Fluid inclusions and rare earth element geochemistry of fluorite from south-central Idaho

Abstract

Epithermal fluorite mineralization of Tertiary age occurs in a northeast-trending belt within the Challis 2 degrees quadrangle of south-central Idaho. Fluorite was collected from the Meyers Cove area and the Bayhorse, Yankee Fork, and Stanley mining districts. Host rocks include the Eocene Challis Volcanics, the Cretaceous Idaho batholith, and the Ordovician(?) Bayhorse Dolomite. Mineralization is structurally controlled and is present as fault and fracture fillings and breccia cements. Argillization and silicification of the wall rocks is common and locally intense.Microthermometric measurements on nearly 400 two-phase (H 2 O + vapor), water-rich inclusions with low vapor to liquid ratios yielded mean homogenization temperatures of 160 degrees C for the Meyers Cove area, 150 degrees C for the Stanley district, and 130 degrees C for the Bayhorse and the Yankee Fork districts. Salinities range from 0 to 1.4 equiv wt percent NaCl.The rare earth element content of the fluorite is quite variable, ranging from 2.6 to 130 ppm, and Ce/Yb ratios reflect the dominant trend of light rare earth element enrichment. Fluorite from the Meyers Cove area and the Bayhorse and Stanley districts have similar Tb/La ratios, indicating crystallization at about the same time. Fluorite from the Yankee Fork district has a lower mean Tb/La ratio which is indicative of earlier crystallization. Chondrite-normalized plots and rare earth element ratios reveal persistent negative Ce anomalies, indicating high oxygen fugacities at the source area. Positive Eu anomalies indicate the release of Eu (super +2) during feldspar destruction, oxidation to Eu (super +3) at the deposition site, and subsequent incorporation in the fluorite. Samples with negative Eu anomalies indicate the presence of divalent Eu.The fluid inclusion, geochemical, geologic, geochronologic, and stable isotope data support deposition in Tertiary hydrothermal systems dominated by fluids of meteoric origin. The most important depositional mechanism is believed to be an increase in the pH of the ore-bearing fluids upon interaction with the wall rocks.