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Gold and Trace Element Zonation in Pyrite Using a Laser Imaging Technique: Implications for the Timing of Gold in Orogenic and Carlin-Style Sediment-Hosted Deposits

Ross R Large^1,, Leonid Danyushevsky¹, Chris Hollit¹, Valeriy Maslennikov², Sebastien Meffre¹, Sarah Gilbert¹, Stuart Bull¹, Rob Scott¹, Poul Emsbo³, Helen Thomas¹, Bob Singh⁴ and Jeffrey Foster¹

¹ CODES ARC Centre of Excellence in Ore Deposits, Private Bag 126, University of Tasmania, Australia 7001
² Institute of Mineralogy, Russian Academy of Science, Urals Branch, Miass, Russia
³ U.S. Geological Survey, Mail Stop 973, Denver, Colorado 80225
⁴ Skygold Ventures, 615-800 West Pender St. Vancouver, British Columbia, Canada V6C 2V6

E-mail: Ross.Large{at}utas.edu.au

Laser ablation ICP-MS imaging of gold and other trace elements in pyrite from four different sediment-hosted gold-arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit: an early synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace elements, in particular, As, Ni, Pb, Zn, Ag, Mo, Te, V, and Se; and a later hydrothermal stage where gold forms as either free gold grains in cracks in overgrowth metamorphic and/or hydrothermal pyrite or as narrow gold-arsenic rims on the outermost parts of the overgrowth hydrothermal pyrite. Compared to the diagenetic pyrites, the hydrothermal pyrites are commonly depleted in Ni, V, Zn, Pb, and Ag with cyclic zones of Co, Ni, and As concentration. The outermost hydrothermal pyrite rims are either As-Au rich, as in moderate- to high-grade deposits such as Carlin and Bendigo, or Co-Ni rich and As-Au poor as in moderate- to low-grade deposits such as Sukhoi Log and Spanish Mountain. The early enrichment of gold in arsenic-bearing syngenetic to diagenetic pyrite, within black shale facies of sedimentary basins, is proposed as a critical requirement for the later development of Carlin-style and orogenic gold deposits in sedimentary environments. The best grade sediment-hosted deposits appear to have the gold climax event, toward the final stages of deformation-related hydrothermal pyrite growth and fluid flow.