A revised classification scheme of pyrite in the Witwatersrand Basin and application to placer gold deposits

Pyrite is the most abundant ore mineral in gold-bearing quartz pebble conglomerates (QPCs) of the Witwatersrand Basin and similar Archaean sedimentary units. Much of the pyrite in Archaean conglomerates is detrital in origin, implying that it survived weathering, transport, and reworking under anoxic conditions. Detrital pyrite is generally found together with authigenic pyrite formed in situ as a result of syn- to post-depositional processes. As pyrite is frequently associated with gold mineralization, many authors have developed different pyrite classification schemes in order to better understand the nature and origin of the mineralization, although the different classifications cannot always be easily compared as they are largely based on inferred pyrite genesis. We propose a revised, entirely descriptive classification scheme for detrital and authigenic pyrite in Archaean QPCs. The scheme was created in order to provide a consistent framework for the petrographic description of pyrite in clastic sedimentary rocks. Detrital pyrite is subdivided into massive, inclusion-bearing, and coarsely crystalline types. Authigenic pyrite includes euhedral, overgrowth, infill, aggregate, and pseudomorphic types. We evaluate the methods used in pyrite classification, propose formation pathways for detrital and authigenic pyrites, and apply the classification scheme to some well-known occurrences of detrital pyrite-bearing conglomerates.