Massive Sulfide Ore Research Articles

Die Bedeutung verschiedener Typen von Magmatismus f�r VHMS Mineralisationen: Die Geochemie der vulkanischen Nebengesteine der massiven Sulfid-Lagerst�tte von Benambra, Victoria, Australien

The Wilga and Currawong Cu-Zn massive sulphide deposits in southeastern Australia are hosted by a deformed sequence of Upper Silurian basaltic to rhyolitic volcanic and sedimentary rocks. The syn-volcanic mineralisation occurs immediately above a thick package of rhyolitic volcanic rocks and volcaniclastic rocks (Thorkidaan Volcanics), and is overlain by relatively thin intercalated sills, intrusive domes and flows of basalt, andesite and dacite (Gibson's Folly Formation). The Thorkidaan Volcanics haveeNd(420Ma) = -2.2 to - 9.8 and are considered to have been derived by partial melting of older crustal rocks, whereas the basalt-andesite-dacite hangingwall sequence hase(Nd(415Ma) = -0.5 to + 2.0 suggesting derivation from a relatively undepleted mantle source. Relatively high-Ti andesitic to dacitic rocks from the Bumble Creek area haveeNd(415Ma) = +5.2 to +5.9 suggesting affinities with Ordovician volcanic rocks elsewhere in the Lachlan Fold Belt. The Thorkidaan Volcanics display a limited silica range (73 to 79 wt.%), but have distinctive minor and trace element variations indicating a substantial fractionation history involving feldspar and several accessory phases. Major and trace element compositions of the basalt-andesite-dacite suite display regular variations consistent with a cogenetic relationship by fractional crystallisation. The basaltic rocks mostly have low TiO2 (< 0.8 wt.%) and other chemical characteristics such as high Zr/Nb and La/Nb which suggest formation in a subduction-related setting; probably an embryonic back-arc basin developed on stretched continental lithosphere, or in small pull-apart basins developed adjacent to a transtensional margin. The magmatic history and paleogeography reflect an extensional tectonic and magmatic cycle comprising uplift, rhyolitic magmatism from crustal melting, extension, subsidence, and penetration of a mantle-derived basalt-andesite-dacite suite up extensional faults to the sea floor. Massive sulphide ores are located exactly at the stratigraphic change from rhyolitic to more mafic mantle-derived magma types. Consideration of the types of mineralisation associated with crustal, S-type granitoids, coupled with thermal constraints limiting the capacity of small bodies of silicic magma to initiate and sustain hydrothermal convection cells of reasonable size, suggests that in the absence of coeval mafic magmatism, S-type crustal-derived silicic volcanic packages are likely to be barren of VHMS deposits. Mineralisation occurs in association with mantle-derived basalt-andesite-dacite suites that either provide the necessary heat to facilitate leaching of the footwall volcanic rocks, or contribute metal-rich hydrothermal solutions during fractional crystallisation, or both.

Setting of Zn-Cu-Au-Ag massive sulfide deposits in the evolution and facies architecture of a 1.9 Ga marine volcanic arc, Skellefte District, Sweden

