Massive Sulfide Veins Research Articles

Petrographical and Geochemical Features of Sulfide Mineralization in the Walash Group, Gallala Area, Kurdistan Region of Iraq

In this article, a Fe-rich sulfide mineralization hosted within the Walash Group in the Gallala area, on the northwestern side of the Zagros Orogen is investigated for the first time. The mineralization occurs as massive sulfide veins and veinlets, as well as disseminated within volcanic lithologies. The objective of the study is to characterize the sulfide-rich mineralization using petrographical and geochemical methods, applied in both the ores and the corresponding host formations, including the characterization of volcanic rock alteration. Petrographic data indicates the presence of Fe-rich sulfide mineralization in altered basaltic andesite to andesite rocks, which are affected by hydrothermal alteration such as actinolization, chloritization, sericitization, and epidotization. The geochemical analysis of the fresh rock samples reveals enrichment of iron in the suture zones, but low copper and zinc concentrations. The mineralogical and textural signatures of sulfide minerals lead to inferring that the ore mineralization origin is volcanogenic to the hydrothermal type of deposits.

Geologic setting of the Pueblo Viejo Au–Ag–Cu-(Zn) mining district, Dominican Republic - Links to volcanic domes and volcanogenic massive sulfide mineralization

Geologic mapping at the Pueblo Viejo Au–Ag–Cu-(Zn) mine, Dominican Republic, and across the surrounding Pueblo Viejo district, reveals the geologic setting at the time of mineralization. Ore deposits consisting of disseminated sulfide, bedded massive sulfide, and high-grade massive sulfide veins formed (∼112 Ma) in a volcanic dome field during a period of extension across an emergent, intraoceanic island arc. Geologic maps reveal the links between volcanic domes and structure, hydrothermal alteration and mineralization, and ore deposits and gossans. Whole-rock geochemical data establish a tholeiitic signature for Early Cretaceous magmatism including the volcanic dome field. Northerly-striking, high-angle faults controlled the emplacement of volcanic domes and served as feeders for hydrothermal solutions. The largest ore deposits formed in a subsiding, marginal marine, volcano-sedimentary basin. Stacked intervals of bedded massive sulfide mineralization indicate that the basin was subsiding while mineralization was underway. Calc-alkaline dikes and plutons in the Pueblo Viejo district intruded at ∼88 Ma, well after mineralization had ended. Low-angle reverse faulting, greenschist facies metamorphism, and milky quartz veining and replacement were also Late Cretaceous (post-mineral) events. Ore deposits in the Pueblo Viejo district are best described as hybrid epithermal – VMS deposits, formed during a period of Early Cretaceous extension, tholeiitic volcanism, and volcanic dome emplacement in a shallow, marginal marine environment. Exploration for Au–Ag–Cu-(Zn) deposits like Pueblo Viejo should focus on intraoceanic island arcs, tholeiitic volcanic dome fields, extensional volcano-sedimentary basins, and VMS mineral occurrences.

Occurrences of Pd–Pt Bismuthotellurides and a Phosphohedyphane-Like Phase in Sulfide Veins of the Monchepluton Layered Complex, Kola Peninsula, Russia

We describe occurrences of platinum-group minerals (PGM) and an uncommon mineral enriched in Cl, and provide a brief review of Cl-bearing minerals associated with basic–ultrabasic complexes. An unusual phosphohedyphane-like phase (~30 µm), close to CaPb4(PO4)3Cl, occurs in one of the PGM-bearing veins of massive sulfides in the Monchepluton layered complex, Kola Peninsula, Russia. These veins consist of varying amounts of pyrrhotite, pentlandite, chalcopyrite, pyrite and accessory grains of galena; they are fairly abundant in the heavy-mineral concentrate, as are small (<0.1 mm) grains of PGM: michenerite, sperrylite, Bi-enriched members of the merenskyite–moncheite series and kotulskite, also rich in Bi. The PGE mineralization is attributed to a low-temperature deposition at the hydrothermal stage. The pyromorphite–phosphohedyphane solid solution likely formed as a secondary phase under conditions of a progressive build-up of oxygen fugacity via oxidation reactions of a precursor grain of galena and involving Ca, as an incompatible component of the sulfides, in a medium of residual fluid enriched in Cl.

The Westwood Deposit, Southern Abitibi Greenstone Belt, Canada: An Archean Au-Rich Polymetallic Magmatic-Hydrothermal System—Part II. Hydrothermal Alteration, Mineralization, and Geologic Model

