Insights from Atom Probe Tomography into Carlin Type Gold Mineralization

Carlin-type gold (CTG) mineralization is one the best studied, yet poorly understood gold mineralization styles in the world. These deposits occur predominantly along NW-SE trends in central Nevada and are characterized by cryptic gold mineralization in host carbonate rocks (Cline et al., 2005, Econ. Geol.). CTG accounts for 9% of worldwide gold production, with all of it currently coming from five mining districts in northern and central Nevada. The discoveries of new CTG deposits in the Yukon Territory and Kyrgyzstan will drastically increase the importance of these deposits in the upcoming years. Despite the vast resource that CTG deposits entail, surprisingly little is known about their formation mechanisms, fluid source, or even the manner in which the gold is hosted. We do know that the gold tends to occur as trace elements within pyrite, which are difficult to study with the “normal” range of geology tools. With the recent application of atom probe tomography to geologic materials we now have the nano-analytical techniques to truly understand these cryptic and globally important deposits. This study combines high-resolution electron probe microanalysis (EPMA) with atom probe tomography (APT) to constrain whether the gold occurs as nano-spheres or is dispersed within the Carlin pyrites. Atom-probe tomography offers the unique capability of obtaining major, minor, trace, and isotopic chemical information at near atomic spatial resolution. We use this capability to investigate both the atomic-scale distribution of trace elements within Carlin-type pyrite rims, as well as the relative differences of sulfur isotopes within the rim and core of gold hosting pyrite. We show that gold within a sample from the Turquoise Ridge deposit (Nevada) occurs within arsenian pyrite overgrowth (rims) that formed on a pyrite core. Furthermore, this As rich rim does not contain nano-nuggets of gold and instead contains dispersed lattice bound Au within the pyrite crystal structure. The spatial correlation of gold and arsenic within our samples is consistent with increased local arsenic concentrations that enhanced the ability of arsenian pyrite to host dispersed gold (Kusebauch et al., 2019, Sci. Adv.). We hypothesize that point defects in the lattice induced by the addition of arsenic to the pyrite structure facilitates the dissemination of gold. The lack of gold-nanospheres in our study is consistent with previous work showing that dispersed gold in arsenian pyrite can occur in concentrations up to ~1:200 (gold:arsenic). We also report a method for determining the sulfur isotopic ratios from atom probe datasets of pyrite (±As) that illustrates a relative change between the pyrite core and its Au and arsenian pyrite rim. This spatial variation confirms the observed pyrite core-rim structure is due to two-stage growth involving a sedimentary core and hydrothermal rim, as opposed to precipitation from an evolving hydrothermal fluid.