兰坪—思茅盆地砂页岩中铜矿床同位素地球化学

ISOTOPIC GEOCHEMISTRY OF COPPER DEPOSITS IN SANDSTONE AND SHALE OF LANPING-SIMAO BASIN, WESTERN YUNNAN

兰坪—思茅中新生代盆地中的铜矿床 ,主要产于由砂岩、粉砂岩和页岩组成的含盐红色碎屑岩建造中。对矿床的同位素组成研究表明 :成矿溶液主要来自大气降水 ,矿化和蚀变作用是在水 /岩比值较低的体系中进行的 ;铅来自赋矿的沉积岩与基底岩石的混合 ;硫、碳和硅则具有多来源的特性。矿床地质特征和同位素组成特点表明 ,所研究的铜矿床与典型砂页岩型铜矿床存在显著差异 ,而与一些地热区的矿化作用相似。说明研究区砂页岩中的铜矿床具有特殊的成矿机理 ,即在经历了沉积成岩作用发生矿质初步聚集后又通过陆相喷流作用而形成工业矿床的。

The copper deposits occurring in Lanping Simao basin of western Yunnan are a new type of copper deposits with many unique features. The cooper deposits exist in Mesozoic and Cenozoic clastic formation composed of sandstone, siltite and argillite. The copper orebodies mostly occur in the host rocks not only in veinlike and network forms but also in bedded quasibedded and lens like forms. Ores have very simple mineral assemblages. Over 20 mineral species have been identified. Besides ordinary sulfides of such copper minerals as chalcopyrite, tetrahedrite, bornite and chalcocite, there are also other minerals like pyrite, galena, sphalerite, azurite, stibnite, quartz, calcite, ferroan dolomite and barite. Ore fabrics are characterized mainly by veinlike, network, brecciated forms and subordinately by bedded, laminated, disseminated forms. Wallrock alterations are mostly silication, calcitization and baritization. δ 34 S values in the copper deposits are -35 6‰～+7 0‰ in sulfides and +7 7‰ ～+16 8‰ in sulfates . δ 13 C values are -22 4‰～-23 7‰ for organic carbon, -7 4‰～+0 8‰ for calcite and ferroan dolomite, -10 8‰～-3 1‰ for CO 2 of inclusion fluids in quartz and -32 1‰～22 2‰ for CH 4 of inclusions in quartz. δ 30 Si values are -0 3‰～0 0‰ in quartz and -0 3‰～+0 1‰ in rocks. Lead isotopic ratios of hydrothermal mine\|rals including sulfides and quartz are 206 Pb/ 204 Pb=18 120～18 962, 207 Pb/ 204 Pb=15 556～15 793, and 208 Pb/ 204 Pb=38 552～39 298, while those of rocks are 206 Pb/ 240 Pb=18 524～19 268, 207 Pb/ 204 Pb=15 587～15 634, and 207 Pb/ 204 Pb=38 431～38 914 Hydrothermal fluids have δ 18 O values of -10 4‰～+8 4‰, which were obtained on the basis of the isotope fractionation equation of quartz water, barite water and calcite water and the δ D values of -137‰～-30‰. The above described isotopic studies show that the ore forming solution was mainly derived from the paleometeoric water. The amount of the hydrothermal fluids in mineralizing and altering systems was relatively small. Lead in the ores came mainly from country rocks and underlying rocks and partly from deeper sources. Sulfur, silicon and carbon were derived from many sources such as the red clastic formation, volcanoclastic rocks and basic rocks. The recent tendencious idea about the genesis of the deposits is that the deposits underwent two ore forming periods, the sedimentary period and the hydrothermal mineralization period. The former was a stage of preliminary enrichment of ore materials and the formation of the source bed, whereas the latter was a stage of the formation of orebodies. Formed in the Mesozoic and Cenozoic clastic rock formation composed of sandstone, siltite and shale, the copper deposits are comparable with typical snadstone and/or shale type copper deposits in other parts of the world.However, the authors have found through the systematic study of the deposits that there are obviously differences between the typical sandstone and/or shale type copper deposits and the studied type. Many features of the copper deposits are similar to those of hydrothermal mineralization areas. It is shown that the copper deposits of the study area have unique metallogenic mechanism.