Abstract The Westwood deposit, located in the Archean Doyon-Bousquet-LaRonde mining camp in the southern Archean Abitibi greenstone belt, contains 4.5 Moz (140 metric t) of gold. The deposit is hosted in the 2699–2695 Ma submarine, tholeiitic to calc-alkaline volcanic, volcaniclastic, and intrusive rocks of the Bousquet Formation. The deposit is located near the synvolcanic (ca. 2699–2696 Ma) Mooshla Intrusive Complex that hosts the Doyon epizonal intrusion-related Au ± Cu deposit, whereas several Au-rich volcanogenic massive sulfide (VMS) deposits are present east of the Westwood deposit. The Westwood deposit consists of stratigraphically stacked, contrasting, and overprinting mineralization styles that share analogies with both the intrusion-related and VMS deposits of the camp. The ore zones form three distinct, slightly discordant to stratabound corridors that are, from north (base) to south (top), the Zone 2 Extension, the North Corridor, and the Westwood Corridor. Syn- to late-main regional deformation and upper greenschist to lower amphibolite facies regional metamorphism affect the ore zones, alteration assemblages, and host rocks. The Zone 2 Extension consists of Au ± Cu sulfide (pyrite-chalcopyrite)-quartz veins and zones of disseminated to semimassive sulfides. The ore zones are spatially associated with a series of calc-alkaline felsic sills and dikes that crosscut the mafic to intermediate, tholeiitic to transitional, lower Bousquet Formation volcanic rocks. The metamorphosed proximal alteration consists of muscovite-quartz-pyrite ± gypsum-andalusite-kyanite-pyrophyllite argillic to advanced argillic-style tabular envelope that is up to a few tens of meters thick. The North Corridor consists of auriferous semimassive to massive sulfide veins, zones of sulfide stringers, and disseminated sulfides that are hosted in intermediate volcaniclastic rocks at the base of the upper Bousquet Formation. The Westwood Corridor consists of semimassive to massive sulfide lenses, veins, zones of sulfide stringers, and disseminated sulfides that are located higher in the stratigraphic sequence, at or near the contact between calc-alkaline dacite domes and overlying calc-alkaline rhyodacite of the upper Bousquet Formation. A large, semiconformable distal alteration zone that encompasses the North Corridor is present in the footwall and vicinity of the Westwood Corridor. This metamorphosed alteration zone consists of an assemblage of biotite-Mn garnet-chlorite-carbonate ± muscovite-albite. A proximal muscovite-quartz-chlorite-pyrite argillic-style alteration assemblage is associated with both corridors. The Zone 2 Extension ore zones and associated alteration are considered synvolcanic based on crosscutting relationships and U-Pb geochronology and are interpreted as being the distal expression of an epizonal magmatic-hydrothermal system that is centered on the upper part of the synvolcanic Mooshla Intrusive Complex. The North and Westwood corridors consist of bimodal-felsic Au-rich VMS-type mineralization and alteration produced by the convective circulation of modified seawater that included a magmatic contribution from the coeval epizonal Zone 2 Extension magmatic-hydrothermal system. The Westwood Au deposit represents one of the very few documented examples of an Archean magmatic-hydrothermal system—or at least of such systems formed in a subaqueous environment. The study of the Westwood deposit resulted in a better understanding of the critical role of magmatic fluid input toward the formation of Archean epizonal intrusion-related Au ± Cu and seafloor/subseafloor Au-rich VMS-type mineralization.

Ni-Cu sulfide mineralization and PGM from the Samapleu mafic-ultramafic intrusion, Yacouba complex, western Ivory Coast

ABSTRACT The mafic-ultramafic Samapleu deposits of the Yacouba complex, which host nickel, copper sulfides, and platinum-group minerals, are located in the Biankouma-Silipou region, western Ivory Coast. These intrusions originate from the mantle and would have been established during the Proterozoic (2.09 Ga) around 22 km deep within the Archean granulites (3.6–2.7 Ga) which at least partially contaminated them. Platinum-group and sulfide minerals from the Samapleu deposits were studied using optical microscopy, scanning electron microscopy, the electronic microprobe, X-ray fluorescence, fire assay, and a Thermo Fisher Scientific Delta S isotope ratio mass spectrometer system. The sulfide mineralization (mainly pyrrhotite, pentlandite, chalcopyrite ± pyrite) is mainly disseminated with, in places, semi-massive to massive sulfide veins. It is especially abundant in pyroxenite horizons with net or breccia textures. The isotopic ratios of sulfur measured from the sulfides (an average of 0.1‰), the R factor (between 1500 and 10,000), and the Cu/Pd ratios indicate a mantle source. Thus, the sulfides would have formed from sulfide liquids produced by immiscibility from the silicate mantle magma under mafic-ultramafic intrusion emplacement conditions and with possible geochemical modification of the magmas by assimilation of the surrounding continental crust. The platinum-group minerals (michenerite, merenskyite, moncheite, Co-rich gersdorffite, irarsite, and hollingworthite) are mainly associated with the sulfide phases. The nature of the platinum-group minerals is indicative of the probable role of late-magmatic hydrothermal fluids during the mineralizing process.

Epithermal mineralization of the Bonanza-Sandy vein system, Masara Gold District, Mindanao, Philippines

The Masara Gold District in southeastern Mindanao island is an area of prolific hydrothermal copper and gold mineralization. This study documents the mineralization characteristics of the NW-trending Bonanza-Sandy epithermal veins to constrain possible hydrothermal fluid sources and ore-forming mechanisms. Epithermal mineralization in the NW veins is divided into three main stages: Stage 1 - massive quartz-sulfide; Stage 2 - massive to amorphous quartz-carbonate (calcite); and Stage 3 - colloform-cockade quartz-carbonate (bladed rhodochrosite). Stage 1 is the main gold mineralization phase, with chalcopyrite, pyrite, sphalerite and galena occurring with native gold and tellurides. Stages 2 and 3 contain invisible gold in the sphalerite, galena, pyrite and chalcopyrite. The deposit exhibits mineralization characteristics typical of intermediate sulfidation epithermal deposits based on the dominant chalcopyrite-pyrite mineral assemblage; illite-muscovite-chlorite alteration mineralogy that point to neutral pH conditions; and sphalerite composition of 2.26 to 8.72 mol% FeS in Stage 1 and 0.55 to 1.13 mol% FeS in Stage 2. The K-Ar age date of illite separates from highly altered diorite porphyry of the Lamingag Intrusive Complex yielded an Early Pliocene age (5.12 ± 0.16 Ma). Hydrothermal fluid exsolved from the magma that formed the Lamingag Intrusive Complex probably formed the ore-forming Stage 1 veins. Stages 2 and 3 involved the deposition of quartz and carbonate veins possibly by boiling hydrothermal fluids. Precious and base metal deposition was controlled by the Masara Fault Zone. Exploration markers for gold mineralization in the Masara Gold District and vicinity include the presence of Lamingag Intrusive Complex and massive sulfide veins.

