Abstract ARCHEAN Cu-Zn deposits are among the most important mineral deposit types in Canada. The Superior province of Canada contains nearly 80 percent of the known Archean Cu-Zn deposits in the world (about 100 of 125 deposits). These deposits are concentrated in 10 separate mining camps, including Sturgeon Lake, Manitouwadge, Mattagami Lake, Chibougamau, Joutel, Val d’Or, Bous-quet, Noranda, Kidd Creek, and Kamiskotia (Fig. 1 and Table 1). A few deposits in rocks of similar age and composition are also known in the Slave province, the Churchill province, and in the Archean of Western Australia, southern Africa, China, and Brazil. Known deposits of this age worldwide account for more than 650 million metric tons (Mt) of massive sulfides, containing 10 Mt of Cu metal, 29 Mt of Zn, 1 Mt of Pb, 33 Mkg Ag, and 750,000 kg Au. The giant Kidd Creek volcanogenic massive sulfide deposit in the western Abitibi subprovince of Canada is the largest known deposit of this age currently in production. The Superior province is the world’s largest exposed Archean craton, occupying an area of more than 1.5 million km 2 , bounded by the Trans-Hudson orogen to the west and the Grenville province to the east. A number of distinct subprovinces are recognized, assembled into east-west-trending granite-greenstone terranes and metasedi-mentary belts (Fig. 1). The granite-greenstone terranes are composed of gneissic rocks of plutonic origin,

Abstract The Kidd Creek mine is an Archean volcanogenic Cu-Zn deposit with total past production and current reserves of more than 138.5 Mt at 2.4 percent Cu, 6.5 percent Zn, 0.23 percent Pb, 90 g/t Ag, and up to 0.15 percent Sn. The massive sulfides occur at the top of a locally thickened felsic volcanic pile, within and overlying a succession of massive rhyolite flows, volcaniclastic rocks, and coarse epiclastic units. The felsic volcanics occupy the core of an anomalous, S-shaped fold structure and attain a maximum thickness of approximately 300 m beneath the deposit. Massive autobrecciated rhyolite occurs at the base of the mine sequence and is interpreted to be a proximal vent facies. The local volcanic basement comprises mainly ultramafic flows, intercalated with minor rhyolite. The ultramafic rocks are interpreted to be early extrusive lavas associated with the development of an extensional rift. Basaltic pillow lavas and breccias occur in the hanging wall of the mine and are extensively intruded by gabbroic sills. South of the mine, this stratigraphy is truncated along the contact with younger, regional metasedi-mentary rocks. Kidd Creek is typical of a class of large volcanogenic massive sulfide deposits that occur within thick successions of permeable felsic volcaniclastic rocks and are dominated by large, stratiform, Zn-rich lenses with laterally extensive zones of ore-grade Cu mineralization. The deposit consists of three main ore-bodies (the North, Central, and South orebodies) that are distributed along an inferred boundary fault of a linear, grabenlike depression. The present deposits have a restored strike length of at least 2 km, indicating remarkable continuity of the hydrothermal system along the length of the graben. The main ore lenses formed by infilling and strata-bound replacement of volcaniclastic rocks, coarse volcanic breccias, and finer grained tuffs that filled the graben. Abundant relics of silicified rhyolite within the massive sulfides, gradational contacts between the massive sulfides and unmineralized fragmental rocks at the margins of the ore zones, and extensive replacement within the hanging-wall breccias confirm that a large part of the deposit formed below the sea floor. Burial of the deposits by mass flows was coincident with mineralization, and subsea-floor deposition of sulfides progressed laterally into the volcaniclastic rocks adjacent to the ore lenses. Metalliferous sediments or exhalative horizons are notably absent, and there is little evidence that widespread venting of high-temperature fluids occurred at the sea floor. Deposition of sulfides within the thick sequence of basin fill ensured that ore-forming fluids were confined to the graben and relatively little metal was lost to high-temperature discharge. The development of the three main orebodies is best explained by a