Discovery of Selenium Mineralisation in Volcanogenic Massive Sulphides of Mesoproterozoic South Delhi Fold Belt in Northwest India

ABSTRACT The 60 km long tract between Pindwara and Deri-Ambaji in the southwestern part of the South Delhi Fold Belt (SDFB) is a volcanosedimentary belt that hosts several Volcanogenic Massive Sulphide (VMS) deposits, notably at Ambaji-Deri (Zn-Pb-Cu), Pipela Cu (-Zn), Danva-Ajari Cu-Zn (-Pb-Au), Basantgarh (Cu ± Zn) and Goliya (Cu ± Zn). The Pipela deposit, however, is largely a stringer zone. A pronounced hydrothermal alteration is a common feature associated with VMS deposits. This paper reports the discovery of selenium (Se) minerals and their high concentrations in sulphide phases that together constitute deposit-grade mineralisation in the Mesoproterozoic SDFB. Selenium forms different minerals with varying content of Se from 0.04 to 10.97% in BiTe-BiTeSeS, PbS-PbSeS and BiPbSeS-BiSeS phases. The in-situ measurements by the LA-ICP-MS method show their high contents in chalcopyrite, pyrite and galena. Sphalerite analysed low selenium. In comparison to massive ores, chalcopyrite (avg. 81.42 ppm) and pyrites (avg. 82.52 ppm) in stringer zones in Pipela analyse high selenium, which mainly occurs due to the substitution of sulphur from these minerals. Galena in Deri gives an average of 73.94 ppm Se, which has significantly contributed to its ore grade value of 48.54 ppm (avg.). The presence of Se minerals has spiked the Se grades in the stringer zone at Pipela. Although only a few samples of ore minerals were studied from the Ajari Cu-Zn deposit, Se values in galena were significantly high, ranging between 62.97 and 162.08 ppm. Looking at the great width (~ 98 m) and lateral and depthward continuity of the high Se-bearing stringer zones, the Pipela deposit appears a promising discovery of Se mineralisation in the SDFB. The Deri deposit, with its good lead content (6.8% Pb), also makes it an extractable grade of selenium from these ores. The grades of this newly discovered selenium mineralisation in the SDFB compare well with global seleniferous VMS deposits.