Sediment-hosted Massive Sulfide Deposits Research Articles

Numerical Simulation of the Self‐Organizational Origin of Concentrically Zoned Aggregates of Siderite and Pyrite in Sediment‐Hosted Massive Sulfide Deposits

AbstractConcentrically zoned pyrite aggregates (CZPA) are common in sediment‐hosted massive sulfide (SHMS) deposits and have been widely used to interpret the ore‐forming processes. There is considerable uncertainty, however, over the formation of aggregates that are oscillatorily zoned and contain randomly‐orientated pyrite microcrystals. Guided by the results of examination of the micro‐textures of CZPA and in‐situ chemical analyses, we conducted a quantitative diffusion‐reaction simulation to assess the mechanism of CZPA formation. Our study shows that oscillatory zoning results from the feedback between the diffusion of reactants and the nucleation‐growth of Fe‐sulfides. Externally derived Fe2+ diffuses into the early diagenetic sediments containing decomposing organic matter (2CH2O + SO42− = 2HCO3− + H2S) and reacts with H2S to form a pyrite layer via an intermediate pathway (Fe2+ + H2S → FeS + 2H+, FeS + H2S → FeS2 + H2). This growth of pyrite layers depletes the local concentration of reactants and suppresses nucleation until the diffusive reaction front advances and another layer is formed. Intermediate phases, for example, mackinawite, nucleate instead of pyrite, because of their greater ease of nucleation due to the low surface tension, and lead to the domination of nucleation over growth. The nucleation of mackinawite and occurrence of siderite in the CZPA are consistent with a low temperature, high pH, anoxic early diagenetic environment. Our study demonstrates that CZPA in SHMS deposits are formed by intrinsic self‐organizational processes rather than by extrinsic changes of ore‐forming fluids. The CZPA in SHMS deposits are thus indicative of their diagenetic origin with Fe2+ infiltrated and diffused from hydrothermal fluids into the sediments.

Thallium isotopes as indicators of ore mineralization at the Zn-rich sediment-hosted massive sulfide TL Deposit, British Columbia, Canada

Few studies have focused on the application of the Tl isotopic system for geochemical exploration. We report ε 205 Tl values of rock samples from the TL Deposit, British Columbia, Canada – a sediment-hosted massive sulfide (SHMS) deposit with characteristics of a Broken Hill-type deposit – and investigate relationships with major and trace element geochemistry. Maps generated using Tl isotope and trace element data indicate that ε 205 Tl values can potentially be used to fingerprint ore mineralization at the TL Deposit. The sources of Tl and other metals (Ag, Pb, Zn) are assessed using Tl isotope data. Measured ε 205 Tl values exhibit positive correlations with Pb, sedimentary exhalative metal index (Zn + 100*Pb + 100*Tl), and the redox proxy, U/Th, and negative correlations with Be, Cd, Ce, La, Ni and Th. Individual lithologies have distinct Tl isotopic compositions. Metal-rich heavily altered samples have relatively high ε 205 Tl values (−5.0 to −2.5 ε -units) reflecting the euxinic conditions of the global Paleoproterozoic ocean and hydrothermal influence. Samples with lower ε 205 Tl values (−15 to −7.6 ε -units) reflect a combination of their mineralogy (phyllosilicate minerals such as biotite and clinochlore), Tl from sediments reflecting the Tl isotopic composition of modern seawater, and possible low-temperature alteration processes. Samples with high Pb and Ag contents have high ε 205 Tl values, indicating a hydrothermal origin of these metals, whereas Zn is highest in samples with low ε 205 Tl values, indicating a low-temperature or sedimentary origin. Thallium isotopes, paired with conventional geochemical data, show promise as a useful tool for exploration of SHMS deposits with Broken Hill-type characteristics. Supplementary material: Major, minor and trace element contents of samples and reference materials, blank values, correlation coefficient values, and XRD patterns are available at https://doi.org/10.6084/m9.figshare.c.6370671 Thematic collection: This article is part of the Geochemical processes related to mined, milled, or natural metal deposits collection available at: https://www.lyellcollection.org/topic/collections/geochemical-processes-related-to-mined-milled-or-natural-metal-deposits

Geology and Genesis of the Giant Gorevskoe Pb-Zn-Ag Deposit, Krasnoyarsk Territory, Russia

