Sulfide Lenses Research Articles

Biogeochemical controls on microbial diversity in seafloor sulphidic sediments

The ultimate fate of hydrothermal sulphides on the seafloor depends on the nature and rate of abiotic and microbially catalysed reactions where sulphide minerals are exposed to oxic seawater. This study combines organic and inorganic geochemical with microbiological measurements across a suboxic transition zone of highly altered sulphidic sediments from the Trans-Atlantic Geotransverse hydrothermal field to characterize the reaction products and microbial communities present. There is distinct biogeochemical zonation apparent within the sediment sequence from oxic surface layers through a suboxic transition zone into the sulphide material. The microbial communities in the sediment differ significantly between the biogeochemical horizons sampled, with the identified microbes inferred to be associated with Fe and S redox cycling. In particular, Marinobacter species, organisms associated with circumneutral Fe oxidation, are dominant in a sulphide lens present in the lower core. The dominance of Marinobacter-related sequences within the relict sulphide lens implies that these organisms play an important role in the alteration of sulphides at the seafloor once active venting has ceased.

Lithogeochemistry as a tracer of the tectonic setting, lateral integrity and mineralization of a highly metamorphosed Mesoproterozoic volcanic arc sequence on the eastern margin of the Namaqua Province, South Africa

The Areachap Group represents the medium- to high-grade metamorphic and deformed remnants of a Mesoproterozoic (ca. 1.24–1.30 Ga) volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the early stages of the Namaqua-Natal Orogeny. Few regional geochemical studies of the succession have been undertaken and despite recent geochronological work from throughout the outcrop belt there is still uncertainty as to the lateral integrity and correlation of the succession from one area to another. This uncertainty has been aided by minimal outcrop and extensive deformation and dislocation of one area from another. This study uses trace element geochemistry combined with Sm–Nd-isotopic data from the northern and southern ends of the succession to test its lateral integrity. Minor differences, including the abundance of certain lithologies, varying influences of a crustal component and differences in age between different areas are apparent, but the succession, as a whole, displays lateral integrity as a continental island arc. All lithologies are characterised by enrichment in Th, U, Pb and the large ion lithophile elements relative to primitive mantle, along with Nb–Ta depletion relative to La, and strong light rare earth element (LREE) enrichment relative to heavy REE depletion, which, along with positive ε Nd(t) values of mafic gneisses from both ends of the succession, indicates mantle-derivation within a subduction-related environment with a substantial amount of crustal assimilation. Despite some minor geochemical and isotopic differences in the lithological architecture on a local scale the Areachap Group exhibits coherent geochemical characteristics along its entire strike length. Localised remobilization of elements, particularly in areas characterised by granulite grade metamorphism, as well as in the altered footwall zones of volcanogenic massive sulphide lenses, is present. These alteration trends, along with primary geochemical signatures are remarkably well preserved despite the high grade of metamorphism, as illustrated in clearly defined hydrothermal alteration trends on the alteration box plot, and serves as an efficient tool to delineate areas that have undergone hydrothermal alteration spatially associated with massive sulphide ore bodies. The systematic association of hydrothermal alteration and sulphide ore bodies provides support for the isochemical character of rock compositions during high-grade metamorphism and the lack of extensive post-metamorphic alteration processes.

Sulfur isotope characteristics of metamorphosed Zn–Cu volcanogenic massive sulfides in the Areachap Group, Northern Cape Province, South Africa

Zn- and Cu-rich massive sulfide ores of volcanogenic origin [volcanogenic massive sulfide (VMS) deposits] occur as stratiform/stratabound lenses of variable size hosted by gneisses, amphibolites, and schists of the Areachap Group, in the Northern Cape Province of South Africa. The Areachap Group represents the highly deformed and metamorphosed remnants of a Mesoproterozoic volcanic arc that was accreted onto the western margin of the Kaapvaal Craton during the ∼1.0–1.2 Ga Namaquan Orogeny. Sulfur isotope data (δ34S) are presented for 57 sulfide separates and one barite sample from five massive sulfide occurrences in the Areachap Group. Although sulfides from each site have distinct sulfur isotope values, all δ34S values fall within a very limited range (3.0‰ to 8.5‰). Barite has a δ34S value of 18.5‰, very different from that of associated sulfides. At one of the studied sites (Kantienpan), a distinct increase in δ34S of sulfides is observed from the massive sulfide lens into the disseminated sulfides associated with a distinct footwall alteration zone. Sulfide–sulfide and sulfide–barite mineral pairs which recrystallized together during amphibolite- and lower granulite facies metamorphism are not in isotopic equilibrium. Sulfur isotope characteristics of sulfides and sulfates of the Zn–Cu ores in the Areachap Group are, however, very similar to base metal sulfide accumulations associated with modern volcanic arcs and unsedimented mid-ocean ridges. It is thus concluded that profound recrystallization and textural reconstitution associated with high-grade regional metamorphism of the massive sulfide ores of the Areachap Group did not result in extensive sulfur isotopic homogenization. This is similar to observations in other metamorphosed VMS deposit districts and confirms that massive sulfide ores remain effectively a closed system for sulfur isotopes for both sulfides and sulfates during metamorphism.

Origin of Mineralizing Fluids of the Sediment-Hosted Navachab Gold Mine, Namibia: Constraints from Stable (O, H, C, S) Isotopes

The Navachab gold mine in the Damara belt of central Namibia is characterized by a polymetallic Au-Bi-As-Cu-Ag ore assemblage, including pyrrhotite, chalcopyrite, sphalerite, arsenopyrite, bismuth, gold, bismuthinite, and bismuth tellurides. Gold is hosted by quartz sulfide veins and semimassive sulfide lenses that are developed in a near-vertical sequence of shelf-type metasedimentary rocks, including marble, calcsilicate rock, and biotite schist. The sequence has been intruded by abundant syntectonic lamprophyre, aplite, and pegmatite dikes, documenting widespread igneous activity coeval with mineralization. The majority of quartz from the veins has δ 18O values of 14 to 15 per mil (V-SMOW). The total variations in δ 18O values of the biotite schist and calcsilicate rock are relatively small (12–14‰), whereas the marble records steep gradients in δ 18O values (17–21‰), the lowest values being recorded at the vein margins. Despite this, there is no correlation between δ 18O and δ 13C values and the carbonate content of the rocks, indicating that fluid-rock interaction alone cannot explain the isotopic gradients. In addition, the marble records increased δ 13C values at the contact to the veins, possibly related to a change in the physicochemical conditions during fluid-rock interaction. Gold is interpreted to have precipitated in equilibrium with metamorphic fluid ( δ 18O = 12–14‰ δ D = −40 to −60‰) at peak metamorphic conditions of ca. 550°C and 2 kbars, consistent with isotopic fractionations between coexisting calcite, garnet, and clinopyroxene in the alteration halos. The most likely source of the mineralizing fluid was a midcrustal fluid in equilibrium with the Damaran metapelites that underwent prograde metamorphism at amphibolite- to granulite-facies grades. Although there is no isotopic evidence for the contribution of magmatic fluids, they may have been important in contributing to the overall hydraulic regime and high apparent geothermal gradients (ca. 80°C/km−1) in the mine area.

