安徽月山矿田夕卡岩型矿床形成的水岩作用

Water/rock Interaction During Formation of Skarn-type Deposits in Yueshan Orefield, Anhui Province

文章分析了安徽月山矿田夕卡岩型矿床形成过程中水岩作用的类型和特征。估算和讨论了水岩作用过程中的质量_体积变化和动力学问题。结论认为 :水岩作用对流体成矿系统中夕卡岩和蚀变矿化作用的发生、发展 ,成矿流体的形成 ,成矿物质的富集 ,矿床定位等具有重要意义 ;水岩作用的水岩比值 (W/R)小于 0 .1;

The Yueshan orefield is a typical skarn-hydrothrmal Cu-Au orefield in the Middle and Lower Yangtze Valley metallogenic belt. The skarn rocks and ores in the skarn ore deposits show spatial zoning. Wall-rock alterations are well developed around the orebodies. Their formation can be subdivided into three stages: the early stage, comprising scapolitization, diopsidization, garnatization, albitization, zoisitization, tremolitization and actinolitization; the middle stage, composed of epidotization, chloritization, silicification and carbonization; the last stage, consisting of carbonization, silicification, argillation, sericitization and chloritization. Analyses in this paper indicate that the skarn rocks are metasomatic ones formed by the water-rock interaction between the magmatic dominant fluids and the wall rocks. In the ore-forming process of the skarn ore deposits, the Yueshan intrusive body underwent alteration through high to lower temperature of water-rock interaction processes. The concentrations of such elements as Si, K, Fe, Mn and Mg got higher in the hydrothrmal solution during alteration, which seems to be of great importance in the enrichment of ore-forming material in the fluid ore-forming system. There were no sharp changes in the volume of dioritic rocks after the high temperature alteration; nevertheless, after the low to moderate temperature alteration, except carbonization, the rock volume increased over 20 percent, and some space was thus provided for the location of orebodies. Infiltration is the major transportation form of the ore-forming elements, and is also a necessary condition for the formation of large-size orebodies. The zonal structures of skarn rocks, mineralizations and alterations are products of time-space separation of the mineral and element associations through a series of dissolution-deposition reactions, and also a reflection of complex results of such processes or factors as the fluid infiltration velocity, the rate of homogeneous phase reactions, the rate of multiphase reactions, and the velocity of dissolution and deposition waves during the water-rock interaction. The water-rock ratios in this orefield were lower than 0.1, and the water-rock interactions took place in an open fluid system. The concentration gradient accelerated the fluid migration and the further water-rock interaction.