Emplacement of sharp-walled sulfide veins during the formation and reactivation of impact-related structures at the Broken Hammer Mine, Sudbury, Ontario

Broken Hammer is a hybrid Cu–Ni–Platinum Group Element (PGE) footwall deposit located in Archean basement rocks below the impact-induced Sudbury Igneous Complex (SIC), Canada. The deposit consists of massive chalcopyrite veins surrounded by thin epidote, actinolite, and quartz selvedges and low-sulfide, high-PGE mineralization consisting of disseminated chalcopyrite (<5%) and platinum group minerals, associated with Ni-bearing chlorite overprinting alteration patches of epidote, actinolite, and quartz. The veins are grouped into five steeply-dipping sets, striking northeast-, southwest-, southeast-, south-, and east–west, which were emplaced along impact-related fractures that were reactivated multiple times during stabilization of the crater floor. Early reactivation of the fractures created pathways for the migration of hydrothermal fluids from which quartz and chlorite precipitated sealing the fractures. Renewed slip shattered the quartz–chlorite veins into fragments that were incorporated in massive sulfide veins that crystallized from fractionated sulfide melts or from high temperature (400–500 °C) hydrothermal fluids, which migrated outward into the basement rocks from a cooling and crystallizing SIC melt sheet. Hydrothermal fluids syn-genetic with the epidote–actinolite–quartz alteration distributed the PGE into the footwall rocks, or late hydrothermal fluids associated with the Ni-bearing chlorite leached Ni and PGMs from the sulfide veins and redistributed them to form low-sulfide, high-PGE zones in the footwall rocks. During post-impact tectonic events, slip at temperatures below the brittle–ductile transition for chalcopyrite (<200 °C to 250 °C) produced striations along the vein margins. The Broken Hammer deposit exemplifies how Cu–Ni–PGE footwall deposits formed by the reactivation of impact-related fractures that provided conduits for the migration of melts and hydrothermal fluids.

High-sulfidation veins in the Jiama porphyry system, South Tibet

The giant Jiama polymetallic ore deposit is a porphyry–skarn system in the eastern Gangdese porphyry belt of the Tibetan Plateau. Steeply dipping high-grade massive sulfide veins are classified as lode-type high-sulfidation (HS) mineralization comprising mainly Cu-sulfosalts in Au-poor and Au-rich veins. Au-poor veins contain an As-rich pyrite–enargite–tennantite assemblage, whereas Au-rich veins contain an Sb-rich enargite–luzonite–tetrahedrite–chalcostibite–watanabeite assemblage. The coexistence of pyrite, dendritic muscovite, and euhedral quartz in the Cu-sulfosalts indicates slightly acidic or near-neutral formation conditions. Most Au- and Ag-bearing minerals are inclusions of tellurides (hessite, sylvanite, and petzite) in tennantite–tetrahedrite, and watanabeite. Au and Ag concentrations in tennantite–tetrahedrite, watanabeite, and chalcostibite are much higher than in enargite and luzonite, confirming that they were controlled by intermediate-sulfidation state Cu–As–Sb–S solid solutions, with Au tending to be enriched in Sb-rich minerals compared with As-rich solid solutions. The HS veins overlying the known porphyry intrusion and cutting skarn orebodies in the Jiama deposit constitute a porphyry–skarn–high sulfidation system. The occurrence of HS veins suggests that channels in paleo-fumaroles and the mineralization that resulted may be ascribed to expansion of magmatic vapor. HS veins close to the porphyry intrusion exposed at the surface indicate that shallow epithermal disseminated mineralization was likely removed by rapid regional uplift and erosion of the Gangdese porphyry belt.

The Chineysky gabbronorite-anorthosite layered massif (NorthernTransbaikalia, Russia): its structure, Fe-Ti-V and Cu-PGE deposits, and parental magma composition

The 1858 ± 17 Ma Chineysky layered anorthosite-gabbronorite massif is located in the southern part of the Siberian platform, within the Kodaro-Udokan metallogenic province of Northern Transbaikalia. The Chineysky Massif outcrops over approximately 130 km2 and contains Russia’s largest V ore resources, hosted within titanomagnetite-rich layers, concentrated in the Magnitny and Etyrko deposits. The titanomagnetite ore reserves were estimated at 2 billion tons with 30 % Fe and 10 % TiO2 on average. In addition, two large Cu-PGE deposits—Rudny and Kontactovy—are hosted in the contact rocks between the intrusion and the sandstone floor rocks. A distinctive feature of the Chineysky sulfide ore is their Cu-enriched composition with much lesser amounts of nickel and cobalt (Cu/Ni/Co ~ 76:7:1). The sulfide ore contains up to 355 ppm PGE and 30 ppm Au + Ag. Three types of sulfide mineralization have been distinguished: (1) endo-contact disseminated sulfides within gabbronorite, (2) exo-contact impregnations in sandstone, and (3) veins of massive sulfides in the exo-contact sandstone. The ore consists predominantly of chalcopyrite, with less abundant pentlandite, pyrrhotite, Co-Ni arsenides and sulfoarsenides, linneite-group minerals, sphalerite, cubanite, and millerite. In addition, many rare minerals were recognized in the ores, including PGM (sperrylite, michenerite, merenskyite, etc.). Using the latest version of the COMAGMAT-5 program, the parental magma temperature (~1150 °C), its composition (~55 wt.% SiO2, 5.8 % MgO), and the most primitive olivine (Fo77) and plagioclase (An69) compositions were calculated. According to the model, titanomagnetite starts to crystallize at T < 1133 °C (fO2 = NNO + 0.5), triggering sulfide liquid immiscibility when the silicate magma had ~0.15 to 0.1 wt.% S.

