Volcanogenic Massive Sulfide Districts Research Articles

Zircon and the role of magmatic petrogenesis in the formation of felsic-hosted volcanogenic massive sulfide (VMS) deposits: a case study from the mid-Paleozoic Yukon-Tanana terrane, northern Canadian Cordillera

Magmatism is a critical component in sustaining hydrothermal convection and metal transport during the formation of volcanogenic massive sulfide (VMS) deposits. Previous studies of magmatic petrogenesis in VMS systems have demonstrated that ore-related volcanic rocks have distinct whole-rock geochemical and isotopic signatures (i.e., high HFSE, REE, Th, εHf-Nd, zircon saturation T) relative to barren volcanic rocks, which supports models of elevated crustal heat flow during periods of ore deposition; however, the petrologic characteristics and intrinsic parameters (e.g., T, fO2) related to these magmatic events in VMS districts remain poorly understood. Arc–back-arc assemblages from the mid-Paleozoic Yukon-Tanana terrane are well-characterized and include the Finlayson Lake VMS district, which is host to several felsic-hosted deposits (e.g., Kudz Ze Kayah, GP4F, Wolverine) that were generated in a peri-Laurentian continental back-arc tectonic setting. In this study, zircon from back-arc and coeval arc rocks in the Yukon-Tanana terrane was used as a proxy for primary magma formation conditions that generated VMS-proximal and VMS-distal stratigraphy. Our results indicate that zircon grains in VMS-proximal environments have unique textural, geochemical, and isotopic characteristics (e.g., low-aspect ratios, greater abundance of zircon-phosphate intergrowths, Th/U > 1, Zr/Hf > 80, Tzrc > 780 °C, εHfi > –7) that are clearly distinguished from zircon in VMS-distal rocks in both the back-arc and arc settings (Th/U < 1, Zr/Hf < 80, Tzrc < 780 °C, εHfi < –7). These signatures correlate to VMS-proximal magmas that were hotter, less fractionated, and contained greater juvenile melt contributions compared to VMS-distal magmas and reflect a series of high-flux magmatic events that directly correspond to the early tectonic development of Yukon-Tanana terrane. Moreover, this study underscores the importance of mineral-scale petrology, geochemistry, and geochronology in defining the primary magmatic conditions that generated VMS-related felsic rocks and highlights the utility of zircon as a prospectivity tool in both grassroots and brownfields VMS exploration.

Multispectral discrimination of spectrally similar hydrothermal minerals in mafic crust: A 5000 km2 ASTER alteration map of the Oman–UAE ophiolite

Multispectral remote sensing of hydrothermal alteration in volcanogenic massive sulfide (VMS) ore systems in mafic crust is relatively uncommon, in part due to the short-wave infrared spectral similarity of several key alteration minerals: epidote, chlorite, actinolite, and serpentine. In this study, we developed regional mosaic generation and classification workflows for Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) imagery to discriminate these minerals over the entire crust of the Semail ophiolite (Oman–UAE). Spectral discrimination was achieved through adaptation of the ASTER (pre-)processing workflow to the specific mapping targets, available datasets, and location of this study. Necessary steps included the pre-selection of ASTER scenes without residual atmospheric water features, mosaic normalization based solely on overlapping target outcrops, correcting cross-mosaic ramp errors, and alteration map classification based on image-derived reference data. The resulting alteration map, validated through comparison with field mapping and sampling, is the most areally extensive continuous survey of hydrothermal alteration yet presented for oceanic crust, providing a renewed framework for research and mineral exploration of Earth's largest ophiolite. Our map confirms that the vast majority of the upper oceanic crust is regionally altered to a spilite type secondary mineral assemblage. Localized areas of epidosite alteration, marking focused hydrothermal flow paths, are confined to the upper oceanic crust, whereas areas of previously unrecognized but intense actinolite alteration are common in both the lower and upper oceanic crust. Our methodological developments expand the standard considerations necessary for regional geological mapping using infrared image mosaics. They further demonstrate the underappreciated capability of multispectral data for mapping spectrally similar rock types. Although the specifics of the method are necessarily optimized for the Oman–UAE ophiolite, re-optimization based on local reference data should allow similar results to be achieved in other well-exposed mafic-hosted VMS districts.

