A geochemical study of the Sweet Home Mine,Colorado Mineral Belt,USA: hydrothermal fluid evolution above a hypothesized granite cupola |
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Authors: | Volker Lüders Rolf L Romer H Albert Gilg Robert J Bodnar Thomas Pettke Dean Misantoni |
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Institution: | (1) GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany;(2) Lehrstuhl für Ingenieurgeologie, TU München, Arcisstr. 21, 80290 Munich, Germany;(3) Department of Geosciences, Virginia Tech, Blacksburg, VA 24061, USA;(4) Institute of Geological Sciences, University of Bern, Baltzerstrasse 1 + 3, CH-3012 Bern, Switzerland;(5) Colorado Calumet Company Inc., P.O. Box 1169, Golden, CO 80402, USA |
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Abstract: | Deposition of quartz–molybdenite–pyrite–topaz–muscovite–fluorite and subsequent hübnerite and sulfide–fluorite–rhodochrosite
mineralization at the Sweet Home Mine occurred coeval with the final stage of magmatic activity and ore formation at the nearby
world-class Climax molybdenum deposit about 26 to 25 m.y. ago. The mineralization occurred at depths of about 3,000 m and
is related to at least two major fluid systems: (1) one dominated by magmatic fluids, and (2) another dominated by meteoric
water. The sulfur isotopic composition of pyrite, strontium isotopes and REY distribution in fluorite suggest that the early-stage
quartz–molybdenite–pyrite–topaz–muscovite–fluorite mineral assemblage was deposited from magmatic fluids under a fluctuating
pressure regime at temperatures of about 400°C as indicated by CO2-bearing, moderately saline (7.5–12.5 wt.% NaCl equiv.) fluid inclusions. LA-ICPMS analyses of fluid inclusions in quartz
demonstrate that fluids from the Sweet Home Mine are enriched in incompatible elements but have considerably lower metal contents
than those reported from porphyry–Cu–Au–Mo or Climax-type deposits. The ore-forming fluid exsolved from a highly differentiated
magma possibly related to the deep-seated Alma Batholith or distal porphyry stock(s). Sulfide mineralization, marking the
periphery of Climax-type porphyry systems, with fluorite and rhodochrosite as gangue minerals was deposited under a hydrostatic
pressure regime from low-salinity ± CO2-bearing fluids with low metal content at temperatures below 400°C. The sulfide mineralization is characterized by mostly
negative δ34S values for sphalerite, galena, chalcopyrite, and tetrahedrite, highly variable δ18O values for rhodochrosite, and low REE contents in fluorite. The Pb isotopic composition of galena as well as the highly
variable 87Sr/86Sr ratios of fluorite, rhodochrosite, and apatite indicates that at least part of the Pb and Sr originated from a much more
radiogenic source than Climax-type granites. It is suggested that the sulfide mineralization at the Sweet Home Mine formed
from magmatic fluids that mixed with variable amounts of externally derived fluids. The migration of the latter fluids, that
were major components during late-stage mineralization at the Sweet Home Mine, was probably driven by a buried magmatic intrusion. |
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Keywords: | Mo porphyry deposits Climax Sweet Home Colorado Mineral Belt Rhodochrosite Fluid inclusions Stable isotopes Lead isotopes Strontium isotopes |
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