Wt Percent NaCl Equiv Research Articles

The Role of Sandstone Diagenesis and Aquifer Evolution in the Formation of Uranium and Zinc-Lead Deposits, Southern McArthur Basin, Northern Territory, Australia

The Paleo- to Mesoproterozoic McArthur basin in the Northern Territory, Australia, is filled with mixed carbonate-siliciclastic successions and felsic and mafic volcanic units that contain authigenic minerals that formed during burial diagenesis. Petrographic investigations indicate that some of these lithologic units were cemented early by quartz overgrowths and became diagenetic aquitards, whereas secondary porosity was developed in others that became diagenetic aquifers for basinal brines. The authigenic minerals in both lithology types preserve geochemical information about the types of fluids that occurred in the basin, with those that resided in the diagenetic aquifers having similar geochemical composition to those indicated by alteration minerals in the McArthur River Pb-Zn deposit and the Westmoreland U deposits. Quartz overgrowths formed early during shallow burial in all clastic lithologic units and are particularly common in the well-sorted, marine sediments; these became diagenetic aquitards due to the porosity-occluding cement. In the calcareous units, early dolomite replaced calcite and aragonite at shallow burial depths. Fluid inclusion microthermometry indicates that most quartz overgrowths formed between 80° and 125°C from a fluid with a salinity between 0 and 13.1 wt percent NaCl equiv. The isotopic composition of dolomite indicates δ18Ofluid of −4.5 ± 1.5 per mil and δ13Cfluid of −6.9 ± 1.0 per mil, suggesting formation from a mixed marine and meteoric fluid. Illite cemented by early quartz has a 40Ar/39Ar age of 1738 ± 10 Ma in one diagenetic aquitard, which reflects the age when the host sediment was deposited and confirms that quartz cementation occurred early during burial. With increased burial, the poorly sorted sandstones and conglomerates that were initially poor aquifers at or near the surface experienced framework grain dissolution and the creation of secondary porosity at depth. As a result, the sediments of the Westmoreland Conglomerate, the lower Yiyintyi Sandstone, significant proportions of the Warramana Sandstone, and most of the sandstone interbeds of the Gold Creek Volcanics became diagenetic aquifers and conduits for basinal brines. Illite, dolomite, and chlorite formed late in the paragenesis and filled the secondary porosity. Quartz veins formed last and primarily in faults and fractures. The dolomite and quartz veins contain fluid inclusions that have low eutectic temperatures indicative of Ca2+- and Na+-dominated brines and salinities between 18.5 and 30.9 wt percent NaCl equiv. Illite crystallinity data and chlorite chemical compositions indicate formation between 150° and 250°C. The isotopic compositions of illite and chlorite indicate δ18Ofluid values between 2.9 and 9.1 per mil and δDfluid values between −61 and −25 per mil, which suggest formation from a mixed meteoric and marine fluid. These δ18Ofluid values are indistinguishable from those determined from the isotopic composition of alteration minerals at the McArthur River Pb-Zn deposit (5 ± 5‰) or synore illite at the Westmoreland uranium deposit (4 ± 2‰), suggesting that the diagenetic aquifers and the deposits may be linked. Argon ages of illite from the diagenetic aquifers indicate that fluid migration began as early as 1680 Ma and continued to approximately 1541 Ma, coinciding with and extending past the time when the McArthur River Pb-Zn deposit (1640 Ma) and the Westmoreland U deposits (1655–1606 Ma) formed. The results presented in this study show that clastic sediments dominated by poorly sorted sandstone and conglomerate facies evolved through burial diagenesis to become aquifer lithologic units that hosted basinal brines with chemical characteristics and ages similar to those that are reported from the Pb-Zn and U deposits of the southern McArthur basin.

Zinc Deposits and Related Mineralization of the Burketown Mineral Field, Including the World-Class Century Deposit, Northern Australia: Fluid Inclusion and Stable Isotope Evidence for Basin Fluid Sources

The stratiform Century Zn-Pb deposit and the discordant Zn-Pb lode deposits of the Burketown mineral field, northern Australia, host ore and gangue minerals with primary fluid inclusions that have not been affected by the Isan orogeny, thus providing a unique opportunity to investigate the nature of the ore-forming brines. All of the deposits are hosted in shales and siltstones belonging to the Isa superbasin and comprise sphalerite, pyrite, carbonate, quartz, galena, minor chalcopyrite, and minor illite. According to Pb model ages, the main ore stage of mineralization at Century formed at 1575 Ma, some 20 m.y. after deposition of the host shale sequence. Microthermometry on undeformed, primary fluid inclusions hosted in porous sphalerite shows that the Zn at Century was transported to the deposit by a homogeneous, Ca 2+ - and Na + -bearing brine with a salinity of 21.6 wt percent NaCl equiv. δD fluid of the fluid inclusion water ranges from −89 to −83 per mil, consistent with a basinal brine that evolved from meteoric water. Fluid inclusion homogenization temperatures range between 74° and 125°C, which are lower than the 120° to 160°C range calculated from vitrinite reflectance and illite crystallinity data from the deposit. This discrepancy indicates that mineralization likely formed at 50 to 85 Mpa, corresponding to a depth of 1,900 to 3,100 m. Transgressive galena-sphalerite veins that cut stratiform mineralization at Century and breccia-filled quartz-dolomite-sphalerite-galena veins in the discordant Zn-Pb lodes have Pb model ages between 1575 and 1485 Ma. Raman spectroscopy and microthermometry reveal that the primary fluid inclusions in these veins contain Ca 2+ , Na + , but they have lower salinities between 23 and 10 wt percent NaCl equiv and higher δD fluid values ranging from −89 to −61 per mil than fluid inclusions in porous sphalerite from Century. Fluid inclusion water from sphalerite in one of the lode deposits has δ 18 O fluid values of 1.6 and 2.4 per mil, indistinguishable from δ 18 O fluid values between −0.3 to +7.4 per mil calculated from the isotopic composition of co-existing quartz, dolomite, and illite. The trend toward lower salinities and higher δD fluid values relative to the earlier mineralizing fluids is attributed to mixing between the fluid that formed Century and a seawater-derived fluid from a different source. Based on seismic data from the Lawn Hill platform and paragenetic and geochemical results from the Leichhardt River fault trough to the south, diagenetic aquifers in the underlying Calvert superbasin appear to have been the most likely sources for the fluids that formed Century and the discordant lode deposits. Paragenetically late sphalerite and calcite cut sphalerite, quartz, and dolomite in the lode deposits and contain Na + -dominated fluid inclusions with much lower salinities than their older counterparts. The isotopic composition of calcite also indicates δ 18 O fluid from 3.3 to 10.7 per mil, which is larger than the range obtained from synmineralization minerals, supporting the idea that a unique fluid source was involved. The absolute timing of this event is unclear, but a plethora of Pb model, K-Ar, and 40 Ar/ 39 Ar ages between 1440 and 1300 Ma indicate that a significant volume of fluid was mobilized at this time. The deposition of the Roper superbasin from ca. 1492 ± 4 Ma suggests that these late veins formed from fluids that may have been derived from aquifers in overlying sediments of the Roper superbasin. Clear, buck, and drusy quartz in veins unrelated to any form of Pb-Zn mineralization record the last major fluid event in the Burketown mineral field and form distinct outcrops and ridges in the district. Fluid inclusions in these veins indicate formation from a low-salinity, 300° ± 80°C fluid. Temperatures approaching 300°C recorded in organic matter adjacent to faults and at sequence boundaries correspond to K-Ar ages spanning 1300 to 1100 Ma, which coincides with regional hydrothermal activity in the northern Lawn Hill platform and the emplacement of the Lakeview Dolerite at the time of assemblage of the Rodinia supercontinent.