The Skellefte mining district occurs in an Early Proterozoic, mainly 1.90-1.87 Ga (Svecofennian) magmatic province of low to medium metamorphic grade in the Baltic Shield in northern Sweden. The district contains over 85 pyritic Zn-Cu-Au-Ag massive sulfide deposits and a few vein Au deposits and subeconomic porphyry Cu-Au-Mo deposits. The massive sulfide deposits mainly occur within, and especially along the top of, a regional felsic-dominant volcanic unit attributed to a stage of intense, extensional, continental margin arc volcanism. From facies analysis we interpret the paleogeography of this stage to have comprised many scattered islands and shallow-water areas, surrounded by deeper seas. All the major massive sulfide ores occur in below-wave base facies associations; however, some ores occur close to stratigraphic intervals of above-wave base facies associations, and the summits of some volcanoes that host massive sulfides emerged above sea level. Intense marine volcanism was superceded at different times in different parts of the district by a stage of reduced volcanism, uplift resulting in subregional disconformities, and then differential uplift and subsidence resulting in a complex horst and graben paleogeography. Uplift of the arc is attributed to the relaxation of crustal extension and the emplacement of granitoids to shallow crustal levels. A few massive sulfide ores formed within the basal strata of this second stage. The horst and graben system was filled by prograding fluvial-deltaic sediments and mainly mafic lavas, and during this stage the Skellefte district was a transitional area between renewed arc volcanism of more continental character to the north, and subsidence and basinal mudstone-turbidite sedimentation to the south. This whole volcanotectonic cycle occurred within 10 to 15 m.y.We define 26 main volcanic, sedimentary, and intrusive facies in the Skellefte district. The most abundant facies are (1) normal-graded pumiceous breccias, which are interpreted as syneruptive subaqueous mass flow units of pyroclastic debris, (2) porphyritic intrusions, and (3) mudstone and sandstone turbidites. Facies associations define seven main volcano types, which range from basaltic shields to andesite cones and rhyolite calderas. Despite this diversity of volcano types, most massive sulfide ores are associated with one volcano type: subaqueous rhyolite cryptodome-tuff volcanoes. These rhyolite volcanoes are 2 to 10 km in diameter, 250 to 1,200 m thick at the center, and are characterized by a small to moderate volume rhyolitic pyroclastic unit, intruded by rhyolite cryptodomes, sills, and dikes. Massive sulfide ores occur near the top of the proximal (near vent) facies association. The remarkable coincidence in space and time between the ores and this volcano type indicates an intimate, genetic relationship between the ores and the magmatic evolution of the volcanoes.Many of the massive sulfide ores occur within rapidly emplaced volcaniclastic facies and are interpreted to have formed by infiltration and replacement of these facies. Some of the ore deposits have characteristics of both marine massive sulfides and subaerial epithermal deposits. We suggest that massive sulfides in the Skellefte district span a range in ore deposit style from deep-water sea-floor ores, to subsea-floor replacements, to shallow-water and possibly subaerial synvolcanic replacements. Facies models are provided for the mineralized rhyolite volcanoes and volcanological guides are provided for exploration for blind ores within these volcanoes.

Ore petrology of volcanogenic massive sulphide ores in the Ural Mountains, Russia

Ore petrology of volcanogenic massive sulphide ores in the Ural Mountains, Russia

The influence on EM-anomalies of the position of the generator in a Turam loop

The Bainaimiao Cu–Mo–Au deposit, located at the northern margin of the North China Craton (NCC) in central Inner Mongolia, is a large Cu-dominant deposit. Orebodies are hosted in strongly deformed and metamorphosed granodiorite porphyry intrusions or in EW-trending shear zones in greenschist and sericite felsic schist of the Bainaimiao Group. Mineralization can be divided into early, middle and late stages, characterized by quartz–pyrite, polymetallic quartz–sulfides and carbonate–quartz, respectively. Hydrothermal activity led to pervasive silicification, K-feldspar, biotite and chlorite–epidote–carbonate alteration.Eight samples from Cu–Mo-bearing quartz veins and massive sulfide ores in greenschist were collected from the VIII orebody in the North ore belt and molybdenite separates were selected for Re–Os analyses. The concentrations of Re and 187Os range from 216.6 to 1051 ppm and 2302 to 4913 ppb, respectively. All samples give Re–Os model ages between 441.2 and 445.9 Ma and yielded an isochron age of 445.0 ± 3.4 Ma (MSWD = 0.26), with an initial 187Os of − 4 ± 17 ppb. A granodiorite porphyry sample for SHRIMP U–Pb dating was collected from the underground of the XIII orebody of the North ore belt. Spots from 14 zircon grains have 206Pb/238U values between 426.2 and 462.9 Ma and form a tight cluster on Concordia yielding a weighted mean 206Pb/238U age of 445 ± 6 Ma. This is considered to be the best estimate of crystallization age of the granodiorite porphyry. The Re–Os isochron age of the molybdenite is consistent with the SHRIMP U–Pb age of the granodiorite porphyry, which indicates that the deposit is a porphyry system of Paleozoic age.According to geodynamic setting and geological history of the northern margin of the NCC, the deposit was probably related to magmatic activity of the Middle Paleozoic Ondor Sum subduction–accretion complex-Bainaimiao arc. During the Devonian, the Bainaimiao arc was accreted to the northern margin of the NCC. Collision between the Bainaimiao arc and NCC led to regional metamorphism of the ore-hosting terrane and ore metals were remobilized in this process. Gold mineralization was probably overprinted onto the earlier Cu–Mo mineralization.

