云南鲁甸地区二叠纪玄武岩中铜矿床的碳氧同位素对成矿过程的指示

Carbon and Oxygen Isotope Components in the Permian Basalt-hosted Copper Deposit in Ludian Area, Yuanan: Implication for the Mineralization Process

过去已知在峨眉山玄武岩顶部气孔状熔岩中含有杏仁状自然铜,但很少构成连续的铜矿体。虽然先后探明了几十个小型铜矿或矿化点,但始终未取得找矿突破。最近,在该熔岩层上下的凝灰岩中鉴别出难识别矿化类型,即与沥青和有机质碳紧密伴生的富铜矿石,甚至在气孔状熔岩中发现呈杏仁体含铜沥青,因而可能构成大规模矿化。在矿体中还见到葡萄石—绿泥石—方解石杏仁体和细网脉,与沥青团块抑或共生抑或稍晚于后者。通过对沥青和方解石的碳和碳氧同位素的测定,前者的δ^(13)C_(PDE)值变化在-32.6‰～-30.9‰之间,后者的δ^(18)O_(SMOW)和δ^(13)C_(PDB)值分别为19.0‰～23.0‰和-18.4‰～-13.5‰。初步认为这些沥青为火山喷发后异地石油贯入及挥发的结果。与矿石密切相伴的葡萄石—绿泥石一方解石为矿化过程的蚀变产物。表明含铜流体流经沥青和有机碳富集这些地球化学障时卸载成矿。

Although the amygdaloidal native copper aggregate was known in vesiculate lava at the top of the Emeishan basalts, economic copper ore deposits had not been discovered until two years ago. A unusual mineralizing types, native copper aggregate associated closely with asphalt and organic carbon, were discovered in tuff and the vesiculate lava at the top and bottom of the. Permian Emeishan large basalt layer in the Ludian area, northeastern Yunnan province. We can see copper-bearing asphalt amygdaloidales in the basalt lava and stockwork of native copper in the carbon-rich tuff. There area so much carbon in the ores that people only can see the native copper aggregate after splitting and polishing the ores. Amygdaloidal calcite aggregate and fine stockwork of calcite-prenite-chlorite are superimposed on the asphalt and carbon-rich ores. We analyzed carbon isotopes of the asphalts and carbon and oxygen isotopes of the calcite and obtained δ13CPDB values of the former ranging from - 32. 6‰ to - 30. 9‰, and δ13CPDB and δ18OSMOW values of the latter ranging from 19. 0‰ to 23. 0 ‰ and from -18. 4 ‰ to -13. 5 ‰,respectively. Therefore, we suggest that the asphalts resulted from the injection and then volatilization of heterochthonous petroleum after volcanic eruption. And the prehnite-chlorite-calcite mineral assemblage is the consequence of alteration during the mineralization process, and copper-bearing fluids unloaded and mineralized by flowing through the geochemical barrier composed of enriched asphalt and organic carbon.