Massive Sulfide Ore Research Articles

Cherts from the Yangla copper deposit, western Yunnan Province: geochemical characteristics and relationship with massive sulfide mineralization

Four layers of cherts were found for the first time in the Yangla copper deposit, western Yunnan Province. The cherts possessed the following geochemical characteristics: 1 Low TiO2 and Al2O3 contents, but high ore-forming element (e.g. Cu, Au, Ag) contents; 2 low total REE contents and clear negative Eu anomalies when normalized to chondrite similar to the REE contents and distribution patterns of associated massive sulfide ores; 3 silicon isotopic compositions of cherts in the Yangla deposit being the same as cherts and geyserite of hot-water sedimentary origin; 4 lead and sulfur isotopic compositions of cherts in the Yangla deposit being similar to those of the massive sulfide ores in the Yangla deposit; 5 Rb-Sr isochron age of cherts from the Yangla deposit being identical with that of host strata. Hence, we conclude that the cherts in the Yangla deposit are of hot-water sedimentary origin, which have a close relationship with the massive sulfide ores. The discovery of hydrothermal cherts from the Yangla copper deposit provides further evidence for the hydrothermal exhalative origin of the massive sulfide deposits.

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.

Exploration geophysics at the Pyhäsalmi mine and grade control work of the Outokumpu Group

The power of geophysics is often realized while surveying barren exploration holes. Integrated interpretation of borehole electromagnetic (EM) and lithogeochemical data led to the discovery of a new volcanogenic massive sulfide (VMS) ore deposit at 500 m depth in the Pyhäsalmi area, which belongs to the Main Sulfide ore belt in Finland. In the deep exploration program, wide‐band multifrequency EM ground surveys were successfully used to detect both new ore lenses and geological structures. Mise‐a‐la‐masse (MAM) borehole and ground surveys as well as borehole EM surveys were effectively used to correlate intersections between drill holes and to locate new orebodies. The latest modeling of MAM data resulted in an exploration target at 700 m depth. The use of geophysics for exploration has been extended to mine production at Outokumpu. Geophysical logging detects ore‐waste boundaries, reduces expensive core drilling, and obtains physical property information quickly on ore intersections. Depending on ore type, geophysical borehole logging can also be applied to classify mineralization, interpret lithology, and sometimes to transform physical responses to metal grades in ore. At the Pyhäsalmi zinc‐copper‐sulfur mine, density logging in percussion boreholes is used to locate mineable ore boundaries and to classify drillhole intersections as massive or semimassive sulfide ore types. Pyrrhotite‐bearing zones are separated from other sulfides by inductive conductivity logs. The use of geophysical logging for grade estimation and control has been most effective in the nickel mines at Enonkoski, Finland, and Namew Lake, Canada (using conductivity logs), and in the Kemi chromium mine, Finland (using gamma‐gamma density logs).

Crosshole seismic imaging for sulfide orebody delineation near Sudbury, Ontario, Canada

Crosshole seismic instrumentation based on a piezoelectric source and hydrophone detectors were used to gather seismograms between boreholes at the McConnell orebody near Sudbury, Ontario. High‐frequency seismograms were recorded across rock sections 50 to 100 m wide containing a continuous zone of massive sulfide ore. First‐arrival traveltimes obtained from a detailed scan were used to create a P-wave velocity tomogram that clearly delineated the ore zone. Refraction ray tracing on a discrete layer model confirmed the main features of the tomogram. The survey demonstrated that it is possible to conduct cost‐effective, high‐resolution crosshole seismic surveys to delineate ore bodies on a scale useful for planning mining operations.