Time series studies of vent fluids from the TAG and MARK sites (1986, 1990) Mid-Atlantic Ridge: a new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulphide orebodies

Abstract The hot springs at TAG and MARK on the Mid-Atlantic Ridge have been resampled after an interval of four years (1986–1990). The fluid compositions show the same temporal stability as observed elsewhere, e.g. 21°N, East Pacific Rise (EPR) and the Guaymas Basin. Although the MARK fluids have no chemical characteristics that would distinguish them from those on the faster spreading ridges of the Pacific, TAG has pronounced differences. The depletion in B and the small shift in δ 11 B indicate a substantial degree of reaction at intermediate temperatures along the recharge path that is unique to TAG. In addition, the TAG mound contains a large cluster of sphalerite-rich ‘onion domes’ that have been formed from an approximately 5:1 mixture of the primary hydrothermal end-member fluid and seawater that is formed within the deposit. This has resulted in the extensive precipitation of FeS within the mound and a resulting decrease in pH to values below 3. The low pH causes the large-scale remobilization of Zn from the interior of the deposit and its reprecipitation on the surface as the domes. Such compositional zoning is a common feature of ophiolite-type massive sulphide ore bodies and probably results by the same mechanism. The end-member data from both hydrothermal areas fall on the mixing planes defined by the EPR and Juan de Fuca data in a new three-component mixing model, indicating the presence of a phase-separated brine pool at depth under these Mid-Atlantic Ridge systems which is very similar in composition to those on the Pacific ridges.

Nd and Pb isotope ratios of the Abitibi greenstone belt: new evidence for very early differentiation of the Earth

Nd and Pb isotopic compositions were determined for 39 samples from mafic and felsic volcanic and subvolcanic units that underlie volcanogenic massive sulfide deposits in the Noranda and Matagami districts of the Abitibi greenstone belt. At Noranda, Pb and Nd isotopic data plot along a single linear array which implies derivation from an isotopically homogeneous source (average ϵ Nd = +2.5 ). At Matagami, in contrast, the Pb and Nd data define two linear arrays. The more compositionally evolved rhyolites have less radiogenic Pb isotopic compositions and higher average initial ϵ Nd values ( ϵ Nd = +3.2 ) than the more primitive compositions ( ϵ Nd = +2.4 ). We interpret these isotopic differences at Matagami, as well as those between Noranda and Matagami, as not due to assimilation of crustal components, but rather primary mantle heterogeneity in the Abitibi mantle at 2.7 Ga. This implies that, although the Abitibi mantle was broadly uniform in terms of initial ϵ Nd values ( +2.5 ± 1), it was characterized by small heterogeneities on both local and regional scales in the Archean. On the basis of Pb and Nd isotopic data, we conclude that the Pb isotopic compositions of galena in Noranda and Matagami ores represent initial Pb isotopic compositions of the underlying volcanic rocks. Pb isotopic compositions of such massive sulfide ores throughout the Abitibi greenstone belt plot along a coherent linear PbPb array and Noranda and Matagami galenas are the most and least radiogenic examples, respectively. This array is nearly coincident with the geochron at 2.7 Ga, using an age for the Earth of 4.53 Ga, and indicates not only that the Abitibi mantle was isotopically heterogeneous at 2.7 Ga but also that these heterogeneities were developed within the first few hundred million years of Earth's history. Available data on initial ϵ Nd and initial 207 Pb 204 Pb for the Superior Province are negatively correlated, implying that this early UPb fractionation was accompanied by SmNd fractionation and involved incompatible element depletion of the mantle.

