EVOLUTION OF THE MAGMATIC-HYDROTHERMAL SYSTEM AT THE SUMMITVILLE HIGH-SULFIDATION EPITHERMAL AU DEPOSIT, COLORADO

The Summitville high-sulfidation epithermal Au deposit is located in southwestern Colorado. The deposit is comparably small (560,000 oz Au), but preserves the full vertical range of mineralization styles from porphyry-type veins at depth to surficial sinter deposits. Early workers suggested that the deposit formation involved early alteration of the host rocks by a high-temperature vapor, followed by the deposition of Cu and Au minerals from a low-temperature liquid. In contrast, more recent models proposed that a vapor caused both alteration and mineralization, emphasizing the importance of metal transport by vapors. The present study utilized a combination of optical microscopy, fluid inclusion petrography and microthermometry, and CL microscopy to clarify the nature of the mineralizing fluids.

Residual quartz at Summitville formed early in the paragenesis through intense acid alteration of the host rocks. The residual quartz grains are crosscut by abundant secondary vapor inclusion trails. The precipitation of large euhedral enargite crystals postdated the residual quartz formation. The enargite contains primary liquid inclusions that have salinities of 7.3-7.7 wt% NaCl equiv. and homogenize at Th of 260-280°C. Small euhedral quartz crystals line open space within the residual quartz and locally overgrow the enargite crystals. Primary liquid inclusions in these crystals have salinities of 0.9-2.6 wt% NaCl equiv. and Th of 195-245°C. Microthermometric data on one euhedral quartz crystal suggests a cooling trend from the core of the crystal towards its rim. The quartz crystals are overgrown by euhedral pyrite containing Au-bearing growth bands and late luzonite. In addition, a distinct late barite-native gold association was recognized.

The present study provides unequivocal evidence that the ore minerals at Summitville formed from a liquid, not a vapor. It is envisaged here that this mineralizing liquid directly formed through a process of active degassing of a magma chamber at depth and does not represent the product of earlier processes of phase separation. Ascent of this mineralizing liquid into the epithermal environment was likely accompanied by cooling and dilution with ambient groundwater.