ALBERT

Description: The discovery of metal-depositing hot springs on the sea floor, and especially their link to chemosynthetic life, was among the most compelling and significant scientific advances of the twentieth century. More than 300 sites of hydrothermal activity and sea-floor mineralization are known on the ocean floor. About 100 of these are sites of high-temperature venting and polymetallic sulfide deposits. They occur at mid-ocean ridges (65%), in back-arc basins (22%), and on submarine volcanic arcs (12%). Although high-temperature, 350°C, black smoker vents are the most recognizable features of sea-floor hydrothermal activity, a wide range of different styles of mineralization has been found. Different volcanic substrates, including mid-ocean ridge basalt, ultramafic intrusive rocks, and more evolved volcanic suites in both oceanic and continental crust, as well as temperature-dependent solubility controls, account for the main geochemical associations found in the deposits. Although end-member hydrothermal fluids mainly originate in the deep volcanic basement, the presence of sediments and other substrates can have a large effect on the compositions of the vent fluids. In arc and backarc settings, vent fluid compositions are broadly similar to those at mid-ocean ridges, but the arc magmas also supply a number of components to the hydrothermal fluids. The majority of known black smoker vents occur on fast-spreading mid-ocean ridges, but the largest massive sulfide deposits are located at intermediate- and slow-spreading centers, at ridge-axis volcanoes, in deep backarc basins, and in sedimented rifts adjacent to continental margins. The range of deposit sizes in these settings is similar to that of ancient volcanic-associated massive sulfide (VMS) deposits. Detailed mapping, and in some cases drilling, indicates that a number of deposits contain 1 to 5 million tons (Mt) of massive sulfide (e.g., TAG hydrothermal field on the Mid-Atlantic Ridge, deposits of the Galapagos Rift, and at 13°N on the East Pacific Rise). Two sediment-hosted deposits, at Middle Valley on the Juan de Fuca Ridge and in the Atlantis II Deep of the Red Sea, are much larger (up to 15 and 90 Mt, respectively). In the western Pacific, high-temperature hydrothermal systems occur mainly at intraoceanic back-arc spreading centers (e.g., Lau basin, North Fiji basin, Mariana trough) and in arc-related rifts at continental margins (e.g., Okinawa trough). In contrast to the mid-ocean ridges, convergent margin settings are characterized by a range of different crustal thicknesses and compositions, variable heat flow regimes, and diverse magma types. These variations result in major differences in the compositions and isotopic systematics of the hydrothermal fluids and the mineralogy and bulk compositions of the associated mineral deposits. Intraoceanic back-arc basin spreading centers host black smoker vents that, for the most part, are very similar to those on the mid-ocean ridges. However, isotopic data from both the volcanic rocks and the sulfide deposits highlight the importance of subduction recycling in the origin of the magmas and hydrothermal fluids. Back-arc rifts in continental margin settings are typically sediment-filled basins, which derive their sediment load from the adjacent continental shelf. This has an insulating effect that enhances the high heat flow associated with rifting of the continental crust and also helps to preserve the contained sulfide deposits. Large hydrothermal systems have developed where initial rifting of continental crust or locally thickened arc crust has formed large calderalike sea-floor depressions, similar to those that contained major VMS-forming systems in the geologic record. Hydrothermal vents also occur in the summit calderas of submarine volcanoes at the volcanic fronts of arcs. However, this contrasts with the interpreted settings of most ancient VMS deposits, which are considered to have formed mainly during arc rifting. Hydrothermal vents associated with arc volcanoes show clear evidence of the direct input of magmatic volatiles, similar to magmatic-hydrothermal systems in subaerial volcanic arcs. Several compelling examples of submarine epithermal-style mineralization, including gold-base metal veins, have been found on submarine arc volcanoes,and this type of mineralization may be more common than is presently recognized. Mapping and sampling of the sea floor has dramatically improved geodynamic models of different submarine volcanic and tectonic settings and has helped to establish a framework for the characterization of many similar ancient terranes. Deposits forming at convergent margins are considered to be the closest analogs of ancient VMS. However, black smokers on the mid-ocean ridges continue to provide critically important information about metal transport and deposition in sea-floor hydrothermal systems of all types. Ongoing sea-floor exploration in other settings is providing clues to the diversity of mineral deposit types that occur in different environments and the conditions that are favorable for their formation.

Description: 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 (Hébécourt 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 Hébécourt 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.

Description: Sedimentcovered oceanic rifts represent a tectonic set­ting for economically important ancient sea-floor massive ulfide deposits (Goodfellow et al., l993a; Slack, 1993). In general, ancient sea-floor sulfide deposits hosted by sedi­mentary rocks are fewer in number and an order of mag­nitude larger than deposits hosted solely by volcanic rocks.

