Paper No. 2
Presentation Time: 9:15 AM
GOLD-BEARING ARSENIAN PYRITE FROM THE CRIPPLE CREEK DISTRICT, CO: IMPLICATIONS FOR HYDROTHERMAL FLUID EVENTS DURING PRE-TELLURIDE GOLD MINERALIZATION
Recent research involving ore characterization and paragenesis of Au-Te ores in the Cripple Creek district has focused on the nature and contribution of gold-bearing arsenian pyrite to the Au endowment of this world-class deposit. Detailed imaging and microanalytical characterization has demonstrated that several types of arsenic zoning occur within auriferous pyrite in the district. These include simple zoning, oscillatory zoning, and complex zoning, demonstrating a broad range of textures with respect to arsenic and gold enrichment. Styles of zoning within complexly zoned pyrites include sector zoning, diffuse zoning, and internal truncation of zones. Internal truncations are interpreted to have formed through dynamic processes of dissolution and reprecipitation, resulting in secondary enrichment of arsenic and gold along the perimeters of occluded grains. Electron Microprobe analyses have shown that pyrite from Au-enriched ore zones contain up to 5.1 elemental wt % arsenic and 989 ppm gold. Gold and arsenic concentrations within individual pyrite crystals show an inverse relationship with iron and sulfur, respectively. The molar ratio of Au:As is predominantly below a value of 0.02, indicating that most of the gold is structurally bound. Rare values above 0.02 indicate the presence of nano-scale inclusions of native gold. Pyrite crystals that are inferred to contain nano-scale inclusions of native gold are generally confined to ore zones that exhibit moderate to strong dissolution textures.
Imaging by SEM and QEMSCAN® has shown that gold-bearing arsenian pyrites are spatially associated with high-temperature biotite alteration. Within brecciated diatremal host rocks, biotite occurs as pervasive groundmass flooding and also has preferentially replaced phenocrysts within coherent clasts of phonolite and related alkaline igneous rocks. Biotite alteration predates the formation gold telluride minerals, and the lower temperature potassium feldspar alteration type associated with gold telluride formation. In addition, initial conservative estimates indicate that more than 10% of the total gold grade from specific sampling sites may be attributed to the presence of gold-bearing arsenian pyrite. This ore type may therefore have a significant contribution to the total gold budget of the deposit.