Interaction Of Magmatic Fluids Research Articles

Eocene magmatism and associated Fe-Cu mineralization in northeastern Turkey: a case study of the Karadağ skarn

ABSTRACT The metallogenic belt containing numerous skarn mineralized systems starting from Eastern Europe continues through Turkey and Iran. The study area in the Eastern Pontides is within this metallogenic belt. The 44.39 ± 0.53 Ma Karadağ Intrusion sourced skarn mineralization is located along the granitoid-to-limestone contact. The Karadağ Intrusion is an arc-like pluton with medium–high K calc-alkaline and peraluminous–metaluminous affinity. The Karadağ Fe–Cu skarn mineralization (Gümüşhane, Turkey), an exoskarn, is hosted by Late Cretaceous limestone and consists of early garnet, clinopyroxene, and orthoclase with late epidote and chlorite. Primary ore minerals are magnetite and haematite, accompanied by minor amounts of chalcopyrite and pyrite. Pyroxene comprises mostly diopside and less hedenbergitic diopside and johannsenite. Wollastonite is very rare. Garnet is andradite and grossular–andradite. Thus, all these results imply that the Karadağ Fe–Cu skarn mineralization formed under relatively oxidized and Fe-rich conditions. The homogenization temperatures of the fluid inclusions vary from 189°C to 405°C for quartz and 186°C to 430°C for calcite. The salinity contents in the fluid inclusions are within 0.2–9 wt% NaCl equivalent for quartz and 0.4–14.8 wt% NaCl equivalent for calcite. First melting temperatures between −36.5 and −47.4°C were attained with the presence of NaCl–FeCl2± CaCl2 in the initial phase of fluid inclusions. The first melting in the later stage of fluid inclusions is between −3.8 and −17.2°C, thereby, supporting the existence of NaHCO3-Na2CO3-KCl-type salt composition. The δ18O values of minerals are 1.7–10.3‰ for garnet, 11.1–14.6‰ for quartz, 6.4 to 6.8‰ for epidote, and 2.8–3.8‰ for magnetite in the Karadağ skarn mineralization. The δ18O value of epidote indicates magmatic fluid, whereas those of garnet, magnetite, and quartz suggest magmatic fluids together with less meteoric effect. The δ18O values in limestone (18.7 to 22.3‰), marble (8.7‰), and calcite (10.5 to 12.2‰) were depleted relative to typical δ18O contents of marine carbonates (20 to 30‰). Moreover, the C isotope values of calcite (−5.4 to −1.4‰), marble (−2.1‰), and limestone (1.0 to 1.2‰) reflect those of metamorphic rocks and skarns (−4.0 to 3‰ and −18 to 2‰). Mineral paragenesis, fluid inclusion, and stable isotopic signatures are consistent with ore mineralization caused by the interaction of magmatic fluids with carbonate host rocks. Karadağ Fe-Cu skarn mineralization, associated with Karadağ postcollisional magmatism, is an example of skarn mineralized system in the Eastern Pontides.

The Role of Magmatic and Hydrothermal Fluids in the Formation of the Sasa Pb-Zn-Ag Skarn Deposit, Republic of Macedonia

