Difference study of greisen formation between open and closed magmatic systems:Example from four representative W-and Li-bearing deposits in the Nanling metallogenic belt
Greisen is a prevalent rock type that formed during the granitic magmatism and mineralization and is a primary ore body for non-ferrous and rare metals.At present,there are three types of understanding to its generation:Metasomatism between the granitic melt and external fluid,interaction between the granitic melt and magmatic hydrotherm,and a special magmatic rock with very high Li-F contents.To further explain the formation mechanism of greisen,four Late Jurassic magmatic-hydrothermal deposits in the Nanling metallogenic belt were selected in this work.They are Taoxikeng quartz vein-type W deposit,Shizhuyuan skarn-greisen type W-Sn-Mo-Bi-F deposit,Maoping quartz vein-greisen type W-Sn-Li deposit,and Xianghuapu granite-type W-Ta-Nb-Li deposit.By studying the different categories of mica from the granite,greisenized granite,and greisen through the EMPA and LA-ICP-MS analyses,we found that:(1)The mica in greisen from the Taoxikeng and Shizhuyuan deposits are phengite;while the mica in greisen from the Maoping and Xianghuapu deposits are zinnwaldite;(2)From the lowly fractionated granite→ the highly fractionated granite→greisen,the SiO2,Al2O3 compositions in mica from the Taoxikeng and Shizhuyuan deposits take a turn rather than always increasing or decreasing trend,but these contents in mica from the Maoping and Xianghuapu deposits show a consistently changing tendency,it shows that they have certain differences in magma evolution.We considered that the reasons for above changes is due to:(1)the greisens in the Taoxikeng and Shizhuyuan deposits were formed during the interaction between the granitic melt and the external fluid in the open system of magma reservoir;(2)the greisens in the Maoping and Xianghuapu deposits were generated during the stage of melt-fluid coexistence,and the fluids came from the magmatic exsolution and minor addition of the meteoric water.Therefore,we suggested that:(1)the W-and Li-element enrichment all required the highly fractionated magmas which experienced a certain degree of crystal fractionation;and(2)whether the magmatic system is open or not is an important condition for the formation of W-or Li-bearing mineralization.The formation of tungsten orebodies needs the addition of external fluid,while the formation of lithium orebodies is the result of the rare-metal extreme enrichment through magmatic differentiation.
GreisenTrace elements in micaSkarn-type W depositQuartz vein-type W-Li depositGranite-type Li depositLate JurassicThe Nanling metallogenic belt
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