High-Precision CA-ID-TIMS U-Pb Zircon Geochronology of Felsic Rocks in the Finlayson Lake VMS District, Yukon: Linking Paleozoic Basin-Scale Accumulation Rates to the Occurrence of Subseafloor Replacement-Style Mineralization

Abstract Felsic igneous complexes and associated volcano-sedimentary rocks in continental back-arc environments host large-tonnage and/or high-grade volcanogenic massive sulfide (VMS) deposits. The emplacement mechanisms, style, and preservation of these deposits is thought to be partially dependent on depositional rates of the host lithofacies (i.e., discrete volcanic eruptions) relative to the setting of massive sulfide genesis on the seafloor as mounds and/or via subseafloor replacement of existing strata. The localization and occurrence of subseafloor replacement-style VMS deposits is therefore strongly influenced by the characteristics of the volcano-sedimentary facies in the hosting basin and the rates of their emplacement; the latter are poorly constrained in the literature due to the difficulty of obtaining high-precision dates that make this possible in Phanerozoic and older rocks. New high-resolution U-Pb geochronology and detailed regional stratigraphic investigation indicate that Devonian-Mississippian volcanic rocks and associated VMS mineralization in the Yukon-Tanana terrane in the Finlayson Lake district, Yukon, Canada, were erupted or emplaced during distinct time periods (ca. 363.3, 362.8, and 355.2 Ma) in two discrete submarine basins: the Kudz Ze Kayah formation and the Wolverine Lake group. The VMS deposits in both settings are contained within intrabasinal rocks that accumulated at rapid rates of ~350 to 2,000 m/m.y. over 0.6 to 1.4 m.y. Locally, these rates reach peak rates up to 7,500 m/m.y. in the Wolverine Lake group, which are interpreted to reflect facies deposition by mass transport complexes or turbidity currents. These new dates indicate that rapid accumulation of volcanic rocks in the back-arc basins was critical for localizing subseafloor replacement-style mineralization and the development of the Zn-enriched GP4F, Kudz Ze Kayah, and Wolverine VMS deposits. Rapid depositional processes observed in these deposits and their host basins are interpreted to have an important role in developing highly porous and permeable, water-saturated lithofacies that provide optimal conditions for enhancing zone refining processes and subsequent preservation of massive sulfide mineralization, which are key in the development of high-grade and large-tonnage VMS deposits. It is herein suggested that quantitative basin-scale accumulation rates, as a result of new U-Pb geochronological methods and increased precision combined with detailed stratigraphic and facies analysis, may provide important perspectives on the formation of continental back-arc basins and the localization of VMS deposits in other continental margin environments globally.

Geochemistry and petrogenesis of Paleoproterozoic rhyolite-hosted zinc-rich metamorphosed volcanogenic massive sulfide deposits in the eastern Betul Belt, central India

The Palaeoproterozoic Betul Belt in central India contains several small base metal (Zn-Cu and Zn-Pb-Cu) deposits that are hosted by felsic volcanics which form part of a bimodal volcanic suite. Although a volcanogenic massive sulfide (VMS) origin has been attributed to these deposits, they are not well-defined based on modern classification criteria. Regionally, the volcanic facies association around the Bhuyari zinc prospect includes massive rhyolite, pillowed basalt, rhyolitic autobreccia, hyaloclastite and peperite that indicate volcanism and sedimentation in relatively deep water. At the deposit-scale, the mineralization is hosted by intensely altered massive quartz-porphyritic rhyolite and associated tremolite-carbonate rock. Alteration facies mapping show a zoned alteration system with an inner phlogopite-biotite-chlorite-garnet-rich core (BPG schist; interpreted as having a chlorite-rich protolith) and an outer quartz-muscovite-K-feldspar zone (QMK schist; sericite-rich protolith). Metamorphism was largely isochemical and the mineral assemblages are amphibolite facies equivalents of hydrothermally altered protoliths. Tremolite-carbonate rocks are spatially close to mineralization and are interpreted either as metaexhalites or intensely altered rhyolite and probably had talc-dolomite protoliths. Carbon and oxygen isotope signatures of carbonate show derivation from seawater-dominant hydrothermal solutions. Tremolite-carbonate and some BPG schist show enrichment in HREE indicating hydrothermal mobilization of REE. Geochemically, the least-altered rhyolite has transitional magmatic affinities with moderate negative Nb anomaly and strong negative Sr, and Ti anomalies. Mantle upwelling and the associated high-heat source generated high-temperature, high-silica rhyolites with elevated HSFE and REE contents. Emplacement of felsic melts at depth provided heat that drove hydrothermal fluid circulation and formation of zinc-rich mineralization and associated hydrothermal alteration zones. Pb-model ages of galena give ~1800 Ma using the Stacey and Kramers (1975) model for VMS mineralization in Bhuyari. The Bhuyari deposit possibly formed in a continental back-arc rift environment during the amalgamation of Bastar and Bundelkhand cratons along the Central Indian Tectonic Zone. The high HFSE-REE contents of Bhuyari rhyolites are similar to other highly prospective VMS districts formed by rifting of evolved continental crust. The recognition of metamorphosed VMS deposit at Bhuyari has implications for further exploration of such deposits along the Paleoproterozoic Betul Belt and similar-aged deformed terrains in India and elsewhere.