Chalcophile element (Ni, Cu, PGE, and Au) variations in the Tamarack magmatic sulfide deposit in the Midcontinent Rift System: implications for dynamic ore-forming processes

The Tamarack magmatic sulfide deposit is hosted by the Tamarack Intrusive Complex (1105.6 ± 1.2 Ma) in the Midcontinent Rift System. The most important sulfide mineralization in the Complex occurs in the northern part, which consists of two separate intrusive units: an early funnel-shaped layered peridotite body containing relatively fine-grained olivine (referred to as the FGO Intrusion) at the top, and a late gabbro-troctolite-peridotite dike-like body containing relatively coarse-grained olivine (referred to as the CGO Intrusion) at the bottom. Disseminated, net-textured, and massive sulfides occur in the base of the FGO Intrusion as well as in the upper part of the CGO Intrusion. The widest part of the CGO Intrusion also hosts a large semi-massive (net-textured) sulfide ore body locally surrounded by disseminated sulfide mineralization. Small massive sulfide veins occur in the footwall of the FGO Intrusion and in the wall rocks of the CGO dike. The sulfide mineralization is predominantly composed of pyrrhotite, pentlandite, and chalcopyrite, plus minor magnetite. Pyrrhotite containing the highest Ni and Co contents occurs in the FGO disseminated sulfides and in the CGO semi-massive sulfide ores, respectively. The most important platinum-group minerals associated with the base metal sulfides are sperrylite (PtAs2), sudburyite (PdSb), and michenerite (PdBiTe). Nickel shows a strong positive correlation with S in all types of sulfide mineralization, and Cu shows a strong positive correlation with S in the disseminated sulfide mineralization. At a given S content, the concentrations of Pt, Pd, and Au in the CGO disseminated sulfides are significantly higher than those in the FGO disseminated sulfides. The semi-massive sulfide ores are characterized by significantly higher IPGE (Ir, Os, Ru, and Rh) concentrations than most of the massive sulfide ores. With few exceptions, all of the various textural types of sulfide mineralization collectively show a good positive correlation between Pt and Pd, and between individual IPGE. At a given Pt or Pd content, however, the semi-massive sulfide ores have higher IPGE contents than the disseminated sulfide samples. Modeling results show that the variations in PGE tenors (metals in recalculated 100 % sulfide) in the Tamarack magmatic sulfide deposit are mainly controlled by variable R factors (magma/sulfide-liquid mass ratios) during sulfide-liquid segregation and subsequent monosulfide solid solution (MSS) fractionation during cooling. The initial contents of Ir, Pt, and Pd in the parental magma, estimated from the metal tenors of the disseminated sulfides, are 0.2, 2, and 1.8 ppb, respectively, which are ∼1/5 of the values for the PGE-undepleted primitive basalts of the Midcontinent Rift System. The variations of PGE tenors in the semi-massive and massive sulfide ores can be explained by MSS fractional crystallization from sulfide liquids. Extreme variations in the PGE contents of the massive sulfides may also in part reflect metal mobility during post-crystallization hydrothermal processes. The higher PGE tenors for the disseminated sulfides in the CGO dike relative to those in the FGO Intrusion are consistent with formation in a dynamic conduit where the early sulfide liquids left in the conduit by the FGO magma were subsequently upgraded by the subsequent surge of the CGO magma. The relatively low PGE tenors for the semi-massive and massive sulfides can be explained by lack of such an upgrading process for the sulfide due to their distal locations in a migrating conduit.

The Samapleu mafic-ultramafic intrusion and its Ni-Cu-PGE mineralization: an Eburnean (2.09 Ga) feeder dyke to the Yacouba layered complex (Man Archean craton, western Ivory Coast)

AbstractThe Yacouba layered complex intrudes the Archean (3.5–2.7 Ga) Kenema-Man craton in the Samapleu-Yorodougou area, western Ivory Coast. In Samapleu area, the complex was recognized in drill holes at three locations: Samapleu Main (SM); Samapleu Extension 1 (E1) and Yorodougou (Yo). It comprises websterites, peridotites and gabbro-norites arranged symmetrically with mafic layers at the center and ultramafic layers at both margins. The complex is inclined at 70–80° to the SE. The thickness of individual layers varies from 2 to 60 m and the total thickness is 120 to 200 m. At the E1 site, the complex extends to depths &amp;gt; 500 m.Contacts with the country rock gneiss are characterized by a hybrid zone that is a few meters thick and composed of plagioclase-orthopyroxene bearing metabasites, and locally (E1 site) a metamorphic assemblage of sapphirine-cordierite-sillimanite-spinel ± rutile. This assemblage is attributed to contact metamorphism during intrusion of the complex in the lower crust at a depth of about 25 km. Zircons in country rock gneisses and granulites, as well as in the hybrid facies, yield Archean ages of ~ 2.78 Ga, similar to ages reported in the Man craton. Rutiles in the hybrid zone give a U-Pb age of 2.09 Ga, which is interpreted as the age of contact metamorphism and emplacement of the intrusion.The Samapleu Main and Samapleu Extension 1 sites contain Ni and Cu sulfide deposit with reserves estimated as more than 40 million tons grading 0.25% Ni and 0.22% Cu (Sama Nickel-CI, August 2013). The Ni-Cu mineralization is composed of pentlandite, chalcopyrite, pyrrhotite and rare pyrite, which is disseminated mainly in pyroxenite or occurs as subvertical and semi-massive to massive sulfide veins. The sulfide textures range from matrix ore, net-textured, droplets or breccia textures. Zones enriched in PGM, particularly Pd, are associated with the sulfides and several chromite bands are also present. These observations suggest that an immiscible sulfide liquid formed from a parental silicate liquid and percolated through the crystal pile.The parental melt composition, determined using the Chai and Naldrett [1992] method, has a SiO2-rich mafic composition with 53% SiO2 and 10% MgO. This result, the presence of the hybrid zone, and the trace-element signature determined using the Bedard [1994] method, suggest a mantle-derived basaltic parental magma that had assimilated abundant continental crust.These observations indicate that Samapleu intrusion corresponds to a magmatic conduit of the Yacouba complex as at Jinchuan (China), Voisey’s bay (Canada), Kabanga (Tanzania) or Nkomati (South Africa).

