Base Metal Sulfide Mineralization Research Articles

The genesis of Pb‐Zn sulphide occurrences in the Lower Devonian Buchan Group, Victoria

Six minor sulphide occurrences hosted by the Lower Devonian Buchan Group have been investigated. Sulphide minerals and associated phases are hosted by both dolostone and limestone lithologies along stylolitized bedding planes, cross‐cutting fractures, low angle minor faults and in cavities. Mineralization was closely associated with minor structures of inferred Tabberabberan age (Middle Devonian), which it appears to have post‐dated, but was largely strata‐bound in nature. The mineralogy of the occurrences is simple and characterized by the following generalized paragenesis which reflects the increasing oxidation state and pH of the mineralizing fluids: pyrite (pseudomorphous after marcasite)‐galena ± sphalerite ± pyrite‐dolomite ± barite‐calcite ± fluorite ± dolomite. The sulphur isotope composition of sulphide minerals varies from ‐32.1 to +4.1‰, with iron and base metal sulphide minerals forming two distinct populations around ‐25‰ and 0‰, respectively. A single barite sample gives a sulphur isotope co...

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.

Hydrothermal W-Mo mineralization of the Cheongyang mine, Republic of Korea: A fluid inclusion and stable isotope study.

The Cheongyang W-Mo mine, about 120km south of Seoul, is composed of fissure-filling quartz veins within Cretaceous granite porphyry (70.5±1.6 Ma) and Precambrian banded biotite gneiss. The spatial distribution of mineralized veins exclusively within or near the granite porphyry and related acidic dykes indicates that W-Mo mineralization was associated with intrusion of the granite porphyry. The major ore minerals are wolframite, scheelite and molybdenite. Ore mineralization can be divided into three distinct stages: greisen, vein (early W-Mo, and late base-metal sulfides), and vug (carbonates). Fluid inclusion data indicate that the hydrothermal system at Cheongyang evolved from initial high temperatures (near 400°C) to later lower temperatures (near 150°C) from fluids with salinities between 0 and 6 wt. % eq. NaCl. Deposition of W-Mo minerals occurred at temperatures between 300° and 400°C mainly as a result of pH increase due to vapor loss by fluid boiling. Later deposition of base-metal sulfides and carbonates was the result of increasing influx of cooler meteoric waters. Fluid inclusion evidence of boiling during the early W-Mo mineralization indicates pressures 100 to 260 bars. Fluid inclusion data for the Cheongyang W-Mo deposits are similar with those of W-Mo deposits in the Hwanggangri district, and are quite different with those of deposits in the Pusan-Yangsan district. This difference in temperature and compositions of hydrothermal fluids for Korean vein-type W-Mo deposits may reflect different pressure-depth conditions of W-Mo mineralization associated with late Cretaceous granitic activity in Korea. Sulfur isotope compositions of sulfide minerals are consistent with an igneous source of sulfur with a δ34SΣs value near 4 per mil. There is a systematic decrease in calculated δ18Owater values with increasing paragenetic time in the Cheongyang hydrothermal system, from values of ≈8 per mil for greisen formation and W-Mo mineralization, to ≈3 per mil for base-metal sulfide mineralization, to ≈-2 per mil for carbonate mineralization in vugs. There is a small increase in δ18Owater values from greisen formation (6.3-6.8‰) to W-Mo mineralization (7.0-7.8‰), likely indicating boiling of ore fluids during W-Mo mineralization. The trend of overall decrease in δ18Owater values with time is interpreted to indicate progressive meteoric water inundation of an early magmatic W-Mo hydrothermal system.

