K-silicate Alteration Research Articles

A Reevaluation of the Timing and Temperature of Copper and Molybdenum Precipitation in Porphyry Deposits

Abstract The timing and temperature at which copper-iron and molybdenum sulfide deposition occurs in porphyry deposits remain controversial. Petrologic estimates indicate that veins and wall-rock alteration zones containing copper-iron sulfides form in a wide temperature range from ~350° to 650°C. Most sulfides are hosted in potassium(K)-silicate–altered rock and quartz A veins or in early-halo alteration selvages formed above ~450°C. In contrast, cathodoluminescence (CL) imaging of A veins indicates that copper-iron sulfides are contained within a primary lucent (bright and gray)-CL quartz and are crosscut by microfractures filled with younger dull (dark and medium-gray)-CL quartz in direct contact with copper-iron sulfides. These observations have been interpreted as supporting late copper-iron sulfide introduction together with dull-CL quartz at moderate temperatures of ~300° to 450°C, based on fluid inclusion estimates. We provide new CL, QEMSCAN, and petrographic data and images of vein quartz as well as petrologic data of altered wall rock from Haquira East (Peru), Encuentro (Chile), and Batu Hijau (Indonesia) porphyry deposits, which were formed at conditions ranging from deep to shallow (~2–10 km). At all three deposits, dull-CL quartz in microfractures is ubiquitously observed crosscutting all generations of high-temperature lucent-CL quartz veins. Each lucent-CL vein type hosts distinct sulfide populations, crosscuts the others, and coexists in space within the copper and molybdenum ore zones. Within this ore zone, the dull-CL quartz only contains copper-iron sulfides where it transects old A veins and early halos, molybdenite where it transects young molybdenite-bearing quartz veins, and both copper-iron sulfides and molybdenite in younger B veins. Furthermore, where the dull-CL quartz crosscuts igneous or barren (deep) quartz veins, it typically lacks copper and molybdenum. Therefore, dull-CL quartz has no particular spatial or genetic affinity with copper-iron sulfides or molybdenite. We propose that copper was introduced and precipitated at high temperatures in stability with K-silicate alteration. In shallow porphyry deposits, most copper was introduced with lucent-CL quartz in A veins, likely formed via adiabatic decompression from magmatic lithostatic to hydrostatic conditions at ~450° to 600°C. In deep deposits, most copper is introduced with quartz-poor early halos, likely formed at a temperature range similar to that of A veins but during an early stage of retrograde silica solubility. The inferred timing and temperature of copper precipitation are consistent with available solubility experiments for copper-bearing solutions that suggest copper precipitation may start at a high temperature of ~600°C, and ~90% precipitates before it cools down to ~400°C. Much of the molybdenum is introduced and precipitated with discrete pulses of molybdenite-bearing quartz veins that crosscut and postdate copper-bearing A veins and early halos and, to a lesser degree, with B veins that may carry both copper and molybdenum. Whereas molybdenite-bearing and barren (deep) quartz veins form at relatively high temperatures of ~550° to 650°C, copper-molybdenum–bearing B veins likely form at lower temperatures near ~500°C. Copper precipitation and local copper remobilization from older veins and halos continued during the formation of copper-iron sulfide veinlets, named C veins, and during the precipitation of dull-CL quartz following K-silicate alteration. C veins and even younger pyrite-rich D veins may have chlorite or sericite selvages and are composed of dull-CL quartz that formed at ~450° and 300° to 450°C, respectively. Microfractures form through all lucent-CL quartz veins because of the thermal contraction of high-temperature quartz at the onset of sustained cooling after K-silicate alteration has ceased. The fluid that migrated through these microfractures was initially in retrograde silica solubility, which causes dissolution and corrosion of the older lucent-CL quartz. The formation of C veins may overlap in time with the initial stage. At a later stage and temperatures below <450°C, the fluid precipitates dull-CL quartz in microfractures and dissolution zones within older lucent-CL quartz. In copper-iron sulfide-bearing A and B veins and molybdenite-bearing quartz veins, corroded lucent-CL quartz and the younger dull-CL quartz infill can often be observed in contact with older sulfides because quartz sulfide grain boundaries are preexisting discontinuities, and they are preferentially opened during volume contraction. Collectively, these observations and estimates are consistent with silicate phase petrology and numerous observations that most copper-iron sulfides precipitate in K-silicate alteration zones or in early halos with K-feldspar-muscovite-biotite assemblages.

Porphyry copper mineralization triggered by sulfate reduction and alkali metasomatism: Constraints from an experimental investigation

The potassium silicate (K-silicate) alteration zone is the main ore contributor in porphyry copper deposits worldwide. Knowledge of element behaviors in the alteration and mineralization processes is essential for an improved understanding of porphyry copper mineralization, but they are still not well understood. In this study, we reacted synthetic Cl-rich fluids, containing K, Na, Cu, Mo, Zn, etc., with andesite in a complex experimental system to simulate the shallow porphyry copper mineralization process. We aimed to bridge the gap between simple experimental studies and complex natural systems and to evaluate the contribution of sulfate reduction to porphyry ore formation and its relationship with early alkali metasomatism. The results show that increasing temperature (from 300 to 500 °C) enhances the K-silicate alteration by promoting ion-exchange reactions, and the K-feldspar is mainly formed by the transformation of plagioclase via a dissolution-reprecipitation processes. The low-salinity vapor phase has a stronger capacity for K-silicate alteration than the liquid phase at similar temperatures. In addition, increasing temperature from 300 to 500 °C favors sulfate reduction to further enhance metal sulfide precipitation. The limited availability of reduced sulfur in the fluid causes preferential precipitation of Cu-(Mo) sulfides, while most of the Zn is soluble in the fluid, and Cu precipitation as sulfides in the vapor is much more efficient than in the coexisting liquid. The overlap between the K-silicate alteration zone and the mineralization triggered by sulfate reduction in porphyry copper deposits is controlled by several concomitant factors, e.g., relatively high temperature (e.g., at 400−500 °C), vapor formation, and decompression. Moreover, K-silicate alteration would further promote mineralization by changing fluid compositions, e.g., removing K from the fluid.

