Relationship of water-rock interactions to base- and precious metal mineralization, Bayhorse district, Idaho.

Abstract

Stable isotope, fluid inclusion, whole-rock geochemical, phase equilibria and field studies are used to quantify the relationship of regional water-rock interactions to ore-forming processes in the Bayhorse Cu-Pb-Ag(-F) district. These studies indicate that the base- and precious-metal mineralization and the fluorite mineralization formed from two temporally and chemically distinct meteoric water-dominated hydrothermal systems: one associated with Cretaceous, granitic plutonism and the other with Eocene Challis volcanism, respectively. Fluid inclusion and stable isotope evidence indicate that neither the Pb-Zn-Ag (205 to 282$\sp\circ$C, 5 to 15 equivalent weight % NaCl; $\delta\sp{18}$O$\sb{\rm H\sb2O}$ = 3.9 to 8.3, $\delta$D$\sb{\rm H\sb2O}$ = $-$135 to $-$144 per mil) nor the fluorite (98 to 146$\sp\circ$C; 0.0 to 1.1 equivalent weight % NaCl; $\delta\sp{18}$O$\sb{\rm H\sb2O}$ = $-$19.5 to $-$17.1, $\delta$D$\sb{\rm H\sb2O}$ = $-$127 to $-$148 per mil) deposits resulted from basin-derived brines, akin to those responsible for Mississippi Valley-type deposits, nor from magmatic waters. The geological and geochemical differences between these mineralizations are due to differences in temperature, and the nature and degree of water-rock interactions. Mass-balance calculations demonstrate the feasibility of producing the base- and precious-metal mineralization solely through meteoric water-rock interactions. This mineralization is typified by two mineral associations: Stage I, siderite + tetrahedrite; and Stage II, quartz + galena. Stage I $\delta\sp{18}$O$\sb{\rm H\sb2O}$ and $\delta$D$\sb{\rm H\sb2O}$ values range from 8.4 to 11.2 and from $-$55 to $-$95 per mil, respectively, whereas Stage II values range from 3.9 to 8.3 and from $-$112 to $-$145 per mil, respectively. Whole-rock $\delta\sp{18}$O and $\delta$D values from the phyllites define an isotopically depleted zone (75 km$\sp2$) around a granitic stock and are modeled to be the result of high temperature interactions with ancient meteoric waters at water/rock ratios (weight fractions) ranging from 0.002 to 0.09. Isotopic data indicate a metasedimentary source for the hydrothermal carbon, whereas either a heterogeneous metasedimentary or a mixed igneous/metasedimentary source is indicated for sulfur. Gibbs free energies for end-member tetrahedrites and tennantites were derived from unpublished $\Delta$H$\sp{\rm o}\sb{\rm f}$ values and estimates for entropy. These values facilitate the use of these phases in better defining the conditions surrounding their formation in terms of temperature, and volatile and non-volatile activities.