Abstract The Bathurst Mining Camp hosts 45 volcanic sediment-hosted massive sulfide deposits and 95 occurrences, including the supergiant Brunswick 12 deposit with a geologic resource of 230 million metric tons (Mt) grading 7.66 wt percent Zn, 3.01 wt percent Pb, 0.46 wt percent Cu, and 91 g/t Ag. Ten of these have been brought into production with a total production to 1999 of 128 Mt with an average grade of 2.87 wt percent Pb, 6.58 wt percent Zn, 0.93 wt percent Cu, and 82 g/t Ag; only the Brunswick 12 deposit remains in production. The Bathurst Mining Camp deposits belong to an ECONOMICally important subgroup of sea-floor hydrothermal deposits referred to as volcanic-sediment-hosted massive sulfide deposits. The Bathurst Mining Camp is interpreted to be a sediment-covered back-arc continental rift referred to as the Tetagouche-Exploits back-arc basin, which opened by rifting of Ganderian continental crust in the Early ordovician and closed by northwestward-directed subduction during the Late ordovician to Early Silurian.The Bathurst Mining Camp was intensely deformed and metamorphosed during multiple collisional events related to east-dipping subduction of the basin. Peak metamorphic conditions vary from 325° to 400°C and 6 to 7 kbars. The present distribution and shape of massive sulfide deposits and associated sulfide stringer zones are mainly controlled by D 1 and D 2 structures. The volcanic sedimentary sequence has been subdivided into the Tetagouche, California Lake, and Sheep- house Brook Groups. These groups reflect different parts of the back-arc basin that were tectonically juxtaposed in a west-dipping subduction-obduction complex. The bimodal volcanic pile in each of the groups evolved from felsic to mafic dominated through time due to crustal thinning during the rifting process. The ambient water column conditions within the Tetagouche basin alternated from stratified with anoxic bottom waters during the deposition of regionally extensive Arenigian black shale of the Knights Brook, Patrick Brook, Nepisiguit Falls, and Spruce Lake Formations, to well-oxygenated during the deposition of Llanvirian maroon shales and cherts of the Little River and Boucher Brook Formations, to anoxic during deposition of Caradocian black shales. Four hydrothermal events spanning 12 to 14 m.y. have been recognized, and from oldest to youngest these are the Chester (478 Ma), Caribou (472–470 Ma), Brunswick (469–468 Ma), and Stratmat (467–465 Ma) horizons. The Stratmat and Brunswick horizons both occur in the Tetagouche Group, whereas the Caribou and Chester horizons are hosted by the California Lake and Sheephouse Brook Groups, respectively. There are two major types of iron formation in the Bathurst Mining Camp. Type 1 is a carbonate-oxide-silicate iron formation that is spatially associated with most massive sulfide deposits of the Brunswick ore horizon and extends several kilometers from the sulfide deposit along mineralized horizons. Siderite, magnetite,Fe/Mn, Ba/A1, P/A1, Zn, Pb, Cu, Ag, As, Au, Bi, Cd, Co, Mo, Se, Sb, Sn, In, T1, and Eu/Eu* increase systematically with proximity to ore deposits and define vectors for mineral exploration. None of the other mineralized horizons in the Bathurst Mining Camp contain Algoma-type iron formations. Type 2 consists of widely distributed Llanvirian Fe-Mn oxides that are not spatially associated with massive sulfide deposits and formed during a period of oxygenated seawater conditions. Most deposits are zoned vertically and laterally from a high-temperature, vent-proximal, veined, and brecciated core to vent-distal hydrothermal sediments as follows: (1) vent complex (pyrrhotite + magnetite + pyrite + chalcopyrite + quartz ± sphalerite ± galena), (2) bedded ores (pyrite + sphalerite + galena ± chalcopyrite), and (3) bedded pyrite (pyrite ± sphalerite ± galena). This mineral zonation is accompanied by the following chemical changes: proximal (high Cu, Co, Bi, Cu/Cu + Pb + Zn); and distal (increased Zn, Pb, Ag, Au, Cd, Sn, In, As, Sb, Tl, and Hg; low Cu/Cu + Pb + Zn). The vent complex is commonly underlain by a highly deformed sulfide stringer zone that extends hundreds of meters beneath deposits and consists of veins and impregnations of sulfides, silicates, and carbonates that cut hydrothermally altered volcanic and sedimentary rocks. Hydrothermal alteration is widespread (1-5 km laterally and hundreds of meters vertically) and is zoned from the core to the margins of upflow zone as follows:Zone 1— quartz + Fe-rich chlorite + pyrrhotite + chalcopyrite (>300°C); zone 2— Fe-rich chlorite + sericite ± pyrite; zone 3—Fe-Mg chlorite + sericite + albite; and zone 4— albite + Mg-rich chlorite. Accompanying these mineralogical changes is a marked increase of Si, Fe, Mg, CO 2 , S, Zn, Pb, Cu, Cd, As, Sb, and Hg and a decrease of Na and Ca in zone 1; and an Mg and Na increase and a Ca decrease in the outer alteration zones. The massive sulfide deposits formed from low-salinity and high-temperature (>300°C) buoyant hydrothermal fluids, which explains the vent-proximal nature of most deposits. The reduced sulfur in Bathurst Mining Camp deposits originated mostly from an ambient-reduced water column. Variations among deposits of different age are controlled by the global secular δ 5 34 S curve for sedimentary sulfate and sulfide. The base metals were probably derived from both hydrothermal and magmatic fluids, whereas elements such as Sn, In, Au, As, and Sb probably originated from magmatic fluids. The Bathurst Mining Camp deposits share many of the attributes of SEDEX deposits, including large size, metal contents (Zn + Pb + Cu + Ag), hydrothermal architecture, anoxic sea-floor environment, and perhaps ambient seawater biogenic sulfur source. The large size of many Bathurst Mining Camp deposits, and volcanic sediment-hosted massive sulfide deposits in general, reflects a number of factors including: (1) hydrothermal architecture consisting of a hydrothermal reservoir capped by impervious fine-grained sediments, (2) the longevity of the hydrothermal system, (3) focused discharge from long-lived vent sites, (4) formation during a major hiatus in volcanism, (5) anoxic bottom waters that facilitated the total capture of metals in buoyant hydrothermal fluids, and (6) direct magmatic input of metals, particularly in the case of large deposits.

