云南个旧锡铜多金属矿区云母^(40)Ar/^(39)Ar年龄及其地质意义

^(40)Ar/^(39)Ar geochronologic study of mica in the Gejiu tin-copper polymetallic ore district, Yunnan province, and its geological significance

云南个旧是全球最大的锡铜多金属矿区,主要成矿作用是与燕山期花岗岩密切相关的岩浆-热液体系。矿区内铜矿的主要矿床类型为变玄武岩型层状铜矿和接触带型铜矿。赋存于花岗岩体的凹陷部位,接触带型铜矿体和氧化型矿体的精确年龄尚未有报道。以老厂矿田内与铜矿体同期的等粒花岗岩脉中的黑云母和与氧化矿同时形成的白云母作为研究对象,利用常规^(40)Ar/^(39)Ar同位素定年方法,获得了黑云母和白云母的^(40)Ar/^(39)Ar坪年龄分别为82.47±0.49Ma和76.17±0.42Ma,相应的正等时线年龄为82.38±0.48Ma和76.07±0.66Ma,反等时线年龄为82.38±0.49Ma和76.07±0.73Ma。结合野外地质接触关系和矿区内其他年代学结果认为,黑云母的^(40)Ar/^(39)Ar年龄82.38±0.48Ma可以代表接触带型铜矿体的形成年龄,也揭示了新山花岗岩体形成后的快速冷却作用过程;白云母的^(40)Ar/^(39)Ar年龄76.07±0.73Ma指示了氧化型矿体的形成年龄,也记录了矿区内与甲介山同期的南北向断裂的晚期活动时限。该年龄与个旧锡铜多金属矿床的成矿时代一致。

Gejiu is one of the largest polymetallic tin ore concentration areas in the world and is a granite-related magmatic-hydrothermal system. There are two main kinds of orebodies in the south area of the Gejiu deposit. One is stratoid orebody, and the other is contact orebody. The contact orebody and oxidized orebody, which closely occur in the depression of granite body, have not been given accurate ages. This paper presents new （40）Ar/ （39）Ar biotite age for the contact copper orebody and muscovite age for the oxidized orebody from the contact zone of Xinshan granite intrusion in the Gejiu tin-copper deposit so as to constrain the timing and dura-tion of mineralization after the formation of Xinshan granite body. The biotite and muscovite yielded （40）Ar/ （39）Ar plateau ages of 82.47±0.49 Ma and 76.17±0.42 Ma, normal isochronal ages of 82.38±0.48 Ma and 76.07±0.66 Ma, and inverse isochronal ages of 82.38±0.49 Ma and 76.07±0.73 Ma, respectively. Combined with field geological contact relationships and the other geological dating results,the authors have reached the conclusions that the （40）Ar/ （39）Ar age of biotite is closely related to ore deposition, and constrains the formation of the contact copper orebody at 82.38±0.48 Ma, and also records the rapid cooling process after the Xinshan granite formation.The （40）Ar/ （39）Ar age of 76.07±0.73 Ma in muscovite indicates the age of the formation of oxidized orebodies, and also records the late stage of movements of the N-S trending faults in the east segment of the Gejiu deposit. These dating results are coincident with the data obtained from the Gejiu polymetallic tin-copper deposit, and hence the tin-copper deposits belong to the same structure-magma-fluid system.