Skarn mineralogy and revolution of reduced fluid in the Lamasu Cu-Zn deposit,West Tianshan,Xinjiang
Lamasu deposit,a relatively early discovered copper porphyry-skarn deposit in western Central Asian Metallogenic Domain,is located in the West Tianshan,Xinjiang.The melanite and Ti-rich grossularite were discovered by petrography and major-trace elements analysis on those skarn minerals in skarn and skarnoid marble.EPMA mapping and major-element quantitative analysis on andradite with zoning in skarn have showed the inhomogenetic core and coupled relation of Al with Fe in zoning.The oriental test from central core to marginal zone of andradite by in-situ LA-ICPMS has shown the subsequent downturn trend of Σ REE and the shaking trend of δEu.Melanite and Ti-rich grossularite are in core-mantle structure,and it shows the decreasing offO2 with the shift of fluid composition from super rich in Ti to rich in Ti.The in-situ LA-ICPMS trace elements test on Ti-rich grossularite has showed higherΣREE compared to andradite,in terms of 58.4 × 10-6～408.4 × 10-6 for Ti-rich grossularite and 28.2 × 10-6～108.7 × 10-6 for andradite.Ti-rich grossularite has strong positive Eu anomaly(δEu:11.3～15.0).Ti-rich grossularite and andradite have higher amount of LREE than HREE.The pyroxene in Lamasu Cu deposit is diopside.Fe is enriching and Mg is losing during the altered process of diopside,and the original diopside turns into baikalite-Fe baikalite after altered.With the deepening of alteration,the altered diopside turns into ferrohornblende,and ferritschermakite is precipitated in the retrogradation fluid directly.Original diopside together with altered diopside,ferrohornblende and ferritschermakite is in light-dark structure with varied lightness under SEM.The micro texture and major-trace elements of skarn minerals have showed that the fluid performing alteration is rich in Cl,REE and Ti,and it is acidic.In early alteration,the fluid is oxidized,and it turns reduced in late stage.The reduced material of CH4 and C2H6,and low fO2 of pyrrhotite-magnetite assemblage imply that Lamasu Cu-Zn deposit could be a reduced porphyry-skarn Cu deposit.
Reduced melaniteTi-rich grossulariteCore-mantle structureLamasu Cu deposit
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维普