Geochemistry of mineralized and barren, late proterozoic felsic volcanic rocks in southeastern Cape Breton Island, Nova Scotia (Canada)

Abstract Late Proterozoic volcanic rocks of southeastern Cape Breton Island, Nova Scotia (Canada), comprise part of the Avalon Composite Terrane of the Canadian Appalachians. They occur in several fault blocks (from southeast to northwest: Coastal, Stirling, East Bay Hills and Coxheath). The blocks are composed mainly of Late Proterozoic volcanic suites which are predominantly bimodal and were emplaced in a volcanic arc setting. The basaltic rocks range from tholeiitic (Coastal and Stirling) to calc-alkalic (East Bay Hills and Coxheath). The felsic volcanic rocks of the Stirling block host a massive sulphide deposit but no economic mineralization has been discovered in the volcanic rocks of the other blocks. The felsic volcanic rocks display significant differences in composition amongst the fault blocks. The Coastal and Stirling blocks contain low-K rhyolites with low-k abundances of strongly incompatible trace elements and relatively flat chondrite-normalized REE patterns. These rocks were probably derived by the melting of mafic igneous rocks compositionally similar to tholeiitic basalts of Archean greenstone belts or island arc tholeiites. Compared to the Coastal block rocks, the Stirling rhyolites are depleted in Eu, Sr and Al 2 O 3 indicating extensive feldspar fractionation. Trace element characteristics of the Stirling block rhyolites are closely comparable to those of the Archean felsic metavolcanic rocks from the Superior Province of the Canadian Shield which are preferentially associated with massive sulphide mineralizations. The similarities suggest that the low-K felsic volcanic rocks with flattish REE patterns and negative Eu and Sr anomalies are also indicative of a promising environment for massive base-metal sulphide mineralization in volcanic belts at least as young as Late Proterozoic. The rhyolites of the Coxheath block have calc-alkalic affinities, higher contents of strongly incompatible elements than the Coastal and Stirling rhyolites and their REE are patterns are enriched in light REE. The East Bay Hills felsic tuffs resemble the Coxheath rhyolites. They both could be derived by melting of a source with a composition of the average lower crust.

The physical and chemical structure of the Oku-aizu geothermal system, Japan.

The physical and chemical characteristics of the Oku-aizu geothermal system, northeast Japan, are reviewed, with these data used to construct a geochemical model of the system. The exploration of this area started in 1974, with a total of 39 wells being drilled through fiscal year 1990. A total of 509 t/h of dry steam (165°C) was confirmed during the simultaneous production test, equivalent to about 55 MW of electric power. The bedrock of the production zones are Miocene formations composed mainly of rhyolitic lavas and pyroclastics. Pre-Tertiary basement unconformably underlie these formations. The uppermost portion of the system is comprised of a Quaternary lacustrine deposit which was penetrated by shallow rhyolite intrusions at 0.2 to 0.5 Ma. This lacustrine deposit is thought to be related to a subsidence structure. Two different types of hydrothermal alteration are recognized. One is related to submarine volcanic activity in the Miocene, and is composed of chlorite and sericite. The other is a product of the present geothermal system, and has advanced argillic alteration zones at the surface and an argillic zone at depth. Alteration minerals observed in the advanced argillic zone are smectite and mordenite with a minor amount of sericite, alunite and kaolinite. Those in the argillic zone are smectite, interlayered illite-smectite, kaolinite, zeolite and K-feldspar. Anhydrite is common in the deeper portion of the system at higher temperature. The highest measured temperature is 341°C, and temperatures higher than 300°C are quite common in production zones. Isotherms at −1200 m ASL open to the SE, which may indicate the direction of the heat source. From the deepest drilled levels to about −200 m ASL, the thermal gradient is small, indicating the existence of non-boiling convection over this depth range. The uppermost part of the convection cell is limited by a sealed zone caused by argillic alteration and a Miocene mudstone formation. The system is considered to have had little boiling in its natural state before exploration. However, it is presently boiling around geothermal wells due to the pressure drop caused by reservoir fluid discharge. The composition of the reservoir fluid is characterized by high salinity (about 2 wt%) and a large amount of non-condensible gas (about 1 wt% CO2 and 250 mg/kg H2S). This, as well as its very high underground temperature, is consistent with isotopic indications of a magmatic component to the discharge of the system. This idea is also concordant with the B/Cl and Br/Cl ratios. Small amounts of base metal sulfide mineralization (pyrite, sphalerite, galena, chalcopyrite etc.) are present at the level of the production zone. This is considered to be a product of the present geothermal system, judging from its mode of occurrence. Significant concentrations of precious metals (Au ≤ 116 mg/kg, Ag ≤ 3.49 wt%) were found in sulfide scales collected from surface two-phase lines.