Whether short-lived or prolonged duration of multistage combined magmatic and hydrothermal events in the giant Chalukou porphyry Mo deposit, China

The duration (or time scales) of multistage combined magmatic and hydrothermal events provides key information constraining the metallogenic history of porphyry deposits, but the duration (time scale) of the mineralising hydrothermal process remains uncertain. Here, we report zircon U–Pb isotopic age, fluorite Sm–Nd isochron ages, and Ti-in-quartz diffusion chronometry for the giant Chalukou porphyry Mo deposit with reserves of 2.46 Mt @ 0.087 wt% Mo in NE China. The deposit is genetically related to hydrothermal fluids associated with a Late Jurassic (148–147 Ma) porphyritic magmatic event coincident with our new fluorite Sm–Nd isochron age of 148 Ma. Porphyry Mo mineralisation is characterised by the superposition of multiple generations of crosscutting quartz-bearing veins including: quartz veins with minor magnetite and hematite associated with K-silicate alteration (A-veins), quartz-molybdenite veins with K-feldspar alteration halos that host the bulk of the Mo mineralisation (B-veins), and late base-metal mineralisation with phyllic alteration (D-veins), as well as post-mineralisation veins with argillic alteration. Hydrothermal veins from the Chalukou porphyry Mo deposit contain multiple generations of quartz that are distinct in their Ti concentrations and scanning electron microscope-cathodoluminescence images. The A-veins preserve subhedral to euhedral characteristics with alternating CL bright and dark growth zoning. The A-veins probably formed at a near-lithostatic pressure of ∼ 1.35 kbar and contain CL-bright quartz assaying 31–71 ppm Ti with Ti-in-quartz temperatures averaging 603 ± 30 °C, and CL-dark quartz assaying 6–25 ppm Ti with Ti-in-quartz temperatures averaging 496 ± 36 °C. The mineralised B-veins also CL-dark quartz, although they display less growth zoning with lower Ti assays of 0.4–27.9 ppm. These veins have near 0.7 kbar pressure and Titanium-in-quartz temperature averaging 390 ± 79 °C. The D-veins are dominated by CL-dark quartz and systematically lower Ti concentrations of 0.4–8 ppm, being lower than the relatively latter vein generations, formed at a temperature of 348 ± 58 °C and pressure of 0.4 kbars. The Ti diffusion in quartz has become a successful method constraining time scales and duration of magmatic and hydrothermal processes. Ti diffusion modelling indicates that the A-veins were deposited and cooled in a period between 2500 and 50,000 years. This shows that the earliest magma-related hydrothermal fluids activities formed within tens of thousands of years.

Genesis of the Yanghuidongzi Cu deposit, NE China: Constraints from H-O-Pb isotopic compositions and geochronological study

• The Yanghuidongzi Cu deposit classified as a porphyry type Cu deposit. • H-O isotopes indicate a mixing of magmatic water and meteoric water for ore fluids. • Pb-Re isotopes indicated a deep seated magma mixed with crustal materials for ore metals. • The Yanghuidongzi Cu deposit formed during Early Jurassic . The newly-discovered Yanghuidongzi porphyry Cu deposit is located in the southeastern part of Heilongjiang Province in NE China and represents one of the rare Jurassic porphyry Cu deposits within the easternmost segment of the Central Asian Orogenic Belt (CAOB). The Cu mineralization occurs as veins and disseminations hosted by granodiorite porphyry, associated with typical porphyry copper deposit alteration zoning, which is characterized by from an inner K-silicate zone through to a quartz–sericite zone to an outer propylitic zone, the K-silicate alteration and quartz–sericite alteration are closely related to mineralization. LA-ICP-MS zircon U-Pb data, sericite Ar-Ar and Molybdenite Re-Os data, and H-O and Pb isotopic compositions are used to constrain the ore-forming age and genesis of the Yanghuidongzi Cu deposit. LA-ICP-MS U-Pb dating of zircons from three granodiorite porphyry samples yielded weighted mean 206 Pb/ 238 U ages of 194.3 ± 1.2 Ma, 192.0 ± 2.0 Ma, and 194.6 ± 2.1 Ma respectively, whereas zircon U-Pb age of granodiorite in the study area yielded weighted mean 206 Pb/ 238 U age of 199.5 ± 1.5 Ma that are interpreted to be the emplacement age of these intrusions. 40 Ar- 39 Ar dating of sericite from a quartz-pyrite vein and a quartz-polymetallic sulfide vein yielded plateau ages of 193.9 ± 2.1 Ma and 193.6 ± 1.9 Ma, respectively, whereas five molybdenite samples yielded a Re-Os isochron age of 194.8 ± 2.1 Ma, with a weighted mean model age of 193.6 ± 1.2 Ma. Mineralization ages are consistent with the emplaced ages of the granodiorite porphyry within the analytical uncertainties, indicating that the Cu mineralization is genetically related to the granodiorite porphyry. The fluid δ 18 O fluid and δ 18 D fluid values range from −4.22‰ to 6.5‰ and from −92.8‰ to −65.7‰, respectively, indicating that the ore fluids were mixtures of magmatic and meteoric water. The 206 Pb/ 204 Pb, 207 Pb/ 204 Pb, and 208 Pb/ 204 Pb values of the pyrite and chalcopyrite are 17.547–18.327, 15.446–15.505, and 37.408–38.113, respectively, suggesting a common, magmatic source for the sulfur. Taking Re (6.75–29.53 ppm) data into consideration, the sources of ore-forming materials may be from a mixture of crustal and mantle sources. Combining our new data with the Jurassic regional tectonic setting, the Yanghuidongzi Cu deposit and the related granitoids formed in a compressional tectonic setting are associated with the subduction of the Paleo-Pacific Plate beneath the Eurasian continent.

Geology, geochemistry and geochronology of Lindero porphyry gold deposit in the Southern Puna plateau, Argentina

The Lindero gold deposit is located in the Southern Puna plateau, northwest Argentina. The deposit is centered in a cluster of six subvolcanic intrusions emplaced at the margin of the Arizaro Basin. Three alteration types were recognized: (i) Ca–Na silicate (clinopyroxene + magnetite + K-feldspar + quartz + calcite ± plagioclase), (ii) K-silicate (K-feldspar + quartz + magnetite ± biotite ± anhydrite) and (iii) chlorite-calcite alteration. The highest ore grades are linked to the K-silicate alteration. The proven plus probable reserves of Lindero are 84,226 t with average grades of 0.63 g/t Au and 0.11% Cu. A previous study assigned Lindero to the iron oxide copper-gold (IOCG) deposit type but many features of Lindero suggest that it is a porphyry gold deposit, including (i) the temporal and spatial link between alteration and the intrusive bodies, (ii) the alteration distribution pattern, particularly the small volume of rock affected by Ca–Na silicate alteration, (iii) the Au-rich and Cu-poor mineralization style.The magmatic complex at Lindero comprises an early-mineral unit (FPD), four inter-mineral units (CPD1, PBFD, CPD2 and DDP) and one post-mineral unit (PMI). In-situ U/Pb SIMS dating of the oldest (FPD), an intermediate (PBFD) and the youngest (DDP) intrusive units, confirms a middle Miocene age. The weighted mean ages of the oldest and youngest units are indistinguishable, with 15.36 ± 0.13 Ma (n = 21) and 15.47 ± 0.11 Ma (n = 16), respectively. Individual ages from each unit range by ~1 m.y. and the overall spread of zircon ages is 15.92 ± 0.23 to 14.44 ± 0.33 Ma. We suggest that emplacement of the subvolcanic stocks took place within this span time, likely at the lower end of this range (15.0–14.4 Ma). Two 40Ar/39Ar ages of hydrothermal biotites from the K-silicate alteration (14.99 ± 0.16 Ma and 14.93 ± 0.12 Ma) indicate that hydrothermal alteration began practically simultaneously with the emplacement of the porphyry units.All of the intrusive units are similar compositionally. They show a fine- to medium-grained porphyric texture (1–4 mm) comprising plagioclase, amphibole, clinopyroxene and scarce quartz phenocrysts (40–55 vol % of phenocrysts) in a K-feldspar ± quartz microcrystalline (0.02–0.07 mm) groundmass, except in the post-mineral unit which has a cryptocrystalline groundmass. Whole-rock analyses reveal a narrow range of dioritic composition (58.6–61.9 wt % SiO2) and high-K calc-alkaline character for all units. Trace element features (low Ba/Nb ratios, high Nb) of the Lindero magmas indicate a back-arc affinity, similar to those from the Southern Puna and distinct from the Central Volcanic Zone (CVZ) frontal arc. The Sr and Nd isotope ratios (87Sr/86Sr = 0.706042 to 0.706607; 143Nd/144Nd = 0.512501 to 0.512582) from Lindero intrusives are also similar to Southern Puna back-arc volcanic rocks. The Pb isotope ratios of Lindero (206Pb/204Pb = 18.79 to 18.83; 207Pb/204Pb = 15.60 to 15.63; 208Pb/204Pb = 38.66 to 38.74) overlap with both back-arc and arc magmas in the CVZ. The narrow age range, spatial association and uniform chemical and isotopic composition of Lindero porphyry units suggest that were derived from a common magma source, which underwent fractionation and/or crustal assimilation before emplacement as suggest by the low concentrations of Mg, Cr, Ni and Sr. The Lindero porphyry units show chemical and isotopic similarities with those from porphyry gold deposits in the Maricunga belt, Chile, and with the porphyry copper deposits of Argentina located in a back-arc setting; however, they differ from porphyry copper deposits in the frontal-arc setting of Chile, notably by the lack of an adakite-like signature (high Sr/Y ratio).

