Quartz-sulfide Veins Research Articles

Gold Mineralization at the Anomaly A Deposit, Clarence Stream Area, Southwestern New Brunswick: Distal Deposits of a Syn-Deformational Intrusion-Related Gold System?

Gold-bearing mineralization at Anomaly A occurs in polydeformed Ordovician turbidites of the Kendall Mountain Formation of the St. Croix belt in southwestern New Brunswick, and is hosted by WSW-trending, shallow-dipping, brittle–ductile high-strain zones associated with the latest stage of the second of four regional deformation events. Gold-bearing syn-deformational stockwork, massive, and multiply brecciated quartz-sulfide veined zones form irregular pods and sheets up to several meters thick, which contain arsenopyrite, pyrrhotite, pyrite, stibnite, and a variety of rare minerals that are gold-bearing. Mineralized zones are enveloped by locally auriferous, weakly to intensely altered wall rock. Vein paragenesis includes: (1) early (pre- to syn-D 2 ) quartz-chlorite veins that are largely the product of migration of quartz by pressure solution; (2) crosscutting, multiphase, vuggy quartz-sulfide veins that contain most of the gold and are interpreted to be structurally controlled; (3) late, crosscutting, nondeformed, non-gold-bearing veins containing laumontite, chlorite, muscovite, quartz, fluorite, and base metal sulfides. Carbonate-poor alteration associated with zones of gold mineralization is lithology-specific, and appears to reflect overprinting of multiple events. Major elements removed during alteration include Si, Na, Mn, and Mg. The minor and trace elements removed include Co, Li, Sc, and possibly Sr, Ni, and Zn. Minor and trace elements added include Au, Ag, As, Sb, S, and possibly F, Cu, Pb, and Zr. On the basis of proximity, time constraints, and a similar ore mineral assemblage, gold mineralization at Anomaly A is interpreted to be a distal equivalent of the intrusion-related gold mineralization at the nearby Clarence Stream Main zone deposit.

Mineralogy and geochemistry of El Dorado epithermal gold deposit, El Sauce district, central-northern Chile

The El Dorado Au-Cu deposit is located in an extensive intra-caldera zone of hydrothermal alteration affecting Upper Cretaceous andesites of the Los Elquinos Formation at La Serena (≈ 29°47′S Lat., 70°43′W Long., Chile). Quartz-sulfide veins of economic potential are hosted by N25W and N20E fault structures associated with quartz-illite alteration (+supergene kaolinite). The main ore minerals in the deposit are pyrite, chalcopyrite ± fahlore (As/(As + Sb): 0.06−0.98), with electrum, sphalerite, galena, bournonite-seligmanite (As/(As + Sb): 0.21−0.31), marcasite, pyrrhotite being accessory phases. Electrum, with an Ag content between 32 and 37 at.%, occurs interstitial to pyrite aggregates or along pyrite fractures. Pyrite commonly exhibits chemical zonation with some zones up to 1.96 at.% As. Electron probe microanalyses of pyrite indicate that As-rich zones do not exhibit detectable Au values. Fluid inclusion microthermometry shows homogenization temperatures between 130 and 352 °C and salinities between 1.6 and 6.9 wt.% NaCl eq. Isotope data for quartz, ankerite and phyllosilicates and estimated temperatures show that δ18O and δD for the hydrothermal fluids were between 3 and 10‰ and between −95 and −75‰, respectively. These results suggest the mineralizing fluids were a mixture of meteoric and magmatic waters. An epithermal intermediate-sulfidation model is proposed for the formation of the El Dorado deposit.

Yerranderie a Late Devonian Silver–Gold–Lead Intermediate Sulfidation Epithermal District, Eastern Lachlan Orogen, New South Wales, Australia

