Stable isotope and fluid inclusion investigations of epithermal vein and porphyry molybdenum mineralization in the Rico mining district, Colorado

Abstract

The Rico mining district, western San Juan Mountains, Colorado, contains epithermal vein deposits, carbonate replacement deposits, and a large zone of porphyry-style molybdenum mineralization. Historically, the vein and replacement deposits have produced significant amounts of silver, lead, and zinc, with minor gold and copper. All the mineralization formed nearly contemporaneously about 5 m.y. ago. The veins and replacement deposits occur in Paleozoic and Mesozoic sedimentary rocks that have been uplifted into the Rico dome, which is cored by a horst of Precambrian greenstone and quartzite. The porphyry molybdenum mineralization (40 million tons of 0.31% Mo) is 1,500 m beneath the surface in the east end of the district and consists of stockwork veining in Precambrian quartzite and greenstone and in Pennsylvanian sedimentary rocks. The epithermal veins occur above and peripheral to the porphyry mineralization. Widespread high silica, alaskite porphyry dikes were also emplaced at the same time as the porphyry mineralization and are probably related to the source intrusion for the molybdenum mineralization and the heat source for the hydrothermal system that produced the epithermal and replacement deposits. Fluid inclusions in quartz from the veins homogenize at temperatures in the range 200 degrees to 300 degrees C and yield salinities less than 5 equiv wt percent NaCl. The delta D values of quartz inclusion fluids (- 112 to - 121ppm) and calculated delta 18 O values of fluids in equilibrium with the quartz (-3 to -17ppm) show that the epithermal vein fluids were 18 O-shifted meteoric waters. Vein samples with large 18 O shifts also exhibit very saline fluid inclusions, indicating that the fluids which experienced the greatest degree of water-rock interaction contain the highest concentration of dissolved components. The porphyry fluid (delta D = -90 to -104ppm, delta 18 O = +2ppm) was derived from mixed magmatic and meteoric sources. Primary inclusions in the porphyry veinlets homogenize to both liquid and vapor in the temperature range 350 degrees to 420 degrees C, suggesting that boiling occurred during their formation. Trains of secondary inclusions are abundant in the veinlets and show that a later lower temperature fluid encroached upon the porphyry system. No evidence for mixing of magmatic and meteoric fluids was found in any of the epithermal veins. The porphyry mineralization formed early in the history of the Rico hydrothermal system. Epithermal vein formation occurred later than the porphyry mineralization, and the meteoric-hydrothermal fluid collapsed into the porphyry core of the system during the retrograde stages of the hydrothermal system.