Understanding Ore Deposit Scale Controls On AU Enrichment in Carlin-Type AU Mineralization: A High-Resolution Study of The Rita K Deposit

Controls on the distribution of gold (Au) within the Rita K Carlin-type deposit are poorly understood. The primary Au-bearing carbonate unit Wispy, is a bioturbated muddy limestone with debris flow clasts that range in size but are typically >5 cm occurring in the lower sections. The orebody is described as a syncline-anticline pair that hosts high-grade Au ore in the fold hinges. The spatial relationship between Au mineralization and fold hinges suggests that shortening due to folding is an important mechanism producing Au-enriched zones. However, the exact mechanism of Au enrichment and deposit-scale controls on Au distribution and fluid evolution through time remains unknown. The goal of this research is to investigate mechanisms causing variations in Au content throughout the Rita K deposit to inform future exploration and mining practices for Carlin-type deposits. Determining the relationship between structure, fluid flow, Au enrichment, and alteration is fundamental in building an ore deposit model at Rita K. For this study, we have selected two core holes based on the presence of debris flow deposits and vein networks to investigate fluid pathways at Rita K by utilizing a combination of bulk-rock geochemistry, reflected and transmitted light petrography, etching studies, micro-X-ray fluorescence mapping, and electron probe micro-analysis. Bulk-rock geochemistry data from our high-resolution study shows that debris flow samples are highly silicified, with an average of 86 wt.% (n=24, ±1σ, 1.34 wt.%) SiO2, when compared to vein network samples that average 39 wt.% (n=17, ±1σ, 10.0 wt.%) SiO2. The distribution of Si in micro-X-ray fluorescence element maps indicates that while vein networks facilitate fluid flow, debris flow deposits promote intergranular fluid flow enhancing wall rock interaction, sulfidation and Au deposition. Moreover, element mapping of pyrite grains challenges the well-established model that all arsenian pyrite rims on preexisting pyrite grains have a high Au content. Bulk rock geochemical data shows no correlation between Au and As in both debris flow samples (r2=0.002) and vein network samples (r2=0.12). The lack of correlation between Au and As suggests that intergranular fluid flow enhances fluid rock interaction leading to the formation of arsenian pyrite rims both with and without Au, and that pyrite rims are not necessarily required for the formation of Carlin-type Au deposits.