Identification of Zn-Bearing Micas and Clays from the Cristal and Mina Grande Zinc Deposits (Bongará Province, Amazonas Region, Northern Peru)
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Abstract:
Zn-bearing phyllosilicates are common minerals in nonsulfide Zn deposits, but they seldom represent the prevailing economic species. However, even though the presence of Zn-bearing clays is considered as a disadvantage in mineral processing, their characteristics can give crucial information on the genesis of the oxidized mineralization. This research has been carried out on the Mina Grande and Cristal Zn-sulfide/nonsulfide deposits, which occur in the Bongará district (Northern Peru). In both of the deposits, Zn-bearing micas and clays occur as an accessory to the ore minerals. The XRD analyses and scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM-EDS) investigations revealed that the Zn-bearing micas that are occurring in both deposits mostly consist of I/S mixed layers of detrital origin, which have been partly altered or overprinted by sauconite during the supergene alteration of sulfides. Sporadic hendricksite was also identified in the Cristal nonsulfide mineral assemblage, whereas at Mina Grande, the fraipontite-zaccagnaite (3R-polytype) association was detected. The identified zaccagnaite polytype suggests that both fraipontite and zaccagnaite are genetically related to weathering processes. The hendricksite detected at Cristal is a product of hydrothermal alteration, which is formed during the emplacement of sulfides. The complex nature of the identified phyllosilicates may be considered as evidence of the multiple processes (hydrothermal and supergene) that occurred in the Bongará district.Keywords:
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Supergene or hypogene alteration of a pentlandite-chalcopyrite-pyrite assemblage has resulted in the formation of a number of secondary sulfides. Violarite formed first and occurs pseudomorphically after pentlandite. Alteration later intensified, resulting in the dissolution of violarite, chalcopyrite and pyrite and the concomitant deposition of bravoite, idiomorphic pyrite, anhedral pyrite, marcasite, chalcopyrite, hematite, quartz and carbonate. Bravoite, in contrast to violarite, does not occur pseudomorphically after pentlandite. This relationship is considered to be of general application. The nickel content of each sulfide was determined using an electron microprobe analyzer. Relationships between idiomorphic pyrite and bravoite show that an immiscibility gap may exist between pyrite, with up to 1.5 percent Ni, and bravoite, with 11 to 12 percent Ni.
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All the major worldwide direct-shipping iron ore deposits associated with banded iron formations (BIF) are characteristically deeply weathered. They extend to considerable depths below the water table and show well-preserved primary structures and textures, but characteristically most deposits contain no evidence of chert bands being present prior to weathering. Recent studies have found evidence of hydrothermal and/ or metamorphic influences in the development of certain ore deposits and new genesis models such as the supergene-modified hypogene model have been postulated for major high-grade iron ore deposits. Nevertheless, there are many high-grade deposits that show no evidence of hypogene alteration and for which a hypogene or metamorphic genesis is unreasonable that are automatically ascribed to supergene enrichment, commonly erroneously attributed to lateritic weathering in tropical environments. Laterite (sensu lato) is a soil formation in which primary textures are destroyed and is underlain by a pallid zone showing the preservation of chert and the depletion, not enrichment, of iron oxides and thus is totally incompatible with the formation of the high-grade ore deposits. Various theories and models that purported to explain the conditions under which such a uniquely BIF-related dissolution of quartz and residual accumulation of hematite could occur by supergene processes typically conflict with current understanding of groundwater hydrology, chemistry, weathering processes and soil formation.Supergene enrichment of ore is universal in the leaching of gangue minerals such as iron silicates, carbonates and apatite and supergene enrichment of BIF to low-grade ore is common in near surface environments above the water table such as ferrugenised BIF outcrops, detrital ore deposits, and some shallow ore deposits that have been subjected to prolonged exposure to fresh meteoric water. In all cases of supergene enrichment traces of the chert bands are visible and the dissolution or replacement processes for the removal of quartz are clear, in direct contrast to the most important deep saprolite ore deposits that show no trace of chert bands.The widespread acceptance of an inappropriate and untenable supergene enrichment model inhibits search for the true origin of the ore and our ability to predict and find concealed high-grade ore deposits.
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