Magmatic Fluids in the Breccia-Hosted Epithermal Au-Ag Deposit of Rosia Montana, Romania

The breccia-hosted epithermal Au-Ag deposit of Rosia Montana is located 7 km northeast of Abrud, in the northern part of the South Apuseni Mountains, Romania. Estimated total reserves of 214.91 million metric tons (Mt) of ore at 1.46 g/t Au and 6.9 g/t Ag (10.1 Moz of Au and 47.6 Moz of Ag) make Rosia Montana one of the largest gold deposits in Europe. At this location, Miocene calc-alkaline magmatic and hydrothermal activity was associated with local extensional tectonics within a strike-slip regime related to the indentation of the Adriatic microplate into the European plate during the Carpathian orogenesis. The host rocks of the magmatic complex consist of pre-Mesozoic metamorphosed continental crust covered by Cretaceous turbiditic sediment (flysch). Magmatic activity at Rosia Montana and its surroundings occurred in several pulses and lasted about 7 m.y. Rosia Montana is a breccia-hosted epithermal system related to strong phreatomagmatic activity due to the shallow emplacement of the Montana dacite. The Montana dacite intruded Miocene volcaniclastic material (volcaniclastic breccias) and crops out at Cetate and Cârnic Hills. Current mining is focused primarily on the Cetate open pit, which was mapped in detail, leading to the recognition of three distinct breccia bodies: the dacite breccia with a dominantly hydrothermal matrix, the gray polymict breccia with a greater proportion of sand-sized matrix support, and the black polymict breccia, which reached to the surface, contains carbonized tree trunks and has a dominantly barren clastic matrix. The hydrothermal alteration is pervasive. Adularia alteration with a phyllic overprint is ubiquitous; silicification and argillic alteration occur locally. Mineralization consists of quartz, adularia, carbonates (commonly Mn-rich), pyrite, Fe-poor sphalerite, galena, chalcopyrite, tetrahedrite, and native gold and occurs as disseminations, as well as in veins and filling vugs within the Montana dacite and the different breccias. The age of mineralization (12.85 ± 0.07 Ma) was determined by 40 Ar- 39 Ar dating on hydrothermal adularia crystals from vugs in the dacite breccia in the Cetate open pit. Microthermometric measurements of fluid inclusions in quartz phenocrysts from the Montana dacite revealed two fluid types that are absent from the hydrothermal breccia and must have been trapped at depth prior to dacite dome emplacement: brine inclusions (32–55 wt % NaCl equiv, homogenizing at T h > 460°C) and intermediate density fluids (4.9–15.6 wt % NaCl equiv, T h between 345°–430°C). Secondary aqueous fluid inclusion assemblages in the phenocrysts have salinities of 0.2 to 2.2 wt percent NaCl equiv and T h of 200° to 280°C. Fluid inclusion assemblages in hydrothermal quartz from breccias and veins have salinities of 0.2 to 3.4 wt percent NaCl equiv and T h from 200° to 270°C. The oxygen isotope composition of several zones of an ore-related epithermal quartz crystal indicate a very constant δ 18 O of 4.5 to 5.0 per mil for the mineralizing fluid, despite significant salinity and temperature variation over time. Following microthermometry, selected fluid inclusion assemblages were analyzed by laser ablation-inductively coupled-plasma mass spectrometry (LA-ICPMS). Despite systematic differences in salinity between phenocryst-hosted fluids trapped at depth and fluids from quartz in the epithermal breccias, all fluids have overlapping major and trace cation ratios, including identical Na/K/Rb/Sr/Cs/Ba. Consistent with the constant near-magmatic oxygen isotope composition of the hydrothermal fluids, these data strongly indicate a common magmatic component of these chemically conservative solutes in all fluids. Cu, Pb, Zn, and Mn show variations in concentration relative to the relatively non-reactive alkalis, reflecting the precipitation of sulfide minerals together with Au in the epithermal breccia, and possibly of Cu in an inferred subjacent porphyry environment. The magmatic-hydrothermal processes responsible for epithermal Au-Ag mineralization at Rosia Montana are, however, not directly related to the formation of the spatially associated porphyry Cu-Au deposit of Rosia Poieni, which occurred about 3 m.y. later.

Petrographic, Geochemical, and Fluid Inclusion Evidence for the Origin of Siliceous Cap Rocks Above Volcanic-Hosted Massive Sulfide Deposits at Myra Falls, Vancouver Island, British Columbia, Canada

Massive sulfides at the Myra Falls volcanic-hosted massive sulfide (VHMS) camp, Vancouver Island, British Columbia, Canada, are overlain by white chert, black chert, argillite, and siltstone. White chert is best developed above the Battle orebody, where it forms a siliceous caprock (3-5 m thick) above the massive sulfides. There is a gradational lateral change from white chert above massive sulfides to black chert and unaltered argillite, 100 to 150 m south of the Battle orebody. Chert horizons are also located above the Ridge and Extension ore zones, but only minor chert lies above the HW orebody, which is instead overlain by a thick argillite sequence. The chert and argillite share similar sedimentologic and petrologic features, including abundant parallel laminations, interbedded turbidites, radiolarian-rich layers, soft-sediment deformation, scours, flame structures, and small phosphatic concretions. These features indicate that white and black chert formed as a replacement of mudstone rather than as exhalative or biogenic deposits. Silicification occurred early in the depositional history of the fine-grained sediments and was contemporaneous with some ore formation. Early syndepositional features are still visible in the chert, with primary pore spaces such as radiolarian tests filled by silica, rutile, apatite, and minor sulfides displaying open-space crystal growth. The presence of minor ore-clast breccias above the orebody indicates that at least parts of the Battle orebody were exposed on the sea floor. Metal zoning is observed in the cap-rock horizon above the Battle orebody, with higher Cu, Zn, and Cd contentrations in chert directly above massive sulfides, higher Pb, Sb, and Ag contentrations in black chert at the edge of the siliceous cap rocks and lower metal concentrations in the distal argillite. Primary fluid inclusions in spherical quartz patches in chert above the Battle orebody indicate that hydrothermal fluids passing through sediment were between 135 to 250C and had salinities ranging from 3.0 to 12.1 wt percent NaCl equiv. These data are similar to those for fluid inclusions measured in quartz interstitial to sulfides in the underlying Battle orebody, which have temperatures of homogenization ranging from 140 to 250C and salinities from 3.0 to 12.4 wt percent NaCl equiv. Fluid inclusions in the Battle orebody display a slight increase in salinity and homogenization temperature with depth, which may reflect the overprinting of earlier high temperature stages by cooler fluids as the hydrothermal system waned or varying degrees of mixing between hydrothermal fluids and seawater. A minimum depth for deposition of the cap rocks (>200 m) is estimated, based on sedimentologic features such as fine parallel laminations and interbedded sandstone turbidites, which indicate deposition below storm wave base. Greater water depths (1,000-1,500 m) are suggested by the lack of evidence of boiling in fluid inclusions. Low O2 concentrations in the bottom water of the Battle basin are suggested by the absence of bioturbation, lack of fossils of benthic fauna, degree of pyritization values >0.90, elevated Zn, Pb, Cu, Cd, As, Sb, Ag, Ba, and V, low Fe and Mn, and V/(V + Ni) > 0.8 in the unaltered argillite. Paleosea-floor reconstructions indicate that the Battle and HW orebodies formed in small basins along a northwest-trending ridge. The finegrained sediments were deposited in depocenters within paleotopographic lows. Hydrothermal fluid densities, estimated from fluid inclusions at Myra Falls, range from 0.88 to 1.05 g/cm3 and are higher than for many other VHMS deposits. However, they are close to the density of seawater at 2C and a 2,000-m depth (1.028 g/cm3). Replacement textures in the siliceous cap rocks above the Battle deposit, the sheetlike morphology of the siliceous cap rocks, and lateral metal zonation indicates that diffuse lateral flow of hydrothermal fluids through the porous sea-floor sediments was more important than the buoyant rise of hydrothermal fluids into the water column. However, buoyant venting during formation of the Battle deposit is indicated by positive Eu anomalies and elevated metal values in argillite of the Battle basin, reflecting the wide dispersal of plume particulates. The range of fluid densities indicate that the hydrothermal fluids emerging onto the sea floor and flowing laterally through porous sea-floor sediments would have varied from buoyant to neutrally buoyant, to negatively buoyant.

Formation of the Auriferous and Barren North Dipper Veins in the Sigma Mine, Val d'Or, Canada: Constraints from Structural, Mineralogical, Fluid Inclusion, and Isotopic Data

