Large-scale alteration effects in the Rico paleothermal anomaly, Southwest Colorado

Abstract

A major 4 Ma palcothermal anomaly is present around the Silver Creek stockwork Mo deposit near Rico, southwestern Colorado. The anomaly extends at least 8 km outward from the deposit and was formed from heated, meteoric water circulating around an intrusion related to the deposit. The top of the 4 Ma Mo deposit is located near the center of the anomaly, more than 1 km beneath the present ground surface. Near-surface contemporaneous vein and replacement base and precious metal deposits are peripheral to the Mo system; they were the source of historic production in the Rico mining district.The thermal and chemical effects of the palcothermal anomaly are well developed in 65 Ma hornblende latite sills that extend throughout the district. They provided an essentially uniform material in which to measure the effects of water-rock interaction. Hydrothermal alteration in the latite resulted in several alteration facies. A texturally destructive facies proximal to the center of the system consists of quartz-illite-calcite and chlorite-epidote assemblages and is restricted to a vertical cylindrical zone, 3 km in diameter, directly above the stockwork Mo deposit. Peripheral to this, the latite is altered to a propylitic assemblage, with primary textures preserved. Temperature gradients in the altered latite, measured from tetrahedralsite AI in hydrothermal chlorite and the degree of partial annealing of fission tracks in apatite and zircon, are symmetrically distributed about the stockwork Mo deposit and show that temperature increased gradationally toward the heat source. The chlorite tetrahedral AI geothermometer indicates that temperatures were less than 200øC in the distal part of the system and greater than 300øC in the center of the palcothermal anomaly, directly over the stockwork Mo system. Whole-rock b•sO values of the latite decrease from near fresh-rock values of about 9.0 per mil to as low as -4.0 per mil above the Mo deposit. Mass exchange between fluid and latite is extensive above the Mo deposit, where nearly all of the Na•O has been removed from the rocks. The extent and distribution of thermal and chemical effects in a vertical cylindrical volume of rock above the heat source suggests that this marks the pathway for an upwelling geothermal plume, 3 km wide and at least 2 km high. A horizontal outflow plume moved laterally away from the rising plume 1.5 to 2 km above the stockwork Mo system. Water/rock ratios within the plume area are greater than 0.5 on a molar oxygen basis, but are less than 0.45 in the propylitized latite outside the plume. All mineralization in the Rico district is related to this 4 Ma meteoric-magmatic hydrothermal system. The deep stockwork Mo deposit is beneath the plume and the shallower veins and replacement deposits are at the margins of the plume.

Large-Scale Stable Isotope Alteration Around the Hydrothermal Carbonate-Replacement Cinco de Mayo Zn-Ag Deposit, Mexico

“…The stable isotope compositions of natural fluids and rocks are generally different, so mineral replacement reactions occurring during hydrothermal rock alteration often result in distinctly modified isotope ratios (δ 18 O, δ 13 C) of the affected rock relative to its precursor (Taylor and Epstein, 1963;Taylor, 1974Taylor, , 1979Larson and Taylor, 1986a;Klein and Criss, 1988;Cathles, 1993). Mapping the spatial distribution of stable isotope alteration has been successfully employed to constrain the size of fossil hydrothermal systems and the controls on fluid migration, their origin, volume, and compositional evolution (Larson and Taylor, 1986b;Larson et al, 1994;Campbell and Larson, 1998). Hydrothermal ore deposit formation involves large volumes of disequilibrium fluids that interact with the country rock, causing halos of isotopic alteration (e.g., δ 18 O and δ 13 C) that show values distinct from more distal, unaltered rocks.…”

“…The spatial extent of isotopic alteration and isotopically distinctive carbonate veins usually extends far beyond zones of visible alteration and metal enrichment (e.g., Criss et al, 1985). Therefore, stable isotope mapping represents a potentially powerful prospecting tool complementing other mineral exploration techniques (e.g., mineralogy, lithogeochemistry, geophysics) (see also Criss and Taylor, 1983;Larson et al, 1994). The extent and degree of isotopic alteration of a given host rock depends on the fluid transport mode (pervasive or channelized), fluid and rock composition, fluid volume, and temperature-dependent reaction kinetics (e.g., Lassey and Blattner, 1988;Frimmel, 1992).…”

“…Historically, the main reasons for the limited practical deployment of stable isotope mapping in mineral exploration were unacceptably high costs of data collection and long turn-around times. Consequently, stable isotope studies of hydrothermally altered country rocks have been limited to purely academic research despite the potential for improving mineral exploration in some environments (Criss and Taylor, 1983;Dilles et al, 1992;Jamtveit et al, 1992;Larson et al, 1994;Murakami and Nakano, 1999;Barker et al, 2013;Hickey et al, 2014;Millonig et al, 2017).…”