Abstract The Bathurst Mining Camp is made up of several different tectonic blocks and slivers—the Fournier, California Lake, Tetagouche, and Sheephouse Brook blocks and the blueschist and Bamford Brook slivers. These blocks and slivers are characterized by unique Arenig-Caradoc volcanic stratigraphies, indicating they represent widely separated, ensialic to ensimatic portions of the Tetagouche-Exploits back-arc basin. Their structural juxtaposition took place during the Ashgill-Ludlow closure of the Tetagouche-Exploits back-arc basin. The Tetagouche-Exploits back-arc basin formed in response to rifting of the northwest-facing Popelogan arc, although the extension-rifting history was diachronous and involved multiple stages. The rifting responsible for the California Lake block (ca. 472–468 Ma) took place before the rifting of the Tetagouche (ca. 467–465 Ma) and Sheephouse Brook blocks (ca. 466–464 Ma). These three blocks have ensialic to transitional crust and share a similar pre-Arenig basement consisting of Miramichi Group deep-water sandstones and shales. Oceanic to transitional back-arc crust is preserved in the Fournier block and the blueschist and Bamford Brook slivers. The various blocks and slivers were sequentially incorporated into the Brunswick subduction complex during closure of the back-arc basin. Massive sulfide deposits in the Brunswick mining camp mainly occur in the California Lake, Tetagouche, and Sheephouse Brook blocks. Radiometric age dating indicates that massive sulfide deposition took place during a protracted period of ca. 12 m.y., although occurring at different intervals in each block. High heat flow due to extension and/or rifting of the Popelogan arc combined with anoxic bottom conditions in the associated basins seem to be a prerequisite for formation of the large massive sulfide deposits. The oldest known massive sulfides are hosted by early Arenig (ca. 478 Ma) dacites of the Clearwater Stream Formation of the Sheephouse Brook Group, which may represent the earliest stages of extension of the Popelogan arc. The main massive sulfide bodies in the California Lake Group are represented by the middle to upper Arenig (ca. 472–470 Ma) Caribou-type deposits, whose formation coincides with the early stages of rifting of the California Lake block from the Popelogan arc. The large, massive sulfide deposits in the Tetagouche Group (referred to as the Brunswick-type deposits) are typically hosted by, or intimately associated with, the pyroclastic and tuffaceous sedimentary rocks that occur near or at the top of the Nepisiguit Falls Formation (ca. 469–468 Ma). The Brunswick-type deposits formed during extension rather than rifting of the Tetagouche block from the Popelogan arc, because mafic volcanism does not accompany felsic volcanism in the Nepisiguit Falls Formation. Rifting-related mafic volcanism becomes abundant in the overlying Flat Landing Brook Formation (ca. 467–465 Ma), the rhyolites of which host a few small massive sulfide deposits. Some contemporaneous massive sulfide mineralization occurs also in the coeval feldspar-porphyritic rhyolites of the Sheephouse Brook Group.