The Sigma-Lamaque mine (>100 metric tons (t) of gold) is an example of an Archean greenstone belt gold deposit formed by processes related to multistage hydrothermal fluid circulation. The earliest recognized vein systems are steeply to moderately dipping fault-fill veins within shear zones and contemporaneous subhorizontal veins, which were overprinted by the late North Dipper vein system. The latter strikes east-west, dips moderately to the north, and exhibits complex internal geometry with features similar to fault-fill and extensional veins, and therefore is interpreted as extensional shear veins. This late vein system was considered one of the major sources of gold in the shallow underground and open-pit ore reserve estimations. These late North Dipper veins were studied in detail in the Sigma pit where they contain ore-grade gold and are hosted by the calc-alkaline tuffs of the Val d’Or Formation and in the North zone where they are barren and hosted by the tholeiitic volcanic rocks of the Jacola Formation. In the Sigma pit, these veins have apparent thicknesses of 0.8 to 3 m and very irregular contacts with the vein walls. They comprise irregular zones of milky quartz filling, disrupted blocks of layered quartz and tourmaline and slabs of foliated pyrite-rich wall rock. Layers with tourmaline and quartz crystals oriented perpendicular to the vein walls are observed close to the vein walls. The veins contain minor amounts of calcite, pyrite, rutile, and traces of chlorite, muscovite, pyrrhotite, chalcopyrite, galena, bismuth tellurides, and gold. Locally, the sulfide content can attain 10 modal percent of the vein mineralogy. In the veins, native gold (with 6 wt % Ag) occurs rarely associated with the main vein-filling minerals, commonly as inclusions in pyrite (main vein-filling stage), and most commonly filling fractures in pyrite, tourmaline, and quartz, where it is alloyed with 8 to 17 wt percent Ag and is associated with calcite and Bi tellurides (late auriferous stage). The proximal wall-rock alteration related to the auriferous North Dipper veins overprints the metamorphic assemblage and is characterized by abundant fine-grained plagioclase, calcite, quartz and minor chlorite, tourmaline, and pyrite in the tuffs and more abundant muscovite in the intrusive feldspar porphyry. In the wall rock, gold with 6 to 28 wt percent Ag is found locally filling fractures in pyrite, where it is associated with a late generation of calcite. The barren North Dipper vein also comprises mainly quartz and tourmaline but lacks calcite and gold, has a very low content of sulfides ( Aqueous-carbonic fluid inclusions with salinities less than 8 wt percent (NaCl equiv) and homogenization temperatures between 228° and 440°C, and carbonic fluid inclusions with less than 0.2 mol percent CH 4 coexisting in the same healed fractures, are common in the auriferous North Dipper veins but very rare in the barren vein where aqueous fluid inclusions have lower temperatures (dominantly 125°–225°C). Crush leaching and laser ablation time-of-flight inductively coupled mass spectrometry (LA-TOF-ICPMS) of fluids in the inclusions indicate elevated concentrations of Ni, Cu, Sb, Pb, and Ag in the fluids in the auriferous veins compared to the aqueous fluids in the barren veins. This suggests different sources for the metals in the fluids associated with the gold, possibly including both igneous and sedimentary rocks. Calculated oxygen isotope equilibration temperatures for quartz-tourmaline pairs from the auriferous main vein-filling stage are 323° to 370°C. This is within the range documented in fluid inclusions related to the late auriferous stage, indicating that the temperature of the auriferous hydrothermal fluids did not change appreciably during the evolution of the system. Calculated δ 18 O H 2 O values for the mineralizing fluids are 9.5 per mil, consistent with either low water/rock ratio systems associated with high-grade metamorphism or a magmatic origin for the fluids. Gold included in pyrite may have precipitated during the main vein-filling stage due to the decrease of H 2 Sconcentration in the hydrothermal fluid following the deposition of sulfides. However, gold in fractures in the main vein-filling minerals precipitated later, due probably to dilution of the auriferous aqueous-carbonic fluids by mixing with aqueous fluids. The absence of ore-grade gold in the barren North zone vein is probably due to the lack of circulation of auriferous aqueous-carbonic fluids in the North zone, located distal to the first-order Larder-Lake Cadillac fault, which may have served as the main regional conduit for the auriferous aqueous carbonic fluids.

MINERALOGY, FLUID CHARACTERISTICS, AND DEPOSITIONAL ENVIRONMENT OF THE PALEOCENE EPITHERMAL Au-Ag DEPOSITS OF THE EL BARQUENO DISTRICT, JALISCO, MEXICO

The El Barqueno epithermal vein district is located in the southwestern part of the Sierra Madre Occidental of Mexico. It is made up of several polymetallic (Au-Ag-Cu-Zn-Pb) epithermal veins hosted by a volcanoplutonic assemblage of andesitic rocks, granites, and granodiorites, which belong to the Lower Volcanic Supergroup of the Sierra Madre Occidental, within the Jalisco block. The veins trend from 45° to 100°, dipping to the southeast and south, and steepening with depth. The maximum strike length of the veins at El Barqueno is about 5 km, and the maximum vein thickness is about 10 m (avg 2–3 m), narrowing with depth. The paragenetic sequence indicates two main mineralization stages. Stage I occurs as breccias with base metal sulfides and virtually no gold or silver. Stage II consists of crustiform quartz bands that form symmetrical vein fillings and contain the precious metal mineralization. Rhombohedral adularia crystals and quartz pseudomorphs after bladed calcite are widespread in stage II, providing evidence for boiling. Gold- and silver-bearing metallic minerals locally precipitated directly on adularia crystals, an association indicating boiling as the main mechanism of metal deposition. Conductive cooling and mixing also may have been important. The temperature and salinity of the mineralizing fluids ranged from 101° to 252°C and 0.2 to 19.1 wt percent NaCl equiv, respectively. Whereas temperatures cluster in the range of 120° to 160°C, salinities have a bimodal distribution, most of them falling below 3 or higher than 10 wt percent NaCl equiv. These two arrays of salinity values only locally reflect mixing and may represent different sources of mineralizing fluids, possibly dominated by meteoric and magmatic sources, respectively. An adularia separate from crustiform quartz bands of stage II was dated by 40 Ar/ 39 Ar geochronology at 57.91 ± 0.44 Ma (isochron age). Thus, El Barqueno has the oldest absolute age of any epithermal deposit in Mexico and suggests the existence of a previously unrecognized Paleocene metallogenic event in southwestern Mexico associated with several types of ore deposits, including iron oxide-copper-gold, porphyry-type skarn and epithermal deposits. This further suggests that older rocks beneath the Upper Volcanic Supergroup of the Sierra Madre Occidental may be prospects for other epithermal veins and that exploration for epithermal deposits and related mineral deposit types should be focused in the oldest outcrop portions of the Sierra Madre Occidental.

The Environment of Vein Formation and Ore Deposition in the Purisima-Colon Vein System, Pachuca Real del Monte District, Hidalgo, Mexico

The Environment of Vein Formation and Ore Deposition in the Purisima-Colon Vein System, Pachuca Real del Monte District, Hidalgo, Mexico

Genesis of Pyrite-Au-As-Zn-Bi-Te Zones Associated with Cu-Au Skarns: Evidence from the Big Gossan and Wanagon Gold Deposits, Ertsberg District, Papua, Indonesia

The Ertsberg district hosts multiple skarn and porphyry-related deposits, which together comprise one of the largest Cu-Au resources in the world. Earlier skarn Cu-Au deposits at Big Gossan and 2 km along strike to the northwest at Wanagon Gold are overprinted by distinctive late-stage pyrite, sphalerite, arsenopyrite, and native gold with local Bi and Te minerals. The Wanagon Gold deposit contains an estimated 2 million ounces (Moz) of gold; reserves at Big Gossan are 33 million tonnes (Mt) at 2.63 percent Cu, 0.92 g/t Au, and 15.72 g/t Ag. Phlogopite from the Big Gossan occurrence is younger than 2.82 ± 0.04 Ma, based on a new 40Ar/39Ar age from the Big Gossan skarn, and K-feldspar from the Wanagon Gold deposit has a 40Ar/39Ar age of 3.62 ± 0.05 Ma. A K-Ar date (3.81 ± 0.06 Ma) from the Wanagon sill constrains formation of the overprinting skarn Cu-Au and late-stage Wanagon Gold deposit to a period of ca. 0.2 m.y. At Big Gossan, earlier skarn Cu-Au mineralization displays three-dimensional mineralogical, chemical, and temperature zonation. The high-temperature core (defined by low Zn/Cu) plunges to the northwest and is open at depth. Highest Cu grades and greatest development of the overprinting pyrite-Au-As-Zn-Bi-Te association occur to the northwest coincident with northeast-striking faults. Pyrite-Au-As-Zn-Bi-Te occurrences are also distributed in faults and fractures to the north and south of the Big Gossan skarn Cu-Au deposit. At Wanagon Gold, leaching of skarn and sandstone preceded introduction of the pyrite-Au-As-Zn-Bi-Te occurrences. In the sandstone, the pyrite-Au-As-Zn-Bi-Te mineralization was accompanied by K-feldspar (adularia) and minor quartz gangue. In carbonate rocks, no leaching or secondary K-feldspar is apparent; instead, sulfides are accompanied by quartz and dolomite gangue. The {delta}34S of sulfide from skarn Cu-Au and overprinting pyrite-Au-As-Zn-Bi-Te occurrences at both deposits range from –0.7 to +5.1 per mil. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses show that later pyrite overprinting both occurrences is distinct from that in the earlier skarn Cu-Au deposits and contains up to 60 ppm Au, 2 percent As, 680 ppm Bi, and 40 ppm Te. The mineralogy of the overprinting occurrences includes native gold, argentian tetrahedrite and tennantite, a silver-antimony sulfide, and Bi and Te-(Ag-Au) minerals including cosalite, bismuthinite, petzite, hessite, altaite, and tetradymite. The fineness of native gold varies with sulfide association. The lowest fineness gold (737–863) occurs with Pb minerals (galena and sulfosalts), and the higher fineness gold (904–974) occurs trapped within pyrite or in association with bismuthinite. Fluid inclusions in sphalerite and quartz in the Big Gossan pyrite-Au-As-Zn-Bi-Te occurrence have an average salinity of 8 wt percent NaCl equiv and an average homogenization temperature of 245°C. Stable isotopes indicate that the inclusion fluids were magmatic. However, a direct genetic relationship to earlier skarn Cu-Au mineralization is not obvious. The pyrite-Au-As-Zn-Bi-Te occurrences are considered to have formed from a fluid with a different composition, possibly the magmatic precursor to fluids commonly recognized in low- and high-sulfidation epithermal deposits that develops at shallow levels and contains significant nonmagmatic (i.e., meteoric) water.