A Study of the Falcon Concentrator

A B6 Falcon Concentrator was tested on several systems: a synthetic magnetite-silica ore, a massive sulphide gold–eopper ore, a fine gold-pyrite-silica flotation tail, and a high-grade gold ore in a low sulphide gangue (cyclone overflow).The Falcon demonstrated its ability to achieve separation in all cases, but with varying degrees of success. For the magnetite-silica system, separation proceeded until the concentrate bed overloaded, at a mass of 300–400 g of magnetite. Recovery increased with decreasing particle size, down to 25–37μm, and in some cases even below 25 μm. Bowl geometry (angle) and gangue size distribution were the most significant operating variables. Results with the massive sulphide ore were the poorest, with gold recoveries typically around 10–20% at very low upgrading ratios, 1:1–5:1. Gold recovery from the fine flotation tail ranged from 30 to 70% at upgrading ratios of 5–15. The high-grade gold cyclone overflow yielded a wide range of gold recoveries, with a very consistent relationship between recovery and mass processed. With a highgrade feed, a concentrate of 0.5% Au could be produced at a recovery of 50%. Résumé Un concentrateur Falcon, type B6, a été utilisé pour le traitement de plusieurs échantillons: un mélange synthétique de magnétite-silice, un minerai or et cuivre de sulfures massifs, un rejet de flottation très fin contenant de l'or, de la pyrite et de la silice, et un minerai d'or à haute teneur contenant peu de sulfures (sur-verse de cyclone).Le Falcon a pu enrichir les produits traités, avec plus ou moins de succès, selon le cas. Pour le mélange magnétite-silice, la séparation se poursuit tant que la bande de concentré n'est pas saturée, ce qui se produit lorsque de 300–400 g de magnétite ont été récupéré. La récupération augmente à mesure que la taille des particules de magnétite diminue, jusqu'à 25–37 μm, et même dans certains cas, le –25 μm. La géométrie du rotor et la granulométrie de la gangue sont les variables opératoires qui ont le plus d'effet sur la récupération. C'est le minerai de sulfures massifs qui a donné les pires résultats, soit des récupérations en or de 10–20%, et des rapports d'enrichissement très bas, variant de 1:1–5:1. Pour le rejet de flottation fin, de 30–70% de l'or fut récupéré, les rapports d'enrichissement variant entre 5:1 et 15:1. En traitant le minerai d'or à haute teneur, nous avons pu, en augmentant la masse traitée, augmenter la teneur du concentré, mais au prix d'une chute de la récupération. Le point d'opération optimum semblait être autour d'une teneur de concentré de 0.5% Au, qui pouvait être atteinte avec une récupération de 50% de l'or.

Microstructural timing of the Rosebery massive sulphides, Tasmania: evidence for a metamorphic origin through mobilization of disseminated base metals

ABSTRACTMicrostructural timing relationships indicate that the Rosebery massive sulphide ore, western Tasmania, Australia, formed by metasomatic replacement of ‘sericite’ schist during a Devonian deformation event (D3). This interpretation is contrary to earlier volcanogenic‐based interpretations, but accords with the discordant position and inferred structurally controlled emplacement of the orebody. The main timing criteria are: overprinting of S3 by the late ore minerals, replacement textures in undeformed mineral parageneses, and a D3 structural control from the microscopic to the macroscopic scales. The consistent observation of these criteria in the orebody and the complete lack of pre‐D3 ore argue against in situ dissolution of a primary orebody and local redeposition of sulphides by replacement.D3 deformation at Rosebery is inferred to have been characterized by heterogeneous cleavage‐parallel extension, which resulted in localized microfracturing and selective replacement of zones of maximum strain rate. Continuous shifts in the strain‐rate distribution pattern during progressive mineralization led to the compositional ore banding.Published Pb‐isotope data point towards a Cambrian source rock for the orebody. This suggests a metamorphogenic origin by regional‐scale dissolution of dispersed volcanogenic metals, focused solution transfer and concentrated redeposition in a structural trap.

The early Archaean Nondweni greenstone belt, southern Kaapvaal Craton, South Africa, Part I. Stratigraphy, sedimentology, mineralization and depositional environment