Tatyanaite, a new platinum-group mineral, the Pt analogue of taimyrite, from the Noril'sk complex (northern Siberia, Russia)

Tatyanaite, a new mineral from Noril9sk (Siberia), is the Pt analogue of taimyrite. It is defined as the member(s) of the tatyanaite-taimyrite solid-solution series with Pt > Pd. Tatyanaite solid solution occurs in massive sulphide ore, which consists of chalcopyrite and subordinate pentlandite, pyrrhotite, and cubanite (or isocubanite). It occurs as central zones of large, elongate grains (up to ∼1 mm) and as aggregates of smaller grains associated with Ag-Au alloys. The associated minerals include unusually Pt-rich taimyrite [(Pd 1.25 Pt 0.86 )(Cu 0.85 Ni ) (Sn 1.01 Sb 0.02 )], atokite-rustenburgite, paolovite, froodite, sperrylite, maslovite, and galena. Cryptic zoning (Pt increases and Pd decreases toward the centre) and polysynthetic twins are characteristic. In reflected light, tatyanaite is pink with lilac tinge in air. Bireflectance is weak to distinct, from brownish pink to pinkish lilac. Anisotropy is distinct to moderate, from light brown to dark blue. Reflectance percentages in air and (in oil) are, for R 1 and R 2 , 470 nm 42.8, 44.1 (32.8, 33.3), 546 nm 49.5, 50.0 (37.6, 38.8), 589 nm 51.8, 54.6 (38.9, 39.9), and 650 nm 55.6, 56.8 (41.6, 44.2). It is ductile; the microhardness is VHN 20 = 292-348, mean of 327 kg/mm 2 . The average of nine electron-microprobe analyses gave Pt 45.38, Pd 19.53, Cu 10.62, Ni 0.15, Fe 0.03, Sn 23.02, Sb 0.27, sum 99.0 wt.%, corresponding to [(Pt 4.76 Pd 3.75 ) ∑8.51 Cu 0.48 ] ∑8.99 (Cu 2.94 Ni 0.05 Fe 0.01 ) ∑3.00 (Sn 3.96 Sb 0.05 ) ∑4.01 [or to (Pt 1.19 Pd 0.94 )(Cu 0.85 Ni 0.01 Fe ) (Sn 0.99 Sb 0.01 )]. The powder pattern is similar to that of synthetic Pd 9 Cu 3 Sn 4 , and, by analogy with the latter, it was indexed for an orthorhombic cell with a = 7.89(1) A, b = 4.07(1) A and c = 7.73(1) A, and V = 248(1) A 3 . The three strongest lines in the pattern are 2.283 (10, 212), 2.163 (4, 203) and 1.369 (3, 323). Tatyanaite-taimyrite formed from a late-stage liquid rich in noble metals, Cu and Sn.

Age and sources of the ore at Tharsis and Rio Tinto, Iberian Pyrite Belt, from Re-Os isotopes

Re-Os isotopes were used to constrain the source of the ore-forming elements of the Tharsis and Rio Tinto mines of the Iberian Pyrite Belt, and the timing of mineralization. The pyrite from both mines has simila]r Os and Re concentrations, ranging between 0.05–0.7 and 0.6–66 ppb, respectively. 187Re/188Os ratios range from about 14 to 5161. Pyrite-rich ore samples from the massive ore of Tharsis and two samples of stockwork ore from Rio Tinto yield an isochron with an age of 346 ± 26 Ma, and an initial 187Os/188Os ratio of about 0.69. Five samples from Tharsis yield an age of 353 ± 44 Ma with an initial 187Os/188Os ratio of about 0.37. A sample of massive sulfide ore from Tharsis and one from Rio Tinto lie well above both isochrons and could represent Re mobilization after mineralization. The pyrite Re-Os ages agree with the paleontological age of 350 Ma of the black shales in which the ores are disseminated. Our data do not permit us to determine whether the Re-Os isochron yields the original age of ore deposition or the age of the Hercynian metamorphism that affected the ores. However, the reasonable Re-Os age reported here indicates that the complex history of the ores that occurred after the severe metamorphic event that affected the Iberian Pyrite Belt massive sulfide deposits did not fundamentally disturb the Re-Os geochronologic system. The highly radiogenic initial Os isotopic ratio agrees with previous Pb isotopic studies. If the initial ratio is recording the initial and not the metamorphic conditions, then the data indicate that the source of the metals was largely crustal. The continental margin sediments that underlie the deposits (phyllite-quartzite group) or the volcanic rocks (volcanogenic-sedimentary complex) in which the ores occur are plausible sources for the ore-forming metals and should constrain the models for the genesis of these deposits.