Mineralogical and Isotopic Zonation in the Sur-Sur Tourmaline Breccia, Rio Blanco-Los Bronces Cu-Mo Deposit, Chile: Implications for Ore Genesis

The Sur-Sur tourmaline breccia is located in the southeast part of the Rio Blanco-Los Bronces porphyry copper- molybdenum deposit, central Chile. The breccia hosts approximately one-quarter of the total resource of 57 Mt of fine copper at Rio Blanco. The breccia is hosted within, and contains altered clasts of, granodiorite from the 12 to 8 Ma San Francisco batholith, which intruded a sequence of Miocene volcanic and volcaniclastic rocks. A series of weakly mineralized to barren felsic porphyries cut the breccia and indicate a minimum age of approximately 6 Ma for mineralization at Sur-Sur. The Sur-Sur breccia dike is at least 3 km long, 0.2 km wide, and has a vertical extent of at least 1 km. The breccia has been cemented by early biotite and anhydrite at depth and by tourmaline and specularite at higher altitudes. These early-formed cements have been overgrown and in some cases replaced by chalcopyrite, magnetite, pyrite, and quartz. Mineralogical zonation in the breccia includes a transition from biotite cement and related biotite alteration upward to tourmaline cement and quartz-sericite-tourmaline alteration at approximately 3,000-m elevation. Iron-oxide minerals are also zoned, with a transition upward from a magnetite-dominated zone below 3,330 m to a specularite-dominated zone above 3,600 m. Pyrite is the dominant sulfide at altitudes above 4,000 m. Secondary liquid-rich, vapor-rich, and hypersaline fluid inclusions are preserved in quartz and tourmaline cement. Measured homogenization temperatures are mostly between 300 degrees and 450 degrees C, and salinities range from 0 to 69 wt percent NaCl equiv. Sulfur isotope compositions of sulfide cement range from -4.1 to +2.7 per mil. The lowest delta 34S(sulfide) values are in samples from between 3,700- and 4,000-m elevation, where they correspond to the highest copper grades in the tourmaline breccia. This high-grade zone also contains abundant specularite (locally replaced by magnetite). Modeling of sulfate-sulfide equilibrium indicate that approximately 150 degrees C of cooling over a vertical interval of 100 m would be required to account for the zonation of sulfide isotope compositions at Sur-Sur, making conductive cooling an unlikely ore-forming mechanism. Measured 206Pb/204Pb values of lead in anhydrite cement in the Sur-Sur tourmaline breccia and the Rio Blanco magmatic breccia range from 17.558 to 18.479. 207Pb/204Pb values range from 15.534 to 15.623, and 208Pb/204Pb values range from 37.341 to 38.412. The lead in anhydrite is considerably less radiogenic than that indicated by values obtained previously for lead in sulfide ores and igneous host rocks at Rio Blanco-Los Bronces. The source of lead in anhydrite must have been from rocks external to the main magmatichydrothermal system, probably the Precordilleran basement. A magmatic-hydrothermal explosion from a deep-seated crystallizing intrusion triggered breccia formation at Sur-Sur. Hydrostatic pressures catastrophically exceeded lithostatic load plus the tensile strength of the confining granodiorite, leading to widespread brecciation and subsequent invasion by large volumes of magmatic gas and hypersaline brine. The low-density gas phase (carrying H2O, SO2, HCl, and B2O3) separated physically from the dense copper-bearing brine and flushed through the breccia column first, where it condensed into ground waters of uncertain derivation. Anhydrite, specularite, and tourmaline were deposited from this lowsalinity, acidic, oxidized hybrid solution. Subsequent upwelling of magmatic-hydrothermal brine resulted in sulfide deposition. High-grade copper deposition is interpreted to have occurred in response to mixing of the oxidized, acidic water with the copper-bearing magmatic-hydrothermal brine.

Na-Fe-Ca Alteration and LREE (Th-Nb) Mineralization in Marble andGranitoids of Sierra de Sumampa, Santiago del Estero, Argentina

The Jasimampa area in the Sierra de Sumampa de Santiago del Estero, Argentina, contains light rare earth element (LREE: Th-Nb) mineralization associated with several stages of carbonates and widespread fenitization of marble, granite (496 ± 2.9 Ma), and alkaline and siliceous igneous comendite dikes (U/Pb zircon age of 390 Ma) derived from fractionation of oceanic island basalts. This is the third discovery of LREE mineralization in Argentina and marks a new alkaline intracontinental magmatic event of Devonian age. Albite (Ab 97-99 ) + aegirine (Ae 70-92 Ja 1-11 Q 6-19 )± hematite ± Mg-arfvedsonite ± fluorine-rich apatite and late Ca-Fe-Mn carbonates ± monazite ± pyrochlore overprint and crosscut granite and comendite outcrops in a 15 km 2 area; locally, the Na-rich assemblage is pervasive and results in total destruction of host rocks. Na-Fe metasomatism near granite and comendite produced aegirine (Ae 86-91 Ja 1-8 Q 6-11 )± Mg-arfvedsonite and minor albite (Ab 98-99 ) in adjacent marble, with multiple stages of late Ca-Mn-Mg-Fe carbonates associated with iron and manganese oxides, hematite, and traces of sulfides (sphalerite, pyrite, galena). Monazite-(Ce) is the main LREE-bearing mineral and occurs with minor ferrocolumbite, pyrochlore, strontiopyrochlore, and apatite associated with a second generation of fine-grained hematite, barite, and celestine. Structural controls on Na-Fe-Ca alteration and LREE mineralization in granitoid and marble protoliths include brittle-ductile shear zones, brittle fracture systems, and a series of monolithologic and heterolithologic breccias with hydrothermal aegirine, carbonate, and chalcedony matrices that occur at granite-marble-comendite contacts. Fragments of multiple breccia types within breccias indicate multiple episodes of brecciation, alteration, and mineralization. The fluid responsible for early fenitization of granite and comendite was aqueous-carbonic with CO 2 of relatively high density (between 0.85 and 0.95 g/cm 3 ), salinity near 9 wt percent NaCl equiv, and minimum temperatures in the range of 250° to 280°C. Pervasively altered comendite and granite have high total REE content (1,750 ppm and 2,126 ppm, respectively), mainly LREE. Early banded and mottled aegirine ± arfvedsonite replacements of marble have high absolute REE abundance (2,513–4,663 ppm) compared to the least altered marble (476 ppm). Subsequent hydrothermal stages with carbonates, hematite, pyrolusite, monazite, and subordinate celestine and barite caused alteration of marble that was accompanied by an increase in absolute REE abundance, up to 15.45 wt percent, and high Sr (10.5 wt %), Th (4,390 ppm), and Ba (1.8 wt %). Alteration and mineralization at Jasimampa are the result of late-stage carbonatitic magma fractionation and fluid exsolution. This is indicated by the strongly alkaline character of early alteration, the composition of the hydrothermal carbonates, apatite, and LREE-Sr- and Ba-rich minerals, the chondrite-normalized REE patterns showing strong LREE enrichment without an Eu anomaly, and the replacement of marble by assemblages with very high Ba, Th, LREE, and Sr, and strong K, Zr, and Ti depletions. The alteration style and mineralization at Jasimampa are similar to the giant Fe-LREE-Nb deposit Bayan Obo and other hydrothermal LREE deposits hosted in carbonatites of China.