Gold in the Brunswick No. 12 volcanogenic massive sulfide deposit, Bathurst Mining Camp, Canada: Evidence from bulk ore analysis and laser ablation ICP─MS data on sulfide phases

The 329-Mt Brunswick No. 12 volcanogenic massive sulfide deposit (total resource of 163 Mt at 10.4% Zn, 4.2% Pb, 0.34% Cu, and 115 g/t Ag) is hosted within a Middle Ordovician bimodal volcanic and sedimentary sequence. Massive sulfides are for the most part syngenetic, and the bulk of the sulfide ore occurs as a Zn–Pb-rich banded sulfide facies that forms an intimate relationship with a laterally extensive Algoma-type iron formation and defines the Brunswick Horizon. Zone refining of stratiform sulfides is considered to have resulted in the development of a large replacement-style Cu-rich basal sulfide facies, which is generally confined between the banded sulfide facies and an underlying stringer sulfide zone. Complex polyphase deformation and associated lower- to upper-greenschist facies regional metamorphism is responsible for the present geometry of the deposit. Textural modification has resulted in a general increase in grain size through the development of pyrite and arsenopyrite porphyroblasts, which tend to overprint primary mineral assemblages. Despite the heterogeneous ductile deformation, primary features have locally been preserved, such as fine-grained colloform pyrite and base and precious metal zonation within the Main Zone. Base metal and trace element abundances in massive sulfides from the Brunswick No. 12 deposit indicate two distinct geochemical associations. The basal sulfide facies, characterized by a proximal high-temperature hydrothermal signature (Cu–Co–Bi–Se), contains generally low Au contents averaging 0.39 ppm (n = 34). Conversely, Au is enriched in the banded sulfide facies, averaging 1.1 ppm Au (n = 21), and is associated with an exhalative suite of elements (Zn–Pb–As–Sb–Ag–Sn). Finely laminated sulfide lenses hosted by iron formation at the north end of the Main Zone are further enriched in Au, averaging 1.7 ppm (n = 41) and ranging up to 8.2 ppm. Laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) analyses of pyrite (n = 97) from the north end of the Main Zone average 2.6 ppm Au and range from the detection limit (0.015 ppm) to 21 ppm. Overall, these analyses reveal a distinct Au–Sb–As–Ag–Hg–Mn association within pyrite grains. Gold is strongly enriched in large pseudo-primary masses of pyrite that exhibit relict banding and fine-grained cores; smaller euhedral pyrite porphyroblasts, and euhedral rims of metamorphic origin surrounding the pyrite masses, contain much less Au, Sb, Ag, As, and Sn. Arsenopyrite, occurring chiefly as late porphyroblasts, contains less Au, averaging 1.0 ppm and ranging from the detection limit (0.027 ppm) to 6.9 ppm. Depth profiles for single-spot laser ablation ICP-MS analyses of pyrite and arsenopyrite display uniform values of Au and an absence of discrete microscopic inclusions of Au-bearing minerals, which is consistent with chemically bonded Au in the sulfide structure. The pervasive correlation of Au with Sn in the Zn–Pb-rich banded sulfide facies suggests similar hydrothermal behavior during the waxing stages of deposition on the seafloor. Under high temperature (>350oC) and moderate- to low-pH conditions, Au and Sn in hydrothermal fluids would be transported as chlorocomplexes. An abrupt decrease in temperature and aH2S, accompanied by an increase in fO2 and pH during mixing with seawater, would lead to the simultaneous destabilization of both Au and Sn chlorocomplexes. The enrichment of Au in fine-grained laminated sulfides on the periphery of the deposit, accompanied by sporadic occurrences of barite and Fe-poor sphalerite, supports lower hydrothermal fluid temperatures analogous to white smoker activity on the flanks of a large volcanogenic massive sulfide system. In lower temperature (<350oC) and mildly acidic hydrothermal fluids, Au would be transported by thiocomplexes, which exhibit multifunctional (retrograde–prograde) solubility and a capacity to mobilize Au to the outer parts of the sulfide mound. The sluggish nature of this low-temperature venting together with larger variations in ambient fO2 could lead to a sharp enrichment of Au towards the stratigraphic hanging wall of massive sulfide deposits.

Processes of high-T fluid–rock interaction during gold mineralization in carbonate-bearing metasediments: the Navachab gold deposit, Namibia

The Navachab gold deposit in the Damara belt of central Namibia is hosted by a near-vertical sequence of amphibolite facies shelf-type metasediments, including marble, calc-silicate rock, and biotite schist. Petrologic and geochemical data were collected in the ore, alteration halos, and the wall rock to evaluate transport of elements and interaction between the wall rock and the mineralizing fluid. The semi-massive sulfide lenses and quartz–sulfide veins are characterized by a complex polymetallic ore assemblage, comprising pyrrhotite, chalcopyrite, sphalerite, and arsenopyrite, native bismuth, gold, bismuthinite, and bismuth tellurides. Mass balance calculations indicate the addition of up to several orders of magnitude of Au, Bi, As, Ag, and Cu. The mineralized zones also record up to eightfold higher Mn and Fe concentrations. The semi-massive sulfide lenses are situated in the banded calc-silicate rock. Petrologic and textural data indicate that they represent hydraulic breccias that contain up to 50 vol.% ore minerals, and that are dominated by a high-temperature (T) alteration assemblage of garnet–clinopyroxene–K-feldspar–quartz. The quartz–sulfide veins crosscut all lithological units. Their thickness and mineralogy is strongly controlled by the composition and rheological behavior of the wall rocks. In the biotite schist and calc-silicate rock, they are up to several decimeters thick and quartz-rich, whereas in the marble, the same veins are only a few millimeters thick and dominated by sulfides. The associated alteration halos comprise (1) an actinolite–quartz alteration in the biotite schist, (2) a garnet–clinopyroxene–K-feldspar–quartz alteration in the marble and calc-silicate rock, and (3) a garnet–biotite alteration that is recorded in all rock types except the marble. The hydrothermal overprint was associated with large-scale carbonate dissolution and a dramatic increase in CO2 in the ore fluid. Decarbonation of wall rocks, as well as a low REE content of the ore fluid resulted in the mobilization of the REE, and the decoupling of the LREE from the HREE. The alteration halos not only parallel the mineralized zones, but may also follow up single layers away from the mineralization. Alteration is far more pronounced facing upward, indicating that the rocks were steep when veining occurred. The petrologic and geochemical data indicate that the actinolite–quartz– and garnet–clinopyroxene–K-feldspar–quartz alterations formed in equilibrium with a fluid (super-) saturated in Si, and were mainly controlled by the composition of the wall rocks. In contrast, the garnet–biotite alteration formed by interaction with a fluid undersaturated in Si, and was mainly controlled by the fluid composition. This points to major differences in fluid–rock ratios and changes in fluid composition during alteration. The alteration systematics and geometry of the hydrothermal vein system are consistent with cyclic fluctuations in fluid pressure during fault valve action.

