Abstract Sulfide deposits in the Bathurst Mining Camp, where hosted by felsic volcanic and/or fine-grained sedimentary rocks in both the footwall and hanging wall, are associated with discrete gravity highs. Even when the local gravity field is influenced by signals produced by neighboring mafic-intermediate volcanic rocks or mafic intrusions, the signal related to a deposit can invariably be distinguished, either as itself or as a filtered enhancement. The presence of distinct signatures is primarily a function of the proximity of these deposits to the bedrock surface. Generally they are located no more than a few meters beneath the ubiquitous glacial overburden. The potential of the gravity method for finding more deeply buried sulfide deposits in the Bathurst Mining Camp is demonstrated by discovery of the Maybrun mineralized zone under roughly 180 m of younger sedimentary cover. Gravity anomalies related to near-surface deposits range in amplitude from the exceptionally large 4-mGal amplitude, premining signature of the 12 million metric ton (Mt) Brunswick 6 deposit to very small anomalies, such as the 0.25-mGal amplitude anomaly associated with the 1.1 Mt Key Anacon deposit. There is no linear relationship between tonnage and amplitude for the deposits of the Bathurst Mining Camp, although a linear relationship, albeit with a moderate degree of scatter, is apparent between amplitude and deposit thickness. The largest deposit in the Bathurst Mining Camp, the giant 122 Mt Brunswick 12 orebody, produces a signal of just 1.0-mGal amplitude, an apparent inconsistency induced by large volumes of dense mafic volcanic rocks in the hanging wall. These produce a positive gravity signature that partially obscures the signal produced by the deposit, which otherwise would have an amplitude of about 3.5 mGal. In addition to being a source of interference, mafic volcanic rocks and mafic intrusions are the main cause of ambiguity in selecting singular gravity anomalies for further exploration.

Abstract The Bathurst Mining Camp has a long history of discovery and mineral development with the first volcanogenic massive sulfides (VMS) being discovered and drilled at Orvan Brook in 1938. However, the camp did not gain national and international prominence until the discovery of the Brunswick Mining and Smelting 6 deposit was announced in 1953. After that, the Bathurst Mining Camp saw a number of important VMS “firsts” in North America; namely, the first deposits to be described in terms of a syngenetic sea-floor model, the first discovery by airborne electromagnetic surveying, the first discovery by stream-silt geochemistry, the first routine application of gravity surveys to screen ground electromagnetic anomalies, the first (in Canada) heap- and vat-leach operations to recover gold from gossans, and the first major mining camp to be described in terms of ensialic, back-arc–basin-depositional and subduction-related accretionay models. To date, a total of 141 massive sulfide occurrences, including 45 deposits, have been discovered in the Bathurst Mining Camp, a subcircular area approximately 60 km in diameter. About half of the discoveries were made in the 1950s and resulted from the use of geophysics, although geochemistry and prospecting were responsible for many. Later discoveries in the camp can be attributed to improved technology and a better understanding of the stratigraphy and structure. Production to the end of 1998 was over 130 million metric tons (Mt) from 12 deposits. The total massive sulfide resources, including production, from the known deposits in the Bathurst Mining Camp are estimated to be over 500 Mt. The geologic picture of the Bathurst Mining Camp has evolved dramatically since the first massive sulfide discovery. During the early 1950s, the epigenetic period, the geology of the camp was virtually unknown but by the end of the decade five informal units were recognized in the Ordovician Tetagouche Group. The sulfide deposits were considered to be replacement bodies genetically related to Devonian granites. By the 1960s, the syngenetic period, the picture was much the same although the Tetagouche Group was being interpreted in terms of geosynclinal theory and the sulfides were considered to be a facies of iron-formation. By the 1970s, the Kuroko period, plate tectonic theory had been applied to the Bathurst Mining Camp and it was being interpreted as an ensialic arc-related to easterly subduction. The stratigraphic framework of the Tetagouche Group, though informal, and significance of the polydeformational structures were more fully appreciated, and the sulfide deposits were considered to be genetically linked to convective circulation of seawater in proximity to calc-alkaline rhyolite domes. During the 1980s, the VMS period, the geologic picture of the Bathurst Mining Camp started to change because of new mapping and lithogeochemical studies. The Tetagouche Group was interpreted to have formed in an ensialic back-arc rift, with much of its structural complexity related to its amalgamation in an accretionary wedge above a westerly dipping subduction zone. The sulfides were considered to be exhalative, deposited in mounds or brine pools, and to have formed from convective circulation of seawater around subvolcanic intrusions. The deposits were divided into two groups with proximal and distal types in each. In the 1990s, the EXTECH period, the Tetagouche Group was redefined and formally subdivided, largely based on lithogeochemistry and geochronology. Many rocks previously included in this group were reassigned to new groups including the California Lake, Fournier, Miramichi, and Sheephouse Brook Groups. The new data and interpretations resulting from the EXTECH project are described in the papers that follow in this volume.

