Trace Elements and Sulfur Isotopes of Sulfides at the Leon Sediment‐Hosted Gold Deposit, Ratatotok District, North Sulawesi, Indonesia: Constraints on the Genesis of Ore Formation

The Leon gold deposit in the Ratatotok district, North Sulawesi, Indonesia, is hosted by the Middle to Late Miocene Ratatotok Limestone Formation overlying Early to Middle Miocene volcanic rocks. This study aims to elucidate the mode of occurrence of ores, and the conditions and genesis of ore formation of the deposit, based on morphological characteristics, trace elements, and sulfur isotope compositions of sulfide minerals. Four stages of sulfide mineralization associated with sedimentary and hydrothermal processes were identified. Stage 1 is characterized by disseminated framboidal pyrite (pyrite 1; a number after mineral name hereafter indicates its mineralization stage) of sedimentary origin. Stages 2–4 record hydrothermal overprinting, with successive formation of euhedral to subhedral pyrite 2 and marcasite 2, euhedral to anhedral pyrite 3 and arsenopyrite 3, and anhedral pyrite 4. Pyrite 1, pyrite 2, and marcasite 2 have lower Au and As contents, < 0.24 ppm Au and < 1.13 wt% As, compared to those of pyrite 3 with 69 ppm Au and 7.04 wt% As. Much higher Au and As contents were observed in arsenopyrite 3 in the limestone (ave. 110 ppm Au and 44.1 wt% As), the brighter and darker arsenopyrite 3 in the porphyritic andesite (90 ppm Au and 44.3 wt% As, and 80 ppm Au and 41.3 wt% As, respectively), and pyrite 4 (ave. 140 ppm Au and 4.41 wt% As). Gold in the sulfides was likely deposited from Au‐unsaturated hydrothermal fluids at the Leon deposit as gold occurs only as solid solution (Au + ) in the sulfides. Gold and As were incorporated into pyrite as coupled partitioning. Temperature and sulfur fugacity (in atm) during the mineralization of arsenopyrite 3 were ~303°C–399°C and log f S 2 = −10.6 to −6.6. δ 34 S CDT values of hydrothermal pyrite in stages 2–4 vary from −3.1‰ to +1.2‰, indicating a predominant magmatic source for S. The Leon deposit is a sediment‐hosted gold deposit with probably a distal intrusion‐centered setting.