17:15 〜 19:15
[HRE12-P05] Element mobility during the formation of the Batubesi skarn Fe-Sn deposit in Belitung Island, Indonesia
キーワード：mass-balance analysis, element mobility, geochemistry, Batubesi, skarn
The Batubesi skarn Fe-Sn deposit, located in the Southeast Asian tin belt on the eastern edge of Belitung Island, Indonesia, is characterized by its association with metasandstone and claystone of the Carboniferous-Permian Kelapa Kampit Formation. Skarn mineralization in the deposit is attributed to the intrusion of Triassic-Jurassic biotite granite. Prograde skarn minerals include garnet, diopside, and hedenbergite, while retrograde skarn is characterized by the presence of epidote and chlorite. Massive orebodies typically associated with garnet can be subdivided into two main groups, massive magnetite and massive arsenopyrite. The massive magnetite is composed predominantly of magnetite, fluorite, and ferrosilite with minor cassiterite. In contrast, the massive arsenopyrite is characterized by abundant arsenopyrite, pyrrhotite, and fluorite with minor cassiterite and trace amounts of pyrite and chalcopyrite.
A mass-balance calculation was conducted to quantify element mobility during the skarn formation. Mean concentrations were used in order to substantially reduce the influence of compositional variation in the samples. In this calculation, a sample of the biotite granite was used as representative of unaltered rocks (Ci0), while some samples of the prograde endoskarn were used as altered rocks (Cia). Gallium and Eu were selected as immobile elements, with respective volume factors (fv) of 0.99 and 1.01, to define an isocon line, where the correlation coefficient (r2) value is maintained at 0.999. Our calculation indicates a notable 37.1 % increase in the total mass of the prograde skarns compared to the unaltered granite. Calcium shows a significant mass gain of 46.7 wt% during the formation of prograde skarns, as indicated by the abundant garnet and diopside. A significant gain is also represented by Fe2O3 (21.6 wt%), As (6414 ppm), S (2818 ppm), Sn (1657 ppm), and Cu (918 ppm), resulting in the formation of massive magnetite and massive arsenopyrite ores. Fluorine-rich hydrothermal fluids, as evidenced by the occurrence of fluorite in this area, would have enhanced the mobility of those elements.
A mass-balance calculation was conducted to quantify element mobility during the skarn formation. Mean concentrations were used in order to substantially reduce the influence of compositional variation in the samples. In this calculation, a sample of the biotite granite was used as representative of unaltered rocks (Ci0), while some samples of the prograde endoskarn were used as altered rocks (Cia). Gallium and Eu were selected as immobile elements, with respective volume factors (fv) of 0.99 and 1.01, to define an isocon line, where the correlation coefficient (r2) value is maintained at 0.999. Our calculation indicates a notable 37.1 % increase in the total mass of the prograde skarns compared to the unaltered granite. Calcium shows a significant mass gain of 46.7 wt% during the formation of prograde skarns, as indicated by the abundant garnet and diopside. A significant gain is also represented by Fe2O3 (21.6 wt%), As (6414 ppm), S (2818 ppm), Sn (1657 ppm), and Cu (918 ppm), resulting in the formation of massive magnetite and massive arsenopyrite ores. Fluorine-rich hydrothermal fluids, as evidenced by the occurrence of fluorite in this area, would have enhanced the mobility of those elements.