Abstract The Gorevskoe Pb-Zn-Ag mine is currently the largest producer of Pb and Zn in Russia, exploiting one of the largest sediment-hosted Pb-Zn deposits worldwide. Despite its size and economic importance, the Gorevskoe deposit remains poorly understood. It is located on the western margin of the Siberian craton within the Yenisei Ridge, a Neoproterozoic orogenic belt. Mineralization consists of three tabular orebodies that are in turn composed of multiple stacked stratiform to strata-bound lenses of galena-pyrrhotite-sphalerite-rich massive sulfide ore, hosted in organic-rich marine metalimestones and calcareous slates of Stenian to Tonian age (1,020 ± 70 Ma). Extensive Fe-Mg-Mn-carbonate alteration haloes surround the ore lenses. The Pb isotope signature of the deposit is consistent with derivation of Pb, and probably all associated metals, from an evolved crustal source at the time of formation of the host rocks. The sulfur isotope compositions of pyrrhotite, sphalerite, galena, arsenopyrite, and pyrite (δ34S = 16.0–20.4‰) do not vary considerably across the deposit and are within the range reported for contemporaneous seawater, indicating complete reduction of marine sulfate as the main source of sulfur. The available geologic and geochemical data indicate that the Gorevskoe deposit belongs to the sediment-hosted massive sulfide (SHMS) class of Zn-Pb deposits, with an affinity to Selwyn-type deposits. Hydrothermal mineralization appears to be temporally related to rifting and distal mafic volcanism in a passive margin setting. Geologic relationships suggest that the orebodies formed in a diagenetic environment. Furthermore, the predominance of primary pyrrhotite over pyrite as the major iron sulfide, the presence of abundant siderite, and the relatively homogeneous sulfur isotope signature of the ores indicate highly reducing conditions during ore formation. They also constrain the character of the metal-bearing fluid to be similarly reducing, and of moderate temperature (200°–300°C). Gorevskoe may thus be regarded as one of the world’s largest Selwyn-type SHMS deposits.

A NEW SUBSEAFLOOR REPLACEMENT MODEL FOR THE MACMILLAN PASS CLASTIC-DOMINANT Zn-Pb ± Ba DEPOSITS (YUKON, CANADA)

Abstract Sedimentary exhalative (SEDEX) deposits are a subset of sediment-hosted massive sulfide deposits and provide our dominant resource of Zn. In the SEDEX model, base metals (Zn, Pb, Fe) are hydrothermally vented into sulfidic (euxinic) seawater and deposited coevally with the organic-rich mudstone host rock, resulting in laterally extensive layered mineralization. In the Selwyn Basin (Canada) at Macmillan Pass, two deposits (Tom, Jason) are well preserved in a succession of Upper Devonian mudstones and are considered type-characteristic examples of the SEDEX deposit model. As with a number of SEDEX deposits, at Macmillan Pass barite is abundant in the succession hosting hydrothermal mineralization. Early work presented a hydrothermal model for barite formation, in which barite coprecipitated with base metal sulfides in a redox-stratified water column. Recently, however, studies have both proposed an alternative diagenetic model for barite formation and provided more precise constraints on the chemistry of the hydrothermal fluid that entered the vent complexes. Here, we present a new model for Macmillan Pass in which sulfide mineralization occurred entirely within the subsurface. The introduction of hot (300°C) hydrothermal fluids into the shallow subsurface (<1-km depth) resulted in the thermal degradation of organic matter and generated CO2; this promoted barite dissolution, which both provided a source of sulfate for thermochemical sulfate reduction and increased the porosity and permeability within the system. Importantly, there was clear potential for the development of positive feedbacks and self-organization between diagenetic and hydrothermal processes, resulting in highly efficient ore-forming systems. In contrast to the SEDEX model, alteration footprints will be controlled by the mass transfer involved in (barite) replacement reactions rather than hydrothermal venting, and exploration criteria at a district scale should strongly favor highly productive continental margins.

Siliciclastic-hosted zinc mineralization in the Proterozoic Vazante – Paracatu District, Brazil: Implications for metallogeny and sources of metals in sediment-hosted base metal systems

The Mesoproterozoic Upper Vazante Sequence is a mixed carbonate – siliciclastic marine succession, deposited between 1300 and 1100 Ma, which contains a variety of carbonate-hosted Zn silicate and Zn – Pb sulfide deposits in central Brazil. Carbonate-hosted Zn ± Pb mineralization is interpreted to have formed in two stages: one prior to the major deformation event and a second stage during the Brasiliano Orogeny (850 to 550 Ma). All known carbonate-hosted Zn-deposits and occurrences in the 250 km – long Vazante – Paracatu District are underlain by the Serra do Garrote Formation, a 500 + m-thick package made up mostly of phyllites and carbonaceous phyllites (>1% Total organic carbon) with minor interlayered meta-arenitic rocks. Our study revealed that siliciclastic rocks from the Serra do Garrote Formation contain horizons enriched in Zn (up to 0.46%).Siliciclastic-hosted zinc mineralization in the Serra do Garrote Formation is associated with pre-orogenic hydrothermal sulfide – quartz – chlorite layers and veinlets, which preferentially formed in carbonaceous and pyritic phyllites belonging to two subunits. The siliciclastic-hosted mineralization in these layers and veinlets is folded and crenulated (pre-orogenic). The geochemical signature of zinc-enrichment includes Cd, Cu, Hg, In, V ± Sb, Se, Mo, Re. Based on textural relationships and chemical U – Th – Total Pb dates of pre-orogenic and syn-orogenic monazite, sulfide mineralization in the Serra do Garrote Formation formed between 1200 and 650 Ma. Textural features indicate that base metals were locally remobilized within the Serra do Garrote Formation during the Brasiliano Orogeny, precipitating new generations of syn-orogenic sphalerite, chalcopyrite and pyrite. Mineralized zones in the Serra do Garrote Formation are distributed throughout the basin and have a similar geochemical signature as the overlying syn-orogenic carbonate-hosted base metal mineralization. However, there is no compelling evidence of depletion in the Serra do Garrote Formation, at the scale of this study, to suggest the ore-related metals have been leached from these siliciclastic rocks to be concentrated in the carbonate-hosted base metal deposits during the Brasiliano Orogeny. The findings of this study suggest that the Serra do Garrote Formation can be a potential target for siliciclastic-hosted base metal mineralization, similar to other Proterozoic sediment-hosted massive sulfide deposits.