Age and depositional environment of the Draa Sfar massive sulfide deposit, Morocco

The Draa Sfar mineralization consists of two main stratabound orebodies, Sidi M’Barek and Tazacourt, located north and south of the Tensift River (“Oued Tessift”), respectively. Each orebody is comprised by at least two massive sulfide lenses. The hosting rocks are predominantly black shales, although minor rhyolitic rocks are also present in the footwall to the southern orebody. Shales, rhyolitic volcanic rocks, and massive sulfides are all included into the Sarhlef Series, which is recognized as one of the main stratigraphic units of the Moroccan Variscan Meseta. Hydrothermal activity related with an anomalous thermal gradient, together with a high sedimentation rate in a tectonically driven pull-apart marine basin, favored the accumulation of organic-rich mud in the deepest parts of the basin and the sedimentary environment suitable for massive sulfide deposition and preservation. This took place by replacement of the hosting unlithified wet mud below the sediment–water interface. Geochemical data suggest a sedimentary environment characterized by oxic water column and anoxic sediment pile with the redox boundary below the sediment–water interface. The low oxygen availability within the sediment pile inhibited oxidation and pyritization of pyrrhotite. Biostratigraphic analysis, based on the palynological content of the hosting black shales, restricts the age of the sulfides to the Asbian substage (mid-Mississippian). This age is consistent with earlier geochronological constraints.

The Zn-Pb deposits of Casario (Ligurian Alps, NW Italy): Late Palaeozoic sedimentary-exhalative bodies affected by the alpine metamorphism

In the Ligurian Alps (South-Western Italian Alps), Zn-Pb deposits occur within late Palaeozoic meta-sedimentary units belonging to the Briançonnais Zone near Casario (Tanaro valley). Different types of sulphide-rich, lens-shaped mineralizations are recognized: sphalerite-galena massive sulphide bodies, pyrite-rich lenses and sulphide-rich quartz–carbonate-chloritoid granofels. Sulphide lenses and host rocks are affected by at least three ductile deformation phases and by a polyphase alpine metamorphism, whose climax conditions are estimated, based on P-T pseudosection calculations, at T = 300-325 °C and P = 0.55-0.60 GPa. In all the mineralized lenses the ore minerals are represented, in variable amount, by Fe-poor sphalerite, galena, pyrite and arsenopyrite (± tetrahedrite, chalcopyrite and pyrrhotite); the gangue consists of quartz, carbonate (sideritemagnesite ± rhodochrosite s.s.), Fe-chloritoid, muscovite-phengite and chlorite. The mineralizations are associated with chloritoid – carbonate micaschists displaying a finely bedded texture, with sharp between-bed compositional contrast, which suggests their exhalative origin. In spite of the tectono-metamorphic overprint, some pre-metamorphic features of the hydrothermal system are still recognized, like relics of the hydrothermal feeding system, primary growth textures and sulphide-rich microbreccias. These massive sulphide lenses, which share many characters with the SEDEX deposits, testify to the occurrence of an exhalative event of Upper Carboniferous age previously unrecognized in the Ligurian Briançonnais Unit.

Volcanic Stratigraphy, Alteration, and Sea-Floor Setting of the Paleozoic Feitais Massive Sulfide Deposit, Aljustrel, Portugal

The Feitais deposit is one of six polymetallic, volcanic-hosted massive sulfide deposits located adjacent to the town of Aljustrel in the Iberian Pyrite Belt of southern Portugal. The deposit occurs on the upright limb of the Feitais anticline, within the upper part of the regional Volcano-Sedimentary Complex of Devonian to Mississippian age and near the contact with the overlying Culm Flysch Group. It is stratigraphically underlain by felsic tuffs and flows and overlain by a unit of feldspar-bearing felsic tuff that passes upward into chert, then argillite, and finally graywacke turbidites. The massive sulfide lens is about 1,000 m in length, 500 m wide, and up to 100 m thick. Although mainly fine-grained pyrite, it commonly has a Cu-rich base that passes upward through a low-grade pyritic zone to a Zn-Pb–enriched upper portion. Sulfide minerals include sphalerite, galena, chalcopyrite, tetrahedrite, and minor arsenopyrite. Copper stockwork veins in rhyolite underlie the thicker sections of the deposit and appear to be spatially associated with growth faults that controlled basin geometry. These faults also influenced the distribution of tuffaceous and cherty (exhalative) units that overlie the sulfides. In terms of mined plus reserve ores, the Feitais deposit was estimated in 1998 to contain 54.5 Mt grading 3.72 percent Zn, 0.42 percent Cu, 1.2 percent Pb, and 44 g/t Ag (Leistel et al., 1998). Recent drilling has identified a new Zn resource in the upper part of the massive sulfide lens consisting of a measured and indicated resource of 15.22 Mt grading 6.00 percent Zn, 0.21 percent Cu, 1.85 percent Pb, and 67.4 g/t Ag; and a deeper Cu resource straddling the lower part of the massive sulfide lens and the underlying stockwork mineralization and consisting of an indicated resource of 4.87 Mt grading 0.88 percent Zn, 2.12 percent Cu, 0.24 percent Pb, and 13.5 g/t Ag. The deposit is open downplunge to the northwest for 200 m. It is truncated by the northeast-striking, steeply dipping Represa fault. The deposit is offset right laterally about 500 m along this fault, north of which it is called the Estacao deposit. Application of lithogeochemical methods to 170 new and 80 published analyses of the Feitais host rocks led to the chemical identification of five main felsic rock types that have distinctive immobile element signatures. These signatures are maintained even where the rocks are strongly altered. The deposit is underlain mainly by a thick (>150 m) unit of rhyolite A, the most fractionated of the felsic rocks. Rhyolite X occurs structurally below rhyolite A in two deep footwall holes. Rhyolite B occurs mainly downdip of the massive sulfide lens but also is present as meter-scale intervals near the base of the sulfide lens. The deposit is overlain stratigraphically by 5 to 30 m of tuffaceous rhyolite C, which is followed by 2 to 20 m of Fe-bearing chert. A centimeter-scale crystal tuff bed with a distinctive felsic composition overlies the cherts and forms a marker horizon that can be traced at least 700 m along the strike of the deposit. The massive sulfide lens becomes thicker downdip to the northeast, then terminates laterally against an uplifted block of footwall rhyolite A capped by rhyolite B. Field relationships and isopach data suggest that an asymmetrical sea-floor graben bounded by a synvolcanic fault was filled by massive sulfides, followed by rhyolite C, then chert, with the chert also extending laterally beyond the graben margin. Footwall and hanging-wall felsic volcanic rocks are altered to chloritesericite-pyrite-quartz-carbonate assemblages and contain stockwork chalcopyrite-pyrite-quartz-carbonate veins. Hydrothermal alteration is strongest in the upper 50 m of rhyolite A below the orebody but extends at least 200 m into the footwall along the main synvolcanic fault zone. Rhyolite C above the orebody is also strongly altered. These areas have experienced significant additions of Fe and Mg, with variable gains or losses of Si. Barium is enriched for up to a few tens of meters above and laterally from the orebody. Dominating the central part of the Aljustrel area is a quartz-feldspar (QF) porphyry that shows primary enrichment in Fe, Ti, and V relative to the rhyolites. At Feitais, this lithology was only encountered deep in the footwall, structurally below rhyolite X. No andesitic or mafic extrusive rocks have been identified. The QF porphyry is chemically unrelated to the four rhyolite types. The rare earth element (REE) patterns of the rhyolites show moderate enrichment in the light REE, with near-flat middle to heavy REE. The rhyolites are of tholeiitic to transitional affinity, with moderately high Zr (180–380 ppm) but low Nb (7–15 ppm) contents. These collective features are consistent with a rift-related, continental-margin setting, behind an arc, for the rhyolite volcanism.

