Massive Sulfide Lenses Research Articles

Geochronology and metallogenic mechanism of the Ashele copper–zinc deposit in the Altay, Xinjiang, Northwest China: Insights from pyrite Re–Os dating, trace elements and sulfur isotopes

The Ashele Cu–Zn deposit is the largest volcanogenic massive sulfide (VMS) type deposit in the Xinjiang Central Asian Orogenic Belt (XCAOB). This deposit is located in the Devonian volcanic–sedimentary basin on the South Altay, Xinjiang. The Nos. I and II mineralized belts are the largest in the ore district consist of two parts: concordant massive sulfide lenses and discordant vein-type sulfide mineralization. This study mainly focuses on the mineralization during the submarine exhalative–sedimentary period. In situ trace element analysis showed that sphalerite is an important host for In and Cd, and tennantite is the main host for Au, Ag, As, Cd, Sb, Zn, Bi, and Se in base metal sulfides system. The concentration fluctuation of Bi and Se may be caused by galena. Pyrite and chalcopyrite are not enriched with specific elements, except for occasional inclusions of other minerals. Seven stratified pyrite samples display a Re–Os weighted average age of 389.1 ± 9.9 Ma (MSWD = 0.15). This data can represent the formation time of the No. I mineralized belt. The geothermometer of sulfur isotope shows sulfide mainly formed at 113 to 362 °C (avg. 214 °C), and sphalerite formed at 133 to 194 °C (avg. 175 °C) based on GGIMFis geothermometer. The δ34S values are between −1.84 to 5.51 ‰, suggestting sulfur mainly comes from magmatic sulfur, followed by seawater sulfate.

The Artulândia prospect area: A newly discovered Paleoproterozoic Zn–Pb–Cu-(±Ag–Bi) VHMS mineralization in Campinorte Magmatic Arc, Central Brazil

The Artulândia prospect area is a recently-mapped, non-auriferous VHMS mineralization within Paleoproterozoic rocks in Central Brazil. The mineralization is hosted in a felsic-dominated bimodal volcano-sedimentary sequence that formed during the onset of Campinorte Magmatic Arc. The Zn–Pb–Cu-(±Ag–Bi) VHMS mineralization occurs as massive sulfide lenses and sulfide disseminations. Surrounding the sulfide lenses, two proximal hydrothermal assemblages are identified: (i) Tremolite-spessartine (±clinochlore-epidote) assemblage, associated with the sphalerite-pyrite aggregates and (ii) Clinochlore-carbonate (±tremolite-talc-serpentine) assemblage encompassing chalcopyrite, galena, and sphalerite. Besides these two assemblages, three others are considered distal to the mineralization, and interpreted to represent the stringer zone: (i) Biotite-spessartine (±chamosite-tourmaline) assemblage, with disseminated sphalerite + pyrite + galena, (ii) Biotite-sericite-carbonate (±chamosite) assemblage, with disseminated pyrite + sphalerite and (iii) Biotite-carbonate assemblage, with pyrite and traces of sphalerite intergrown with biotite lamellae. The mineralogy of the assemblages results from the Paleoproterozoic, syn-volcanic metamorphism (Neoproterozoic, Brasiliano Orogeny) of hydrothermal alteration minerals. The mineralogy and mineral chemistry data of the assemblages show that proximal assemblages formed under great influence of Fe–Mg–Ca-rich fluids. The highest grades of base metals are restricted to the sulfide lenses: Zn (1.4% wt.), Pb (1.0% wt.%), and Cu (1.8% wt.). Anomalous contents of Mo (>180 ppm), Bi (>700 ppm) and Ag (>60 g/t) occur in the proximal hydrothermal assemblages. Gold grades as high as 270 ppb are directly related to chalcopyrite-enriched zones. The δ34S measurements in pyrite vary from +8.77‰ to +10.99‰. These values are compatible with Paleoproterozoic deposits in which the isotope composition reflects a mix of seawater and leached sulfur from host rocks.

Mineralogy and mineral chemistry of the ABM replacement-style volcanogenic massive sulfide deposit, Finlayson Lake district, Yukon, Canada

The ABM deposit is a bimodal-felsic, replacement-style volcanogenic massive sulfide deposit (VMS) that is hosted by back-arc affinity rocks of the Yukon–Tanana terrane in the Finlayson Lake VMS district, Yukon, Canada. Massive sulfide zones occur as stacked and stratabound lenses subparallel to the volcanic stratigraphy, surrounded by pervasive white mica and/or chlorite alteration. Remnant clasts of volcanic rocks and preserved bedding occur locally within the massive sulfide lenses and indicate that mineralization formed through subseafloor replacement of pre-existing strata. Three mineral assemblages occur at the ABM deposit: (1) a pyrite–chalcopyrite–magnetite–pyrrhotite assemblage that is associated with Cu–Bi–Se–Co-enrichment and occurs at the center of the massive sulfide lenses; (2) a pyrite–sphalerite assemblage, which occurs more commonly towards lens margins and is enriched in Zn–Pb–Ag–Au–Hg–As–Sb–Ba; and (3) a minor assemblage comprising chalcopyrite–pyrrhotite–pyrite stringers associated with pervasive chlorite alteration, which occurs mostly at the sulfide lens margins. Petrographic observations of preserved primary, zone refining, and metamorphic textures in combination with in situ geochemistry show that the pyrite–sphalerite assemblage formed at lower temperatures (< 270 °C) than the other two mineral assemblages (~ 270–350 °C), and that mineral chemistry in all mineral assemblages was affected by greenschist facies metamorphism, although the effects are limited to recrystallization, small-scale remobilization (< 1 m) and trace element redistribution.

