江西九瑞坳下地区矽卡岩的矿物化学、氧同位素特征及成矿潜力探讨

Mineral chemistry,oxygen isotopic compositions and mineralization potential of skarns in the Aoxia area,Jiurui ore district,Jiangxi Province

江西九瑞矿集区是长江中下游成矿带上重要的铜金资源产地,近年来地勘单位在该区西南部的坳下远景区发现物化探异常,并布置钻探工作,在奥陶系白云岩与隐伏黑云母花岗斑岩侵入体的接触带上,揭露了矽卡岩和矿化。本文以这些矽卡岩为研究对象,运用电子探针及同位素质谱仪测定矿物的化学成分和氧同位素组成,在此基础上探讨了热液的物理化学条件,流体来源及演化过程。结果表明,形成坳下矽卡岩的热液流体,进蚀变阶段温度为516~663℃,平均597℃,氧同位素值(δ^(18)O_(Fluid))为9.3‰,偏还原性;退蚀变阶段温度为263~317℃,平均290℃,氧同位素值(δ^(18)O_(Fluid))为-4.9‰,偏氧化性。与九瑞的武山、东雷湾、邓家山等矽卡岩矿床相比,坳下镁矽卡岩组分更高,发现了以往九瑞地区的矽卡岩矿床中未见的橄榄石、金云母。经综合分析,认为高盐度的、较热的、氧化性弱的岩浆热液,与稀释的、较冷的、氧化性强的大气降水的流体混合过程,是坳下矽卡岩及其矿化的形成机制。这一过程,对形成有价值的矽卡岩型矿床非常有利,在九瑞矿集区后续的勘查工作中,应关注侵入体与奥陶系白云岩接触带形成的镁矽卡岩相关矿床,以及志留系和奥陶系地层硅钙界面处受到岩浆热液叠加形成的层状矿体。

Jiurui ore district is an important Cu-Au resource producing area in the Middle-Lower Yangtze River Metallogenic Belt. In recent years, some geophysical and geochemical exploration and drilling were carried out in the Aoxia area, southwestern Jiurui district. Skarns and related mineralization were found at the contact zone between intrusion and dolomite strata. In this study, we report mineral chemistry and oxygen isotopic compositions of the skarns, in an attempt to unravel the physico-chemical conditions, source and evolution process of hydrothermal fluid, and to discuss the mineralization potential in the Aoxia area. Prograde alteration stage shows formation temperatures of 516~663℃ （average 597℃） with fluid oxygen isotopic value （δ18OFluid） of 9.3‰ and more reducing condition. Whereas, the retrograde alteration stage shows lower temperatures of 263~317℃ （average 290℃） and lower δ18OFluid value of -4.9‰, but more oxidizing condition. Comparing to other skarn deposits in the Jiurui district, such as the Wushan, Dongleiwan and Dengjiashan, the Mg-skarn content is higher in Aoxia. Fluid mixing of magmatic hydrothermal and meteoric fluid is an important controlling factor for the formation of the Aoxia skarns and related mineralization. These data suggest a high potential for mineralization in the Aoxia area, and the contact zone between the granitoid intrusion and dolomite strata, and the boundary of sandstone and dolomite strata are good targets and should be paid more attention in future exploration in the district.