ISOTOPE GEOCHEMISTRY STUDY OF DONGCHUAN COPPER DEPOSITS IN MIDDLE YUNNAN PROVINCE, SW CHINA: II. DATING THE AGES OF MINERALIZATIONS BY Pb-Pb AND 40 Ar- 39 Ar METHODS

Qiu Huaning Sun Dazhong Zhu Bingquan Chang Xiangyang (Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640)  

It is hard to determine the ore-forming ages of Dongchuan copper deposits because of the complexity of mineralizations in the deposits and the difficulty to select suitable minerals for the conventional isotopic dating methods. In this study, we dated the copper ores from the Luoxue Formation of the Kunyang Group in Tangdan, Dongchuan, using the Pb-Pb isochron method, and determined the fluid inclusions in a quartz sample (DC-51-1Q) from a copper-quartz vein from the same formation by the 40 Ar- 39 Ar crushing in vacuo technique. The Pb-Pb isochron age of the copper ores,(794±73)Ma,may indicate that the copper deposit the Luoxue Formation was formed or reworked in the Jinning-Chengjinag Period.Just at the quartz from the Lushui tin-tungstem deposit (H N Qiu, 1996 ), the quartz samole DC-51-1Q by 40 Ar- 39 Ar crushing in vacuo analysis forms a smoothly “falling-down ” age soectrum fron as old as 1994 Ma at the first stage to as young as ～ 710 Ma at the last three stages.Tge date yield an isochron line on the 40 Ar/ 36 Ar- 39 Ar/ 36 Ar diagram with the isochron age of (712±33) Ma and the initial 40 Ar- 39 Ar ratio of 935±62. The falling-down age spectrum and the high initial 40 Ar-/ 36 Ar ratio suggest that the fluid inclusions contain high concentration of excess 40 Ar within the fluid inclusions , especially within the larger ones.The osochron age of 712 Ma imllies that the Jinning-Chengjiang orogeny was accompanied with the fluid circulation which formed the copper-quarta vein. the fact that the quartz was successfully determined with the 40 Ar/ 36 Ar technique by crushing in vacuo indicates that the close K-Ar system was kept well enough within the fluid inclusions.Although the quartz experienced a geologic history for as long as 700 Ma,it ws still suitable for 40 Ar- 39 Ar method to determine the ore-forming age of Dongchuan copper deposits.

【Fund】： 国家科委攀登计划超大型矿床A项和B项 国家自然科学基金
【CateGory Index】： P597

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