Links between seawater paleoredox and the formation of sediment-hosted massive sulphide (SHMS) deposits — Fe speciation and Mo isotope constraints from Late Devonian mudstones

Many models of sediment hosted massive sulphide (SHMS) deposit formation invoke basin restriction events that resulted in long-term stagnation and anoxia, in which sulphidic (euxinic) conditions ultimately prevailed. Euxinic conditions are then thought to provide a chemical trap for hydrothermally exhaled base metals. Here, we present Fe speciation and Mo isotope data for organic-rich mudstones from two drill-holes intersecting Upper Devonian strata, deposited along the passive margin of ancestral North America. One drill-hole intersects a 35 m thick sequence of SHMS mineralisation, while the other intersects correlative, un-mineralised strata. All samples have high FeHR/FeT values (>0.38), indicating water-column anoxia. For the majority of samples in the un-mineralised drill-hole, the levels of pyritisation fall below the threshold typically used to define euxinic conditions (FePY/FeHR ≤ 0.70). In contrast, higher levels of pyritisation in the mineralised drill-hole (median FePY/FeHR = 0.86) likely developed via diagenetic pyrite enrichment. Whereas Pb and Zn are negatively correlated with Mo, Mo-U co-variation is consistent with Fe (oxyhydr)oxide particulate shuttling in the water-column. In addition, a weak correlation between TOC/P and Mo provides further evidence that Mo was sourced via authigenic, rather than hydrothermal, processes. The δ98Mo values (+0.66 to +1.02‰) are uniform between both drill-holes, and substantially lower than constraints for Late Devonian seawater (+1.5 to +2.0‰), consistent with Mo adsorption to Fe (oxyhydr)oxides. Collectively, the data provide evidence that local seawater was dominantly ferruginous (anoxic, non-sulphidic) at Macmillan Pass. Regional variability in the extent of ferruginous (low TOC/P) and euxinic (high TOC/P) conditions likely contributed to a balance between P regeneration and P enrichment that maintained nutrient availability and productivity in the Selwyn Basin during the Late Devonian. We argue that high primary productivity and enhanced organic carbon burial are key variables for promoting sulphate reduction in the sub-surface. Moreover, how such conditions are maintained over long periods of basin evolution is more important for producing effective metal traps in SHMS systems, rather than a specific, localised redox condition of seawater (i.e. euxinia).

The north-subducting Rheic Ocean during the Devonian: consequences for the Rhenohercynian ore sites

Base metal mining in the Rhenohercynian Zone has a long history. Middle-Upper Devonian to Lower Carboniferous sediment-hosted massive sulfide deposits (SHMS), volcanic-hosted massive sulfide deposits (VHMS) and Lahn-Dill-type iron, and base metal ores occur at several sites in the Rhenohercynian Zone that stretches from the South Portuguese Zone, through the Lizard area, the Rhenish Massif and the Harz Mountain to the Moravo-Silesian Zone of SW Bohemia. During Devonian to Early Carboniferous times, the Rhenohercynian Zone is seen as an evolving rift system developed on subsiding shelf areas of the Old Red continent. A reappraisal of the geotectonic setting of these ore deposits is proposed. The Middle-Upper Devonian to Early Carboniferous time period was characterized by detrital sedimentation, continental intraplate and subduction-related volcanism. The large shelf of the Devonian Old Red continent was the place of thermal subsidence with contemporaneous mobilization of rising thermal fluids along activated Early Devonian growth faults. Hydrothermal brines equilibrated with the basement and overlying Middle-Upper Devonian detrital deposits forming the SHMS deposits in the southern part of the Pyrite Belt, in the Rhenish Massif and in the Harz areas. Volcanic-hosted massive sulfide deposits (VHMS) formed in the more eastern localities of the Rhenohercynian domain. In contrast, since the Tournaisian period of ore formation, dominant pull-apart triggered magmatic emplacement of acidic rocks, and their metasomatic replacement in the apical zones of felsic domes and sediments in the northern part of the Iberian Pyrite belt, thus changing the general conditions of ore precipitation. This two-step evolution is thought to be controlled by syn- to post-tectonic phases in the Variscan framework, specifically by the transition of geotectonic setting dominated by crustal extension to a one characterized by the subduction of the supposed northern slab of the Rheic Ocean preceding the general Late Variscan crustal shortening and oroclinal bending.