The Sasa Pb-Zn-Ag deposit belongs to the group of distal base metal skarn deposits. The deposit is located within the Serbo-Macedonian massif, a metamorphosed crystalline terrain of Precambrian to Paleozoic age. The mineralization, hosted by Paleozoic marbles, shows a strong lithological control. It is spatially and temporally associated with the calc-alkaline to shoshonitic post-collisional magmatism that affected the Balkan Peninsula during the Oligocene–Miocene time period and resulted in the formation of numerous magmatic–hydrothermal ore deposits. The mineralization at the Sasa Pb-Zn-Ag deposit shows many distinctive features typical for base metal skarn deposits including: (1) a carbonate lithology as the main immediate host of the mineralization; (2) a close spatial relation between the mineralization and magmatic bodies of an intermediate composition; (3) a presence of the prograde anhydrous Ca-Fe-Mg-Mn-silicate and the retrograde hydrous Ca-Fe-Mg-Mn ± Al-silicate mineral assemblages; (4) a deposition of base metal sulfides, predominately galena and sphalerite, during the hydrothermal stage; and (5) a post-ore stage characterized by the deposition of a large quantity of carbonates. The relatively simple, pyroxene-dominated, prograde mineralization at the Sasa Pb-Zn-Ag skarn deposit represents a product of the infiltration-driven metasomatism which resulted from an interaction of magmatic fluids with the host marble. The prograde stage occurred under conditions of a low water activity, low oxygen, sulfur and CO2 fugacities and a high K+/H+ molar ratio. The minimum pressure–temperature (P–T) conditions were estimated at 30 MPa and 405 °C. Mineralizing fluids were moderately saline and low density Ca-Na-chloride bearing aqueous solutions. The transition from the prograde to the retrograde stage was triggered by cooling of the system below 400 °C and the resulting ductile-to-brittle transition. The brittle conditions promoted reactivation of old (pre-Tertiary) faults and allowed progressive infiltration of ground waters and therefore increased the water activity and oxygen fugacity. At the same time, the lithostatic to hydrostatic transition decreased the pressure and enabled a more efficient degassing of magmatic volatiles. The progressive contribution of magmatic CO2 has been recognized from the retrograde mineral paragenesis as well as from the isotopic composition of associated carbonates. The retrograde mineral assemblages, represented by amphiboles, epidote, chlorites, magnetite, pyrrhotite, quartz and carbonates, reflect conditions of high water activity, high oxygen and CO2 fugacities, a gradual increase in the sulfur fugacity and a low K+/H+ molar ratio. Infiltration fluids carried MgCl2 and had a slightly higher salinity compared to the prograde fluids. The maximum formation conditions for the retrograde stage are set at 375 °C and 200 MPa. The deposition of ore minerals, predominantly galena and sphalerite, occurred during the hydrothermal phase under a diminishing influence of magmatic CO2. The mixing of ore-bearing, Mg-Na-chloride or Fe2+-chloride, aqueous solutions with cold and diluted ground waters is the most plausible reason for the destabilization of metal–chloride complexes. However, neutralization of relatively acidic ore-bearing fluids during the interaction with the host lithology could have significantly contributed to the deposition. The post-ore, carbonate-dominated mineralization was deposited from diluted Ca-Na-Cl-bearing fluids of a near-neutral pH composition. The corresponding depositional temperature is estimated at below 300 °C.

Geology, mineralogy and evolution of iron skarn deposits in the Zanjan district, NW Iran: Constraints from U-Pb dating, Hf and O isotope analyses of zircons and stable isotope geochemistry

The Zanjan iron skarn deposits are located in the northwest Central Iranian terrane, along the Soltaniyeh mountains. The deposits are hosted by late Proterozoic to late Cambrian carbonate rocks (Soltaniyeh, and Mila Formations) and by Permian (Ruteh Formations), intruded by the Arjin granite, the Khakriz granodiorite and the Inche-Rahbari quartzmonzonite (53±1Ma to 55±0.4Ma, early Eocene). These granitoids show high-K calc-alkaline, metaluminous affinity and have similar geochemical features, including strong enrichment of LILEs and LREEs, and depletion in high field strength elements (HFSEs) and HREE. The Arjin, Khakriz and Inche-Rahbari granitoid samples show zircon εHf(t) values of −0.7 to +5.8 (TDM2=1052–1640Ma) and δ18O values of +5.55‰ to +7.42‰ suggesting partial melting of older crustal material then mixing with juvenile mantle-derived melts. The thermal anomaly induced by underplating mafic melts resulted in partial melting of continental crust and formation of Arjin, Khakriz and Inche-Rahbari magmas emplaced during several magmatic pulses at 53–55Ma.The Zanjan iron skarn deposits are magnesian (Arjin and Inche-Rahbari) and calcic (Khakriz) with endoskarn and exoskarn facies. In these skarn facies forsterite, garnet and pyroxene comprise the main prograde skarn minerals, whereas magnetite, serpentine, amphiboles of the tremolite-ferroactinolite series (sometimes asbestiform), epidote, talc and calcite are the principal retrograde skarn minerals. Iron exoskarns formed at the contacts of the intrusions parallel to the fabric of the host rocks and in networks of crosscutting veins and veinlets. The assemblage of ore minerals consists of magnetite, hematite (specularite), pyrite, chalcopyrite and bornite; in addition, minor galena and sphalerite occur in the Khakriz deposit. The sulfur isotopic compositions in the Arjin deposit range from +15.99 to +18.35‰ and in the Inche-Rahbari and Khakriz deposits from +2.96 to 5.30‰. The oxygen and carbon isotope ratios of the fresh or partly altered limestones from Soltaniyeh and Mila Formations reveal δ13C values of −0.64 to +0.03‰ and δ18O composition of +13.12 to +24.41‰ typical of marine carbonate. The isotopic compositions of the hydrothermal calcite (skarn calcite) range from −5.32 to −2.01‰ for carbon isotope and 3.29 to 20.67‰ for oxygen isotope. Complex water-rock interaction between the ore-bearing magmatic fluids and sedimentary wall rocks was responsible for sulfur, oxygen and carbon mixing. Field observations, mineral paragenesis, and stable isotopic signatures in these deposits are consistent with ore mineralization formed by the interaction of magmatic fluids with both cooler meteoric water and carbonate host rocks.

