Abstract

The Sultan Mountain mine, in the western San Juan Mountains of Colorado, has produced Cu-Pb-Zn-Ag-Au ores from the mid-1870s until the 1950s. Production was from veins filling faults and fissures along the southern margin of the Silverton caldera. The principal host rock to the veins is a quartz monzonite stock.Five periods of hypogene mineralization have been recognized: (I) early quartz-pyrite; (II) quartz-pyrite; (III) rhodochrosite-siderite; (IV) main ore-stage chalcopyrite, tetrahedrite, galena, sphalerite, and gold; and (V) quartz-fluorite. Evidence of open-space filling (banding, crustification, vugs) is widespread.Heating studies of fluid inclusions in quartz, rhodochrosite-siderite, sphalerite, and fluorite indicate temperatures were approximately 200 degrees C for stages I to IV and 186 degrees C for stage V. Stages I, II, and IV show evidence of boiling. Crushing studies indicate that high-pressure gas, probably CO 2 , is present in the fluid inclusions. Freezing point depression estimates of salinity, corrected for CO 2 , indicate a range of 13.6 to 1.3 wt percent NaCl equiv. These data together with P-V-T data for saline solutions and P (sub CO 2 ) = 38 bars give a minimum depth of formation of 600 m.The delta 18 O values, at 200 degrees C, of water calculated for oxygen isotope composition of quartz range from -5.63 to -8.8 per mil and the values for rhodochrosite-siderite are -0.05 and -0.35 per mil. The delta 18 O values of whole rocks adjacent to the Champion and Alletha veins range from 2.65 to 6.10 per mil. These data suggest hydrothermal fluids were predominantly composed of meteoric water which underwent isotopic exchange with the quartz monzonite. The delta 13 C values for rhodochrosite-siderite range from -6.9 to -7.3 per mil. Carbon isotope data are inconclusive as to the source of the carbon. The delta 34 S values for sulfides range from -2.9 to -6.1 per mil, and these limited data suggest a Paleozoic evaporite source for most of the sulfur. Stable isotope data indicate isotopic control of the fluid by the surrounding country rocks.Geologic and geochemical studies suggest a model of Sultan Mountain ore deposition in which predominantly meteoric hydrothermal solutions, restricted mostly to the Sultan Mountain area, scavenged metals from the Precambrian basement and the Tertiary volcanic and intrusive rocks. Sulfur was derived mostly from solution of Paleozoic evaporites; however, the sulfur isotope data indicate some of the sulfur may have been derived from the quartz monzonite stock. The primary mechanism of precipitation of the ore was an increase in pH as a result of boiling.

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