Distribution and composition of gold in porphyry gold systems: example from the Biely Vrch deposit, Slovakia

The Biely Vrch deposit in the Western Carpathians is assigned to the shallow, sulfide-poor porphyry gold deposit type and has an exceptionally low Cu/Au ratio. According to 3-D geochemical models, there is a limited spatial correlation between Au and Cu due to the primary introduction of gold by a salt melt and Cu by low-density vapor. Despite a rough spatial correlation of gold grades with quartz stockwork intensity, gold is hosted mostly by altered rock, exclusively in native form. Three main gold mineral assemblages were recognized here. In the deepest parts of the system, the K- and Ca-Na silicate gold assemblage is associated with minerals of high-temperature alteration (plagioclase, K-feldspar, actinolite), with gold grades and fineness depending on depth and potassium content of the host rock: K-silicate alteration hosts the lowest fineness gold (~ 914), whereas Ca–Na silicate alteration has the highest (~ 983). The intermediate argillic gold assemblage is the most widespread, with gold hosted mainly by chlorite, illite, smectite, and interstratified illite–chlorite–smectite minerals. The gold fineness is mostly variable (875–990) and inherited from the former gold mineral assemblages. The latest advanced argillic gold assemblage has its gold mostly in kaolinite. The extremely high fineness (~ 994) results from gold remobilization by late-stage aqueous magmatic-hydrothermal fluids. Uncommon bonanza-grade appears where the earlier gold mineral assemblages were further enriched by this remobilized gold. Primary precipitation of gold occurred during ascent and cooling of salt melts at 450 to 309 °C, mostly during retrograde quartz solubility.

Zircon and molybdenite geochronology and geochemistry of the Kalmakyr porphyry Cu–Au deposit, Almalyk district, Uzbekistan: Implications for mineralization processes

Copper, gold and molybdenum mineralization of the Kalmakyr porphyry deposit in Uzbek Tien Shan occurs as stockworks, veinlets and disseminations in the phyllic and K-silicate alteration zones developed predominantly in a middle to late Carboniferous intrusive complex composed of monzonite and granodiorite porphyry. Zircon U–Pb dating yielded an age of 327.2±5.6Ma for the ore-hosting monzonite and an age of 313.6±2.8Ma for the ore-bearing granodiorite porphyry. Re–Os dating of seven molybdenite samples from stockwork and veinlet ores yielded model ages from 313.2 to 306.3Ma, with two well-constrained isochron ages of 307.6±2.5Ma (five stockwork ores) and 309.1±2.2Ma (five stockwork ores and two veinlet ores), respectively. These results indicate that Cu–Au mineralization post-dated the emplacement of the monzonite, started right after the emplacement of the granodiorite porphyry, and lasted for ca. 7Ma afterward. The geochronological and geochemical data suggest that the Kalmakyr deposit was formed in a late Carboniferous mature magmatic arc setting, probably related to the latest subduction process of the Turkestan Ocean beneath the Middle Tien Shan. The εHf(t) values of zircon grains from the monzonite vary from +11 to +1.7, with an average of +5.1, and those of zircon grains from the granodiorite porphyry range from +5.7 to −1.8, with an average of +2.4. These data indicate that the magma of both monzonite and granodiorite porphyry was derived from partial melting of a thickened lower crust with input of mantle components and variable crustal contamination, and that there was more mantle contribution to the formation of the monzonite than the granodiorite porphyry. The high rhenium concentrations of molybdenite (98–899ppm) also indicate major mantle contribution of rhenium and by inference ore metals. The relatively high EuN/EuN∗ values (average 0.68), Ce4+/Ce3 values (average 890) and Ce/Nd values (average 36.8) for zircon grains from the granodiorite porphyry than those from the monzonite (average EuN/EuN∗=0.33, average Ce4+/Ce3=624, average Ce/Nd=3.9) suggest that the magma for the syn-mineralization granodiorite porphyry has higher oxygen fugacity than that for the pre-mineralization monzonite. Based on these data, it is proposed that while the monzonite was emplaced, the oxygen fugacity and volatile contents in the magma were relatively low, and ore metals might disperse in the intrusive rock, whereas when the granodiorite porphyry was emplaced, the oxygen fugacity and volatile contents in the magma were increased, favoring copper and gold enrichment in the magmatic fluids. The Kalmakyr deposit formed from a long-lived magmatic-hydrothermal system connected with fertile magmatic sources in relation to the subduction of the Turkestan Ocean beneath the Middle Tien Shan.

Geology, geochemistry and tectonic settings of the molybdenum deposits in South China: A review

