Structure and geochemistry of the lead-zinc-pyrite deposits at Zawar, Rajasthan, India

Abstract

Zawar, the only lead-zinc producing mine in India, I located in the lowest part of the upper Aravalli, ascrbed to the Archaen system. The rock types exposed in the area are dolomites, quartzites, phyllites and schists, which have suffered lo-grade regional metamorphim and which display high dip (60º - 80º). Chemical, optical and X-ray investigations were carried out on biotites. Optical determinations suggest that the biotites belong to meroxene and lepidomelane varieties, while the X-ray analysis indicate that they are in 1M [or 3T?] polymorphic state. Intergradations among the different rock types are very common. Petrographic studies reveal to sets of deformations (pre-crystalline and post-crystalline). Mineralisation is found only in dolomite; although there is no consistent structural control of mineralisation, there is an obvious control by lithology. No wall-rock alteration exclusively due to mineralisation has been observed; the presence of any associated minerals can be accounted for as derived from the host rocks during regional metamorphism. Although the main ore deposit follows the E-W trend, for most of the area, the regional trend is N-S. This structural anomaly is explained by cross-folding resulting from pressures acting at right angles to the main Aravalli forces. Cross-folding appears to have been accompanied by a local rise in temperature leading to recrystallization and remobilization of minerals. Recrystallization in ore texture is well-evidenced by the triple junctions at 120 in a single phase aphalerite matrix; galena also shows similar feature. In mochia-Balaria area these recrystallization of sulphides in cracked crystals of microcline are seen. Banding of galena and sphalerite, is interpreted as due to deformation of mixed soft and hard sulphites. It is, therefore, concluded that mineralisation pre-dated the metamorphism. Hence, no meaningful paragenetic sequence can be discerned among the sulphides. From the X-ray fabric analysis of ores and rocks, some support comes for a pre-metamorphic emplacement of the mineralization. Ore minerals and host rocks both reveal preffered orientation. Probably the ore was in place even before the first metamorphic episode. Where it is localized in shear planes and tensional openings, metamorphism is considered to have caused it to migrate to sites of lower pressure. Geochemical analysis of trace elements near the ore body suggests a progressive enrichment in chalcophile elements and a corresponding impoverishment in lithoplhile elements as the sulphate mineralization increases. High Sr- and Rb- contents accompany Pb-high and Zn-poor zones. The dispersion patterns of trace elements indicate a wall-rock aureole of approximately 20ft. in doloriite. As alternatives to primary wall-rock alteration two possibilities are considered: (1) primary dispersion in sediments (2) secondary dispersion through pore fluids during metamorphisas. The 'secondary' porosity-permeability values show an increase with depth, may prehaps be correlated with increasing grain size and dolomitization; these are the result of post-mineralization iiietamorphism and movement. The deposit, is interpreted as a tectonically metamorphosed, reraobilized ore, where a syngenetic submarine hydrothermal origin for the galena, blende and pyrite seems to fit the facts of Mineralogy and geochemistry best, although the possibility of a pre-metamorphic epigenetic origin can not be completely ignored.