Sphalerite Grains Research Articles

Tennantite decomposition: Evidence from the Kedabek copper deposit, Azerbaijan

A microscopic study of ores of the Kedabek deposit revealed unusual microtextures of intimate intergrowths of fine grains of chalcopyrite, fahlore (tetrahedrite), arsenopyrite and sphalerite. These textures result from fahlore (tennantite) decomposition. A detailed study of the chemical composition of earlier and later generations of ore minerals and the bulk composition of the products of decomposition of tennantite suggest that the decomposition of tennantite is due to an increase in the activity of antimony. The reason for such an increase is the crystallization of earlier generations of arsenopyrite and tennantite which caused a depletion of arsenic and an increase in the antimony/arsenic ratio in residual solutions.

Formation of accessory sphalerite by thermochemical sulphate reduction in Lower Paleozoic carbonate rocks (St. Lawrence Lowlands, Québec, Canada)

Sulphur isotope compositions of accessory sphalerite grains in carbonate rocks of the St. Lawrence Lowlands are characterized by a wide range of values (-5.4 to +19.9%c). The variations correspond to systematic trends when results are considered from site to site. δ 34S: (1) increases from the base to the top of the carbonate succession; (2) decreases from the center of the basin towards the margin; and (3) increases northeastward along the margin. Such variations of the isotopic composition at a scale of tens of kilometers must reflect controls well beyond the centimetric scale at which precipitation of the sulphides occurred. Petrographic features of these accessory sulphides suggest a late origin, in temperature ranges high enough to sustain thermochemical sulphate reduction. The onset of this reaction is thought to be responsible not only for the sphalerite precipitation, but also for the systematic δ 34S variations observed: ongoing reduction would shift ΣSO 4/ ΣH 2S in pore fluids to lower values, with concurrent δ 34S increase of the reduced species, the trend being possibly enhanced by ongoing sulphide precipitation. Thus, at a given site, first precipitates would show a relatively low δ 34S, and late precipitates, stratigraphically higher in younger rocks, a relatively high δ 34S, explaining point (1) above. Lower subsidence towards the margin would delay the onset of reduction and precipitation, resulting in relatively low δ 34S for these sites compared to those towards the center, thus explaining point (2). The Beekmantown Group, bearing evaporitic sulphates, is absent in the northeastern part of the Lowlands. The long distance from sulphate source beds probably favored fractionation of the reservoir of dissolved sulphur. In addition, the reduced availability of sulphate might have contributed in reaching low ΣSO 4/ ΣH 2S ratios more rapidly in the northeastern than in the southwestern part of the margin, thus explaining point (3) above.

Sphalerites in EH chondrites: I. Textural relations, compositions, diffusion profiles, and pressure-temperature histories

Sphalerite-bearing assemblages in the unequilibrated EH chondrites Qingzhen, Yamato 691, and Indarch were studied to understand their pressure and temperature histories. Sphalerite in Qingzhen and in Yamato 691 are zoned with descending FeS concentration to troilite. Compositions of sphalerite cores in Qingzhen and Yamato 691 are similar (47.03–49.6 mol% FeS in Qingzhen versus 47.4–49.7 mol% FeS in Yamato 691). The FeS contents of the sphalerite rims to troilite are comparable (45.98 mol% in Qingzhen, 42.3 mol% in Yamato 691). Exsolution lamellae of troilite were found in several sphalerite grains in Qingzhen. The FeS content in these grains also decreases to a value as low as 45.4 mol% FeS near the contact to the lamellae. Secondary homogeneous sphalerite, formed during a late metamorphic episode in Yamato 691 (800 M.Y. ago), is characterized by low FeS content (43.2–42.3 mol% FeS). The descending concentration profiles in sphalerites in Qingzhen and Yamato 691 were produced by re-equilibration of the sphalerite with the neighbouring troilite at high pressures during late metamorphic episodes. However, the compositions of sphalerite cores may reflect primordial compositions after sphalerite condensation but before its burial in the parent asteroid. We obtain closure temperatures between 639 K and 728 K, and 650 K and 732 K for sphalerites in Qingzhen and Yamato 691, respectively. From compositions of sphalerite rims we estimate that the re-equilibration took place at 0.18 kbar in Qingzhen and at 0.77-0.95 kbar in Yamato 691. Qingzhen and Yamato 691 were metamorphosed at different times and much later after accretion (1.4 B.Y. and 800 M.Y., respectively).Sphalerite in Indarch displays complex textures indicative of multiple metamorphic events. Its FeS-content varies between 53.0 and 56.0 mol%. The grains contain open pores and are subdivided by platelets arranged parallel to ( 111 ) planes of the original sphalerite. This is strongly suggestive of sphalerite-wurtzite inversion and release of a volatile species (83) in a violent event.