AbstractFelsic volcanic units of the Early Devonian Bindook Volcanic Complex host the Yerranderie epithermal silver–gold–lead district 94 km west–southwest of Sydney. Mineralization in the district forms part of a fault‐controlled, intermediate sulfidation, epithermal silver–gold–base metal vein system that has significant mineral and alteration zonation. Stage 1 of the mineral paragenesis in the veins developed quartz and carbonate with early pyrite, whereas stage 2 is a crustiform banded quartz–pyrite–arsenopyrite assemblage. Stage 3, the main stage of sulfide deposition, comprises early sphalerite, followed by a tetrahedrite–tennantite–gold assemblage, then a galena–chalcopyrite–native silver–pyrite assemblage, and finally a pyrargyrite–polybasite–pearceite assemblage. Stage 4 involves the deposition of quartz veins with minor (late) pyrite and stage 5 is characterized by siderite that infilled remaining voids. Mineral zonation occurs along the Yerranderie Fault, with bornite being restricted to the Colon Peaks–Silver Peak mine area, whereas arsenopyrite, which is present in both the Colon Peaks–Silver Peak and Wollondilly mine areas, is absent in other lodes along the Yerranderie Fault. The Yerranderie Fault, which hosts the major lodes, is surrounded by a zoned alteration system. With increasing proximity to the fault the intensity of alteration increases and the alteration assemblage changes from an outer quartz–muscovite–illite–(ankerite) assemblage to a quartz–illite–(pyrite–carbonate) assemblage within meters of the fault.40Ar/39Ar dating of muscovite from the alteration zone gave a 372.1 ± 1.9 Ma (Late Devonian) age, which is interpreted to be the timing of the quartz–sulfide vein formation. Sulfur isotope values for sulfides range from 0.1 to 6.2‰ with one outlier of −5.6 δ34S‰. The results indicate that the initial ore‐forming fluids were reduced, and that sulfur was probably sourced from a magmatic reservoir, either as a direct magmatic contribution or indirectly through dissolution and recycling of sulfur from the host volcanic sequence. The sulfur isotope data suggest the system is isotopically zoned.

Nature and origin of the BIF-hosted São Bento gold deposit, Quadrilátero Ferrífero, Brazil, with special emphasis on structural controls

The orogenic banded iron formation (BIF)-hosted Au mineralization at São Bento is a structurally-controlled, hydrothermal deposit hosted by Archean rocks of the Rio das Velhas greenstone belt, Quadrilátero Ferrífero region, Brazil. The deposit has reserves of 14.3 t Au and historical (underground) production of 44.6 t Au between 1987 and 2001. The oxide-facies São Bento BIF is mineralized at its lower portion, where in contact with carbonaceous, pelitic schists, particularly in the proximity of sulfide-bearing quartz veins. Shear-related Au deposition is associated with the pervasive, hydrothermal sulfidation (mainly arsenopyrite) of the Fe-rich bands of the São Bento BIF. Auriferous, sulfide- and quartz-rich zones represent proximal alteration zones. They are enveloped by ankerite-dominated haloes, which reflect progressive substitution of siderite and magnetite within the BIF by ankerite and pyrrhotite, respectively. The São Bento BIF was intensely and extensively deformed, first into open, upright folds that evolved into tight, asymmetric, isoclinal folds. The inverse limb of these folds attenuated and gave way to sheath folds and the establishment of ductile thrusts. Mineralized horizons at São Bento result from early structural modifications imposed by major transcurrent and thrusts faults, comprising the Conceição, Barão de Cocais and São Bento shear zones. Dextral movement on the SW–NE-directed Conceição shear zone may have generated splays at a compressional side-stepping zone, such as the São Bento shear zone, which is the structural locus for the São Bento gold mineralization. Relaxation of the Conceição shear zone under more brittle conditions resulted in the development of dilatational zones where gold–sulfide–quartz veins formed. These structures are considered to have been generated in the Archean. Geochronological data are scarce, with Pb–Pb analyses of refractory arsenopyrite and pyrite from bedded and remobilized ore plotting on a single-stage growth curve at 2.65 Ga. A later compressional, ductile deformation of unknown age overprinted, rotated and flattened the original, N60E-directed structure of the whole rock succession, with development of planar and linear fabrics that appear similar to Proterozoic-aged structures. Fluid inclusion studies indicate low salinity, aqueous fluids, with or without CO 2 and/or CH 4, with extremely variable CO 2/CH 4 ratios, of probable metamorphic origin. Fluid evolution shows a paragenetic decrease in the carbonic phase from 10–15% to 5%, and increase in the H 2O/(CO 2 + CH 4) and CO 2/CH 4 ratios, suggesting important interaction with carbonaceous sediment. Trapping conditions indicate a temperature of 300 °C at 3.2 kbar.

Textural and fluid inclusion constraints on the origin of the banded-iron-formation-hosted gold deposits at Maevatanana, central Madagascar

