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

Malte Stoltnow,Samuel Niedermann,Stefan De Graaf,Volker Lüders

doi:10.1007/s00126-022-01102-6

Abstract

AbstractDeep 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.

Highlights

The Colorado Mineral Belt (CMB) hosts a number of economic molybdenum porphyry deposits, such as the worldclass Climax and Urad-Henderson deposits (Fig. 1)
The classification of fluid inclusions (FIs) as primary, pseudosecondary, and secondary and the definition of fluid inclusion assemblages (FIA; coevally trapped fluid inclusions along features such as crystal planes, growth zones, or healed microfractures) followed the criteria suggested by Roedder (1984) and Goldstein and Reynolds (1994)
We identified three types of fluid inclusions hosted in gangue and ore minerals of the early-stage mineralization: type 1— vapor-rich aqueous carbonic, three-phase FIs in milky quartz and fluorite; type 2—vapor-rich, two-phase FIs in milky quartz and fluorite; and type 3—aqueous, two-phase FIs in milky quartz, fluorite, pyrite, huebnerite, and sphalerite (Table 2)

Summary

Introduction

The Colorado Mineral Belt (CMB) hosts a number of economic molybdenum porphyry deposits, such as the worldclass Climax and Urad-Henderson deposits (Fig. 1). The formation of Climax-type Mo deposits in the CMB is related to the emplacement of highly evolved calc-alkaline granitic and subvolcanic rhyolite porphyry melts The ore is concentrated in ore shells that consist of stockwork vein and greisen molybdenite–fluorite–quartz–pyrite–topaz–sericite mineralization accompanied by intense host rock alteration (e.g., Audétat and Li 2017 and references therein; ESM Fig. 1). Multiple intrusions lead to different overlapping ore shells and individual Climax-type deposits (Wallace et al 1968; Mutschler et al 1981; White et al 1981; Bookstrom 1989; Carten et al 1993; Wallace 1995; Ludington and Plumlee 2009).

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