar Cheshmeh, Meiduk). Rocks of the Takab belt are also distinctly more alkaline than the Urumieh-Dokhtar rocks. Viewed in the context of plate tectonic models for Neogene closure of a Neo-Tethys ocean between the Afro-Arabian and Eurasian plates, these differences may reflect a normal subduction zone origin for the Urumieh-Dokhtar arc and contained porphyry Cu deposits but a collisional origin for the slightly younger Takab belt volcanic rocks and epithermal gold deposits. The Sari Gunay deposit may thus be classified as a collision-related alkalic-type epithermal system, although it is less alkaline than classic deposits of this group such as Porgera, Emperor, or Cripple Creek (the latter apparently being related to postcollisional or back-arc extensional rather than syncollisional processes).

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization

Sandstone Diagenesis in the Mount Isa Basin: An Isotopic and Fluid Inclusion Perspective in Relationship to District-Wide Zn, Pb, and Cu Mineralization