Composition of biotite within the Wushan granodiorite, Jiangxi Province, China: petrogenetic and metallogenetic implications

The Wushan skarn copper deposit is genetically associated with the Wushan granodiorite. In this study, we investigate the petrography and mineralogy of biotites within the Wushan granodiorite. We also determine the formation conditions of these biotites and discuss the significance of these minerals in terms of petrogenesis and mineralization. Electron microprobe analysis shows that biotites within the Wushan granodiorite are Magnesio-biotites that contain relatively high Mg and Ti concentrations and low Fe and Al concentrations. The ionic coefficient of [Al.sup.VI] in these biotites ranges from 0.03 to 0.19, with ΣFeO/(ΣFeO + MgO) ratios that range from 0.531-0.567 and MgO concentrations that range from 12.80-14.06 wt%. These results indicate that the Wushan granodiorite is an I-type granite. The Wushan biotites crystallized at temperatures (T) of 720°C-750°C, with oxygen fugacity (f[O.sub.2]) conditions of 11.6 to-12.5 and pressures (P) of 0.86-1.03 kb. These conditions are indicative of a crystallization depth (H) of 2.84-3.39 km. These data also indicate that the Wushan granodiorite developed under conditions of high temperature and high oxygen fugacity, suggesting that the Wushan granodiorite is prospective for magma-hydrothermal mineralization and that this granodiorite probably contributed to the formation of the Wushan skarn copper deposit. El deposito de skarn cuprifero de Wushan esta asociado geneticamente con la granodiorita de Wushan. En este estudio se investiga la petrografia y mineralogia de biotitas de la granodiorita de Wushan. Se determinan tambien las condiciones de formacion de estas biotitas y se discute la significacion de estos minerales en terminos de petrogenesis y mineralizacion. Un analisis de microsonda a electrones muestra que las biotitas de la granodiorita de Wushan son biotitas de magnesio que contienen altas concentracionesrelativas de Mg y Ti y bajas de Fe y Al. El coeficiente iconico de [Al.sup.VI] en estas biotitas oscila entre 0,03 y 0,19, con indices SFeO/(SFeO + MgO) que oscilan entre 0,531-0,567 y concentraciones de MgO que van desde 12,80 a 14,06 wt%. Estos resultados indican que la granodiorita de Wushan es de granito tipo I. Las biotitas de Wushan se cristalizaron a temperaturas (T) de 720°C-750°C, con condiciones de fugacidad del oxigeno (f[O.sub.2]) de-11,6 a-12,5 y presion (P) de O,86 a 1,03 kb. Estas condiciones indican una profundidad de cristalizacion (H) de 2,84-3,39 kilometros. Los datos tambien indican qu