The Xitieshan volcanic sediment-hosted massive sulfide deposit, North Qaidam, China: Geology, structural deformation and geochronology

The Xitieshan deposit (~64Mt at 4.86% Zn, 4.16% Pb, 58g/t Ag, and 0.68g/t Au) is hosted by the Middle to Late Ordovician Tanjianshan Group of the North Qaidam tectonic metallogenic belt, NW China. This belt is characterized by island arc volcanic, ultra-high pressure (UHP) metamorphic and ophiolitic rocks. The Tanjianshan Group constitutes a succession of metamorphosed bimodal volcanic and sedimentary rocks, which are interpreted to have formed on the margin of a back-arc ocean basin between the Qaidam block and the Qilian block.Four stratigraphic units are identified within the Ordovician Tanjianshan Group. From northeast to southwest they are: 1) unit a, or the lower volcanic-sedimentary rocks, comprising bimodal volcanic rocks (unit a-1) and sedimentary rocks (unit a-2) ranging from carbonates to black carbonaceous schist; 2) unit b, or intermediate-mafic volcaniclastic rocks, characterized by intermediate to mafic volcaniclastic rocks intercalated with lamellar carbonaceous schist and minor marble lenses; 3) unit c, a purplish red sandy conglomerate that unconformably overlies unit b, representing the product of the foreland basin sedimentation during the Early Silurian; 4) unit d, or mafic volcanic rocks, from base to up, comprising the lower mafic volcaniclastic rocks (unit d-1), middle clastic sedimentary rocks (unit d-2), upper mafic volcaniclastic rocks (unit d-3), and uppermost mafic volcanic rocks (unit d-4). Unit a-2 hosts most of the massive sulfides whereas unit b contains subordinate amounts.The massive stratiform lenses constitute most of the Xitieshan deposit with significant amount of semi-massive and irregularly-shaped sulfides and minor amounts in stringer veins. Pyrite, galena and sphalerite are the dominant sulfide minerals, with subordinate pyrrhotite and chalcopyrite. Quartz is a dominant gangue mineral. Sericite, quartz, chlorite, and carbonate alteration of host rocks accompanies the mineralization.U-Pb zircon geochronology yields three ages of 454Ma, 452Ma and 451Ma for the footwall felsic volcanic rocks in unit a-1, sedimentary host rocks in unit a-2 and hanging-wall unit b, respectively. The Xitieshan deposit is considered to be coeval with the sedimentation of unit a-2 and unit b of the Tanjianshan Group. The Xitieshan deposit has been intensely deformed during two phases (main ductile shear and minor ductile-brittle deformation). The main ductile shear deformation controls the general strike of the ore zones, whereas minor deformation controls the internal geometry of the ore bodies. 40Ar-39Ar age of muscovite from mylonitized granitic gneisses in the ductile shear zone is ~399Ma, which is interpreted to date the Xitieshan ductile shear zone, suggesting that Early Devonian metamorphism and deformation post-dated the Tanjianshan Group.The Xitieshan deposit has many features similar to that of the Bathurst district of Canada, the Iberian Pyrite Belt of Spain, the Wolverine volcanogenic massive sulfide deposit in Canada. Based on its tectonic setting, host-rock types, local geologic setting, metal grades, geochronology, temperatures and salinities of mineralizing fluid and source of sulfur, the Xitieshan deposit has features similar to sedimentary exhalative (SEDEX) and VMS deposits and is similar to volcanic and sediment-hosted massive sulfide (VSHMS) deposits.

The Importance of Siliceous Radiolarian-Bearing Mudstones in the Formation of Sediment-Hosted Zn-Pb ± Ba Mineralization in the Selwyn Basin, Yukon, Canada

Sediment-hosted massive sulfide (SHMS) mineralization occurred in three main episodes during the evolution of the Selwyn Basin, Yukon, and Northwest Territories, Canada. In the Macmillan Pass district (Tom and Jason deposits), the Upper Devonian strata of the Earn Group host the youngest, least deformed examples of SHMS mineralization. The Earn Group comprises gray to black, homogeneous mudstones, which have been deformed in the Cordilleran fold-and-thrust belt, are weathered and poorly exposed; this makes stratigraphic correlations and the recognition of prospective units very challenging. The host rock (siliceous mudstone) and mineralized intervals at the Tom and Jason Zn-Pb ± Ba deposits have been sampled from drill core. Major element chemistry and total organic carbon (TOC) data are presented for the host-rock samples from two drill holes (TYK-5, n = 18; 76-17, n = 19). All mudstones contain between 2 and 5 wt % TOC and 67 to 83 wt % SiO2. Subtle alteration, marked by potassium loss (K2O/Al2O3 50% radiolarians by volume) within the mineralized horizons and in barren, time equivalent strata, and therefore are a useful stratigraphic marker unit in otherwise homogeneous, featureless fine-grained mudstones. The presence of highly siliceous, carbonaceous mudstone units (with preserved radiolarians) suggests that in the Late Devonian there is a link between biological productivity in the water column, the resultant sedimentation, and the SHMS deposits. These biosiliceous lithologies may be particularly favorable host rocks to SHMS mineralization for the following reasons: (1) high porosities during early diagenesis preserve volume and enhance permeabilities in partially lithified sediments, which are important parameters for subseafloor replacement mineralization; (2) these units are rich in organic materials due to their deposition in productive settings, and have a high capacity for the generation of sulfide via sulfate reduction. Therefore, we suggest there is a link between times of high biological activity and productivity in oceans, sediments derived from those processes, and SHMS deposits.