Genesis of the Wolverine Volcanic Sediment-Hosted Massive Sulfide Deposit, Finlayson Lake District, Yukon, Canada: Mineralogical, Mineral Chemical, Fluid Inclusion, and Sulfur Isotope Evidence

The Wolverine deposit is a high-grade (~6.2 Mt at 12.7% Zn, 1.3% Cu, 1.6% Pb, 370.9 g/t Ag, and 1.8 g/t Au) volcanic and sedimentary rock-hosted massive sulfide deposit in the Finlayson Lake area, Yukon Territory. The area has the potential to become an important mining camp and currently contains more than 30 million tons (Mt) of massive sulfide ore in five deposits. A succession of metamorphosed early Mississippian felsic volcanic and sedimentary rocks that are interpreted to have formed on the margin of an ensialic back-arc ocean basin between the Yukon-Tanana terrane and the ancestral North American craton hosts the Wolverine deposit. The massive sulfide ore occurs at a single horizon that marks a significant change in the character of the volcanism from quartz- and feldspar-phyric rhyolite of the footwall to aphyric rhyolite and chemical sedimentary exhalative rock of the hanging wall. The deposit consists of two discrete Zn-Pb-Ag massive sulfide lenses situated at the same stratigraphic level connected by strata-bound semimassive, replacement-style Zn-Pb-Ag mineralization. Copper-rich massive sulfide commonly replaces the Zn-Pb-Ag massive sulfide at the base of the lenses and in the footwall replacement zones beneath the massive mineralization. Several zones of sulfide stringer veins and chlorite-sericite-carbonate alteration occur within permeable footwall volcaniclastic rocks. Alteration minerals preferentially associated with massive sulfide mineralization include Ba-rich phengitic mica, biotite, Mg-rich chlorite and siderite. Microthermometric measurements of fluid inclusions in the massive sulfide bodies and accompanying stringer zones indicate formation at temperatures of 235° to 353°C. Primary, aqueous, liquid-rich fluid inclusions in hydrothermal quartz from quartz-sulfide stringer veins contain low-salinity fluids (2.1–8.5 wt % NaCl equiv; mean = 6.0). There is no evidence of fluid boiling in the inclusions, and based on the average fluid salinity and temperature of homogenization, mineralization took place under a minimum water depth of ~1,000 m. In situ analyses of δ34SV-CDT in sulfide minerals from massive sulfide lenses (means 0.8‰) and sulfide stringer veins (mean 12.0‰) display a bimodal distribution. Lower δ34S values characterize sulfides near the top of the lenses versus higher (more positive) values in underlying veins may reflect different processes of reduction of seawater sulfate. Sulfur in sulfides from the tops of the lenses was likely derived mainly from bacterial reduction of seawater sulfate, whereas sulfur in the stringer veins and at the base of the lenses was mainly from inorganic reduction of seawater sulfate. Sulfide minerals precipitated both on the sea floor and in the subsea floor from hydrothermal fluids that vented from multiple mounds. The presence of black shales and the strongly positive sulfur isotope compositions of the sulfides provide evidence that the ambient bottom water in the basin in which the Wolverine deposit formed was likely anoxic. This environment may have contributed to the preservation of the sulfide mounds. Based on its tectonic setting, host-rock types, local geologic setting, metal grades, age, and source(s) of sulfur, the Wolverine deposit best exemplifies the volcanic sediment-hosted massive sulfide (VSHMS) class of syngenetic massive sulfide deposits in the district.

Geology, Fluid Inclusions, and Oxygen Isotope Geochemistry of the Baiyinchang Pipe-Style Volcanic-Hosted Massive Sulfide Cu Deposit in Gansu Province, Northwestern China

The Baiyinchang massive sulfide Cu deposit (Zheyaoshan and Huoyanshan mines) is hosted by an early Cambrian, submarine, felsic volcanic succession within an extrusive cryptodome associated with an overlying basaltic flow, in a Late Proterozoic-early Paleozoic submarine volcanic belt in the north Qilian orogen, northwestern China. The deposit is comprised of two mineralized zones: a 30-cm-thick, strata-bound Zn-rich sulfide lens associated with hematitic Fe-Mn cherts, and an underlying, discordant massive ore-dominated sulfide zone enveloped by a hydrothermal alteration pipe that is zoned from chlorite in the center to quartz-sericite at the margin. The discordant sulfide zone accounts for 90 percent of the Cu reserves of the Zheyaoshan mine. It consists of four main ore types: (1) pipelike pyrrhotite-pyrite ± chalcopyrite ore, (2) massive sulfide ore, (3) a disseminated ore halo, and (4) footwall stringer ore. The pyrrhotite-pyrite ± chalcopyrite pipe has an elliptical shape in plan and is 30 × 50 m across. The pipe partially replaces the overlying massive pyrite lens and extends downward at least 150 m, to be gradually replaced by chalcopyrite-rich stringer veins and chalcopyritebearing quartz veins surrounded by a discordant hydrothermal alteration envelope. Massive chalcopyrite-pyrite lenses discordant to volcanic bedding, containing relict patches of felsic volcanic host rocks, are commonly enveloped by a disseminated sulfide halo within a chloritized volcanic unit. These features suggest that Zheyaoshan is a pipe-style deposit that formed mainly by subsea-floor replacement of volcanic host rocks. Studies of fluid inclusions indicate that there are four types: (1) type I two-phase, aqueous fluid inclusions, (2) type II daughter mineral-bearing, multiphase fluid inclusions, (3) type III CO2-rich fluid inclusions, and (4) type IV CH4-rich fluid inclusions. Type II inclusions have high homogenization temperatures (Th) ranging from 320° to 430°C, contain high salinity fluids (31‐38 wt % NaCl equiv), and coexist with CO2-rich fluids found in vapor-rich, high-Th (up to 487°C), moderate salinity (10‐16 wt % NaCl equiv) inclusions in the discordant sulfide zone and associated altered rocks, suggesting a possible contribution of a magmatic fluid to the hydrothermal system. The coexistence of vapor-rich, high-Th (>300°C) and aqueous, low-Th (<220°C) type I fluid inclusions in the stringer zone suggests that heated seawater mixed with magmatic fluid (gas) in the feeder zone. Most type I fluid inclusions in the massive chalcopyrite-pyrite body and in the strongly chloritized pipe have a low Th (62°‐225°C) and high salinities (15.0‐23.0 wt % NaCl equiv), suggesting that a dense brine zone developed in fractures in the subsea floor where sulfides accumulated by open-space filling and replacement of host volcanic rocks. Eleven quartz samples from the overlying discordant sulfide zone yielded a restricted range of δ18O values between 8.8 and 11.1 per mil, from which we calculate that the corresponding hydrothermal fluids had δ18O values ranging from ‐5.3 to +3.1 per mil over a temperature range of 160° to 278°C. Whole-rock δ18O values

Formation of the Tharsis Massive Sulfide Deposit, Iberian Pyrite Belt: Geological, Lithogeochemical, and Stable Isotope Evidence for Deposition in a Brine Pool

Formation of the Tharsis Massive Sulfide Deposit, Iberian Pyrite Belt: Geological, Lithogeochemical, and Stable Isotope Evidence for Deposition in a Brine Pool

Geologic Setting, Mineralogy, and Geochemistry of the Early Tertiary Au-Rich Volcanic-Hosted Massive Sulfide Deposit of La Plata, Western Cordillera, Ecuador