long-lived, low-temperature hydrothermal system punctuated by several higher temperature pulses of Cu-rich fluid. Focusing of the fluids was caused by intense silicification of the rhyolite above and adjacent to the main upflow zone. Extensive lateral flow occurred within the bedded volcaniclastic rocks, and the highest temperature fluids appear to have occupied a number of high-level aquifers beneath the deposits. These are marked by conformable lenses of chlorite alteration, semimassive chalcopyrite, and strata-bound chalcopyrite stringer mineralization. The larger alteration envelope is broadly conformable to the ore lenses and consists of quartz and sericite, together with chlorite, Fe-rich carbonate, and minor tourmaline. Two main ore suites occur at Kidd Creek: a low-temperature, polymetallic suite enriched in Zn, Ag, Pb, Cd, Sn, Sb, As, Hg, ±Tl, ± W, and a higher temperature suite of Cu, Co, Bi, Se, In, ± Ni. The massive ores consist mainly of pyrite, pyrrhotite, sphalerite, and chalcopyrite, together with galena, tetrahedrite, ar-senopyrite, and cassiterite, in a quartz and siderite gangue. However, more than 60 different ore minerals and ore-related gangue minerals are present, including complex assemblages of Co-As sulfides, Cu-Sn sulfides, Ag minerals, and selenides. Tin is present as cassiterite in the upper part of the massive sphalerite lenses and as stannite in the underlying chalcopyrite-rich ores. Despite the high Ag content of the deposit, Kidd Creek is remarkably Au poor. The ores exhibit a close chemical affinity with their immediate felsic host rocks, including strong coenrichments of Ag, Pb, As, Sn, W, and F However, the complex metal assemblage suggests that a more primitive mafic suite may also have played a role in metal supply. The extensive metagraywackes to the south of the mine are younger than Kidd Creek and therefore could not have been a source for metals. An abundance of pyrrhotite, arsenopyrite, high Fe sphalerite, and Fe-rich chlorite indicates predominantly low fO 2 –fS2 conditions, and the abundant siderite in the ore indicates that the hydrothermal fluids were highly enriched in CO 2 . Sulfur isotope compositions range from -2.4 to +3.3 per mil, with the bulk of the massive sulfides having S 34 S values close to 0 per mil. The mineralogy and bulk composition of the Kidd Creek ores bear a closer resemblance to those of many Phanerozoic Zn-Cu-Pb deposits (e.g., Bathurst, Neves Corvo) than to other Archean Cu-Zn deposits. The predominance of Zn-rich ores (ca. 70–80 Mt) implies that most of the deposit formed at low temperatures (ca. 250°C). Solubility modeling indicates that a large hydrothermal system at relatively low temperatures would have been sufficient to account for about 75 percent of the metals. The significant enrichments in Ag, Pb, and Sn reflect not only the abundance of felsic volcanic rocks in the mine sequence but also the sustained, low-temperature venting history of the deposit. In contrast, the Cu-rich ores appear to have been introduced during relatively short-lived, hydrothermal pulses at much higher temperatures. The higher temperatures most likely coincided with discrete felsic magmatic events that occurred at several intervals during the ∼3.5 m.y. history of the volcanic complex. The late-stage introduction of Cu may indicate that the Cu-rich fluids evolved separately from the lower temperature, con-vective part of the hydrothermal system. This model is supported by the presence of a high-grade bornite zone in the South orebody, which represents a massive influx of Cu metal at peak hydrothermal temperatures late in the development of the Cu stringer zone. Kidd Creek resembles sulfide deposits that are currently forming in young, intraoceanic back-arc rifts, such as the Lau basin, and this may be an appropriate modern analogue for the Kidd Creek setting. The combination of voluminous mafic-ultramafic flows in the footwall of the deposit, punctuated by anomalous felsic volcanism, and the extensive deposits of coarse epiclastic rocks and volcaniclastic sediments suggest that Kidd Creek formed within a subsiding rift basin. The importance of a plumelike source for the ultramafic melts and the longevity of the hydrothermal system may indicate that rifting occurred above a stationary hot spot.