More than a trace of oxygen: Ichnological constraints on the formation of the giant Zn-Pb-Ag ± Ba deposits, Red Dog district, Alaska

Sediment-hosted massive sulfide (SHMS) deposits are an important source of global zinc resources, and the Red Dog Pb-Zn-Ag ± Ba district in Alaska (USA) contains giant deposits of this type. The existing model for ore formation at Red Dog involves early diagenetic replacement of sediment deposited in a restricted basin with stratified suboxic bottom waters. We present new observations of trace fossils Schaubcylindrichnus ichnospecies (isp.) and Chondrites isp. in several Red Dog deposits. The presence of the trace fossils, the size of the largest burrows, and the pervasiveness of the ichnofabric indicate that at least some intervals of the host sediment were deposited in an oxygenated middle to outer shelf environment. The burrow linings and infill are replaced by barite, hydrothermal quartz, and sulfide minerals, and the lack of compaction suggests that mineralization was diagenetically early. To reconcile these data with those from previous regional sedimentological and lithogeochemical studies, we propose a new model whereby the ore-hosting sediment was deposited in a shelfal setting in which redox conditions were affected by a fluctuating oxygen minimum zone. The strong spatial correlation between bioturbation and Red Dog SHMS deposits suggests that the presence of trace fossils may have played an important role in controlling the flow of ore-forming fluids by increasing host sediment permeability.

Distribution, Mineralogy, and Geochemistry of Selenium in Felsic Volcanic-Hosted Massive Sulfide Deposits of the Finlayson Lake District, Yukon Territory, Canada

Three significant base metal deposits were recently discovered in the Finlayson Lake district, Yukon Territory, Canada; however, widely varying selenium contents (e.g., 1,100 ppm avg Se at Wolverine, 200 ppm avg Se at Kudz Ze Kayah, and 7 ppm avg Se at GP4F) have hindered their rapid exploitation. Observations and data from the deposits in the Finlayson Lake district show a strong positive correlation between high-temperature (>300°C), copper-rich sulfide assemblages at the base of volcanic-hosted massive sulfide (VHMS) and the selenium content of all sulfide minerals. Thermodynamic calculations for selenide and sulfide minerals indicate significantly higher temperatures of formation and lower solubilities of selenide versus sulfide minerals, which is consistent with petrographic observations and can explain the distribution of selenium in zone-refined VHMS and volcanic sediment-hosted massive sulfide deposits. Volumetric mass-balance calculations indicate that some selenium could be sourced from contemporaneous volatile degassing of SeO 4 , its rapid reduction to H 2 Se, and subsequent entrainment into hydrothermal ore fluid. However, estimates for the Kudz Ze Kayah and Wolverine deposits suggest an additional large Se reservoir. Volumetric mass-balance estimates indicate that contemporaneous volcanic rocks and black shales are likely selenium reservoirs for the Kudz Ze Kayah and Wolverine deposits, respectively.

Distinctive features of Late Palaeozoic massive sulphide deposits in South China

More than 20 sediment-hosted massive sulphide deposits occur in Late Palaeozoic basins in South China. These deposits are accompanied by a certain amount of volcanic rocks in the host sequence and are economically important for their Cu, Pb, Zn, Au and Ag reserves. The deposits and their host strata were commonly intruded by Mesozoic granitoids. Remobilisation of sedimentary ores and magmatic hydrothermal overprinting processes resulted in the coexistence of massive sulphides with vein-, skarn- and porphyry-type orebodies in the same region or within a single deposit. The ore-containing basins occur in different tectonic settings. The Lower Yangtze basin occurs on a passive continental margin, where the deposits are high in Cu and Au with minor Pb and Zn and recoverable Ag, Co and Mo. The ores have a lower concentration of radiogenic lead, and δ 34S values close to zero. Fluid inclusions are highly saline and Na-rich. Fluids and metals of the Lower Yangtze Region are interpreted to have been derived essentially from deep sources including the Precambrian basement. By contrast, basins of the Nanling Region formed in an intracontinental setting developed on a folded Caledonian basement. These deposits are higher in Pb, Zn, Sn and W, as well as Cu, with recoverable Ag, Sb, Hg, U, Bi, Tl and Mo. The ores are characterised by a higher concentration of radiogenic lead and a wide variation of δ 34S composition. Fluid inclusions have lower salinities and higher K +/Na + ratios. Fluids are considered to have been sourced substantially from seawater by convection. Metals for the Nanling deposits were essentially derived from the Caledonian basement by leaching. The contrast in ore composition between these two regions appears to have been controlled by differences in basement composition of the ore-forming basins.