Involvement of magmatic fluids at the Laloki and Federal Flag massive sulfide Cu–Zn–Au–Ag deposits, Astrolabe mineral district, Papua New Guinea: sulfur isotope evidence

We present the first sulfur (S) isotope data of sulfides, sulfates, pyrite in host mudstone, and bulk sulfur of gabbroic rocks from the Laloki and Federal Flag massive Cu–Zn–Au–Ag deposits in the Astrolabe mineral district, Papua New Guinea. Early-stage pyrite–marcasite, chalcopyrite, and sphalerite from Laloki display wide range of δ34S values from −4.5 to +7.0 ‰ (n = 16). Late-stage pyrite, chalcopyrite, and sphalerite have restricted δ34S values of −1.9 to +4.7 ‰ (n = 16). The mineralizing stage these correspond to had moderately saline (5.9–8.4 NaCl eq. wt%) mineralizing fluids of possible magmatic origin. A single analysis of late-stage barite has a value of δ34S +17.9 ‰, which is likely similar to coexisting seawater sulfate. Pyrite from the foot-wall mudstone at Laloki has very light δ34S values of −36.1 to −33.8 ‰ (n = 2), which suggest an organic source for S. Pyrite–marcasite and chalcopyrite from Federal Flag show δ34S values of −2.4 to −1.9 ‰ (n = 2), consistent with a magmatic origin, either leached from intrusive magmatic rocks or derived from magmatic–hydrothermal fluids. The very narrow range and near-zero δ34S values (−1.0 to +0.6 ‰) of bulk gabbroic samples is consistent with mantle-derived magmatic S. Sulfur isotope characteristics of sulfides and sulfates are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and sedimented mid-ocean ridges. The most reasonable interpretation is that the range of the sulfide and sulfate δ34S values from both Laloki and Federal Flag massive sulfide deposits is indicative of the complex interaction of magmatic fluids, seawater, gabbroic rocks, and mudstone.

Geochemical Constraints on the Genesis of the Scheelite Dome Intrusion-Related Gold Deposit, Tombstone Gold Belt, Yukon, Canada