Evolution of the Hydrothermal System Associated with the ABM Replacement-Style Volcanogenic Massive Sulfide Deposit, Finlayson Lake District, Yukon, Canada

Abstract The ABM deposit, Finlayson Lake district, Yukon, Canada, is a bimodal-felsic, replacement-style volcanogenic massive sulfide (VMS) deposit (19.1 Mt @ 6.6 wt % Zn, 0.9 wt % Cu, 2.0 wt % Pb, 1.4 g/t Au, and 148 g/t Ag) hosted by Late Devonian continental back-arc–related volcanosedimentary rocks of the Kudz Ze Kayah formation. The VMS-related hydrothermal alteration associated with the deposit extends &amp;gt;1 km beyond the mineralization. Zones of pervasive sericite and chlorite alteration occur proximal to the massive sulfide lenses (&amp;lt;50 m) both in the hanging wall and the footwall, and zones of pervasive sericite and moderate sericite ± chlorite alteration extend laterally from the mineralization and into the hanging wall and footwall for hundreds to thousands of meters. Geochemical data and petrographic observations indicate that feldspar destruction and formation of white mica and chlorite were the main alteration processes. In both the hanging wall and footwall to the mineralization, base (e.g., Zn, Cu, Pb) and trace metals (e.g., Sb, Tl, Mo) form halos with elevated values up to 100 and 200 m, respectively. The paragenesis and the formation conditions of the hydrothermal alteration were determined through petrography of hydrothermal alteration assemblages and their crosscutting relationships, electron microprobe analyses of the compositions of white mica, chlorite, and carbonate, and illite-chlorite geothermometry. These data suggest that the sericite ± chlorite assemblage was the earliest and most extensive phase of the hydrothermal alteration that formed at temperatures around 215° ± 30°C. Overprinting the sericite-chlorite assemblage is the pervasive sericite assemblage that formed at temperatures around 250° ± 15°C. The pervasive chlorite assemblage formed at temperatures around 320° ± 10°C and overprints the preceding sericite-rich assemblages. Microprobe analyses of white mica and chlorite generally show that Mg-rich varieties are more common proximal to mineralization and formed earlier in the deposit paragenesis than Fe-rich varieties. Mineralogy derived from short-wave infrared data for mica and chlorite, however, shows no clear spatial trends across the deposit due to complex overprinting relationships between alteration minerals.

Tracing the Source of Hydrothermal Fluid in Ophiolite-Related Volcanogenic Massive Sulfide Deposits: A Case Study from the Italian Northern Apennines

The Italian Northern Apennines contain several Fe-Cu-Zn-bearing, Cyprus-type volcanogenic massive sulfide (VMS) deposits, which significantly contribute to the Cu resources of Italy. The massive sulfide lenses and related stockwork mineralizations are hosted by several levels (from basalt to serpentinite) of the unmetamorphosed ophiolitic series; therefore, this region offers perfect locations to study the ore-forming hydrothermal system in detail. A combination of fluid inclusion microthermometry, Raman spectroscopy, electron probe analyses (chlorite thermometry) and stable and noble gas isotope geochemistry was used to determine the fluid source of the VMS system at Bargone, Boccassuolo, Campegli, Casali–Monte Loreto, Corchia, Reppia and Vigonzano. This question of the fluid source is the focus of modern VMS research worldwide, as it has a direct influence on the metal content of the deposit. The obtained temperature and compositional data are both in the typical range of VMS systems and basically suggest evolved seawater origin for the mineralizing fluid. Modification of seawater was most commonly due to fluid–rock interaction processes, which happened during long-lasting circulation in the crust. The role of a small amount of magmatic fluid input was traced only at the lower block of Boccassuolo, which may be responsible for its higher ore grade. This fluid origin model is evidenced by O, H and C stable isotopic as well as He, Ne and Ar noble gas isotopic values.

Polymetallic nanoparticles in pyrite from massive and stockwork ores of VMS deposits of the Iberian Pyrite Belt

