Rare-Earth Elements and Genesis of Lamprophyres in the Laowangzhai Gold Orefield, Yunnan Province

摘要

The Lowangzhai super-large gold orefield, which is situated in northern Mt. Ailao tectonic zone, Yunnan Province, is a typical gold orefield where lamprophyres are temporally and spatially related to gold mineralization. Major element data show that lamprophyres in the orefield are of alkalic series and can be divided into potassic and K-rich calc-alkaline lamprophyres. The rocks are enriched in rare-earth elements as compared with the primary mantle and mid-ocean ridge basalts(MORB). Modelled calculations by the least squares method of Petro-logical Mixing show that the mantle source for the lamprophyres is enriched in rare-earth elements. The geotectonic development of western Yunnan, Sr and Nd isotopic compositions, incompatible element patterns and linear programing calculations indicate that the fluids were derived from dehydration of submaine sediments which are enriched in ALK, LREE and incompatible elements and then were carried to mantle wedges as a result of plate subduction. That is the main factor leading to the formation of a metasomatic fertile mantle in the area studied.