Mineralogy & Texture of the Askot Sulfides
Mineralogical and textural studies of the sulfides are based on the microscopic examination of 120 polished sections in reflected light. Replacement is the dominant process in the formation of sulfide deposits of Askote. Replacement has affected not only the minerals of the host rock, but also the earlier formed ore and gangue minerals. The resulting textures are varied. Only a few minerals, chiefly the hard and early formed ones, e.g. pyrite and arsenopyrite have developed well defined crystal outlines. Vein replacements are quite common in hard minerals like quartz and earlier formed pyrite, magnetite and arsenopyrite. These minerals, shattered by small movements contemporaneous with mineralization, are commonly invaded along the fractures by younger minerals. The invading mineral penetrates the host laterally from the fractures into the mineral. In advanced stages of vein replacement, only ragged residuals of the invaded mineral remain.
Mineralogy of the sulfides is complex - there being a considerable variation in the quantities, form, and textures of the individual minerals. Two phases of mineralization are recognised. The first generation sulfides, viz., pyrite, arsenopyrite, sphalerite, galena and chalcopyrite, occur in minor amounts and are replaced by the second generation arsenopyrite, sphalerite, galena, chalcopyrite, cubanite, gudmundite, and pyrrhotite. In the massive sulfide zone all the sulfide minerals are intimately intergrown whereas in the disseminated zone, only pyrite, arsenopyrite, sphalerite, galena and chalcopyrite are found.
The crystals vary in size from tiny specks to more than 20 mm in length. The size of crystals is largest in the centre of orebody and gradually decreases towards the fringe. The mineral shows a tendency towards euhedral crystallization regardless of its age relations. Its paragenetic sequence is determined on the basis of replacement along fractures. Acicular shape of arsenopyrite is indicative of selective rapid growth in one direction indicating the direction of flow of mineralizing solutions.
Porphyritic texture is exhibited by arsenopyrite. The growth of large crystals of arsenopyrite is due to their enhanced force of crystallization.
Intergrowths of arsenopyrite with pyrrhotite (fig. 2) are interpreted to be decomposition products of the early formed Fe- and As-rich fahlores (tenantite).
Graphic textures resulting from the replacement of sphalerite by chalcopyrite and galena are rare. Tiny islands of sphalerite in chalcopyrite and galena adjacent to large sphalerite grains are evidence of perepheral replacement of the latter (fig. 3). The distribution of the sphalerite islands are indicative of the shape and size of the original sphalerite grain.
Inclusions of galena and chalcopyrite of first generation in sphalerite of second generation are quite common. Phenocrysts (the term merely indicates large mineral grains in the midst of a fine grained surrounding) of sphalerite are generally to be seen. More commonly, however, it occurs as fine grained undeformed aggregates.
Small inclusions of dust like and especially star shaped particles and crosses of sphalerite are contained in chalcopyrite. These are generally interpreted as products of exsolution. The sphalerite stars are generally branched (fig. 4) following the cleavage planes of chalcopyrite.
Second generation galena penetrates sphalerite along fractures and also replaces the latter along grain boundaries. Graphic textures resulting from the replacement of galena by other minerals are rare. Spheroidal texture exhibited by inclusions of bismuth in second generation galena (fig. 5) are interpreted as inclusions of molten bismuth in a growing crystal of galena. Bismuth is molten above 2800o C.
Argentite occurs as tiny cubic or triangular crystals about 5-10 microns in diameter in galena. These are interpreted to be products of exsolution. The tiny argentite crystals are generally to be found at the periphery of the galena crystal or outside it (fig. 6), indicating thereby that they have been expelled from the galena lattice by diffusion.
Argentite can make up as much as 10 percent of the mixed crystal with galena at high temperatures. Below a temperature of 179o C the symmetry of argentite, which is cubic at high temperature with a galena structure, inverts to a monoclinic symmetry (Ramdohr, 1969). Hence, the expulsion of exsolved argentite is logical in view of the great difference between the symmetries of the two minerals at lower temperatures. Exsolution segregation at grain boundaries has frequently been reported (Ramdohr, 1969). In very fine grained aggregates, and with great velocity of migration and very slow cooling, all unmixed substances can migrate outwards, and therefore the diagnostic texture can disappear entirely.
Oriented intergrowths of chalcopyrite and pyrrhotite (fig. 7) are interpreted as a decomposition product of a high temperature chalcopyrite (chalcopyrrhotite). Second generation chalcopyrite is one of the most abundant sulfide minerals and replaces almost all other sulfides. Xenoblats of chalcopyrite are quite common, and the mineral occasionally forms interfingering textures with galena and sphalerite. Cellular island shaped forms of first generation chalcopyrite are quite common in later minerals. Pseudoeutectic intergrowths between idiomorphic to subidiomorphic gudmundite and chalcopyrite are also common (fig. 7). First generation chalcopyrite penetrates into first generation sphalerite and galena, replacing these minerals along fractures. Shredded textures are produced by the replacement of sphalerite by chalcopyrite. Graphic textures resulting from the replacement of earlier minerals by chalcopyrite are rare.
Cubanite lamellae occur as exsolution bodies in second generation chalcopyrite. The lamellae are oriented parallel to the crystallographic planes (111) (fig. 8). Small inclusions of dust-like and especially star-shaped particles and crosses of sphalerite are contained in chalcopyrite (fig. 4). The sphalerite stars are generally branched, following the cleavage planes of the host. They are generally elongated in the (111) planes of chalcopyrite. These star-shaped skeletal crystals of sphalerite are interpreted to be products of exsolution. At high temperatures chalcopyrite can dissolve up o 17 percent sphalerite, whereas chalcopyrite and cubanite are miscible in all proportions. Pyrrhotite is often found in the form of Stringers and plates in chalcopyrite (fig. 7). In some cases the pyrrhotite stringers may be the decomposition products of exsolved cubanite lamellae in chalcopyrite.
Spindle-shaped inversion twins are quite common in second generation chalcopyrite (fig. 9). The boundaries of the twin lamellae are not parallel all over the mineral grain.
They commonly form intergrowth networks and are hardly accompanied by strain and translation. These twins are irregular in shape and are strongly interwoven and unevenly distributed. Such twins are termed inversion twins (Ramdohr, 1969) and are the result of cooling of high temperature minerals.
Graphic intergrowths of gudmundite and cubanite are quite common. Serrate boundaries of gudmundite, tongues of cubanite penetrating into gudmundite, and transition from the graphic association to the unquestioned peripheral replacement indicate that such graphic intergrowths are products of replacement of gudmundite by cubanite, rather than simultaneous precipitation.
10. Other Minerals
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