Deciphering the Complex Fluid History of a Greenstone-Hosted Gold Deposit: Fluid Inclusion and Stable Isotope Studies of the Giant Mine, Yellowknife, Northwest Territories, Canada

Mesothermal, greenstone-hosted gold deposits are typically products of complex hydrothermal systems that involved multiple fluids at various times throughout their histories. The Giant mine is an example of such an extremely complicated system, including (1) multiple, local and regional gold-depositing events; (2) at least two styles of gold ore introduction in the mine area, including both refractory, sulfide-hosted and free-milling, vein-hosted ores, whose relative timing is enigmatic; (3) fluid overprinting associated with deposition of multiple generations of postore vein- and vug-filling minerals; and (4) postore deformation and recrystallization of ore veins, especially along faults. Three main stages of quartz-carbonate mineralization are recognized in the Giant mine. Stage I encompasses deposition of dominant refractory, sulfide-hosted ores and subordinate free-milling, vein-hosted gold ores. Refractory, metavolcanic rock-hosted orebodies are connected to metasedimentary rocks east of the mine by an east-dipping alteration zone characterized by a depletion in Na and enrichments in K, Ag, As, S, and Sb. Quartz veins within the wall-rock alteration zone have δ 18 O values that decrease systematically from 14.7 per mil in deeper metasedimentary rocks toward 11.6 per mil in shallower metabasalts in the mine. This decrease is interpreted to indicate that 18 O-enriched ore fluids originated in deeper, metasedimentary rocks and reacted extensively with wall rocks along the entire extent of their flow paths before depositing dominantly refractory gold ores within more 16 O-enriched, shallower, Ti-rich tholeiitic metabasalts. Quartz ± carbonate veins related to these gold ores were deposited from H 2 O-CO 2 -NaCl fluids with T h values of 180° to 360°C and salinities of 4 to 9 wt percent NaCl equiv. Evidence of sporadic fluid unmixing indicates that gold was deposited at temperatures near 350°C and pressures of 1 to 2 kbars. The δ 18 O values (SMOW) of vein quartz (11.6–14.7‰) and calcite (8.6–14.1‰) within the ore-related wall-rock alteration zone indicate deposition from fluids ( δ 18 O water = 4.4–9.8‰) that equilibrated with metasedimentary and metavolcanic rocks during greenschist metamorphism. The δ 18 O values (8.6–11.4‰) of vein quartz that locally contains free gold, hosted in metavolcanic rocks outside of the ore-related alteration zone, indicate deposition from fluids with lower δ 8 O water values of 2.8 to 5.6 per mil. The difference in ranges of δ 18 O water values may indicate that multiple events were responsible for gold-bearing quartz veining, or alternatively, that fluids depositing veins within and outside of the alteration zone represented distinct fluid reservoirs that evolved through reaction with isotopically distinct rocks in their source regions (i.e., 18 O-enriched metasedimentary vs. 16 O-enriched metavolcanic rocks). Postore (stage II) carbonate veins associated with minor Pb-Zn-Sb-Ag mineralization were deposited from highly saline NaCl-CaCl 2 brines with T m values of –32° to –36°C and T h values of 72° to 273°C. The δ 18 O values of these carbonates (20.6–26.1‰) indicate that their parent brines ( δ 18 O water = 9–14‰) also equilibrated with metasedimentary and metavolcanic rocks. Subsequent dissolution of these carbonate veins created abundant vuggy porosity. Late (stage III), primarily vug-filling dolomite ± stibnite, overgrown by scalenohedral calcite, was deposited from dilute fluids ( h values of 95° to 115°C. The δ 18 O values of dolomite (17.4 toward 13.4‰) and latest calcite (10.9‰) indicate deposition from progressively less evolved meteoric waters with decreasing δ 18 O values from 1.1 toward –6.9 per mil. Refractory ores in the main alteration zone and gold-bearing quartz veins contained therein formed from chemically similar CO 2 -H 2 O-NaCl ore fluids whose oxygen isotope compositions are consistent with a metasedimentary source. These two styles of gold mineralization appear to be part of the same mineralizing event in which the style of mineralization was dictated by the mechanism of ore deposition. Refractory, sulfide-hosted gold mineralization resulted from reaction of mineralizing fluids with metavolcanic wall-rock Fe 2+ . Free-milling, gold-bearing quartz vein ores were deposited as a result of fluid unmixing, with loss of H 2 S to the vapor phase. Gold-bearing quartz veins outside of the main wall-rock alteration zone in the Giant mine were also deposited by unmixing of similar H 2 O-CO 2 -NaCl fluids. However, these fluids were isotopically distinct from those within the alteration zone and may not be part of the refractory, sulfide-hosted gold-depositing event. They may instead represent a separate mineralizing event whose fluids and ore-forming constituents were derived solely from within metavolcanic rocks. This overprinting of one event on the other may have been a necessary condition for the development of world-class gold deposits in the Yellowknife district. The important roles of both metavolcanic and metasedimentary source rocks may explain why some smaller greenstone belts, with limited volumes of metavolcanic rocks, can host substantial economic gold mineralization and has important implications for regional resource evaluation and exploration for gold deposits in greenstone belts.

Nature of Hydrothermal Fluids at the Shale-Hosted Red Dog Zn-Pb-Ag Deposits, Brooks Range, Alaska

The Red Dog Zn-Pb-Ag district in the western Brooks Range, northern Alaska, contains numerous shale-hosted Zn-Pb sulfide and barite deposits in organic-rich siliceous mudstone and shale, chert, and carbonate rocks of the Carboniferous Kuna Formation. The giant Red Dog shale-hosted deposits consist of a cluster of four orebodies (Main, Qanaiyaq, Aqqaluk, and Paalaaq) that lie within distinct thrust panels that offset a single ore deposit during the Mesozoic Brookian orogeny. These Zn-Pb-Ag-barite orebodies contain one of the world’s largest reserves and resources of zinc. Fluid inclusions in samples of vein sphalerite, which accounts for about 20 percent of the ore in the Main deposit, and quartz that composes the bulk of the extensive silicification in the ore deposit, were studied by microthermometry, Raman spectrometry, and ion chromatography. The study of fluid inclusions in the vein sphalerite was limited by the intense postore deformation of the ore deposits. However, four primary aqueous fluid inclusion assemblages in vein sphalerite yield temperatures of homogenization of 115° to 120°C, 123° to 127°C, 110° to 120°C and 175° to 180°C. More abundant final-melting temperatures indicate that the fluid inclusions in sphalerite have salinities of about 14 to 19 wt percent NaCl equiv. The fluid inclusion electrolyte data show that the ore fluid responsible for the vein sphalerite derived its salinity from the evaporation of seawater. Considering the salinity of the fluid inclusions together with the electrolyte data, it is possible that the evaporative was initially about 30 wt percent saline fluid and that it mixed with a more dilute fluid somewhere along its flow path. The temperature, salinity, and electrolyte composition of vein sphalerite in the Red Dog deposits are remarkably similar to those characteristics in sphalerite veins near the Century zinc deposit, Australia. Together, these data compose the majority of information on the temperature and composition of sphalerite in deposits of this type. On the basis of data describing fluid inclusions in sphalerite and the geologic setting of the ore deposits, a brine model is suggested for the Red Dog deposits. In this model, brines were produced in evaporative environments in supratidal carbonate facies of the Lisburne Group less than 100 km from the Red Dog deposits. These reflux brines may have infiltrated the underlying rocks of Endicott Group or fractured metasedimentary basement rocks. In the absence of a local heat source at the Red Dog deposits, the temperature of the ore fluids (~100° to <200°C) requires that the fluids circulated at depths between ~ 2.4 and 7.4 km. In the Red Dog area, the metalliferous fluids ascended into the organic-rich rocks of the Kuna Formation, probably along zones of active extensional faults or breaches in the shale aquitards overlying the aquifers in the Endicott Group. Fluid inclusions were also studied in the abundant quartz that constitutes the majority of the silica rock in the ore deposits. This postore quartz extensively replaced barite and was traditionally thought to be part of the main ore event. Primary fluid inclusion assemblages contain two-phase aqueous inclusions, single-phase inclusions of dense methane, or both. Primary assemblages that contain single-phase, dense-methane inclusions together with two-phase aqueous inclusions yield consistent homogenization temperatures that provide unequivocal evidence for the coeval trapping of immiscible gas and aqueous fluids. The densities of the methane inclusions, together with the temperature of homogenization of coexisting aqueous fluid inclusions, show that these fluid inclusions were trapped between pressures of 800 and 3,400 bars and temperatures between 187° and 214°C. The pressures obtained provide unequivocal evidence that the quartz formed after ore deposition in the Carboniferous because such high fluid pressures could only have been produced from thrust loading during the Mesozoic Brookian orogeny. The observed large variation in pressure is best explained by transient fluid pressures from hydrostatic to lithostatic conditions during thrust loading. The 3,400 bars pressure corresponds with about 12 km of lithostatic burial, whereas the lower pressures (800 bars) correspond with about 8 km of hydrostatic pressure. Because of their low salinity (0–5 wt % NaCl equiv) the electrolyte compositions of the quartz fluid inclusions do not constrain their origin.