Re–Os isotopic constraints on the genesis of the Sally Malay Ni–Cu–Co deposit, East Kimberley, Western Australia

Sulphide mineralisation and a cyclic suite of unmineralised troctolite and peridotite from the Sally Malay intrusion, East Kimberley, Western Australia, a possible Voisey's Bay analogue, were analysed for their trace element and platinum group element (PGE) abundances, Sm–Nd and Re–Os isotopic compositions. Massive sulphides have low common Os concentrations and high Re/Os ratios, similar to massive sulphide ores from the Duluth Complex and Voisey's Bay, and are consistent with low R factors (∼200) ( R factor=effective mass of silicate melt with which a given mass of sulphide melt has equilibrated). All Re–Os isotopic data from massive sulphides and unmineralised sections from the Sally Malay intrusion do not form a single isochron, possibly the result of variations in the R factor within the ore system. However, samples with only disseminated or matrix sulphides yield a Model 3 isochron of 1893±57 Ma, within error of the magmatic 1844±3 Ma U-Pb zircon age, suggesting that these samples have remained isotopically closed. Massive sulphide ores yield exceptionally radiogenic initial Os isotopic compositions ( γ Os=+950 to +1300), whereas disseminated sulphide mineralisation and unmineralised troctolite and peridotite yield lower γ Os values (+60 to +470). The Os isotopic data suggest that the Sally Malay intrusion and associated Ni–Cu–Co mineralisation were derived from crustally contaminated mantle melts. The Re–Os isotopic data support a two-stage model for the development of the Sally Malay intrusion ores. The first stage required crustal contamination of a basaltic melt at depth which promoted sulphide-saturation and the formation of an immiscible sulphide melt phase. The second stage involved transport of the sulphides in the host basaltic magma, resulting in a low R factor and a low metal tenor. Passive deposition of the magma produced a basal sulphide layer with a very radiogenic crustal Re–Os isotopic composition. The variable initial Os isotopic composition is likely to reflect a variable R factor, which renders isochron systematics difficult to interpret.

Development and application of new technology for the treatment of complex massive sulphide ores case study—Faro lead/zinc concentrator—Yukon

Faro is one of the largest lead/zinc mines in North America. Since start-up in the late 1960's the concentrator has processed more than 80 million tonnes of ore. The mine and concentrator are located 35 km north of the town of Faro, central Yukon, in the heart of the Anvil Range plateau. The Anvil Range plateau hosts a series of six fairly large orebodies, spread over a 40 km distance. Three of the ore bodies (Faro 1, 2 and 3) have been mined out. Two of the remaining three ore bodies (Vangorda and Grum) have been treated for the past six years. The ores are massive, finely-disseminated sulphides, which are difficult to treat. Because Grum and Vangorda ores do not respond well to the treatment methods used for Faro ores, a new method had to be developed and introduced in the plant. In the early 1980's, detailed laboratory studies were conducted to identify flowsheet and reagent schemes that would provide satisfactory plant metallurgical results. Using a new approach, based on a parametric model and Lakefield's data base of metallurgical know-how, a treatment process was introduced in the plant for treatment of Vangorda ore in 1986, and for the more refractory Grum ore in 1990. This one is currently being processed. This paper describes the development and application of the improved process for different ore types.

Radiocarbon-dated ferricrete provides a record of natural acid rock drainage and paleoclimatic changes

Ancient deposits of ferricrete (stratified iron oxyhydroxide or clastic sediment cemented by stratified iron oxyhydroxide) are present in headwater streams draining an historic mining district in the Rocky Mountains, USA. These deposits, which form under conditions of low pH and high metal concentrations, are present in streams draining many other mineralized areas. Radiocarbon dating of wood fragments entombed within the ferricrete indicates dates of deposition as old as 8840±50 years before present (b.p.). The ferricrete deposits provide evidence that natural, metal-rich acid rock drainage has been occurring in this mineralized region for thousands of years, due to the weathering and oxidation of exposed or near-surface massive sulfide ore deposits. The dating and chemical composition of the ferricrete deposits have applications to the environmental remediation of historic mining sites and to environmental issues related to natural background chemistry. In addition, the radiocarbon dates of the ferricrete deposits correlate closely with warm–wet periods in the Holocene record, showing that ferricrete deposits may represent a previously untapped source of paleoclimatic information.