La Plata is, economically, the most important of several volcanic-hosted massive sulfide (VHMS) deposits of the Western Cordillera of Ecuador, with preliminary estimated reserves of 840,000 metric tons (t) at 4.8 g/t Au, 54 g/t Ag, 4.1 percent Cu, 0.7 percent Pb, and 4.2 percent Zn. Like the other VHMS deposits of the Western Cordillera, it is hosted by the Early Tertiary Macuchi unit, which represents an oceanic island-arc sequence, composed of submarine volcanic and volcaniclastic sequences of basaltic to andesitic compositions, accreted to the continent along the Chimbo-Toachi shear zone in the late Eocene. La Plata displays a typical VHMS vertical zonation, from bottom to top, of a pyrite stockwork, disseminated sulfides, massive sulfide lenses, and a thin red jasper bed with exhalative-sedimentary textures. The massive sulfide lenses are located within footwall andesitic volcanic and volcaniclastic rocks that are pervasively altered to a quartz-pyrite-sericite assemblage, and immediately below a hanging-wall sequence consisting of at least two basaltic to basaltic-andesitic lava flows, each capped by autoclastic breccia intervals. The hanging-wall rocks, especially the autoclastic breccias, are hematite altered up to 300 m above the ore level. Hanging-wall volcanic rocks have geochemical features typical of arc tholeiites. Dolerite dikes in the hanging wall have middle-ocean ridge basalt (MORB)-type geochemical features likely indicating deposit formation in an intra-arc extensional environment. The massive sulfide lenses are situated mainly on the eastern side of an antiform with footwall rocks at the core and hanging-wall rocks on both flanks. Lithologic contacts, volcanic strata, and axial planes of folds have a consistent north-south strike. Footwall rocks at the core of the antiform are locally affected by meter-thick, north-south–trending subvertical shear bands that result in an intense tectonic foliation. Deformation structures are absent in the hanging-wall rocks suggesting that the footwall rocks, due to a different rheology, were ductile during deformation. Centimeter- to decimeter-sized massive sulfide clasts are incorporated into a mylonitic matrix of altered footwall rocks within these shear bands that contain dextral movement indicators. The larger massive sulfide lenses (up to 100 m long) probably result also from tectonic dismembering, within the footwall rocks, of a former, larger, massive sulfide body. The deformation is probably related to the Chimbo-Toachi dextral shear zone situated a few kilometers to the east of La Plata. The main ore minerals are pyrite, chalcopyrite, sphalerite, bornite, and galena, with minor tennantite and digenite. The gangue minerals are barite, quartz, and sericite. Mineralogical banding, defined by chalcopyrite-rich layers alternating with pyrite- and/or sphalerite-rich layers, is mostly the result of tectonic injection of ductile chalcopyrite, bornite, and galena within shear bands of cataclastic pyrite and sphalerite. Pyrite and sphalerite also show mild recrystallization textures. Deformation is heterogeneous as a consequence of strain partitioning, resulting in portions of the massive sulfide lenses being essentially undeformed and preserving primary VHMS textures, such as framboidal pyrite. Ag-poor gold is microscopically visible and can reach dimensions of few hundreds of microns. Whereas it is likely that some gold at La Plata has been reprecipitated during the shear event, the lack of gold in structural positions indicating an epigenetic origin with respect to the VHMS sulfides (e.g., in veins), suggests that gold was present in the original VHMS ore. Sulfur isotope compositions of the sulfides are homogeneous and consistent with sulfur derivation from leaching of the host volcaniclastic sequence, although a direct magmatic contribution cannot be ruled out. Strontium and sulfur isotope compositions of the late-stage barite are consistent with mixing between hydrothermal fluid and seawater. The very low Fe contents of sphalerite (0.27 ± 0.40 mol % FeS) suggest precipitation from fluids characterized by high a S2 and a O2. Because gold solubility as a bisulfide complex is higher in fluids with high a S2 and a O2, this might explain the Au-rich nature of the La Plata VHMS deposit. Additionally, the association of La Plata with magmatic rocks derived from a depleted mantle could indicate primary enrichment of gold in the source rocks. These characteristics resemble those of Au-rich VHMS deposits and submarine vents of the southwestern Pacific, including the occurrence within an intraoceanic island arc, a similar fluid chemistry and sulfide para-genesis, in contrast to other VHMS deposits of the Central Andes. This may have ultimately resulted from the Mesozoic to Cenozoic geodynamic evolution of the Northern Andes, which differs from that of the Central Andes and, being characterized by multiple accretions of both oceanic plateaus and island arcs, is similar to that of the southwestern Pacific region.

Geological Fluid Mapping in the Tongling Area: Implications for the Paleozoic Submarine Hydrothermal System in the Middle‐Lower Yangtze Metallogenic Belt, East China

Abstract: The Tongling area is one of the 7 ore‐cluster areas in the Middle‐Lower Yangtze metallogenic belt, East China, and has tectonically undergone a long‐term geologic history from the late Paleozoic continental rifting, through the Middle Triassic continent‐continent collision to the Jurassic‐Cretaceous intracontinental tectono‐magmatic activation. The Carboniferous sedimentary‐exhalative processes in the area produced widespread massive sulfides with ages of 303–321 Ma, which partly formed massive pyrite‐Cu deposits, but mostly provided significant sulfur and metals to the skarn Cu mineralization associated with the Yanshanian felsic intrusions.To understand the Carboniferous submarine hydrothermal system, an area of about 1046 km2 was chosen to carry out the geological fluid mapping. Associated with massive sulfide formation, footwall sequences 948 m to 1146 m thick, composed of the Lower Silurian‐Upper Devonian sandstone, siltstone and thin‐layered shale, were widely altered. This hydrothermal alteration is interpreted to reflect large‐scale hydrothermal fluid flow associated with the late Paleozoic crustal rifting and subsidence. Three hydrothermal alteration types, i.e., deep‐level semiconformable silicification (S1), fracture‐controlled quartz‐sericite‐pyrite alteration (S2–3), and upper‐level sub‐discordant quartz‐sericite‐chlorite alteration (D3), were developed to form distinct zones in the mapped area. About 50‐m thick semiconformable silicification zones are located at ∼1‐km depth below massive sulfides and developed between an impermeable shale caprock (S1) and the underlying Ordovician unaltered limestone. Comparisons with modern geothermal systems suggest that the alteration zones record a sub‐seafloor aquifer with the most productive hydrothermal fluid flow. Fracture‐controlled quartz‐sericite‐pyrite alteration formed transgressive zones, which downward crosscut the semiconformable alteration zones, and upwards grade into sub‐discordant alteration zones that enveloped no economic stringer‐stockwork zones beneath massive sulfides. This transgressive zone likely marks an upflow path of high‐flux fluids from the hydrothermal aquifer. Lateral zonation of the sub‐discordant alteration zones and their relationship to overlying massive sulfide lenses suggest lateral flows and diffusive discharging of the hydrothermal fluids in a permeable sandstone sequence. Three large‐sized, 14 middle‐small massive sulfide deposits, and 40 massive sulfide sites have been mapped in detail. They show regional strata‐bound characters and two major styles, i.e., the layered sheet plus strata‐bound stringer‐style and the mound‐style. Associated exhalite and chemical sedimentary rock suites include (1) anhydrite‐barite, (2) jasper‐chert, (3) Mg‐rich mudstone‐pyrite shale, (4) barite lens, (5) siderite‐Fe‐bearing dolomite, and (6) Mn‐rich shale‐mudstone, which usually comprise three sulfide‐exhalite cyclic units in the area.The spatial distribution of these alteration zones (minerals) and associated massive sulfides and exhalites, and regional variation in δ34S of hydrothermal pyrite and in δ18O‐δ34C of hanging wall carbonates, suggest three WNW‐extending domains of fluid flow, controlled by the basement faults and syn‐depositional faults. Each fluid domain appears to have at least two upflow zones, with estimated even spacing of about 5–8 km in the mapped area. The repeated appearance of sulfide‐sulfate or sulfide‐carbonate rhythmic units in the area suggests episodically venting of fluids through the upflow conduits by breaking the overlying seals of the hydrothermal aquifer.