Mineralogical characterization of silver flotation concentrates made from zinc neutral leach residues

Silver flotation concentrates prepared from high-silver (1480 ppm Ag) and low-silver (300 ppm Ag) neutral leach residues have been examined mineralogically to determine the phases present and to elucidate the behavior of silver during zinc processing. The flotation concentrates consist principally of sphalerite although lesser amounts of zinc ferrite and PbSO4, as well as traces of other phases, also are present. In the high-silver flotation concentrate, silver occurs mostly as Ag2S or (Ag, Cu)2S rims on sphalerite although (Ag, Cu)2S inclusions within sphalerite also are present. Trace amounts of a Cu-Ag-S-Cl phase are present on rare copper oxide grains, and this silver-bearing phase may be a fine mixture of Ag2S, AgCl, and Cu2S. In the low-silver flotation concentrate, silver occurs mostly as Ag2S although traces of silver-bearing CuS and Cu2S also are present. The Ag2S occurs as <1 μm particles disseminated in elemental sulfur-silica gel patches, as discontinuous rims or isolated patches on sphalerite grains, and as tiny free particles. Silver chloride was not detected. These studies suggest that silver dissolves during neutral leaching and subsequently reacts with sphalerite or other sulfides to form silver sulfide.

Sulfur Isotopes Related to Sedimentation Conditions for Metalliferous Black Shales of Pennsylvanian Age: ABSTRACT

Sulfur isotope compositions were determined for pyrite and sphalerite grains isolated from 8 metalliferous Missourian, Desmoinesian, and Atokan black shales from Missouri, Illinois, Kansas, and Indiana. The younger Missourian shales from the Forest City and Illinois basins contain consistently light sulfur as expected for euxinic conditions, but sulfides from the older shales show heavier and more erratic sulfur values. These isotope patterns suggest that younger shales accumulated slowly offshore, although older shales may have been deposited more quickly in shallower water. Isotope values, which also correlate with heavy metal patterns (e.g., higher molybdenum values for the easternmost older shales), may therefore reflect gradually deepening conditions for the epicontinental Pennsylvanian seas of the United States Mid-Continent. End_of_Article - Last_Page 246------------

Sphalerite geobarometry in the Cu-Fe-Zn-S system

Phase relations have been determined among sphalerite, pyrite, pyrrhotite, and intermediate solid solution (iss) from 439 degrees to 725 degrees C at 1 bar and 353 degrees to 760 degrees C at 5 kb. The extents of solid solution in the four-phase assemblage are described as functions of temperature (in Kelvins) by the following equations: log mole % ZnS in iss (1 bar) = 2.795 - 1,632 T (super -1) , log mole % ZnS in iss (5 kb) = 2,659 - 1,608 T (super -1) , log mole % CuS in pyrrhotite (1 bar) = 2,546 - 1,866 T (super -1) , log mole % CuS in sphalerite (1 bar) = 4.202 - 3,755 T (super -1) , and log mole % CuS in sphalerite (5 kb) = 4.084 - 3,791 T (super -1) . The pressure dependence (in kb) of the composition of sphalerite in equilibrium with pyrite and pyrrhotite with or without iss can be expressed for the temperature-independent portion of the sphalerite geobarometer as P = 42.30 - 32.10 log mole % FeS.Because of the low solubility of copper in sphalerite in the four-phase assemblage below 650 degrees C, the presence of chalcopyrite in natural ores should be of no consequence to sphalerite geobarometry over its temperature-independent portion. Nevertheless, sphalerites which are inhomogeneous in FeS and which contain abundant blebs of chalcopyrite in excess of the amount that can be reasonably accounted for by an equilibrium process of solubility and subsequent exsolution are a common feature of many sulfide ores. Such sphalerite grains should be avoided in applying the geobarometer. The chalcopyrite blebs may be a product of a premetamorphic sphalerite-chalcopyrite intergrowth or represent true exsolution from sphalerite in any one of (1) a former sphalerite + pyrrhotite + iss equilibrium, (2) a stable four-phase assemblage at temperatures above the temperature-independent portion of the geobarometer, or (3) a metastable four-phase assemblage in which pyrite was inert. Use of the sphalerite geobarometer requires a thorough understanding of the textural relationships and discrimination of equilibrium from disequilibrium or low-temperature reequilibrated assemblages.

Environment of ore deposition at Cerro de Pasco, Peru

Environment of ore deposition at Cerro de Pasco, Peru

Electron-probe check of fe-distribution in sphalerite grains of the Nagybörzsöny hydrothermal ore deposits, Hungary

Micron-scale iron-distribution patterns in sphalerite grains derived from two contrasting mineral assemblages of the Nagyborzsony hydrothermal ore deposits (“high temperature” stockwork and “low temperature” veins), were determined with electron microprobe, pointcounter and continuous chart-recorder measurements along radial profiles. A significant oscillation of iron-content ranging from 8.1 to 18 per cent for stockwork and 4.3 to 15.8 per cent for veins took place during crystal growth. Homogeneous crystal cores are characteristic for vein-filling, while replacement occurring in deeper levels of the stockwork results in mosaic cores. Frequent oscillation of iron content is relevant to top-sections of the ore bodies, copious supply being recorded in recurring shells, whereas scanty, unilateral ones produce crescents arranged in onion-like structure.