Chemistry of streams draining grassland and forest catchments at Plynlimon, mid-Wales

Abstract The chemistry of streamwater, bulk precipitation, throughfall and soil waters has been studied for three years in two plantation forest and two moorland catchments in mid-Wales. Na and CI are the major ions in streamwater reflecting the maritime influence on atmospheric inputs. In all streams, baseflow is characterised by high pH waters enriched in Ca, Mg, Si and HCO3. Differences in baseflow chemistry between streams reflect the varying extent of calcite and base metal sulphide mineralization within the catchments. Except for K, mean stream solute concentrations are higher in the unmineralized and mineralized forest catchments compared with their respective grassland counterparts. In the forest streams, storm flow concentrations of H+ are approximately 1.5 times and Al four times higher than in the moorland streams. Annual catchment losses of Na, Cl, SO4, NO3, Al and Si are greatest in the forest streams. In both grassland and forest systems, variations in stream chemistry be explained by mixing...

Sediment geochemistry and base metal sulphide mineralisation in the Quidong area, Southeastern New South Wales, Australia

Geochemically anomalous, pyritic sediments occur directly above a Mid Silurian unconformity in the Quidong area of southeastern New South Wales. The composition of these sediments reflects derivation from a mixture of: (a) feldspar- and mica-depleted detritus reworked from underlying quartz-rich flysch; (b) Mg-rich clay or chlorite precipitated from hydrothermal exhalations; and (c) pyrite formed by reaction of iron in clays or oxides with reduced sulphur derived largely from sea-water sulphate and possibly a magmatic source. Three types of base metal sulphide mineralisation occur at Quidong including: (a) weak syngenetic concentrations in the pyritic sediments; (b) stratabound and fault-controlled bodies of massive sulphides hosted by the pyritic sediments and containing higher grade Pb, Zn and Cu; and (c) small vein and cavity fillings of galena, barite and other minor sulphides in overlying limestones. All types of mineralisation are related to hydrothermal activity which occurred during and after deposition of the pyritic facies. The geochemistry of the immediately underlying basement rocks and Pb isotope data indicate that the source of the metal-bearing fluids was deeper in the crust and probably related to widespread partial melting and magmatic processes. The sulphidic sediments and stratabound sulphide deposits represent syngenetic-epigenetic, sediment hosted mineralisation developed in a shallow marine environment, distal from major volcanic centers. This style of mineralisation has not previously been described from the region. It has some similarities to the Irish-Alpine type spectrum of deposits best known in Europe.

The distribution of chlorine and iodine in soil in the vicinity of lead mining and smelting operations, Bixby area, S.E. Missouri, U.S.A.

Iodine and Cl are enriched in soils in the vicinity of the Magmont and Buick lead mines near Bixby, southeastern Missouri. The enrichments, up to 5.6 ppm I and 305 ppm Cl, are against regional background of 1.26 ppm I and 41 ppm Cl. The area of highest I and Cl is thought to reflect a zone of base metal sulphide mineralization occurring about 400 m below the surface. Iodine and Cl are also enriched in soils immediately adjacent to a tailings pond, hence these elements would appear to be leached from this source. A zone of enhanced I values (up to 2.65 ppm I) to the north of a lead smelter is superimposed on a much larger zone of lead enrichment (up to 12,000 ppm Pb) and is thought to represent I released from sulphide ores on smelting.

Sulfur, oxygen, carbon, and strontium isotope geochemistry of the Sarrabus-Gerrei mining district, southeastern Sardinia, Italy

