楚雄盆地六苴铜矿床容矿砂岩孔隙演化对成矿的制约

六苴铜矿床是典型的陆相红层盆地砂岩型铜矿床, 具有明显的浅紫过渡带控矿与金属矿物分带特征, 砂岩粒间孔隙为金属矿物主要赋存空间。通过对赋矿砂岩层各岩性段的碎屑含量、颗粒分选性、胶结物特征、孔隙类型及孔隙度、渗透率等的统计与分析, 结果表明, 上白垩统马头山组六苴下亚段(K₁ml1)的中细粒长石石英砂岩具有高碎屑含量、低分选系数、高孔渗系数等特征, 为有效的流体迁移通道。K₁ml1砂岩层局部含丰富的有机质, 在中成岩阶段可演化为烃源岩, 形成富有机质的酸性-还原流体。该流体与碱性-氧化流体在砂岩透水通道中形成稳定对流, 在砂岩中可形成由紫到浅的铁质、钙镁质、钙硅质、硅质胶结的胶结物分带。在水-岩相互作用中, 酸性-还原流体起溶解砂岩早期的铁质、泥晶碳酸盐胶结物及还原硫酸盐的作用, 由此形成粒间孔隙并提供还原硫, 从而为矿质沉淀提供空间和硫源;碱性-氧化流体则提供铜离子并控制金属硫化物、碳酸盐胶结物的沉淀。生烃作用减弱时, 碱性-氧化流体越过稳定对流的平衡面, 使硅质胶结的浅色砂岩溶蚀, 形成溶蚀孔洞, 进一步提供容矿空间, 并导致金属矿物发生交代作用。砂岩各成岩阶段的水-岩相互作用是控制孔隙和胶结物生成及矿质沉淀的主要因素。

The Pore Evolution of Ore-bearing Sandstone and Its Restriction to Mineralization in Liuju Copper Deposit in Chuxiong Basin,Yunnan

Based on Liuju copper deposit is one of the typical terrestrial sandstone-type deposits located in the Chuxiong red-bed basin of southwest China. The mineralization occurred in the interface between purple sandstone and grey sandstone of medium-fine grained sandstone strata, sandwiched between two purple silty mudstones. Besides, there coexists a horizontal metallic mineral zonation in this transitional zone from purple sandstone to grey sandstone:hematite, chalcocite, bornite, chalcopyrite and pyrite. Aiming at the relationship between mineralization and characteristics of cement and pore in sandstone, this paper has carried out the analysis of the statistical data of detrital contents, sorting coefficient, cement characteristics, pore types and the test results of porosity and permeability of each lithologic layer. The results shows that, the medium-fine grained feldspar quartz sandstone in the first Liuju sub-member of upper Cretaceous Matoushan group (K₁ml1) was the most effective channel for fluid migration because of the high detrital content, low sorting coefficient, high porosity and high permeability. The place where organic matters was rich in locally in the sandstone strata of K₁ml1 was changed into hydrocarbon source rocks during the middle diagenetic stage. The diagenetic pore water beared organic matters could evolve into acid reducing fluid by chemical reactions with wallrock. The acid reducing fluid and the alkaline oxidizing fluid from purple sandstone could form stable convection in the fluid migration channel of sandstone. Under the control of pH and Eh, cement zonation of ferruginous facies, calcite-dolomite carbonate facies, siliceous carbonate facies, siliceous facies formed in the sandstone strata from purple part to grey part. In the process of the water-rock interaction, the acid reducing fluid with hydrocarbons played a part in dissolving ferruginous and carbonate cements formed in the early diagenetic stage, which could open up inter grain pore for precipitation of copper sulfides. Besides, the process could also provide sulfur source by sulfate reduction. While, the alkaline oxidizing fluid was controlling the precipitation of the carbonate cements and copper sulfides in the interface of physicochemical conditions, which was the controlled conditions to the coexistence of cement zonation and metallic mineral zonation. When the hydrocarbon generation declined, the alkaline oxidizing traversed the neutral plane of Eh and pH value and conducted the corrosion of siliceous cements and quartz grains in grey sandstone, which could open up solution pore for more mineralization room and cause metasomatism between metallic sulfides. The water-rock interaction is the main factor of controlling the formation of pore for mineralization room and precipitation of cements and copper sulfides in diagenetic stages of sandstone strata.