德兴斑岩铜矿成矿热液来源及其演化——花岗闪长斑岩的氧同位素制约

THE ORIGIN AND EVOLUTION FOR THE ORE-FORMING FLUIDS OF TONGCHANG PORPHYRY COPPER DEPOSIT, DEXING: OXYGEN ISOTOPIC CONSTRAINS OF GRANODIORITES

作者研究了德兴铜厂花岗闪长斑岩的氧同位素组成特征在垂直方向上的变化规律，及蚀变作用对花岗闪长斑岩全岩和单矿物氧同位素组成的影响。结果表明，从地表到深部，花岗闪长斑岩全岩和长石单矿物的δ１８ Ｏ 值总体上有逐渐降低的趋势，反映花岗闪长斑岩受到已演化了的大气降水水岩氧同位素交换作用的影响。水岩交换作用对花岗闪长斑岩氧同位素组成的影响在浅部和深部是不同的，这主要受控于水岩交换温度和交换水的初始氧同位素组成等因素。石英的氧同位素组成及变化特征不同于全岩和长石，其值与岩石的蚀变作用有关。从花岗闪长斑岩的氧同位素组成及其变化规律可以推论，由大气降水演化为德兴斑岩铜矿成矿热液是可能的。

Tongchang porphyry copper deposit, together with the Zhushahong and Fujiawu porphyry deposits, makes up Dexin copper field the largest copper field in China. It is located in the northeast part of Jiangxi province, China.The orebodies are mainly found in the contact zone between the Proterozoic Jiulin group and granodiorite porphyry body. This paper focuses on the oxygen isotope composition and variation pattern of the granodiorites in vertical direction, and their relation with the alteration of granodiorite in Tongchang porphyry copper deposit, Dexin. The results show that the δ 18 O values of granodiorite whole rocks vary from 6.4‰ to 10.7‰, and the feldspar separates have the δ 18 O values of 6.5‰～8.9‰. There exits certain variation pattern for the oxygen isotope composition of the granodiorite whole rocks and feldspar separates, namely, the δ 18 O values of the whole rocks and feldspar separates decrease with the depth from the surface to the deeper levels.Through oxygen isotope exchange calculation between meteoric water and granodiorite using Taylor's model, it can be inferred that the meteoric water with a value of -8.5‰ circulated in the wall rocks of granodiorite firstly, and then continued to flow from deeper levels to the surface in mining area. Because of different interaction temperatures and δ 18 O values of the interacted water at the shallower and deeper positions, the granodiorite whole rocks have different variation patterns: the altered rocks of the shallower levels usually have higher δ 18 O values, while the lower δ 18 O values of the granodiorites are found in the deeper positions.The oxygen lsotope compositions and their variation patternz of quartz sampltes are different from those of whole rocks and feldspqr separates. Their δ 18 O values (8.4-9.8‰) are not related to depth, but related to the extent of alteration. The quartz samples of intensely altered granodiorites have the highest δ 18 O values. The increment of quartz duing alteration of feldspars is responsible for their oxygen isotope composition change. It can be inferred that the ore forming fluids of Tongchang porphyry copper deposits were derived from the evolved meteoric water.