Formation of modern and Paleozoic stratiform barite at cold methane seeps on continental margins: Comment and Reply

34 S values within 2‰ of contemporaneous seawater sulfate, and (3) associated polymetallic sulfi de deposits. These criteria take into account spreading-ridge hydrothermal systems that can deposit barite in association with volcanogenic massive sulfi de deposits of Zn, Cu, and Pb. They do not apply to sedimentary-exhalative hydrothermal systems that can form both bedded barite deposits and the sediment-hosted massive sulfi de deposits of Zn, Pb, and Ag (±barite) that account for the bulk of the world’s known Zn and Pb reserves. Whereas spreading-ridge hydrothermal systems arise from seawater convection through hot volcanic rocks, sedimentary-exhalative hydrothermal systems arise from the ascent of basinal brines through sedimentary sequences into intracratonic or continental margin seas. There are barite deposits throughout the world that are clearly products of sedimentary-exhalative hydrothermal systems and show none of the criteria set forth by Torres et al. as diagnostic of a hydrothermal origin. To cite just a few examples, at Jason and Tom in western Canada, at Meggen and Rammelsberg in Germany, and at Red Dog in northwest Alaska, barite occurs both intergrown with sulfi de minerals and as barite-only deposits above or lateral to massive sulfi de bodies. At all these localities, the barite contains Sr that is more radiogenic than contemporaneous seawater Sr (Turner, 1991; Maynard et al., 1995; Ayuso et al., 2000) refl ecting scavenging of Sr and other elements from continental detritus in underlying sediments. The barites do not show δ 34

40Ar/39Ar Dating of Zn-Pb-Ag Mineralization in the Northern Brooks Range, Alaska

The 40 Ar/ 39 Ar laser step-heating method potentially can be used to provide absolute ages for a number of formerly undatable, low-temperature ore deposits. This study demonstrates the use of this method by determining absolute ages for Zn-Pb-Ag sediment-hosted massive sulfide deposits and vein-breccia occurrences found throughout a 300-km-long, east-west–trending belt in the northern Brooks Range, Alaska. Massive sulfide deposits are hosted by Mississippian to Pennsylvanian(?) black carbonaceous shale, siliceous mudstone, and lesser chert and carbonate turbidites of the Kuna Formation (e.g., Red Dog, Anarraaq, Lik (Su), and Drenchwater). The vein-breccia occurrences (e.g., Husky, Story Creek, West Kivliktort Mountain, Vidlee, and Kady) are hosted by a deformed but only weakly metamorphosed package of Upper Devonian to Lower Mississippian mixed continental and marine clastic rocks (the Endicott Group) that stratigraphically underlie the Kuna Formation. The vein-breccias are mineralogically similar to, but not spatially associated with, known massive sulfide deposits. The region’s largest shale-hosted massive sulfide deposit is Red Dog; it has reserves of 148 Mt grading 16.6 percent zinc, 4.5 percent lead, and 77 g of silver per tonne. Hydrothermally produced white mica in a whole-rock sample from a sulfide-bearing igneous sill within the Red Dog deposit yielded a plateau age of 314.5 Ma. The plateau age of this whole-rock sample records the time at which temperatures cooled below the argon closure temperature of the white mica and is interpreted to represent the minimum age limit for massive sulfide-related hydrothermal activity in the Red Dog deposit. Sulfide-bearing quartz veins at Drenchwater crosscut a hypabyssal intrusion with a maximum biotite age of 337.0 Ma. Despite relatively low sulfide deposition temperatures in the vein-breccia occurrences (162°–251°C), detrital white mica in sandstone immediately adjacent to large vein-breccia zones was partially to completely recrystallized. The 40 Ar/ 39 Ar age spectra and inverse isochron plots of the multicomponent whole-rock sandstone samples are more complex than those of single minerals. However, different minerals have different Ca/K and Cl/K ratios and closure temperatures, and these properties were used to identify portions of spectra dominated by argon release from specific minerals. 40 Ar/ 39 Ar laser step-heating analyses of Late Devonian sandstone whole rocks produced spectra that record a two-stage resetting history: a Carboniferous hydrothermal event first and later Mesozoic to Tertiary events, which are in agreement with geologic constraints. The 40 Ar/ 39 Ar ages and the similar mineralogy, lead isotope composition, and relative stratigraphic positions support the interpretation that the shale-hosted massive sulfide deposits and most vein-breccia occurrences are temporally and genetically related, and that they are different expressions of Carboniferous basinal dewatering.