Characteristics and Evolution of the Hydrothermal Fluid in the North Zone High-Grade Area, Porgera Gold Deposit, Papua New Guinea

The ~20-million-ounce (Moz) Porgera gold deposit, Papua New Guinea, is hosted by 6-m.y.-old alkalic intrusions and Cretaceous sedimentary rocks in which the intrusions were emplaced. Gold-bearing veins occur in three stages: (1) magnetite-sulfide-carbonate ± quartz veins with minor gold (prestage I), (2) base metal-sulfide-carbonate ± quartz ± Au veins (stage I), and (3) quartz-roscoelite-pyrite-gold veins and breccias (stage II). Stage II veins are economically the most significant. Quartz-roscoelite-pyrite-gold veins form high-grade zones associated with the Roamane fault (a late normal fault that crosscuts the intrusive complex) and in the footwall to the fault (the North zone). The North zone mineralization is the main focus of this study. The quartz-roscoelite-pyrite-gold assemblage occurs in three texturally distinct styles: (1) thin (1–5 mm) veinlets in which roscoelite-pyrite-gold are more abundant than quartz and in which roscoelite and gold also occur in the wall rock; (2) veins (5 mm to 10 cm) in which roscoelite-pyrite-gold with minor quartz form a band at the vein edges, followed by coarse-grained quartz and the vein centers commonly filled with anhydrite and carbonate; and (3) breccia veins and breccias in which wall-rock fragments are rimmed by roscoelite-pyrite-gold and minor quartz, followed by vuggy quartz infilling. Fluid inclusions from quartz in these veins and breccias are mostly liquid rich, and average salinities in individual samples range from 7.5 ± 1.0 to 9.6 ± 0.2 wt percent NaCl equiv. In five of 27 samples, an additional cluster of salinities between 4.4 and 6.2 wt percent NaCl equiv was observed. These relatively low salinity inclusions occur toward the vein center and are less abundant than high-salinity inclusions that occur toward the vein margins. Three samples exhibit a continuous salinity trend from 4.5 to 10.2 wt percent NaCl equiv. For samples where CO 2 analyses were available average corrected salinities range from 5.1 to 8.0 wt percent NaCl equiv. Average homogenization temperatures (T h ) of individual samples range from 127° ± 12° to 167° ± 25°C. The average T h of the low-salinity inclusions (145° ± 9°C) is marginally lower but overlaps with that of all high-salinity inclusions (152° ± 17°C). Gas chromatographic analyses showed that the high-salinity fluid contains up to 2 mol percent CO 2 , 0.11 mol percent CH 4 , 0.065 mol percent N 2 , and traces of C 2 H 4 , C 2 H 6 , and COS. Concentrations of Cl – (310–609 mM/l), Br – (0.28–0.75 mM/l), Li + (1.25–8.80 mM/l), Na + (462–1126 mM/l), K + (0–81 mM/l), Mg 2+ (0–7.0 mM/l), and Ca 2+ (0–185 mM/l) were determined by ion chromatography. The δ 18 O values of quartz range from 13.9 to 18.3 per mil, and calculated δ 18 O H 2 O values range from –1.2 to 4.1 per mil. The δ D H 2 O values lie between –77 and –52 per mil. The calculated isotopic composition of the hydrothermal fluid lies between that of magmatic and that of meteoric water. The δ 18 O values of carbonates range from 15.2 to 16.6 per mil. Carbon isotopes were analyzed on carbonates ( δ 13 C = –3.3 to –2.4‰), altered and unaltered sedimentary rocks (–5.4 to –4.0 and –1.5 to 0.0‰, respectively), and organic carbon in shales (–23.6 to –16.8‰). The δ 34 S pyrite values range from –9.4 to 6.1 per mil, and δ 34 S anhydrite values range from 12.4 to 17.5 permil. The petrographic and analytical results suggest that an ascending fluid interacted with the sedimentary rocks and/or fluid hosted by the sedimentary rocks at depth (rather than at the site of ore deposition). This interaction is suggested by the presence of organic-derived volatiles and high \(NH_{4}^{+}\) concentrations in inclusion fluid. In some samples, two fluids were involved in stage II vein formation. Vapor-rich fluid inclusions indicate that the hydrothermal fluid boiled locally, and continuous salinity trends suggest that fluid mixing occurred in some stage II veins. The analytical data and paragenetic information were used to estimate element concentrations in the Porgera hydrothermal fluid that were used for thermodynamic reaction path modeling using the software CHILLER. This fluid was subjected to boiling, mixing with sedimentary formation water, cooling, and fluid-rock reaction. During boiling, quartz, pyrite, gold, and K-feldspar formed; mixing resulted in the deposition of pyrite, gold, and mica; reaction with diorite produced the observed wall-rock alteration assemblage; and cooling formed quartz plus minor mica, pyrite, gold, and kaolinite. Observations, analytical data, and modeling results suggest that more than one process was involved in stage II vein formation in different locations and at different times. Boiling, mixing, and fluid-rock reaction, all accompanied by cooling, occurred intermittently over the entire depth extent of the North zone, and the size of Porgera may be a result of all of these processes occurring at the deposit.

The Gandy and Abolhassani Epithermal Prospects in the Alborz Magmatic Arc, Semnan Province, Northern Iran

The Gandy and Abolhassani epithermal precious and base metal deposits occur in the Torud-Chah Shirin mountain range in the Alborz magmatic belt of northern Iran. The mountain range is considered to be part of the Paleogene Alborz volcanic arc. The exposed rocks in the study area consist of a volcaniclastic sequence of thin-bedded siltstones and sandstones, lapilli tuffs, volcanic breccias, and intermediate lava flows at Gandy, and mostly andesitic flows at Abolhassani. The flows are middle to upper middle Eocene age and they show a typical arc geochemical signature, with low concentrations of Nb, Ta, Zr, Hf, and Ti. Variable hydrothermal alteration occurs in scattered outcrops, covering about 4 km^2 at Gandy and 1 km^2 at Abolhassani. The Gandy and Abolhassani areas are about 3 km apart, and each contains a small abandoned Pb-Zn mine. Mineralization at Gandy occurs in quartz sulfide veins and breccias and is accompanied by alteration halos of quartz, illite, and calcite up to 2 m wide. The mineralization is divided into three main stages: brecciation (I), fracture filling (II), and crustiform banding (III). Stage I is economically important in terms of precious metal content. Stage II consists of four substages and contains the majority of base metal ore with quartz, calcite, and barite. Native gold is commonly found within partially oxidized pyrite and secondary iron (hydr)oxides such as goethite in stage I and coexists with galena and chalcopyrite in stage II. The final stage is dominated by quartz and calcite. Mineralization in the Abolhassani veins occurred in three main stages. The first two stages, which are economically important, contain similar mineral assemblages, including quartz, calcite, barite, galena, sphalerite, pyrite, and chalcopyrite, whereas the final stage is dominated by quartz and calcite. No gold grains were found in the Abolhassani samples. The average (max) assays from 14 channel samples of Gandy veins are 14.5 (68.3) g/t Au, 30.6 (161) g/t Ag, 3.1 (13) wt percent Pb, 0.84 (3.8) wt percent Zn, and 1.0 (6.3) wt percent Cu. For comparison, the values from 19 channel samples of Abolhassani veins are 0.85 (6.0) g/t Au, 29.5 (115) g/t Ag, 6.4 (16.5) wt percent Pb, 1.2 (5.2) wt percent Zn, and 0.83 (7.7) wt percent Cu. Fluid inclusion and sulfur isotope compositions were analyzed for the sulfide-sulfate assemblages of stage II at Gandy and stages I and II at Abolhassani. In both cases fluid inclusion assemblages were examined mostly in subhedral crystals of sphalerite. The average homogenization temperatures (T_h) and salinities of fluid inclusion assemblages from Gandy range from 234° to 285°C, with a peak at about 250°C and 4.2 to 5.4 wt percent NaCl equiv. These T_h values are in good agreement with isotopic temperatures from two sphalerite and galena pairs (236° and 245°C). The temperature and salinity values in fluid inclusion assemblages from the Abolhassani deposit range from 234° to 340°C and 6.7 to 18.7 wt percent NaCl equiv. Sulfide pairs of sphalerite-galena do not give reasonable isotopic equibrium temperatures at Abolhassani. Comparison of Th versus ice melting (T_(m(ice))) values for the two deposits indicates the presence of a moderate-salinity fluid (5–6 wt % NaCl equiv) of similar temperature (~250°C) in each deposit but with a higher temperature and salinity component also present at Abolhassani. The base metal-rich mineralization at Abolhassani may thus have been caused by the periodic injection of this higher salinity fluid. The Abolhassani deposit has a higher average Ag/Au ratio (~35) and Pb + Zn concentration (up to 7.6 wt %) than Gandy (Ag/Au ~ 2 and 3.9 wt %), consistent with this interpretation. The minimum depth of formation was at least 430 m below the paleowater table for Gandy and possibly as much as 600 m at Abolhassani. The lower grades of gold and the presence of higher salinity fluids at Abolhassani suggest that the occurrence of higher grade precious metal zones is unlikely at greater depth. By contrast, Gandy may have potential at depth for extensions of the high-grade gold veins. Exploration in the region should focus on areas with geologic evidence for relatively little posthydrothermal erosion, <200 to 300 m, thus increasing the preservation potential of epithermal veins with high gold grades, similar to those at Gandy. Results from Gandy and Abolhassani may aid exploration and assessment of the numerous, untested epithermal and related prospects along this 1,800-km-long volcanic belt in northern Iran.