The Scheelite dome intrusion-related gold deposit, western Selwyn basin, Yukon, is hosted in hornfelsed metasedimentary strata that lie adjacent to the exposed apices of a monzogranite to quartz monzonite plutonic complex of the mid-Cretaceous Tombstone-Tungsten magmatic belt, Tintina gold province, Alaska and Yukon. A variety of mineralization styles occur throughout a 10- × 3-km east-trending corridor and include reduced Au- and W-rich skarns, Au-, W- and Ag-Pb-Zn-Sb–rich quartz tension-vein arrays, and multiphase fault veins and isolated zones of Au-rich sericite-carbonate altered rock. Integrated U-Pb SHRIMP data for magmatic zircon and Ar-Ar data for magmatic and hydrothermal biotite indicate that gold mineralization occurred within 1 to 2 m.y. of magma emplacement. Fluid inclusion, oxygen isotope, and arsenopyrite geothermometry data indicate that hydrothermal minerals formed at depths of 6 to 9 km over a temperature range from 550°C. High-temperature Au-rich skarns formed at >400°C, whereas vein-hosted mineralization formed at 280° to 380°C. In skarns, Au is strongly associated with enrichments of Bi, Te, W, and As, whereas a variety of Au-rich veins occur, with As-rich (type 1), and Te- and W-rich (type 2) end members. Silver-Pb-Zn-Sb veins are typically Au poor and represent the latest and lowest temperature phase in the hydrothermal paragenesis. The fluid inclusion data indicate that all mineralization styles were formed from low-salinity (≤4 wt % NaCl equiv) aqueous-carbonic fluids, consistent with the composition of fluid inclusions within infilled miarolitic cavities in the intrusive rocks. However, the nonaqueous fluid was predominantly CH 4 in skarn, CO 2 in Au-Te and Au-W veins, and a fluid with roughly equal amounts of CO 2 , CH 4 , and N 2 in Au-As and Ag-Pb-Zn-Sb veins. Oxygen isotope data are consistent with a mineralizing fluid of predominantly magmatic origin that was variably modified to more positive δ 18 O values during interaction with 18 O-enriched metasedimentary strata. Sulfur isotope data suggest two possible sources of sulfur, a magmatic source characterized by δ 34 S values of approximately −5 to 0 per mil and sulfur from the metasedimentary country rocks characterized by more negative δ 34 S values of approximately −15 to −10 per mil. Collectively the data indicate that gold at Scheelite Dome was deposited from a magmatic-hydrothermal system. Interaction of magmatic fluids with graphitic hornfels rocks resulted in reduction of the ore fluids, higher CH 4 /CO 2 ratios, and modification of the oxygen and sulfur isotope values of the ore fluids toward those of the metasedimentary hornfels. Progressive reduction and cooling of hydrothermal fluids, in addition to phase separation in vein-hosted mineralization, were the mechanisms for gold deposition. Compared to other intrusion-related gold deposits associated with the Tombstone-Tungsten magmatic belt magmatism, exposed mineralization at Scheelite Dome is predominantly hosted by hornfelsed metasedimentary rocks. This results in more diverse mineralization styles and a greater spread of isotope and fluid inclusion data.

8: Precambrian geodynamics and ore formation: The Fennoscandian Shield

Compared with present-day global plate tectonics, Archaean and Palaeoproterozoic plate tectonics may have involved faster moving, hotter plates that accumulated less sediment and contained a thinner section of lithospheric mantle. This scenario also fits with the complex geodynamic evolution of the Fennoscandian Shield from 2.06 to 1.78 Ga when rapid accretion of island arcs and several microcontinent–continent collisions in a complex array of orogens was manifested in short-lived but intense orogenies involving voluminous magmatism. With a few exceptions, all major ore deposits formed in specific tectonic settings between 2.06 and 1.78 Ga and thus a strong geodynamic control on ore deposit formation is suggested. All orogenic gold deposits formed syn- to post-peak metamorphism and their timing reflects the orogenic younging of the shield towards the SW and west. Most orogenic gold deposits formed during periods of crustal shortening with peaks at 2.72 to 2.67, 1.90 to 1.86 and 1.85 to 1.79 Ga. The ca. 2.5 to 2.4 Ga Ni–Cu ± PGE deposits formed both as part of layered igneous complexes and associated with mafic volcanism, in basins formed during rifting of the Archaean craton at ca. 2.5 to 2.4 Ga. Svecokarelian ca. 1.89 to 1.88 Ga Ni–Cu deposits are related to mafic–ultramafic rocks intruded along linear belts at the accretionary margins of microcratons. All major VMS deposits in the Fennoscandian Shield formed between 1.97 and 1.88 Ga, in extensional settings, prior to basin inversion and accretion. The oldest “Cyprus-type” deposits were obducted onto the Archaean continent during the onset of convergence. The Pyhäsalmi VMS deposits formed at 1.93 to 1.91 Ga in primitive, bimodal arc complexes during extension of the arc. In contrast, the Skellefte VMS deposits are 20 to 30 million years younger and formed in a strongly extensional intra-arc region that developed on continental or mature arc crust. Deposits in the Bergslagen–Uusimaa belt are similar in age to the Skellefte deposits and formed in a microcraton that collided with the Karelian craton at ca. 1.88 to 1.87 Ga. The Bergslagen–Uusimaa belt is interpreted as an intra-continental, or continental margin back-arc, extensional region developed on older continental crust. Iron oxide–copper–gold (IOCG) deposits are diverse in style. At least the oldest mineralizing stages, at ca. 1.88 Ga, are coeval with calc-alkaline to monzonitic magmatism and coeval and possibly cogenetic subaerial volcanism more akin to continental arcs or to magmatic arcs inboard of the active subduction zone. Younger mineralization of similar style took place when S-type magmatism occurred at ca. 1.80 to 1.77 Ga during cratonization distal to the active N–S-trending subduction zone in the west. Possibly, interaction of magmatic fluids with evaporitic sequences in older rift sequences was important for ore formation. Finally, the large volumes of anorthositic magmas that characterize the Sveconorwegian Orogeny formed a major concentration of Ti in the SW part of the Sveconorwegian orogenic belt under granulite facies conditions, about 40 million years after the last regional deformation of the Sveconorwegian Orogeny, between ca. 930 and 920 Ma.