THE LENGSHUIQING Ni-Cu DEPOSIT, SICHUAN, SOUTHWESTERN CHINA: ORE CHARACTERISTICS AND GENESIS

Lengshuiqing is a Ni–Cu sulfide deposit related to the Neoproterozoic Panxi arc, in southwestern China. The mineralization is associated with five separate but genetically related intrusions. The magmatic rocks from Lengshuiqing range from peridotite and olivine pyroxenite to gabbro, diorite, quartz diorite, granodiorite and granite. The sulfide ore occurs in the ultramafic rocks and in four distinct facies: (1) disseminated to net-textured (the dominant type), (2) blebby (disseminated globular), (3) breccia matrix ore, and (4) rare veins of massive sulfides. We present the first geochemical data on the individual types of ore from Lengshuiqing. The disseminated to net-textured ore has Ni/Cu = 3, and average Ni tenor of 4.9%. The fractionation of monosulfide solid-solution induced the occurrence of high Ni tenors in the massive ore (6–8% Ni). The PGE tenors of all ore types are less than 1 ppm. The mantle-normalized values of the chalcophile elements show PGE depletion relative to Ni and Cu, which can be explained by a pre-emplacement equilibration of the magma in the presence of immiscible sulfides. The IPGE and Pt are depleted relative to Pd and Rh, suggesting the retention of some PGE as alloys in the mantle source. The formation of the ore is explained in conjunction with the conduit-related origin of the host intrusions. Sulfide immiscibility probably occurred in a staging chamber as a result of the assimilation of SiO 2 -rich crustal material. The segregated sulfide droplets, together with olivine crystals, were transported to the shallower intrusions at Lengshuiqing, which were likely to have been wide magmatic conduits. The decrease of magma velocity in the intrusions at Lengshuiqing determined the accumulation of sulfide melt and olivine crystals, and the formation of the disseminated to net-textured ore. Late pulses of volatile-rich magma produced breccias and led to the spatial redistribution of the sulfide melt, which generated breccia matrix ore.

Conditions of formation of the Mavrokoryfi high-sulfidation epithermal Cu–Ag–Au–Te mineralization (Petrota Graben, NE Greece)

The Mavrokoryfi Cu–Ag–Au–Te prospect, northeastern Greece, consists of atypical, high-sulfidation mineralization where precious metals were introduced contemporaneously with advanced argillic alteration from magmatic vapors. It occurs as veins of massive sulfides in zones of silicic and advanced argillic alteration spatially associated with an andesitic lava dome and hyaloclastites. Mineralogical data demonstrate an unusual ore and gangue mineralogy that is compatible with formation under very oxidizing conditions (logfO2 values of >−31.8) at temperatures of 200°C to 250°C. Oxidizing conditions favored the formation of hypogene lead sulfates (anglesite and barian celestite) instead of galena. Selenian acanthite, cadmian freibergite, and argentian goldfieldite are the main carriers of silver in the deposit and are reported in Greece for the first time. They were deposited at logfS2 of −9 to −7 and logfTe2 values of −9 to −12.5 (250°C). Ag-poor goldfieldite at Mavrokoryfi has up to 3.7 apfu Te and is the most Te-rich goldfieldite yet reported. The mineralization is accompanied by aluminum–phosphate–sulfate minerals of magmatic-hydrothermal origin and an unusual Pb-enrichment. Ore-forming components were likely derived from andesite porphyries.

Footwall-type Cu-Ni-PGE Mineralization in the Broken Hammer Area, Wisner Township, North Range, Sudbury Structure