The Maevatanana deposits consist of gold-bearing quartz–sulphide veins crosscutting banded iron formation (BIF) within a metamorphosed 2.5 Ga greenstone belt. The host rocks are dominated by a sequence of migmatites, gneisses, amphibolites, magnetite-rich quartzites and soapstones, intruded by large granitoid batholiths (e.g. the 0.8 Ga Beanana granodiorite). In the mineralised rocks, pyrite is the dominant sulphide, in addition to accessory chalcopyrite and galena. Outside the immediate ore zone, the BIF is dominated by quartz + magnetite ± hematite, accompanied by cummingtonite, albite and biotite. Gold occurs as globular grains (usually <500 μm) within quartz crystals close to the sulphides and as invisible inclusions within pyrite and chalcopyrite (up to 2,500 ppm Au content). Fluid inclusion textural and microthermometric studies indicate heterogeneous trapping of a low-salinity (∼3.6 wt.% eq. NaCl) aqueous fluid coexisting with a carbonic fluid. Evidence for fluid-phase immiscibility during ore formation includes variable L/V ratios in the inclusions and the fact that inclusions containing different phase proportions occur in the same area, growth zone, or plane. Laser Raman spectroscopy confirms that the vapour phase in these inclusions is dominated by CO2 but shows that it may contain small amounts of CH4 (<1 mol%), H2S (<0.05 mol%) and traces of N2. Fluid inclusion trapping conditions ranged from 220 to 380°C and averaged 250°C. Pressure was on the order of 1–2 kbar. The abundant CO2 and low salinity of the inclusions suggest a metamorphic origin for the fluid. Likewise, the presence of H2S in the fluid and pyritisation of the wall-rock indicate that gold was likely transported by sulphide complexing. Fluid immiscibility was probably triggered by the pressure released by fracturing of the quartzites during fault movements due to competence differences with the softer greenstones. Fracturing greatly enhanced fluid circulation through the BIF, allowing reaction of the sulphide-bearing fluids with the iron oxides. This caused pyrite deposition and concomitant Au precipitation, enhanced by fluid phase separation as H2S partitioned preferentially into the carbonic phase.

The Tianger (Bingdaban) shear zone hosted gold deposit, west Tianshan, NW China: Petrographic and geochemical characteristics

The Wangfeng–Tianger–Saridala ore district consists of over 20 orebodies with total proven gold reserves exceeding 15 tonnes. The district is located on the EW-striking Tianger (Bingdaban) shear zone in west Tianshan Mountains, northwest China. The lensoid orebodies are distributed along the shear zone in a region about 25 km in length and 1 km in width. Enhanced fluid flow and fracturing controlled location and orientation of mineralized zones during deformation along narrow shear bands oriented oblique to the overall shear zone margins. Fabrics in fractures and strain shadow overgrowths generally plunge steeply, parallel to the lineation in the surrounding cataclasite matrix consisting of new grains of quartz and minor amounts of micas. Native gold occurs in mylonitized granites or mylonitized sulfide–mica–quartz veins. Gold grade is closely related with the occurrence of sulfide–quartz veins. Pyrite and pyrrhotite in gold-bearing sulfide–quartz veins contain inclusions of native gold. Chondrite-normalized rare earth element (REE) distribution patterns for pyrite from the orebodies show light REE enrichments and negative Eu anomalies. Relationships between gold contents and Th/U, Zr/Hf and (La/Yb) N values for pyrites imply that gold-rich pyrites are different from gold-poor pyrites, thus indicate that the pyrites in the orebodies have a different origin compared with those in wall rocks. A Rb–Sr isochron for the mylonitized Au-bearing quartz veins indicates an age of 224 ± 14 Ma with an initial 87Sr/ 86Sr ratio of 0.7294 ± 0.0089 (MSWD = 1.1). This age is conformed by 40Ar/ 39Ar dating on muscovites (220.9 to 222.5 Ma), which is interpreted as the age of ore-formation in the Tianger deposit. High initial 87Sr/ 86Sr ratios (0.7294), high δ 34S values (+ 11.2 to + 16.5‰), low δ 18O H2O values (1.67 to 3.07‰) and low δD values (− 84 to − 104‰) indicate that the ore-forming fluid was unrelated to any magmatic process.

U-Pb and Re-Os Geochronologic Evidence for Two Alkalic Porphyry Ore-Forming Events in the Cadia District, New South Wales, Australia