South China Block (SCB) is the area including the Yangtze Craton and the Huanan Orogen where scattered Precambrian terranes are usually regarded as segments of Cathaysia Land. It is the third most important molybdenum metallogenic province in China, next to the Qinling-Dabie area and Northeast China, containing 29 Mo-only or Mo-dominated, 9W-Mo(-Sn-Bi) and 8 Cu-Mo deposits. These 46 deposits are located mainly in: (1) the Lower Yangtze River Belt of the northeastern Yangtze Craton, (2) the Northern Jiangnan Orogenic Belt that is generally considered a Meso-Neoproterozoic magmatic arc complex accreted onto the southeastern margin of the Yangtze Craton, (3) the Wuyi-Yunkai Orogenic Belt characterized by local exposures of Proterozoic metamorphic terranes and the more widespread Sinian (Uppermost Proterozoic) to Triassic sedimentary sequences, and (4) the Southeast Coastal Volcanic Belt characterized by Yanshanian andesitic to felsic volcanic rocks. Their genetic types are dominated by porphyry and skarn mineral systems, with only a few quartz-vein systems. The orebodies form veins, lens, cylindrical shapes, pipes, or irregular in shape, usually controlled by faults at various scales and volcanic-subvolcanic complexes. The host-rocks are variable in lithologies, including granites, porphyries, volcanic breccias and tuffs, clastic sediments and carbonate rocks, but the high-grade orebodies are usually hosted in carbonate-shale sequences. Hydrothermal mineralization processes can be generally divided into four stages, from early to late, they are (1) K-feldspar-quartz veins or veinlets, (2) quartz-molybdenite stockworks, (3) quartz-polymetallic sulfide stockworks, and (4) quartz-carbonate-fluorite veinlets. Fluid-rock interaction as exemplified by wallrock alteration evolved from K-silicate alteration (K-feldspar-quartz-mica), through phyllic (quartz-sericite-chlorite-epidote), to propylitic or argillic alteration, with skarn alteration typically occurring in skarn-type mineral systems. Hydrothermal mineral assemblages vary between two end-members, namely the dry system formed by CO2-rich fluids and marked by quartz, K-feldspar, fluorite, carbonate and epidote, and the wet system mainly originated from CO2-poor fluids and composed of biotite, sericite and chlorite. The ore-forming fluids are magmatic in origin and show high-temperature and high-salinity. The melt-fluid systems forming Cu-Mo deposits are more oxidizing than those forming the W-Mo or Mo deposits, as suggested by accessory minerals in granitoids and daughter minerals in fluid inclusions. The Cu-Mo deposits are related to the I-type granitic rocks (adakite-like), whereas the W-Mo and Mo-only systems are related to granitic rocks of S- or A-types, although all of them show high K contents. Available isotope ages show that the Mo and Mo-bearing deposits were predominantly formed in the early Yanshanian Orogeny (170–134Ma), followed by the late Yanshanian Orogeny (110–92Ma) and the Caledonian Orogeny (450–410Ma). The Caledonian Mo-mineralization has been observed only in the Wuyi-Yunkai Orogenic Belt and related to the collision between the Yangtze Craton and the pre-Devonian Huanan Orogen or terranes separated from the Cathaysia Land, linked to the assembly of the Gondwana supercontinent. The Early Yanshanian mineralization affected the entire Huanan Orogen and the eastern Yangtze Craton, and resulted from the syn- to post-collisional tectonism following the closure of eastern Paleo-Tethys. The Late Yanshanian Mo deposits mainly occur in the Southeast Coastal Volcanic Belt and the southeastern margin of the Wuyi-Yunkai Orogenic Belt, and are related to the westward subduction of the Paleo-Pacific plate. The skarn-type mineral systems generally show lower Re contents than the porphyry-type deposits in a same tectonic unit, suggesting that carbonate host-rocks have lower Re contents than the causative porphyries. The Re contents in molybdenites from porphyry or porphyry-skarn Cu-Mo systems are >50ppm, mainly >100ppm, suggesting a source significantly contributed by the mantle; whereas the Re contents in molybdenites from the Mo-only or W-Mo-dominated deposits are <100pm, mainly <50ppm, indicating a genetic relation to the crust-sourced granitic magmatism.

Xiongcun, Tibet: A telescoped system of veinlet-disseminated Cu (Au) mineralization and late vein-style Au (Ag)-polymetallic mineralization in a continental collision zone

The Xiongcun Cu–Au deposit occurs in the western part of the Gangdese porphyry and skarn Cu–Au mineralization belt in the Himalayan–Tibetan Orogen and was formed in an intracontinental orogenic environment related to the India–Asia collision. Sericite 40Ar/ 39Ar dating of the Xiongcun deposit indicates that ore formation occurred around 38.11 ± 0.90 Ma, much earlier than the main ore-forming ages (20 to 14 Ma) of the Gangdese Cu–Au metallogenic belt. The potassium feldspar K–Ar age of the sulfide-bearing pegmatoid dyke in the Xiongcun district is 47.62 ± 0.70 Ma. These ages indicate that the mineralization in Xiongcun is related to relaxation (ca. 52 to 40 Ma) and subsequent uplift (40 to 38 Ma) of the orogen. In the Xiongcun deposit, mineralization, hosted by Jurassic pyroclastic rocks and controlled by NW-striking faults, can be divided into two styles, i.e. veinlet-disseminated Cu ± Au mineralization and vein-style Au ± Ag-polymetallic mineralization. The former is associated with local remnant K-silicate alteration, extensive phyllic alteration and peripheral widespread propylitization. The latter, overprinting the former, is associated with strong silicification, chloritization and kaolinization. The early veinlet-disseminated Cu ± Au mineralization is similar to porphyry-type mineralization and has a mineral assemblage of quartz–sericite–pyrite–chalcopyrite–bornite–magnetite–pyrrhotite–sphalerite ± rutile ± anhydrite. The late vein-style Au ± Ag-polymetallic mineralization, characterized by a mineral assemblage of quartz–pyrite–pyrrhotite–sphalerite (rich in Fe)–chalcopyrite–galena–native gold–electrum–chlorite–kaolinite ± barite ± calcite ± siderite ± hematite, belongs to a sulfide-rich low-sulfidation-type epithermal system. The H, O and S isotopic compositions and high salinities of fluid inclusions indicate a magmatic contribution and increasing contents of meteoric water in the mineralized system from early to late stage. The wide range of homogenization temperatures (380 to 120 °C), homogenization pressures (45.92 to 1.94 bar) and salinities (36.61 to 1.23 wt.% NaCl equiv.) suggest rapid uplift and are consistent with the geological setting. Because of uplift, the hydrothermal system of Xiongcun evolved from a porphyry system to an epithermal system, resulting in an immature porphyry system and the telescoping of late vein-style Au ± Ag-polymetallic mineralization onto the early veinlet-disseminated Cu ± Au mineralization.

Geochemical Characteristics and Significance of Major Elements, Trace Elements and REE in Mineralized Altered Rocks of Large-Scale Tsagaan Suvarga Cu-Mo Porphyry Deposit in Mongolia

The alteration types of the large-scale Tsagaan Suvarga Cu-Mo porphyry deposit mostly comprise stockwork silicification, argillization, quartz-sericite alteration, K-silicate alteration, and propylitization. The mineralized and altered zones from hydrothermal metallogenic center to the outside successively are Cu-bearing stockwork silicification zone, Cubearing argillized zone, Cu-Mo-bearing quartz-sericite alteration zone, Cu-Mo-bearing K-silicate alteration zone, and propylitization zone. The K-silicate alteration occurred in the early phase, quartz-sericite alteration in the medium phase, and argillization and carbonatization (calcite) in the later phase. Ore-bearing-altered rocks are significantly controlled by the structure and fissure zones of different scales, and NE- and NW-trending fissure zones could probably be the migration pathways of the porphyry hydrothermal system. Results in this study indicated that the less the concentrations of REE, LREE, and HREE and the more the extensive fractionation between LREE and HREE, the closer it is to the center circulatory hydrothermal ore-forming and the more extensive silicification. The exponential relationship between the fractionation of LREE and HREE and the intensity of silicification and K-silicate alteration was found in the Cu-Mo deposit studied. The negative Eu anomaly, normal Eu, positive Eu anomaly and obviously positive Eu anomaly are coincident with the enhancement of Na 2O and K 2O concentrations gradually, which indicated that Eu anomaly would be significantly controlled by the alkaline metasomatism of the circulatory hydrothermal ore-forming system. Therefore, such characteristics as the positive Eu anomaly, the obvious fractionation between LREE and HREE and their related special alteration lithofacies are suggested to be metallogenic prognostic and exploration indications for Tsagaan Suvarga-style porphyry Cu-Mo deposits in Mongolia and China.

