HYDROTHERMAL PRECIOUS-METAL AND SULFIDE NANOPARTICLE TRANSPORT AND DEPOSITION AND THE GENESIS OF BONANZA EPITHERMAL ORES

In the early decades of the 20^th Century, Waldemar Lindgren understood that bonanza epithermal ores were intimately associated with coeval magmas, and that some of these ores had peculiar textures consistent with colloidal transport of precious metals in these systems. Perhaps if he had lived another couple of decades, he might have deduced a connection between his twin hypotheses. We have revisited some of Lindgren’s key deposits where he developed his ideas (Silver City, Idaho; National District, Nevada) and have had the opportunity to investigate more recently discovered and similar ores in the northern Great Basin (Sleeper, Midas, and Ivanhoe). So perhaps now we can evaluate whether Lindgren’s hypotheses were correct, and if they are consistent with new findings about vapor-phase transport of metals by magmatic fluids, and if so, do the bonanza ores provide evidence of these processes? We think the answers to those questions are yes, yes, and yes. Evermore precise geochronology of ores and associated magmas, new isotopic techniques not previously available, and particularly, advances in the understanding of magma and metal behavior now seemingly confirm Lindgren’s early ideas about magmas releasing metals to ore-forming environments. The how and the where this happens perhaps is just now coming to light. We propose that a metal-rich low-density magmatic fluid separates from a cooling magma chamber or a denser magmatic fluid and that these fluids are close to saturation with respect to metallic minerals . As these fluids rise and cool from near magmatic temperature, metallic nanoparticles (<10^-7m) nucleate and are entrained in these super-critical magmatic fluids. Further cooling as these fluids move up to the epithermal environment causes additional particles to nucleate, earlier formed ones to grow, and ultimately they are locally deposited in bonanza epithermal ores (low- and high-sulfidation). Metallic alloys (or alloy-like phases such as naumannite, Ag₂Se) aggregate by forming self-organized three-dimensional “fractal dendrites”, whereas sulfides anneal and recrystallize to form “normal-” appearing mineral textures in bonanza ores. Episodic release of these fluids is recorded in 10s-100s of bands enriched in ore minerals in some of the bonanaza ores.