ABSTRACT

This paper analyzes the lamprophyres from the Laowangzhai gold deposits in Yunnan Province, China, which have close temporal-spatial relations with gold mineralization, for their major elements, trace elements and rare earth elements (REE), as well as their Sr, Nd isotopic compositions and the contents of fixed ammonia (NH₄⁺). The analytical results of major elements indicate that the lamprophyres in this region are potassic, calc-alkaline lamprophyres. Their M [Mg/(Mg + Fe²⁺)] and the contents of the transition elements reflect that the rocks possess the characteristics of primary magmas. As compared with the mid-ocean ridge basalts (MORB), the lamprophyres in this region are enriched in large ion lithophile elements (LILE) and high field strength elements (HFSE). Their REE distribution patterns are of the LREE-enrichment type and their trace element distribution patterns are of the strong incompatible element enrichment type, indicating that the rocks originated from a fertile mantle. As compared with the modern values of the primitive mantle, the lamprophyres in this region have relatively high ⁸⁷Sr/⁸⁶Sr (0.70644∼0.70895) and relatively low ¹⁴³Nd/¹⁴⁴Nd (0.512436∼0.512524). The contents of NH₄⁺ in the rocks (74.34 ppm∼468.7 ppm) are obviously higher than those of other types of mantle-source rocks (1 ppm∼27 ppm), but lower than those of the carbonaceous country rocks (799.6 ppm∼1742 ppm) in the ore field. Various lines of evidence demonstrate that the lamprophyres which possess the above Sr, Nd isotopic signatures and the contents NH₄⁺ of could not be derived from the magma with MORB characteristics during its ascending, or derived from the magma chamber, which had both been contaminated by crustal materials with high ⁸⁷Sr/⁸⁶Sr, low ¹⁴³Nd/¹⁴⁴Nd and high contents of NH₄⁺, instead, the rocks are the results of partial melting of the fertile mantle. It is the main mechanism of formation of the fertile mantle in this region that metasomatism of the mantle mixed with crustal materials by fluids resultant from dewatering of crustal materials brought into the mantle during subduction and LREE-, LILE- and HFSE-rich fluids from the deep mantle (including the astrosphere).