Abstract Gold is an important commodity in Zn-Pb-Cu-Ag–type massive sulfide deposits of the Middle Ordovician Bathurst Mining Camp, New Brunswick. The mean gold content of massive sulfides from 43 deposits is 0.85 ppm and ranges from a detection limit of 0.002 to 6.86 ppm. The 230 Mt Brunswick 12 deposit has a range of 0.55 to 0.72 ppm Au (Ag/Au ≅ 100) in the mill feed, although concentrations greater than 2 ppm occur in some ore lenses. Overall, Au exhibits a moderate positive Spearman Rank correlation with Ag (r’ = 0.53), As (r’ =0.57), and Sb (r’ = 0.65); correlations with base metals and other hydrothermal elements are generally low. Gold in massive sulfide deposits is concentrated in two sulfide facies with distinct element associations: vent complex sulfides with an Au + Bi + Co ± Cu association, and bedded sulfides with an Au + Sb + As ± Ag association. The 69.5 Mt Caribou massive sulfide deposit, for example, has an average Au grade of 1.72 ppm Au, with values greater than 6 ppm in the vent complex, which underlies a stratiform bedded sulfide facies. Gold exhibits a Bi + Co ± Cu association in the vent complex of the Caribou, Heath Steele B and C, and Orvan Brook deposits, and its concentration increases toward the footwall of these deposits. Gold is preferentially enriched in bedded sulfides of the Stratmat, Wedge, Armstrong A, Restigouche, and Murray Brook deposits and generally increases in content along with Sn and Ag toward the stratigraphic hanging wall. The enrichment of Au in the bedded sulfide facies and underlying vent complex suggests that Au was transported and deposited by different Au complexes under evolving fluid conditions. High-temperature (>350°C) and acidic hydrothermal fluids circulating deep within the mound and footwall transported gold as the complex. An increase in pH, which accompanied chalcopyrite replacement of bedded sulfides in the vent complex, and a decrease in temperature as hydrothermal fluids progressed upward through the mound are mechanisms that most likely led to the precipitation of gold from the complex. The precipitation of Au during zone refining in the high-temperature core of the fluid upflow zone is also reflected by the Au + Bi + Co ± Cu association. Gold transport in lower temperature (<300°C) and mildly acidic fluids circulating within the bedded sulfide facies was likely dominated by the complex. The precipitation of Au, which is reflected by a sharp enrichment of Au with Sb, Sn, and As toward the stratigraphic hanging wall of massive sulfide deposits, may have resulted from a decrease in aH2S and a corresponding increase in fo 2 and pH during mixing with seawater. The Au(HS) 0 complex, dominant under higher temperature and more acidic conditions than , may also have contributed to gold transport within the sulfide deposits.

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 T he M ining and mineral processing industry is important to the Canadian economy and in 2001 contributed $35.1 billion, or 3.7 percent, to the Gross Domestic Product and employed approximately 376,000 Canadians (Minerals and Metals Sector, Natural Resources Canada). However, over the past decade, Canada’s base metal reserves have declined by more than 25 percent, and significant new discoveries will be required if Canada’s role as a major base metal producer is to be maintained into the twenty-first century. The Bathurst Mining Camp is one of Canada’s most important base metal mining districts, accounting in 2001 for 30 percent of Canada’s production of Zn, 53 percent of Pb, and 17 percent of Ag. In 1999, the Bathurst Mining Camp accounted for 32 percent of the Zn, 80 percent of the Pb, and 25 percent of the Ag reserves (Minerals and Metals Sector, Natural Resources Canada). The value of production from the Bathurst Mining Camp in 2001 exceeded $500 million and accounted for 70 percent of total mineral production in New Brunswick. Approximately 2,000 people are directly employed by the mining industry in the Bathurst Mining Camp. Without the discovery of new ore reserves, however, production will decline and will cease within about 10 yr at current production rates, and with it the principal source of economic activity in northeastern New Brunswick will also disappear. To address the major decline of mineral resources in Canada’s economically important mining districts, EXTECH (Exploration and Technology) projects were established by the Geological Survey of Canada.

