Impacts of Magmatic-Hydrothermal Alteration in the Leadville Limestone, Southwest Colorado

Abstract

Carbon dioxide reservoirs in the Mississippian Leadville Limestone have produced ∼826 billion cubic feet of gas. One of the leading hypotheses on the origin of these gas reservoirs is the thermal recrystallization and hydrothermal alteration of the Leadville Limestone caused by intrusion of Cenozoic magmas and associated fluids. This study investigated the interaction between the Leadville Limestone and hydrothermal fluids sourced from the Sultan stock, a 26Ma granite near Silverton, Colorado. Interpreting the impact of Cenozoic intrusions onto the carbonate strata is important to supplement ongoing research, expedite recovery and provide more accurate extraction within producing fields of the Four Corners region. Well-exposed outcrops of the Leadville Limestone were studied over 8 km away from the stock contact in five localities. Seven distinct facies within the unit as well as karst infill of the Pennsylvanian Molas Formation were examined to understand facies-selective hydrothermal alteration and recrystallization. Field and carbon and oxygen isotope data were collected to assess the physical and chemical changes in the Leadville Limestone by thermal recrystallization and hydrothermal alteration. Our research indicates that hydrothermal alteration from magmatic fluids promoted the formation of secondary dolomite, iron oxides and sulfides, and quartz. In some locations the Leadville Limestone proximal to the stock was metamorphosed to the assemblage wollastonite-garnet-epidote or coarse-grained marble. Hydrothermal dolomite occurs as selective and non-selective replacement, as well as filling fractures and voids. Carbonate isotope data of dC13 shows a range from −7 to +5, and dO18 ranges from +5 to +25. All of the samples from sections show a spread in dC13, but several sections show a noticeable decrease in dO18 values, possibly due to alteration and silicification adjacent to fractures. Recrystallization caused by thermal-hydrothermal processes reduced primary and secondary porosity and permeability. Post-diagenetic fractures were developed providing 1-3% porosity within peloidal grainstone facies. Coarser grained facies and upper member grainstone show a higher degree of susceptibility to alteration and reduction in porosity and permeability. Additional geochemical data is currently being collected to be presented with research.

AAPG Datapages/Search and Discovery Article #90189 © 2014 AAPG Annual Convention and Exhibition, Houston, Texas, USA, April 6–9, 2014