This paper reports the first-ever study on nanoscale mineralogy in pyrite from the volcanogenic massive sulfide (VMS) deposits of the Iberian Pyrite Belt, southwestern Iberian Peninsula. It targeted colloform-textured grains formed at low temperature in the distal part of a polymetallic (Pb-Zn) massive sulfide lens hosted in felsic volcanoclastic rocks from the Masa Valverde deposit, and euhedral-textured grains (re)-deposited by higher temperature fluids in the Co-Au rich stockwork hosted in black shales of the Filón Norte orebody of the Tharsis deposit. The results acquired by a combination of techniques for mineral microanalysis and characterization (i.e., reflected light, FE-SEM, EPMA, LA-ICP-MS, HRTEM-STEM and TEM-EDS) show that trace amounts of metals (Au, Ag, As, Pb, Sb, Cu, Co) are incorporated as both lattice-bound and into nanoparticles (NPs). The mode of occurrence is strongly related with the evolutionary history of the mineralization. In the colloform pyrite collected from the massive sulfide lens, a rhythmic banding/oscillatory zonation with up to 3 wt% As, 5,000 ppm Pb, 1,070 ppm Sb and 750 ppm Cu is defined by the coexistence of several nano-sized layers (5 to 100 nm) and NPs (<100 nm) containing all these metals. The NPs include galena [PbS], tetrahedrite [(Cu,Fe)12Sb4S13)] and arsenopyrite [FeAsS] that exhibit euhedral and less frequently anhedral (i.e., droplet-like) morphologies being both randomly and preferentially oriented with respect to As-rich pyrite bands they are usually associated with. These features suggest formation of the NPs via direct deposition from the hydrothermal fluid(s) or low-temperature melts entrained in them as well as exsolution of trace elements originally dissolved in the As-rich pyrite structure. Additionally, some of these NPs are connected to late fractures disrupting the chemical zoning in colloform pyrite documenting a third genetic type of NPs related to late infiltration of fluids post-dating pyrite formation. In contrast, euhedral pyrite from the stockwork form well-developed homogeneous grains with discrete porous areas relatively depleted in Fe (45.20 wt%), and As (8,800 ppm) but enriched in Co (5,900 ppm). At the nanoscale, Co-enriched domains show patchy zoning defined by irregular distribution of Co– and As-rich bands of 200–500 nm in thickness. These nanometer Co– and As-rich bands are often disrupted by micron-to-nano-sized polycrystalline Au-Ag-Hg particles that fill voids in porous areas. Contact morphology anatomy between Co-rich pyrite and inclusions suggests that the Au-Ag-Hg particles are negative crystals occupying spaces originated in pyrite by coupled dissolution-reprecipitation reaction. Likewise, HRTEM observations along such pyrite-inclusion contacts show the existence of polycrystalline matrices in both pyrite and Au-Ag-Hg inclusions, the former consisting of nano-sized domains of arsenian pyrite and/or arsenopyrite in As-free pyrite and the Au-Ag-Hg inclusions made up of multiple crystal domains including nano-crystallites of Au0/Ag0 or electrum. Recognition of crystalline nanodomains and NPs in these polycrystalline matrices raises the possibility that Au NPs or nanomelts already present in the hydrothermal fluid catalyzed the formation of these heterogeneous crystals.

The Westwood Deposit, Southern Abitibi Greenstone Belt, Canada: An Archean Au-Rich Polymetallic Magmatic-Hydrothermal System—Part I. Volcanic Architecture, Deformation, and Metamorphism

Abstract The Westwood deposit (4.5 Moz Au) is hosted in the 2699–2695 Ma Bousquet Formation volcanic and intrusive rocks, in the eastern part of the Blake River Group, southern Abitibi greenstone belt. The Bousquet Formation is divided in two geochemically distinct members: a mafic to intermediate, tholeiitic to transitional lower member and an intermediate to felsic, transitional to calc-alkaline upper member. The Bousquet Formation is cut by the synvolcanic (2699–2696 Ma) polyphase Mooshla Intrusive Complex, which is cogenetic with the Bousquet Formation. The deposit contains three strongly deformed (D2 flattening and stretching), steeply S-dipping mineralized corridors that are stacked from north to south: Zone 2 Extension, North Corridor, and Westwood Corridor. The North and Westwood corridors are composed of Au-rich polymetallic sulfide veins and stratabound to stratiform disseminated to massive sulfide ore zones that are spatially and genetically associated with the calcalkaline, intermediate to felsic volcanic rocks of the upper Bousquet Formation. The formation of the disseminated to semimassive ore zones is interpreted as strongly controlled by the replacement of porous volcaniclastic rocks at the contact with more impermeable massive cap rocks that helped confine the upflow of mineralizing fluids. The massive sulfide lenses are spatially associated with dacitic to rhyolitic domes and are interpreted as being formed, at least in part, on the paleoseafloor. The epizonal, sulfide-quartz vein-type ore zones of the Zone 2 Extension are associated with the injection of subvolcanic, calc-alkaline felsic sills and dikes within the lower Bousquet Formation. These subvolcanic intrusive rocks, previously interpreted as lava flows, are cogenetic and coeval with the intermediate to felsic lava flows and domes of the upper Bousquet Formation. The change from fractional crystallization to assimilation- and fractional crystallization-dominated processes and transitional to calc-alkaline magmatism is interpreted to be responsible for the development of the auriferous ore-forming system. The Westwood deposit is similar to some Phanerozoic Au ± base metal-rich magmatic-hydrothermal systems, both in terms of local volcano-plutonic architecture and inferred petrogenetic context. The complex volcanic evolution of the host sequence at Westwood, combined with its proximity to a polyphase synvolcanic intrusive complex, led to the development of one of the few known large Archean subaqueous Au-rich magmatic-hydrothermal systems.

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.

The Paleoarchean Northern Mundo Novo Greenstone Belt, São Francisco Craton: Geochemistry, U–Pb–Hf–O in zircon and pyrite δ34S-Δ33S-Δ36S signatures