Mineralogy and geochemistry of metalliferous sedimentary rocks from the Upper Cretaceous VMS deposits of the Eastern Pontides (NE Turkey)

Metalliferous sedimentary rocks, despite being limited in scope, are important stratigraphic horizons within the volcanogenic massive sulfide (VMS) deposits in the Upper Cretaceous volcanic belt of the Eastern Pontides. The metalliferous sedimentary rocks compose a distinctive genetic class. The abundance of such rocks in the Eastern Pontide district provides a basis for study and description that may be more widely applied in other VMS districts. The metalliferous sedimentary rocks generally form layers less than 1.5 m thick above stratiform massive sulfide ores. The metalliferous sediments are largely composed of quartz and hematite. While the Si ± Fe content of sediments directly overlying the stratiform massive sulfide ores is high, an increase in the amount of carbonate is observed in sediments that are not directly overlying the ores. The metalliferous sedimentary rocks were formed by the mixing of mainly chemical components and to a smaller degree detrital components in various proportions due to sedimentation processes occurring on the seafloor. Of the major constituents of the metalliferous sediments, Si, Fe, Cu, Zn, Pb, Sb, and Au generally have a hydrothermal origin, whereas Al, Zr, and Ti are from detrital volcanic and volcaniclastic components. The high levels of U and V indicate a seawater origin, pointing to a submarine environment for the formation of the Eastern Pontide metalliferous sedimentary rocks. The composition of the metalliferous sediments of Eastern Pontide VMS deposits implies that these sediments carry a continental-arc provenance signature. Silica (±iron)-rich rocks in the Eastern Pontides can be used as guide horizons in exploring for volcanogenic massive sulfide deposits in the district since they formed close to the sulfide ores and their areal extents are much greater than those of the sulfide ores. In comparison with their analogues on a global scale, these rocks are broadly comparable to silica- and iron-rich sedimentary rocks in the VMS-bearing districts of the Urals.

Genesis of the Zinkgruvan stratiform Zn-Pb-Ag deposit and associated dolomite-hosted Cu ore, Bergslagen, Sweden