The Cadia district, located in the eastern Lachlan fold belt of New South Wales, Australia, comprises four gold-copper porphyry deposits (Ridgeway, Cadia Quarry, Cadia Hill, and Cadia East) and two related ironcopper- gold skarn deposits (Big Cadia and Little Cadia). These deposits formed along a northwest-striking, arctransverse structural corridor within the intraoceanic Macquarie arc. This arc is composed of a belt of Early Ordovician to Early Silurian mafic to intermediate volcanic, volcaniclastic, and intrusive rocks of calc-alkalic to alkalic composition. The Cadia porphyry deposits are temporally and genetically associated with composite intrusive complexes of alkalic monzodiorite to quartz monzonite. U-Pb dating of igneous minerals from the intrusions and Re-Os dating of hydrothermal molybdenite from the deposits has revealed the presence of two temporally discrete events of magmatism and related porphyrystyle mineralization in the Cadia district. The monzonitic intrusive complex related to mineralization at Ridgeway and a quartz monzonite porphyry stock that lies immediately southwest of the Cadia Quarry deposit are early Late Ordovician (456-454 Ma). In contrast, the quartz monzonite porphyry stock that hosts the Cadia Quarry and Cadia Hill orebodies and an intermineral quartz monzonite porphyry dike at Cadia East are Early Silurian (~438 Ma). Re-Os molybdenite ages determined for quartz-sulfide veins within the early Late Ordovician quartz monzonite porphyry confirm multiple episodes of mineralization associated with the Cadia Quarry deposit, suggesting a complex history between 460 and 450 Ma. At Cadia Hill, Cadia Quarry, and Cadia East, a widespread event of porphyry gold-copper mineralization is recorded at about 443 to 441 Ma, based on three Re-Os molybdenite ages from sheeted quartz sulfide veins. Similar ages for magmatic zircon from the host intrusions support a link between dated intrusions and mineralization. These new ages are in general agreement with the ages of other porphyry gold-copper and related epithermal deposits of the Eastern Lachlan fold belt and help to constrain the relationship between the Cadia region and the evolution of the Lachlan fold belt. It is notable that the high-grade Ridgeway deposit is up to 18 m.y. older than any alkalic porphyry deposit yet discovered in the Macquarie arc, a feature that may be important for continued exploration success in this region.

Modeling of geochemical processes occurred during the formation of the Schlema deposit, Erzgebirge. II. Formation of pre-uranium veins

By means of computer modeling, it was shown that the pre-uranium quartz and quartz-sulfide veins of the Schlema deposit with their typomorphic assemblages could be formed at the expense of cooling of hot pore solutions from the Aue granite cupola underlying the mineralized zones. As was shown previously [1], the ore metal content of these pore solutions changes with decreasing temperature and pressure in the granite-water system and an increase in the intensity of granite “flushing” with a water phase. The dependence of the composition of precipitated material on the proportions of chlorides and carbon dioxide in the pore solutions was evaluated in the models. It was shown that silica mobilization from the granite and redeposition were accompanied by the oxidation of ferrous iron in the granite and extraction of sulfide sulfur in molal amounts similar to or even higher than those of silica. This provided conditions for uranium migration after these stages and formation of hydrothermal uranium ores.

Indium Enrichment in the Meng'entaolegai Ag-Pb-Zn Deposit, Inner Mongolia, China

Abstract. The Meng'entaolegai In-rich Ag-Pb-Zn deposit is located in the eastern part of Inner Mongolia. It is one of the In-richest deposits in China. Large amounts of quartz and sulfide minerals constitute a hydrothermal quartz-sulfide vein deposit within a Hercynian acidic granite massif, which occupies an area of about 400 km2. Thirty-six orebodies, controlled strictly by the E-W trend faults, are found in the orefield of 6 km in length from east to west and 200 to 1,000 m in width from south to north. The ore minerals are mainly galena, sphalerite and pyrite, and subordinate chalcopyrite, arsenopyrite, cassiterite and stannite with many Ag-minerals. The gangue minerals are mainly quartz, calcite, sericite and chlorite. Economic components of the deposit are dominated by Pb andZn (reserves of Pb and Zn are 0.17 Mt and 0.37 Mt, and their grades are 1 % and 2.3 %, respectively), with Ag, Sn, In and Cd (1,800 t Ag, >2,000 t Sn, >500 t In and 1,800 t Cd) as by-products. Indium is highly enriched in ores and its contents are 9 to 295 ppm in ores and 85 to 2,660 ppm in sphalerite. Analytical results show that the ore-forming fluid of this deposit contains 0.8–3.5 ppm In and 4–36 ppm Sn, and the two elements show a very good positive correlation with a correlation coefficient of 0.8672, while the correlation between In and Zn in the ore-forming fluids, with a correlation coefficient of 0.5723, is not as good as that between In and Sn. This indicates that indium has an affinity with tin in the ore-forming fluids. The authors think that this is probably the main reason why those In-rich deposits spread over the world are simultaneously enriched in tin.