Discussion: Sulphur isotope composition of the Brunswick No. 12 massive sulphide deposit, Bathurst Mining Camp, New Brunswick: implications for ambient environment, sulphur source, and ore genesis

The authors (W.D. Goodfellow and J.M. Peter) report an impressive amount of chemical data on the Brunswick No. 12 orebody, one of the world’s largest volcanic-associated massive sulphide deposits. They have proposed that most of the sulphur in the massive sulphide ore and the pyrites in adjoining sediments of the Patrick Brook Formation was probably derived from an overlying, anoxic seawater column in which sulphate reduction had taken place, the resulting H2S having mixed with venting metal-rich fluids. The principal lines of evidence brought in to support this idea are (1) the sediments of the Patrick Brook Formation underlying the footwall sedimentary and volcanic rocks of the Nepisiguit Falls Formation, which hosts the orebody, have S/C values and Mn and Fe contents that indicate anoxic conditions in the seawater during sedimentation; (2) the δ34S values of the sulphides in the ore types and the underlying feeder zone are similar to those of pyrites in the Patrick Brook Formation and to coeval pyrites in the Selwyn Basin; and (3) there is no alternative source of reduced sulphur because there would be inadequate sulphate remaining in deep circulating seawater to provide sufficient reduced sulphur for ore formation, and there is little evidence for the presence of evaporite sulphate in the deep footwall sequence. The significance of their major lines of evidence, and also those of Lentz et al. (1996), is discussed below.

Weak elastic‐wave scattering from massive sulfide orebodies

Massive sulfide ore deposits, traditionally the target of electromagnetic or potential‐field geophysical investigations, can potentially be recognized and mapped using seismic methods based on their scattering response. To characterize the seismic expression of massive sulfide orebodies, I review formulas that describe elastic‐wave scattering from isolated inclusions in the far‐field and weak‐scattering limits (the Born approximation) and conduct a series of numerical tests. The minerals pyrite, sphalerite, and galena are used as the basis for these numerical modeling studies because they are relatively abundant and collectively span the full range of observed velocities and densities for ore rocks. The assumption of weak scattering, on which the Born approximation rests, is verified empirically in a representative example, by examining in‐situ wavefield measurements from a vertical seismic profiling experiment in northern Québec, Canada. For simplicity, orebodies are modeled as ellipsoidal inclusions in a homogeneous medium. The mathematical formalism leads to decoupling of factors for “composition” and “shape.” At seismic frequencies (∼50 Hz) and for ore deposits of sufficient size to represent economically viable targets (more than 109 kg), the shape factor effectively controls the scattering response. In this regime, negligible backscattering from spherical inclusions is predicted by the theory. In the case of flattened ellipsoids, which represent a more realistic orebody morphology, the shape‐factor effect leads to significant backscattering that is focused in the direction of specular reflection. For noisy 2-D data, numerical modeling indicates that these characteristics can cause scattering from an isolated dipping orebody to resemble reflections from a planar interface. For 3-D seismic surveys, however, isolated scattering bodies will produce a diagnostic pattern of concentric circular diffractions in unmigrated time‐slice sections.

Thermobarometry of the Bleikvassli Zn-Pb-(Cu) deposit, Nordland, Norway

The Bleikvassli massive sulfide ore deposit is hosted by Proterozoic pelitic, quartzofeldspathic, and amphibolitic rocks of the Uppermost Allochthon of the Scandinavian Caledonides. Staurolite-garnet-biotite and kyanite-staurolite-biotite assemblages indicate that metamorphism reached the kyanite zone of the amphibolite facies. Geothermobarometry was conducted on rocks in and around the deposit using a variety of silicate and sulfide calibrations. Temperature determinations are most reliant on the garnet-biotite exchange reaction, with analyses obtained from 259 garnet rims and adjacent biotite. Results from nine calibrations of the garnet-biotite geothermometer are considered, but compositional limitations of many calibrations involving high Ca and Mn contents in garnet and AlVI and Ti in biotite make many of the coexisting mineral pairs unsuitable. Average temperatures calculated from the two calibrations that most closely address the garnet-biotite compositions observed at Bleikvassli are 584 °C ± 49 °C and 570 °C ± 40 °C. The application of two calibrations of the garnet-staurolite geothermometer on a limited number of samples yields 581 °C ± 2 °C and 589 °C ± 12 °C, assuming aH2O=0.84, based upon calculations of the modal proportions of gaseous species. Pressure determinations are less constrained. Phengite and plagioclase-biotite-garnet-muscovite geobarometers give average pressures of approximately 5.0 kbar and 8.5 ± 1.2 kbar, respectively. Pressures obtained from the sphalerite-hexagonal pyrrhotite-pyrite barometer average 7.7 ± 1.0 kbar. In consideration of these results, the peak metamorphic conditions at the Bleikvassli deposit are estimated to be 580 °C and 8 kbar.

