Development of Framboidal Pyrite During Diagenesis, Low-Grade Regional Metamorphism, and Hydrothermal Alteration

Abstract

Distribution patterns, microstructural relationships, and compositional variations for framboidal pyrite in the Paleozoic sedimentary host rocks to gold deposits on the northern Carlin trend, Nevada, and at Fosterville in central Victoria, collectively demonstrate that it is not all of synsedimentary-early diagenetic origin. Framboidal pyrite also nucleated in hydrothermal veins formed at several kilometers depth and along dissolution surfaces (stylolites and crenulation cleavages) formed during metamorphism at up to anchizonal grade (i.e., prehnitepumpellyite facies). There are no obvious differences in either the size range or internal morphology of framboids formed in these various settings, suggesting that a single, relatively simple crystallization path may operate over a wide depth range in the upper crust. At Fosterville, anomalously metal-rich (including Au) framboidal pyrite is most abundant adjacent to laminated shear veins that developed by slip along bedding surfaces during folding of the sedimentary host rocks. Microstructural relationships and Pb isotope compositions for the framboidal pyrite, and other similarly distributed, early-formed sulfides, support development in response to the localized, episodic influx of metal-enriched H 2S-bearing hydrothermal fluids during regional deformation between 440 and 400 Ma. These findings may have implications for other similarly deformed metasedimentary successions, where the discovery of Au-bearing framboidal pyrite has recently been cited as evidence for significant Au enrichment during sedimentation. © 2009 Society of Economic Geologists, Inc.