Greenstone belts contain several clues about the evolutionary history of primitive Earth. Here, we describe the volcano-sedimentary rock association exposed along the eastern margin of the Gavião Block, named the Northern Mundo Novo Greenstone Belt (N-MNGB), and present data collected with different techniques, including U–Pb–Hf–O isotopes of zircon and multiple sulfur isotopes (32S, 33S, 34S, and 36S) of pyrite from this supracrustal sequence. A pillowed metabasalt situated in the upper section of the N-MNGB is 3337 ± 25 Ma old and has zircon with εHf(t) = −2.47 to −1.40, Hf model ages between 3.75 Ga and 3.82 Ga, and δ18O = +3.6‰ to +7.3‰. These isotopic data, together with compiled whole-rock trace element data, suggest that the mafic metavolcanic rocks formed in a subduction-related setting, likely a back-arc basin juxtaposed to a continental arc. In this context, the magma interacted with older Eoarchean crustal components from the Gavião Block. Detrital zircons from the overlying quartzites of the Jacobina Group are sourced from Paleoarchean rocks, in accordance with previous studies, yielding a maximum depositional age of 3353 ± 22 Ma. These detrital zircons have εHf(t) = −5.40 to −0.84, Hf model ages between 3.66 Ga and 4.30 Ga, and δ18O = +4.8‰ to +6.4‰. The pyrite multiple sulfur isotope investigation of the 3.3 Ga supracrustal rocks from the N-MNGB enabled a further understanding of Paleoarchean sulfur cycling. The samples have diverse isotopic compositions that indicate sulfur sourced from distinct reservoirs. Significantly, they preserve the signal of the anoxic Archean atmosphere, expressed by MIF-S signatures (Δ33S between −1.3‰ to +1.4‰) and a Δ36S/Δ33S slope of −0.81 that is indistinguishable from the so-called Archean array. A BIF sample has a magmatic origin of sulfur, as indicated by the limited δ34S range (0 to +2‰), Δ33S ~ 0‰, and Δ36S ~ 0‰. A carbonaceous schist shows positive δ34S (2.1‰–3.5‰) and elevated Δ33S (1.2‰–1.4‰) values, with corresponding negative Δ36S between −1.2‰ to −0.2‰, which resemble the isotopic composition of Archean black shales and suggest a source from the photolytic reduction of elemental sulfur. The pillowed metabasalt displays heterogeneous δ34S, Δ33S, and Δ36S signatures that reflect assimilation of both magmatic sulfur and photolytic sulfate during hydrothermal seafloor alteration. Lastly, pyrite in a massive sulfide lens is isotopically similar to barite of several Paleoarchean deposits worldwide, which might indicate mass dependent sulfur processing from a global and well-mixed sulfate reservoir at this time.

Ore mineralogy and trace element (re)distribution at the metamorphosed Lappberget Zn-Pb-Ag-(Cu-Au) deposit, Garpenberg, Sweden

Qualitative and quantitative mineralogical and textural investigations were undertaken for the different sulfide minerals in the 1.89 Ga Lappberget deposit, with a focus on sphalerite and pyrite. Three sphalerite types were identified and associated with: (1) main massive sulfide mineralization, (2) Fe-Mn skarn ore, and (3) Ag-rich fissure veins. Sphalerite-1 contained a relatively higher amount of trace elements (Fe, Co, Ge, Cu, Sn, and Au) but is lower in Mn/Fe, Ga, and Hg compared to sphalerite-2. Evidence of tectono-metamorphic modification of sulfides is observed throughout the deposit, such as the formation of growth twins and deformation twins in sulfides, the development of high-angled triple-point junctions in recrystallized pyrite and sphalerite, and the formation of ‘ball ore’ or ‘durchbewegung’ textures within mineralized shear zones. The recrystallization of pyrite and galena may have led to the re-distribution of fluid-mobile elements (As, Cu, Zn, Pb, Ag, and Au), some of which were deposited in discordant Ag-rich fissure veins peripheral to the main massive sulfide lenses. Trace element signatures of sulfides in Lappberget are compatible with the other metamorphosed and deformed, subseafloor volcanogenic carbonate replacement (SVALS)-type deposits in the Bergslagen mining district.

Stratigraphy and U–Pb detrital zircon provenance, Crixás Greenstone Belt, Goiás-Brasil: Gold-bearing vs barren siliciclastic rocks

The Crixás Greenstone Belt, located in an Archean-Paleoproterozoic terrane within the Neoproterozoic Tocantins Province, hosts an important gold deposit in Brazil with an endowment of 7 Moz currently operated by AngloGold Ashanti. Its stratigraphy is composed of an Archean komatiite and basalt basal sequence followed by a Paleoproterozoic sedimentary sequence composed of carbonaceous phyllites, dolomite-rich marbles with minor siliciclastic lenses (Ribeirão das Antas Formation) followed by a uniform siliciclastic sequence (Córrego Geral Formation). At the mining area, the entire sequence is stratigraphically overturned and metamorphosed under greenschist to lower amphibolite facies conditions. The orebodies are structurally controlled by thin-skinned low-angle thrust faults, and are mainly hosted by the sedimentary sequence in dilation zones named locally, from the shallowest to the deepest, as Structure Palmeiras, Structure IV and Structure III. The gold mineralization occurs in association with disseminated sulfides, massive sulfide lenses or quartz veins. Structure III is the deepest mineralized structure with gold mostly occurring in quartz veins cutting carbonaceous phyllites. Siliciclastic lenses occurring within the Structure III interval show variable gold grades, while siliciclastic rocks from the uniform deeper sequence just below Structure III are barren. The chronology of the sedimentary deposition, metamorphism and mineralization is still poorly understood. Detailed drill core logging, petrography and U–Pb geochronology of detrital zircons from siliciclastic rocks were carried out in order to better understand the relationship between the two metasedimentary packages and gold distribution. Petrographically there are no relevant differences between Au-bearing siliciclastic rocks (Ribeirão das Antas Formation) and barren siliciclastic rocks (Córrego Geral Formation), except for the presence of sulfides in the former. The new dataset of detrital zircon U–Pb ages revealed a ubiquitous Paleoproterozoic peak in all samples, with nearly no Archean contribution. K-feldspar and lithic angular fragments, zircons with low to medium roundness, and the U–Pb results indicate a Rhyacian provenance from a proximal source. Neoproterozoic overgrowths are found on detrital zircons from the barren siliciclastic interval of the Córrego Geral Formation, providing unequivocal evidence of a Neoproterozoic event around 600 Ma that affected the rocks. The Neoproterozoic partial reset, possibly related to the overriding of the Mara Rosa Magmatic Arc onto the Crixás Greenstone Belt, is unlikely to be linked to the mineralization considering that zircons from the barren rocks are the most thermally affected by this event. The Au mineralization is more likely related to Paleoproterozoic events, a time period known as a major global episode of orogenic gold deposit formation.

