Characterization of Carlin-Type Auriferous Arsenian Pyrite from the Goldstrike Property using EMP, SIMS, and VESPERS Synchrotron u-XRF: Constraints to Ore Deposition Mechanisms

Abstract

The Goldstrike property, located in northern Nevada within the Carlin Trend, contains one of the largest Carlin-type Au deposits in the world. The vast majority of this mineralization, formed in the Eocene, is in the form of Au-bearing, trace element-rich arsenian pyrite, either as very fine grains, overgrowths on earlier pyrite, or as reported in this study, patchy zones with high As values. Eight samples characteristic of Ore I and Ore II - ore types defined by Almeida et al. (2010) - were selected and analysed using electron microprobe, secondary ion mass spectrometry, and synchrotron !-XRF. !-XRF is a non-destructive technique for the elemental analysis of these samples with additional structural analysis capability. Although Ore I and Ore II yield similar Au values in whole rock analyses, and in the samples selected in this study, Ore II yielded much lower Au and trace element values in pyrite than Ore I. However, free gold was found in an Ore II sample, which explained their similar gold grade in whole rock. Two compositional trends were identified based on the ratio of Au and As in auriferous pyrite from both Ore I and Ore II: 1) those above an Au/As ratio of 0.007, characterized by elevated Ag, Au, As, Cu, Hg, Sb, and Tl that trend positively with respect to Au. The maximum value of Au ranges from 0.12 to 0.15at% (0.56 to 0.68wt%), occurring at an As concentration of 2.5 to 4.3at% (5 to 7.4wt%), and 2) those below an Au/As ratio of 0.007, characterized by As above the optimal range associated with lower Au and lower trace element concentrations. The peak in As corresponds well with the theoretical maximum amount of As that can be incorporated as a solid solution in pyrite (~6wt%) before the structure changes to a two-phase pyrite-arsenopyrite system. The less structurally stable solid solution has a more reactive surface that is more amenable to adsorption of other trace elements, including Au, especially with increased As.