The Sarrabus-Gerrei mining district consists of several hydrothermal ore deposits hosted within black shale, metasandstone, and metavolcanic (porphyroid) and granitoid rocks of Paleozoic age. Ore consists of base metal sulfide minerals (chiefly galena and sphalerite) in a gangue of barite, fluorite, calcite, and quartz. Stable isotope compositions were determined for 69 base metal sulfide minerals, 33 barite, 23 calcite, and 3 quartz samples from 16 deposits throughout the district; some synsedimentary pyrite from the black shale also was analyzed. In addition, 87 Sr/ 86 Sr analyses of 14 barite, 6 calcite, and 11 different country-rock samples were performed.Barite displays delta 34 S = 14.1 to 21.9 per mil, delta 18 O = 9.3 to 18.8 per mil, and 87 Sr/ 86 Sr = 0.7103 to 0.7131. No significant difference in delta 34 S values exists among barite from the various host rocks, but barite from the black shale is depleted in 18 O with respect to barite from the sandstone, porphyroids, and granitoids. Barite from the granitoids has the highest (0.7128-0.7131) and the lowest (0.7103-0.7111) 87 Sr/ 86 Sr ratios; these latter values are close to the initial Sr isotope ratios of granitoids from the Sarrabus district. Moreover, barite always has a lower 87 Sr/ 86 Sr ratio than coexisting calcite, suggesting that the two minerals are not cogenetic. Sulfide minerals show delta 34 S values between -8.0 and +5.1 per mil. Galena and sphalerite from the Baccu Arrodas deposit within the black shale are appreciably depleted in 34 S compared to those from other shale-hosted deposits; galena and sphalerite from these latter deposits and all other deposits are quite uniform in 34 S. Sulfur isotope temperatures calculated for most barite-galena and barite-sphalerite pairs range from 247 degrees to 390 degrees C. At these temperatures, water in equilibrium with barite should have delta 18 O values in the following ranges: 6.2 to 7.9 per mil for the Baccu Arrodas deposit within the black shale, 9.7 to 16.0 per mil for deposits within the sandstone, and 10.5 to 17.9 per mil for deposits within the granitoid. Most calcite samples have delta 13 C values from -8.2 to -4.8 per mil, compatible with a magmatic origin of the carbon.The data are consistent with the following genetic model: the Baccu Arrodas ore deposit within the black shale formed from metamorphic fluids, probably related to the tectonism and metamorphism of the Hercynian orogeny about 365 m.y. ago. The main source of sulfur could have been pyrite present in the host rock. Barite in the other black shale-hosted deposits may have the same origin as at Baccu Arrodas, but the sulfide ore should have formed predominantly from magmatic sulfur at the time of emplacement of granitic plutons about 300 m.y. ago. The ore deposits within the other host rocks can be ascribed to this magmatic event. Barite and sulfide minerals were deposited from mostly magmatic water; sulfur was provided to the ore-forming fluids by granitic magmas, with a minor component from other wall rocks. Late-stage calcite and quartz samples may have crystallized from water with a meteoric component.

Alteration and base metal sulfide mineralization in the porphyries of the Banke Complex, Northern Nigeria

Alteration and base metal sulfide mineralization in the porphyries of the Banke Complex, Northern Nigeria

Trace-element geochemistry of ore-associated and barren, felsic metavolcanic rocks in the Superior Province, Canada

