Description: The 2.67 Ga Hackett River volcanogenic massive sulfide (VMS) deposits located in the northeastern Slave province, Nunavut, Canada, are among the largest undeveloped massive sulfide resources in Canada and are silver rich compared to other such deposits of similar age, with Ag grades up to 3,000 g/t. The deposits are hosted by the Ignerit Formation of the felsic to intermediate calc-alkaline Hackett River Group metavolcanic rocks that are part of the province-wide supracrustal Yellowknife Supergroup. One of the most economically significant of the Hackett River deposits is the Hackett River Main zone (Main zone), which consists of two parts: a stratigraphically lower chalcopyrite-rich stringer zone and an upper massive to semimassive polymetallic sulfide lens. The mineralization is subdivided into five types based on mineralogy, textures, and approximate stratigraphic position: (1) disseminated footwall sulfides, (2) copper-rich stringer sulfides, (3) pyrite-poor sphalerite-pyrrhotite-chalcopyrite mineralization at the top of the stringer zone, (4) mineralization in calc-silicate–altered calcareous tuff units, and (5) sphalerite-pyrite massive sulfide. In type 1 mineralization, disseminated pyrite, pyrrhotite, and sphalerite contain negligible Ag and in type 2, Bi-Ag-(Pb) sulfides, Ag-Bi-Se–enriched galena and chalcopyrite are the dominant Ag hosts. Within type 3, Ag-rich tetrahedrite (freibergite) and galena are the main Ag hosts. In type 4, Ag is hosted in disseminated electrum and freibergite, and within type 5 mineralization, freibergite hosts 99% of the Ag. Overall within the Main zone, Ag-rich freibergite contains 79.4% of the Ag, whereas chalcopyrite hosts 6.3% and galena contains 1.8%. Trace minerals such as electrum host the remainder of the Ag, and these have a limited spatial distribution. Zone refining is the most important control on the distribution of Ag within the Main zone and the principal controls on Ag residence are mineralizing fluid temperature, deposit-scale relative redox conditions, sulfidation state, location of the mineralization relative to the hydrothermal conduit, and the ratio of Bi to Sb in the mineralizing fluid available for coupled substitution. Within the freibergite and chalcopyrite, Ag directly substitutes for Cu and replaces Pb in galena by coupled substitution with Bi and, to a lesser extent, Sb. Lower temperatures 〈ca. 250°C and more oxidizing conditions favored partitioning of Ag into freibergite and less oxidizing conditions favored galena as a host. At higher temperatures, 〉ca. 250°C, the most reducing conditions favored incorporation in Ag-Bi-rich galena (plus Se) and Bi-bearing sulfides or Ag-rich chalcopyrite under lesser reducing conditions.

Description: Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples (δ3He, pH, dissolved Fe and Mn, and particulate chemistry) have been collected from the Rumble II West volcano, southern Kermadec arc, New Zealand. Rumble II West is a caldera volcano with an ∼3-km-diameter summit depression bounded by ring faults with a resurgent central cone. Rocks recovered to date are predominantly mafic in composition (i.e., basalt to basaltic andesite) with volumetrically lesser intermediate rocks (i.e., andesite). On the basis of its size, geometry, volcanic products, and composition, Rumble II West can be classified as a mafic caldera volcano. Rumble II West has a weak hydrothermal plume signature characterized by a small but detectable δ3He anomaly (25%). Time-series light scattering data though, obtained from vertical casts and tow-yos, do show that hydrothermal activity has increased in intensity between 1999 and 2011. Massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite, with lesser sphalerite, pyrite, and traces of galena. The weak hydrothermal plume signal indicates that the volcano is in a volcanic-hydrothermal quiescent stage compared to other volcanoes along the southern Kermadec arc, although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past. Of the volcanoes along the Kermadec-Tonga arc known to host massive sulfides (i.e., Clark, Rumble II West, Brothers, Monowai, Volcano 19, and Volcano 1), the majority (five out of six) are dominantly mafic in composition and all but one of these mafic volcanoes form moderate-size to large calderas. To date, mafic calderas have been largely ignored as hosts to sea-floor massive sulfide deposits. That 75% of the presently known massive sulfide-bearing calderas along the arc are mafic in composition (the dacitic Brothers volcano is the exception) has important implications for sea-floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land.

