Deformation of Pyrite at Varying Metamorphic Grades in Sediment-Hosted Base Metal Sulphide Deposits of Rajasthan, India

The ubiquitous iron sulphide, pyrite, occurs in trace amounts in rocks or may form massive pyritic ore bodies with all perceivable gradations in between. Very often it shares the deformational and metamorphic history of its host rocks. Textural characteristics of pyrite, and its behavior in natural ores and in experimental conditions under varying temperature and pressure, have therefore been studied by different workers from time to time. An overview of these studies shows that there is a mismatch between the experimentally achieved deformation mechanisms at different temperature and pressure, and the observed brittle or ductile behaviour of pyrite in naturally deformed sulphide bodies. An attempt is made here to analyze the deformation behaviour of pyrite under different temperature-pressure conditions, by studying pyritic ores in three sediment-hosted Pb–Zn sulphide deposits of Rajasthan (Balaria-Zawar, Rajpura-Dariba and Rampura-Agucha), occurring in broadly similar geological settings, but deformed and metamorphosed at different grades (upper greenschist, middle amphibolite and upper amphibolite to granulite facies respectively). Observations of hand specimens and optical microscopy of pyritic ores from Rajasthan have shown that the mineral behaved in a macroscopically ductile manner—not only in the form of mesoscopic and microscopic folding of layers, but also by distortions, bending and stretching of individual grains. In general, pyrite plasticity increases with temperature as revealed by more definitive evidences of plastic deformation in higher metamorphic grade deposits (e.g. Rajpura-Dariba and Rampura-Agucha) than in lower grade Balaria ores from the Zawar ore district. However, the Balaria ore, characterized by the coexistence of framboidal (sedimentary-diagenetic) and idiomorphic (metamorphic) pyrite, is more intensely folded. Higher grade ores may, on the other hand, induce more grain growth and thereby are likely to lose the evidence of plastic deformation through polygonization and grain coarsening. This may be one reason behind the apparent scarcity of plastic deformation textures observed in pyrite from naturally deformed and metamorphosed sulphide ore deposits.