THE Ni Cu PGE SULFIDE ORES OF THE KOMATIITE-HOSTED FORTALEZA DE MINAS DEPOSIT, BRAZIL: EVIDENCE OF HYDROTHERMAL REMOBILIZATION

The Fortaleza de Minas Ni–Cu–PGE sulfide deposit is hosted by Archean komatiitic rocks of the Morro do Ferro greenstone belt, near the southwestern margin of the Sao Francisco craton, Minas Gerais state, Brazil. The deposit contains 6 million tonnes of ore with an average grade of 2.2 wt% Ni, 0.4% Cu, 0.05% Co and 1.2 ppm PGE+Au, and comprises (i) a main orebody, which is metamorphosed, deformed and transposed along a regional shear zone, consisting mainly of disseminated, brecciated and stringer sulfide ores that are interpreted to be of early magmatic origin, and (ii) PGE-rich discordant veins that are hosted in N–S- and NE–SW-trending late faults that cross-cut the main orebody. The discordant PGE-rich ore (up to 4 ppm total PGE) is characterized by thin, discontinuous and irregular veins and lenses of massive sulfides hosted by serpentinite and talc schist, and is relatively undeformed if compared with the early types of ore. It is composed mainly of pyrrhotite, pentlandite, chalcopyrite, magnetite, carbonates, and amphiboles, with minor cobaltite–gersdorffite, sphalerite, ilmenite, and quartz, and rarely maucherite (Ni11As8), tellurides and platinum-group minerals (PGM). Omeeite, irarsite, sperrylite, and Ni-bearing merenskyite are the main PGM, followed by minor amounts of testibiopalladite and an unknown phase containing Ru, Te, and As. The PGM occur either included in, or at the margins of, sulfides, sulfarsenides, silicates and oxides, or filling fractures in pyrrhotite, pentlandite, and chalcopyrite, suggesting that they started to precipitate with these minerals and continued to precipitate after the sulfides were formed. The mantle-normalized metal distribution of the two samples of discordant veins shows distinct patterns: one richer in Ni–Pd–Ir–Rh–Ru–Os and another with higher amounts of Cu–Pt–Bi. Both are strongly depleted in Cr if compared with the metamorphosed magmatic ore of this deposit, which follows the general Kambalda-type magmatic trend. On the basis of structural, mineralogical and geochemical evidence, we propose that the PGE-rich discordant ore may have formed by remobilization of metals from the deformed, metamorphosed magmatic orebody (which shows a depleted pattern in these elements) by reduced (pyrrhotite – pentlandite – pyrite are stable), neutral to alkaline and carbonic fluids (carbonate-stable). The PGE may have been transported as bisulfide complexes, and precipitated as tellurides (mainly Pd) and arsenides (Pt, Rh, Ru, Os, Ir) in the late N–S and NE–SW-trending faults owing to a decrease in the activity of S caused by the precipitation of sulfides in the veins.

The LaRonde Penna World-Class Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi, Quebec: Mineralogy and Geochemistry of Alteration and Implications for Genesis and Exploration

Gold-rich volcanogenic massive sulfide (VMS) deposits consist of synvolcanic banded and concordant massive sulfide lenses and adjacent stockwork feeder zones in which gold concentration in g/t Au exceeds the combined Cu, Pb, Zn grades in wt percent and thus constitutes the main commodity. The Agnico-Eagle LaRonde Penna deposit is a world-class Au-rich VMS (production, reserves and resources of 58.8 Mt at 4.31 g/t Au) located in the eastern part of the Blake River Group of the Abitibi greenstone belt. The deposit comprises four stacked massive sulfide lenses within the upper member of the Bousquet Formation (2698–2697 Ma). The two main ore zones, 20 North and 20 South, are sheetlike, massive to semimassive polymetallic sulfide lenses and stringer zones. Both lenses and sulfide stringers are deformed and transposed by the main foliation. The 20 North lens (Zn-Au-Ag-Cu-Pb) is the main orebody. It is subdivided into two zones: the 20N Au and 20N Zn zones. The 20N Au zone is a transposed and ribbon-textured gold- and copper-rich pyrite-chalcopyrite stringer zone overlain to the south by a 10- to 30-m-thick massive pyrite-sphalerite-galena lens (20N Zn zone). The 20 South lens is an 8- to 10-m-thick gold- and zinc-rich massive sulfide and stringer zone located about 10 to 15 m below the Cadillac Group sedimentary rocks (<2687 Ma). At depth (≥1,900 m below surface), the 20 North and 20 South lenses grade into aluminous zones composed mainly of quartz-pyrite-kyanite-andalusite-muscovite-Zn–rich staurolite assemblages that host transposed sulfide stringers and local semimassive to massive Au-rich pyrite and chalcopyrite layers. The synvolcanic hydrothermal alteration now corresponds to mappable upper greenschist-lower amphibo-lite–grade metamorphic assemblages. The footwall of the 20 North lens is characterized by a large discordant to semiconformable distal quartz-biotite ± garnet assemblage, which transitions laterally into a proximal quartz-garnet-biotite-muscovite zone. The abundance of pink Mn-rich garnet porphyroblasts increases toward the 20N Au zone. The hanging wall of the 20 North lens is characterized by a meter-thick zone of fracture-controlled pink alteration composed of quartz, biotite, rutile and/or anatase, and titanite associated with barren sulfide stringers. The garnet-rich assemblage in the footwall records gains in MnO, Fe2O3(total), and MgO and losses of Na2O. In the hanging wall, Fe2O3(total), S, and CO2 were added to the rocks with a slight increase in K2O, and CaO. At depth (≥ 1,900 m), the Au-rich aluminous replacement zone is a (up to 30 m) thick, highly strained zone composed of a quartz-pyrite-kyanite-andalusite-chalcopyrite-gold assemblage. All oxides except Al2O3, SiO2, and Fe2O3(total) were strongly leached. The metamorphosed hydrothermal alteration associated with the 20 South lens is characterized by a pink quartz-biotite-rutile-titanite assemblage very similar to that in the hanging wall of the 20 North lens. Toward the ore zone, the pink assemblage is gradually replaced by a proximal quartz-muscovite-green mica-pyrite assemblage, which hosts the sulfide mineralization. The aluminous alteration at LaRonde Penna is interpreted to be the metamorphic equivalent of an advanced argillic alteration and has many similarities to that of metamorphosed high-sulfidation systems and particularly a class of Au-rich VMS characterized by aluminous alteration. The LaRonde Penna and Bousquet 2-Dumagami deposits are interpreted to represent one large hydrothermal system in which variable contributions of hydrothermally modified seawater and magmatic volatiles contributed to the different styles of alteration and mineralization. The study illustrates that diverse styles of Au-rich VMS can coexist within the same deposit. In terms of exploration, almost all sulfide lenses or hydrothermal alterations minerals are located at or near volcanic hiatuses within the Bousquet Formation. These hiatuses represent major exploration targets especially when located in the upper part of the Bousquet Formation. The aluminous alteration zones have accommodated most of the postore strain due to their ductility and are transformed into schists. Consequently, the alteration product coincides spatially with the deformation zones despite the lack of a genetic relationship. Quartz-and Mn-rich garnet-biotite assemblages and/or aluminous schists with anomalous gold and/or zinc in intermediate to felsic transitional to calc-alkaline volcanic or volcaniclastic rocks located underneath a sedimentary cover represent excellent targets for Au-rich VMS in metamorphosed terranes.