The Nondweni greenstone belt is an exceptionally well preserved ∼3.4 Ga succession of predominantly volcanic rocks near the southern margin of the Kaapvaal Craton. Nondweni Group rocks show both similarities and differences to the Barberton Sequence, of similar age, 300 km to the north. In contrast with the latter, none of the Nondweni lavas exceed 23% MgO and spinifex textures are exclusively of pyroxene type. The characteristics of the lavas and magma genesis are discussed in Part II of this series of papers. Three Formations are recognized in the Nondweni Group: the lowermost Mogongolozi Formation comprises basaltic andesites, komatiitic andesites and basalts with intercalated coarser-grained pyroxenites and gabbros. The Toggekry Formation is a felsic unit of quartz-sericite schists with small CuZn massive sulphide ore bodies of volcanogenic origin. The uppermost Witkop Formation comprises mainly basalts, komatiites and komatiitic basalts intercalated with chert layers and capped by a clastic sedimentary succession. The total exposed thickness of the Nondweni Group is 16,500 m is the result of duplication of stratigraphy and tectonic thickening during early D1 thrusting in the bedding plane of much thinner lithological units. Tuffaceous sediments comprise a relatively minor component of the stratigraphy but are indicative of depositional environment. Banded chert layers are largely derived from accumulations of accretionary lapilli tuffs, although a diverse origin is indicated for some varieties. At the base of the Witkop Formation a number of undeformed, banded, coarse-grained graphitic quartzite bodies, entirely devoid of sedimentary structures, are considered to have originated as hydrothermal deposits resulting from fumarolic activity. Selective brecciation of layers resulted from explosive release of fluids and gases. The textures of these bodies are in marked contrast to the highly rounded pseudo-conglomerates from the clastic sedimentary succession at the top of the Witkop Formation. This ∼500 m thick succession passes upwards through reworked tuffs and alluvial fan deposits, into a zone of evaporites and stromatolites. Swallow-tail twin crystals of barite, and an evaluation of crystal interfacial angles, are indicative f replacement of primary evaporative gypsum. Most primary features of the Nondweni greenstone belt suggest a shallow-water or subaerial depositional environment, and together with characteristics of the magma composition, indicate a tectonic setting in a subduction-related continental margin setting.

Early proterozoic black shales of the Timeball Hill Formation, South Africa: volcanogenic and palaeoenvironmental influences

Black shales in the Early Proterozoic Timeball Hill Formation exhibit a widespread dark grey colour due to disseminated iron minerals, predominantly limonite after pyrite, with subordinate thin beds and laminae more intensely pigmented by finely disseminated flakes of carbonaceous material. Andesitic-basaltic volcanism is thought to have provided the source of iron and sulphur for the ferruginous colouration in a basal and uppermost black shale facies. Sulphate-reducing bacteria around volcanic vents possibly produced the organic matter for the darker beds. Turbiditic rhythmites and succeeding fluviodeltaic sandstones overlie the basal black shale facies and associated Bushy Bend lavas. In the south west of the basin both rhythmites and sandstones have black shales as either thin interbeds or matrix material suggesting the possibility of continued fumarolic emissions in this region. A repetition of the rhythmite facies again shows an association with subordinate thin black shale interbeds in the southwest of the basin and this facies is succeeded by a second occurrence of the black shale facies, underlying the Hekpoort Andesite Formation. The observed association of black shales with turbiditic rhythmites, lavas lacking pillow structures and fluviodeltaic sandstones suggests water depths which varied from shallow to a few hundreds of metres. Some potential for volcanogenic massive sulphide and sedimentary exhalative ore deposits exists in the black shale as there is volcanic rock associated at the base of the formation.

Anisotropic velocity tomography: A case study in a near‐surface rock mass

Cross‐borehole data were acquired in the surface crown pillar of a massive sulfide ore mine. The data consist of five, two‐dimensional (2-D), cross‐borehole panels, each with approximately 900 source‐receiver pairs. The panels were located within the crown pillar at either side of and within a major subvertical fault zone that intersects the orebody. An initial analysis of the data indicates that the bedrock containing the orebody is seismically anisotropic. A rigorous analysis of the traveltimes using anisotropic velocity tomography confirms the initial assessment that anisotropy exists within the crown pillar rock mass. Anisotropic velocity tomography is the generalization of tomographic methods to anisotropic media. As in any geophysical problem, the data are insufficient to completely resolve the distributions of the rock properties at all scale lengths; we use external constraints on the roughness of the final solution to ensure an algebraically well‐posed problem. Plots of the data residuals (the “traveltime surfaces”) are an essential tool in determining an optimal level of constraint. Of equal importance are plots of the relationship between the solution roughness and the rms level of the residuals. The final results of anisotropic velocity tomography are a set of images (tomograms) of the velocity and selected anisotropy parameters for the five panels. Our images do not contain the distortions typically exhibited when using isotropic tomography in anisotropic media. The velocity tomograms clearly show the geometry of the overburden contact at the top of the bedrock. The anisotropy tomograms show a decrease in anisotropy with depth on two of the panels. They also show a decrease in anisotropy with proximity to the fault zone. These features of the seismic velocity anisotropy are consistent with observations of fracture orientation and distribution. The results of the crosshole data interpretation contribute to the overall site investigation and provide a reliable interrogation of the bulk properties of the rock mass.

