A geochemical study of the Sweet Home mine, Colorado Mineral Belt, USA: formation of deep hydrothermal vein–type molybdenum greisen and base metal mineralization

Abstract

Deep hydrothermal Mo, W, and base metal mineralization at the Sweet Home mine (Detroit City portal) formed in response to magmatic activity during the Oligocene. Microthermometric data of fluid inclusions trapped in greisen quartz and fluorite suggest that the early-stage mineralization at the Sweet Home mine precipitated from low- to medium-salinity (1.5–11.5 wt% equiv. NaCl), CO2-bearing fluids at temperatures between 360 and 415 °C and at depths of at least 3.5 km. Stable isotope and noble gas isotope data indicate that greisen formation and base metal mineralization at the Sweet Home mine was related to fluids of different origins. Early magmatic fluids were the principal source for mantle-derived volatiles (CO2, H2S/SO2, noble gases), which subsequently mixed with significant amounts of heated meteoric water. Mixing of magmatic fluids with meteoric water is constrained by δ2Hw–δ18Ow relationships of fluid inclusions. The deep hydrothermal mineralization at the Sweet Home mine shows features similar to deep hydrothermal vein mineralization at Climax-type Mo deposits or on their periphery. This suggests that fluid migration and the deposition of ore and gangue minerals in the Sweet Home mine was triggered by a deep-seated magmatic intrusion. The findings of this study are in good agreement with the results of previous fluid inclusion studies of the mineralization of the Sweet Home mine and from Climax-type Mo porphyry deposits in the Colorado Mineral Belt.

Effects of Metasomatism on Granite-Related Mineral Systems: A Boron-Rich Open Greisen System in the Highiş Granitoids (Apuseni Mountains, Romania)

“…On the one hand, however, the paragenetic sequences of greisen systems follow some general rules [1- 3,5]. Deposition of the ore minerals typically starts with an Sn-W oxide phase, followed by common sulfides (e.g., porphyry Cu-Au-Mo-ore deposits), and then a late oxide phase characterized by Fe-oxides associated with carbonates [1- 9,36]. On the other hand, tourmalinization, epidotization, and hematitization are also observed in the contact metamorphic aureole in the study area (Figure 8); therefore, we suggest that this mineral assemblage, including epidote and apatite, is closely related to the late vein-depositing stage of the greisenization processes (Figure 9).…”

“…Related late to postmagmatic metasomatic processes have a close relationship with the economical enrichment of volatile components such as B, F, and Cl, and metallic elements such as Li, Be, Mo, Sn, W, Bi, U, and rare earth elements (REE). The greisen ores contain a characteristic mineral assemblage comprising quartz, muscovite, tourmaline, topaz, and fluorite, usually adjacent to quartz-cassiterite-'wolframite' veins (see, e.g., [1][2][3][4][5][6][7][8][9]). The paragenetic sequences of greisen-related mineralizations appear to follow some general rules that could be widely applicable to most greisen deposits [1-3].…”