内蒙古北山老硐沟金矿石榴子石地球化学特征

老硐沟金矿是北山成矿带东段发现的中型金矿床，是多期次多阶段成矿作用叠加的产物，矿床成因类型复杂。矿床共分为5个矿段，其中Ⅲ矿段以矽卡岩矿体为主。矽卡岩矿物以石榴子石为主，可分为早、晚两期，早期石榴子石更具震荡环带。通过详细的镜下观察和电子探针对两期石榴子石进行了系统研究，早期石榴子石核部以钙铝榴石组分为主，向边部为钙铁-钙铝过渡组分；晚期石榴子石以钙铁榴石为主。石榴子石化学成分特征表明，早期矽卡岩化阶段，热液环境为中酸性、弱氧化—弱还原环境；后期铁质含量增多，氧逸度增加，热液环境碱性、氧化性增强。老硐沟金矿Ⅲ矿段石榴子石为钙铁-钙铝榴石系列，属热液交代成因，早期多形成钙铝-钙铁榴石，伴随铜矿化，晚期热液环境变化，钙铁榴石增多，黄铁矿化、毒砂矿化增多，造成金富集成矿。

Geochemical characteristics of the garnets from the Laodonggou gold deposit,Beishan, Inner Mongolia

The Laodonggou gold deposit is a medium-sized gold deposit discovered in the eastern section of the Beishan metallogenic belt. It resulted from the superposition of multi-phase and multi-stage metallogenesis. Due to its complex deposit genesis, the deposit has five sections, of which skarn orebodies dominate Section III. Skarn minerals are mainly garnet, which can be divided into early and late phases, with early garnet having more oscillatory ring bands. We systematically studied the two types of garnet by detailed microscopic observation and electron microprobe. The core of the early garnet primarily consists of essonites, and the edge is calcium–iron to calcium–aluminum composition. Andradites dominate the late garnet. The chemical composition of garnet indicates that the early skarnization stage is under a moderately acidic and weakly oxidizing–to–weakly reducing hydrothermal environment, while the iron content increases in the later stage and the oxygen escape increases, resulting in increased alkalinity and oxidation. The garnets in Section Ⅲ, originating from hydrothermal metasomatism, belong to the andradite–essonite series. Essonites and andradites prevail in the early stage, accompanied by copper mineralization. With the late hydrothermal environmental changes, andradites increase, and pyrite mineralization and arsenopyrite mineralization increase as well, resulting in gold-rich mineralization.