Gold Mineralization And Zonation In The State Of Kelantan

Gold mineralization in Kelantan is mainly distributed in the central part of the state, bounded by Stong Igneous Complex and Senting Granite on the west, Kemahang Granite in the north and Boundary Range Granite in the east. The gold mineralization is more common in Triassic sedimentary rocks followed by Permian metasedimentary rocks. The oldest Silurian-Ordovician metasedimentary rocks only contain some insignificant gold mineralization, whilst the youngest Cretaceous-Jurassic sedimentary rocks are devoid of gold mineralization. Basically, the gold mineralization are identified as volcanogenic massive sulphides, skarns and hydrothermal quartz vein deposits. Six types of hydrothermal quartz veins can be observed, viz. low sulphide quartz veins, high sulphide quartz veins, quartz veins in sheared granite zones, quartz veins at the boundary of sedimentary rocks, structurally controlled quartz veins in volcanic-sedimentary rocks and metamorphic segregation quartz veins. The main factors contributing succession of gold mineralization are source rocks, heating chamber as well as depositional structures. The principal source rocks are Permian-Triassic volcanic rocks that are associated with sedimentary rocks. The heating chamber that induced the hydrothermal fluids is the granitoind bodies that intruded under the volcanic-sedimentary rocks, whilst the structures which allow the infiltration and deposition of gold are shear zones and fault zones originating from depth. Based on the type of ore deposits, geochemical data and geological setting, the study area can be divided into five zones, namely gold zone (hydrothermal veins), gold-base metal zone (volcanic exhalative), gold-silver­ mercury zone (hydrothermal veins), base metal-gold zone (massive sulphides) and silver-lead-zinc zone (skarn and massive sulphides).

Himalayan Cu–Mo–Au mineralization in the eastern Indo–Asian collision zone: constraints from Re–Os dating of molybdenite

We present new Re–Os molybdenite age data on three porphyry Cu–Mo–Au deposits (Yulong, Machangqing, and Xifanping). These deposits are associated with the Himalayan adakitic magmatism that occurred in a continental collision environment, controlled by large-scale Cenozoic strike-slip faults in the eastern Indo–Asian collision zone. Three distinct episodes of Cu–Mo–Au mineralization are recognized. At Yulong, Re–Os isotopic data of four molybdenite samples from sulfide-quartz veins in the quartz–sericite alteration zone yield an isochron with an age of 40.1±1.8 Ma (2σ), coincident to a zircon sensitive high-mass resolution ion microprobe (SHRIMP) age of 40.9±0.1 Ma for the host monzogranite. The molybdenite Re–Os dates, together with K–Ar, Rb–Sr, U–Pb, and 40Ar/39Ar dates on the pre- and intra-ore porphyries, suggest that Cu–Mo–Au mineralization formed during the late stage (∼40 Ma) of regional porphyry magmatism, but hydrothermal activity probably lasted to at least ∼36 Ma. At Machangqing, molybdenite Re–Os data from the K–silicate and quartz–sericite alteration zones yield an isochron with an age of 35.8±1.6 Ma (2σ), which is identical to the zircon SHRIMP and bulk-rock Rb–Sr ages (35∼36 Ma) of the host granite, but older than bulk-rock K–Ar dates (31∼32 Ma) for associated Au-bearing quartz syenite with advanced argillic alteration. At Xifanping, five molybdenite samples from the K–silicate alteration zone yield the youngest Re–Os isochron age in the area, at 32.1±1.6 Ma (2σ). The Re–Os molybdenite dates here are younger than K–Ar ages (33.5∼34.6) for hydrothermal biotite and actinolite. There is a positive correlation between the absolute age of the deposits and their Cu and Au reserves in the eastern Indo–Asian collisional zone. Episodic stress relaxation probably caused multiple magmatic intrusions, which most likely resulted in three episodes of Cu–Mo–Au mineralization in the eastern Indo–Asian collision zone.

Mineralogy and PT formation conditions of lead-zinc ore at the Dzhimidon deposit, north Ossetia

The mineralogy and PT formation conditions of the Dzhimidon Pb-Zn deposit in the Sadon ore district are considered. The deposit is localized in metamorphic rocks of the Buron Formation, which pertain to the pre-Jurassic basement (lower structural stage) and are cut through by Upper Paleozoic granitoids, and in the Lower Jurassic terrigenous sequence (upper structural stage). Orebodies as quartz-sulfide veins are mainly hosted in the metamorphic rocks. Galena, sphalerite, chalcopyrite, pyrite, pyrrhotite, and arsenopyrite are the most abundant sulfides, while quartz, carbonates, chlorite, sericite, and feldspar are gangue minerals. The bismuth mineralization identified at this deposit for the first time is represented by diverse phases of the Ag-Pb-Bi-S system. Five stages of the ore deposit formation are recognized: a premineral stage (quartz-feldspar), three ore-bearing stages (pyrite-arsenopyrite, pyrrhotite-chalcopyrite-sphalerite, and arsenopyrite-sphalerite-galena), and a postmineral stage (quartz-calcite); each stage comprises one or several mineral assemblages. The study of fluid inclusions in quartz, calcite, and sphalerite of the premineral, ore-forming, and postmineral stages has shown that the ore was deposited mainly from Na chloride solution with a salinity varying from >22 to <1.0 wt % NaCl equiv at a temperature from 460 to ∼120°C and 430–290 bars pressure. The third stage was characterized by an abrupt increase in temperature and by the appearance of Mg(Fe,Ca) chloride solutions equally with Na chloride fluids, presumably owing to the emplacement of granite porphyry.

