西藏知不拉矽卡岩型铜矿床矿物学特征及地质意义

Mineralogical Characteristics of Zhibula Skarn-Type Cu Deposit in Tibet and Their Geological Significance

西藏冈底斯知不拉矽卡岩型铜矿床位于驱龙超大型斑岩型铜钼矿床以南约2km,矽卡岩及矿体主要呈层状-似层状赋存于下侏罗统叶巴组凝灰岩和大理岩中,具有凝灰岩-石榴子石化凝灰岩-石榴子石矽卡岩-辉石矽卡岩-(硅灰石化)大理岩的空间分带特征.石榴子石从早期到晚期以及从凝灰岩到大理岩方向均具有暗棕红色-棕红色-绿色(褐色)-淡黄褐色的变化特征,以钙铁榴石和钙铝榴石为主,辉石主要为透辉石,少量为锰钙辉石.靠近凝灰岩的石榴子石Al、Ti含量较高,靠近大理岩的石榴子石Fe、Mn含量较高.石榴子石环带特征明显,浅色环带富钙铁,暗色环带富钙铝,由核部向边缘整体显示钙铁组分逐渐增加、钙铝组分相对减少的趋势.知不拉层状-似层状矽卡岩型矿体是由深部隐伏岩浆结晶分异的含矿热液在温度与压力的驱动下沿凝灰岩和大理岩的岩性分界面选择性交代形成,属于岩浆热液接触交代型矿床而非层控或喷流成因层矽卡岩型矿床,应与驱龙斑岩铜钼矿床属于同一套斑岩成矿系统.

The Zhibula skarn-type Cu deposit is located about 2 km away from the south of Qulong superqarge porphyry CwMo deposit in Gangdese metallogenic belt, Tibet. The skarn and ore body are mainly distributed as layer-stratoid in the tuff and marble of Yeba Group in Lower Jurassic, which show zoning characteristics： tuff-garnert tuff-garnet skarn-pyroxene skarn- （wollastonite） marble. From early to late stages, or from tuff to marble, the colors of garnets change from dark brownish red to brownish red, followed by green（brown）, and pale brownish yellow These garnets are mainly grossular and andradite. The end members of pyroxenes are dominated by diopside, with minor johannsenite. Garnets near tuff have higher A1, Ti contents, whereas those nearthe marble have relatively more enriched Fe, Mn contents. Garnets show distinctive light and dark oscilla- tory-zoning. Light zoning is distinctively enriched withcalcium-iron components, and dark zoning is enriched with calcium- aluminum components. These garnets generally show increasing calcium-iron components and relatively reducing calcium- aluminum components from the core to outer zones. The Zhibula layer-stratoid skarn type ore body are developed due to thedeep buriedore-bearinghy drothermal migration with differential magmatic crystallization along the interface, or migration along fractured rock bands driven by temperature and pressure differences. In addition, selective metasomatic alteration between tuff and marble or crystalline limestone contributed to the formation of this ore deposits. Therefore, this ore deposit belongs to the magmatic hydrothermal contact metasomatic type rather than stratabound or exhalative forming layer skarn-type, and has the same forming mechanism with Qulong porphyry Cu-Mo deposit.