Zinkgruvan, a major stratiform Zn-Pb-Ag deposit in the Paleoproterozoic Bergslagen region, south-central Sweden, was overprinted by polyphase ductile deformation and high-grade metamorphism (including partial melting of the host succession) during the 1.9–1.8Ga Svecokarelian orogeny. This complex history of post-ore modification has made classification of the deposit difficult. General consensus exists on a syngenetic-exhalative origin, yet the deposit has been variably classified as a volcanogenic massive sulfide (VMS) deposit, a sediment-hosted Zn (SEDEX) deposit, and a Broken Hill-type (BHT) deposit. Since 2010, stratabound, cobaltiferous and nickeliferous Cu ore, comprising schlieren and impregnations of Cu, Co and Ni sulfide minerals in dolomitic marble, is mined from the stratigraphic footwall to the stratiform Zn-Pb-Ag ore. This ore type has not been fully integrated into any of the existing genetic models. Based on a combination of 1) widespread hematite-staining and oxidizing conditions (Fe2O3>FeO) in the stratigraphic footwall, 2) presence of graphite and reducing conditions (Fe2O3<FeO) in the ore horizon and hangingwall and 3) intense K-feldspar alteration and lack of feldspar-destructive alteration in the stratigraphic footwall, we suggest that both the stratiform Zn-Pb-Ag and the dolomite-hosted Cu ore can be attributed to the ascent and discharge of an oxidized, saline brine at near neutral pH. Interaction of this brine with organic matter below the seafloor, especially within limestone, formed stratabound, disseminated Cu ore, and exhalation of the brine into a reduced environment on the sea floor produced a brine pool from which the regionally extensive (>5km) Zn-Pb-Ag ore was precipitated.Both ore types are characterized by significant spread in δ34S, with the sulfur in the Cu ore and associate marble-hosted Zn mineralization on average being somewhat heavier (δ34S=−4.7 to +10.5‰, average 3.9‰) than that in the stratiform Zn-Pb-Ag ore (δ34S=−6 to +17‰, average 2.0‰). The ranges in δ34S are significantly larger than those observed in syn-volcanic massive sulfide deposits in Bergslagen, for which simple magmatic/volcanic sulfur sources have been invoked. Mixing of magmatic-volcanic sulfur leached from underlying volcanic rocks and sulfur sourced from abiotic or bacterial sulfate reduction in a mixing zone at the seafloor could explain the range observed at Zinkgruvan.A distinct discontinuity in the stratigraphy, at which key stratigraphic units stop abruptly, is interpreted as a syn-sedimentary fault. Metal zonation in the stratiform ore (decreasing Zn/Pb from distal to proximal) and the spatial distribution of Cu mineralization in underlying dolomitic marble suggest that this fault was a major feeder to the mineralization. Our interpretation of ore-forming fluid composition and a dominant redox trap rather than a pH and/or temperature trap differs from most VMS models, with Selwyn-type SEDEX models, and most BHT models. Zinkgruvan has similarities to both McArthur-type SEDEX deposits and sediment-hosted Cu deposits in terms of the inferred ore fluid chemistry, yet the basinal setting has more similarities to BHT and felsic-bimodal VMS districts. We speculate that besides an oxidized footwall stratigraphy, regionally extensive banded iron formations and limestone horizons in the Bergslagen stratigraphy may have aided in buffering ore-forming brines to oxidized, near-neutral conditions. In terms of fluid chemistry, Zinkgruvan could comprise one of the oldest known manifestations of Zn and Cu ore-forming systems involving oxidized near-neutral brines following oxygenation of the Earth’s atmosphere.

Lithological discrimination based on statistical analysis of multi-sensor drill core logging data in the Matagami VMS district, Quebec, Canada

In mineral exploration, traditional core logging is largely based on the visual inspection of drilled cores, a process which is often subjective and non-reproducible. However, a number of physical, chemical and mineralogical properties of rocks can now be measured at high spatial resolution on drill cores. The resulting large multiparameter databases can help geologists to quantitatively discriminate between lithologies, study hydrothermal alteration, and potentially vector towards mineralization. Multivariate statistical methods are important tools to assist geologists in interpreting such large databases. We present an application of model-based cluster analysis in the Matagami base metal mining district, more specifically to improve lithological discrimination at the zinc-rich Bracemac and McLeod volcanogenic massive sulfide (VMS) deposits. The model-based cluster analysis method is able to efficiently discriminate different geological units encountered, even in cases where two units are visually similar or in the presence of strong hydrothermal alteration.

What processes at mid-ocean ridges tell us about volcanogenic massive sulfide deposits

Episodic seafloor spreading, ridge topography, and fault movement at ridges find (more extreme) analogs in the arc and back-arc setting where the volcanogenic massive sulfide (VMS) deposits that we mine today were formed. The factors affecting sulfide accumulation efficiency and the extent to which sulfides are concentrated spatially are the same in both settings, however. The processes occurring at mid-ocean ridges therefore provide a useful insight into those producing VMS deposits in arcs and back-arcs. The critical observation investigated here is that all the heat introduced by seafloor spreading at mid-ocean ridges is carried out of the crust within a few hundred meters of the ridge axis by ∼350°C hydrothermal fluids. The high-temperature ridge hydrothermal systems are tied to the presence of magma at the ridge axis and greatly reduce the size and control the shape of axial magma intrusions. The amount of heat introduced to each square kilometer of ocean crust during its formation can be calculated, and its removal by high-temperature convection allows calculation of the total base metal endowment of the ocean basins. Using reasonable metal deposition efficiencies, we conclude that the ocean floor is a giant VMS district with metal resources >600 times the total known VMS reserves on land and a copper resource which would last >6,000 years at current production rates.

