The role of carbonaceous "indicator" slates in the genesis of lode gold mineralization in the western Lachlan orogen, Victoria, southeastern Australia

"Indicator" slates have long been considered to represent a useful explorat ion guide for turbidite-hosted mesothermal lode gold mineralization in cent ral Victoria. This assumption has been based on an apparent close spatial r elationship between high gold grades and the proximity of thin, commonly py ritic and carbon-rich marker units, Detailed studies in a number of gold de posits throughout central Victoria, however, reveal that highest gold grade s do not necessarily coincide with the presence of carbonaceous units. In m any places where gold mineralization is associated with carbon-rich matter, the high C accumulations are the result of epigenetic remobilization durin g hydrothermal alteration and ore genesis. Petrographic, geochemical, and s table isotope IC. O, Si investigations into the origin and nature of the ca rbonaceous matter-mainly amorphous bitumens and pyrobitumen of organic orig in, with biological fragments and rare graphite of both detrital and metamo rphic origin-demonstrate that black shales within the Cambro-Ordovician suc cession ill central Victoria lacked the vital constituents to provide (1) a primary sink for precious metals, and (2) whereas the presence of carbonac eous matter was likely to affect the evolution of epigenetic hydrothermal f luids and, locally, may have facilitated gold enrichment, carbon-rich sedim entary rocks were not crucial for ore genesis on the deposit scale. Instead , the size of the hydrothermal cell, physicochemical conditions of the asce nding fluids, and dynamic fault-valve behavior played far more significant roles in controlling gold precipitation. The importance of these processes has implications for exploration targeting sediment-hosted, lode- and disse minated-style gold mineralization in the western Lachlan orogen and in slat e belt provinces elsewhere.