Magmatic and hydrothermal evolution of the Cripple Creek gold deposit, Colorado, and comparisons with regional and global magmatic-hydrothermal systems associated with alkaline magmatism

摘要

The Cripple Creek district, Colorado is renowned for its epithermal gold telluride veins which have produced over 21 million ounces of gold from an intensely altered alkaline diatreme complex (total production + economic resources of >900 tons Au, geologic resource 1000 tons). Gold mineralization principally occurs as telluride minerals hosted by swarms of narrow veins composed of quartz ± fluorite, carbonate, adularia, pyrite > barite/celestite and accessory base metal sulfides. Mineralized hydrothermal breccias are also found in the district, along with low-grade, bulk tonnage resources that are targets of current mining activities. Newly recognized is a complex history of hydrothermal alteration and magmatism that led up to, and continued past the development of gold mineralization. These include the development of large volumes of low-temperature, alkali feldspar-rich styles of alteration, and more restricted volumes of high temperature, pyroxene and biotite-rich types. Gold mineralization is associated with voluminous K-feldspar + pyrite ± carbonate alteration that largely postdates igneous activity, and these are widely developed in the upper ∼1000 m of the volcanic complex. These follow a complex magmatic history characterized by at least three cycles of recharge. Although voluminous sulfate (anhydrite) and sulfide-rich styles of mineralization were also developed in the latest stages of hydrothermal activity, a remarkable aspect of Cripple Creek is the distinct underdevelopment of acid styles of alteration; feldspar and carbonate-rich styles of alteration predominate at all levels of exposure. The link between alkaline magmatism and gold deposits has been long recognized, but relatively recent discoveries of large, high grade deposits (Ladolam, Philippines, Porgera, Papua New Guinea), along with continued production from districts like Cripple Creek, encourages continued exploration. Salient characteristics shared by these deposits include telluride-rich mineralization accompanied by extensive carbonation, and voluminous K-metasomatism. Likewise, hydrolytic (acid) alteration tends to be poorly developed in many alkaline systems. This has important environmental implications, as the high acid buffering potential makes these deposits environmentally favorable to mine. The potential in alkaline systems for large and high grade deposits, coupled with the common lack of recognition of their distinctive styles of alteration and mineralization, makes these a compelling exploration target.