The LaRonde Penna Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi Greenstone Belt, Quebec: Part I. Geology and Geochronology

The LaRonde Penna Au-rich volcanogenic massive sulfide (VMS) deposit is the largest Au deposit currently mined in Canada (58.8 Mt at 4.31 g/t, containing 8.1 Moz of Au). It is part of the Doyon-Bousquet-LaRonde mining camp located in the eastern part of the Blake River Group of the Abitibi greenstone belt which is host to several of the world’s most important, present and past, Au-rich VMS deposits (e.g., Horne, Quemont, Bousquet, Bousquet 2-Dumagami). The LaRonde Penna deposit consists of massive to semimassive sulfide lenses (Au-Zn-Ag-Cu-Pb), stacked in the upper part of a steeply dipping, south-facing homoclinal volcanic sequence composed of extensive tholeiitic basaltic flows (Hebecourt Formation) overlain by tholeiitic to transitional, mafic to intermediate, effusive and volcaniclastic units at the base (lower member of the Bousquet Formation) and transitional to calc-alkaline, intermediate to felsic, effusive and intrusive rocks on top (upper member of the Bousquet Formation). The mafic to felsic volcanism of the Hebecourt Formation and of the lower member of the Bousquet Formation formed an extensive submarine basement or platform on which the intermediate to felsic rocks of the upper member of the Bousquet Formation were emplaced at restricted submarine eruptive centers or as shallow composite intrusive complexes. The submarine felsic volcanic rocks of the upper member of the Bousquet Formation are characterized by dacitic to rhyodacitic autoclastic (flow breccia) deposits that are cut and overlain by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes. This volcanic architecture is thought to have been responsible for internal variations in ore and alteration styles, not only from one lens to another, but also along a single mineralized horizon or lens. In the upper part of the mine, the 20 North lens comprises a transposed pyrite-chalcopyrite (Au-Cu) stockwork (20N Au zone) overlain by a pyrite-sphalerite-galena-chalcopyrite-pyrrhotite (Zn-Ag-Pb) massive sulfide lens (20N Zn zone). The latter was formed, at least in part, by replacement of footwall rhyodacitic autoclastic deposits emplaced within a subbasin located between two rhyolite domes or cryptodomes. The 20N Zn zone tapers with depth in the mine and gives way to the 20N Au zone. At depth in the mine, the 20N Au zone consists of semimassive sulfides (Au-rich pyrite and chalcopyrite) enclosed by a large aluminous alteration halo on the margin of a large rhyolitic dome or cryptodome. U-Pb zircon geochronology gives ages of 2698.3 ± 0.8 and 2697.8 ± 1 Ma for the footwall and hanging-wall units of the 20 North lens, respectively. Thus, the formation of the 20 North lens was coeval with other VMS deposits in the Bousquet Formation and in the uppermost units of the Blake River Group. Although deformation and metamorphism have affected the primary mineral assemblages and the original geometry of the deposit, these events were not responsible for the different auriferous ore zones and alteration at LaRonde Penna. Studies of the LaRonde Penna deposit show that the hydrothermal system evolved in time and space from near-neutral seawater-dominated hydrothermal fluids, responsible for Au-Cu-Zn-Ag-Pb mineralization, to highly acidic fluids with possible direct magmatic contributions, responsible for Au ± Cu-rich ore and aluminous alteration. The different ore types and alteration reflect the evolving local volcanic setting described in this study.

The LaRonde Penna Au-Rich Volcanogenic Massive Sulfide Deposit, Abitibi Greenstone Belt, Quebec: Part II. Lithogeochemistry and Paleotectonic Setting

The Au-rich massive to semimassive sulfide lenses of the LaRonde Penna deposit (58.8 Mt at 4.31 g/t Au) are stacked in a steeply dipping, southward-facing homoclinal volcanic sequence forming a continuous, differentiated volcanic succession composed of two main formations: the ca. 2700 Ma Hebecourt Formation and the 2701 to 2698 Ma Bousquet Formation, which corresponds to the uppermost segment of the Blake River Group. The Hebecourt Formation is composed of regionally extensive LREE-depleted ([La/Sm]N ∼0.9) tholeiitic, basaltic to andesitic, massive to pillowed flows that formed a submarine stratum on which the Bousquet Formation was emplaced. The Bousquet Formation is further divided into a lower member and an upper member. The lower member of the Bousquet Formation is composed of feldspar and quartz-phyric tholeiitic felsic (Zr/Y ∼3.4, Zr/TiO2 ∼860) sills and extensive effusive and volcaniclastic mafic to intermediate and tholeiitic to transitional rocks. The upper member is mainly characterized by submarine, coalesced dacitic to rhyodacitic autoclastic flows that are cut and/or covered by rhyodacitic and rhyolitic domes and/or partly extrusive cryptodomes and by intermediate to mafic sills and dikes. Mafic to intermediate and tholeiitic to transitional (Zr/Y ∼2.3–5) rocks of the Bousquet Formation are characterized by a low Zr/TiO2 ratio (<60), moderately enriched chondrite-normalized LREE and MREE ([La/Sm]N ∼2.2–2.7) patterns, flat HREE ([Gd/Lu]N ∼1.2–2) patterns, and negative Nb, Ta, Zr, and Hf anomalies. Felsic transitional to calc-alkaline (Zr/Y ∼5–8) rocks of the upper member of the Bousquet Formation are characterized by a moderate Zr/TiO2 ratio (∼250–615), high incompatible element contents, LREE-enriched patterns ([La/Sm]N ∼3.2–6.6), flat HREE patterns ([Gd/Lu]N ∼1–1.4), pronounced negative Nb, Ta, and Ti anomalies, and positive Zr and Hf anomalies. The Nd isotope signature of six separate LaRonde Penna deposit host units (eNd ∼3–3.4) suggests that they were generated by partial melting of depleted upper mantle and/or juvenile material (mafic crust) or by a combination of those two processes. The sequence is interpreted to reflect the progression from diapirism of depleted upper mantle associated with underplating by mafic-ultramafic magma and assimilation and magmatic differentiation (assimilation-fractional crystallization) at midcrustal levels in subsidiary magmatic chambers within a ca. 2721 Ma, relatively thick, juvenile or immature mafic ± felsic arc–back-arc crust in an intermediate setting between back-arc basin and volcanic-arc environments. This setting, compatible with the inferred geodynamic setting for the southern Abitibi belt, could be responsible, at least in part, for the Au enrichment of the volcanic massive sulfide (VMS) deposits of the Doyon-Bousquet-LaRonde mining camp. This study shows that Archean HREE-depleted and high Th, transitional to calc-alkaline dacite, rhyodacite, and rhyolite, referred to as FI and FII type, such as those associated with the LaRonde Penna deposit, can be important hosts for VMS and Au-rich VMS and may be as prospective as the tholeiitic or FIII-type rhyolite-bearing sequences.