Evolution of the subseafloor hydrothermal system associated with the Ming VMS deposit, Newfoundland Appalachians, and its controls on base and precious metal distribution

The ~ 487 Ma Ming volcanogenic massive sulfide (VMS) deposit consists of four subparallel, elongated, semi-massive to massive sulfides lenses (the 1807, 1806, Ming North, and Ming South zones) hosted in rhyodacite of the Rambler Rhyolite formation, Newfoundland Appalachians. A discordant Cu-rich Lower Footwall zone underlies the semi-massive to massive sulfide lenses. Alteration associated with mineralization can be divided into nine facies that formed in three paragenetic stages: (1) weak quartz–calcite ± spessartine, quartz–sericite, and quartz–sericite–chlorite alteration (stage 1); (2) quartz–chlorite, quartz–chlorite–sulfide, and quartz–chlorite–sericite assemblages (stage 2); and (3) quartz–sericite–sulfide and localized Mn-rich carbonate assemblages (stage 3). A thin syngenetic silica-rich layer immediately overlies part of the VMS deposit and likely formed during the early stages. The volcanic architecture and synvolcanic faults controlled the lateral distribution of extrusive rocks and hydrothermal alteration. Precipitation of the high temperature, discordant to semi-conformable Cu-rich chloritic assemblages (stockwork), was laterally restricted to one of these synvolcanic faults and the transition from coherent- to volcaniclastic-dominated lithofacies. Lower temperature, sericitic assemblages (stages 1 and 3) are controlled by the distribution of volcaniclastic rocks and generally form the immediate footwall to the semi-massive to massive sulfide lenses. Lithogeochemical mass balance calculations illustrate the alteration minerals and mineralization: chlorite-rich assemblages—gains in SiO2, Fe2O3t, MgO, Cr, Ni, and Cu and losses in Na2O, MnO, and CaO and sericite-rich assemblages—gains in K2O, Zn, and Ag and losses in MnO, MgO, CaO, Na2O, and Y. Calcium- and magnesium-rich alteration assemblages are restricted to the northwest fringe of the deposit, distal to the main chloritic and sericitic alteration, and have elemental gains in P2O5, Y, and losses in K2O. The late stage 3 quartz–sericite–sulfide assemblage overprints most assemblages, hosts sphalerite–galena–sulfosalt–Ag–Au-rich veins, and is spatially associated with coherent volcanic rocks. The less permeable nature of these rocks is interpreted to have acted as a physical barrier for ascending metal-rich hydrothermal fluids. Results from the detailed reconstruction of the hydrothermal architecture and paragenetic evolution of the Ming deposit suggest that precious metals were introduced during the waning stage of the hydrothermal system, associated with decreases in temperature and pH of the hydrothermal fluids.

The early Paleozoic Huangtupo VMS Cu—Zn deposit in Kalatag, Eastern Tianshan: Implications from geochemistry and zircon U—Pb geochronology of volcanic host rocks

The Tianshan in China forms the southern part of the Central Asian Orogenic Belt (CAOB) and also hosts important polymetallic resources. Here we investigate the Huangtupo medium–size copper–zinc deposit, located in the central part of the Kalatag copper Polymetallic Belt with a view to evaluate the nature and genesis of the early Paleozoic volcanogenic massive sulfide (VMS) Cu—Zn deposits. This deposit is correlated to intermediate–felsic volcano–sedimentary sequences of the early Paleozoic Daliugou Formation which are mainly composed of andesite, dacite, and rhyolite with minor basalt, as well as their pyroclastic equivalents. The ore bodies are hosted in the contact zone between andesitic volcanic breccia and tuff representing an eruptive hiatus between the two volcanic cycles with massive sulphide lens at the top and stockwork zone at lower part of the Daliugou Formation. We report new zircon U—Pb data, major and trace element geochemistry and Sr—Nd isotopes from the andesite, dacite and andesitic tuff in the Huangtupo deposit. Zircon from the dacite and the andesite occurring above the ore bodies yield and age of 440.1 ± 1.2 Ma and 439.7 ± 0.95 Ma, and that from the andesitic tuff in footwall shows and age of 442.0 ± 1.1 Ma. All samples show typical geochemical signatures of subduction–related calc–alkaline magmas with slight enrichment in LILE and LREE but depletion in Nb and Ti content relative to primitive mantle. The andesites are characterized by high MgO (7.6–10.8 wt%) contents and Mg# values (64–72) with low Sr/Y and La/Yb ratios, belonging to sanukite. Both dacite and tuff have low MgO (<2.5 wt%) and Al2O3 content (15–16.4 and 11.3–15.3 wt%) with Eu negative anomalies (Eu/Eu* = 0.71–0.75 and 0.44–0.6), showing their low–MgO affinity. All the volcanic rocks display similar Nd and Sr isotopic signature, with εNd(t) of 6.5 to 7.3 and moderately high (87Sr/86Sr)i ratios of 0.705274 to 0.706977. The geochemical characteristics and regional geology indicate that these volcanic rocks were formed in an intra–oceanic arc during southward subduction of the oceanic lithosphere in the northern part of the Eastern Tianshan. They were most likely derived from the partial melting of the mantle wedge with input and interaction with subducted–related melts and/or fluids. We also propose that the Huangtupo VMS deposit is analogous to Kuroko type VMS deposits developed within intra–oceanic rift.