Sulfur and oxygen isotopic record in sulfate and sulfide minerals of early, deep, pre-Main Stage porphyry Cu–Mo and late Main Stage base-metal mineral deposits, Butte district, Montana

Typical porphyry-type Cu–Mo mineralization occupies two connected domal centers, the eastern Pittsmont and western Anaconda domes, that predate and largely underlie the well-known, throughgoing, Main Stage polymetallic veins of Butte. Among the sulfur-bearing minerals recovered from deep drill core of this early pre-Main Stage hydrothermal assemblage are anhydrite, chalcopyrite, pyrite, and molybdenite in veinlets bordered by K-silicate alteration, and pyrite from slightly younger quartz–pyrite veinlets with ‘gray-sericitic’ alteration selvages. The ranges of δ 34S values for minerals of the K-silicate assemblage are 9.8–18.2‰ for anhydrite ( n=23 samples), 3.0‰ to 4.7‰ for molybdenite ( n=6), 0.4‰ to 3.4‰ for pyrite ( n=19), and −0.1‰ to 3.0‰ for chalcopyrite ( n=13). Sulfate–sulfide mineral fractionation is consistent with an approach to isotopic equilibrium, and calculated temperatures for mostly coexisting anhydrite–sulfide pairs (anhydrite–molybdenite, n=6, 545 to 630 °C; anhydrite–pyrite, n=13, 360 to 640 °C; and anhydrite–chalcopyrite, n=8, 480 to 575 °C) are broadly consistent with petrological, alteration, and fluid-inclusion temperature estimates. The δ 34S values for pyrite ( n=25) in veinlets of the ‘gray-sericitic’ assemblage range from 1.7‰ to 4.3‰. The δ 34S values for sulfides of the pre-Main Stage K-silicate and ‘gray-sericitic’ assemblages are similar to those of most Main Stage sulfides, for which 281 analyses by other investigators range from −3.7‰ to 4.8‰. Sulfide–sulfide mineral pairs provide variable (−175 to 950 °C) and less reliable temperature estimates that hint of isotopic disequilibria. The sulfide data, alone, suggest a conventionally “magmatic” value of about 1‰ or 2‰ for Butte sulfur. However, the high modal mineral ratios of sulfate/sulfide, and the isotopic systematics of the early K-silicate assemblage, suggest that pre-Main Stage fluids may have been sulfate-rich ( X SO 4 2− ≈0.75) and that total sulfur was isotopically heavy ( δ 34S ΣS≈10‰), which would have required an evaporitic crustal component to the relatively oxidized granitic parental magma that was the source of the hydrothermal fluids and sulfur. Modeling of brine–vapor unmixing of a 10‰ fluid, reduction of sulfate, and vapor loss suggest that these processes may have formed the isotopically heavier (14‰ to 18‰) anhydrite of the western and shallower Anaconda Dome, contrasting with the lighter and more numerous values (9.8‰ to 12.9‰) for anhydrite of the eastern and deeper Pittsmont Dome. Such a process might also have been able to produce the sulfide isotopic compositions of the younger ‘gray-sericitic’ and Main Stage zones, but the limited data for sulfates permit δ 34S ΣS compositions of either 2‰ or 10‰ for these later fluids. Oxygen isotopic data for late Main Stage barite (−0.3‰ to 12.4‰, n=4 samples) confirm variable meteoric water contributions to these fluids, and the data support either the absence of, or limited, sulfate–sulfide isotopic equilibrium in these samples. The δ 34S values for sulfate–sulfur of barite are markedly variable (4.4‰ to 27.3‰), and the unusual 34S depletion indicates sulfur formed by oxidation of H 2S.

Porphyry Cu–Au mineralisation associated with a multi-phase intrusion, and related replacement fronts in limestones in an island arc setting near the Gümüşhane village (Artvin) in the Eastern Black Sea Province (Turkey)

The Gümüşhane area near Artvin is highly characteristic due to a significant hydrothermal alteration zone genetically associated with a microdiorite stock and its late-stage derivatives in the form of porphyry plugs that intruded into all the pre-Middle Eocene lithologies. The porphyry intrusion is multi-stage, intermediate to felsic in composition, and divided into pre-ore feldspar porphyries and quartz–feldspar porphyries, syn-ore feldspar–amphibole porphyries, and post-ore feldspar porphyries. Sericitic alteration is dominant, but K-silicate alteration is also observed and is characterised mainly by secondary feldspars, biotites, quartz, anhydrite, magnetite and pyrite veinlets. The central part of the alteration and the porphyry system where syn-ore feldspar–amphibole porphyry outcrops coincides with intense quartz stock-working with anomalous but uneconomical Cu and Au values. In this zone, pyrite is ubiquitous and is accompanied by chalcopyrite and lesser sphalerite, sulphosalts, molybdenite, bornite and magnetite. Chemical analyses of surface and drill core samples show that overall Au and Cu values are around 0.5 ppm and 0.3%, respectively, and that they are in the ranges of 1–2 ppm and 0.3–0.4%, respectively, in the densest stock-work zones. Multi-phase intrusion of porphyries into Late Cretaceous limestone also caused replacement fronts along re-crystallised limestone and porphyry contacts. This type of mineralisation is enriched in Mn, Zn, Cu, Au and Ba, and contains on average 4.7% MnO, 3.2% FeO, 3.1% Zn, 0.95% Cu, 0.3% Pb, 200 ppb Au, 900 ppm Bi and 660 ppm Ba, present in Fe- and Mn-oxides, pyrite, chalcopyrite, sphalerite and lesser bornite, sulphosalts and gold. Supergene oxidation is well developed in these zones. Hydrothermal alteration mass change calculations reflect an intimate relationship between the two types of mineralisation. The lithologies near mineralised fronts are highly depleted in MnO and Zn, and the intensity of depletion is reduced away from the contact into the porphyry. The leached elements are added to the re-crystallised limestone block in which MnO and Zn show an 81-fold and 472-fold increase, respectively, over the least altered limestone. Au and Cu are enriched along the contact both in the porphyry and in the limestone, implying that they may have been derived either from other lithologies in the vicinity or the magma itself.

