Syngenetic gold in western Victoria: occurrence, age and dimensions

Abstract

In western Victoria, a widespread stratiform style of gold enrichment in Palaeozoic black mudstone and chert—clearly different from the classic mesothermal quartz vein deposits of the Victorian goldfields—has been confirmed by whole-rock geochemistry and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS). This enrichment pre-dates compaction, deformation and low-grade metamorphism of the sedimentary host-rocks, and therefore possibly developed diagenetically during slow deposition of the thin carbonaceous black mudstone beds and the thinner layers of chert. These paired strata have been documented at many locations in three regional outcrop areas of chevron-folded Cambrian and Lower Ordovician metasediments in the Stawell and Bendigo Zones, where they are interbedded with quartz-rich turbidites. The layers were named ‘indicators’ by the early miners, who found locally rich nuggety gold deposits at intersections between these layers and mesothermal quartz veins. Gold is present in euhedral pyrite crystals in both black shale and chert. LA-ICPMS analysis of individual pyrite crystals in the indicator beds shows that pyrite is enriched in Au, As, Sb, Se, Te and Bi. The Au content of pyrite varies from 0.03 to 2.69 ppm with a mean of 0.58 ppm and shows a positive correlation with As, which varies from 1000 to 6000 ppm. Many pyrite crystals show enrichment of gold in their cores and depletion in their rims, confirming the likely syngenetic or syndiagenetic accumulation of gold during pyrite formation in the sediments. Prior to regional metamorphism, folding and faulting, the many indicator strata in the outcrop areas were parts of an extensive marine sequence of Late Cambrian and Early Ordovician age. The former primary source of this mineralisation is considered to have been one or more contemporaneously exhalative submarine hydrothermal systems. Thus, the older Palaeozoic sediments of the western Lachlan Fold Belt were significantly enriched in syngenetic gold in the Early Palaeozoic, at least 40 million years before emplacement of the quartz – gold vein deposits of the goldfields.

Key words:

• exhalative
• gold
• indicator beds
• Lachlan Fold Belt
• mineralisation
• Palaeozoic
• platinum-group minerals
• syngenetic

Acknowledgements

This work has developed from discussions with many colleagues at various locations over several years, and includes the assistance and input of ideas of Sergio Martini at Compania. Pesquisa Recursos Minerais, Rio de Janeiro, and Stafford McKnight at the University of Ballarat who also provided indicator samples for the LA-ICPMS research. A very helpful critique was provided at an early stage of documentation by Nicholas Hayward, WMC Resources Ltd, and later much constructive comment by H. J. Harrington, Australian National University, Canberra, both of whom are acknowledged with thanks. Peter D'Auvergne of Ballarat Goldfields NL kindly provided the drillcore sample of Figures 4 –  7. Detailed reviews and constructive criticism by Ross Cayley and Rick Squire considerably improved the manuscript. Assistance in the field, in sample preparation, and with a fund of local geological and historical knowledge was provided by Alan Svanosio, Newlyn, Victoria, who introduced BLW to the indicators. Development work on the LA-ICPMS analytical technique for trace elements in pyrite at CODES University of Tasmania by Mark Norman and Leonid Danyushevsky is gratefully acknowledged.