Geology and mineralogy of massive sulfide ores from the central Okinawa Trough, Japan

Geology and mineralogy of massive sulfide ores from the central Okinawa Trough, Japan

A fluid inclusion and light element stable isotope study of the gold-bearing quartz vein system, Falun, Sweden

The Falun gold quartz vein mineralization is located ca 230 km NW of Stockholm, Sweden, within the Early Proterozoic volcano-sedimentary sequence of Bergslagen. The mineralization consists of a system with subparallel quartz veins that crosscut the alteration zone to the Falun massive sulphide deposit. Early barren and late gold-bearing quartz veins follow tectonic structures postdating the formation of the massive sulphide ore. Both generations of veins are epigenetic to the massive sulphide ore and were formed by hydrothermal processes. Fluid inclusion study of the gold-bearing quartz veins indicates a low-moderately saline fluid (0.3 to 17.4 equiv wt% NaCl). Heterogeneous trapping is indicated by coexisting inclusions showing a variable CO2 content from 100% CO2 ± CH4 to 100% aqueous fluid. Temperatures of total homogenization also show a wide spread from 116–350°C with a slightly bimodal distribution with peaks at ca 180°C and 280°C. MeasuredδD values — 69 to — 63%0 (SMOW), of inclusion fluid and calculatedδ18O values of hydrothermal fluids — 7.5 to — 1.4%0 (SMOW), strongly suggest a meteoric origin for the fluids. The quite consistentδD values and the range inδ18O values indicate that major water-rock interaction led to the evolution inδ18O of the hydrothermal fluids.

Phase relations in the Fe-Zn-S system to 5 kbars and temperatures between 325 degrees and 150 degrees C

Sphalerite, pyrite, and pyrrhotite were recrystallized together in a saturated ammonium iodide solution at 150 degrees to 325 degrees C and 0.4 to 5.1 kbars. The data were obtained from ten sets of experiments comprising gold capsules stacked consecutively within the bores of cold seal pressure vessels which were heated for periods of between 300 and 750 days. Five capsules contained magnetite as an additional reactant.A P-T-composition map has been constructed for sphalerite buffered by pyrite and hexagonal pyrrhotite. The upper P-T range for the present study includes the lower temperature boundary for the sphalerite geobarometer which has been located near 285 degrees C at 0.5 kbars and 300 degrees C at 5 kbars. The provisional lower P-T range for the present data extends from approximately 180 degrees C at 0.5 kbars to 190 degrees C at 5 kbars. Within the investigated P-T interval, isobaric composition curves for sphalerite show a progressive decrease in iron content with increasing pressure and decreasing temperature.Hexagonal pyrrhotite was the only iron monosulfide identified in the experimental reaction products and was found to increase in iron content with increasing pressure. Its occurrence in these long-term experiments supports earlier predictions that monoclinic pyrrhotite may be a metastable phase down to low temperatures. Hexagonal pyrrhotite may stably invert to monoclinic pyrrhotite at temperatures below 150 degrees C under isotropic stress conditions; however, at higher temperatures monoclinic pyrrhotite appears to be favored by anisotropic stress conditions which produce lattice strain. Under these conditions it may form and persist metastably as the preferred phase below the sphalerite geobarometer boundary. The presence of magnetite does not affect the compositions of sphalerite or pyrrhotite under the conditions of study.The present calibration may be applied to hydrothermal vein deposits provided codeposition of phases can be proven; however, it is expected to have limited application for massive sulfide ores that have prograded or retrograded to temperatures around or below 300 degrees C. This is because the calibration requires buffering by pyrite and hexagonal pyrrhotite, whereas most lower temperature assemblages appear to have been buffered by pyrite and monoclinic pyrrhotite over a range of temperatures, pressures, and anisotropic stress conditions. This latter buffer has restricted sphalerite compositions between around 11 and 12 mole percent FeS.

