Mineralogical and geochemical fingerprints of alteration associated with the Cripple Creek alkaline-magmatic Au-Te deposit, Colorado

Abstract

The Cripple Creek alkaline-magmatic Au-Te deposit, located in southern Colorado, USA, represents a world-class gold deposit. The deposit formed within an alkaline volcanic complex dominated by a central diatreme that records a complex history of brecciation, alteration and mineralizing events. The origin of gold and tellurium in the diatreme remains enigmatic. Questions also remain about why near-mine satellite intrusions of similar magmatic affinities and inferred age are weakly mineralized or barren. The broad aim of this study was to investigate controls on mineralization by characterizing the type, intensity and timing of alteration events in the satellite intrusions and comparing the results to alteration within gold-bearing mineralized samples from within the Cripple Creek diatreme. A complex history of overprinting mineral growth and alteration could be deciphered that is interpreted to have formed as a result of an evolution of fluid composition and temperature during at least two separate fluid influx events. Alteration of trachyte in the district is described in terms of intensity of potassic alteration that controls the preservation of pre-potassic alteration events and is texturally destructive when developed intensely. Partial replacement of igneous sanidine by albite and growth of secondary amphibole is preserved in weakly altered trachyte and is part of an early sodic alteration event. With increasing intensity of potassic alteration primary igneous textures and albite are completely erased due to replacement by potassium feldspar. Potassium feldspar is overprinted by sericite, which occurs pervasively in the groundmass and is concentrated around vugs formed in intensely altered trachyte. Kaolinite partially replaces both sericite and potassium feldspar and is also most commonly formed around vugs and along fractures. This progression in alteration mineralogy from a potassium feldspar-dominated groundmass, to abundant formation of kaolinite after sericite, is interpreted to reflect the cooling path of a single hydrothermal fluid that was most likely of magmatic origin. It is possible that this fluid evolved from the same fluid that caused early sodic alteration. In intensely potassium-altered zones adjacent to breccia, potassium feldspar, sericite, and kaolinite are overprinted by biotite and Fe-hydroxide minerals that typically form along fractures in trachyte. Biotite and associated, most likely supergene, Fe-hydroxide minerals are interpreted to have formed from a hot, Fe-rich hydrothermal fluid, likely sourced from a different magmatic fluid than that responsible for the sodic-potassic alteration in these rocks. Weakly altered intrusive rocks, which formed the breccia of the satellite bodies, are preserved in the center of the breccia complexes. They are silica-poor, and relatively high in Fe compared with trachyte. These silica-poor rocks may have affinities similar to lamprophyre rocks that intrude the main diatreme. Textural relationships of clasts and matrix suggest the breccia formed after the potassic alteration event, but before both trachyte and silica-poor intrusive rocks were altered by a high-T, Fe-bearing hydrothermal fluid. It is possible this was related to a deeper, basic magmatic body. Au-telluride minerals were observed in biotite-Fe-hydroxide altered fractures, indicating a possible correlation to fluids formed in association with basic magmatic activity. Furthermore, based on comparisons between rocks of the satellite intrusive and the diatreme, gold mineralization was probably primarily controlled by the exposure to fluids, which was limited in the satellite bodies by poor availability of major fluid pathways.