Abstract The Eagle’s Nest Ni-Cu-PGE deposit was discovered in the McFaulds Lake area of the James Bay lowlands of northern Ontario, Canada, in 2007 by Noront Resources Ltd. It is a magmatic sulfide deposit hosted by mafic and ultramafic rocks interpreted to be a feeder conduit beneath an extensive complex of sills and related volcanic rocks, which range in composition from dunite through ferrogabbro to rhyolite. The complex, called the Ring of Fire, has been dated at 2734.5 ± 1.0 Ma and it was emplaced into 2773.37 ± 0.9 Ma felsic plutonic rocks. The felsic rocks form a sill complex structurally beneath metasedimentary and metavolcanic rocks considered to have formed along a passive margin at ca. 2800 Ma within the Oxford-Stull domain of the North Caribou superterrane in the Archean Superior province. In its original configuration, the Eagle’s Nest deposit formed in a shallowly plunging or subhorizontal keel structure at the base of a dike-like chonolith, but subsequent deformation has turned it into a vertically plunging rod of sulfide mineralization along the northwestern margin of a north-south–striking dike. The most magnesian chilled margin is a picrite with MgO content near 18 wt percent, placing it at the boundary between komatiite sensu stricto and komatiitic basalt. Modeling suggests that the parental magma contained at least 22 percent MgO and was derived from previously melt depleted mantle. Sulfide saturation was attained following extensive contamination of the magma, resulting in the accumulation of a slurry of olivine crystals with variable amounts of interstitial sulfide melt and postcumulus orthopyroxene at the base of the conduit, locally producing significant pools of massive sulfide at or near the lower contact. The sulfide segregation occurred at moderate degrees of sulfide supersaturation from a magma rich in chalcophile elements, leading to high base and precious metal tenors in the resulting deposit. Minor fractionation of the sulfide magma is evidenced by the dispersion of massive and net-textured sulfide compositions along a tie-line between Ni-rich monosulfide and Cu-rich intermediate sulfide solid solutions, as well as by minor quantities of vein-hosted massive sulfide with extremely enriched base and precious metal tenors throughout the deposit. The former are interpreted as sulfide cumulates, whereas the latter are possible remnants of highly evolved sulfide liquids. Extensive metamorphic remobilization of Pt is considered to be responsible for wholesale depletion of Pt in much of the massive sulfide and for the local generation of sulfide veins carrying >1,100 ppm Pt.

Abstract The Caribou deposit consists of a west-east en echelon array of six stratiform massive sulfide lenses that extend for 1,500 m horizontally and 1,200 m vertically. This deposit is second only to the Brunswick 12 deposit in terms of size, with total massive sulfide resources estimated at 70 million tons (Mt). The sulfides are hosted within a Middle Ordovician (472–470 Ma) sequence of felsic volcanic and sedimentary rocks that have been intensely deformed and metamorphosed to greenschist facies during subduction and continental collision. The tectonic setting is interpreted as a back-arc continental rift. Laminated black pyritic shale with high S/C ratios, low MnO contents, and positive δ 34 S values, and the absence of sea-floor sulfide oxidation, indicate ambient anoxic and H 2 S-rich bottom waters before the onset of hydrothermal activity. The sulfides at Caribou are divided into three major hydrothermal facies: (1) bedded sulfides, (2) vent complex, and (3) sulfide stringer zone. Sedimentary textures are best developed in the bedded sulfide facies, which contains abundant colloform and laminated framboidal pyrite. The vent complex consists of clasts (<10 cm in diam) of fine-grained pyrite with magnetite, chalcopyrite, pyrrhotite, dark green chlorite, quartz, and ferroan carbonates in the matrix. The Caribou massive sulfides are zoned both vertically and laterally from the core of the vent complex to the vent-distal bedded sulfide facies as follows: Cu, Bi, and Co contents, Cu/(Cu + Pb + Zn) ratios, and Eu/Eu * values decrease, and Zn, Pb, Ag, Au, Sn, In, As, Sb, Mo, Cd, Hg, and Ga increase. The vent complex is also characterized by Fe-rich sphalerite, Ag-rich galena, and As-poor pyrite compared to the composition of these minerals in the bedded sulfide facies. The sulfide stringer zone consists of quartz + pyrite + pyrrhotite + chalcopyrite veins, representing the core of fluid discharge that is surrounded by an outer, lower temperature hydrothermal alteration composed of Mg-rich chlorite + albite. Altered felsic volcanic rocks in both the stratigraphic footwall and hanging wall show the same general trend of increasing FeO + MgO/Na 2 O + CaO toward the massive sulfides and underlying the sulfide stringer zone. This widespread Mg alteration is explained by entrainment of heated seawater into the hydrothermal fluid discharge conduits. Thermochemical modeling suggests that the Caribou deposit formed from high-temperature, low f O 2 fluids that deposited a pyrrhotite + chalcopyrite + quartz assemblage in the sulfide stringer zone and overlying the vent complex. The reaction of this primary assemblage with heated entrained seawater caused the replacement of pyrrhotite by pyrite and/or magnetite at lower temperatures. The high-temperature pyrrhotite-chalcopyrite-quartz vein assemblage, inferred low salinities (<8 wt % NaCl), and low SiO 2 /Al 2 O 3 ratios in the bedded sulfides indicate that the Caribou hydrothermal fluids probably formed buoyant plumes from which sulfides precipitated and rained to the sea floor to form sedimentary sulfides. The capture of sulfide precipitates at Caribou approached 100 percent because ambient reduced and H 2 S-rich bottom waters promoted sulfide precipitation and prevented sulfide oxidation. The lack of a sulfur isotope discontinuity between bedded sulfides and host sediments is consistent with an ambient, anoxic water column origin for a major component of reduced sulfur.