Hydrothermal source rocks of the Meng'entaolegai Ag-Pb-Zn deposit in the granite batholith, Inner Mongolia, China: Constrained by isotopic geochemistry

The Meng'entaolegai Ag-Pb-Zn-In polymetallic deposit is located in the eastern part of Inner Mongolia. It consists of a hydrothermal quartz-sulfide vein deposit hosted within a Hercynian granite massif which is about 400 km2 in size. More than 40 orebodies are found in the orefield which is 6 km in length from east to west and 200 to 1,000 m in width from south to north. All orebodies are controlled by E-W trending faults. Economic resources are dominated by Pb and Zn (reserves of Pb and Zn are 0.17 Mt and 0.37 Mt, and their grades are 1% and 2.3%, respectively), with Ag, Sn, In and Cd (1,800 t Ag, >2,000 t Sn, >500 t In and 1800 t Cd) as by-products. The δ34SCDT values of sulfides range from -1.7‰ to +4.6‰, averaging 1.4‰, indicating that sulfur is magmatic in origin. The H-O isotopic compositions (δD values from -52.8‰ to -66.9‰ and δ18OH2O values from 4.8‰ to 7.9‰) of the fluid inclusion water in quartz show that the ore-forming fluid has a mainly magmatic source. The lead isotopic compositions of ore are relatively homogeneous and are significantly lower than those of feldspars and whole rock of the Hercynian Meng'entaolegai granite and the Yanshanian Duerji granite. Neither these granites nor the regional basement Precambrian metamorphic rocks provided ore with lead. By comparison, ore lead isotopic composition is the same as that of E-W trend diorite dykes which intruded the granites and this indicates that the ore lead is probably derived from magamatism which is related to these dykes. We considered therefore that although orebodies occurred within the Hercynian Meng'entaolegai granite, the deposit origin is not related to this stage of magmatism.

Intrusion-Hosted Mineralization in the Charters Towers Goldfield, North Queensland: New Isotopic and Fluid Inclusion Constraints on the Timing and Origin of the Auriferous Veins

Auriferous quartz sulfide veins of the Charters Towers goldfield are mainly hosted in oxidized I-type granitoids of the Ravenswood batholith. K-Ar and Ar-Ar isotope age data of hydrothermal muscovite from alteration envelopes of veins at Charters Towers and the Hadleigh Castle mine (~40 km east of Charters Towers) are indistinguishable within error, suggesting broadly synchronous gold deposition between 410 and 404 Ma and across a significant segment of the Ravenswood batholith. Published geochronological data indicate that several granitoid bodies were emplaced into the Ravenswood batholith at the time of gold mineralization. Despite their association in space and time with igneous rocks, the gold deposits lack an obvious causative intrusion. Moreover, published lead isotope studies indicate that the lead in the ore was not acquired from any of the exposed intrusions. A distal origin of the ore-forming fluids also may be inferred from nitrogen isotope values of hydrothermal sericite, suggesting wall-rock interaction with metamorphic fluids or fluids that were in equilibrium with metamorphic rocks during the paragenetic stage of pyrite and arsenopyrite deposition (stage II). Veins of the Charters Towers goldfield contain three different types of fluid inclusions, which are distinguishable by petrography and microthermometry. This study is the first to report saline fluid inclusions in sphalerite (18.9–28.3 wt % NaCl equiv) and vein quartz (20.9–24.7 wt % NaCl equiv), trapped during the stage of gold deposition (stage III). The range of temperatures and salinities, particularly of the saline inclusions in sphalerite, could indicate mixing between hotter, more saline fluids (e.g., deep-seated magmatic) and cooler, more dilute solutions (e.g., modified meteoric) as the cause of gold deposition. The geological and geochemical data are not compatible with derivation of fluids, metals, and ligands from individual plutons. Similarity of host rocks, ore element associations, alteration assemblages, structural controls, and tectonic settings strongly suggest that the auriferous veins of the Charters Towers goldfield belong to a group of granitoid-hosted lode gold deposits that are generally classified as orogenic.

