CONSTRAINING THE MAGMATIC-HYDROTHERMAL FLUID EVOLUTION FROM PROXIMAL TO DISTAL SETTINGS BY FLUID INCLUSION AND ISOTOPIC ANALYSES OF ORE AND GANGUE MINERALS

Magmatic-hydrothermal systems form a variety of ore deposits at different proximities to upper-crustal hydrous magma chambers, ranging from greisenization in the roof zone of the intrusion, porphyry mineralization at intermediate depths to epithermal vein deposits near the surface. The physical transport processes and chemical precipitation mechanisms vary between deposit types and are often still debated. For this study, we investigate the fluid evolution from proximal to distal settings at the Sweet Home Mine, Colorado (8 km from the world-class Climax Mo deposit), which has been assumed to be underlain by a magmatic intrusion and is overlain by polymetallic mineralization of the recently explored Detroit City Mine.

The Sweet Home Mine hosts greisen-like alteration overprinted by huebnerite (Hub) and sulfide–Fl–rhodochrosite (Rds) mineralization at 26 to 25 Ma. δ³⁴S compositions of pyrite as well as Sr isotopes and REY distributions in fluorite suggest that early Qz–Mo–Py–Toz–Ms–Fl assemblages were deposited from magmatic fluids under a fluctuating pressure regime at 400°C as indicated by CO₂-bearing, moderately saline (7.5–12.5 wt.% NaCl equiv.) fluid inclusions. Later sulfide mineralization was deposited under a hydrostatic pressure regime from low-salinity±CO₂-bearing fluids below 400°C. This stage is characterized by mostly negative δ³⁴S values for sulfides and sulfosalts, highly variable δ¹⁸O values for rhodochrosite due to variable mixing of magmatic fluids with meteoric water, and low REE contents in fluorite.

The overlying newly studied Detroit City Mine comprises 1) an early Qz–Mo–Py–Ser–Fl greisen stage, 2) a late greisen (?) Hub–Sp stage, 3) a Cu-sulfosalts–Cu-sulfides-Sp–Gn–Fl–gemmy Rds stage, and 4) a late Fl–Ap–Rds stage. In this presentation, we will show first results from new analyses of fluid inclusions, radiogenic and stable isotopic compositions to decipher the evolution of the ore-forming fluids. In particular, we investigate whether sulfide mineralization at the Detroit City Mine has formed by mixing of magmatic fluids with variable amounts of externally derived fluids, as proposed for the underlying Sweet Home Mine.