Aqueous and liquid petroleum inclusions in barite from the Walton Deposit, Nova Scotia, Canada; a Carboniferous, carbonate-hosted Ba-Pb-Zn-Cu-Ag deposit

The Walton Ba-Pb-Zn-Cu-Ag deposit, with a historical (1941-1978) production of 4.4 Mt of barite and 0.36 Mt of massive sulfide ore, is hosted by sideritized, Visean carbonates of the Macumber Formation which forms the basal member of the marine Windsor Group in southern Nova Scotia. Strata-bound mineralization occurs as massive barite which gives way to barite and sulfide mineralization at depth; locally, mineralization transects bedding and may occur within the underlying terrestrial clastics of the Tournaisian Horton Group. Barite occurs in a variety of forms (massive to bladed, fine to coarse grained) and colors (white, red, orange, and shades of these) and overlaps the deposition of sulfide minerals.All barite types are characterized by the presence of abundant aqueous and liquid petroleum fluid inclusions that are of dominantly equant shape, of primary and secondary origin, and < or = 100- to 150-mu m size. Inclusions are classified as (1) L- and V-rich aqueous with rare solid phases, (2) mixed CO 2 and CH 4 (rare), and (3) liquid petroleum with rare bitumen. It is not possible to establish a chronology for the different fluids and all types may, in fact, be closely related temporally and spatially. The occurrence of three-phase aqueous-petroleum inclusions, indicative of heterogeneous trapping, indicates contemporaneity of the aqueous and petroleum inclusions. The aqueous inclusions are dominated by high-salinity types (20-28 wt % NaCl equiv), but intermediate- and low-salinity types also occur. Observations of first melting (eutectics at -55 degrees and -35 degrees C) and final melting (ice and hydrohalite) indicate that fluid compositions varied in terms of NaCl/(NaCl + CaCl 2 ) (< or = 0.1-1.0) and NaCl/(NaCl + MgCl 2 ) (0.2-0.84) ratios. Preliminary SEM-EDS analysis of decrepitate mounds confirms the presence of Na, Ca, and Mg and also indicates K, Fe, and Mn in solution. The lack of clathrate phases suggests that, if present, condensed gases (e.g., CH 4 , CO 2 ) occur in only minor to trace amounts in aqueous inclusions. Homogenization temperatures cover a broad range with maximum temperatures of ca. 300 degrees C.Liquid petroleum inclusions, with a blue-white color under ultraviolet light, occur in isolated populations with uniform L/V ratios characterizing a particular group. Homogenization temperatures range from ca. 100 degrees to 300 degrees C, similar to the range for aqueous inclusions but within a group a much narrower range is recorded. The high homogenization temperatures and rare occurrence of bitumen in the inclusions predicates rapid, postentrapment cooling of the area (i.e., within a few kiloyears), otherwise degradation of the petroleum would have occurred. The delta 34 S signatures of sulfides at Walton suggest that mineralization occurred in the presence of liquid petroleum with the petroleum causing reduction of aqueous sulfate to H 2 S.Collectively the fluid inclusion data indicate that mineralization occurred at ambient conditions of 250 degrees to 300 degrees C and pressures of ca. 400 bars, as constrained for different inclusion types. The liquid petroleum is considered to have been generated by thermogenic processes via interaction of the heated, highly saline mineralizing fluid with organic-rich beds within the Horton Group (Horton Bluff Formation) and was subsequently entrained as an immiscible phase within the aqueous fluid. Generation of the hydrothermal petroleum at Walton is analogous to the present-day occurrence of liquid petroleum vented from black smokers on the ocean floor (e.g., Guaymas basin).