Description: Volcanogenic massive sulfide (VMS) deposits typically contain significant proportions of magma-derived chalcophile (Cu affinity) and siderophile (Fe affinity) elements such as Au, Cu, V, Zn, Mo, Bi, Sb, and As that relate to the composition of associated (host) magmatic rocks. Here, we combine new and published trace element data for lavas recovered from 15 volcanic centers along the Kermadec arc. The data show that mafic back-arc and arc-front lavas are enriched in most of the chalcophile and siderophile elements when compared with mid-ocean ridge basalts (MORB). Elevated (Cu, Zn, V, Mo, Pb)/Yb, Ba/La, As/Ce, and Sb/Pr ratios indicate that the chalcophile and siderophile elements are either transported into the mantle wedge via hydrous fluids derived from the subducting slab, or are liberated from residual mantle wedge sulfides that are oxidized by hydrous fluids. Lower ratios of (Cu, Zn, Mo, Sb, and Pb)/(MREE, HREE) in basalts from the Kermadec back arc (Havre Trough) when compared to the arc front suggests decreasing slab-related input into the mantle source away from the arc front. Unusually high contents of LILE, Ag, Sn, Mo, Th, LREE, MREE, Nb, Zr, Hf, and positive trends in (Ag, Sn)/Yb with Th/Yb, Hf/Y, (La/Sm)N, but low Sr/Y, in dacites from the Brothers volcanic center, southern Kermadec arc, indicate the additional transport of Ag and Sn via a solute-rich supercritical fluid, or via a sediment-derived melt. Magmas generated through partial melting of a sub-arc mantle metasomatized by hydrous melts thus appear to play an important role in the formation of Cu-Au-Ag−rich arc-type VMS deposits.

Description: The Howard’s Pass district (HPD) comprises 14 Zn-Pb sedimentary exhalative (SEDEX) deposits and is located within the Selwyn basin, Yukon, Canada. Although the HPD is renowned for its large accumulation of base-metal sulfides, in places the Late Ordovician to Early Silurian host rocks also contain abundant carbonate-bearing fluorapatite (CBFA). This mineral is present stratigraphically below, within, and above the SEDEX deposits and occurs as fine-grained layers that are interbedded with cherty carbonaceous mudstone. Electron probe microanalysis and laser ablation-inductively coupled plasma-mass spectrometric analysis reveal that mineral compositions and rare earth element-yttrium (REE-Y) systematics, respectively, are remarkably similar throughout the stratigraphic succession. North American Shale Composite (NASC)-normalized La/Sm and La/Yb ratios indicate that the original REE compositions in CBFA have undergone only minor compositional modification subsequent to deposition. Uniformly negative Ce anomalies indicate that the mineral formed in analogous manner to modern and ancient sedimentary phosphorites under suboxic bottom-water conditions. Europium anomalies are mostly absent, indicating that reduced, slightly acidic high-temperature hydrothermal fluids were not a major source of REE-Y to CBFA. The chemical homogeneity of the mineral irrespective of its stratigraphic position indicates that a common process was responsible for its deposition within the sedimentary rocks of the HPD. On the basis of the similarity of the REE patterns to modern and ancient phosphorites, and the absence of positive Eu anomalies, we conclude that the CBFA is of hydrogenous origin, and not hydrothermal as suggested by previous workers. As such, phosphorite formation in the HPD is casually related to SEDEX Zn-Pb deposit formation.

Description: A high-Cr mineral assemblage is observed in the unusual Matoush uranium deposit (Quebec, Canada), which is associated with the bimodal Matoush dike intruding Otish Basin sandstones. In addition to uraninite, both silicate (chromium-dravite and chromphyllite) and oxide (eskolaite) high-Cr minerals are present in major amounts. While high-Cr dravite and high-Cr muscovite have been described previously, eskolaite, Cr 2 O 3 , has not been studied extensively. The highest Cr contents observed in the Matoush silicates, obtained by electron microprobe, are comparable with the highest ever documented in the literature, with Cr 2 O 3 of up to ca . 22 wt.% in chromphyllite and ca . 37 wt.% in chromium-dravite. Complete solid solutions among hematite (Fe 2 O 3 ), eskolaite, and karelianite (V 2 O 3 ) are theoretically possible, but until now only complete Cr-V substitution had been documented in natural samples. Matoush eskolaite contains significant amounts of Fe, with up to 37% wt.% Fe 2 O 3 substitution, and minimal V substitution (typically under 0.5 wt.% V 2 O 3 ). A hydrated Fe-Cr oxide is also observed closely associated with eskolaite at Matoush, having the same Cr:Fe:V ratio as eskolaite, but with significant assumed H 2 O. Rietveld analysis of X-ray powder diffraction data is best fit using a model that includes eskolaite and a second hydrated crystalline Fe-Cr oxide with a similar atomic structure to eskolaite, but a significantly larger c-axis dimension.