Linking mineral and fluid inclusion paragenetic studies: The Batman deposit, Mt. Todd (Yimuyn Manjerr) goldfield, Australia

The quartz–sulphide vein system of the Batman deposit at the Mt. Todd Mine, Australia, formed during east–west extension of a north–northeasterly trending structure system, early in D 2 during retrograde contact metamorphism associated with cooling of the Tennysons leucogranite. Four distinct fluid inclusion types have been identified in gold-bearing quartz–sulphide veins of the deposit: (Type I) low to moderate salinity aqueous primary inclusions (1 to 20 wt.% NaCl equivalent) with small amounts of CO 2; (Type II) high salinity Na–Ca–Cl aqueous secondary inclusions (35 to 50 wt.% total NaCl–CaCl 2), containing halite, carbonate and trona daughter minerals, and minor CO 2–CH 4 vapour; (Type III) liquid and vapour secondary fluid inclusions (28 to 34 or 17 to 18 wt.% NaCl equivalent) with CH 4 vapour and discrete gold and bismuth grains and associated growth zones; (Type IV) vapour-rich CO 2 ± (CH 4) secondary inclusions with minor NaCl liquid (> 20 wt.% NaCl equivalent). A consistency in bulk fluid composition suggests a common parent fluid and a common parent source for the fluid inclusions. Previous paragenetic and textural studies have established a sequence in sulphide and silicate precipitation. The mineral paragenesis has been linked to the trapping chronology of Types I–IV fluid inclusions in order to constrain the fluid conditions during (micro)-tectonism and the mineralising process. The link is possible because two mineral phases exhibit a temporal genetic relationship with the formation of fluid inclusions: (1) Growth zones in simple open space quartz fill contain tourmaline crystals in intimate association with Type I liquid–vapour fluid inclusions; (2) Gold is intimately associated with bismuth in Type III vapour–liquid fluid inclusions, either as discrete inclusions within the trails, or wetting the inclusions. The progressive change in the composition of Types I–IV is consistent with the evolution of a retrograde contact metamorphic fluid in wall rock lithologies.

Geological, mineralogical and geochemical characteristics of the Radzimowice Au?As?Cu deposit from the Kaczawa Mountains (Western Sudetes, Poland): an example of the transition of porphyry and epithermal style

The sheeted quartz–sulfide veins of the Radzimowice Au–As–Cu deposit in the Kaczawa Mountains are related to Upper Carboniferous post-collisional potassic magmatism of the composite Zelezniak porphyry intrusion. Multiple intrusive activity ranges from early calc-alkaline to sub-alkaline and alkaline rocks and is followed by multiple hydrothermal events. Early crustally derived dacitic magma has low mg# ( 3 ppm Au is about 1:5 (geometric mean). Hydrothermal alteration started with sericitization, pyritization, and kaolinitization in vein selvages followed by alkaline hydrothermal alteration of propylitic character (illitization and chloritization), albitization and carbonatization. The mineralization of the Radzimowice deposit is considered as related to alkaline magmatism and is characterized by the superposition of low-sulfidation epithermal mineralization on higher-temperature and deeper-seated mesothermal/porphyry style.

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 Characteristics of Auriferous Quartz Veins in Archean and Paleoproterozoic Greenstone Belts of Eastern and Southern Finland

Fluid inclusions in auriferous quartz veins in five deposits in the Archean (2.9–2.7 Ga) and Paleoproterozoic (2.0–1.75 Ga) greenstone belts of eastern and southwestern Finland have been characterized on the basis of their composition, reconstructed isochores, and decrepitation behavior. The dominant fluids associated with gold mineralization are H2O-CO2 ± CH4 ± halite (Archean) and H2O-CO2-CH4 or H2O-CH4 (Paleoproterozoic) in composition; phase ratios were generally uniform H2O/CO2 in the Archean but were variable in the Paleoproterozoic. In the Archean Au ± sulfide quartz veins, the mineralizing fluids contain less CH4 (<30 mol %) and have higher salinities (up to 39 wt % NaCl equiv) than the corresponding fluids in the Paleoproterozoic Au-sulfide-quartz veins (up to 38 mol % CH4 and below 13 wt % NaCl equiv). Additionally, the estimated pressure-temperature conditions for gold mineralization differ markedly between the two domains. Mineralization occurred in a pressure-temperature range of 300° to 500°C at 2 to 3.5 kbars in the Archean domain and 200° to 300°C at 1 to 2 kbars in the Paleoproterozoic domain. Decrepigrams for the Archean and Paleoproterozoic auriferous quartz veins show different characteristics and permit classification of the different fluid inclusion populations on a regional scale.