Sulphide mylonites from the Renström VMS deposit, Northern Sweden

AbstractSulphide mylonites are fine-grained massive sulphides which have deformed in a plastic manner. In the Renström Zn-Pb-Cu-Ag-Au VMS deposit, one of several operating mines in the Early Proterozoic Skellefte District in Northern Sweden, shear-zone metamorphism has resulted in the development of mylonitic fabrics within the sulphides. The massive sulphide ore is hosted in a shallow submarine to subaerial volcano-sedimentary sequence which has been variably metamorphosed and deformed. Initially, the sequence underwent burial metamorphism which was followed by an amphibolite grade regional metamorphic event at pressures of around 7.5 kbar and temperatures of 540-600°C This has been overprinted by a retrogressive metamorphic event at greenschist facies (at ca. 400°C with concomitant ductile deformation. Finally the area was uplifted to shallower crustal levels with associated cataclastic deformation.Both the regional and dynamic metamorphic events have resulted in the development of specific textures in the sulphide ores. Textural evidence indicates that pressure solution has been mainly responsible for the plastic deformation in pyrite, while the weaker sulphide minerals such as pyrrhotite, chalcopyrite and galena have generally recrystallised in response to the high strains.Sulphide mylonites are probably common rocks in many polydeformed massive sulphide deposits like Renström. They may have previously been misinterpreted as primary depositional textures.

Graphic galena-clausthalite solid solution in low Fe sphalerite from the Trout Lake massive sulfide ores, Flin Flon, Manitoba

In an extensive ore petrology study of the Trout Lake massive sulfide deposit (Healy and Petruk 1988) masses of graphic galena-clausthalite solid solution in low Fe sphalerite were observed in a sample of massive sphalerite ore (sample TLN.1-87). This texture was noted with particular interest in view of the sparse data on textures and intergrowths of clausthalite (Ramdohr 1980; Picot and Johan 1982) and the cue by Barton and Bethke (1987) to seek other minerals forming inclusions in Fe-poor sphaleritc that is replacing Fe-rich sphalerite. Semiquantitative electron microprobe analyses (using an MAC V at the Univ. of Manitoba) of the galena-clausthalite intergrowth gave a wide range in Se concentrations, with Se varying from <0.05 to 23 wt percent (end-member clausthalite contains 27.6 wt percent Se). Healy and Petruk (1989) reported that the Trout Lake ores assayed 380 ppm Pb and 49 ppm Se and inferred that chalcopyrite, pyrite, pyrrhotite, and galena (including clausthalite) are the principal Se-bearing minerals. Subsequently, Pinard and Petruk (1989) determined that the average Se content of Trout Lake galena was 0.85 wt percent Se, with values ranging from 0.1 to 3.4 wt percent Se. They also found grains of galena-clausthalite solid solution with Se values up to 11.2 wt percent Se. This paper presents and interprets the results of electron microprobe analyses of galena-clausthalite and sphalerite in sample TLN. 1-87.

Geologic and geochemical controls on the mineralogy and grain size of gold-bearing phases, eastern Australian volcanic-hosted massive sulfide deposits

The mineralogy and occurrence of gold varies in a systematic manner in volcanic-hosted massive sulfide deposits of eastern Australia. In deposits where it is concentrated with zinc at or near the top of the massive sulfide deposit, gold occurs in auriferous arsenopyrite and pyrite in the center of the lens, and electrum of variable fineness (300-900) in the baritic top of the lens. The former occurrence results from precipitation of the gold into the lattice of arsenopyrite and coprecipitation of submicroscopic gold grains in arsenic-poor pyrite. The latter occurrence results from direct precipitation of electrum from thio complexes owing to the decrease in a (sub H 2 S) caused by dilution and oxidation as hydrothermal fluids mix with seawater.In deposits where gold is concentrated with copper at the base of the sulfide lens and in the stringer zones, the gold occurs as electrum of high fineness (750-1,000) or as gold tellurides that are commonly associated with chalcopyrite and/or pyrite. This occurrence is caused by the direct precipitation of gold from chloro complexes owing to an increase in pH or a decrease in temperature as the hydrothermal fluids move up through the sulfide body.Gold in auriferous pyrite is readily mobilized during deformation. As the pyrite fractures, gold migrates via solution-precipitation mechanisms to cracks and grain boundaries to form electrum by combination with silver, which is released from sulfide minerals in a similar manner. With increasing stress the electrum is released from pyrite to recrystallize with other easily remobilized minerals such as galena. During this process, electrum grain size increases.Thermodynamic modeling of the variation in electrum fineness shows good agreement with variations measured in massive sulfide ores. For gold transported as a thio complex, the predicted fineness of 190 to 990 for temperatures of 200 degrees to 300 degrees C and typical volcanogenic fluids matches the observed variation in deposits of the Zn-Au association. The very high finenesses observed in deposits of the Cu-Au association can be modeled assuming gold chloro complex transport and high Ag/Au ratios in the hydrothermal fluids.