Description: The Lemarchant deposit is a Cambrian bimodal felsic Zn-Pb-Cu-Ag-Au volcanogenic massive sulfide (VMS) deposit located in the Central Mobile Belt, Newfoundland, Canada. Despite regional greenschist metamorphism and faulting, primary mineralogy and mineral textures are well preserved. The deposit is a type example of a precious metal-enriched VMS deposit in the Appalachians. Mineralization consists of a stratiform massive sulfide zone that lies at the contact between a rhyolitic footwall and a basaltic hanging wall. A stringer sulfide zone that is hosted in footwall rhyolite breccia underlies the massive sulfide lens. The stratiform sulfide zone contains massive baryte that is heterogeneously replaced by sphalerite and pyrite, lesser galena, and trace chalcopyrite. The stringer zone contains chalcopyrite, pyrite, and lesser sphalerite and galena. Sphalerite ranges in color from white (low-Fe) to red (high-Fe). The palest sphalerite (〈1 mol.% FeS) occurs in the baryte-rich stratiform zone and is associated with early stage exhalative mineralization, intermediate sulfidation epithermal suite minerals (tetrahedrite, bornite, colusite, Ag-bearing gold, covellite) and sulfide minerals enriched in the epithermal trace element suite (Au, Ag, As, Bi, Co, Cr, In, Mo, Ni, Sb, Sn, Te). Darker sphalerite (4.7–13.6 mol.% FeS) in the stratiform zone overprints early stage mineralization and occurs with chalcopyrite; high-Fe sphalerite is also present in the stringer zone. Early exhalative/epithermal-type VMS mineralization was deposited from low temperature (150–250 °C), oxidized, acidic to near-neutral hydrothermal fluids with high sulfur activity. Early mineralization was likely deposited in relatively shallow water (〈1500 mbsl) that intermittently boiled and precipitated Au in the stratiform zone. Late-stage polymetallic, Cu-rich VMS mineralization was deposited from higher temperature (〉300 °C), less oxidized, near-neutral hydrothermal fluids, likely in deeper water (〉1500 mbsl). Abundant epithermal suite minerals and epithermal trace element suite-enriched sulfides at Lemarchant suggest a direct magmatic contribution to the hydrothermal fluid.

Description: Volcanogenic massive sulfide (VMS) deposits typically contain significant proportions of magma-derived chalcophile (Cu affinity) and siderophile (Fe affinity) elements such as Au, Cu, V, Zn, Mo, Bi, Sb, and As that relate to the composition of associated (host) magmatic rocks. Here, we combine new and published trace element data for lavas recovered from 15 volcanic centers along the Kermadec arc. The data show that mafic back-arc and arc-front lavas are enriched in most of the chalcophile and siderophile elements when compared with mid-ocean ridge basalts (MORB). Elevated (Cu, Zn, V, Mo, Pb)/Yb, Ba/La, As/Ce, and Sb/Pr ratios indicate that the chalcophile and siderophile elements are either transported into the mantle wedge via hydrous fluids derived from the subducting slab, or are liberated from residual mantle wedge sulfides that are oxidized by hydrous fluids. Lower ratios of (Cu, Zn, Mo, Sb, and Pb)/(MREE, HREE) in basalts from the Kermadec back arc (Havre Trough) when compared to the arc front suggests decreasing slab-related input into the mantle source away from the arc front. Unusually high contents of LILE, Ag, Sn, Mo, Th, LREE, MREE, Nb, Zr, Hf, and positive trends in (Ag, Sn)/Yb with Th/Yb, Hf/Y, (La/Sm)N, but low Sr/Y, in dacites from the Brothers volcanic center, southern Kermadec arc, indicate the additional transport of Ag and Sn via a solute-rich supercritical fluid, or via a sediment-derived melt. Magmas generated through partial melting of a sub-arc mantle metasomatized by hydrous melts thus appear to play an important role in the formation of Cu-Au-Ag–rich arc-type VMS deposits.

Description: Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples ( 3 He, pH, dissolved Fe and Mn, and particulate chemistry) have been collected from the Rumble II West volcano, southern Kermadec arc, New Zealand. Rumble II West is a caldera volcano with an ~3-km-diameter summit depression bounded by ring faults with a resurgent central cone. Rocks recovered to date are predominantly mafic in composition (i.e., basalt to basaltic andesite) with volumetrically lesser intermediate rocks (i.e., andesite). On the basis of its size, geometry, volcanic products, and composition, Rumble II West can be classified as a mafic caldera volcano. Rumble II West has a weak hydrothermal plume signature characterized by a small but detectable 3 He anomaly (25%). Time-series light scattering data though, obtained from vertical casts and tow-yos, do show that hydrothermal activity has increased in intensity between 1999 and 2011. Massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite, with lesser sphalerite, pyrite, and traces of galena. The weak hydrothermal plume signal indicates that the volcano is in a volcanic-hydrothermal quiescent stage compared to other volcanoes along the southern Kermadec arc, although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past. Of the volcanoes along the Kermadec-Tonga arc known to host massive sulfides (i.e., Clark, Rumble II West, Brothers, Monowai, Volcano 19, and Volcano 1), the majority (five out of six) are dominantly mafic in composition and all but one of these mafic volcanoes form moderate-size to large calderas. To date, mafic calderas have been largely ignored as hosts to sea-floor massive sulfide deposits. That 75% of the presently known massive sulfide-bearing calderas along the arc are mafic in composition (the dacitic Brothers volcano is the exception) has important implications for sea-floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land.