Boron isotope geochemistry of hydrothermal ore deposits in China: a preliminary study

A preliminary boron isotope study of tourmaline was carried out for a number of sediment-hosted massive sulfide deposits and granite-related hydrothermal Sn deposits in China. Tourmalines from the Yindongzi-Tongmugou (Devonian) and the Qingchengzi (Proterozoic) Pb-Zn deposits show δ11B values of −7.6 to −13.7‰, which are within the typical δ11B ranges of massive sulfide deposits worldwide. At Dachang Sn-polymetallic deposit, the hydrothermal fluids responsible for the formation of banded tourmalinites have remarkable different δ11B values (−1 to −5‰) from those of quartz-tourmaline veins near the granite contacts (∼ −10‰), which indicate their different origins. Together with other geological and geochemical evidence, the banded tourmalinites at Dachang are of submarine hydrothermal exhalative origin. Overall, the boron isotopic compositions of tourmaline from massive sulfide deposits are mainly controlled by source rock compositions, and non-marine evaporites may have played an important role in the massive sulfide ore formation in same cases. Other controlling factors include temperatures, water/rock ratios, and seawater entrainments.Tourmalines from the granite-related Yidong Sn deposit display a gradual increase in δ11B value from within the granite (−13.9‰), to the contact (−12.7 to −11.9‰) and the country rocks (−11.4 to −11.3‰). It is suggested that these variations reflect the boron isotope fractionation during magmatic degassing and magmatic-hydrothermal ore-forming processes with 11B preferentially partitioned into hydrothermal solutions. The similar and limited δ11B variations (−19.5 to −13.9‰) in granitic tourmalines from the granite-related Sn deposits in South China suggest that these Sn-rich granites may have similar source rocks from the basement of metasedimentary rocks and volcanic rocks in this region.

Drilling of sediment-hosted massive sulphide deposits at the Middle Valley and Escanaba Trough spreading centres: ODP Leg 169

Drilling of sediment-hosted massive sulphide deposits at the Middle Valley and Escanaba Trough spreading centres: ODP Leg 169

Proterozoic Pb isotope evolution in the Belt-Purcell Basin; constraints from syngenetic and epigenetic sulfide deposits

Sediment-hosted massive sulfide and Ag-Pb-Zn veins are two major deposit types which contain large concentrations of lead in the Middle Proterozoic Belt-Purcell basin. New Pb isotope data on the Sullivan sediment-hosted massive sulfide deposit and the St. Eugene Ag-Pb-Zn vein deposit are combined with existing data to provide a comprehensive overview of Proterozoic Pb isotope evolution in the Belt-Purcell basin. The Pb isotope ratios for sulfides in the Sullivan deposit form a linear array, the Sullivan array. Bedded sulfides plot within this array and no systematic change in Pb isotope ratios with stratigraphic position (time) is observed. The Sullivan array results from the mixing of leads from the upper crustal and/or orogen Pb reservoirs, represented by metasedimentary rocks of the lower Aldridge Formation in the footwall of the Sullivan deposit, with, perhaps, leads derived from the Precambrian basement of the Belt-Purcell basin. A zone within the massive sulfides of the Sullivan deposit has been altered to chlorite-pyrite-calcite. The center of the alteration zone is comprised of massive pyrite-calcite which has more radiogenic Pb isotope compositions, the Sullivan alteration. The Sullivan alteration apparently formed from the same Pb reservoirs as the Sullivan array, perhaps as a product of the East Kootenay orogeny (<1370 Ma), after a period of up to 108 m.y. which allowed radiogenic Pb growth. Ag-Pb-Zn occurrences in the Belt-Purcell basin form an array in Pb isotope space, the Coeur d'Alene array, which is not collinear with the Sullivan array; it is characterized by Pb isotope compositions less radiogenic than those of the Sullivan array. The Coeur d'Alene array is a mixture of leads derived from the upper crustal and mantle Pb reservoirs. It is proposed that mantle lead was introduced into the Belt-Purcell basin at 1468 Ma by the mafic Moyie sills. Leads in sediment-hosted massive sulfide deposits in middle Aldridge rocks and leads of the Coeur d'Alene array in epigenetic veins formed after intrusion of the Moyie sills but before East Kootenay orogeny metamorphism.

Sulfur isotope variability in sediment-hosted massive sulfide deposits as determined using the ion microprobe SHRIMP; II, A study of the H.Y.C. Deposit at McArthur River, Northern Territory, Australia