Transient electromagnetic surveys in the Okiep district

In the Okiep District early miners produced massive sulfide ore from some five deposits. Some of these deposits later contributed to the reserves of disseminated ore mined during modern times. It is unreasonable to assume that all of the massive sulfide bodies present within the area are intersected by the erosion surface and thus were discovered by the early miners. Consequently, blind massive sulfide ore bodies could be present and may have large quantities of disseminated ore associated with them. The transient electromagnetic method is ideally suited to exploring for massive sulfide bodies, and six test surveys were carried out at various sites in the district. Four of these surveys were unsuccessful whereas, at the remaining two sites, excellent anomalies were recorded. At Ezelsfontein East Extension an anomaly was recorded indicative of a massive sulfide body at shallow depth and of generally flat attitude. This anomaly has a time constant of 15 ms and the interpreted body was confirmed by a limited diamond drilling program. A deep, flat‐lying conductor was interpreted from the TEM results at Fonteintjie West Prospect. This anomaly, with a time constant of 0.6 ms, has limited size. Diamond drilling confirmed the presence of submassive to massive sulfide mineralization at this locale. Neither of these two drilled prospects had economic mineralization.

Rare earth elements and hydrothermal ore formation processes

The geochemical behaviour of REE is influenced by nearly all important hydrothermal ore formation processes including fluid-rock interactions, fluid precipitations, adsorption and scavenging onto particles, and changes in fluid temperature, pressure, pH, Eh, alkalinity and ligand concentration. Destabilization of REE complexes in response to these physicochemical changes and possible chemical-crystallographic controls determine the concentration and distribution of REE within hydrothermal minerals, mineraloids and amorphous phases. Alteration assemblages of intrusive related mineralisations may exhibit a wide range of REE distributions and REE fractionation trends. The REE distribution within these hydrothermally altered lithologies will depend on the REE concentration in the rock and the fluid, the partitioning behaviour of the REE between the rock phases and the fluid, and the types of alteration reactions which take place. Reactive rocks such as carbonate-rich lithologies of skarn deposits may gain or loose significant amounts of REE during fluid-rock interactions. REE geochemical investigations combined with fluid inclusion and stable isotope studies may point to the origin of alteration products and the source (s) of ore fluids. Ancient massive sulphide ores commonly exhibit anomalous Eu concentrations similar to many recent submarine hydrothermal precipitates. The recognition of hydrothermal fluid, seawater, sedimentary or volcanically derived REE distributions within ores, chemical sediments and hydrothermally altered lithologies may help to constrain genetic modelling of massive sulphide deposits. Care should be taken with the interpretation of REE distributions from metamorphosed hydrothermal ore deposits becase metamorphic and diagenetic alteration reactions may change the REE distributions. Mobility of REE may occur during diagenesis of carbonate-rich rocks and during the development of shear zones and migmatites. In contrast, very large fluid-rock ratios are necessary to cause significant changes in REE patterns of silicate-rich rocks during diagenesis and regional metamorphism.

Tourmalinites in supracrustal rocks in the Bjørnesund area, West Greenland

In the mid-Archaean supracrustal rocks of Bjørnesund south of Fiskenæsset metre wide anthophyllite-rich zones are found hosted in mafic volcanics. These zones, which are locally associated with sulphides, are interpreted as alteration zones from hydrothermal solutions which circulated through the volcano-sedimentary pile; in places they contain thin tourmalinites. The tourmaline composition and geologic setting of the occurrences closely resemble stratabound tourmalinites of submarine hydrothermal origin which elsewhere are associated with metalliferous deposits. It is concluded that the presence of the Bjørnesund tourmalinites associated with anthophyllite zones indicates that hydrothermal processes were once active in the depositional environment where the Bjørnesund supracrustals were formed; in other regions such processes are known to have generated massive sulphide ore bodies.