Chromite is developed as an accessory mineral in a 1,700-m-thick sequence of lower and lower critical zone cumulates in the area south of Potgietersrus and as a major constituent in a number of chromitite layers within this sequence. The lower zone consists of a 1,500-m-thick sequence of ultramafic rocks characterized by varying proportions of olivine, orthopyroxene, and chromite, which allows the pile of cumulates to be subdivided into 37 cyclic units. The critical zone, on the other hand, is only 350 m thick, of which only the lower 150 m contains chromite-bearing lithologies.Electron microprobe analyses of chromite and coexisting silicates indicate a strong positive correlation between the Mg/(Mg + Fe (super +2) ) and Cr/(Fe (super +2) + Fe (super +3) ) ratios of chromite and the Mg/(Mg + Fe (super +2) ) ratio of the mafic silicates. The composition of the chromite is also strongly influenced by the mineralogical composition of the host rock and probably reflects conditions within the parental magma during crystallization of these rocks. The analytical data have also shown the TiO 2 content of the disseminated chromite to be the most informative indicator of the degree of differentiation of the magma.A detailed investigation of the chromite across the contact of a thin chromitite layer has shown that the chromitite layers crystallized in response to an influx of magma and that chromite crystallization could have been enhanced by an increase in the f (sub O 2 ) and a rise in the Cr 2 O 3 content of the liquid, thereby causing an increase in the Cr 2 O 3 content of the chromite within the chromitite layers.Postcumulus modification of the chromitite layers is extensive and primary textural features have been obliterated essentially by sintering. Sintering took place in the presence of a reactive Mg-rich liquid which resulted in an increase in the Mg/Mg + Fe (super +2) ratio of the chromite. This process resulted in a densification of the chromitite layers from an estimated initial porosity of 30 to 45 percent to less than 10 percent.
The texture, mineralogy and composition of chromite in the upper chromitite of the Muskox intrusion, in the Northwest Territories, have been studied in two 0.5-meter sections of drill core. The principal rock-type is an orthopyroxenite that contains cumulus olivine, orthopyroxene and chromite, and the intercumulus minerals clinopyroxene and plagioclase. The minor minerals ilmenite and biotite are found, together with a number of accessory minerals, in pockets that are interpreted as sites of late intercumulus melt. The chromitite seam is up to 10 cm thick and contains chromite with a narrow range in composition: 0.64 2 ) = -9.1. The disseminated chromite in the orthopyroxenite shows a much greater range in composition, and increases in Fe (super 2+) /(Fe (super 2+) +Mg), Fe (super 3+) /(Fe (super 3+) +Al+Cr), Ti and Ni with stratigraphic height above the massive chromitite. The chromite in the Muskox chromitite is significantly higher in Fe (super 3+) , Ti and Fe (super 2+) /(Fe (super 2+) +Mg) than chromite in the Bushveld, Stillwater and Great Dyke chromitites; furthermore, the Muskox chromitites formed much higher in the stratigraphic section of the layered series than in these other intrusions. The Muskox chromitites are considered to have formed late in the magmatic history of the intrusion as a result of mixing of a fractionated magma with a more primitive magma and a component due to wall-rock assimilation.
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.
The lower zone of the Potgietersrus limb of the Bushveld Complex consists of a succession of ultramafic rocks, at least 1,600 m thick, that can be subdivided into 37 cyclic units. Ni, Cu, and Pt mineralization is developed at two levels in the sequence, namely, a zone of mineralization within the Volspruit pyroxenite subzone and mineralization associated with the chromitite layers of the Drummondlea harzburgite-chromitite subzone. Of the two, the mineralization in the Volspruit subzone is more significant, in that values over the basal 6 m of the mineralized zone are on the order of 0.27 percent Ni, 0.19 percent Cu, 3.09 ppm Pt, and 2.56 ppm Pd. The composition of the coexisting silicates suggests that separation of an immiscible sulfide liquid was brought about by a drop in temperature of the magma, possibly caused by the emplacement of less primitive, cooler magma that mixed with the residual, crystallizing, lower zone magma.In the Drummondlea subzone, sulfide mineralization is developed within the upper parts of the chromitite layers and the overlying olivine-rich rocks. In this case, separation of sulfide liquid evidently took place in response to the crystallization of large quantities of chromite as a result of which the FeO content of the magma was lowered, as well as its sulfur-carrying capacity.
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