Abstract The bornite zone of the Kidd Creek mine is a high-grade, replacement body occupying the core of the chalcopyrite stockwork and massive chalcopyrite lens of the South orebody. The bornite zone has produced nearly 340,000 metric tons (t) of Cu-rich ore, averaging close to 19 wt percent Cu. The mineralization consists of massive and semimassive bornite that has replaced massive chalcopyrite at the base of the South orebody. Bornite stringer mineralization occurs below the massive bornite ore and has replaced chalcopyrite in the preexisting stockwork zone of the massive chalcopyrite lens. Detailed miner-alogical studies indicate that the bornite ores were part of the late-stage hydrothermal paragenesis of the South orebody, and they are interpreted to have formed from a high-temperature pulse of Cu-rich fluids, late in the history of the Kidd Creek hydrothermal system. The bornite ores contain a complex assemblage of Cu, Co, Bi, Se, Ag, As, and Ni minerals and exhibit strong geochemical enrichments in these elements compared to other Cu-rich ores. The principal ore minerals include bornite, tennantite, digenite, enargite, mawsonite, carrollite, and numerous Ag-Bi se-lenides and Bi-Pb-Cu-Se sulfides. Tennantite-rich ores are concentrated along the contact between the massive bornite and overlying massive chalcopyrite ores and define the original replacement front. Se concentrations in the bornite are up to 1 wt percent, and the main zone of bornite mineralization is surrounded by a broad halo of Se enrichment (300–1,600 ppm). Se/S ratios in the bornite ores are among the highest recorded in any volcanogenic massive sulfide deposit. Important concentrations of Sn, In, W, and Pb appear to have been inherited from preexisting Cu- and Zn-rich sulfides during emplacement of the bornite zone. Similarities in the mineralogy and bulk composition of the bornite ores and that of massive chalcopyrite and chalcopyrite stringer ores elsewhere in the deposit suggest that they formed under similar conditions at close to peak hydrothermal temperatures (ca. 350°–400°C). However, high concentrations of Cu, Co, Bi, and Se in the bornite zone suggest that these ores were products of a uniquely enriched end-member fluid. Massive bornite formed in response to increasing a Cu+ /a Fe2+ by the replacement of pyrite and chalcopy-rite. Although late pyrite porphyroblasts are present at the margins of the bornite zone, bornite + pyrite is not an equilibrium assemblage in the ores. This suggests that the bornite did not form simply by oxidation or sulfidation of preexisting chalcopyrite. Mass balance considerations also indicate that the bor-nite ores could not have formed solely by leaching of Cu metal from the massive chalcopyrite ores, as in other bornite-rich sulfide deposits. The requirement for a massive influx of Cu at close to peak hy-drothermal temperatures suggests that the bornite zone originated during a single high-temperature pulse of Cu-rich fluid, rather than by incremental addition of Cu over a sustained period of lower temperature upflow. A Cu-rich source in the deep geothermal reservoir or from a subvolcanic magma is necessary to account for the metal enrichment. The bornite ores exhibit a complex postmineralization history dominated by the effects of regional thermal metamorphism, structural remobilization, and late-stage, metamorphic hydrothermal fluids. Regional metamorphism affected the bornite ores more than any other part of the Kidd Creek deposit because of the low thermal stabilities of minerals in the Cu-rich part of the Cu-Fe-S system and their strong tendency to reequilibrate at metamorphic temperatures. Repeated heating of the Cu-rich minerals above their maximum thermal stabilities and subsequent reequilibration of complex solid solutions during retrograde cooling resulted in extensive sulfide-sulfide reactions and the widespread development of postmetamorphic textures (i.e., myrmekitic intergrowths, exsolution lamellae, reaction rims). To a large extent, the present mineralogy of the bornite ores is a product of exsolution from nonstoichiometric solid solutions formed during metamorphism. The bornite ores were also poorly buffered against metamorphic reactions between the ore minerals, owing to the absence of a stable Fe-S-O assemblage (e.g., pyrite-pyrrhotite-magnetite). The release of sulfur during retrograde cooling caused widespread sulfidation reactions among the ore minerals and the growth of abundant, large pyrite porphyroblasts in the halo of the bornite zone. The present high sulfidation assemblages (e.g., tennantite-enargite) are metamorphogenic and do not represent conditions during the hydrothermal emplacement of the bornite ores. Late-stage metamorphic fluids were strongly localized at the margins of the bornite zone and also promoted the growth of a distinctive meta-morphic assemblage of Mg-rich chlorite, phlogopite, and dolomite in the tennantite-rich ores.