The H.Y.C. lead-zinc mineralization at McArthur River has long been regarded as a classic example of the sediment-hosted massive sulfide deposit category. Its pristine and fine-grained nature, however, has made it difficult to study as well as mine. A key issue concerning the genesis of this deposit has been to define the role of the host sediment pyrites (Py 1 and its overgrowths of Py 2 ) in relation to the localization of the base metal mineralization. Utilization of the SHRIMP ion microprobe for in situ analyses of the sulfur isotope compositions of Py 1 Py 2 , and base metal sulfides in the orebodies has shown that growth of Py 1 and Py 2 , with delta 34 S values ranging from -13 to +15 and -5 to +45 per mil, respectively, likely involved H 2 S formed by biogenic processes. On average, the delta 34 S values of Py 2 are about 15 per mil higher than those of Py 1 , suggesting that the pyrite types may often have formed sequentially from single batches of sulfate in an environment which became closed with respect to sulfate supply through time. In detail, the isotopic systematics of the pyrites can be quite complex with as much as 50 per mil or so variation in a distance of millimeters so that multiple events may be required to account for their formation. Such an origin for pyrite is entirely consistent with a geologic setting offering a restricted sulfate supply such as a semiemergent lacustrine or coastal lagoonal environment as described by Williams and Logan (1986).The sulfur isotope variability exhibited by the main base metal sulfides (sphalerite, galena, and chalcopyrite) is found to be more restricted than of either pyrite type, ranging from -5 to +14 per mil and excluding the few late-stage veins, the range is limited to -5 to +8 per mil. The delta 34 S values of the main-stage base metal sulfides may be lower, the same as, or higher than nearby Py 1 though they are generally lower than adjacent Py 2 . These data would suggest that formation of the base metal sulfides was distinct from that of Py 1 or Py 2 precipitation and that localization of the base metal sulfides did not involve biogenic sulfide either directly through use of residual microbially generated H 2 S( (sub (aq)) )or by replacement of Py 1 or Py 2 . The lack of isotopic communication between pyrite and base metal sulfides combined with the fundamental textural constraints of overgrowth and crosscutting relationships, preclude the onset of base metal mineralization prior to the cessation of pyrite growth. However, soft-sediment deformation features imply that in many cases mineralization occurred before the sediment was completely lithified.Linkage of the isotopic and textural characteristics with the geologic setting (Williams, 1979; Williams and Logan, 1986) produces a model where the base metal sulfide mineralization formed epigenetically (or diagenetically, i.e., Gustafson and Williams, 1981) from permeability-controlled migration of basinal brines along bedding. The presence of marcasite in the main-stage mineralization suggests that the brines were relatively low in pH and temperature. Occurrence of euhedral ankerite and prismatic quartz with the base metal sulfides implies the dolomitic sediment underwent extensive recrystallization during hydrothermal mineralization which included addition of Fe, CO 2 , and Si as well as the more obvious Zn, Pb, Cu, and possibly H 2 S. It is possible that early fluids were dominated by Fe, CO 2 , and SO 4 whereas later, warmer fluids carried more Zn, Pb, Cu, CO 2 , as well as H 2 S. In light of these new data and interpretations, all earlier models for the formation of the H.Y.C. deposit which involve syngenetic accumulation of fallout from hydrothermal brine pools and/or those which incorporate biogenically produced reduced sulfur in base metal mineralization must be rejected.

Origins of stratiform and stratabound Fe–Cu–Zn horizons in the Lower Proterozoic Moran Lake Group, Labrador Central Mineral Belt

Zn, Cu, and Fe are concentrated as stratiform and stratabound sulphide-rich beds in the Lower Proterozoic Warren Creek Formation, Moran Lake Group, central Labrador. Upper Member sedimentary rocks have a hydrothermal-like Fe enrichment but a dominantly hydrogenous signature as indicated by high Al2O3 relative to SiO2, and high Al and Fe relative to Mn. The Upper Member shales and sulphide-rich beds were deposited as Fe-rich pelagic sediments. The paucity of Mn and abundance of Fe in typical shale samples and lack of Cu, Pb, and Zn fractionation in stratiform massive sulphide beds that contain up to 4702 ppm Zn, 533 ppm Cu, and 15 ppm Pb suggest that deposition occurred in restricted brine pools (i.e., Cu and Zn were precipitated rapidly and were not fractionated). Stratabound sphalerite mineralization containing &gt; 3.7% Zn and 121 ppm Cu (but no Pb) was deposited in a porous lithology at the top of the Warren Creek Formation and represents a unique style of metal concentration.The stratiform deposits probably formed by advection of low-temperature connate waters in a situation typical of sediment-hosted exhalative mineralization (SEDEX). The potential for ore-grade metal concentration is apparently low because metal associations (Fe,Cu,Pb,Zn,Ba) are unlike those of sediment-hosted massive sulphide deposits, the sediments have a dominantly hydrogenous rather than hydrothermal signature, and the absolute grades of known occurrences are very low. The stratabound Zn deposit was probably formed by converting Zn-rich brines (≤ 200 °C) trapped during development of a hydrothermal convection system during a period of increased geothermal gradient. The potential for this type of occurrence in the Warren Creek area to reach economic grade is limited because the convection cells were shallow, ephemeral, and without the metal associations of sediment-hosted massive sulphide deposits.

Sulfur isotope variability in sediment-hosted massive sulfide deposits as determined using the ion microprobe SHRIMP; I, An example from the Rammelsberg orebody; discussion

Sulfur isotope variability in sediment-hosted massive sulfide deposits as determined using the ion microprobe SHRIMP; I, An example from the Rammelsberg orebody; discussion