Hydrothermal Alteration and Cu‐Au Mineralization at Nena High Sulfidation‐type Deposit, Frieda River, Papua New Guinea

Abstract. The Nena Cu‐Au deposit, located in the Frieda River mineral district of northwestern mainland Papua New Guinea, is a composite structurally‐lithologically controlled high sulfidation (HS) system. Its hydrothermal alteration and Cu‐Au mineralization are presented in this paper. Initially propylitized andesitic volcanics veined by epithermal quartz were pervasively superimposed by zoned HS alteration. The zonation grades from vuggy silica core to sulfur‐rich, pyritic silica‐alunite halo followed by pyrophyllite‐dickite‐kaolinite interval and finally to thin illite‐smectite margin, suggesting progressive decrease in temperature and increase in pH. This zonation is enveloped by chlorite‐epidote‐calcite‐gypsum alteration.The acid altered rocks were then invaded by multiple phases of pyrite, subsequently crosscut by quartz, vein alunite and barite. Then sequential deposition of bladed covellite, enargite, luzonite and stibioluzonite occurred from the NW to the SE portions of the deposit, forming a zonation suggestive of progressive decrease in temperature, sulfur fugacity and sulfidation stage. Most ore mineralization occurs in the vuggy silica core. Gold mineralization commenced from the transition of enargite to luzonite and continued throughout the stibioluzonite stage. Associated with gold deposition are Au‐rich pyrite, tennantite‐tetrahedrite, chalcopyrite‐bornite, native tellurium, electrum, calaverite, bismuthinite and galena. Native sulfur occupied the remaining cavities and represents the waning stage of the hydrothermal system.Fluid inclusions studies distinguished magmatic (>300–350d̀C, 9–15 wt% NaCl equiv.) and meteoric (<150–200d̀C, 1–2 wt% NaCl equiv.) fluids (Holzberger et al., 1996). Temperatures and salinities of fluid inclusions from barite associated with Cu sulfides show a general decrease from NW (330d̀C, 9–15 wt% NaCl equiv.) to SE (172d̀C, 10 wt% NaCl equiv.) parts of the deposit, indicating gradual entrainment of ground water (Hitchman and Espi, 1997). Interaction of magmatic fluids with meteoric water accompanied by changes in temperature, salinity, acidity and oxidation state of the resultant fluids is interpreted to have been the main cause of metal precipitation. Finally, supergene processes generated Au zone with an underlying chalcocite‐covellite‐digenite blanket over the primary sulfides at depth. Gold occurs as lattice constituent in scorodite, limonite‐goethite and jarosite. Chalcocite is more abundant and widespread than other Cu sulfides. Acidic fluids deposited powdery alunite and kaolinite, vein alunite and amorphous silica. Weakly secondary biotite‐quartz altered porphyry located below the known HS Cu‐Au deposit contains chalcopyrite‐bornite and is overprinted by quartz‐alunite‐pyro‐phyllite‐pyrite assemblage. This feature indicates close temporal, spatial and genetic relation between the two deposit types.

Fluid inclusion studies of the Chinkuashih high-sulfidation gold–copper deposits in Taiwan