IP tomography test survey in the Barrier Main Lode, Broken Hill, Australia

In both the domestic and overseas mineral exploration projects of the Metal Mining Agency of Japan (MMAJ), many holes have been drilled in order to core for geological logging and laboratory analysis. However drill holes themselves are seldom used to obtain additional information except for occasional geophysical logging such as resistivity, self-potential and induced polarisation (IP). Therefore the MMAJ has been developing an IP tomography data acquisition system since 1992 for the purpose of effectively utilising drill holes, reducing exploration cost and improving accuracy of geophysical exploration.This system is based on time-domain measurement and adopts a pole-dipole electrode configuration to avoid coupling effects compared to a pole-pole array and to deploy the electrodes more easily than a dipole-dipole array. It consists of a transmitter, receiver, personal computer, controller, non-polarisable electrodes, ground cables and borehole cables, and has the following features:It acquires resistivity and chargeability data at 10 sites simultaneously.It acquires very accurate data because of an operational amplifier at each electrode in the borehole cable and the relay box.It acquires the data automatically through the controller, computer and relay boxes.In 1997, the MMAJ conducted an IP tomography test survey in order to examine the applicability of the system for mineral exploration in the Barrier Main Lode, located northeast of Broken Hill, NSW, Australia. The quality of the acquired data was excellent because of high sensitivity of the system and absence of culture noise such as power lines. The data was analysed with the inversion technique based on the alpha centers method and the non-linear least square method. In the reconstructed resistivity and chargeability images, the ore body appears as a low resistivity zone and as a high chargeability zone. The reconstructed images are consistent with the result of IP logging although the reconstructed resistivity is lower than that obtained by IP logging. It is confirmed that the reconstructed chargeability image can delineate a massive sulphide ore deposit such as the Barrier Main Lode more clearly than the reconstructed resistivity image.

Seamount formation and associated caldera complex and hydrothermal mineralization in ancient oceanic crust, Troodos ophiolite (Cyprus)

The inter-graben zone between the Solea and Mitsero grabens in the northern part of the Troodos ophiolite (Cyprus) displays igneous, tectonic, and physiographic features that are characteristic of seamount volcanism at modern oceanic spreading centers. In this zone the 200-m-high Alestos hill volcanic edifice consisting of pillow lavas with preserved lava tubes and cone sheet intrusions is interpreted to have developed as a seamount fed by a late-stage, off-axis shallow-level gabbroic pluton that was intrusive into young oceanic crust. Concentric cone sheet intrusions and associated faults and shear zones in and around the Alestos hill seamount collectively form a seafloor caldera complex, which probably formed during the extrusion of lavas and subsequent cauldron subsidence over the gabbroic pluton. The ENE-trending elongated massive sulfide ore deposit formation in the Alestos–Memi area represents a focused upflow zone for hydrothermal fluids driven by the same off-axis gabbroic intrusion responsible for formation of the Alestos hill seamount. The ENE-trending and up to 150-m-wide Alestos–Memi fault zone dissects the elongated ore body in a dextral sense and might have provided a high permeability conduit in the upper crust for upwelling hydrothermal flow at the time of magmatism. The geometry of this fault zone and its kinematic relations with a N–S-striking Alestos Forest normal fault in the west are suggestive of its transfer fault origin, reminiscent of mineralized transfer faults found at the Mid-Atlantic Ridge. The active TAG hydrothermal mound with massive sulfide deposits that is located on a small volcanic dome 3 km off-axis and along a transfer fault in the Trans-Atlantic Geotraverse (TAG) area at 26°N provides a modern analogue for the Alestos hill seamount and associated faulting and mineralization in the ancient oceanic crust of the Troodos ophiolite. These findings indicate that off-axis magmatism played a major role in crustal accretion, hydrothermal alteration, and ore mineralization during the late-stage evolution of the Troodos ophiolite.