Abstract Quaternary mapping and drift prospecting studies were conducted in the Bathurst Mining Camp between 1993 and 1999 as part of the EXTECH-II project. Regional surficial mapping (1:50,000 scale) and till sampling were conducted in the Nepisiguit Falls area, where a total of 227 basal till samples were collected, at approximately 2-km spacing. More closely spaced samples were collected around the Halfmile Lake (520 samples) and Restigouche (265 samples) massive sulfide deposits. Ice-flow indicators (striations, grooves, and roche moutonnées) together with till fabric analyses, till clast provenance studies, till geochemistry, and the distribution of boulder erratics indicate that early glacial transport was primarily in an easterly direction. This ice flow was followed by northeastward- and southeastward- flowing ice. Minor late-stage, local ice flows followed in various directions. Evidence of north-northeast and southeast ice flows preceding the eastward flow exists in the eastern parts of the Bathurst Mining Camp. Bedrock outcrop is scarce and most of the area (>99%) is covered by unconsolidated material (till, glaciofluvial, organic, alluvial, and colluvial deposits) and preglacially weathered bedrock (regolith). A generally thin (<2 m) layer of sandy-clay greenish- to yellowish-brown, locally derived basal till covers most of the area. Geochemically anomalous till within the Bathurst Mining Camp includes east-northeastward-trending dispersal trains from the Halfmile Lake, Restigouche, and Stratmat mineral deposits. Indium, Sn, and As, and, to a lesser extent, Cu, Pb, Ag, and Zn in the <0.063-mm fraction of basal till are the best indicators of glacial dispersal in the Bathurst Mining Camp. Most till geochemical dispersal trains are short (<500 m) fans and ribbons and trend in an east-northeast direction in the northern, central, and eastern parts of the Bathurst Mining Camp and in a southeast direction in the southern part of the Bathurst Mining Camp.

Abstract The Bathurst Mining Camp, which hosts over 45 massive sulfide deposits, formed in a back-arc rift that developed on a passive margin sequence of turbiditic and hemipelagic sediments overlying Ganderian continental crust. The provenance of these clastic sediments changed from Avalonian basement during the prerift passive margin stage and the early stages of back-arc rifting (Arenig) to obducted ultramafic rocks during later sedimentation (upper Llanvirn). The water column conditions alternated from stratified with anoxic bottom waters during the deposition of regionally extensive Tremadoc and Arenig black shale of the Knights Brook, Patrick Brook, Nepisiguit Falls, and Spruce Lake Formations, to well-oxygenated during the deposition of Llanvirn maroon shales and cherts, and anoxic during deposition of the Caradoc black shales. Evidence for anoxic conditions include the absence of bioturbation, the high S/C ratios and low Mn contents of black shales, the lack of seawater oxidation associated with most sea-floor sulfide deposits, and the highly positive δ 34 S values for sedimentary pyrite that are consistent with closed system, Raleigh fractionation of sulfur isotopes in a restricted basin. Although the metal/reduced sulfur ratio of the hydrothermal fluids cannot be precisely constrained, highly positive δ 34 S values for massive sulfides, and the similarity of these values to coeval pyrite in host sediments, indicate that reduced sulfur in the ambient water column played a major role in the precipitation of sulfides. In addition, a water column with reduced and stagnant bottom waters enhanced the efficiency of sulfide precipitation near hydrothermal vents. Unlike modern black smokers, where over 95 percent of the metals is lost to the oxygenated water column, metals carried in buoyant hydrothermal plumes in an anoxic water column would precipitate as sulfides and settle to the sea floor to form sedimentary sulfide deposits. Massive sulfides are rare in the Llanvirn due to the change to oxidizing sea-floor conditions. As a result, maroon shale and chert containing hydrothermal Fe-Mn oxides occur over large areas of the Bathurst Mining Camp, similar to modern Fe-Mn oxyhydroxides along oceanic rifts. Mass-balance calculations show that the flux of hydrothermal Zn is comparable to that which generated the Bathurst massive sulfide deposits, although in the oxygenated ocean most of the metals were dispersed over a wide area. The redox conditions in the Bathurst back-arc rift have therefore exerted a fundamental control on the formation and preservation of massive sulfide deposits. The role of a reduced water column in forming and preserving massive sulfide deposits may also explain why volcanogenic massive sulfide (VMS) deposits cluster at particular times in the Earth’s history when the oceans were stratified. Although most Bathurst sulfide deposits are associated with felsic volcanic rocks, the occurrence of metalliferous oxidized sediments in younger mafic volcanic sequences suggests that this association may be controlled as much by the ambient sea-floor environment as by the supply of metals from hydrothermal sources.

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.