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Petrophysical properties and geochemical characteristics of crystalline rocks: implications for interpreting geophysical anomalies in the Cripple Creek mining district, Colorado
Wood, Patrick James Lombard
Wood, Patrick James Lombard
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2023
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The study evaluated whether legacy gravity and magnetic geophysical data acquired over the world-class Cripple Creek deposit in Colorado can be used to map Oligocene alkalic satellite intrusions and Proterozoic basement host rock based on physical property contrasts. Geochemical data for alkalic satellite intrusions and Precambrian host rock indicate that FeO, TiO2, and MgO contents explain physical property contrasts. Alteration effects density where rock types have experienced significant biotite alteration, sulfidation, advanced argillic alteration, Fe oxide-hydroxide formation, or intense leaching by surficial weathering. Physical property measurements were tested against magnetic and gravity data over the Cripple Creek district to incorporate surrounding alkalic intrusions. These data, constrained by geological information, suggests that the district borders a large, zoned granodiorite-gneiss complex to the southeast.
High susceptibility bodies are located along the northern, southwestern, and southeastern edges of the central diatreme. The magnetic anomaly north of the basin reflects a continuation of the biotite plagioclase gneiss that outcrops west of the Cripple Creek diatreme. A high magnetic anomaly (~400 nT) occurs west of the diatreme and is interpreted to reflect the aeromagnetic response of high magnetic susceptibility (~20.0 x 10-3 SI) biotite plagioclase gneiss. The nature of the magnetic signal in the southeast is indicative of magnetite-rich Ajax gneiss (~1.0 vol. % = ~30.0 x 10-3 SI; ~400ï€800 nT) and topographic influence at Big Bull Mountain. Low signals are sourced from altered diatreme breccias and become more extreme with increased alteration intensity over the Cresson Pipe, Globe Hill Pipe, and Altman. The aeromagnetic dataset maps subtle magnetic low anomalies over pyroxene phonolite at Grouse Mountain, Straub Mountain, and Little Pisgah Peak intrusions hosted in Ajax augen gneiss. Magnetic and gravity lows are observed over low density, low magnetic susceptibility (<1.00 x 10-3 SI) Pikes Peak granite, marking a lithologic contact along the northern border of the district.
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