On the Size and Spacing of Volcanogenic Massive Sulfide Deposits within a District with Application to the Matagami District, Quebec

Volcanogenic massive sulfide (VMS) districts are typically ~40 km in diameter and contain about a dozen regularly spaced Zn-Cu orebodies, one or two of which contain more than half of the district’s resources. We numerically investigate this deposit size and spatial distribution by calculating zinc transport across the sea floor, first above sills of simple geometry, and then above the Bell River Complex at Matagami. For sills with simple rectangular geometry (i.e., a constant thickness), convection is strongest at the edge. The edge convection induces a progression of convection cells above the sill, although the sill cools mainly by the retreat of its thermal edges toward the sill center. If the rock permeability is a function of temperature that is maximized at 375°C, the resulting vents occur at stable, discrete locations that proxy for ore deposits. The fraction of sill heat vented at or above 300°C and the potential metal resources of the districts are greatest if the host permeability is 10−15 m2 (1 millidarcy) and the sill top is at 1 km depth. The simulations suggest that, as the host permeability increases, a single large deposit will progressively dominate a host of smaller ones. The Matagami simulations are based on a sill that tapers from 6.5 to 0 km thickness over a distance of 30 km. In the model, convection occurs both above the sill and along its underside, and metal is extracted from both sides of the cooling sill. The spacing of the resulting discharge sites is similar to that observed in actual VMS districts and, where direct comparison is possible, the mass of metal deposited is similar for an accumulation efficiency of ~3 percent.

Rhyolite Geochemical Signatures and Association with Volcanogenic Massive Sulfide Deposits: Examples from the Abitibi Belt, Canada

The relationship between rhyolite geochemistry and volcanogenic massive sulfide (VMS) mineralization has been proposed as an exploration tool to discriminate prospective felsic volcanic centers. The most widely used classification discriminates between four types of rhyolite: FI, FII, FIIIa, and FIIIb. The FI rhyolites are calc-alkaline, with strongly fractionated REE patterns and strongly negative Ta and Nb anomalies. They are usually considered barren, unless associated with FII or FIII felsic volcanic rocks. The FII rhyolites are calc-alkaline to transitional with moderately fractionated REE patterns and moderate Ta and Nb anomalies. They range from barren to having a high potential to host VMS mineralization. The FIIIa and FIIIb rhyolites are tholeiitic and show weakly fractionated REE patterns and weak to absent Nb and Ta anomalies. They have the highest potential to host VMS mineralization. The FIIIb rhyolites are high-temperature rhyolites with flat REE patterns and no Ta or Nb anomalies. The Abitibi greenstone belt, especially in Quebec, is well known for its abundant and diverse VMS deposits. Representative samples of VMS-associated rhyolites within and outside of mining districts, including the classic Noranda VMS district, were analyzed for major and trace elements to validate their proposed favorability for hosting VMS deposits. Results indicate that all of the rhyolite types are prospective, but mineralization may differ from the classic Noranda-type VMS deposit. The FI-type rhyolites appear to be particularly associated with gold-rich VMS deposits, such as the world-class Laronde deposit, and are more prospective for Cu-Au replacement and vein-type deposits. The FII-type rhyolites account for about 70 percent of rhyolites in the Abitibi belt. Although considered less prospective, some districts dominated by FII rhyolites, such as Val-d’Or and Selbaie, have collectively produced in excess of 100 million metric tons (Mt) of ore. Deposits in these districts mainly consist of sulfide veins and disseminated ore with low Cu and Zn grades and are associated with abundant and highly vesicular volcaniclastic rocks that display a compositional continuum from andesite to rhyolite. Other weakly mineralized FII districts (e.g., Hunter mine, Gemini-Turgeon) are characterized instead by bimodal flow-dome sequences. The FIIIa-type rhyolites occur mainly in the Noranda district and form flow-dome complexes in bimodal sequences with associated Noranda-type VMS mineralization. In small felsic centers (Joutel, Normetal, Chibougamau, Quevillon) that show a volcanic evolution from FIIIa to FII to FI affinities, VMS deposits are directly associated with FIIIa rhyolites, thus demonstrating the usefulness of rhyolite geochemistry for exploration in these areas. The FIIIb rhyolites are rare in the Abitibi belt, with most occurring in the Matagami district where they are associated with Zn-Cu VMS deposits. Based on this analysis, we suggest that a combination of rhyolite geochemistry, volcanic facies, and style of the mineralization may be more meaningfully applied in exploration than rhyolite type alone, particularly in the case of FI and FII rhyolites.