Porphyry-copper mineralisation in the central Srednogorie zone, Bulgaria

The porphyry-copper systems in the central part of the Srednogorie zone (Bulgaria) are represented by three major deposits (Elatsite, Medet and Assarel) and several smaller deposits and occurrences, all of them within the Panagyurishte ore district. The hydrothermal systems are related to Late Cretaceous calc-alkaline igneous complexes. Ore mineralisation is developed predominantly in the apical parts of subvolcanic and intrusive bodies as well as within the volcanic and basement metamorphic rocks. Several of the porphyry systems are spatially associated with shallow-level intermediate and high-sulphidation volcanic-hosted epithermal deposits of economic importance, such as the major gold-copper mine at Chelopech located 10 km from the Elatsite porphyry-copper deposit. Mineralisation processes in the porphyry deposits start with intensive hydrothermal alteration of the wall rocks. K-silicate alteration is characteristic for pre-ore hydrothermal activity in all of them, and it is located mostly in their central parts. Propylitic alteration is prominent in the Medet and Assarel deposits. The Assarel deposit is located in the central part of a palaeovolcanic structure and shows a large spectrum of pre-ore alterations, including propylitic, sericitic, and advanced argillic assemblages. The initial stages of the hydrothermal systems are characterised by high temperatures (>550–500 °C) and highly saline (50–20 wt% NaCl equiv.) and vapour-rich fluids of likely magmatic origin. The composition of the fluids gradually changes from H2O–NaCl±FeCl2 to H2O–NaCl–KCl and H2O–NaCl-dominated as the fluids cool, react with wall rocks, and may become diluted with meteoric water. Fe-Ti-oxide mineral associations were formed early in all deposits, later followed, in the Elatsite deposit, by an assemblage of bornite, chalcopyrite, platinum group element (PGE) phases, Co-Ni thiospinels, Ag- and Bi-tellurides, and selenides. The main ore stage in all deposits is dominated by chalcopyrite ± pyrite, associated with propylitic alteration at Medet, K-silcate-sericite alteration at Elatsite, and chlorite-sericite alteration at Assarel. Specific geochemical features of the ore systems are marked by precipitation of numerous fine-grained mineral inclusions in chalcopyrite and pyrite (palladoarsenide, Pd-rammelsbergite, Cu-Ni sulpharsenides in Elatsite; Co-and Ni-pyrite, carrolite, vaesite, sulvanite and colusite in Medet; enargite, As-sulvanite, colusite, goldfieldite, wittichenite, calaverite and aikinite in Assarel). Native gold and electrum are observed as small grains, except in Elatsite where larger aggregates up to several millimetres are found. Molybdenite is present as small flakes within chalcopyrite, or forms felted aggregates within thin quartz veinlets in the Elatsite and Medet deposits, and very rarely at Assarel. Thin veinlets of quartz-galena-sphalerite are found at the upper parts of the deposits. Supergene alteration of the primary sulphide minerals is developed in the Assarel deposit where a thick blanket of secondary copper mineralisation (chalcocite, covellite) has formed. In the other two deposits, secondary mineral associations (Cu and Fe oxides and hydroxides) are limited to near the surface. Differences in the deposits studied suggest that Elatsite and Medet are formed probably at deeper levels of the porphyry systems, whereas Assarel is developed at rather shallower and lower-temperature conditions typical of the apical part of a subvolcanic body intruding its volcanic superstructure.

The Machangqing copper-molybdenum deposits, Yunnan, China ? an example of Himalayan porphyry-hosted Cu-Mo mineralization

The Machangqing copper-molybdenum deposits occur in the Shanjian fold belt, Yunnan Province, China. Two types of ores are distinguished: (1) Cu-Mo quartz veinlets in magnetite-series granite porphyry; and (2) Cu-Mo skarns occurring at the contact between the Ordovician sedimentary sequence and the granite porphyry. With decreasing temperature and\(a_{K^ + } /a_{H^ + } \) of hydrothermal fluids initially in equilibrium with K-feldspar, the following alteration patterns developed within the porphyry, from the center outwards: silicification, K-silicate, phyllic and argillic alteration. The paragenetic sequence of alteration minerals observed in the Cu-Mo skarns resulted from decreasing temperature and/or increasing\(a_{Ca^{2 + } } /a_{H^ + }^2 \) of the hydrothermal fluids initially in equilibrium with grandite garnet. Fluid inclusions in quartz suggest boiling during the mineralization. The mineralization temperatures based on filling temperatures and salinities of quartz are in the following ranges: about 265° to 400 °C and 5.0 to 14.6 wt.% NaCl eq. for the Cu-Mo veinlets; and 200° to 500 °C and 10.2 to 42.0 wt.% NaCl eq. for the Cu-Mo skarns. As is evident from log fo2-pH diagrams, ores of the early stage of mineralization in the Cu-Mo skarns, characterized by the assemblage magnetite + pyrite + rare pyrrhotite + K-feldspar + quartz, were deposited from highly alkaline and high temperature fluids. With decreasing temperature and fo2, the pH of the ore fluids was shifted towards slightly alkaline to neutral, with the resultant formation of the main stage ores, characterized by the assemblage chalcopyrite + pyrite + molybdenite + sphalerite + K-feldspar +sericite (muscovite) + epidote + uartz. Very minor amounts of ore minerals, including matildite, bismuthinite and electrum, are associated with a late stage of ore formation.

Constraints on Magma Degassing beneath the Far Southeast Porphyry Cu-Au Deposit, Philippines

K-silicate alteration in the Far Southeast (FSE) porphyry Cu–Au INTRODUCTION deposit formed at ~1·4 Ma, concentric to dikes of quartz diorite Magmas become saturated with volatile components as porphyry. At ~2 km paleodepth the hydrothermal system consisted they ascend in the crust, and either erupt or crystallize of magmatic hypersaline liquid and vapor at 550°C and Ζ50 at depth. Saturated magmas that remain at depth exsolve MPa (lithostatic pressure). Advanced argillic alteration formed at an aqueous fluid that forms a hydrothermal system on the same time over the deposit at Ζ1 km paleodepth from acidic separation from the magma chamber (Burnham, 1979). condensates of the vapor. At [1·3 Ma, K-silicate alteration was Although part of the exsolved fluid (a low-density vapor) overprinted by 350°C magmatic liquid (~5 wt % NaCl equiv.) may discharge to the surface from passively degassing at hydrostatic pressure. Sericite alteration and much of the Cu–Au volcanoes (Hedenquist, 1995), part will remain at depth, mineralization formed at this later stage. Evolution of the magmatic and this dense, residual magmatic liquid may be asfluid composition was simulated with a magma-chamber cryssociated with hydrothermal mineralization in the portallization model. Homogeneous crystallization during early stage phyry ore environment (Hedenquist & Lowenstern, convection is assumed, whereas at 50 vol. % crystals the chamber 1994). becomes stagnant and crystallizes from rim to core over a narrow In this paper we use evidence from a porphyry ore crystallization interval. The model calculation, based on a magma deposit in the Philippines to determine the composition chamber with 2 km thickness at 6 km depth (150 MPa) and of the fluid that exsolved from the deeper parent magma. [800°C (saturated melt composition at 30 vol. % crystals, 5 The evolving composition of the magmatic fluid at the wt % H2O, 0·2 wt % Cl and dD of –40‰), can reproduce the depth of the ore body can then be used to place constraints chemical and isotopic compositions of the early and late magmatic on the nature of the evolved magma chamber far below fluids. The most critical factor controlling the compositional evolution the porphyry ore body. This approach differs from that of the model hydrothermal system is the transition from convective of other studies where a portion of the parent magma to stagnant magma-chamber crystallization. There is also a sharp chamber is available to study directly (e.g. at Yerington, decrease in the rate of fluid exsolution associated with this transition, Nevada; Dilles, 1987). which can account for the thermal collapse of the FSE porphyry The Far Southeast (FSE) porphyry Cu–Au deposit system, from K-silicate to sericite alteration. and overlying Lepanto epithermal Cu–Au ore body are

