Lithology, geochemistry, and structure of Moke Creek sulphide deposit host rocks, Otago Schist, New Zealand

Abstract

Three types of greyschist (fissile, dark grey, and light grey thickly segregated types) and subordinate greenschist make up most of the Otago Schist hosting the Moke Creek sulphide deposit. Greyschist in the study area is mostly of Torlesse Terrane affinity, with the exception of a sliver of Caples Terrane affinity schist preserved in the footwall of the Moonlight Fault. The three greyschist types are geochemically indistinguishable; however, the former two types contain higher contents of graphite, which may explain their darker field appearance. The greenschists are shown to represent metamorphosed MORB and within-plate basalts. Schist immediately hosting the Moke Creek sulphide deposit consists of both muscovite-rich and subordinate epidote-rich varieties. The former variety contains up to 6.4 wt% K2O, whereas the epidote-rich variety is depleted in K2O but contains >5 wt% CaO. It is suggested that sericitisation during hydrothermal alteration associated with sulphide deposition produced the mica-rich schist. The Moke Creek sulphide deposit shares many similarities with Besshi-type sulphide deposits formed by seafloor hydrothermal activity. The Moke Creek sulphide deposit may have been formed as vein-dominated roots to exhalative sulphide deposits. The sulphide layers and associated hydrothermal alteration zones were subsequently deformed into parallelism with lithological layering in the Otago Schist. © 2002 Taylor & Francis Group, LLC.