Age and Paleotectonic Setting of Volcanogenic Massive Sulfide Deposits in the Guerrero Terrane of Central Mexico: Constraints from U-Pb Age and Pb Isotope Studies

The Guerrero terrane of central and western Mexico comprises volcanic and sedimentary strata of mainly Mesozoic age, with geochemical signatures indicating formation in juvenile back-arc to slightly more evolved arc environments. At least 60 volcanogenic massive sulfide (VMS) deposits have been discovered thus far within the Guerrero terrane. They comprise two main belts: the Coastal belt mainly comprising deposits in the Zihuataneo subterrane (including the Cuale-Bramador district), and the Central belt, which includes deposits in the Teloloapan subterrane (Campo Morado, Tlanilpa-Azulaquez, Tizapa, Rey de Plata, etc.), the Guanajuato subterrane (El Gordo, Las Gavalinas), and the Zacatecas subterrane (San Nicolas and El Salvador deposits). The age of host rocks for the VMS mineralization in the various subterranes was poorly constrained prior to this study. Eleven new U-Pb zircon ages for volcanic host rocks from the Campo Morado, Tlanilpa-Azulaquez, Leon-Guanajuato, and San Nicolas-El Salvador VMS districts, together with recently determined U-Pb ages from the Cuale area, demonstrate that VMS deposits in the Guerrero terrane range from latest Middle Jurassic to early Early Cretaceous (Callovian to Valanginian) in age. The oldest ages were obtained from the Cuale district (total age range of 157.4 ± 4.1 Ma for three units) and the youngest are from the Tlanilpa-Azulaquez district (total age range of 139.7 ± 2.5 Ma for two samples). Volcanogenic massive sulfide deposits in the Central belt resemble those of the bimodal-siliciclastic deposit type, whereas those in the Coastal belt are more similar to the bimodal-felsic type. Published geochemical and radiogenic isotope data from the various subterranes suggest that VMS deposits in the Coastal belt and most of those in the Central belt are hosted within juvenile to slightly evolved arc settings. The San Nicolas and El Salvador deposits of the Zacatecas subterrane are the only deposits that unequivocally formed in a back-arc setting. Our new U-Pb dating results, together with existing stratigraphic, geochemical, and radiogenic isotope data from throughout the Guerrero terrane, are consistent with the Guerrero terrane having developed as a west-facing continental margin arc that was built on mainly oceanic crust (and overlying Early Mesozoic siliciclastic fans shed from the east) along the western margin of nuclear Mexico. Slab rollback in the Middle Jurassic led to the development of a “Rocas Verdes-type” continental back-arc inboard of the arc, which is now preserved as the Arperos basin in the central and southern Guerrero terrane and the host rocks for the San Nicolas and El Salvador deposits farther to the north in southeastern Zacatecas State. We have determined Pb isotope compositions of sulfides from VMS deposits and several other styles of mineralization from throughout the Guerrero terrane. A compilation of these and previously published data shows that isotopic compositions from the various VMS deposits fall in a relatively confined field that overlaps the average upper crustal growth curve. This suggests that metals were mainly sourced from continentally derived sedimentary strata that are interlayered with or immediately underlie the host volcanic section and/or from the host volcanic rocks themselves (or subvolcanic intrusive equivalents) that had assimilated significant quantities of upper crustal material either at the point of origin or during subsequent ascent and eruption. Lead isotope compositions from the Francisco I Madera deposit in Zacatecas State indicate that most of the sulfides in this deposit are epigenetic rather than syngenetic in origin and are likely Tertiary in age.

Fluid Temperature and Salinity Characteristics of the Matagami Volcanogenic Massive Sulfide District, Quebec