Footwall-type Cu-Ni-PGE (Platinum-group element) deposits are a major focus of recent exploration in the Sudbury mining camp, owing to their higher Cu and precious metal contents compared to orebodies situated along the lower contact of the Sudbury Igneous Complex. The most important footwall-type orebodies occur in the Onaping-Levack area of the North Range, but economic deposits are found along the East Range of the structure as well. Recently, promising occurrences were also discovered in Wisner Township of the North Range. In this paper we report results from two main footwall-type Cu-Ni-PGE occurrences (the Broken Hammer zone and the South zone) of Wisner Township. Both discoveries are situated in intensely brecciated zones of the footwall (Sudbury Breccia), 1,300 and 500 m, respectively, north of the present Sudbury Igneous Complex-footwall contact. The mineralization consists of (1) massive sulfide veins, (2) disseminated/replacement sulfides, and (3) silicate-quartz–dominated veins. Although massive sulfide veins account for the major part of the resource, sulfide-poor disseminations may have similarly high precious metal contents. Ore assemblages are dominated by chalcopyrite, millerite, magnetite, and/or pyrite, quartz, and hydrous silicates. These are accompanied by platinum-group minerals (PGM) and other trace minerals, which commonly occur as composite grains within sulfides and hydrous silicates. These include minerals typical of footwall-type deposits (e.g., merenskyite, michenerite, hessite), but also minerals found only in a few Sudbury deposits (e.g., clausthalite, sopcheite, naumannite, bohdanowiczite). An unnamed CuPdBiS 3 mineral is one of the most abundant PGM at Broken Hammer and might be a Pd-equivalent of muckeite (CuNiBiS 3 ). Statistical investigation of metal distribution patterns using grab sample and drilling data sets from both localities indicate important differences between the two occurrences and the various mineralization styles (e.g., increasing Cu/(Cu+Ni) and decreasing Pt/(Pt+Pd) ratio with increasing sulfide content; metal concentrations recalculated to 100% sulfides decrease by up to two orders of magnitude for all metals from the disseminated to the massive sulfides). Fluid inclusion studies revealed three fluid generations which are very similar to those described from well known footwall-type deposits of the Onaping-Levack area. Primary polyphase fluid inclusions in Cu-Ni-PGE–bearing assemblages represent a highly saline (30–40 wt % NaCl equiv) magmatic-hydrothermal system with a high temperature (min 450°–500°C) and a lower temperature (min 300°–350°C) stage. A later low-temperature Ca-rich fluid (min 100°–200°C) is most probably part of a post-Sudbury Igneous Complex regional fluid flow in which temperature was controlled by the geothermal gradient and not related to ore-forming processes. The fluid inclusion data indicate that mobilization of highly saline and relatively high temperature (400°–500°C) Sudbury Igneous Complex-related magmatic-hydrothermal fluids similar to those described from the Onaping-Levack embayment also occurred in the Wisner footwall. Although our studies do not rule out the possibility of an initial magmatic sulfide melt emplacement, they prove that magmatic-hydrothermal fluids played a significant role in the transport and deposition of metals.

The As-Contaminated Elqui River Basin: a Long Lasting Perspective (1975–1995) Covering the Initiation and Development of Au–Cu–As Mining in the High Andes of Northern Chile

The Elqui watershed (northern Chile) constitutes a highly contaminated river system, with arsenic exceeding by up to three orders of magnitude the average for river waters. There are three main reasons that explain this contamination: (1) the regional geology and hydrothermal (mineralizing) processes that developed in this realm during Miocene time; (2) the later unroofing-erosion-oxidation-leaching of As-Cu rich sulfide ores, a process that have been taking place for at least 10,000 years; and last but not least (3) mining activities at the high-altitude (>4000 m above sea level) Au-Cu-As El Indio mine, from the late 1970s onwards. The El Indio mineral deposit hosted large veins of massive sulfides, including the important presence of enargite (Cu(3)AsS(4)). The continuous natural erosion of these veins and their host rocks (also rich in As and Cu) during Holocene time, led to important and widespread metal dispersion along the river system. During the studied pre mining period (1975-1977), the high altitude river Toro waters already showed very large As concentrations (0.36-0.52 mg l(-1)). The initiation of full scale mining at El Indio (1980 onwards) led to an increase of these values, reaching a concentration of 1.51 mg l(-1) As in 1995. During the same year other rivers of the watershed reached peak As concentrations of 0.33 (Turbio) and 0.11 mg l(-1) (Elqui). These figures largely exceed the USEPA regulations for drinking water (0.01 mg l(-1) As), and about 10% of the total As data from the river Elqui (and 70% from the river Turbio) are above the maximum level allowed by the Chilean law for irrigation water (0.1 mg l(-1) As).

Platinum-Group Element and Re-Os Isotope Variations of the High-Grade Kilvenjarvi Platinum-Group Element Deposit, Portimo Layered Igneous Complex, Finland

The Kilvenjarvi deposit is a high-grade platinum-group element (PGE) deposit located in the basement below the Portimo Layered Igneous Complex, Finnish Lapland. It consists of a stockwork of massive Cu-Fe-Ni sulfide veins, up to a meter wide, which cut metamorphic banded gneiss, amphibolite, and granite. The veins are surrounded by intense sulfide disseminations that are accompanied by an alteration of the basement mineral assemblage to chlorite, calcite, orthoamphibole, monazite, and an unidentified hydrous ferroaluminous silicate. The veins locally carry in excess of 110 ppm PGE, and neighboring rocks in the basement that contain disseminated sulfides locally carry 25 ppm PGE. The PGE are hosted primarily by michenerite (PdBiTe) and sobolevskite (Pd[Bi,Te]), with only very few grains of merenskyite, sperrylite, vysotskite-braggite, paolovite, cooperite, and isomertieite. The massive sulfide veins host two distinct mineral assemblages that reflect an event of sulfur addition. The primary assemblage of chalcopyrite, pentlandite, and pyrrhotite is similar to that of common orthomagmatic sulfides and appears to represent the crystallization product from a sulfide melt. However, the primary minerals have been replaced extensively by a low-temperature secondary assemblage of chalcopyrite, pyrite, marcasite, Ni-bearing pyrite, violarite, and millerite. The disseminated sulfides in the basement lithologic units comprise only the secondary assemblage and are notably richer in chalcopyrite than the massive sulfide veins. The sulfides are associated with chlorite and calcite, indicating that they were likely introduced by carbon-rich (CO 2 or CH 4 ) hydrothermal fluids. The Kilvenjarvi deposit is extremely fractionated in PGE compared to unmineralized rocks and PGE occurrences within the Portimo Complex. Os and Ir concentrations are very low, and Pd/Ir ratios reach 2.5 × 10 5 . The massive and disseminated sulfides can be distinguished by their slightly different PGE patterns. Most notably, the massive sulfides are richest in Pd, and the disseminated sulfides are richest in Pt and Au. This fractionation appears to be consistent with an initial concentration in a magmatic sulfide melt followed by hydrothermal redistribution. The Re-Os isotopes have been significantly affected by Re mobility and cannot be used for dating. However, platinum-group mineral separates from the Kilvenjarvi deposit have nearly uniform isotopic compositions ( 187 Os/ 188 Os = 0.18–0.20) that are invariably higher than rocks from the Portimo Complex. Thus, it is evident that the Kilvenjarvi deposit had a significantly higher crustal contribution (possibly accounting for up to 18% of the Os) than parental magmas for the Layered series of the Portimo Complex. Contamination and fractional sulfide crystallization within the Portimo Marginal series may have been responsible for this isotopic signature. The deposit has some similarities to the various offsets associated with the Sudbury structure, Canada. A primary magmatic origin of the sulfides and PGE is evident from the mineralogy, geochemistry, and isotope geochemistry. However, the deposit has been subjected to extensive hydrothermal alteration involving carbon-rich fluids and resulting in a partial redistribution of the sulfides, PGE, and Re, extensive recrystallization, and a partial resetting of the Re-Os isotope system.