FLUID INCLUSION AND ISOTOPE EVIDENCE FOR THE ORIGIN OF THE UPTON Ba-Zn-Pb DEPOSIT, QUEBEC APPALACHIANS, CANADA

The Upton Ba-Zn-Pb deposit is hosted by the Early Ordovician crinoidal limestone of the Upton Group in the southern Quebec Appalachians. Mineralization consists of barite and minor amounts of sphalerite, pyrite, galena, and chalcopyrite. The paragenesis includes (1) development of secondary porosity and precipitation of subhedral barite, bladed barite, and barite rosettes; (2) precipitation of fracture-filling barite; (3) filling of fractures and remaining voids in the limestone by a sparry calcite; and (4) precipitation of sulfides, quartz, organic matter, and bitumen in the fractures and along the stylolites. Fluid inclusions were studied in pre-barite syntaxial calcite overgrowths, in subhedral and bladed barite, and in post-barite calcite, sphalerite, and quartz. The homogenization temperatures and salinities of the fluid inclusions are greater than 50°C (single-phase liquid inclusions) and 3.9 to 6.9 wt percent NaCl equiv for syntaxial calcite cement, 42.1° to 96.9°C and 4.7 to 8.1 wt percent NaCl equiv for subhedral and bladed barite, 101.4° to 142°C and 5.1 to 7.5 wt percent NaCl equiv for fracture-filling barite, 82.4° to 132.2°C and 18.3 to 19.7 wt percent NaCl equiv for sphalerite, and 121.8o to 164.2oC and 1.7 to 4.2 wt percent NaCl equiv for quartz. Bitumen is present throughout the paragenesis, but primary CH4 inclusions only occur in quartz. H2S- and CH4-enriched gases were detected in barite and calcite by quadrupole mass spectrometry (QMS) analysis, occurring as secondary fluid inclusions. The 87 Sr/ 86 Sr ratios of the barite range from 0.70654 to 0.70781, lower than Early Ordovician seawater. Based on these results and previous C-O-S isotope data, we propose that the Upton Ba-Zn-Pb deposit was formed by mixing of two fluids, a marine water-dominated, SO 2‐ 4 rich fluid, and an 18 O-enriched and 87 Sr-depleted basinal brine, which carried Ba, Zn, and Pb. The reduced sulfur required for the precipitation of sulfides was derived from the thermochemical reduction of sulfate by hydrocarbons. The late precipitation of sulfides after barite may be related to the sluggishness of the sulfate reduction. The mineralization occurred during the Middle to Late Ordovician Taconian orogeny, and tectonic burial and compression is thought to have initiated the circulation of fluids, which would have been channeled through the sedimentary pile along thrust faults and vertical fractures and migrated into the confined crinoidal limestone.

Fluid Characteristics and Genesis of Early Neoproterozoic Orogenic Gold-Quartz Veins in the Harnas Area, Southwestern Sweden

Gold-quartz veins occurring in the Mjosa-Vanern ore district, southeast Norway and southwest Sweden, represent early Neoproterozoic members of the orogenic gold type of deposit. The Harnas gold-quartz veins, in the central part of the ore district, are steeply dipping veins hosted in a local, west-northwest–east-southeast–trending brittle shear zone, which transects the north-south–trending deformational fabric in the surrounding greenschist grade orthogneisses. This deformation and subsequent vein formation occurred at around 1.0 Ga in a late phase of the Sveconorwegian (Grenvillian) orogeny. Fluid inclusions show that the ore-bearing vein system at Harnas developed essentially in three successive stages: a quartz stage at a depth of ≈ 4 km, a pyrite-gold stage at a shallower crustal level (≈1.5 km) after rapid exhumation of the area, and finally a galena stage. All stages involved fracturing subparallel to the strike of the host shear zone. During the first two stages, the ore fluid was an aqueous H 2 O-CO 2 fluid with a salinity of 4 to 10 wt percent NaCl equiv and a temperature of ≈200°C, whereas in the galena stage it was a purely aqueous fluid with a similar salinity and a temperature of ≈150°C. Oxygen and sulfur isotope results imply a predominantly metamorphic origin for the ore fluid and suggest that important ore constituents, such as lead and sulfur, were derived from the regional orthogneisses. Other gold-anomalous quartz veins in the Harnas area, as well as the Brustad gold-quartz vein in the northernmost part of the Mjosa-Vanern ore district, show some variation in fluid composition. However, aqueous fluid inclusions containing CO 2 and calcite were identified in all veins. This, and other similarities, strongly suggests that the veins throughout the district were formed contemporaneously and were controlled by deformation that, at least in part, affected the entire Mjosa-Vanern region. It is inferred from geologic evidence and pressure estimates that veins began to form during the final phase of Sveconorwegian continent-continent collision and were completed during incipient rapid exhumation of the thickened crust. A set of barren quartz-calcite veinlets, which crosscut the ore-bearing veins at Harnas, is unrelated to the ore formation. These veinlets were deposited from a surface-derived, low-temperature, saline aqueous fluid during some significantly later, but regionally extensive, hydrothermal event.

Mineralization and Fluid Inclusion Study of the Shizhuyuan W-Sn-Bi-Mo-F Skarn Deposit, Hunan Province, China

The Shizhuyuan deposit, China, is a world-class W-Sn-Bi-Mo-F skarn deposit hosted by Devonian limestone in the thermal aureole of the Qianlishan granite complex. The Qianlishan complex comprises five separate intrusions, including fine-grained porphyritic biotite granite (182–187 Ma, granite 1), medium-grained biotite-K feldspar granite (158–163 Ma, granite 2), fine-grained biotite and K feldspar granite (granite 3), granitic porphyry (144–146 Ma, granite 4), and diabase (142 Ma). The upper part of the granite complex is characterized by extensive greisen alteration. Skarn zones distributed around the intrusions are mainly calcic, consisting of garnet, garnet-pyroxene, vesuvianite-garnet, and wollastonite-vesuvianite, progressively outward from the intrusion. Following the primary skarn formation, some of the skarn zones underwent retrograde alteration. The high garnet/pyroxene ratio and the diopside-rich and andradite-rich compositions of the pyroxene and garnet indicate that the skarn belongs to the oxidized type. Mineralization consists of Sn-Be veinlet ore (type I) in marble and porphyry, massive W-Bi-Mo-Sn skarn ore (type II), stockwork W-Sn-Bi-Mo-F ore (type III), and W-Sn-Mo-Bi greisen ore (type IV), mainly associated with granite 2. Emplacement of granite 2 was accompanied by late, intense fracturing characterized by stockwork mineralization (type III), which was superimposed on massive skarn and greisen zones. The stockwork ore consists mainly of greisen and skarn veins and veinlets with scheelite, wolframite, molybdenite, cassiterite, bismuthinite, and fluorite. The Sm-Nd method has been used to analyze the Sm-Nd concentration, and data for pyroxene and garnet in the massive skarn (type II ore) associated with granite 2 yields an isochron age of 157 Ma. This age corresponds closely to the age of granite 2. Fluid inclusion studies of four types of ore samples reveal four types of inclusions: aqueous two-phase inclusions, H2O-CO2 inclusions, gas-rich inclusions, and daughter mineral-bearing inclusions containing halite or calcite. Homogenization temperatures of fluid inclusions in skarn minerals range from 350° to 535°C. The homogenization temperatures of fluid inclusions in greisen and stockwork are lower than that in skarn, and range from 200° to 360°C. The fluid inclusion data indicate that there are two types of fluids associated with the massive skarn and greisen, having salinities from 26 to 41 wt percent NaCl equiv, and from 1 to 21 wt percent NaCl equiv, respectively. The high-salinity inclusions are daughter mineral bearing, whereas the low-salinity inclusions are dominantly aqueous with a few gas-rich inclusions. The results suggest the evolution of the ore-forming fluids either by immiscibility or by mixing between a high-temperature, high-salinity magmatic water and a low-temperature, low-salinity fluid such as meteoric water, consistent with previously published isotopic studies. The Shizhuyuan deposit is a multi-element skarn, highly enriched in W, Sn, Bi, Mo, and F. This is interpreted to reflect multiple sources and stages of mineralization, which, in turn, may account for the large size of the Shizhuyuan deposit.