Abstract Rhyolites in the vicinity of the Kidd Creek mine are altered to quartz-sericite and quartz-chlorite assemblages, with minor Fe-Mg carbonate and tourmaline. The deposit overlies a large volume of silicified rock that extends for up to 300 m below the ore lenses. The main upflow zone is centered on the thickest accumulation of massive rhyolite, which has been altered to a fine-grained, gray, highly siliceous rock known as “cherty breccia.” At the margins of the main upflow zone, the mine rhyolites are altered mainly to quartz and sericite, forming a distinctive halo around the deposit. However, the alteration extends only a few tens of meters into the hanging wall and is truncated by gabbroic sills intruded across the top of the deposit. The uppermost quartz porphyry unit of the mine sequence shows only minor sericitization and sphalerite staining. Close to the ore zones, quartz, sericite, and chlorite replace the matrix of fragmental rocks and occupy fracture networks in brecciated rhyolite. Discordant zones of chlorite alteration cut the most intensely silicified rocks immediately beneath the massive sulfide lenses and host the main chalcopyrite stringers. Chlorite-rich rocks also occur as strata-bound lenses within the footwall rhyolites where fine-grained, interflow tuffs have been replaced by conformable zones of chalcopyrite stringer mineralization. Quartz-sericite rocks marginal to the chalcopyrite stringer zones are stained by sphalerite and are host to widespread sphalerite stringer mineralization, which is interpreted to be the remnants of an earlier Zn-rich stockwork. Fe-Mg carbonates (ferroan dolomite, ankerite, and siderite) are intimately associated with the ore and occur immediately adjacent to and within the massive sphalerite lenses. Lower CO2/CaO ratios away from the ore zones reflect mainly regional-scale dolomitization of the rhyolites. Immobile element plots indicate significant mass and volume change associated with the alteration. Large mass gains of SiO in the footwall were accommodated by a substantial increase in volume associated with locally intense crackle brecciation of the rhyolite. Bulk SiO 2 contents in these rocks commonly exceed 85 wt percent. Silicification of the mine sequence is coincident with a large zone of alkali depletion that can be traced for several hundred meters laterally away from the ore lenses. The cherty rhyolite in the immediate footwall is stripped of Na 2 O, CaO, Rb, Sr, Ba, ± MgO, whereas quartz-sericite rocks at the margins of the ore lenses (and also in the deep footwall) have gained K 2 O and MgO, as well as Mn, F, Cl, and Li. These enrichments are interpreted to be part of the early synvolcanic alteration during the initial stages of low-temperature, hydrothermal circulation. The addition of MgO at the margins of the ore lenses indicates that these were most likely zones of infiltration of seawater. Quartz-chlorite rocks within these zones exhibit nearly quantitative removal of alkalies and have gained FeO, Mn, B, F, and base metals. Although depleted in the footwall rocks, K 2 O is largely conserved at the mine scale, and the local addition of K 2 O in sericitic rocks in the hanging wall of the deposit is most likely related to the leaching of potassium from immediately beneath the massive sulfides. A negative correlation between K 2 O and FeO (± MgO) confirms that much of the later chlorite formed at the expense of preexisting sericite. Rare earth element (REE) abundances approximate protolith concentrations in zones of silicification and quartz-sericite alteration, but are progressively reduced (up to 80%) in the quartz-chlorite rocks. Samples of black, chloritic rhyolite have flattened REE profiles implying significant light REE mobility at this stage. The distribution and style of alteration at Kidd Creek is a strong function of the original permeability of the volcanic pile and reflects a combination of mainly low-temperature, diffuse flow through the permeable fragmental rocks at the top of the mine sequence and more focused discharge through brecciated zones in the underlying massive rhyolites. The broadly conformable zones of silicification, sericitization, and chloritization contrast sharply with the narrow, chlorite-rich pipes associated with many smaller Archean Cu-Zn deposits. The extent of silicification is thought to be related to pervasive flooding of a large volume of rock by SiO 2 -saturated fluids during a sustained period of low-temperature (<200°-250°C) hydrothermal upflow. The interpreted paragenesis of alteration is consistent with the documented thermal history of the deposit inferred from the mineralization and whole-rock S 18 O values (i.e., sustained low-temperature discharge punctuated by higher temperature upflow) and is similar to that observed in other large, felsic volcaniclastic-hosted massive sulfides (e.g., Horne, Crandon).

