17:15 〜 19:15
[HRE12-P02] Textural and geochemical characteristics of gold-bearing pyrite from the Leon Carlin-type gold deposit in the Ratatotok district, North Sulawesi, Indonesia
キーワード：Leon deposit, Carlin-type, Gold deposit, Ratatotok District, Au-bearing pyrite, As-rich pyrite
The study area is located in Ratatotok district, North Sulawesi, Indonesia. Geology of the Ratatotok district consists of Miocene silty and fossiliferous limestone overlain by Pliocene andesitic volcanic rocks and intruded by porphyritic andesite. Several gold deposits and prospects, such as Mesel, Leon, Yance, and Bulex, have been reported in the district. The Mesel deposit, the biggest deposit in the district showing a similarity to the characteristics of Carlin-type gold deposits, has a reserve of 7.8 million tons at 7.3 g/t Au. The Leon deposit is situated approximately 1 km north-northeast of the Mesel deposit. Several studies have previously been conducted on the Mesel and Leon deposits to characterize their alteration and mineralization. One of the findings was relatively high gold grades despite absence of visible gold. This study aims to elucidate the relationship between textures and invisible gold content in pyrite from the Leon deposit, as well as their origins. Petrographic, SEM, and EPMA analyses were employed to achieve the objectives.
Ore mineralization in the Leon deposit is characterized by pyrite, marcasite, arsenopyrite, stibnite, realgar, and orpiment, with trace amounts of chalcopyrite, cinnabar, and sphalerite. Gold mineralization occurred as dissemination and as calcite-sulfide veinlets within carbonate rocks, particularly limestone, and in porphyritic andesite. We identified three types of pyrite (Pyrite-0, Pyrite-L-1, and Pyrite-L-2) in limestone and two types of pyrite (Pyrite-PA-1 and Pyrite-PA-2) in porphyritic andesite. Pyrite-0 is As-poor and framboidal-shaped pyrite (~4 to 22 µm, with an average of ~8 µm) occurring as accumulations of very fine euhedral to subhedral grains (~0.1 to 3 µm), which are mostly overgrown by later generations of marcasite and pyrite. Pyrite-L-1 is also As-poor and occurs as euhedral to subhedral crystals commonly coexisting with marcasite, both of which overgrow Pyrite-0. This type has a relatively smoother surface than those of Pyrite-0 and Pyrite-L-2. Pyrite-L-2 is significantly enriched in As. It forms the rim of zoned pyrite overgrowing Pyrite-L-1 and marcasite and occurs as euhedral to subhedral, elongated, and skeletal-like grains. This type is relatively more porous and has lower relief under the microscope compared to those of Pyrite-L-1 and marcasite. Pyrite-PA-1 in the porphyritic andesite is As-poor, occurs as euhedral to subhedral grains, overgrows Fe-rich rims of amphiboles, and sometimes contains inclusions of apatite and rutile. In contrast, Pyrite-PA-2 is As-rich and rims Pyrite-PA-1.
Pyrite in the limestone shows a gradual increase of As and Au contents from Pyrite-0, Pyrite-L-1 to Pyrite-L-2, with average values of 0.31 wt.% As and below the limit of detection (70 ppm) Au, 0.60 wt.% As and 80 ppm Au, and 6.5 wt.% As and 150 ppm Au, respectively. Meanwhile, pyrite (Pyrite-PA-1 to Pyrite-PA-2) hosted in the porphyritic andesite exhibit an increase of As contents (average 0.03 wt.% and 3.73 wt.%, respectively) and a decrease in Au contents (average 80 ppm and below the limit of detection, respectively). Gold in pyrite with elevated Au grades in the limestone (Pyrite-L-2) and the porphyritic andesite (Pyrite-PA-1) occurs as a solid solution (Au+) and possibly as nanoparticles (Au0) within pyrite crystals, respectively. Co/Ni ratios of pyrite indicate that most of Pyrite-0 and Pyrite-L-1 were generally derived from sedimentary and hydrothermal sources, whereas Pyrite-L-2, Pyrite-PA-1, and Pyrite-PA-2 exhibit a magmatic-hydrothermal origin.
Ore mineralization in the Leon deposit is characterized by pyrite, marcasite, arsenopyrite, stibnite, realgar, and orpiment, with trace amounts of chalcopyrite, cinnabar, and sphalerite. Gold mineralization occurred as dissemination and as calcite-sulfide veinlets within carbonate rocks, particularly limestone, and in porphyritic andesite. We identified three types of pyrite (Pyrite-0, Pyrite-L-1, and Pyrite-L-2) in limestone and two types of pyrite (Pyrite-PA-1 and Pyrite-PA-2) in porphyritic andesite. Pyrite-0 is As-poor and framboidal-shaped pyrite (~4 to 22 µm, with an average of ~8 µm) occurring as accumulations of very fine euhedral to subhedral grains (~0.1 to 3 µm), which are mostly overgrown by later generations of marcasite and pyrite. Pyrite-L-1 is also As-poor and occurs as euhedral to subhedral crystals commonly coexisting with marcasite, both of which overgrow Pyrite-0. This type has a relatively smoother surface than those of Pyrite-0 and Pyrite-L-2. Pyrite-L-2 is significantly enriched in As. It forms the rim of zoned pyrite overgrowing Pyrite-L-1 and marcasite and occurs as euhedral to subhedral, elongated, and skeletal-like grains. This type is relatively more porous and has lower relief under the microscope compared to those of Pyrite-L-1 and marcasite. Pyrite-PA-1 in the porphyritic andesite is As-poor, occurs as euhedral to subhedral grains, overgrows Fe-rich rims of amphiboles, and sometimes contains inclusions of apatite and rutile. In contrast, Pyrite-PA-2 is As-rich and rims Pyrite-PA-1.
Pyrite in the limestone shows a gradual increase of As and Au contents from Pyrite-0, Pyrite-L-1 to Pyrite-L-2, with average values of 0.31 wt.% As and below the limit of detection (70 ppm) Au, 0.60 wt.% As and 80 ppm Au, and 6.5 wt.% As and 150 ppm Au, respectively. Meanwhile, pyrite (Pyrite-PA-1 to Pyrite-PA-2) hosted in the porphyritic andesite exhibit an increase of As contents (average 0.03 wt.% and 3.73 wt.%, respectively) and a decrease in Au contents (average 80 ppm and below the limit of detection, respectively). Gold in pyrite with elevated Au grades in the limestone (Pyrite-L-2) and the porphyritic andesite (Pyrite-PA-1) occurs as a solid solution (Au+) and possibly as nanoparticles (Au0) within pyrite crystals, respectively. Co/Ni ratios of pyrite indicate that most of Pyrite-0 and Pyrite-L-1 were generally derived from sedimentary and hydrothermal sources, whereas Pyrite-L-2, Pyrite-PA-1, and Pyrite-PA-2 exhibit a magmatic-hydrothermal origin.