Preliminary scientific results of the Creede Caldera Continental Scientific Drilling Program: Stable isotope geochemistry of the Creede Formation and implications for the origin of fluids responsible for hydrothermal vein minerals in the Creede mining district

Abstract

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards or with the North American Stratigraphic Code. Any use of trade, product, or firm names in this report is for descriptive purposes only and does not imply endorsement by the U.S. Government. SULFUR ISOTOPE GEOCHEMISTRY Stable isotope systematics of sulfur-bearing vein minerals from the Creede mining district. The main stage sulfides in the veins at Creede are characterized by a narrow range of 534S values (-2±2%o) typical of volcanic or magmatic sulfur in a Tertiary volcanic environment; the vein barites, however, show a very large range of 534S values from about 4 to 44%o (Rye et al., 1988). The aqueous sulfate and sulfide species in the hydrothermal fluids were not in isotopic equilibrium and each sulfur specie was derived from different sources with profoundly different histories. The isotope data on the barites at Creede are so unusual for a hydrothermal deposit in a Tertiary volcanic environment that we have long known that some factor was present at Creede that normally does not exist in such hydrothermal systems. The sulfur and oxygen isotope data on hydrothermal barites are summarized in Figure 2 (Rye et al., 1988). The isotopic composition of the barites varies from north to south in the district with values from the northern mineralized areas being lower in both 534S and 518O. The data fall in a triangle that can best be interpreted as a mixture of three components. The lower apex (518O = 2%0 ; 534S = 1%0) is interpreted as deep sulfate from the hydrothermal system. The right apex (518O = 20 %0 ; 534S = 35 %o) is a critical one; the combination of large sulfur and oxygen values require that the sulfate went through a bacteriogenic reducing cycle in a sedimentary environment, interpreted to be the Creede Formation. The top apex (818O = 8%0 ; 534S = 48 %o) has even larger sulfur but lower oxygen values. Rye et al. (1988) interpreted the top apex to represent sulfate that had evolved by bacteriogenic reduction but had undergone further isotopic evolution by oxygen isotope exchange with ore fluids and thermochemical reduction of sulfate in the presence of organic matter. This further evolution occurred as lake water or Creede Formation pore fluids mixed with deeply circulating ore solutions in the southern part of the district. Bacteriogenic reduction of sulfate in a sedimentary environment produces diagenetic pyrite if iron is available. Therefore, the formation of diagenetic pyrite in the Creede Formation is a requirement of the hypothesis that the hydrothermal sulfate underwent a bacteriogenic cycle in the Lake Creede sediments. The search for such pyrite was one of the principal objectives of the drilling program. Drilling was critical to such a search because Creede Formation rocks exposed at the surface have been oxidized and any diagenetic pyrite removed by weathering. other component was a nearly fresh meteoric water with a 8D of about-110%o and a 818O of about-15%o, respectively. Thes...