Epithermal high-sulfidation Au–Cu deposits at Chinkuashih in northeastern Taiwan are found both within Pleistocene calc-alkaline volcanic rocks and the Miocene sedimentary rocks. The deposits consist of gold–copper veins of the Penshan group and breccia pipes of the Tushihshan–Changjen group. Primary two-phase inclusions in hydrothermal quartz and secondary two-phase inclusions in hydrothermal quartz as well as igneous quartz phenocrysts probably represent fluids related to mineralization. These inclusions have homogenization temperatures generally between 190°C and 280°C and salinities mostly from 0.5–5.0 wt.% NaCl eq. In the Penshan group, microthermometric data indicate that variable amounts of boiling, mixing and cooling occurred in the hydrothermal system. Based on the Th of inclusions trapped from boiling fluids and the CO 2 contents of the inclusions, the hydrostatic pressure is estimated at 6.9–7.9 MPa. In the Tushihshan–Changjen group, the microthermometry of sulfur gas-bearing secondary two-phase fluid inclusions from hydrothermal quartz shows bimodal contents for a certain Th value, perhaps due to the intermittent incursion of brine at depth by phreatic or phreato-magmatic explosions. The presence of poly-phase (liquid+vapor+halite±sylvite±opaques) fluid inclusions with high Th (307°C–484°C) and high salinities (30–51 wt.% NaCl eq.) in one sample suggests a dense brine of probable magmatic derivation circulated in the Chinkuashih system. Halite-bearing inclusions which were homogenized by vapor disappearance coexisting with low-salinity vapor-rich inclusions in this sample may have been trapped in an immiscibility field at vapor-pressures from 7.5 to 21.5 MPa. Halite-bearing inclusions which were homogenized by halite dissolution after the vapor disappearance may have been trapped under even greater pressures. Fluid inclusion gas in samples were analyzed with a QMS. The results of bulk analyses indicate that these inclusions contain 99.45–99.67 mol% H 2O, 0.27–0.52 mol% CO 2, 0.015–0.066 mol% N 2, 0.0015–0.015 mol% CH 4 and small amounts of SO 2 in three samples. These data suggest that the inclusion fluids are dominantly meteoric waters that interacted with host rocks and accumulated upward-fluxing magmatic volatiles. The fluid inclusion data indicate that the hydrothermal fluids related to the ore deposition of the high sulfidation Au–Cu deposits at Chinkuashih are characterized by the interaction of magmatic fluids with meteoric water under epithermal conditions.

Geology, geochemistry, and origin of high sulfidation Cu-Au mineralization in the Nansatsu District, Japan

The Nansatsu district of southern Kyushu has been the site of calc-alkaline volcanism for the last 10 m.y., shifting eastward with time. Associated hydrothermal activity followed deposition of the volcanic host rocks by about 0.5 m.y. and was characterized by interaction of magmatic fluids with meteoric water under epithermal conditions, resulting in the formation of high sulfidation Cu-Au deposits at Kasuga, Iwato, and Akeshi. The orebodies consist of 95 wt percent SiO 2 and result from leaching of the original andesite lava and pyroclastic flows by acid chloride-sulfate waters. These are inferred to have formed when magmatic vapors containing HCl and SO 2 condensed into meteoric water. The residual silica (now quartz) orebodies are best developed where the host was initially permeable. The margins of the quartz bodies are abrupt, with narrow (1-2 m) halos representing the reaction front of acid waters isochemically dissolving the host rock. The halo comprises alunite (strongly zoned in Na and K, with P-rich cores), dickite, and/or kaolinite+ or -pyrophyllite, grading out into illite and interlayered illite-smectite clays, and finally, propylitic alteration. This pattern is characteristic of deposits of this type throughout the world, for example, at Summitville, Colorado, and Lepanto, Philippines. Mineralization occurred after initial leaching by the vapor condensates, with metals transported by a dense magmatic fluid. Mixing with meteoric water and the subsequent temperature decrease caused the general decrease in grade toward the margin of the quartz bodies; ore grades are restricted to the quartz bodies. Gold is most closely associated with enargite and pyrite; later minerals include covellite and then sulfur. The last stage of activity was steam-heated, with descending waters oxidizing sulfides to goethite and locally remobilizing Au into fractures (this varies in degree between deposits). Erosion exposed the orebodies to supergene weathering, continuing the sulfide oxidation and Au remobilization. Stable isotope results indicate that hypogene alunite formed from a mixture of magmatic fluid (delta 18 O = 7+ or -2 per thousand, delta D = -25+ or -5 per thousand, similar to nearby active volcanic discharges) with local meteoric water. In contrast, the clays in the marginal halo have isotopic compositions indicating a delta 18 O shift of 6 to 8 per mil from local meteoric water values, probably due to water-rock interaction, and the delta 18 O values of residual silica quartz may also be due to meteoric water domination. Fluid inclusion study of postmineralization quartz crystals indicates that the fluids had a salinity of about 1 wt percent NaCl equiv during late quartz growth, though there is evidence in one sample for higher salinity fluid having been present, up to 30 wt percent NaCl equiv (some inclusions contain daughter minerals of halite and sulfur). The T h values of over 1,000 measurements on late quartz from the ore zones indicate that the mean temperature during that stage ranged from 200 degrees C at Akeshi to about 200 degrees C at Iwato and 230 degrees C at Kasuga. The presence of vapor-rich inclusions, some with T hv similar to T hl , indicate the presence at times of a two-phase fluid in the center of the ore zones, with depths of about 150 to 300 m below the paleowater table. The mineralizing fluid was relatively oxidized (sulfide/sulfate ratio about 3:1), close to pyrite-alunite coexistence. Under these redox conditions, a pH of 3 and over a temperature range of 200 degrees to 300 degrees C, AuCl 2 (super -) complexing may dominate over HAu(SH) 2 at salinities above about 2 wt percent NaCl. Several conditions are conducive for high sulfidation mineralization to occur: (1) a crystallizing magma exsolves a fluid, with lower pressure conditions favoring metal fractionation from melt to fluid, (2) the exsolved fluid separates into vapor and saline liquid phases due to immiscibility, with the latter being metal rich, (3) the gas-rich (HCl and SO 2 +H 2 S) vapor ascends to the surface with at least a portion condensed into meteoric water, forming an acid fluid which leaches the host rock to create permeable zones for later mixing, and (4) the dense, metal-bearing fluid also ascends into this leached zoned and precipitates Cu sulfosalts, sulfides, and Au upon mixing with meteoric water. If the saline liquid is not released from its source adjacent to the magma, due to lack of fracturing, or if there is a strong hydraulic gradient caused by high relief, only the vapor-related stage may occur. This will leave leached barren rock which is characteristic of many eroded volcanic terranes.

