New constraints on fluid chemistry and elemental mobility during hydrothermal alteration from thallium isotope systematics of the battle mountain district, North-central Nevada

Abstract

The Battle Mountain area is a major porphyry style Au (–Cu) district in northern Nevada, forming one end of the Battle Mountain-Eureka mineral trend. This trend, along with several others in the north-central Nevada region, hosts some of the world's most prolific gold mineralization, which is oftentimes associated with a suite of trace elements including arsenic, antimony, tungsten, and thallium and can be related to or overprinted by hydrothermal alteration. However, determining the extent of control hydrothermal fluids exert on some of the deposits in the region has been difficult to ascertain. One of the associated trace metals in the region, thallium (Tl), is a highly incompatible element and, as such, dominantly resides in the continental crust, where it can be easily remobilized during hydrothermal alteration. While previous work has demonstrated the effects of hydrothermal alteration on Tl distribution and fractionation, the controls responsible for the observed Tl fractionation during hydrothermal alteration are still poorly characterized but may provide insight into fluid behavior. Here, we present new Tl isotope composition and concentration data for a suite of 54 mineral separates obtained from 43 samples from the Battle Mountain area, which range from unaltered intrusive igneous rocks through varying types and degrees of hydrothermally altered rocks.Measured Tl concentrations vary by more than an order of magnitude, from below detection limit (0.2 ppm in this study) to 2.0 ppm, while ε205Tl ranges between −5.0 and +2.2 (ε205Tl is the deviation of the 205Tl/203Tl isotope ratio of a sample from a standard in parts per 104). Thallium concentrations correlate positively with whole-rock potassium (K), thus show strong increases during K alteration, and demonstrate significantly lower ε205Tl values within K-altered samples. Conversely, during later, overprinting Na–Ca alteration, both K and Tl are removed, resulting in a noted decrease in Tl concentrations coupled with a shift to significantly higher ε205Tl values. It appears that during hydrothermal alteration, 203Tl is more easily (re)mobilized and (re)distributed, which reflects: 1) a first-order hydrothermal alteration control that relates to the transport of Tl during the formation of a new, metasomatic mineral assemblage (particularly the breakdown and/or formation of K-bearing minerals) and 2) a second-order mineralogical control, relating to inter-mineral equilibrium, which also results in a small fractionation effect.