Genesis of the Dadongchang Pb-Zn deposit in the Tengchong area of the western Yunnan:Evidence from the REE Geochemistry
The Dadongchang Pd-Zn deposit in the Tengchong area is one of the representative skam type deposits in the eastern Tengchong Block of the"Sanjiang"metallogenic belt in Southwest China.The skam period(early and late skarn stages)and quartz-sulfide period(early and late sulfide stages)in its metallogenic process have been identified.Calcite is the main gangue mineral in this deposit,and its formation is closely related to the Pb-Zn mineralization.In this paper,REE characteristics of calcites in different stages of main mineralization period(quartz-sulfide period)have been compared with those of vesuvianites and epidotes of the early mineralization period(skam period),and those of the,Dawanshan unit granite,marble and limestone.The results show that calcites in two stages of the main mineralization period have right-declined REE distribution patterns,with ΣREE contents of 84.68×10-6～125.58×1 0-6 and 35.08×10-6～38.24×10-6,LREE/HREE ratios of 1.59～2.04 and 4.36～5.38,(La/Yb)N values of 1.12～1.81 and 4.01～5.72(La/Sm)N values of 2.62～3.16 and 5.09～6.38,and(Gd/Yb)N values of 0.64～0.97 and 1.22～1.30,respectively.Data of two-stage calcites are plotted in the interface fields of magmatic and hydrothermal origins in the Yb/La-Yb/Ca diagram.The Y/Ho ratios of vesuvianites,epidotes,calcites of the early sulfide stage,and marble samples are closely related to those of the Dawanshan unit granite samples,while the Y/Ho ratios of calcites of the late sulfide stage are varied between those of the Dawanshan unit granites and those of wallrocks.It is suggested that the calcite in two stages of the main mineralization period of the deposit is of magmatic-hydrothermal origin,and the ore-forming fluid is mainly derived from the magmatic-hydrothermal fluid,but the ore-forming fluid in the late sulfide stage of the main mineralization period could be strongly contaminated by the Permian limestone strata in the process of water-rock reaction.The 6Eu values are 1.25～1.75 and 0.54～0.78,and the 8Ce values are 0.85～0.94 and 0.94～1.05,respectively,for calcites of the early and the main ore-forming periods.The ore mineral precipitation could be mainly caused by the change of REDOX environments,with a small influence by the water-rock reaction in the late stage.The orebodies in the deposit are relatively close to the early Cretaceous granite porphyry.They occurred mainly in forms of stratiform and stratified bodies in the fractured altered carbonate rocks(skarns),with zonation of skam minerals from the deep to shallow parts.Based on the comprehensive analysis of regional geological characteristics and ore-controlling factors of the deposit,it is considered that the Daddongchang Pb-Zn deposit belongs to the skam Pb-Zn deposit.
rare earth elementscalcite originsource and evolution of the ore-forming fluidthe skarn Pb-Zn deposit
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