Use of Pyrite Microfabric as a Key to Tectono-Thermal Evolution of Massive Sulphide Deposits — An Example from Deri, Southern Rajasthan, India

Anju Tiwary, Mihir Deb and Nigel J. Cook
Department of Geology, Delhi University, Delhi 10007, India
Mineralogical Institute, University of Würzburg, Am Hubland, D-97074 Würzburg, Germany

Abstract: Pyrite is an ubiquitous constituent of the Proterozoic massive sulphide deposit at Deri, in the South Delhi Fold Belt of southern Rajasthan. Preserved pyrite microfabrics in the Zn-Pb-Cu sulphide ores of Deri reveal a polyphase growth history of the iron sulphide and enable the tectono-thermal evolution of the deposit to be reconstructed.

Primary sedimentary features in Deri pyrites are preserved as compositional banding. Regional metamorphism from mid-greenschist to low amphibolite facies is recorded by various microtextures of pyrite. Trails of fine grained pyrite inclusions within hornblende porphyroblasts define S1-schistosity. Pyrite boudins aligned parallel to S1 mark the brittle-ductile transformation of pyrite during the earliest deformation in the region. Isoclinal to tight folds (F1 and F2) in pyrite layers relate to a ductile deformation stage during progressive regional metamorphism. Peak metamorphic conditions around 550°C, an estimation supported by garnet-biotite thermometry, resulted in annealing of pyrite grains, while porphyroblastic growth of pyrite (up to 900 µm) took place along the retrogressive path. Brittle deformation of pyrite and growth of irregular pyritic mass around such fractured porphyroblasts characterize the waning phase of regional metamorphism. A subsequent phase of stress-free, thermal metamorphism is recorded in the decussate and rosette textures of arsenopyrite prisms replacing irregular pyritic mass. Annealing of such patchy pyrite provides information regarding the temperature conditions during this episode of thermal metamorphism which is consistent with the hornblendehornfels facies metamorphism interpreted from magnetite-ilmenite geothermometry (550°C) and sphalerite geobarometry (3.5 kbar). A mild cataclastic deformation during the penultimate phase produced microfaults in twinned arsenopyrite prisms.

Keywords: pyrite • microfabric • metamorphism • massive sulphide • Rajasthan • India

Mineralogical Magazine; April 1998 v. 62; no. 2; p. 197-212; DOI: 10.1180/002646198547576
© 1998, The Mineralogical Society
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