Nature, origin and evolution of the granitoid-hosted early Proterozoic copper-molybdenum mineralization at Malanjkhand, Central India - a reply

At Malanjkhand, Central India, lode-type copper (-molybdenum) mineralization occurs within calcalkaline tonalite-granodiorite plutonic rocks of early Proterozoic age. The bulk of the mineralization occurs in sheeted quartz-sulfide veins, and K-silicate alteration assemblages, defined by alkali feldspar (K-feldspar ≫ albite) + dusty hematite in feldspar ± biotite ± muscovite, are prominent within the ore zone and the adjacent host rock. Weak propylitic alteration, defined by albite + biotite + epidote/zoisite, surrounds the K-silicate alteration zone. The mineralized zone is approximately 2 km in strike length, has a maximum thickness of 200 m and dips 65°–75°, along which low-grade mineralization has been traced up to a depth of about 1 km. The ore reserve has been conservatively estimated to be 92 million tonnes with an average Cu-content of 1.30%. Supergene oxidation, accompanied by limited copper enrichment, is observed down to a depth of 100m or more from the surface. Primary ores consist essentially of chalcopyrite and pyrite with minor magnetite and molybdenite. δ34S (‰) values in pyrite and chalcopyrite (−0.38 to +2.90) fall within the range characteristic of granitoid-hosted copper deposits. δ18O (‰) values for vein quartz (+ 6.99 to +8.80) suggest exclusive involvement of juvenile water. Annealed fabrics are common in the ore. The sequence of events that led to the present state of hypogene mineralization is suggested to be as follows: fracturing of the host rock, emplacement of barren vein quartz, pronounced wall-rock alteration accompanied by disseminated mineralization and the ultimate stage of intense silicification accompanied by copper mineralization. Fragments of vein quartz and altered wall rocks and striae in the ore suggest post-mineralization deformation. The recrystallization fabric, particularly in chalcopyrite and sphalerite, is a product of dynamic recrystallization associated with the post-mineralization shearing. The petrology of the host rocks, hydrothermal alteration assemblages, ore mineral associations, fluid inclusions and the sulfur and oxygen isotopes of ores are comparable to those in Phanerozoic (and reported Precambrian) porphyry-copper systems, and the Malanjkhand deposit has important implications for both metallogenic models for, and mineral exploration in, Precambrian terrains.

Geology, porphyry CuAu, and epithermal CuAuAg mineralization of the Tombulilato district, North Sulawesi, Indonesia

The geology of the Tombulilato district, North Sulawesi, is characterized by an island arc-type volcano-sedimentary pile, > 3400 m thick and of late Miocene(?)-Pleistocene age, which is made up of submarine to subaerial basic to acid volcanic rocks interbedded with marine and continental sedimentary rocks. The sequence is intruded by high-level stocks and dikes, and cut by diatreme breccias of late Pliocene and Pleistocene age, some of which are associated intimately with porphyry CuAu and epithermal CuAuAg mineralization. A main compressive deformation event took place in the Pliocene. Three main mineralization types are present in the district: porphyry CuAu; high-sulfidation epithermal CuAuAg; and low-sulfidation epithermal AuAg. Porphyry CuAu mineralization is present at Cabang Kiri, Sungai Mak, Kayubulan Ridge, and Cabang Kanan. Hypogene CuAu mineralization is associated typically with magnetite-bearing K-silicate assemblages, which are partially to completely obliterated by sericite, illite and chlorite. Copper as chalcopyrite and bornite, and Au show a positive correlation. Multiphase intrusions and alteration-mineralization events are commonplace. Biotite-bearing K-silicate alteration at Cabang Kiri and Kayubulan Ridge yields KAr ages of 2.93±0.06 and 2.36±0.05 Ma, respectively. Cabang Kiri possesses a gold-rich zone (> 1.5 ppm Au) open at depth, and the bulk of the mineralization at Sungai Mak is contained in a supergene chalcocite blanket. High-sulfidation epithermal CuAuAg mineralization is present at Motomboto, which displays an alteration pattern consisting of a core of residual (vuggy) silica, bordered outward by zones of quartz-alunite, quartz-kaolinite, and chlorite. This alteration zoning is interpreted to have formed by magmatic-volatiles charging a meteoric convection cell, with the resulting fluids becoming progressively neutralized outward by fluid-rock interaction. CuAuAg mineralization is contained in the silica core associated with multiphase hydrothermal breccias, which are typically cemented by pyrite, enargite, luzonite, marcasite, and minor sphalerite and galena. Late-stage barite contains trails of secondary liquid + vapor-rich inclusions with T h between 154 and 178°C. Available K-Ar dating on alunite suggests that advanced argillic alteration at Motomboto was active between 1.89 and 0.93 Ma. Low-sulfidation epithermal AuAg mineralization at Kaidundu, and elsewhere in the district, is confined to narrow (to 1 m wide) silica veins. They consist of chalcedony and quartz displaying comb and cockade textures, and abundant open vugs. The veins contain <5 vol.% sulfides (dominantly pyrite), and illitic alteration is restricted to narrow selvages and occluded wallrock fragments. Gold is free in quartz, and also associated with pyrite and tellurides in millimeter-scale bands and disseminations. Crystalline quartz contains primary liquid + vapor-rich fluid inclusions with T h between 166 and 324°C and salinities of up to 0.8 equiv. wt.% NaCl. Preliminary geologic reconstructions suggest that these mineralization types in the Tombulilato area were generated over about 2 M yr (between 2.9 and 0.9 Ma) as part of a district-scale hydrothermal system. Continuous syn-mineralization uplift and erosion, which are interpreted to have removed some 2 km of rock in the last 3 M yr, were responsible for the progressive unroofing of the hydrothermal system and the superposition of epithermal environments over relics of higher temperature, deeper-seated mineralization. In the near-surface environment, intense uplift accompanied formation of a chalcocite blanket at Sungai Mak.