The Discovery of the Romero VMS Deposit and Its Bearing on the Metallogenic Evolution of Hispaniola during the Cretaceous

The recently discovered Romero deposit, located in the Tres Palmas district, Cordillera Central of the Dominican Republic, has probable reserves of 840,000 oz gold, 980,000 oz silver and 136 Mlb copper. Mineralization is hosted by intermediate volcanic and volcaniclastic rocks of the lower stratigraphic sequence of the Cretaceous Tireo formation. The andesitic host rocks yield a U-Pb zircon concordia age of 116 ± 10 Ma. Au–Ag–Cu(–Zn) mineralization is divided into: (1) an upper domain with stacked massive sulfide lenses and sulfide dissemination within a 20-m-thick level of massive anhydrite-gypsum nodules, and (2) a lower domain with a high-grade stockwork mineralization in the form of cm-scale veins with open space fillings of fibrous silica and chalcopyrite, sphalerite, pyrite (+electrum ± Au–Ag tellurides). The δ34S values of sulfides from the upper (−7.6 and +0.9‰) and lower (−2.4 and +5.6‰) domains are consistent with a heterogeneous sourcing of S, probably combining inorganically and organically induced reduction of Albian-Aptian seawater sulfate. Despite this, a magmatic source for sulfur cannot be discarded. The δ34S (+19.2 and +20.0‰) and δ18O (+12.5 and +14.2‰) values of anhydrite-gypsum nodules are also consistent with a seawater sulfate source and suggest crystallization in equilibrium with aqueous sulfides at temperatures higher than 250 °C. These data point to a classification of Romero as a volcanogenic massive sulfide (VMS) deposit formed in an axial position of the Greater Antilles paleo-arc in connection with island arc tholeiitic magmatism during a steady-state subduction regime. Circulation of hydrothermal fluids could have been promoted by a local extensional tectonic regime expressed in the Tres Palmas district as a graben structure.

Petrogenesis of the Rambler Rhyolite Formation: Controls on the Ming VMS Deposit and geodynamic implications for The Taconic Seaway, Newfoundland Appalachians, Canada

The Ming Cu-Zn-Ag-Au volcanogenic massive sulfide (VMS) deposit is hosted by the Upper Cambrian-Lower Ordovician Rambler Rhyolite formation, which consists of a folded northeast plunging felsic dome complex, developed in the uppermost segment of the obducted supra-subduction Baie Verte oceanic tract, in the north central Newfoundland Appalachians. The deposit is overlain by interstratified mafic volcanic flows and volcaniclastic rocks of the Snooks Arm Group. The upper 1 km of the Rambler Rhyolite formation consists of a coherent felsic lithofacies at its base, overlain by a quartz-bearing volcaniclastic-dominated lithofacies (units 1.2 and 1.3). Geochemically, these rocks are light rare earth element (LREE)- and large ion lithophile element (LILE)-enriched ([La/Yb]pn = 4–20; Th = 2.58–4.05 ppm), and high field strength element (HFSE)-depleted (Y = 5.64–6.59 ppm) rhyodacite with calk-alkalic affinities (Th/Yb > 2; Zr/Y > 7). The last pulse of felsic volcanism (unit 1.3) is intimately associated with the massive sulfide lenses and has FI- and FII-type rhyolite signatures (high La/Yb ratios), commonly attributed to deep (≥ 30 km) crustal melting. The eNd(t) values of the rhyolites range from −2.5 to −1.3, indicating the rocks were influenced by crustal material. A 10 percent batch partial melt of a garnet-amphibole metamorphosed normal mid-oceanic ridge basalt (N-MORB) and island arc tholeiite (IAT) crustal material can generate a melt similar to the felsic rocks hosting the Ming deposit; however, the IAT reproduces much better the absolute abundances of LILE (Th, Sr), HFSE (Nb, Zr, Y), and middle rare earth elements (MREE) of the felsic rocks. It is interpreted that the felsic melt came from melting of a subducted slab with IAT affinity, thus explaining the depth of melt generation and FI-FII signatures of the felsic rocks. Moreover, the slab-derived siliceous melt may also have contributed magmatophile elements (for example, Ag, Au, Se, Te, Sn, Sb, Hg) into the hydrothermal system once it had reached near-surface depths, and may explain the epithermal suite element-enrichment in the Ming deposit. This study shows that strongly fractionated felsic volcanic rocks associated with boninites, such as those associated with the Ming VMS deposit, can be important hosts for precious metal-enriched VMS and may be as prospective as tholeiitic felsic volcanic sequences (for example, FIII-FIV-type rhyolite – low La/Yb ratios). The base of the Snooks Arm Group is comprised of spatially restricted sulfide-bearing mafic breccia, overlain by a regionally extensive sedimentary sequence (eNd(t) = +3.1 to +5.5), which is in turn overlain by interstratified high-Mg basalt (eNd(t) = +1.6), Th-enriched back-arc basin basalt (BABB), enriched mid-oceanic ridge basalt (E-MORB; eNd(t) = +6.6), and LREE-enriched/low-Ti tholeiitic tuffs (eNd(t) = −0.5). Four generations of mafic to intermediate dikes and sills cross-cut the Rambler Rhyolite formation and share similar geochemical characteristics to the extrusive rocks of the Snooks Arm Group, suggesting comagmatic relationships. The combination of variations in Nb/Yb, Th /Yb, and eNd(t) values within and between units suggest melts derived from depleted to enriched mantle material with melts synchronously generated by both slab-metasomatized mantle wedge and upwelling back-arc asthenosphere.