Fluid inclusions hosted within various lithologic units of the >40 million metric ton (Mt) Matagami district, Abitibi greenstone belt, preserve samples of Archean volcanogenic massive sulfide (VMS) and postvolcanogenic massive sulfide hydrothermal fluid. Microthermometric measurements on ore-hosted primary two-phase liquid-vapor inclusions from Matagami’s south limb deposits indicate that the VMS hydrothermal fluid was highly saline (16.2 ± 4.7 wt % NaCl-CaCl2 equiv, 1σ, n = 230) and of moderate temperature (trapping temperature, Tt = 208° ± 32°C, 1σ, n = 230). A fluid with these characteristics is capable of transporting ~5 × 10−4 m (30 ppm) Zn as ZnCl3 − and ZnCl4 2− chloride complexes. However, the low temperature of this fluid precluded efficient Cu transport (≤ 3 ppm), which may in part explain the relatively Cu poor nature of the Matagami deposits. Calculated densities of this ore fluid as high as 1.10 g/cm3 are consistent with a bottom-hugging brine model. However, a subset of the data indicate a fluid less dense than ambient seawater, suggesting that buoyant hydrothermal plumes were also present. A microthermometrically determined high CaCl2 content (XNaCl <0.55, molar Na/Ca = 2.3/1) for the VMS ore-hosted primary fluid is consistent with an Archean hydrothermal fluid more Ca-rich than modern-day seawater. Quartz-epidote veins located in the hydrothermal cracking zone of the Bell River Complex host primary liquid-vapor-halite inclusions. These inclusions are interpreted to be samples of the deep-seated equivalent to the VMS ore-hosted hydrothermal fluid described above. Microthermometry indicates that these inclusions trapped a high-temperature brine (Tt = 373° ± 44°C, 1σ , n = 92; 38.2 ± 1.9 wt % NaCl equiv, 1σ , n = 92). We interpret this brine to be a phase-separated product of (modified) model seawater (3.2 wt % NaCl), an exsolved magmatic fluid, or a combination thereof, deep within the hydrothermal system at 650° to 670°C and a near-lithostatic pressure of 90 MPa. Phase separation and subsequent convection lowered the temperature of the brine prior to its entrapment within the hydrothermal cracking zone. The occurrence of high-temperature brine overlain by lower temperature/salinity fluid suggests a two-cell convection model, consistent with metal mass-balance calculations for the south limb. The high salinity of the ore-hosted fluid inclusions indicates two possibilities: (1) a significant amount of brine was incorporated into the upper cell and mixed with heated seawater during convection; (2) Archean seawater itself was very saline and of variable salinity. With the cooling of the Bell River Complex, lower temperature fluids, dominantly of seawater origin, circulated deep within the hydrothermal system. Modified by water-rock interaction, yet not phase separated, these fluids sealed off the remnant permeability of the fracture network of the hydrothermal cracking zone and were locally trapped as secondary liquid-vapor fluid inclusions (homogenization temperature, Th = 242° ± 17°C; 9.1 ± 1.6 wt % NaCl equiv, 1σ , n = 14) hosted within the Bell River Complex quartz-epidote vein material. Post- and/or waning-stage VMS hydrothermal activity is evident from the presence of quartz-epidote veins crosscutting Wabassee Group hanging-wall rocks. Microthermometry on quartz-hosted primary liquid-vapor fluid inclusions suggests that this activity occurred at relatively low temperatures (Th = 76°–177°C, n = 212), over a wide range of salinity (6.0–32.4 wt % NaCl-CaCl2 equiv, n = 212), and with a high apparent CaCl2 content (XNaCl <0.06). These fluid inclusion data illustrate the importance of subsea-floor chemical and physical processes directly related to metal transport and deposition in VMS systems. In particular, phase separation deep within the hydrothermal system is interpreted as a key process for generating saline brines capable of forming significant ore deposits.

Oxygen isotope mapping in the Panorama VMS district, Pilbara Craton, Western Australia: applications to estimating temperatures of alteration and to exploration

Whole rock oxygen isotope data are presented for the Panorama district, in the Archean Pilbara Craton of Western Australia, where near-perfect exposure reveals a cross section through a complete volcanogenic massive sulfide (VMS) hydrothermal alteration system. The δ18O values decrease with depth in the volcanic pile, across semi-conformable alteration zones, to values below 6‰ immediately above a large (180 km2) subvolcanic intrusion. Altered rocks in the upper parts of the subvolcanic intrusion have lower δ18O values (6–8‰) than least altered granite (8‰), apart from sericite–quartz altered zones, which are slightly higher (8–10‰). Corridors of low δ18O values crosscut this regional zonation, and are coincident with transgressive feldspar-destructive alteration zones, which underlie VMS mineralization. The whole rock oxygen isotope distribution patterns are interpreted to represent alteration temperature, where high δ18O values correspond to low temperature alteration and low δ18O values correspond to high temperature alteration. Alteration temperatures, which were calculated using modal alteration mineral abundances and an assumed fluid δ18O, are consistent with this interpretation. Increasing temperatures with depth in the volcanic pile and high temperatures in transgressive corridors leading up to VMS deposits, are consistent with a convective hydrothermal model, in which heat from the subvolcanic intrusion drove seawater through the volcanic pile. Granite-hosted sericite–quartz alteration zones are 18O-enriched, and are tentatively interpreted to have formed from a mixed magmatic-evolved seawater fluid.

Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana Terrane, eastern Alaska Range, Alaska

A belt of volcanogenic massive sulfide deposits extends for over 150 km along the southern margin of the Yukon-Tanana terrane of the eastern Alaska Range. Located north of the Denali fault, the Yukon-Tanana terrane forms a major basement unit in east-central Alaska. The volcanogenic massive sulfide deposits are primarily in the Jarvis Creek Glacier subterrane, one of the highest structural-stratigraphic levels of the Yukon-Tanana terrane. The Jarvis Creek Glacier subterrane consists of a volcanogenic massive sulfide-bearing metavolcanic rock member and a metasedimentary rock member. The volcanogenic massive sulfide deposits and enclosing metavolcanic and metasedimentary schists exhibit two periods of regional metamorphism and penetrative deformation: an older, Early Cretaceous, amphibolite facies event and a younger, mid-Cretaceous lower greenschist facies event.From northwest to southeast, the major volcanogenic massive sulfide districts are the Bonnifield, Trident Glacier, and Delta. The Bonnifield district contains massive sulfide deposits in metavolcanic and metasedimentary rocks that are correlated with the Jarvis Creek Glacier subterrane. The 25-km-long, northwest-trending Trident Glacier district contains several occurrences consisting of massive pyrrhotite and lesser pyrite, minor chalcopyrite with or without sphalerite, and galena and arsenopyrite in south-dipping massive sulfide pods, lenses, and stringers that parallel the foliation of the enclosing schists. Host-rock protoliths were mainly andesite, dacite, rhyodacite tuffs and flows, shale, and limestone, with lesser quartz-rich keratophyre flows and tuffs, volcanic graywacke, and siltstone.The Delta district contains at least 35 deposits in an 800-km 2 area. Thirty-one of the deposits define four mineral trends, 5 to 32 km long, that are subparallel to the west-northwest-striking, southwest-dipping structures and lithologies. The deposits consist of layers and zones containing varying amounts of massive to disseminated pyrite and pyrrhotite, and lesser amounts of chalcopyrite, galena, sphalerite, and arsenopyrite. The larger deposits range between 1.0 and 12.4 million metric tons with combined grades of 5.6 percent total metal (Cu, Pb, Zn), and Ag and Au contents of 56 and 1.9 g/metric ton, respectively. Hostrock protoliths are similar to those found in the Trident Glacier district, but some of the deposits are spatially associated with diorite or gabbro sills that exhibit lower greenschist facies metamorphism and deformation.Sulfur isotope values of 26 pyrrhotite separates from the Trident Glacier district occurrences average 7.7 per mil, the mean value of seven pyrite separates is 8.1 per mil. Forty-one Delta district pyrite separates average 5.9 per mil and ranges of values of the four northwest-trending trends overlap. The relative consistency of sulfur isotope values of iron-bearing sulfides in the Trident Glacier district, and the apparent lithologic relationship of isotope values in the Delta district, may reflect original-rock sulfur isotope values. Alternatively, the 34 S-enriched values may have resulted from the addition of reduced seawater sulfate to the hydrothermal fluid. The sulfide accumulations are interpreted as kuroko-like deposits that formed along a Devonian continental margin igneous arc. Deeper level portions of the arc are exposed in structurally lower levels of the Yukon-Tanana terrane to the north and contain metamorphosed granodiorite and granite plutons of Devonian to Mississippian age. U-Pb isotope data from zircons from metavolcanic rocks hosting the sulfide bodies indicate an age of about 370 Ma. The 207 Pb/ 204 Pb ratios for sulfides from the three districts are similar and indicate lead was in part derived from first-cycle sediments that were eroded largely from Precambrian crystalline rocks. The 207 Pb/ 204 Pb ratios from feldspars in Devonian metavolcanic rocks are similar to those in sulfides and indicate incorporation of radiogenic Precambrian lead into the Devonian magmas. A Precambrian crystalline source for detritus in metasedimentary rocks is indicated by locally abundant quartzite containing approximately 2.3 Ga zircons which were derived from the Hudsonian-age block of the Canadian Shield in southern British Columbia.Cretaceous greenschist facies retrogressive metamorphism and intense penetrative deformation is indicated by reequilibration of U-Pb isotope ratios, metamorphic Rb-Sr mineral isochrons, and K-Ar biotite and muscovite ages of 110 to 115 Ma. The data indicate that the mid-Cretaceous retrogressive metamorphism and associated intense deformation occurred during the accretion of the Yukon-Tanana terrane to Alaska.