Application of the Re‐Os isotopic system to the study of Precambrian magmatic sulfide deposits of Western Australia

Mafic and ultramafic rocks that host magmatic Fe‐Ni‐Cu‐platinum‐group element (PGE) sulfide deposits are an important feature of the Archaean Yilgarn and Pilbara Cratons of Western Australia. We have applied the Re‐Os isotopic geochronometer and tracer to four komatiite‐hosted Ni sulfide deposits, one Merensky Reef‐style Cu‐Ni‐PGE disseminated sulfide accumulation in a mafic layered intrusion, and a Cu‐Ni sulfide‐rich siliceous high‐MgO basalt dyke swarm in order to constrain better the genesis of these magmatic ore systems. Significant post‐crystallisation Re and/or Os mobility is observed in the isotopic data for samples from the 3.3 Ga Ruth Well prospect (Pilbara Craton), for cumulates from the 2.9 Ga Munni Munni Complex (Pilbara Craton), for some massive sulfide ore samples from the 2.7 Ga Kambalda deposit (Yilgarn Craton), and for some disseminated ore samples from the 2.7 Ga Perseverance deposit (Yilgarn Craton). We attribute this open‐system behaviour and disturbance of the Re‐Os system to hydrothermal alteration, sulfide recrystallisation during deformation and metamorphism, and/or metasomatism associated with mesothermal lode‐gold mineralisation. However, less tectonised and metamorphosed ores and a host komatiite lava from Kambalda as well as olivine‐sulfide cumulates from Perseverance and Mt Keith yield a magmatic Re‐Os isochron age of 2706 ± 36 Ma, in good agreement with U‐Pb zircon ages of 2709–2702 Ma for metasediments from Kambalda, demonstrating that the Re‐Os isotopic systematics in these samples have remained closed for 2.7 Ga. This isochron can also be used as a tracer of the magmatic processes that brought the host komatiites to sulfide saturation. Assimilation of sulfidic crustal materials via thermal erosion has been proposed as a mechanism by which the Archaean komatiites at Kambalda, Perseverance and Mt Keith achieved sulfide saturation. However, the isochron for the Yilgarn ores yields a precise chondritic initial γ?? value of ‐0.1 ± 0.3 (per cent deviation in 187Os/188Os from chondritic mantle of the same age), whereas a proposed sulfidic sedimentary contaminant from Kambalda yields an extremely radiogenic initial γ?? value of +900. These Os isotopic data in combination with trace element and Nd isotopic data demonstrate that the major ore‐forming komatiites in Western Australia were derived from chemically depleted mantle without significant contamination (< 3%) by crust either prior to eruption or during turbulent flow at the surface. Thus, ground melting and crustal assimilation may not be as important as previously thought in the genesis of these deposits. However, Re‐Os isotopic data for a 2.4 Ga siliceous high‐MgO basalt dyke at Cowarna Rocks (Yilgarn Craton) yield a radiogenic initial Os isotopic composition (γ?? = + 250), suggesting that sulfide saturation may have been linked to crustal assimilation. Thus, the timing and mechanism of sulfide saturation may be subordinate to dynamic magmatic processes in the localisation of economic concentrations of magmatic sulfide liquids. This geodynamic concept has important implications for the discovery of other styles of magmatic ore deposits, such as Voisey's Bay (Labrador, Canada)‐style sulfide mineralisation, in Australia.

The Ashele Deposit: A Recently Discovered Volcanogenic Massive Sulfide Cu‐Zn Deposit in Xinjiang, China

Abstract: The Ashele Cu‐Zn deposit is a recently discovered volcanogenic massive sulfide deposit in Xinjiang, Northwestern China. It is the largest Cu‐Zn deposit in this type of deposits in China, which were formed in the early period of later Palaeozoic Era. This deposit is hosted within a suit of bimodal submarine volcanic rocks of the Ashele Formation of Lower‐Middle Devonian System formed in an environment of paleocontinental margin rift setting. Lensoid orebodies occur between spilitic rocks developed at footwall and quartz‐keratophyric tuff at hanging wall. Zonation of metal elements in the Ashele mine is one of typical volcanic‐related exhalative Cu‐Zn sulfide deposits in the world. Black ores enriched in Pb, Zn and Ag occurs on the top of the No. 1 orebody in the Ashele deposit, yellow ores enriched in Cu in the middle part, and the chalcopyritization stringer below the massive sulfide ores. Zonation of ore‐structure in the No. 1 orebody is also apparent and corresponds to the zoning of elements, i. e. lamellar and/or banded sulfide‐sulfate ores on the top, massive sulfide ores in the middle, and stockwork veinlets associated with altered breccia pipe on the bottom. Four epochs of mineralization in the Ashele deposit has been recognized. The first period of syngenetic‐exhalative deposition of sulfides is the main epoch of mineralization, and the ores deposited subsequently subjected to thermo‐metamorphism at the second epoch, superimposed by hydrothermal mineralization at the third epoch, and weathered or oxidized at the fourth epoch.More than 100 categories of minerals have been recognized in the Ashele mine, but only pyrite, chalcopyrite, sphalerite, tetrahedrite, galena, barite, quartz, chlorite, sericite, and calcite are dominant, making up various types of ores, and alteration pipes or horizons. Studies of ore petrology suggest that the massive ores were volcanogenic and deposited by exhalative process.

