Apatite and Biotite Geochemistry Constraints on the Petrogenesis of the Granites in Bangka Island, Indonesia: Implications for Tin Mineralization

ABSTRACT Bangka Island, situated within the Southeast Asian tin belt, is a prominent tin‐producing area in Indonesia. Primary tin deposits of Bangka Island are associated with two types of Late Triassic granites, namely biotite granite and hornblende‐biotite granite. In this study, we investigate petrography, bulk chemistry, and mineral chemistry of biotite and apatite for the biotite granite and hornblende‐biotite granite to elucidate their petrogeneses and physicochemical properties in relation to the formation of primary tin deposits in Bangka Island. Both granite types are composed of alkali feldspar, quartz, plagioclase, and biotite, with hornblende occurring only in the hornblende‐biotite granite, and accessory minerals including ilmenite, zircon, apatite, and monazite. They are classified as I‐type and ilmenite‐series, with A/CNK ranging mainly from 1.0 to 1.1 and absence of magnetite, as well as magnetic susceptibility values below 0.16 × 10 −3 SI unit. Apatite in both types of granite occurs as inclusions in biotite and quartz and is classified into three textural types. Apatite‐1 and apatite‐2 are hosted in the cores and rims of biotite, respectively, while apatite‐3 is hosted in quartz. All the types of apatite from both the biotite granite and hornblende‐biotite granite are identified as fluorapatite. Similarly, the chemical compositions of biotite in the biotite granite and hornblende‐biotite granite fall within the Fe‐biotite field. Whole‐rock Sn contents of the biotite and hornblende‐biotite granites vary from 3.0 to 51.0 ppm (av. 19.3 ppm) and 2.0 to 7.0 ppm (av. 3.3 ppm), respectively. The biotite granite has slightly higher silica contents (SiO 2 ~ 75 wt%) and relatively lower Eu anomaly (Eu/Eu* ~ 0.19), compared to those of the hornblende‐biotite granite (SiO 2 ~ 74 wt%; Eu/Eu* ~ 0.31). Both the granites were derived from crustal protoliths, as indicated by FeO T and MgO contents in biotite. Moreover, these granites exhibit comparable crystallization temperatures (zircon saturation temperatures = 728°C–872°C; apatite saturation temperatures = 831°C–939°C; Ti‐in‐biotite temperatures = 739°C–814°C). Al‐in‐biotite geobarometer indicates that the biotite granite and hornblende‐biotite granite crystallized at 69–177 MPa and 37–117 MPa, respectively, suggesting that both types of granite emplaced in the upper crustal level (1.3–6.4 km). A shallow emplacement of the biotite granite strongly favors magma degassing and extensive fluid exsolution in magmatic systems, as reflected by the decrease in calculated H 2 O contents from apatite‐1 to apatite‐3 and high Cl concentrations in magmatic fluids. Higher FeO and MnO and lower CaO contents of apatite, and higher Al T and Fe 2+ /(Fe 2+ +Mg) of biotite in the biotite granite indicate a more advanced degree of fractional crystallization compared to the hornblende‐biotite granite. In biotite granite, relatively low SO 3 contents of apatite and low X Mg , Mg#, and Fe 3+ /(Fe 3+ +Fe 2+ ) of biotite suggest a more reduced magma compared to the hornblende‐biotite granite. The fractionation of reduced magma of the biotite granite would have enhanced the concentration of Sn 2+ in the melt and promoted tin mineralization at shallow depths.