New Perspectives Workshop, September 2014
Presented by AIG Victoria and AusIMM Central Victoria Branch, Romsey Victoria
Stuart W Bull and Ross R Large*
CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Hobart, Australia.
The Ordovician sediments that host the giant Bendigo saddle reef gold deposits consist of a 3 km thick sequence of turbiditic sandstones and interbedded siltstones and mudstones. Sedimentological studies suggest the succession formed within a major deep marine channel-levee complex similar to those described from contemporary continental margin to basin plain settings outboard of major river systems (e.g. the Amazon, Mississippi and Congo). Black shales, which are commonly the immediate host rocks to the epigenetic gold reefs, are interpreted to be over-bank deposits or abandoned channel fills, developed adjacent to active channels which were sandstone-dominated, and had an incised axial thalweg marked by the coarsest-grained sediments present.
Organic carbon content of the black shales at Bendigo varies from 0.2 to 2 wt%, compared to the grey shales, siltstones and sandstones, which vary from 0.05 to 0.2 wt %. Trace elements fall into two main groups; 1) elements that have a linear relationship with aluminium, and are controlled by the detrital clay content (Sn, Ba, Rb, Li, Cs, Mn, Cr and Tl), and 2) elements that show relationships with both aluminium and organic carbon (V, U, Ni, Zn, Cu, Bi, Pb, Se, Ag and Au) and are controlled by both the clay and organic matter content in the carbonaceous shales. The second group of elements are enriched in the black shale facies.
The background gold content of the black shales in the drill holes distal from mineralisation averages 8.9 ppb, compared to the sandstones with 1.5 ppb. Most of the gold in the shales is present in diagenetic pyrite and marcasite, which LA-ICPMS analyses indicate, varies from 5 to 3,850 ppb and averages 370 ppb Au. The geochemical data suggest that this syngenetic gold was most likely sourced by erosion of the hinterland, and transported attached to detrital clay particles or as colloidal
gold, by a high volume feeder river system. High Rb/K ratios in the shales support a highly weathered source typical of a giant river system. By analogy with modern systems, following transport into deep marine channel levee-complexes via continental margin canyons, gold and other redox sensitive trace elements were ultimately trapped by reduction, adsorption and complexation with organic matter in the sub-oxic to anoxic over-bank deposits. Oxidation of much of the organic matter during diagenesis, released the gold and certain trace elements (Ni, Co, Se, Ag, Cu, Bi, Pb), which became incorporated into diagenetic pyrite.
Enrichment of gold in diagenetic pyrite of the black shale facies of the Ordovician turbidites at Bendigo was the first stage in a two-stage process that produced the world-class quartz-gold saddle reef deposits.
Ross Large is a Distinguished Professor of Geology at the University of Tasmania, and the recent past Director of CODES, the ARC Centre of Excellence in Ore Deposits. Ross gained his BSc (Hons) from the University of Tasmania in 1969 and PhD from University of New England in 1973. For ten years Ross worked for Geopeko in mineral exploration in Northern Territory, Queensland and Tasmania. In 1984 he joined the University of Tasmania, and five years later established CODES as a National Key Centre jointly funded by the Australian Research Council, University of Tasmania, the Mining Industry and the State Government. Under his leadership CODES has grown to become recognized as one of the top industry collaborative ore deposit research centers in the world. Ross has over 100 publications in international journals and has gained a number of awards for his research, including the 1983 SEG Lindgren Award, 1989 AusIMM Presidents Award, 2005 Haddon King Medal from AAS, 2010 SEG Silver Medal and the 2011 BJ Skinner Award. He was made a Distinguished Professor at UTAS in 2007. His current research interests are sediment-hosted gold deposits and evolution of the chemistry of the oceans relative to marine ore deposits.