Hydrothermal alteration of middle Proterozoic basalts, Stuart Shelf, South Australia; a possible source for Cu mineralization

Geochemical studies on core material from middle Proterozoic rocks in the vicinity of Mount Gunson, South Australia, have identified the causes of their secondary alteration and revealed a possible relationship between hydrothermal alteration of volcanic rocks and base metal mineralization in the Stuart shelf-Adelaide geosyncline copper province. This province includes the sedimentary stratiform deposit at Mount Gunson and the giant granite breccia-and hematite breccia-hosted Olympic Dam deposit.Drill holes have intersected the Pandurra Formation and an unconformably underlying sequence of tuffs, siltstones, volcanic breccias, and felsic, intermediate, and mafic igneous rocks. Metasediments and a basal brecciated granite were also intersected in one drill hole. The chemistry of the mafic and intermediate volcanics is similar to ultrapotassic and potassic rocks associated with continental tensional regimes (e.g., high K and Fe 2 O 3 /FeO ratios, and relatively high Mg, Ba, Cr, Ni, Rb, Zr, Ce, La, and Y contents). Chemical, mineralogical, and textural features indicate that Si, Ca, Fe, Na, K, and Cu were mobile and Mg, Al, P, Pb, and Zn relatively inert. Copper values (mostly <20 ppm) are lower than values typical of basaltic rocks, suggesting that Cu has been removed from the system during the circulation of hydrothermal fluids and alteration of primary Fe-Ti oxide, Cu sulfide, and ferromagnesian silicate minerals.Fluid inclusion studies show the presence of both NaCl-CO 2 -rich and CaCl 2 -rich fluids. The former are in early replacement segregations of quartz, fluorite, and orthoclase, whereas the Ca-rich fluids are associated with a later, but possibly overlapping, hydrothermal event resulting from the interaction of magmatic fluids and fluids released during decarbonation reactions.The delta 13 C values of vein and amygdule-filling carbonate in the igneous rocks indicate a dominance of magmatic fluids, with variable addition and mixing of carbon from either solution and/or reaction of sedimentary carbon. Sulfide mineral delta 34 S values suggest a magmatic sulfur source. The delta 34 S values of pyrite from drill hole PY3 (1.9 + or - 0.7ppm) are within the range for sulfide minerals from mafic and ultramafic igneous rocks.Geochemical trends associated with Cu (U, Pb, Zn) mineralization in South Australian Proterozoic rocks indicate it was derived from a mafic to intermediate igneous source. Hydro-thermally altered Gawler Range volcanic and subvolcanic rocks in the region are likely source rocks for these metals.