40Ar/39Ar and Re-Os Geochronology of Porphyry Copper-Molybdenum Deposits and Related Copper-Silver Veins in the Collahuasi District, Northern Chile

The Rosario and Ujina porphyry Cu-Mo deposits, together with porphyry copper mineralization at the nearby Quebrada Blanca deposit, constitute the third-largest concentration of copper mineralization associated with the Domeyko fault system in northern Chile. At Rosario, fault-hosted Cu-Ag-rich massive sulfide veins are associated with pyrophyllite-alunite-quartz altered rocks. The Rosario massive sulfide veins cut biotitealbite-magnetite, K-feldspar and illite-chlorite altered rocks associated with porphyry-style copper-molybdenum mineralization. Similar massive sulfide veins occur in the La Grande area, 1 to 2 km south of Rosario. Copper ore at Ujina is associated with a K-feldspar-biotite-altered quartz monzonite intrusion. The K-feldspar-biotite assemblage has been overprinted by white mica-chlorite alteration. New geochronologic data presented in this paper constrain the ages of hydrothermal activity in the Rosario and Ujina deposits. The 40Ar/39Ar biotite, illite, and alunite dates (at 2 sigma error), and a Re-Os molybdenite date (at 0.5% error) are reported for porphyritic rocks and hydrothermal alteration from the Rosario and Ujina mineralized centers. A weighted mean 40Ar/39Ar plateau age of 34.4 plus or minus 0.3 Ma is obtained for igneous biotite in a monzonite porphyry that hosts copper mineralization at the Rosario deposit. Illite and hypogene alunite from separate overprinting alteration events yielded 40Ar/39Ar ages of 34.5 plus or minus 0.5 Ma (plateau age) and 32.6 plus or minus 0.3 Ma (plateau age), respectively. An Re-Os age of 33.3 plus or minus 0.2 Ma for molybdenite at Rosario is slightly younger than the 40Ar/39Ar age of illite, but older than the alunite. A weighted mean plateau age of 32.7 plus or minus 1.6 Ma for hypogene alunite from the La Grande Cu-Ag-(Au) vein south of Rosario is indistinguishable from the age of Rosario alunite. At Ujina, the weighted mean 40Ar/39Ar plateau age of igneous biotite for a monzonite intrusion that hosts copper mineralization is 35.2 plus or minus 0.3 Ma. The monzonite is intruded by postmineralization porphyry dikes of similar composition, from which igneous biotite yielded a 40Ar/39Ar age of 34.7 plus or minus 0.3 Ma. Igneous biotite in the Rosario Porphyry cooled through its closure temperature during and after the formation of illite, implying that porphyry-style ore and alteration minerals in the Rosario deposit had formed by 34.3 Ma. The age of alunite at Rosario and La Grande indicate that a second discrete episode of hydrothermal activity was superimposed, 1.8 plus or minus 0.4 m.y. later, onto the earlier-formed porphyry Cu system. Hydrothermal activity at Ujina is constrained by the 40Ar/39Ar ages of igneous biotite in the premineralization and postmineralization intrusions and occurred during a minimum interval of 0.5 plus or minus 0.4 m.y. The biotite granite at La Profunda, 1.5 km east of Ujina, has a igneous biotite age of 81.2 plus or minus 2.9 Ma, indicating that this intrusion is unrelated to the mineralized Eocene-Oligocene porphyry intrusions at Ujina, Rosario, and Quebrada Blanca. The 40Ar/39Ar data for igneous biotite in the Rosario and Ujina porphyries are cooling ages after multiple hydrothermal events. In most cases, these are minimum hydrothermal ages (i.e., ages of the last high-temperature hydrothermal event to have affected the samples), not magmatic ages. However, the biotite age for the Inca Porphyry is possibly only slightly younger than the intrusion age because biotite has not been affected by later hydrothermal alteration.