Early proterozoic AuCoU mineralization in the Kuusamo district, northeastern Finland

Abstract The Kuusamo volcano-sedimentary belt is part of an Early Proterozoic rift system in the Fennoscandian Shield extending from the Norwegian Sea to Lake Onega. The rocks range in age from 2.5 to 2.0 Ga. Sulphide deposits are confined to the two lowermost sedimentary units, the Sericite Quartzite and Siltstone Formations. These are intruded by albite diabase and syenite dikes and sills. The mineralization is associated with pervasive hydrothermal alteration. Spilitization and albitization predate mineralization and appear to be premetamorphic. The later FeMg-silicate and K-silicate alteration is associated with mineralization in ductile shear zones, whereas carbonatization and silicification dominate in brittle zones. Ore deposits and occurrences can be divided into three groups: uranium deposits, CoAuU-bearing sulphide deposits, and Fesulphide deposits. Uranium deposits are of the sulphide-poor, stratiform sandstone type often associated with an albitization front. The U deposits are also related to albite diabase dikes and alkaline granitic intrusions. The CoAuU deposits can be subdivided into two end members: replacement ores in a ductile deformation zone and breccia ores in a brittle deformation zone. At Juomasuo the sulphide mineralization is located in a FeMg-silicate and K-silicate alteration zone within a ductile shear zone surrounded by a wide albitized zone. The ore reserves are 1 Mt grading 5–6 g/t Au and 0.2% Co. Other enriched elements are Cu, U, W, Mo, Se, Te and Bi. The Konttiaho deposit represents a breccia ore in a brittle deformation zone where the CoAuU mineralization is associated with carbonate and quartz within albitized country rocks. Typical ore minerals in both types are pyrite, pyrrhotite, cobaltite, Co-pentlandite, chalcopyrite, uraninite, molybdenite, scheelite, gold and tellurides. The Fe-sulphide deposits are similar to the CoAuU deposits but lack precious metals. Sulphide mineralization is epigenetic, post peak-metamorphic and related to the Svecokarelian orogeny at 1.8–1.9 Ga. The mineralization is controlled by stratigraphy, antiform structures and shear zones in the central part of the volcano-sedimentary belt. The broad elemental spectrum suggests a complex origin and probably a deep-seated source for the metals.

Epithermal and subjacent porphyry mineralization, Acupan, Baguio District, Philippines: a fluid-inclusion and paragenetic study

The Acupan gold-silver deposit is a major precious-metal producer in the Baguio gold district, Northern Luzon, Philippines. Paragenetic and fluid-inclusion studies have confirmed the presence of two contrasting hydrothermal fluid types, each characterized by distinctive fluid-inclusion populations, ore and gangue associations and alteration assemblages. Mineralization related to both porphyry and epithermal ore-forming events are recognized. The spatial proximity of magmatic-hydrothermal to epithermal mineralization has not previously been described from an adularia-sericite deposit such as Acupan. Two vein stages of porphyry origin (stages I and II) are related to emplacement of a subvolcanic porphyry dacite. Stage I (quartz-magnetite-chalcopyrite) veinlets are associated with K-silicate alteration. Biotite, magnetite and anhydrite are the dominant alteration minerals. ‘White sericite’ and propylitic alteration halos envelop stage II (anhydrite-quartz-pyrite-chlorite) veinlets, which cross-cut the earlier stage I mineralization. The association K-mica-quartz-pyrite-anhydrite-chlorite typifies the white sericite assemblage. Propylitic alteration is dominated by chlorite-pyrite-anhydrite; epidote is notably absent. Paragenetic stages III to V of epithermal character occur together as veins up to one meter wide and are superimposed on porphyry-style mineralization. The highest precious metal concentrations are present in fine-grained ‘grey quartz’ (stage III), recognized predominantly along vein margins in the upper levels of the deposit and as clasts enclosed by later paragenetic stages. Fragments of altered wall rock are hosted by stage III gangue. Stage IVa mineralization is coarse-grained and typified by ‘white quartz’ together with K-mica, adularia, rhodochrosite and mangano-calcite. Opaque minerals include pyrite, sphalerite, chalcopyrite, galena, free gold and gold-silver tellurides. Vuggy, colloform and brecciation textures are all recognized in stage IVa gangue. Later-stage IVb gangue infills central portions of the veins and is dominated by coarse-grained calcite and quartz. Stage V (anhydrite-pyrite-quartz) veins reopen and crosscut stages III and IV. Stage IVb and V veinlets develop locally intense propylitic envelops where they are isolated from the larger veins. Sericitic alteration (K-mica-quartz-pyrite-illite) is in general symmetrically disposed about the epithermal veins in envelopes up to a meter wide, and is surrounded by a large, weakly altered propylitic zone characterized by the mineral association epidote-chlorite-pyrite-carbonate-magnetite. Fluid-inclusion studies reveal that stage I veinlets were deposited from hypersaline (34 to over 74 eq. wt. % NaCl) magmatic fluids. Homogenization temperatures ( T H) ranged from 350°C to in excess of 600°C. Stage II was deposited from saline fluids (31–63 eq. wt. % NaCl), and T H measurements from 300°C to in excess of 600°C were observed. Entrapment temperatures for stage III fluids were between 240°C and 320°C, with salinities from 0 to 2.6 eq. wt. % NaCl. Stage IVa formed from low-salinity fluids (0–5.1 eq. wt. % NaCl) at temperatures between 220°C and 310°C. Deposition of stage IVb occurred between 180°C and 280°C from low-salinity fluids (0–1.7 eq. wt. % NaCl). Temperatures declined approximately 60°C from the beginning of stage III deposition to the end of stage IVb. A late resurgence in temperatures is present during anhydrite deposition (stage V), with T H between 214°C and 299°C and salinities from 0.2 to 3.4 eq. wt. % NaCl. Magmatic-hydrothermal fluids exsolved during crystallization of the subvolcanic porphyry dacite produced stage I and II mineralization. A co-existing magmatic vapor generated by phase separation was ubiquitous throughout these stages. Epithermal mineralization commenced at least 1 Ma after stage II, with near-surface rupturing of a hydrothermal seal producing hydrothermal breccias, widespread boiling and gold mineralization (stage III). Later self-seal/rupture cycles and minor hydrothermal brecciation produced apparent paragenetic reversals of stage IVa gangue sealed by stage III, and stage IVb gangue sealed by IVa. Boiling, recognized at greater depths in stage IVa gangue, was more restricted in extent than for stage III. Gold deposition below the boiling zone in stage IVa was localized at structural intersections and controlled by pressure fluctuations. Stage IVb may have been deposited in response to boiling and/or fluid mixing. Temperature increase and/or fluid mixing are the likely mechanisms for stage V deposition.

Paleomagnetic documentation of the early, high-temperature zone of mineralization at Butte, Montana

Butte quartz monzonite, host rock to mineralization in the Butte district, Montana, was pervasively altered to a high-temperature K-silicate alteration assemblage during pre-Main Stage, porphyry-type mineralization within the deep levels of the district. Magnetization directions of steep, negative inclination (reverse polarity) are characteristic of Butte quartz monzonite within the zone of pervasive pre-Main Stage alteration and mineralization. Butte quartz monzonite in surrounding, less altered parts of the district and farther from Butte contains only steep, positive inclination characteristic directions (normal polarity). Magnetic, petrographic, and radiometric evidence together with the exact correlation of reversed polarity directions with high-temperature pre-Main Stage alteration assemblages suggest that, in the deep, central portion of the Butte district, the original normal polarity thermoremanent magnetization was completely unblocked by heating to temperatures in excess of 578 degrees C. A secondary thermoremanent magnetization was acquired in subsequent cooling in a field of reversed polarity prior to Main Stage, fracture-controlled mineralization. The detection of reversed polarity directions in a portion of the Butte quartz monzonite pluton serves as a method for delineating the extent of a high-temperature (greater than 578 degrees C) thermal perturbation during mineralization at Butte.