Archaean felsic metavolcanic rocks in the Superior Province of the Canadian Shield may be divided into three major groups on the basis of trace-element abundances and ratios. (1) FI felsic metavolcanic rocks are dacites and rhyodacites characterized by steep REE patterns with weakly negative to moderately positive Eu anomalies, high Zr/Y, low abundances of high-field-strength elements (e.g., HREE, Y, Zr, Hf), and high abundances of Sr. Examples occur in the Bowman Subgroup and Skead Group in the Abitibi Belt, in the Kakagi Lake, Lake of the Woods, Shoal Lake, and Sturgeon Lake areas of the Wabigoon Belt, and in the Confederation Lake area of the Uchi Belt. None of these horizons, as known, hosts base-metal sulphide deposits. (2) FII felsic metavolcanic rocks are rhyodacites and rhyolites characterized by gently sloping REE patterns with variable Eu anomalies, moderate Zr/Y, and intermediate abundances of HFS elements and Sr. Examples occur in the Misema Subgroup of the Abitibi Belt, in the Wabigoon Lake and Sturgeon Lake areas of the Wabigoon Belt, and in the Confederation Lake area of the Uchi Belt. Of these horizons, only those in the Sturgeon Lake area host base-metal sulphide deposits, and they exhibit the most pronounced negative Eu anomalies of this group. (3) FIII felsic metavolcanic rocks are rhyolites and high-silica rhyolites characterized by relatively flat REE patterns, which may be subdivided into two types. FIIIa felsic metavolcanic rocks exhibit variable negative Eu anomalies, low Zr/Y, and intermediate abundances of HFS elements and Sr. Examples occur in the Noranda mining district of the Abitibi Belt. FIIIb felsic metavolcanic rocks exhibit pronounced negative Eu anomalies, low Zr/Y, high abundances of HFS elements, and low abundances of Sr. Examples occur in the Kamiskotia, Kidd Creek, Matagami, and Noranda mining districts, the Garrison Subgroup in the Abitibi Belt, and at the South Bay mine in the Confederation Lake area of the Uchi Belt. All of these FIII horizons, with the exception of Garrison, host important base-metal sulphide deposits.These geochemical variations are interpreted to reflect differences in the petrogenesis of the felsic magmas, specifically, their formation or degree of modification in high-level magma chambers, which also influenced the formation of massive base-metal sulphide deposits. Most massive base-metal sulphide deposits in the Superior Province are underlain by subvolcanic magma chambers, which have been interpreted to have supplied heat to drive the ore-forming hydrothermal systems. FIII and some FII felsic volcanic rocks are interpreted to have been derived from these high-level magma chambers, accounting for their distinctive geochemical signatures and their association with massive base-metal sulphide mineralization. In contrast, FI felsic volcanic rocks are interpreted to have been derived from a deeper source and are considered to have escaped significant high-level modification, accounting for their distinctive geochemical signatures and the lack of associated base-metal sulphide mineralization. With certain limitations, the geochemistry of felsic metavolcanic rocks therefore may be used as a guide to identify prospective horizons for massive base-metal sulphide exploration in the Superior Province.

Geology, geomorphology and mineral resources of khodana and adjoining areas (Haryana) and their significance in landuse planning

The area around Khodana, Bhiwani district, Haryana forms a part of the semi-arid region of western India. As vegetation is sparse and mainly xerophytic, photointerpretation technology was effectively utilised in bringing out the broad geology and structural pattern of the mature Precambrian terrain, which were ground-checked and correlated with the results of detailed integrated surveys carried out for base metal sulphide mineralisation in the area.

Regional geochemical prospecting in Calabria, Southern Italy

A governmental regional drainage survey assessed the mineral potential of Calabria in 1974. The Calabria area of southern Italy is a mineralized region in which Penninic and Austro-Alpine nappes, comprising ophiolites, metasedimentary rocks (phyllites, paragneiss, granulites, amphibolites) and granitic and granodiorite plutons, are overthrusting carbonate Apenninic nappes. Pre-Alpine stratabound and vein-type base-metal sulfide mineralization predominate, tectonically redistributed during the Alpine orogeny. A total of 5518 samples of the original 6999 active stream sediments from an area of 10,000 km 2 were analyzed by X-ray fluorescence and atomic absorption for Cu, Pb, Zn, Hg, Fe, Mn, As, W, Mo, Sn, and Be on the minus 180 μm fraction. Follow-up investigation of a 39-km 2 area around Bivongi-Ferdinandea using 760 soil samples analyzed for Fe, Mn, Zn, Cu, Pb, K, Mo, and Au, investigated molybdenite and chalcopyrite occurrences associated with granodiorite. A five-factor R-mode factor analysis reflected geological and mineralization features in the reconnaissance data. Factor 1 (Fe-Mn-Cu-Zn) is related to the Fe and Mn distribution in the stream sediments. Factor 2 (Be-Sn) reflects presence of pegmatites within metamorphic and magmatic rocks. Factor 5 (Mo) is essentially a single-element factor. Factor 4 (As-Pb-Zn-Hg-Cu) reflects mineralization. Factor 3 (W-Ba) does not cluster in association with any specific lithological unit. Significant metal associations, recognized from R-mode factor analysis, were used as independent variables and regressed against the original metal concentration of each sample. Residuals of multiple regression analysis, differentiated features representing mineralization from those related either to lithologic or geochemical parameters. New prospecting targets were identified and areas of known mineral occurrences were evident in the distinction of residuals. R-mode factor analysis of data from the follow-up investigation identified three factors. Factor 2 (Mo-K) reflects Mo mineralization and associated hydrothermally altered areas but does not suggest additional target areas.