The Bisha Volcanic-Associated Massive Sulfide Deposit, Western Nakfa Terrane, Eritrea

The Neoproterozoic Bisha volcanic-associated massive sulfide deposit (VMS) is a large (>39 million metric tons (Mt)) relatively high grade Zn-Cu-Au-Ag deposit in a newly discovered VMS district in western Eritrea. The host stratigraphic section comprises a large, possibly coeval and cogenetic intrusion known as the Bisha Gabbroic Complex, ~3 km downsection, a series of mafic to felsic tuffs with minor flows and minor sedimentary rocks in the proximal footwall, and felsic flows, fine tuffs, and volcanic-derived siliciclastic rocks in the proximal hanging wall. Minor siliceous exhalite units extend for several kilometers along strike and are present upsection. Two other VMS deposits are present along strike: Bisha Northwest, 1.5 km to the north-northwest, and Harena, 9.5 km to the south-southwest; additionally, a large, lower grade VMS deposit, Hambok, is present ~15 km to the southwest. The Bisha deposit is a single, nearly inclusion and dike-free lens, 1.2 km along strike, that has been folded and affected by significant, near-surface oxidation and supergene processes. The deposit and host strata form an overturned synform within a probable, west-over east, kilometer-scale nappe structure. It would appear that the strata have been folded and thrust against the gabbroic complex which behaved as a competent buttress during deformation. The oxidation and supergene enrichment have led to the development of four mineralogically and spatially distinct ore types or zones. The hematite-goethite-quartz gossan, from 0- to ~30-m depth, has high gold, silver, and lead contents, with native gold, argentiferous gold, electrum, anglesite, and lesser cerrusite. The kaolinite-quartz-sulfate zone occurs at ~25- to 35-m depth, and also is enriched in gold, silver, and lead, and has the same ore mineralogy as the gossan; additionally it has minor chalcocite and minor to trace amounts of unusual Zn-Pb-Fe carbonate and sulfate minerals. The supergene sulfide zone is present from ~35- to 65-m depth and has significant copper enrichment, with predominant chalcocite, and lesser digenite, covellite, and bornite, along with pyrite, minor pyrrhotite, anglesite, and gangue minerals. The primary massive sulfide, from ~65- to more than 450-m depth, has a significant volume of zinc-rich ore and a typical VMS sulfide assemblage of pyrite, sphalerite, chalcopyrite, and minor to trace galena, pyrrhotite, tetrahedrite, tennantite, arsenopyrite, barite, and gangue. The average ore and trace metal contents of the four ore types are calculated from an extensive assay and whole-rock geochemical database. The average metal contents for primary massive sulfide are comparable to that of other bimodal-siliciclastic deposits using an extended, primitive mantle-normalized plot. The ore metals, Cu, Pb, Zn, Au, and Ag, are 2.5 to 3 orders of magnitude higher than average continental crust, whereas the trace metals are more variable. Manganese is significantly depleted in comparison to the average crustal value. Moderate to strong chloritic alteration and stringer sulfide veins are present in footwall felsic and intermediate tuffs along the entire strike length, immediately east of the eastern massive sulfide lens. This is high-lighted using a chlorite alteration index, in wt percent: (MgO + FeO)*100/(MgO + FeO + CaO + K2O + Na2O). In addition, the chloritic footwall is depleted in silica, alkali, and alkaline earth elements, and europium. In contrast, these elements are enriched in the hanging-wall tuffs, indicating that the Bisha hydrothermal system continued after deposition of the tuffs. Primary geochemistry of mafic to felsic volcanic and nearby intrusive rocks, including the Bisha Gabbroic Complex, indicate a bimodal igneous suite, with evidence for cogenetic and possibly coeval volcanic and intrusive rocks. The suite has tholeiitic to transitional calc-alkalic affinities, with mantle plume-influenced, primitive arc signatures for least altered basalts. Lead isotope ratios of galena and anglesite mineral separates from the Bisha, Bisha Northwest, and Harena deposits have model ages that range from 790 to 770 Ma. This is in contrast to a lead model age of ~720 Ma for the Adi Nefas VMS deposit in the eastern Nakfa terrane and to galena from a nearby gold vein deposit at ~490 Ma. The model ages for the Bisha area VMS deposits indicate that these deposits and their host rocks formed early during the development of the 850 to 550 Ma Nakfa terrane.

Discrimination of Metamorphic and Metasomatic Processes at the Broken Hill Pb-Zn-Ag Deposit, Australia: Rare Earth Element Signatures of Garnet-Rich Rocks

Garnet occurs in a variety of rock types spatially related to the giant stratiform Paleoproterozoic Broken Hill lead-zinc-silver deposit, Australia. The deposit was metamorphosed to granulite facies conditions and subjected to at least five periods of deformation (D1–D5). Sulfides at Broken Hill are commonly associated with quartz garnetite, garnetite, and blue quartz gahnite-garnet rock. The origin of quartz garnetite and garnetite is controversial. Proposed models include: metamorphism of manganiferous sediments formed by submarine hydrothermal processes that mixed with aluminous pelagic sediments; metamorphism of an original detrital sediment; metasomatic mobilization of Mn between the sulfide lenses and the wall rocks either synpeak (D1–D2) or postpeak metamorphism (D3); and reaction of Mn derived from partially melted sulfide orebodies with the surrounding pelitic gneisses. REE patterns of whole-rock samples of garnetite and quartz garnetite show high light rare earth element/heavy rare earth element (LREE/HREE) ratios. Chondrite-normalized REE patterns, based on analyses of garnet in garnetite and quartz-garnetite and some samples of blue quartz gahnite-garnet rock, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques, generally show very low LREE/HREE ratios, flat HREE signatures, and low overall REE contents (<100 ppm). HREE are incorporated in garnet, whereas LREE likely occur in intergranular material between garnet grains and in monazite, zircon, and apatite inclusions in garnet or along grain boundaries. Garnetite and fine-grained quartz garnetite show positive and negative Eu anomalies, respectively. Coarse metasomatic garnet surrounding D3 quartz veins, and metasomatic garnet in quartz garnetite halos around lead-rich orebodies, remobilized quartz garnetite and some blue quartz gahnite-garnet rocks, show positive and negative Eu anomalies, variable HREE, low to moderate LREE/HREE ratios, and arcuate patterns. The REE signature of garnet in garnetite and quartz garnetite is characteristic of metamorphic garnet with Eu anomalies reflecting relative inputs of detrital to hydrothermal components in a premetamorphic protolith, f O2 conditions, and temperature, whereas arcuate HREE signatures are characteristic of metasomatic garnet that formed postpeak metamorphism. The REE chemistry of garnet is consistent with mineralogical, petrological, and structural studies that demonstrate garnetite and quartz garnetite are metamorphosed sediments that formed by submarine hydrothermal processes.