Abstract

The Midas deposit is the largest known high-grade Au-Ag vein deposit located along the northern Nevada rift. It belongs to a suite of middle Miocene low-sulfidation epithermal systems associated with magmatism and faulting along the rift. Interpretation of new 40Ar/39Ar dates for volcanic rocks and hydrothermal minerals related to gold mineralization and additional isotopic dates throughout the Midas region constrain the timing of volcanic, tectonic, and hydrothermal activity. The Midas hydrothermal system developed following a change from mafic-dominated bimodal volcanism and basin formation to felsic volcanism, folding, and faulting at about 15.6 Ma. From 15.6 to 15.2 Ma, sediments and tuffs were deposited on a relatively impermeable rhyolite flow. During this period, faulting and tilting of the volcanic edifice created pathways for hydrothermal fluids that flowed to the surface forming sinters and hydrothermal breccias. Approximately 200 kyr after the change in volcano-tectonic regime, oblique-slip faulting took place along zones of preexisting weakness, creating dilational zones and additional channelways for mineralizing fluids. At 15.4 Ma, high-grade veins formed in fault zones throughout the region, depositing at least 4 million ounces (Moz) of gold and 40 Moz of silver. The timing of high-grade Au-Ag mineralization is identical to the age of rhyolite intrusions whose source magma chamber likely provided the heat necessary to drive the hydrothermal system. The age of an unaltered tuff that unconformably overlies opalized sediments establishes that tilting of the units and the hydrothermal system had ceased by 15.2 Ma. The temporal and spatial coincidence of rhyolite volcanism, faulting, and high-grade mineralization may reflect the importance of contributions from deeper fluid reservoirs containing magmatic components or highly exchanged meteoric waters.

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