Nanoscale Characteristics of Carlin-Type Auriferous Pyrite from the Nadaleen Trend, Yukon

Abstract Gold deposits of the Nadaleen trend in central Yukon host over 1.7 million ounces (Moz) of Au and share many characteristics in common with Nevada’s Carlin-type deposits, including similar host rock types, structural setting, alteration, and geochemistry, as well as the occurrence of gold in hydrothermal arsenian pyrite. We examined the textures, minor and trace element geochemistry, and δ34S signatures of precursor pyrite and hydrothermal pyrite overgrowths in samples grading over 35 g/t Au from the Sunrise and Conrad deposits. In the Osiris limestone at Sunrise, hydrothermal pyrite occurs as rims ranging from <1 to 5 µm overgrowing subhedral to euhedral sedimentary pyrite grains that are 20 to 100 µm in diameter; as rims (<1 to 3 µm thick) of hydrothermal pyrite that cement together the individual aggregates (measuring <1 to 5 µm) in framboidal pyrite; and as disseminated hydrothermal pyrite grains (<1 µm) that may be single stage. The hydrothermal pyrite in our Sunrise samples contains up to 45 ppm Au, 29 ppm Cu, 1,053 ppm As, and 15 ppm Ag, with δ34S compositions that are 1 to 8‰ higher than the sedimentary pyrite. The hydrothermal pyrite is zoned at the nanoscale, with the highest Au concentrations typically in the outermost portion of the rims. In the Conrad gabbroic dike, hydrothermal pyrite occurs as rims ranging from <1 to 5 µm overgrowing earlier pyrite grains that are 5 to 100 µm in diameter. The inner rims of the hydrothermal pyrite contain up to about 20 ppm Au, 900 ppm As, 60 ppm Ag, and 50 ppm Cu, whereas the outer margins of the hydrothermal pyrite contain up to about 670 ppm Au, 23,400 ppm As, 385 ppm Ag, and 115 ppm Cu. Relatively coarse hydrothermal rims (up to 5-µm) occur on the coarsest grains of precursor pyrite, suggesting that the substrate partially controls the texture of the hydrothermal pyrite, potentially due to the availability of Fe during sulfidation. The δ34S plateau values of the hydrothermal rims range from 1.2 to 11.0‰. Bayesian stable isotope modeling shows that the δ34S compositions of the hydrothermal pyrite can be generated by mixing the locally present sedimentary rocks with locally present magmatic sulfur. The modeling indicates that additional sources are not required, although they cannot be ruled out. At high Au concentrations, the modeling shows that most of the sulfur in the hydrothermal pyrite comes from a magmatic source, potentially from buried plutons visible as aeromagmetic anomalies. The modeling does not differentiate between whether (1) these magmatic rocks contributed sulfur and metals during passive leaching by an amagmatic hydrothermal fluid or (2) cooling magmas exsolved a sulfur- and metal-bearing fluid that led to magmatic-hydrothermal mineralization. We favor the latter interpretation, since the available geochronological evidence suggests that mineralization on the Nadaleen trend occurred during or shortly after Late Cretaceous emplacement of volumetrically limited, mantle-derived gabbroic dikes. Collectively, the evidence supports a Carlin-type origin for the gold deposits on the Nadaleen trend. Continued study is needed to link site-specific characteristics and processes to the regional metallogenic setting.