Description: Recent developments in the measurement of δ34S and δ33S in sulfide minerals provide a new framework to evaluate isotopic equilibrium among sulfide minerals and a new tool to investigate mineralizing processes or sources of sulfur in ore-forming systems. Isotopic equilibrium requires that values of δ34S and δ33S are highly correlated between mineral pairs and that deviations (quantified as Δ33S) from a reference mass-dependent fractionation are the same for both minerals within experimental error. In this contribution, we apply this framework for the first time to natural samples, and evaluate isotopic equilibrium among sulfide minerals from the Archean Kidd Creek volcanogenic massive sulfide deposit in Ontario, Canada. Two of eight sulfide mineral pairs showed clear isotopic disequilibrium, in agreement with interpreted paragenetic relationships; six other pairs have isotopic compositions that are consistent with equilibrium. We discuss the implications of these results for the interpretation of mineral paragenesis and sulfur isotope geothermometry at Kidd Creek. This technique offers new possibilities for evaluating sulfur isotope systematics in Archean (〉2.45 Ga) ore deposits, in which the range of δ34S values is small but non-zero values of Δ33S are common.

Description: 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.

Description: Brothers volcano, which is part of the active Kermadec arc, northeast of New Zealand, forms an elongate edifice 13 km long by 8 km across that strikes northwest-southeast. The volcano has a caldera with a basal diameter of ~3 km and a floor at 1,850 m below sea level, surrounded by 290- to 530-m-high walls. A volcanic cone of dacite rises 350 m from the caldera floor and partially coalesces with the southern caldera wall. Three hydrothermal sites have been located: on the northwest caldera wall, on the southeast caldera wall, and on the dacite cone. Multiple hydrothermal plumes rise ~750 m through the water column upward from the caldera floor, originating from the northwest caldera walls and atop the cone, itself host to three separate vent fields (summit, upper flank, northeast flank). In 1999, the cone site had plumes with relatively high concentrations of gas with a ΔpH of −0.27 relative to seawater (proxy for CO2 + S gases), dissolved H2S up to 4,250 nM, high concentrations of particulate Cu (up to 3.4 nM), total dissolvable Fe (up to 4,720 nM), total dissolvable Mn (up to 260 nM) and Fe/Mn values of 4.4 to 18.2. By 2002, plumes from the summit vent field had much lower particulate Cu (0.3 nM), total dissolvable Fe (175 nM), and Fe/Mn values of 0.8 but similar ΔpH (−0.22) and higher H2S (7,000 nM). The 1999 plume results are consistent with a magmatic fluid component with the concentration of Fe suggesting direct exsolution of a liquid brine, whereas the much lower concentrations of metals but higher overall gas contents in the 2002 plumes likely reflect subsea-floor phase separation. Plumes above the northwest caldera site are chemically distinct, and their compositions have not changed over the same 3-year interval. They have less CO2 (ΔpH of −0.09), no detectable H2S, total dissolved Fe of 955 nM, total dissolved Mn of 150 nM, and Fe/Mn of 6.4. An overall increase in 3He/4He values in the plumes from R/RA = 6.1 in 1999 to 7.2 in 2002 is further consistent with a magmatic pulse perturbing the system. The northwest caldera site is host to at least two large areas (~600 m by at least 50 m) of chimneys and sub-cropping massive sulfide. One deposit is partially buried by sediment near the caldera rim at ~1,450 m, whereas the other crops out along narrow, fault-bounded ledges between ~1,600 and 1,650 m. Camera tows imaged active 1- to 2-m-high black smoker chimneys in the deeper zone together with numerous 1- to 5-m-high inactive spires, abundant sulfide talus, partially buried massive sulfides, and hydrothermally altered volcanic rocks. 210Pb/226Ra dating of one chimney gives an age of 27 ± 6 years; 226Ra/Ba dating of other mineralization indicates ages up to 1,200 years. Formation temperatures derived from Δ34Ssulfate-sulfide mineral pairs are 245° to 295° for the northwest caldera site, 225° to 260°C for the southeast caldera and ~260° to 305°C for the cone. Fluid inclusion gas data suggest subsea-floor phase separation occurred at the northwest caldera site. Alteration minerals identified include silicates, silica polymorphs, sulfates, sulfides, Fe and Mn oxide and/or oxyhydroxides, and native sulfur, which are consistent with precipitation at a range of temperatures from fluids of different compositions. An advanced argillic assemblage of illite + amorphous silica + natroalunite + pyrite + native S at the cone site, the occurrence of chalcocite + covellite + bornite + iss + chalcopyrite + pyrite in sulfide samples from the southeast caldera site, and veins of enargite in a rhyodacitic sample from the northwest caldera site are indicative of high-sulfidation conditions similar to those of subaerial magmatic-hydrothermal systems. The northwest caldera vent site is a long-lived hydrothermal system that is today dominated by evolved sea-water but has had episodic injections of magmatic fluid. The southeast caldera site represents the main upflow of a relatively well established magmatic-hydrothermal system on the sea floor where sulfide-rich chimneys are extant. The cone site is a nascent magmatic-hydrothermal system where crack zones localize upwelling acidic waters. Each of these different vent sites represents diverse parts of an evolving hydrothermal system, any one of which may be typical of submarine volcanic arcs.

Description: 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 Hébécourt Formation and the 2701 to 2698 Ma Bousquet Formation, which corresponds to the uppermost segment of the Blake River Group. The Hébécourt 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 (εNd ∼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.