西藏甲玛铜多金属矿床岩浆-热液过渡阶段的矿物学证据

从矿物学的角度寻找甲玛铜多金属矿床斑岩体岩浆热液出溶的证据,建立起岩浆-热液过渡的详细过程。通过电镜扫面阴极发光对矿物的显微结构进行细致观察,并结合矿物化学特征,得到岩浆-热液过渡阶段的信息:(1)石英斑晶具有强烈的溶蚀结构,蠕虫状石英斑晶中溶蚀湾切断石英第一阶段的生长环带;(2)显微空腔将豆荚状或糖状结构石英与破布状黑云母(较少磷灰石、磁铁矿)、长石等相连接;(3)斑岩中伴生萤石,黑云母和角闪石中的F含量高;(4)斑岩体中流体包裹体与富挥发分熔融包裹体共存。岩浆挥发分作为主要的流体相被挤出岩浆并以气泡的形式聚集在岩浆房外壳顶部,并使已固结的矿物发生自交代。随着易挥发组分的增多,使气体压力大于束缚压力,导致岩浆外壳和邻近围岩突然破裂。岩浆流体沿着围岩裂隙流出并以热液的形式交代围岩。同时,由于减压造成岩浆热液的二次沸腾,使成矿元素沉淀。

Magmatic-hydrothermal transitions evidence from mineralogy of Jiama Cu-polymetallic deposit in Tibet, China

The information about the magmatic to hydrothermal transition is preserved in the porphyry of the Jiama Cu-polymetallic ore deposit in Tibet.It includes the quartz phenocrysts with strong resorption textures such as vermicular zones of igneous groundmass cutting primary quartz cathodoluminescence banding,the pods of saccharoidal quartz connected by graphic quartz-alkali feldspar intergrowths and ragged biotite,the fluorite as an accessory mineral in igneous rocks,high F in hydroxyl sites in igneous biotite and amphibole,and the fluid inclusions and silicate melt inclusions presenting in the porphyry of the Jiama deposit.The authors present detailed petrographic observations of primary igneous features,combined with silicate-melt and fluid inclusion studies,and find physical models of exsolution.Metallic elements are preferentially partitioned into the associated magmatic volatile phase.As the volatile concentration increases,the vapor pressure leads to sudden failure of the carapace and adjacent wall rock-this occurs once the vapor pressure is greater than the confining pressure.Magmatic fluids escape as the cracking of the wall rock continues;these fluids then hydrothermally alter the rock which they pass through.At the same time,caused by the rapid decreasing pressure,the hypersaline fluid boils again.This causes Cu-Mo sulfides to deposit from the fluid.