O DEPÓSITO BAHIA: UM POSSÍVEL EXEMPLO DE DEPÓSITO DE SULFETO VULCANOGÊNICO TIPO BESSHI ARQUEANO EM CARAJÁS

The Bahia ore deposit is hosted by the Archean Igarape Bahia Group which is one of the volcano-sedimentary sequences that have filled the Carajas basin (SE Para state). This group consists of basic metavolcanic, metapyroclastic and clastic metasedimentary rocks, besides banded iron formations and breccias. These rocks are cut by basic dikes and have been altered by hydrothermal fluids to mineral assemblages compatible with the greenschist facies metamorphic conditions. The Bahia deposit is formed by the Acampamento Norte, Acampamento Sul, Furo Trinta and Alemao ore bodies. The first three ones show disseminated mineralization that is hosted mainly by breccias. The last one was recently discovered and is partially made up of massive sulfide lenses. The breccias occur in the verticalized contact between the basic metavolcanic and the clastic metasedimentary rocks and usually display gradational boundaries with the host rocks. The lithoclasts are either from basic metavolcanic rocks or banded iron formations. Their shapes vary form angular to subangular. The matrix is composed of chlorite, siderite, chalcopyrite, quartz, magnetite, tourmaline and calcite. The matrix/clasts ratio is highly variable producing clast-supported to matrix-supported breccias, some of which showing slight foliation and orientation of the lithoclasts. To these breccias it is suggested a phreatic origin whose fragments were reworked and transported into deeper zones as debris flow. Compared to the host rocks, Cu and Au are more enriched in the breccias, in which the sulfides also occur in veins and pods together with quartz and/or siderite. Chalcopyrite and the pyrite are the main sulfides, but in the massive magnetite and sulfide-rich beds bornite is also present. Magnetite is an abundant phase found not only in banded iron formation clasts, but also in the breccia matrix as disseminations or in massive beds. Chloritization, carbonation, magnetitization and sulfidation are the most important types of hydrothermal alteration, although silicification and tourmalinization may also occur. Pervasive Chloritization has affected ali rocks in greater or lesser degree. Microthermometric studies in quartz crystals present in the breccias revealed aqueous fluids chemically represented by the H 2 O-NaCl-CaCl 2 system with variable salinities (5.5-41.5 weight % eq. NaCl). It was also observed pure CO 2 inclusions. The more frequent homogenization temperature ranges were 110-140°C and 150-225°C respectively for the two-phase and three-phase aqueous inclusions. The aqueous fluids have been assumed to be convectively driven sea water whereas the carbonic fluids were probably mantle-derived. The most saline aqueous fluids (>23% wt equiv. NaCl) are believed to have had a magmatic contribution. The geological characteristics of the Bahia deposit allow it to be interpreted as a Besshi-type volcanogenic deposit in which uranium and rare-earth elements, that occur in anomalous concentrations, have been introduced by later events.

Alteration zonation and short wavelength infrared (SWIR) characteristics of the Honghai VMS Cu-Zn deposit, Eastern Tianshan, NW China

The Honghai VMS Cu-Zn deposit is located in the Dananhu-Tousuquan arc, Eastern Tianshan, northwestern China. Orebodies of the Honghai deposit are buried 300m underground and hosted by the Middle Ordovician-Upper Silurian Daliugou Formation felsic to intermediate volcanic-sedimentary rocks. At the footwall and hanging wall of the massive sulfide lens, intensive and pervasive phyllic alterations were found. Detailed drill core logging, petrographic study and electron microprobe analyses had delineated six alteration/mineralization zones, namely (from top to bottom) Zone I (chlorite-albite-white mica-carbonate), II (epidote-chlorite-albite-white mica-carbonate), III (quartz-white mica-pyrite), IV (massive sulfide), V (chlorite-pyrite±white mica) and VI (chlorite-quartz-white mica).Our new data supported the sub-seafloor replacement influence for the Honghai VMS deposit, as evidenced by the presence of (1) sulfide breccias in the immediate host rocks; (2) rapid emplacement of host rocks; (3) strong phyllic alteration in the immediate hanging wall; (4) replacement textures of the massive sulfides; (5) barite as gangue minerals. Moreover, intensive chlorite and epidote alterations and vein mineralization are present in the hanging wall, which are comparable with porphyry-related propylitic alteration. Besides, different mineralogy and mineral chemistry are present between chlorites from the hanging wall and footwall at Honghai, which probably suggested porphyry-style alteration and mineralization overprinting at Honghai.Short wavelength infrared (SWIR) spectral analysis had identified mainly white mica, chlorite and epidote, and minor montmorillonite, calcite, gypsum, kaolinite and prehnite. The wavelengths of the AlOH absorption feature for white mica range from 2194 to 2221nm, suggesting compositions from paragonite, through muscovite to phengite. The wavelengths of FeOH absorption feature for chlorite range from 2249 to 2261nm, indicating compositions from Mg-rich to Fe-rich chlorite. According to SWIR measurements and electron microprobe analyses (EMPA), areas proximal to the massive sulfides are characterized by short wavelength of the AlOH absorption feature and high Alvi content of white mica. Petrography and EMPA showed that chlorite in the ore proximal zones are more Mg-rich than that in ore distal zones. These findings indicated high temperature for the white mica formation and more seawater influx during the chlorite formation in areas near the massive sulfides. This study suggested that SWIR analysis, supported by petrographic study and EMPA, can serve as an effective tool in alteration mapping, and mineral geochemical variations are potential deposit-scale exploration vectoring tools.