Abstract THE PAPERS contained this volume are the first published contributions on the geology and genesis of the Kidd Creek deposit since the early work of Walker and Mannard (1974) and Walker et al. (1975). In the course of this research, many outstanding questions have been answered and many new questions raised. Although there is general agreement among the authors concerning the setting of the deposit and its long-lived hydrothermal system, a number of possible interpretations of the origins of the ore-forming fluids, the source of the metals, and the heat necessary to drive hydrothermal circulation have been proposed. The different interpretations highlight key areas for future research at Kidd Creek. Notwithstanding this diversity, it is encouraging that such a large group of investigators, working independently on many separate aspects of the deposit, should arrive at a positive consensus on its overall geology and genesis. Many of the papers in the volume will be important benchmarks for additional studies at the mine. With development now extending below 6,800 ft and a substantial resource defined as far as 9,800 ft below surface, research at Kidd Creek can be expected to continue long into the future. The improved understanding of the deposit and the region will also provide an important framework for future geologic investigations of volcanogenic massive sulfides elsewhere in the Abitibi and in other parts of the world.

Abstract Volcanic-associated massive sulfide deposits (VMS) are predominantly stratiform accumulations of sulfide minerals that precipitate from hydrothermal fluids at or below the sea floor, in a wide range of ancient and modern geological settings (Figs. 1, 2). They occur within volcano-sedimentary stratigraphic successions, and are commonly coeval and coincident with volcanic rocks. As a class, they represent a significant source of the world's Cu, Zn, Pb, Au, and Ag ores, with Co, Sn, Ba, S, Se, Mn, Cd, In, Bi, Te, Ga, and Ge as co- or by-products. The understanding of ancient, land-based VMS deposits has been heavily influenced by the discovery and study of active, metal-precipitating hydrothermal vents on the sea floor. During the last three decades, excellent descriptions of sea-floor sulfides and related vent fluids and hydrothermal plumes have provided modern analogs for the land-based VMS deposits (Rona, 1988; Rona and Scott, 1993; Hannington et al., 1995). Conversely, the geology and mineralogy of land-based deposits have provided insight into the plumbing systems and sulfide mineral paragenesis of sulfide deposits relevant to sea-floor hydrothermal systems. This volume capitalizes on the complementary nature of ancient, land-based VMS deposits and active, metal-precipitating hydrothermal systems on the sea floor, much as the Reviews in Economic Geology Volume 2 (Berger and Bethke, eds., 1985) did with epithermal deposits and active, subaerial geothermal systems, and draws equally from land-based and sea-floor VMS research.

Abstract The Kidd Creek deposit, located 25 km north of Timmins, Ontario, in the western Abitibi subprovince (Fig. 1) of the Superior province of the Canadian Shield, is a very large and important example of a bimodal-mafic(ultramafic) volcanic-associated massive sulfide (VMS) deposit. It contains >138.7 million tonnes (Mt) with an average grade of 2.35 percent Cu, 6.50 percent Zn, 0.23 percent Pb, and 89 g/t Ag (mined, mineable, probable and possible reserves) to a depth of 2,400 m. There is an additional inferred resource of 17 Mt at 1.85 percent Cu and 8.43 percent Zn to a depth of 3,000 m (Canadian Mining Journal, 1996). Other elements that are, or have been, recovered profitably include S, Se, Cd, Sn, Ge, and In. The deposit was discovered in 1963 by drill-testing a strong airborne electromagnetic conductor in an area with sparse outcrop; it was later found that parts of the deposit came within 6 m of the surface, covered by a relatively thin veneer of glacial till. Open pit mining began in 1966, and production from underground began in 1972. The deposit is currently being mined by underground methods at a rate of ∼3 Mt per year. There are several features that collectively make Kidd Creek unique among bimodal mafic(ultramafic) VMS deposits, or VMS deposits of any type: (1) it is effectively a singular orebody rather than one of many in a district, (2) it has a distinctive stratigraphic footwall of komatiite flows intercalated with high-temperature, high-silica rhyolites, (3) the host rocks possess an unusually high δ 18 O signature (Beaty et al., 1988; Huston et al., 1995), and (4) it is very large. This brief contribution emphasizes the geology at Kidd Creek, largely drawing from studies under a cooperative industry-government research program on the mine over the last five years that has led to an Economic Geology Monograph in press at the time of this writing (Economic Geology Monograph 10).