Recrystallization Softening and Hardening in Sphalerite and Galena

Naturally deformed and work-hardened sphalerite and galena have been experimentally recrystallized and the effects of this process on microhardnesses have been studied in detail. Specifically, the hardnesses of recrystallized material on one hand, and of the remaining unrecrystallized material on the other, have been followed separately from the onset of recrystallization and until this process was complete.For both minerals the effects of recrystallization were (1) an initial sharp fall in hardness where the material recrystallized (2) a lesser but still obvious fall in hardness of the remaining unrecrystallized material, indicating a period of incipient ordering prior to visible recrystallization (3) a hardening--secondary hardening--of recrystallized material as recrystallization proceeded. Secondary hardening of galena may yield a hardness greater than that of the initial work-hardened state. This may however, be a recrystallization preferred orientation effect, and therefore apparent only.The two minerals commence recrystallization somewhat differently. Sphalerite recrystallizes by apparently random nucleation of small strain-free grains within the body of individual original strained grains, whereas the new grains of galena grow from subgrains or by outgrowth of whole grains forming part of the original strained polycrystalline aggregate. Growth of new sphalerite grains is accompanied by the development of numerous twins.

Evolution of thought on ore controls in east Tennessee

All early students of the zinc and barite deposits of East Tennessee, noting the geographic coincidence of the principal mining area with complex late Paleozoic geologic structure, concluded that (1) the ore-bearing breccias were produced by Appalachian tectonic forces and that (2) mineralizing solutions gained access through fault-produced channelways.In 1931, Ulrich suggested that the breccias were produced by collapse and fragmentation resulting from solution of the carbonate rocks. This idea was promptly rejected by most geologists, and for the next 15 years nearly all papers on these deposits contained refutations of Ulrich's hypothesis and proof of the late Paleozoic tectonic origin of the breccias.Recognition by Odell in 1950 of a spatial association between thinning of the lime-stone zone of the Kingsport Formation and the presence of ore bodies, plus the observation that all identifiable blocks in the breccias were displaced downward as much as 30 to 50 feet, led him to suggest that foundering of the rocks over solution-thinned zones produced the breccias. Hydrothermal fluids, rather than ordinary ground water, were thought to be the dissolving agent.Kendall, in 1960, described a stratified varved matrix with detrital sphalerite grains in many breccias at Jefferson City, in which lamination is parallel to bedding of the host Kingsport Formation, indicating that both brecciation and mineralization occurred while the rocks were horizontal. Ruskell about 1960 pointed out the similarity between cross sections of southwest Wisconsin ore bodies and those of East Tennessee when the latter were rotated to a horizontal position. Detailed studies of the post-Knox unconformity led to recognition of a paleokarst topography of considerable relief developed on this surface. Regional stratigraphic studies of the Knox Group by Harris demonstrated the probability of pre-Middle Ordovician exposure of the Kingsport somewhere to the north or northwest, providing a recharge area for ground waters at that time.Detailed mapping of ore bodies, barren areas, and related structures by the mining-company geologists, plus application of the principles of ground-water hydrology and rock mechanics, has led to the theory, generally accepted in the district, of solution-collapse origin of the breccias, probably during Ordovician time.

The george lake zinc deposit, wollaston lake area, northeastern saskatchewan

During a search for the source of a quartzite glacial erratic carrying galena, newly published aeromagnetic maps attracted attention to an unsuspected metasedimentary series. Subsequent prospecting in this belt uncovered quartzite boulders containing finely-disseminated sphalerite together with minor galena, pyrite, and pyrrhotite. These boulders formed a glacial train some 10 miles in length flanked by pyrite-bearing quartzite boulders along a length of 40 miles. Samples of the sphalerite-bearing float assayed up to 14% Zn and 4 1/2% Pb.The surficial geology showed that the valley along which the float was found represents a melt water channel that had exposed the boulders from the till.The source of the float was found to be at the east side of the southwest end of George Lake where bedrock was covered by drift ranging from 0-100 feet in thickness.Surface drilling indicated a deposit of 5 million tons averaging 2.65% Zn and 0.35% Pb, which included a higher grade core of 2.9 million tons averaging 3.67% Zn and 0.53% Pb. These estimated tonnages are based on shallow drilling and are assumed to extend to a maximum depth of 400 feet. The grade of material found at the source of the float was much lower than that indicated by assays of the erratics. This difference was ascribed to irregular sampling of the float.The metal contents of the adjacent rock units are listed and compared with average results from other similar types elsewhere.The trace element content of the sphalerite indicates certain similarities when compared with sphalerites from Mississippi Valley Type deposits.The sulfides are considered to have been deposited syngenetically. Metamorphism subsequently resulted in some minor redistribution of ore minerals as shown by the inclusion of sphalerite grains in metamorphic garnets.

Distribution and Composition of Sulfide Minerals at Balmat, New York

Distribution and Composition of Sulfide Minerals at Balmat, New York