Characterization of sulfide deposits by airborne magnetic and gamma-ray responses in eastern Finland

Integrated interpretation of aerogeophysical data permits geophysical characterization of sulfide deposits in eastern Finland. The structural control and the hydrothermal alteration of the rocks can be discerned from airborne magnetic and radiometric data. The Cu–Co–Zn deposits associated with serpentinite–dolomite–calc-silicate rock–siliceous rock–black shale settings of the Outokumpu type are locally characterized by reduced intensity of magnetization and electrical conductivity and reduced eTh and K radiation. Enhanced eU/eTh anomalies are commonly spatially related to major structural lineaments, such as the boundaries of structurally more coherent bedrock blocks or intersections of faults and shear zones. Electromagnetic surveys can be used to distinguish black shales and serpentinites in the region. The sulfide deposits in eastern Finland can be divided into two groups on the basis of airborne geophysical characteristics: (1) intense magnetization and high electrical conductivity and (2) moderate magnetization and electrical conductivity. In the sulfide deposits of group 1, ore-forming constituents mainly precipitated during primary sedimentation (for example, the black-shale-hosted Ni–Cu–Zn ore at Talvivaara). Group 2 is related to sulfide deposits where major ore-forming processes have affected the associated black shale formation after the sedimentation phase (Outokumpu-type Cu–Co–Zn deposits). In cases where the magnetization and electrical conductivity of black shales are especially weak, ore formation processes probably continued during late metamorphic or post-metamorphic deformation. The theoretical basis for the observed differences in magnetic response between different deposits has to do with the mineralogy of the black shales. Pyrite changes into ferrimagnetic monoclinic pyrrhotite during metamorphism above greenschist facies, especially along tectonic structures. If no further alteration occurs, pyrrhotite-bearing black shale formations appear as highly magnetic and electrically conductive zones on geophysical maps. Wherever intense tectonic deformation, emplacement of quartz–sulfide veins, hydrothermal alteration, or contact metamorphism has affected the black shale formation after sedimentation, both magnetic and electromagnetic responses may be weakened because of diminished amounts of the pyrrhotite-bearing and graphite-rich material. In addition, the presence of antiferromagnetic pyrrhotite of hexagonal crystal structure may decrease the magnetic response. The classification presented here applies specifically to eastern Finland, but may be widely applicable to terranes with abundant black shale formations that have undergone regional metamorphism of upper greenschist facies or higher. Association of sulfide ores with black shales is a common feature in the geological record. Our integrated geophysical method offers a quick way of examining large areas, and it should prove useful in the search for sulfide deposits in other parts of the world.

Structure, Isotopes, and 40Ar/39Ar Dating of the Pengjiakuang Gold Deposit, Mesozoic Jiaolai Basin, Eastern China

The Pengjiakuang gold deposit, located on the northeastern margin of the Mesozoic Jiaolai basin, eastern Shandong Province, China, is controlled by a low-angle detachment fault. Three economic ore bodies and more than 10 mineralized bodies have been proved in recent years. The main ore bodies are made up of gold-bearing breccia, cataclastic rocks, and sulfide-quartz veins in lenses and veins. Gold ores are typically brecciated, veinlet, and disseminated. Lamprophyre dikes are spatially and temporally associated with the gold ore bodies. The isotopic analysis indicates that the δ34S range (6.5-12.89‰) of sulfides in the Pengjiakuang gold deposit is consistent approximately with that of metamorphic rocks, Mesozoic granites, and the Jiaojia and Linglong gold deposits in eastern Shandong. However, there are wide ranges of compositions of δ34S and lead isotopes in the Pengjiakuang ore except for lower 206Pb/204Pb ratios. Analyses of sulfur and lead isotopic compositions suggest that the ores have a multiple origin. Unlike the Jiaojia and Linglong deposits, the oxygen and hydrogen isotopic compositions of fluid inclusions in the Pengjiakuang deposit show a mixed source: dominant meteoric water, and subordinate magmatic water. The Ar-Ar method was adopted to date the Au-bearing quartz. Brecciated ore yields a 40Ar/39Ar age of 118.42 ± 0.25 Ma and an isochron age of about 117.03 ± 0.13 Ma. Veinlet ore yields a 40Ar/39Ar age of 118.70 ± 1.40 Ma and an isochron age of 117.33 ± 0.15 Ma. The results indicate that the Pengjiakuang gold deposit was formed at ∼117.3-118.4 Ma. Biotite from a lamprophyre dike gives an age of 117.49 ± 0.25 Ma and an isochron age of 116.83 ± 0.36 Ma. Clearly, the formation of both lamprophyre dikes and gold mineralization are coeval products of tectonic-magmatic activity. The consistency in metallogenic age between the Pengjiakuang gold deposit, located on the margin of a Mesozoic pull-apart basin, and gold deposits of the northern uplift area in eastern Shandong suggests that both can be ascribed to the same the metallogenic event.