Abstract Volcanogenic massive sulfide deposits contain variable amounts of gold, from an average production grade of about 1 g/t to more than 10 g/t in some deposits (Fig. 1). They include conventional base metal massive sulfides with accessory gold as well as some deposits in which gold is a primary commodity. Deposits of the first group may contain significant amounts of gold, owing to modest grades and large tonnages (e.g., 180 tonnes Au at Flin Flon), but they usually have low gold-to-base metal ratios and gold is recovered mainly as a by-product (Fig. 2). Deposits of the second group are true gold deposits in a strict economic sense. The most important of these, in terms of total contained gold, include the large copper-gold deposit at Horne, in the Noranda district, Quebec; the large pyritic gold deposits in the Bousquet district, Quebec; the Boliden Cu-Au-As deposit in the Skellefte district, Sweden; and the high-grade, polymetallic massive sulfide at Eskay Creek, British Columbia (Table 1 and Fig. 3). In this chapter, we compare the geologic settings, mineralogy, geochemistry, and alteration associated with several of the most gold-rich deposits and examine the possible controls on gold enrichment. An emphasis on deposits found in Canada is evident from the examples listed in Tables 1 and 2. Comprehensive reviews of gold-rich volcanogenic massive sulfide deposits in Australia and models for their genesis have been presented by Large et al. (1989) and references therein. Detailed studies of the important physical and chemical controls on gold enrichment in volcanogenic massive sulfide deposits can also be found in papers by Huston and Large (1989) and Hannington and Scott (1989a). These results are reviewed briefly, but the reader is referred to the original papers for a more thorough discussion. In the second part of the chapter we consider possible links between gold-rich volcanogenic massive sulfide and volcanogenic gold deposits in the epithermal environment and examine active, gold-depositing hot springs, on the sea floor and on land, as possible modern analogs.

Abstract The occurrence and distribution of gold in volcanogenic massive sulfides on the modern sea floor is a function of the physical and chemical characteristics of the hydrothermal fluids and theil ability to become saturated with respect to gold at a high concentration. Evidence from active hydrothermal vents at midocean ridges indicates that high-temperature (350°C) hydrothermal fluids may contain 0.1 to 0.2 μg/kg Au and could transport as much as 500 to 1,000 g Au/yr in a single deposit. However, in the absence of an effective precipitation mechanism, most of the gold in high-temperature vents may be lost to a diffuse hydrothermal plume and related metalliferous sediments. Associated high-temperature, Cu-Fe sulfides and subsea-floor stockwork mineralization are typically gold poor, containing ≤0.2 ppm Au. In contrast, low-temperature (<300°C) sulfides accumulating at or near the sea floor may contain up to 6.7 ppm Au. Gold-rich assemblages are formed at elevated sulfidation states and commonly contain high levels of Zn (>10%), Pb (>0.1%), Ag (>100 ppm), As (>200 ppm), and Sb (up to 500 ppm). High concentrations of H 2 S (up to 8.4 mmole/kg or 285 ppm) in the vent fluids stabilize Au(HS) 2 − complexes down to at least 200°C and account for the enrichment of gold in late-stage, low-temperature sulfides. Gold is precipitated from Au(HS) 2 − by oxidation during high-level mixing of the vent fluids with ambient seawater. The locus of mixing and the extent of sulfide-sulfate reactions are important controls on the site and temperature of gold mineralization. Precipitation of gold from AuCl 2 occurs in one deposit where fluids have acquired significantly elevated salinities. Similar patterns of gold enrichment in examples from ophiolite-hosted Cu pyrite, Phanerozoic Zn-Cu-Pb, and Archean Cu-Zn deposits suggest similar controls on the occurrence and distribution of gold in ancient volcanogenic massive sulfide ores.