HYDROTHERMAL METAL TRANSPORT IN CREEDE CALDERA, SAN JUAN VOLCANIC FIELD, CO: CARBON ISOTOPE CONSTRAINTS FROM FLUID INCLUSIONS

Abundant quantities of ore metals such as gold, zinc, and copper are found in the Colorado Mineral Belt (CMB), a 500km long NE-SW trending region of igneous activity that stretches across Colorado and was formed over millions of years from 60 Ma to 70 Ma (Chapin, 2012). Prime examples of mineralization within the CMB are hosted within the well preserved Creede Caldera. The Creede ore system that formed approximately 28 million years ago after an eruption event created the accommodation space for an ancient lake; later cooling of the sub-caldera intrusion drove the hydrothermal circulation associated with the Creede ores (Bethke and Hay, 2000).

While it is clear that hydrothermal deposition in Creede is linked to precipitation of ores from meteoric waters and magmatic fluid from an underlying pluton (Bethke and Hay, 2000) , the role of the specific ligands that transported ore metals to the site of deposition is less well known. Fluid inclusions (pockets of gas and liquid phases that become trapped within a mineral when it (re)crystallizes) in carbonates from Creede can be rich in hydrocarbons (Bethke and Hay, 2000) which suggests that reduced carbon-bearing species may have played a role in mineralization processes at Creede. In order to examine fluids associated with hydrothermal circulation at Creede, we sampled drill cores CCM-1 and CCM-2 (held at the USGS Core Research Center in Denver) for calcite microveins that cut through sedimentary rocks from Lake Creede over the depth range from ≈600 to ≈2350 feet (near the contact with lake sediments and the underlying Snowshoe Mountain Tuff).

We used a specially designed closed-system crusher to powder ≈0.5 g of rock, and then flowed any gas that was released into a Picarro G2201-i Analyzer. Proof-of-concept measurements indicate that water and methane, but minimal carbon dioxide, were released during crushing. In addition, the methane that was released had δ¹³C values that were lower than local atmospheric methane (≈-47.5 to -47.9 ‰; Niwot Ridge CO) by ≈6 to 10 ‰. We interpret these values to indicate that a methane-rich fluid, potentially of biogenic origin, was involved in hydrothermal mineralization processes at Creede. Data reported during this presentation will be used to evaluate this hypothesis.