西藏知不拉铜多金属矿床地质、石榴子石分带特征及其地质意义

西藏知不拉铜多金属床是冈底斯成矿带东段典型的矽卡岩矿床,石榴子石是矿区最主要的矽卡岩矿物,其颗粒间的空隙是金属矿物的主要赋存部位。本文通过详细的钻孔编录,结合岩矿鉴定及电子探针分析,划分出两种不同类型的石榴子环带,并在垂向上具有明显的分带:产于顶板凝灰岩中的石榴子石以钙铁榴石为主,环带中心颜色深,向外逐渐变浅,由纯钙铁榴石过渡到钙铝榴石;而位于底板大理岩附近石榴子石多以钙铝榴石为主,从环带核部向外颜色变深,化学组成由钙铝榴石向钙铁榴石变化,其它化学成分变化不大。反映该区上下两套不同性质围岩在石榴子石形成过程中所起的作用不同,其中上部凝灰岩主要提供了Fe,底部大理岩则是Ca的来源,热液流体贡献Si、Al及部分Fe,并随着环境和物质成分改变导致环带外侧具有不同于核部的变化趋势。这很好地解释了石榴子石矽卡岩在空间上具有上部为钙铁榴石、向下逐渐过渡到钙铝榴石的空间分带。石榴子石特征及分带显示了其属热液接触交代成因,这为矿床类型的确定提供了依据,也为在该区域内寻找类似矿床指明了方向。

Geological and garnet zoning characteristics of the Zhibula copper polymetallic deposit in Tibet and their significance

The Zhibula copper polymetallic deposit is a typical skarn type deposit in eastern Gangdise metallogenic belt of Tibet, and garnet is the main skarn mineral in the ore district, with most metal minerals occurring between garnet grains. In this paper, such means as detailed borehole logging, rock-mineral identification under microscope and electron microprobe analysis were used to divide the zoning into two different types. There obviously exists vertical garnet zoning as follows: Garnet is mainly andradite in tuff comprising the roof, the garnet center is dark-colored and the color gradually becomes lighter towards the margin, accompanied by the change of the chemical composition from pure andradite to grossular. Garnet is mostly grossular near the marble floor, the color becomes dark from the center to the edge and the main chemical composition changes from andradite to grossular while other chemical components change quite insignificantly. These characteristics suggest that two distinct wall rocks made different contributions to the garnet formation. In the upper part, tuff mainly provided Fe composition, whereas at the bottom, marble contributed Ca ingredient, and hydrothermal fluids contributed Si, Al and some Fe. The rim of the garnet zone is different from the core because the environment and material compositions changed gradually during the whole process. This well explains the vertical garnet skarn zoning characterized by the change of the grossular in the upper part to andradite in the lower part. At the same time, the characteristics and zoning of the garnet show that Zhibula is a metasomatic hydrothermal deposit. The results obtained by the authors not only provide the basic data for the study of deposit genesis but also indicate the criteria in search for similar deposits in this region.