Abstract

Processes responsible for high-grade gold enrichment are of significant economic interest and have long been elusive. Structural and geophysical models postulate that earthquake-induced fault-valve processes are frequently responsible for gold mineralization in structurally controlled hydrothermal systems, although a comprehensive geochemical record of these processes has been lacking. The Porgera gold deposit (Papua New Guinea) provides an opportunity to examine the complex processes surrounding high-grade Au mineralization in structurally controlled hydrothermal systems. New high-resolution trace element and sulfur isotope data obtained in situ by laser ablation–inductively coupled plasma–mass spectrometry and sensitive high-resolution ion microprobe–stable isotope analysis of pyrite across a well-constrained paragenetic sequence reveal a temporal evolution in fluid chemistry. The data uncover a stratigraphy of repeated high-Au negative δ34S and low-Au positive δ34S zones within individual pyrite crystals present in the highest grade gold event (stage II). Results provide the first clear geochemical documentation of rapid, complete switching in ore-forming processes, preserved in individual pyrite crystals throughout an evolving hydrothermal system. It is postulated that geochemical variations are attributable to rapid Au deposition during pressure release due to fault failure followed by switching to more reduced rock-buffered (metal poor) fluids from adjacent sediments as the rupture seals. The cyclicity of this process is reflected in the pyrite zonation seen at the Porgera deposit. This study demonstrates the usefulness of applying contemporary analytical techniques to pyrite in hydrothermal systems to gain new insights into ore genesis and fluid pathways, revealing that deposition of exceptionally high grade ore at the Porgera deposit was the result of multiple events in a fluctuating ore-forming environment.

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