Trace Element Zonation in Carlin-Type Pyrite: Tracking Ore-Forming Processes at the Nanoscale

Abstract

Abstract Carlin-type gold deposits are renowned for hosting gold in finely zoned hydrothermal pyrite, but the characteristics of this zonation are incompletely understood. We use new depth profile techniques in nanoscale secondary ionizing mass spectrometry (NanoSIMS) to characterize the Au, Cu, As, Ag, and δ34S zoning in auriferous pyrite from eight gold deposits in Nevada: Carlin-type pyrite from Carlin, Deep Star, Beast, Turquoise Ridge and Getchell; Eocene dike pyrite from Beast, Betze Post, and Deep Star; and auriferous hydrothermal pyrite from the Lone Tree distal disseminated gold deposit and the Red Dot sedimentary rock-hosted deposit at Marigold. All of the hydrothermal pyrite types are characterized by hundreds of nanoscale zones with varied Cu, As, Ag, and Au. Most samples show concentric zoning, although patchy alteration or sectoral zoning can also be present. The number, sequence, and thickness of the zones is inconsistent throughout the data set. Correlations among the trace and minor elements vary among pyrite types, deposits, and between grains in the same sample. In different grains from the same sample, the Pearson correlation between Au and As varied from strongly negative (–0.7) to no correlation (0.0) to strongly positive (1.0). The sedimentary and magmatic precursor pyrite grain cores contain minor Au, Ag, As, and Cu, as well as Sb where analyzed. These trace elements are universally more enriched in hydrothermal pyrite overgrowths, except for Ag, which can be more enriched in some of the grain cores of magmatic origin. The maximum trace element concentrations in our Carlin-type hydrothermal pyrite are 2,600 ppm Cu and 17,290 ppm As (Turquoise Ridge); 2,050 ppm Ag (Beast); and 1,960 ppm Au (Deep Star). The maximum values from the entire sample suite are in Lone Tree arsenian pyrite with 70,080 ppm As; 9,790 ppm Ag; and 2,022 ppm Au; and Red Dot hydrothermal pyrite with 26,700 ppm Cu. Transmission electron microscopy data indicates that the Au occurs as nanoparticles at Red Dot. We combine new and previously published NanoSIMS δ34S data to show that Carlin-type pyrite grains with high δ34S sedimentary pyrite grain cores have rims with lower δ34S, whereas those with isotopically negative δ34S sedimentary pyrite grain cores have positive δ34S in the rims, due to mixing between sulfur in the sedimentary pyrite and sulfur from a magmatic-hydrothermal fluid. At high Au content, the Carlin-type hydrothermal rim δ34S values are close to the mean (7.1‰) of Tertiary magmas in the Great Basin, and within the range of Eocene mineralizing magmatic-hydrothermal fluids in the region (pyrite in equilibrium with this fluid has a δ34S of 0 to 8.8‰). At Lone Tree the δ34S values of the hydrothermal rims are slightly greater than the pyrite grain cores, and at Red Dot the rims have δ34S that is lower than the cores. The presence of As assisted with incorporation of Au in the Carlin-type pyrite, although Au was inconsistently available during pyrite growth. Our data show a wide range of As/Au molar ratios, indicating that the gold occurs as both Au+1 and Au(0) in different zones of the same grain. Variation in the form of Au may have resulted from fluctuations in the saturation state of Au, temperature changes during pyrite growth, or the presence of electrical potential differences caused by heterogeneous As and Cu concentrations in the pyrite. Local-scale mixing with meteoric fluids resulted in successive hydrothermal pyrite growth zones, iteratively upgrading the Au content of the pyrite to achieve the large Au endowment of the deposits. Despite many commonalities between Carlin-type hydrothermal pyrite and distal disseminated hydrothermal arsenian pyrite at Lone Tree, the metal sources or processes of fluid evolution are not identical. Hydrothermal arsenian pyrite at Red Dot has characteristics intermediate between distal disseminated and Carlin-type pyrite.