云南老王寨金矿区煌斑岩的成因：稀土元素研究

A STUDY ON RARE EARTH ELEMENTS AND THE GENESIS OF LAMPROPHYRES IN LAOWANGZHAI GOLD DEPOSIT AREA, YUNNAN

云南老王寨金矿区煌斑岩与金矿化在时间上、空间上密切共生。主要元素分析表明，本区煌斑岩属钾质－富绅质钙碱性煌斑岩；与原始地幔和大洋中脊玄武岩相比，岩石相对富集稀土元素；模拟计算表明，岩石来源于稀土元素相对富集的交代富集地幔；地质发展史、不相容元素分配型式、Ｓｒ、Ｎｄ同位素组成及有关模拟计算表明，俯冲进入地幔楔的地壳物质脱水形成的流体是引起该区地幔交代作用的主要因素。

The Laowangzhai super-large type gold deposit of the northern Ailao shan structure zone, Yunnan Province, is a typical example where lamprophyres are temporally and spatially related to gold mineralization. The ages of lamprophyres are in the range of 22. 7-49. 0 Ma. They can be subdivided into minettes and kersantites according to mineral assemblage. Major elements show that rocks are alkalic series and potassic and potassiumrich cafe-alkaline lamprophyres. Chondrite-normalized transitional element patterns of lamprophyres are 'W'-shaped which suggests that the rocks be mantle-derived, MORB-normalized incompatible element patterns are 'camelhump'-shaped which are relatively enriched in large ion lithophile elements (LILE) and high field strength elements (HFSE), and have high87Sr/86Sr (0. 70694-0. 70769) but low143Nd/144Nd (0. 512488-0. 512493) ratios. All the characteristics mentioned above suggest that the source of lamprophyres in Laowangzhai gold deposits area be abnormal (rich ) mantle.The REE parameters of lamprophyres are ∑REE 130. 19 357. 75×10-6,LREE115. 80 342. 41×10-6, HREE 12. 64×16. 41×10-6, LREE/HREE 8.10-21. 92, and the chondrite-normalized REE patterns are LREE-rich types. According to the REE contents, all lamprophyres may be subdivided into two groups: the high-∑REE group with ∑REE 236.84-357. 75×10-6, LREE 220. 43-342. 41×10-6, HREE 15. 15-16. 41×10-6, LREE/HREE 13. 43 21. 93; and the low-∑REE group which have ∑REE 130. 19 177. 84×10-6,LREE115. 80 163. 81×10-6, HREE 12. 64 15. 01×10-6, and LREE/HREE 8.10 11. 96. Two groups of rocks are similar in the REE patterns.Modelling calculation of representative samples YD-20 (high-∑REE) and YLW 21 (low-∑REE) shows that two groups of lamprophyres in Laowangzhai gold deposit area are the products of different partial melting degrees (high-∑REE is 7. 5%±and low-∑REE is 13% ±) of a common REE-rich metasomatic mantle. The geological and structural development histroy of western Yunnan, Sr and Nd isotopic compositions, the patterns of incompatible elements and linear programing calculation indicate that the fluid formed by dehydration of submarine sediments which were rich in ALK, LREE and incompatible elements and carried to mantle wedge by subducting plate is the major factor giving rise to formation of metasomatic-rich mantle in the area.