129-14 Geochemistry of Gem Peridot near Badger Creek in Park and Fremont County, Colorado

Abstract:

The Badger Creek area, located just northeast of Salida, Colorado, is a rare occurrence for gem peridot in the state wherein its specific geochemistry is not well documented. The peridot is hosted in Miocene-age basalt where collected samples range from 0.5-2mm in size, although facet grade material of up to 3 carats have been reported. The basalt is divided into 3 types: 1) a lower elevation black, dense unit with mainly microscopic peridot, 2) a mid-elevation gray, softer type hosting most of the gems, 3) an upper red-gray vesicular unit with dominant alluvial peridot with some in-situ within vesicles.

Fourier Transform Infrared Spectroscopy (FTIR) and portable X-Ray Fluorescence (XRF) were used to examine major & trace elemental compositions to evaluate the forsterite-fayalite nature of the peridot & host rocks. Type 1 basalt has an average composition of 40.8 wt% Si, 6.95 wt% Fe, 4.5% wt% Mg. Type 2 basalt has averages of 37.2 wt% Si, 6.2 wt% Fe & 4.7 wt% Mg. Type 3 basalt has averages of 38.4 wt% Si, 7.2 wt% Fe & 2.2% wt% Mg. Average peridot composition with type 2 basalt is 9.7 wt% Si, 2.3 wt% Fe & 10.0 wt% Mg, with average compositions associated with type 3 being 7.6 wt% Si, 1.5 wt% Fe & 7.6 wt% Mg. Results indicate that Badger Creek olivine is predominantly forsteritic. However, when compared with olivine from the prolific San Carlos region of Arizona, the Badger Creek olivine exhibits a higher iron composition.

The whole rock data is indicative of variations in cooling history, Si content & the concentration ratios of Fe and Mg within the basalt. Compared with the San Carlos locality, Badger Creek exhibits a higher Fe/Mg ratio despite overall lower concentrations of both elements. The San Carlos samples also display a more consistent Mg-Fe relationship between olivine & basalt, suggesting consistent equilibration between the mineral & host magma. In contrast, Badger Creek samples exhibit greater Mg–Fe non-linear variability, indicating a more complex petrogenetic history of multiple magma pulses and/or assimilation of crustal material. These differences suggest variations in the magmatic conditions & chemical evolution between the two localities despite similar geologic ages & tectonic settings. Further planned studies include petrography to deduce specific gem growth conditions and fluid inclusion microthermometry.

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