HYDROTHERMAL ALTERATION AND MASS-EXCHANGE IN THE HORNBLENDE LATITE PORPHYRY, RICO, COLORADO

The Rico paleothermal anomaly, southwestern Colorado, records the effe cts of a large hydrothermal system that was active at 4 Ma. This hydro thermal system produced the deep Silver Creek stockwork Mo deposit, wh ich formed above the anomaly's heat source, and shallower base and pre cious-metal vein and replacement deposits. A 65 Ma hornblende latite p orphyry is present as widespread sills throughout the area and provide d a homogeneous material that recorded the effects of the hydrothermal system up to 8 km from the center. Hydrothermal alteration in the lat ite can be divided into a proximal facies which consists of two assemb lages, quartz-illite-calcite and chlorite-epidote, and a distal facies which consists of a distinct propylitic assemblage. Temperatures were gradational vertically and laterally in the anomaly, and decreased aw ay from the central heat source. A convective hydrothermal plume, 3 km wide and at least 2 km high, was present above the stockwork molybden um deposit and consisted of upwelling, high-temperature fluids that pr oduced the proximal alteration facies. Distal facies alteration was pr oduced by shallower cooler fluids. The most important shallow base and precious-metal vein deposits in the Rico district are at or close to the boundary of the thermal plume. Latite within the plume had a large loss of Na2O, large addition of CaO, and variable SiO2 exchange. Dist al propylitized latite samples lost small amounts of Na2O and CaO and exchanged minor variable amounts of SiO2. The edge of the plume is mar ked by steep Na2O exchange gradients. Na2O exchange throughout the pal eothermal anomaly was controlled by the reaction of the albite compone nts in primary plagioclase and alkali feldspars. Initial feldspar alte ration in the distal facies was dominated by reaction of the plagiocla se, and the initial molar ratio of reactants (alkali feldspar albite c omponent to plagioclase albite component) was 0.35. This ratio of the moles of plagioclase to alkali feldspar albite components that reacted evolved to 0.92 as the reaction progressed. Much of the alkali feldsp ar albite component in the proximal facies reacted while the primary p lagioclase was still unreacted, but the ratio for these assemblages in creased to 1.51 when the plagioclase entered the reaction paragenesis. Plagioclase reaction during distal propylitic alteration resulted in pseudomorphic albite mixed with illite and a loss of Na2O. CaO is lost in the distal facies as hornblende reacts to chlorite, although some calcium may be fixed in calcite. CaO is added to the proximal facies a s the quantity of chlorite replacing hornblende increases and epidote and calcite are produced.