Hydrothermal Alteration of Trachyte Satellite Intrusions in the Cripple Creek District

The Cripple Creek deposit located in southwestern Colorado represents one of the largest epithermal gold deposits hosted by alkaline volcanic rocks. A petrographic study of the mostly barren satellite intrusions surrounding the deposit was undertaken to investigate and identify regional alteration patterns. Transmitted light, optical cathodoluminescence, and scanning electron microscopy, integrated with whole-rock geochemical data, revealed a complex history of fluid-rock interaction.
Early albitization of trachyte is preserved within weakly potassic-altered rocks. In more intensely altered rocks, potassium feldspar pervasively replaced the sanidine-dominated groundmass. Late alteration of the trachyte involved the formation of white mica and later kaolinite. The mineralogical changes imply a change in the physiochemical conditions of alteration from early sodic to later potassic alteration, followed by alteration caused at progressively decreasing temperatures and increasing acidity.
Tephrite intrusions within the trachyte satellite bodies have not been influenced by these styles of alteration. The tephrite has been altered to Fe-hydroxide minerals and biotite, most likely caused by influx of Fe-rich fluids. Within the mineralized trachyte of the main diatreme, Au-Te mineralization is at least locally associated with this style of Fe-rich alteration.