Mineralogical and geochemical studies of the Globe Hill, Schist Island, and Wild Horse Extension pits, Cripple Creek gold deposit, Colorado
The Cripple Creek alkaline igneous rock-related low-sulfidation epithermal gold telluride deposit is hosted in a 10 km wide Oligocene alkaline volcanic diatreme in Proterozoic rocks. Periods of phreatic eruptions and progressively more mafic alkaline magma emplacement created the diatreme and breccias that host the gold ore, followed by breccia pipe creation, and later by the emplacement of phonolite and later lamprophyre dikes throughout the diatreme. Pervasive potassic alteration affected much of the diatreme, and is spatially related to the late-stage veins containing native gold and precious metal tellurides.
Previous studies have focused on the high-grade gold veins primarily located toward the center of the diatreme, investigating the paragenesis of minerals with relation to high- and low-grade ore. However, little work has been done to establish elemental and mineralogical aids to exploration for low-grade ore. The current study evaluates the mineralogy and paragenesis of low-grade mineralization in the currently mined WHEX, Globe Hill, and Schist Island pits, and the Galena Embayment along the margin of the diatreme next to the WHEX pit, as well as high-grade mineralization in the WHEX and Cresson pits, to establish a simplified three stage paragenesis for low-grade ore in the Cripple Creek deposit, determine the elements and minerals that vector to ore, and characterize the fluid history of the deposit.
Workers in the district have reported overlapping suites of minerals, including pyrite, in association with high- and low-grade Au mineralization. Pyrite, which can record past fluid conditions through crystal textures and trace element enrichments, is analyzed using scanning electron microscopy, electron microprobe, and laser ablation-inductively coupled plasma-mass spectrometry analyses to compare the trace element compositions of pyrite in low- and high-grade gold mineralization. These analyses reveal a complex trace element pattern with variability in Ag, As, Au, Co, Ni, Sb, and Te in the rims, cores, and zones (area between the core and the rim) of stages 2 (Py2) and 3 pyrite (Py3) and between the studied pits currently being mined. Trace element concentrations in stage 1 pyrite (Py1) are lower than those in Py2 and Py3, and the patterns are uniform. Trace elements form in the structure of pyrite and as micro- and nanoinclusions (e.g., up to 48,220 ppm As, 6,983 ppm Co, 12,250 ppm Ni, and 10,860 ppm Sb). The concentrations of As, Au, and Cu reported here are in general agreement with previous studies of pyrite in the deposit. The patterns of trace element enrichment suggest that the final stage of pyrite growth in high-grade gold telluride veins was enriched in Ag, As, Au, Cu, Pb, Sb, and Te.
The paragenesis and compositional zoning of pyrite associated with high-grade ore contrast with those associated with pyrite from low-grade ore, which preceded the formation of high-grade gold telluride veins. This suggests that metallic mineralization occurred in multiple episodes and that low-grade ore did not form from the same hydrothermal fluids that precipitated the high-grade ore. Early pyrite formed under gentle, non-boiling conditions, followed by a transition to vigorous boiling in Py2 and Py3, ending with non-boiling conditions recorded in the Au and As-rich rims of Py3. This enrichment of Au and As on the rims of pyrite is consistent with other alkaline rock-related epithermal Au deposits, as are the concentrations of trace elements throughout the pyrite grains. The presence of As, Ag, Au, Co, Cu, Mo, Ni, Sb, Te, and Tl in pyrite are considered vectors to high-grade ore in the Cripple Creek deposit.
Mining of and exploration for low-grade ore is focused in the northern part of the diatreme, where little work has been done to identify elements and minerals that have strong relationships with Au mineralization. Correlation coefficients, principal component analysis, hierarchical cluster analysis, and Random Forests were used to analyze major and trace elements compositions of 995 samples of rocks containing low- and high-grade mineralization from drill core in three pits in the northern part of the Cripple Creek diatreme. These methods suggest that Ag, As, Li, Sb, Te, W, and V are pathfinders to Au mineralization. These elements, in conjunction with previous mineralogical studies, indicate that tellurides, fluorite, quartz, carbonates, roscoelite, tetrahedrite, pyrite, sphalerite, muscovite, monazite, bastnäsite, and hübnerite serve as exploration guides to ore. These elements and minerals are consistent with pathfinders to ore identified in other alkaline rock-related epithermal Au deposits.