Geochemistry of the Fluids Related to Epigenetic Carbonate-Hosted Zn-Pb Deposits in the Maestrat Basin, Eastern Spain: Fluid Inclusion and Isotope (Cl, C, O, S, Sr) Evidence

Epigenetic Zn-Pb deposits in the Maestrat basin (eastern Spain) are hosted by dolostones that replace Aptian limestones. The deposits have morphologies ranging from mineralized breccias to stratiform, massive replacements. Sphalerite and galena cementing breccias precipitated as well-formed crystals; whereas, the stratiform replacements are characterized by textures such as colloform and skeletal or dendritic growths. Two mineralized areas are found: Valdelinares and Mas de la Mina-Cedraman. The highest ore grades (up to 11% Zn) and largest deposits (ca. 30,000 t of sulfides) occur as stratiform deposits in the Valdelinares area (Resurreccion and Restauracion mines). In the Valdelinares area, microthermometry of fluid inclusions reveals that ore-stage minerals precipitated by the mixing of two distinct fluids: fluid A, a high salinity brine, with salinity greater than 26 wt percent NaCl equiv and temperatures above120oC, and fluid B, a low salinity fluid, with less than 15 wt % NaCl equiv and temperatures around 40oC. Fluid A-dominated brines are found in mineralized breccias located in the vicinity of major fracture zones; whereas, more dilute fluids are typical of massive ores. In the Mas de la Mina area, the presence of two fluids is less evident, although the ranges in both temperature and salinity also point to mixing. Na-K-Li-Cl-Br systematics of the inclusion fluids indicates that fluids A and B were evaporated seawater that did not significantly interact with K- and Li-rich sediments. The most diluted mixtures (fluid B) are characterized by relatively heavy Cl (δ 37 Cl = 1.11 and 1.19‰) compared to fluid-A dominated brines, which have δ 37 Cl values close to seawater (0‰). The sulfur isotope compositions range from ‐10.5 to 13.4 per mil for sulfides and between 16 and 17.8 per mil for barite. Such values are consistent with mixing of a metal-rich brine with small amounts of 32 S-enriched sulfur and a sulfur-rich fluid with a δ 34 S close to the marine sulfate of Late Cretaceous to early Paleocene age. The C and O isotope compositions of ore-stage calcite (δ 13 C from 0.6 to ‐7.8‰; δ 18 O from 17.2 to 20.9‰) are also explained by the precipitation from different mixtures of fluids A and B. From the geochemical data, it is inferred that fluid mixing played a key role in the formation of the Zn-Pb deposits in the Maestrat basin. Ore precipitation occurred in places where brines ascending through faults interacted with a dilute, sulfur-rich fluid. Deposits with the highest ore grade and size are related to areas where the most extensive mixing occurred.

OVACIK GOLD DEPOSIT: AN EXAMPLE OF QUARTZ-ADULARIA–TYPE GOLD MINERALIZATION IN TURKEY

The Ovac1k gold deposit is located 100 km north of Izmir in western Turkey. It lies adjacent to the east-northeast–trending Bergama graben, and it consists of a series of high-grade gold-bearing epithermal quartz veins hosted by subaerial andesitic-dacitic lava dome facies of lower Miocene age. The region is underlain by Paleozoic metamorphic rocks and limestone, which are cut by medium- to high-level intrusions and overlain by subaqueous and subaerial andesitic to dacitic lava domes that host the Ovac1k gold deposit. Middle to late Miocene extensional tectonic activity was responsible for the formation of north-northeast–south-southwest to northeast-southwest–trending grabens. The extensional activity was accompanied by normal faulting with a later, variable sinistral strike-slip component oriented east-west and northwest-southeast. It is probable that these faults were critical in controlling the development of epithermal quartz veins, both mineralized and unmineralized. Two of four nearly east-west–trending epithermal veins at Ovac1k contain significant Au mineralized areas and display typical low-temperature epithermal textures, including crustiform banding, quartz pseudomorphs after bladed calcite, and multiphase hydrothermal breccias. Veins crop out throughout a maximum strike length of 400 m, and surface widths are up to 35 m. Mineralized areas extend down dip for at least 200 m. Gold appears to be related mainly to earlier-formed quartz-adularia veins and breccias. The gold occurs mainly in clasts of earlier vein material that has been transported from deeper levels in the vein system. Little gold is contained in the matrix of the breccias. The alteration associated with mineralization consists of secondary albite or adularia together with quartz and clays, such as smectite, interlayered illite/smectite and illite/chlorite, illite, and chlorite. Metallic minerals occur generally only in trace amounts and include chalcopyrite, pyrite, arsenopyrite, sphalerite, bornite, chalcocite, covellite, galena, and electrum. The mineralizing fluids had temperatures ranging from 150° to 250°C and salinities between 7.0 and 8.0 wt percent NaCl equiv. The veins have a distinct trace element signature, including high Au, Ag, As, and Sb. High Au and Ag with low As and Sb contents are found in the quartz-adularia veins, whereas low Au and Ag with high As and Sb are found in the silicified hanging-wall and footwall zones. Drilling has confirmed continuity of mineralized rock to a vertical depth of 175 m. To date a resource of 2,980,000 tonnes at 9.0 g/t Au containing 862,000 ounces has been delineated. Other deposits of this type should be sought where late Oligocene to early Miocene volcano-plutonic rocks are cut by major northeast-southeast–trending grabens with associated northwest-southeast– to east-west–oriented normal to sinistral strike-slip faults. Volcanic rocks with strong adularia-sericite alteration and silicification, accompanied by quartz veins with epithermal textures are primary targets.

Genesis of the Giant, Bonanza San Rafael Lode Tin Deposit, Peru: Origin and Significance of Pervasive Alteration

The San Rafael Sn-Cu deposit, located at lat 14°13'58 S, long 70°19'18 W, on the upper slopes of the Cordillera de Carabaya, northern Puno Department, southeastern Peru, yielded 38,182 t of fine tin in 2001 and, with reserves of 14.460 Mt at 5.14 percent Sn, the main San Rafael Lode constitutes, both at present and historically, the largest-known bonanza-grade cassiterite repository. Sn and subordinate Cu mineralization were confined to laterally and vertically extensive brittle shear zones generated by regional tectonism that transect a small 24.65 ± 0.20 Ma, epizonal stock of strongly peraluminous, Lachlan S-type cordierite-biotite monzogranite and granodiorite. Early, barren, stage I quartz-tourmaline veins and breccias were emplaced at 24.10 ± 0.15 Ma by high-temperature (<580°C), boiling, saline fluids that plausibly directly exsolved from the granitic melt, whereas both cassiterite (stage II) and, at higher elevations, chalcopyrite (stage III) ores were precipitated from largely nonboiling, cooler (Th = 215°–420°C), and less saline (0–20 wt % NaCl equiv) fluids at 21.9 to 22.7 ± circa 0.5 Ma. Only narrow (<2 m) envelopes of chlorite and weaker sericite and silica are associated with stages II and III veins, but more than 80 percent of the upper 750 m of the San Rafael stock experienced quasipervasive hydrothermal alteration. Early K metasomatism is recorded by widespread replacement of magmatic plagioclase and perthitic alkali feldspar by Na-free orthoclase, while broadly coeval alteration to albite more erratically affected groundmass plagioclase and the rims of alkali feldspar phenocrysts. Despite the intensity of the alkali metasomatism, only minor redistribution of dispersed, magmatic Sn occurred, largely during alteration to orthoclase. Other petrographically similar granitic bodies in the region experienced more intense Na metasomatism, and they host argentian base metal rather than Sn-dominated veins. Superimposed hydrolytic alteration, most intense within 20 to 30 m of the San Rafael Lodes, converted biotite to Fe-rich chlorite and converted alkali feldspar and plagioclase to fine-grained muscovite, but there is no significant dispersed cassiterite or chalcopyrite. Secondary fluid inclusions trapped in quartz phenocrysts of the granitic rocks range in salinity from 0 to 65 wt percent NaCl equiv and in Th from 200° to 530°C; fluid boiling is evident in the higher-temperature populations. First-melting temperatures and decrepitate analyses reveal, in addition to NaCl, widely variable concentrations of KCl, CaCl2 (attaining 40 wt % of the solute) and, apparently most abundant in proximity to the lodes, FeCl2. Entrapment pressures are estimated to have fluctuated in the range 150 to 615 bars for much of the hydrothermal history, but fluid overpressuring to at least 2 kbar occurred, probably cyclically, during the evolution of the hydrothermal system. The salinity vs. homogenization temperature range for the phenocryst-hosted inclusions closely matches that of primary inclusions in quartz and cassiterite in the lodes and, remarkably, small clusters of the former record the entire thermal and compositional trajectory of the mineralizing fluids. Early magmatic brines are inferred to have permeated much of the stock whereas, circa 2 m.y. later, Sn- and Cu-rich, lower-temperature, lower-pH fluids were largely channeled along the evolving shear zones where they mixed with cool, nonsaline, tectonically driven ground water. In conjunction with fluid neutralization through hydrolytic alteration, this channeling resulted in catastrophic precipitation of botryoidal and coarsely crystalline cassiterite and, subsequently, chalcopyrite. The hiatus between initial retrograde boiling and ore deposition, a feature documented in several world-class, high-grade lithophile-element deposits, may record protracted storage of metal-rich brines at depth, perhaps in association with small volumes of highly fractionated melt, such as is represented by small bodies of tourmaline leucogranite exposed on the upper margin of the San Rafael stock. However, the parallel evolution in temperature and salinity exhibited by the early nonmineralizing and later fertile hydrothermal fluids may also indicate that Sn and Cu were largely introduced into the subjacent magma chamber at a late stage.