The Lithological, Stratigraphical, Structural Control of Quartzites Hosting Sulphide Mineralisation in the Proterozoic Shillong Group, Meghalaya, India

Abstract

The White Quarzite belonging to the Lower Shillong Group, overlying the older Archeans, in the Proterozoic Shillong Group of rocks host the sulphide mineralization. The sulphide minerals are argentiferous galena and subordinate auriferous arsenopyrite, loellignite, gersdorffite, safflorite, pyrite and pyrohotite occurring as bed - like or lens - like ore bodies along the compositions bands. Surface geochemical prospecting revealed Pb values of (maximum) 29%, silver values (maximum) of 0.3%, and gold values ranging from 50 to 260 ppb. The lithological, stratigraphic and structural controls of the sulphide mineralization have been established. The syn-sedimentary/syn-genetic Pb (+Ag) mineralization was remobilized during the F₁ folding. The Pb/(Pb + Zn) ratio of 0.93 and co-existence of detrital monazite and zircon suggests deposition of the sediments in shallow marine condition. The S– isotopic analysis of the samples have revealed δ³⁴ S values with an average of +9.3‰. The positive δ³⁴S value shows derivation of sulphur due to sulphate reduction within a limited reservoir in a sedimentary domain and bears no mantle signature. Sulphide ore minerals are crystalline and unzoned, indicating crystallaization under equilibrium condition. Mineral assemblage of the host rock and mineralized zones indicate biotite-grade of regional metamorphism. Pb–Pb age of the galena being between 1530 and 1550 Ma interpreted the age of the mineralization.