Metamorphism of volcanogenic massive sulphide deposits in the Urals. Ore geology

The Urals VMS province comprises a broad spectrum of variably metamorphosed deposits, from unmetamorphosed to those without any primary ore textures, which are the results of high-grade metamorphic processes. Contact metamorphism near large granite and granodiorite plutons caused the most significant changes of ores, with coarse-grained to pegmatoidal ores with magnetite closest to its contact with the intrusion, followed by pyrrhotite-enriched copper ores, and more distal zinc (±Pb±Ag) mineralisation. Koktau, Tarnyer and Vesenneye deposits are metamorphosed to the hornblende-hornfels and pyroxene-hornfels facies (t=400–800°C, P=1–6kbar). Metamorphism of Tash-Yar, Dzhusinskoe and Krasnogvardeiskoe deposits corresponds to the greenschist and albite-epidote-hornfels facies (t=250–450°C, P=1–4kbar).The regional metamorphism of VMS ores varies from prehnite-pumpellyite facies (t=150–300°C, P=0.5–4kbar) in the South Urals to the epidote-amphibolite and amphibolite facies (t=400–600°C (up to 700°C), P=1–6kbar) in the Karabash area in the Middle Urals. In the Magnitogorsk zone, the metamorphism of host rocks and VMS bodies increases to the north, reaching its peak near the Ufa promontory of the East European platform. With increased metamorphism, the morphology of orebodies evolves from gently dipping thick lenses (Alexandrinskoe and Uzelga fields), to subvertical and folded (Uchaly and Novo-Uchaly deposits) and pseudomonoclinal steeply-dipping vein-like bodies (Karabash district).The massive sulphide transformation in PTX-gradient fields led to partial redistribution of ore material. An enrichment in Cu, Zn, Ag and Au, ±Pb occur in the uppermost parts of large steeply-dipping massive sulphide lenses in wide tectonic zones (e.g., Gai deposit) or as gold-sulphide disseminated bodies near large metamorphosed VMS lenses, distal to a granite pluton (Tarnyer deposit). Partial melting probably occurred in some highly metamorphosed deposits (Tarnyer, Koktau and Mauk). Redeposition of base metals sulphides (chalcopyrite, tennantite, sphalerite, ±bornite, galena), as well as the presence of “visible” gold and tellurides, took place during retrograde metamorphism, which produced a transfer of ore matter towards the low stress areas, such as the outer parts of shear zones, the uppermost parts of steeply-dipping ore lenses, pressure shadows, hinge zones of small folds, and small extension fractures (i.e., Alpine-type veins) in deformed ore body or its immediate surroundings.

Discovery of the Eureka volcanogenic massive sulphide lens using downhole electromagnetics

The Eureka volcanogenic massive sulphide (VMS) lens forms part of the Stockman Project in north-east Victoria. Eureka was discovered in early 2013, heralding the first new VMS mineralised zone to be discovered at the project since 1979. Key drivers to the detection of Eureka included the combination of downhole electromagnetics (EM) and a robust geological model.The lens is located 350 m to the north-east of the Currawong deposit, at a vertical depth of 360 m. Surface EM methods played a significant role in the discovery of the nearby Currawong and Wilga deposits during the late 1970s. Despite this, modern day airborne and fixed-loop transient EM (FLTEM) surveys failed to detect Eureka, most likely due to its depth, moderate conductance and loop-edge effects masking anomalies. The key component in discovering the lens was the interpretation of two subtle downhole transient electromagnetics (DHTEM) responses from 2012 exploration drillholes. These responses were further strengthened by structural and short wave infrared modelling, presenting a compelling multi-component drill target.The lens was discovered soon thereafter, with a discovery intercept of 22.65 m at 1.2% Cu, 0.7% Pb, 3.9% Zn, 43 g/t Ag and 1.3 g/t Au.

Mineralogy and Metal Zoning of the Cambrian Zn-Pb-Cu-Ag-Au Lemarchant Volcanogenic Massive Sulfide (VMS) Deposit, Newfoundland

The Lemarchant deposit is a Cambrian bimodal felsic Zn-Pb-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit located in the Central Mobile Belt, Newfoundland, Canada. Despite regional greenschist metamorphism and faulting, primary mineralogy and mineral textures are well preserved. The deposit is a type example of a precious metal-enriched VMS deposit in the Appalachians. Mineralization consists of a stratiform massive sulfide zone that lies at the contact between a rhyolitic footwall and a basaltic hanging wall. A stringer sulfide zone that is hosted in footwall rhyolite breccia underlies the massive sulfide lens. The stratiform sulfide zone contains massive baryte that is heterogeneously replaced by sphalerite and pyrite, lesser galena, and trace chalcopyrite. The stringer zone contains chalcopyrite, pyrite, and lesser sphalerite and galena. Sphalerite ranges in color from white (low-Fe) to red (high-Fe). The palest sphalerite (<1 mol.% FeS) occurs in the baryte-rich stratiform zone and is associated with early stage exhalative mineralization, intermediate sulfidation epithermal suite minerals (tetrahedrite, bornite, colusite, Ag-bearing gold, covellite) and sulfide minerals enriched in the epithermal trace element suite (Au, Ag, As, Bi, Co, Cr, In, Mo, Ni, Sb, Sn, Te). Darker sphalerite (4.7–13.6 mol.% FeS) in the stratiform zone overprints early stage mineralization and occurs with chalcopyrite; high-Fe sphalerite is also present in the stringer zone. Early exhalative/epithermal-type VMS mineralization was deposited from low temperature (150–250 °C), oxidized, acidic to near-neutral hydrothermal fluids with high sulfur activity. Early mineralization was likely deposited in relatively shallow water ( 300 °C), less oxidized, near-neutral hydrothermal fluids, likely in deeper water (>1500 mbsl). Abundant epithermal suite minerals and epithermal trace element suite-enriched sulfides at Lemarchant suggest a direct magmatic contribution to the hydrothermal fluid.