GENETIC RELATIONSHIPS BETWEEN BASE-METAL SULFIDES AND PLATINUM-GROUP MINERALS IN THE YANGLIUPING Ni Cu (PGE) SULFIDE DEPOSIT, SOUTHWESTERN CHINA

The Yangliuping Ni‐Cu‐(PGE) sulfide deposit in southwestern China is hosted by a series of mafic-ultramafic sills that intrude Devonian strata. Massive sulfides and disseminated sulfides occur at the base of the sills. Minor massive sulfide veins occur in footwall fractures. The major base-metal sulfides (BMS) are pyrrhotite, pentlandite, and chalcopyrite. The most common PGE-bearing sulfarsenide is a Pd-bearing cobaltite‐gersdorffite solid solution. Other PGM identified include sperrylite, testibiopalladite, and Pd-bearing melonite. All grains of the cobaltite‐gersdorffite solid solution and PGM are enclosed in pyrrhotite and pentlandite. Both the PGM and their host BMS have been partially altered in place by hydrothermal fluids. Electron-microprobe analyses show that pyrrhotite and pentlandite contain up to 0.1‐0.5 wt% Pt. Euhedral grains of cobaltite‐ gersdorffite solid solution ( 600°C based on the phase relations of the system FeAsS‐NiAsS‐CoAsS. Sperrylite and testibiopalladite crystals are up to 50‐80 � m in diameter and are compositionally homogeneous. Some testibiopalladite crystals occur in the contact of two pyrrhotite grains. Monosulfide solid-solution was the first phase to crystallize from a sulfide melt, and it then exsolved to pyrrhotite and pentlandite. Pt and Pd became enriched in the residual sulfide liquid, and crystallized as PGM and Pd-bearing cobaltite‐gersdorffite solid solution at a lower temperature. Minor Pt and Pd in Mss were finally expelled from the structure below 800°C.

Exploration Techniques at the Thorn Ag‐Au‐Cu High‐Sulfidation Epithermal Prospect, Northern British Columbia, Canada

Abstract. The Thorn prospect is an El Indio‐style high‐sulfidation epithermal prospect in a little‐explored belt of Late Cretaceous subaerial volcanics and subvolcanic intrusions in northwestern British Columbia, Canada. More than 30 massive sulfide (pyrite‐enargite‐tennantite/tetrahedrite) veins, quartz‐sulfide (quartz‐pyrite‐enargite‐tennantite/tetrahedrite) veins and quartz‐vuggy silica‐alunite veins/breccias fill ENE fractures and faults over an area of 1,600 times 1,900 m. They are hosted within a Late Cretaceous feldspar‐quartz‐biotite granodioritic porphyry stock and flanked by alteration envelopes a few meters to tens of meters wide: an inner envelope of intense sericite‐clay‐pyrite and an outer one of weak clay‐sericite‐chlorite. Where several veins are close together, the sericite‐clay‐pyrite alteration envelopes coalesce into zones up to 150 m wide.The most successful exploration techniques have been prospecting, silt and soil geochemistry, airborne magnetic and electromagnetic surveys and diamond drilling. Silt and soil geochemical anomalies for Au, Ag, As, Cu, Pb, Sb and Zn mark the known veins and indicate prospective areas where veins have not yet been discovered. Magnetics and resistivity outline the gross property‐wide structure and lithologies. Most of the sericite‐clay‐pyrite alteration and known veins are outlined by the &lt;264 ohm‐m resistivity contour, which covers an area of 800 times 2,300 m. Weak EM conductors are thought to represent undiscovered massive sulfide and quartz‐sulfide veins, and their flanking sericite‐clay‐pyrite alteration.

Eloise Cu‐Au deposit, East Mt Isa Block: Structural environment and structural controls on ore

The blind Eloise Cu‐Au deposit is hosted by the subsurface eastward extension of the Proterozoic East Mt Isa Block. The host rocks comprise highly deformed, amphibolite‐grade metasediments and amphibolite of the Soldiers Cap Group. The deposit formed during the waning phase of the Isan Orogeny (D3, 1540–1500 Ma) in a regional Cu‐Au metallogenic event synchronous with emplacement of the Williams Batholith (1540–1490 Ma). The deposit lies within and adjacent to a series of anastomosing shear zones at a local jog within the regional‐scale Levuka Shear Zone, This was a major fluid channel similar to the parallel Mt Dore and Cloncurry Fault Zones to the west. Alteration and mineralisation in the jog were synchronous with ductile‐brittle deformation, and were emplaced in ductile shear zones and cleavage, and brittle stockworks and faults. Brittle mineralisation styles were developed in competent enclaves in more competent primary host rock or silicic/feldspathic alteration. Three principal paragenetic stages are recognised. Stage I albitisation post‐dated peak (D2) metamorphism, and was overprinted by high temperature (>450°C) stage II hornblende + biotite alteration. Stage III sulfide mineralisation fractured and replaced the mafic silicates during a subsequent cooler event (200–450°C). Deposition of stage III ore, in refracture‐infill of stage II veins and selective replacement of stage II alteration, was controlled by sulfidisation of the mafic stage II assemblages. The changing distribution of alteration during the mineralising event (silicic/feldspathic versus hornblende/ phyllosilicate) controlled the competency architecture and in consequence the local mineralisation style. Each mineralisation stage has ductile and brittle styles, each locally overprinting the other. Ductile‐brittle deformation fluctuated in time and space at all scales, producing mineralisation characterised by locally ambiguous textures with respect to the timing of deformation. Massive sulfide veins show durchbewegung texture and breccias are ductilely deformed, yet identical mineralisation assemblages elsewhere occur in veins cross‐cutting cleavage. Ambiguity is compounded by a strong component of replacement in the mineralisation, which produced selvages along cleavage‐parallel veins which mimic cleavage, The paragenesis is determined from unambiguous overprinting relations in the brittle styles, and is then extrapolated to the more equivocal ductile styles.