Geochemical constraints on the genesis of copper mineralization in the Mount Gunson area, South Australia

Late Proterozoic unmetamorphosed rocks on the Stuart Shelf. Two major styles of base metal sulfide mineralization: sandstone-hosted deposits associated with the Pandurra Formation and stratiform dolostone-mudstone-hosted deposits in the Tapley Hill Formation. The concentration of Cu-Pb-Zn mineralization supports the hypothesis that mineralizing fluids flowed up through major zones of crustal weakness and precipitated as Cu-Pb-Zn sulfides in chemically and structurally favorable beds.--Modified journal abstract.

The geological setting of the platinoid and base metal sulfide mineralization in the Platreef of the Bushveld Complex in Drenthe, North of Potgietersrus

On the farm Drenthe, the Platreef consists of a complex assemblage of norites, melanorites, pyroxenites, serpentinites, and xenoliths of dolomite. It is a zone of up to 250 m thick between the basal contact of the complex and the overlying main zone norites. The composition of the orthopyroxene shows a rapid increase from En 60 to En 70 in the lower 30 m of the sequence, and a sharp drop from En 70 to En 60 at the top of the Platreef. This compositional break at the upper contact is due to a change from primary bronzite in the Platreef to inverted pigeonitc in the main zone.High-temperature metamorphic assemblages are preserved in the larger xenoliths, whereas their outer margins and the smaller xenoliths contain retrograde metamorphic mineral assemblages. The igneous rocks display irregularly distributed zones of alteration to bronzite bastitites and serpentinites. The alteration process was probably dominated by degassing of the xenoliths.The distribution and composition of the major sulfide minerals indicate a change from a sulfur-rich to a metal-rich environment during the crystallization of the Platreef. Pyrite, monoclinic pyrrhotite, pentlandite, and chalcopyrite dominate the lower part of the Platreef; and hexagonal pyrrhotite, pentlandite, and chalcopyrite (+ or -cubanite) prevail in the upper portion. Whole-rock analyses show an upward decrease in the Ni/Cu ratio from 1.95 to 1.70. Large variations occur in the distribution and grade of Ni, Cu, and platinum-group elements, although highest values are associated with a zone of serpentinization. The Ni and Cu values are unreliable indicators in predicting Pt and Pd concentrations in zones of major metal enrichment. Ni and Cu correlate strongly while Pt and Pd have more significant correlations with Ni than Cu, although the minerals of the platinum-group elements are primarily associated with chalcopyrite. The Pt/Pd ratio decreases upward in the sequence.The mineralization is considered to have been caused by the breakup of dolomite xenoliths that added CO 2 , H (sub 2>) O, and S to the magma, thus simultaneously lowering the solubility of sulfur in the magma and adding sulfur from an external source, which resulted in the precipitation of an immiscible sulfide liquid.

The origin of a mississippi valley-type deposit in the Viburnum Trend, Southeast Missouri

