Metallogenic mechanism of the rare-element pegmatite dykes in Guanpo area from the East Qinling Orogen
The East Qinling pegmatite district is a famous rare-metal production area,and it is the largest pegmatite district hosting abundant rare element(REL)in the Qinling orogenic belt.The Guanpo area,as the most representative and typical pegmatite high-density area in East Qinling,is featured by a large number of pegmatite dykes,high proportion of REL-mineralized pegmatites,multi types of REL mineralization involved in Li,Be,Nb,Ta and Cs,large lithium-mineralized pegmatite dykes,and various REL minerals.From northwest to southeast,this area developed the Huoyangou Sn-Nb-Ta deposit,Nanyangshan,Qiligou-Qiantai and Caijiagou lithium deposits.The internal structures include weakly zoned pegmatites which are homogeneous and layered,and complexly zoned pegmatites.The REL pegmatite dykes belong to spodumene subtype in complex type dominantly,accompanied with lepidolite subtype in complex type and albite-spodumene type.The REL pegmatite magmas are peraluminous with high contents of Si and alkalis,and they are enriched in large ion lithophile elements(Rb,K)and high field strength elements(Nb,Ta,Hf),and poor in Ba and Ti with low Nb/Ta ratios and strong REE tetrad effects.Thus,the REL pegmatite magmas are highly fractionated and evolved.The mineralogical studies show that the Nanyangshan,Qiligou-Qiantai and Caijiagou REL pegmatites have high degrees of evolution compared with tin-bearing pegmatites in Huoyangou deposit.There is a Late Ordovician-Early Silurian pegmatite magma activity in Guanpo area and the main ore forming times are Late Silurian-Early Devonian(424～408Ma)and Early-Middle Devonian(399～384Ma).The Guanpo REL pegmatite dykes formed in collision,weakening convergence with crust uplift and post-orogenic extension settings,and mainly concentrate in late orogeny.The tourmaline B isotopes and zircon Hf isotopes indicate that the Guanpo REL pegmatite dykes are derived from Proterozoic crustal materials,mainly metasedimentary rocks.There is no genetic correlation between REL pegmatites and granites exposed in this region,and the REL pegmatite dykes might be products of partial melting of the North Qinling unit.The weakly-zoned Sn-Nb-Ta-mineralized pegmatites in Huoyangou deposit did not experience obvious fractional crystallization after emplacement.The magmas of weakly zoned Li-mineralized pegmatites are enriched in lithium and achieved lithium-saturated melt without obvious fractional crystallization after emplacement and the lithium-bearing minerals continued to deposit in fluid stage.The systems of complexly zoned Li-or Li-Be-Nb-Ta-Cs-mineralized pegmatites began to deposit REL-bearing minerals when REL-saturated melt formed by fractional crystallization after emplacement.With continuous evolution of melt and fluxes therein,REL-rich and fluxed melt formed and different assemblages of REL-bearing minerals occurred.The activities of alkali fluids with or without atmospheric water further complete REL mineralization and redistribution.The formation processes for REL pegmatite magma which involved in the first REL enrichment,and magma-fluid evolution processes which further complete REL enrichment,contribute to formation of the REL pegmatites in Guanpo area.The previous studies on mineralogical,geochemical,wall-rock alteration and regional structures supply the possible REL mineralization indicators for REL prospecting in Guanpo area.
REL pegmatiteMagma-fluid evolution processPegmatite magma originMetallogenic mechanismEast Qinling
万方数据
维普