Ancient vent chimney structures in the Silurian massive sulphides of the Urals

Abstract Exceptionally well preserved volcanogenic massive sulphide ores at Yaman Kasy in the Silurian of the southern Urals have yielded not only well-preserved sulphidized vent macrofauna but also fragments of vent chimneys. All fragments show a broad 3-fold mineralogical zonation. An outer zone which forms the chimney/conduit wall comprises largely pyrite and marcasite which is laminated or collomorphic and is commonly porous. In one fragment this zone is characterized by a honeycomb-like structure in the pyrite, infilled by barite, sphalerite and chalcopyrite. Dendrite growth textures branch outwards towards the chimney wall. The middle of the three zones comprises largely pyrite with chalcopyrite and sphalerite as thin veinlets and infillings. The innermost zone is dominated by chalcopyrite. Minor gold and bismuth tellurides occur at the boundary between the middle and the inner zones. The inner zone is interpreted as the hydrothermal conduit lining and in all cases is defined by bladed chalcopyrite, which shows a texture consistent with growth toward an inner open space. The central part of the conduit is now infilled with sphalerite or in one case pyrite, chalcopyrite, silica and minor barite. Fluid inclusion studies indicate the presence of high-temperature (&gt; 350°C) fluids of around 3.5 0.000000e+00quivalent NaCl in the basal parts of the massive sulphide mound with cooler temperatures (&lt; 100°C) recorded in barite from the upper part of the mound. Barite is associated with chimney fragments, as worm tube infillings and as later cross-cutting veins. Preliminary S and Sr isotope data from sulphides and sulphates supports both igneous and seawater sources for sulphur in the hydrothermal system with evidence for seawater circulation and sulphate precipitation beneath the sulphide mound. The results are consistent with a similar model of chimney growth to that proposed for modern vent sites. It is proposed that a high temperature fluid flux through open conduits fed black smoker activity accompanied by lateral fluid diffusion through the chimney wall which mixed with seawater. The result of this is a combination of conductive cooling and fluid mixing leading to precipitation of distinctively zoned mineral assemblages across the vent conduit wall. The occurrence of tellurium, bismuth and precious metal-bearing phases indicates some similarities with the geochemistry of sulphides from other Palaeozoic massive sulphide deposits associated with felsic volcanic centres.

Rb-Sr isotope dating of silicalite from the Dajiangping massive sulfide ore deposit, Guangdong Province

THE Dajiangping pyrite ore deposit located in the Yunfu County of Guangdong Province is asuper-large pyrite ore deposit. Its age has not been defined so far because of the absence of fossil. This note is to use Rb-Sr isotope dating of silicalite to constrain the age of the ore depositand place special emphasis on its geological significance...

Symplectite of pyrite and magnetite in the massive sulphide ore of the Mashan mine, Anhui Province

A symplectite of pyrite and magnetite in the massive sulphide ore of the Mashan mine, Anhui Province, is interpreted to have been formed by their replacing earlier pyrrhotite. The compositions of pyrrhotite, pyrite and magnetite related to this texture are given by electron microprobe analysis. Such a texture is likely to be formed when the ore-forming system reaches the three-phase point of pyrrhotite, pyrite and magnetite from the pyrrhotite stability field. The very small probability for the system to reach this point could be used to account for the rare occurrence of such sympletite in natural ores.