Petrologic and isotopic studies of the base metal sulfide mineralization in the Buick mine have established the following paragenesis: an early stage of fine-grained pyrite and marcasite replaced the dolostone host rock; the iron disulfides and dolostone were replaced by sphalerite during the main stage of sphalerite precipitation (delta 34 S sp 16.5 to 19.0ppm). Following the sphalerite, two main stages of galena were precipitated--a cuboctahedral stage (delta 34 S gn 21.3 to 10.0ppm), during which galena replaced dolostone and sphalerite and grew in vugs in the dolostone and in the sphalerite, and a cubic stage (delta 34 S gn 10.5 to 0.9ppm). The two stages of galena precipitation were separated by a period during which the main stage of sphalerite and the cuboctahedral galena were partially dissolved and bladed marcasite (delta 34 S mc 2.3 to 4.1ppm) was precipitated, and followed by a period of mild etching of the cubic galena. Minor amounts of pale sphalerite (delta 34 S sp 14.0 to 16.5ppm) were then precipitated. White and colorless rhombs of dolomite were precipitated at the same time as some of the main stage of sphalerite, the cubic galena, and the pale sphalerite. After the pale sphalerite was precipitated, chalcopyrite, quartz, and siegenite were precipitated, followed by calcite and finally kaolinite. The paragenetic and textural data for both galena and sphalerite in the Buick mine strongly suggest that these minerals were precipitated and dissolved repetitively during ore formation. Similar textural evidence for galena reported by other workers for different mines in the Viburnum Trend implies that repetitive precipitation and dissolution of galena was probably an important process during the formation of the entire Viburnum Trend.Fluid inclusions in late quartz crystals coprecipitated with chalcopyrite have filling and freezing temperatures within the ranges reported for sphalerite from other parts of the Viburnum Trend, demonstrating that the ore-forming solutions in the Viburnum Trend were probably at temperatures in the range 105 degrees to 141 degrees C (assuming a maximum pressure correction of about 11 degrees C) and were extremely saline, containing about 4.0 to 5.0 molal chloride. Consideration of typical chemical analyses of fluid inclusions from other Mississippi Valley-type deposits suggests that the maximum likely concentration of dissolved CO 2 in the ore-forming solutions was about 0.05 molal. At these concentrations, the ore-forming solutions that travelled to the site of deposition could have had pH9s about two units less than neutral and still have been in equilibrium with dolomite or calcite.The ranges of sulfur and lead isotopic compositions of galena from the Buick mine are very similar to those established by earlier studies of the Old Lead Belt and adjacent subdistricts. However, the correlation of isotopically lighter sulfur with more radiogenic lead is much stronger for the Viburnum Trend, suggesting that galena was precipitated from many solutions, each of which carried lead and sulfur together and had different lead and overall sulfur isotopic compositions.The textural, paragenetic, and isotopic data for the Buick mine cannot be easily explained either by a sulfate reduction model in which sulfate was reduced by organic matter or methane, or mixing models in which base metals and reduced sulfur were transported separately to the site of deposition. Instead, the data strongly support a reduced sulfur model for ore formation according to which the base metals were transported together with reduced sulfur (as H 2 S), in the same solutions, to the sites of deposition. The reduced sulfur model is also supported by a theoretical calculation of the solubilities of galena and sphalerite, which demonstrate that low, but economically significant, concentrations of lead or zinc could have been transported as chloride complexes, together with equivalent concentrations of reduced sulfur, at temperatures of 100 degrees to 150 degrees C and pH9s of one to two units less than neutral, and could have been precipitated by pH increases, cooling, or dilution at the sites of deposition.

Regional reconnaissance exploration rock geochemistry for massive sulphides, New Brunswick, Canada

In the Bathurst District of New Brunswick there are more than 50 known occurrences of base metal sulphide mineralization within an area of Palaeozoic volcanic-sedimentary rocks approximately bounded by the Rocky Turn deposit in the north, the Key Anacon deposit in the east, the Heath Steele deposit in the south, and the Devil's Elbow deposit in the west. Only four of these occurrences are, or have been, producing mines; 19 are classed as “major occurrences”. The area is highly prospective for massive sulphide deposits of the Brunswick Mining and Smelting and Heath Steele type; it would obviously be of considerable importance to define the zones within the sequence where major occurrences should be sought. To determine whether exploration rock geochemistry could be used on a regional reconnaissance scale, 419 samples of rhyolite from an area of 2000 km 2 (at an average density of one sample per 5 km 2) were analyzed for total content of Cu, Pb, Zn, Ca, Mg, K, Na, Fe, and Mn. The data were processed by calculating the geometric mean of all samples in cells of approximately 10 km 2. Contrary to the relations documented on a mine scale (within one kilometre of major deposits), where the clearest halos are given by major elements, it is the ore elements that give the best regional patterns. The producing mines and the most important of the known occurrences all lie in zones where rhyolite contains less than 10 ppm Cu. Element ratios considerably enhance anomalous relations. The Zn:Pb ratio of the sulphides in the main deposits is 2.4–2.8, regardless of grade. It is demonstrated that all present and past producing mines and the most important known major occurrences lie within well-defined zones of Zn:Pb ratios of 2.4–2.8. Similarly, zones where the Pb:Cu ratio is > 3.0 and the Zn:Cu ratio is > 7.0 also define the most important deposits. These ore-element relations derived from a low sample-density survey define priority zones for detailed exploration for significant major massive sulphide deposits.