FLUID CHEMISTRY EVOLUTION AND MINERAL DEPOSITION IN THE MAIN-STAGE CREEDE EPITHERMAL SYSTEM

This paper presents results of chemical speciation and reaction path c alculations that model fluid chemistry evolution and ore deposition in the main-stage Creede, Colorado, epithermal system. An extensive geol ogic, mineralogic, and geochemical framework for mineralization has be en developed by many researchers for the central and southern district vein systems (OH and P veins; central and southern Amethyst and Bulld og Mountain vein systems) and is used to constrain and guide the model ing presented in this paper. Previous studies have shown that the cent ral base metal sulfide-rich and southern barite- and silver-rich Creed e ores were deposited by hydrothermal brines with temperatures as high as 285 degrees C and salinities as high as 13 wt percent NaCl equiv. Fluid inclusion studies indicate that mixing with dilute steam-heated ground waters was the dominant ore deposition mechanism, although boil ing did occur during some stages. Speciation calculations confirm that the hydrothermal fluids, due to their high salinities, were relativel y acidic (pH near 5.5) and transported significant quantities of base metals (i.e., up to 10-2 m total concentrations of Zn) as chloride com plexes. Reaction path calculations show that strong north-south minera logical variations in main-stage mineralization are best accounted for by variable boiling of the hydrothermal brines, followed by lateral m ixing of the brines with overlying dilute, steam-heated ground waters. The extent of boiling prior to mixing was a function of the temperatu re and gas contents of the hydrothermal fluids as they first entered t he district ore zones from depth, and of the thickness of the steam-he ated ground-water column overlying the hydrothermal fluids. The calcul ations indicate that limited amounts of boiling (during stages when th e overlying ground-water column was relatively thick) produced a ite-p yrite-hematite-sphalerite-galena-chalcopyrite +/- adularia assemblage in the central district vein systems (OH and P veins; central Amethyst and Bulldog Mountain vein systems). More extensive boiling (during a stage in which the ground-water table apparently dropped considerably) deposited quartz, fluorite, adularia, and hematite with only minor su lfides in the central district veins. Boiling ceased when the saturati on pressure of the hydrothermal fluids dropped below hydrostatic press ures generated by the overlying ground-water column. Following boiling , lateral mixing with overlying steam-heated ground waters initially p roduced sphalerite- and galena-rich assemblages in the central distric t vein systems. With continued mixing to the south, the hydrothermal f luids deposited abundant barite, subordinate sphalerite, galena, and q uartz, and lesser native silver, acanthite, and sulfosalts in the dist rict's southern vein systems (southern Amethyst, southern Bulldog Moun tain vein systems). Modeling results for Creede and other epithermal f luid compositions show that epithermal ore grades, mineral assemblages , and mineral zoning patterns are strongly influenced by shallow hydro logic processes such as boiling and fluid mixing. As a result, epither mal mineral assemblages and zoning patterns can be used to reconstruct the paleohydrology of the hydrothermal systems from which they were d eposited, and thus provide useful tools for epithermal ore exploration .