湖北鸡笼山岩体主要矿物的成因矿物学意义

GENETIC MINERALOGY SIGNIFICANCE OF MAJOR MINERALS OF JILONGSHAN PLUTON,HUBEI PROVINCE

鸡笼山矽卡岩型铜金矿位于我国长江中下游铁铜成矿带西端,其成矿作用与矿区出露的鸡笼山花岗闪长斑岩关系密切。该岩体主要矿物有斜长石、钾长石、黑云母、角闪石和石英。矿物化学分析结果显示:岩体中的斜长石主要为更长石-中长石;黑云母为镁质黑云母,富Mg2+,Al 3+,K+,贫Ca2+,MF值变化在0.47～0.64之间;角闪石贫Ti 2+(0.35%～1.78%),富Mg2+(5.3%～9.35%)。矿物温压计计算结果显示,岩体固结温度约在720℃～790℃之间,压力约在4×108 Pa～5×108 Pa之间,深度相当于13km～15km形成度,氧逸度(LogfO2)变化范围是-10.5～-14。黑云母及角闪石化学成分显示岩体为I型花岗岩,形成于造山带背景环境下,岩浆具壳幔混合特点。该区在燕山期,上地幔岩石圈隆起,碱性玄武质岩浆先后沿不同方位基底断裂上升侵位,发生结晶分异作用并与地壳物质发生同化混染联合作用,形成闪长质岩浆。在有利的断裂构造条件下,岩浆以较快的速度上升,与地层围岩发生接触交代作用,同时形成矽卡岩矿床。

Jilongshan skarn-host Cu-Au deposit located in west of Middle-Lower Yangtze River Fe-Cu metallogenie belt,its mineralization is closely related to exposed granodiorite porphyry. The main minerals of pluton are plagioclase, potassium feldspar, biotite,amphibole and quartz. Mineral chemical analysis results showed that plagioclase in rock is oligoelase-andesine, biotite is magne-sian biotite and rich in Mg^2＋ ,Al^2＋ ,K^2＋,depleted of Ca^2＋ ,MF changes from 0.47 to 0.64,amphi-bole is short of Ti^2＋ （0.35%-1.78%） and rich in Mg^2＋ （5.3%-9.35%）. Mineral geotempera-ture and geobarometer showed that pluton consolidation temperature is 720 ℃ - 790 ℃ ,pressure is 4×10^8 Pa-5 3×10^8 Pa,which is equivalent to a depth of 13 km-15 km,and oxygen fugacity （Log fo2 ） rangs from -10.5 to -14. Chemical composition of biotite and amphibole suggest the pluton is Ⅰ type granite formed in orogenic zone environment and the mixture of crust and mantle. During the Yanshannian,upper-mantle upwelled and alkaline basaltic magma emplaeed along the different directions basement faults in this district. Then magma experieneing fraetional crystalli-zation and mixing with the basement crust units resulted in the formation of dioritic magma. The dioritie magma migrated along the favourable structures and metasomatized with wall rock, and formed ore deposits simultaneously.