Description: The 2.67 Ga Hackett River volcanogenic massive sulfide (VMS) deposits located in the northeastern Slave province, Nunavut, Canada, are among the largest undeveloped massive sulfide resources in Canada and are silver rich compared to other such deposits of similar age, with Ag grades up to 3,000 g/t. The deposits are hosted by the Ignerit Formation of the felsic to intermediate calc-alkaline Hackett River Group metavolcanic rocks that are part of the province-wide supracrustal Yellowknife Supergroup. One of the most economically significant of the Hackett River deposits is the Hackett River Main zone (Main zone), which consists of two parts: a stratigraphically lower chalcopyrite-rich stringer zone and an upper massive to semimassive polymetallic sulfide lens. The mineralization is subdivided into five types based on mineralogy, textures, and approximate stratigraphic position: (1) disseminated footwall sulfides, (2) copper-rich stringer sulfides, (3) pyrite-poor sphalerite-pyrrhotite-chalcopyrite mineralization at the top of the stringer zone, (4) mineralization in calc-silicate–altered calcareous tuff units, and (5) sphalerite-pyrite massive sulfide. In type 1 mineralization, disseminated pyrite, pyrrhotite, and sphalerite contain negligible Ag and in type 2, Bi-Ag-(Pb) sulfides, Ag-Bi-Se–enriched galena and chalcopyrite are the dominant Ag hosts. Within type 3, Ag-rich tetrahedrite (freibergite) and galena are the main Ag hosts. In type 4, Ag is hosted in disseminated electrum and freibergite, and within type 5 mineralization, freibergite hosts 99% of the Ag. Overall within the Main zone, Ag-rich freibergite contains 79.4% of the Ag, whereas chalcopyrite hosts 6.3% and galena contains 1.8%. Trace minerals such as electrum host the remainder of the Ag, and these have a limited spatial distribution. Zone refining is the most important control on the distribution of Ag within the Main zone and the principal controls on Ag residence are mineralizing fluid temperature, deposit-scale relative redox conditions, sulfidation state, location of the mineralization relative to the hydrothermal conduit, and the ratio of Bi to Sb in the mineralizing fluid available for coupled substitution. Within the freibergite and chalcopyrite, Ag directly substitutes for Cu and replaces Pb in galena by coupled substitution with Bi and, to a lesser extent, Sb. Lower temperatures 〈ca. 250°C and more oxidizing conditions favored partitioning of Ag into freibergite and less oxidizing conditions favored galena as a host. At higher temperatures, 〉ca. 250°C, the most reducing conditions favored incorporation in Ag-Bi-rich galena (plus Se) and Bi-bearing sulfides or Ag-rich chalcopyrite under lesser reducing conditions.

Description: The chemical compositions of uraninites from five major deposit types (tabular, unconformity, vein, metasomatic, and igneous), measured by electron microprobe and LA-ICP-MS, underline the extreme chemical variation of natural uraninite, due to different formation conditions prevailing at each deposit type. Apart from the major elements always analyzed in uraninite (U, Th, Y, REE, Pb, as well as Si, Ca, and Fe), several additional elements are present in uraninite to a significant degree: Mn (overall median value of 6088 ppm), V (4528 ppm), Na (2365 ppm), As (2251 ppm), W (1811 ppm), Mg (441 ppm), Sb (286 ppm), Sr (261 ppm), Ti (235 ppm), Mo (133 ppm), Bi (125 ppm), and Ba (118 ppm). Uraninites from different deposit types have distinct chemical compositions: tabular-type uraninites have the lowest Th, Y, and REE and the highest trace elements, in particular Mg, Mn, and V; sandstone-hosted unconformity-related uraninites have the lowest Y and the highest Fe, Na, Cu, Ni, and Ni; basement-hosted unconformity-related uraninites have the lowest Ca and Fe and the highest Ti, Ni, and W; metasomatism-related uraninites have the lowest Y and the highest Th and Si; and igneous uraninites have the lowest trace elements and the highest Th, Y, REE, Zr, and Hf. The vein-type uraninites have the most variable chemical compositions. In addition to the REE spectra, with only the igneous uraninites displaying a negative Eu anomaly, the chemical compositions of uraninites can be used with high confidence as provenance indicators.