镇雄县羊场磷矿梅树村组含磷岩系地球化学特征与沉积环境OA

The Yangchang phosphate deposit in Zhenxiong County,Yunnan Province,located on the southwestern margin of the Yangtze Block,is the largest concealed high-grade phosphate deposit in Asia.However,the evolution of paleo-seawater chemical conditions during its mineralization period and the synergistic mechanism between phosphorus enrichment and sedimentary environment remain unclear,which restricts the understanding of the source of massive phosphorus in the deposit.Taking borehole ZK0016 of this deposit as the research object,this work conducted petrographic observations under an optical microscope,X-ray fluorescence spectrometry(XRF)analysis of whole-rock major elements,and inductively coupled plasma mass spectrometry(ICP-MS)analysis of trace elements.Combined with characteristic geochemical indicators such as Sr/Cu,Sr/Ba,V/Cr,δCe,Ceanom and U/Th,supplemented by Ba-Sr,SiO2-Al2O3 and lgw(U)-lgw(Th)analysis,this study systematically discriminated the paleoclimate,paleosalinity and redox conditions during the formation of the phosphorus-bearing rocks,and discussed the influence of hydrothermal activity on the mineralization process.The results show that the major elements of the phosphorus-bearing rock series of the Meishucun Formation in the Yangchang phosphate deposit are dominated by P2O5,CaO and SiO2.The trace element composition is characterized by significant enrichment of marine-derived elements such as Ba,Sr and U,and relative depletion of terrigenous elements(e.g.,Cs,Rb,Th)and mantle-derived elements(e.g.,Co,Cr,Ni),indicating that the sediment source is mainly derived from seawater.The REEs generally exhibit relative enrichment of LREEs and depletion of HREEs,with LREE/HREE ratios ranging from 2.55 to 6.24(average 4.49).The PAAS-normalized REE distribution pattern is relatively flat with a slight left-leaning trend,and the total rare earth elements(ΣREE)show a good positive correlation with P2O5.The REEs are mainly enriched by early adsorption,which well preserves the geochemical characteristics of the original sediment.Paleoenvironmental index analysis shows that the Sr/Cu ratios of the phosphorus-bearing rock series range from 21.1 to 224(average 104,all greater than 10).This,combined with the high Sr content(188×10-6～1880×10-6,average 1023×10-6),indicates that the rock series was formed under arid and hot paleoclimate conditions.The magnesium-aluminum ratio(m=100×MgO/Al2O3)ranges from 47.6 to 4799(average 1159),and the Sr/Ba ratio ranges from 0.722 to 4.32(average 2.60,all greater than 0.5).These results,together with the Ba-Sr diagram,confirm that the sedimentary water was in a saline epicontinental sea environment.The V/Cr ratio ranges from 0.486 to 3.75(average 1.55,mostly less than 2.0),the Y/Ho ratio ranges from 54.0 to 83.9(average 67.1),the δCe value ranges from 0.178 to 0.853(average 0.355),and the Ceanom value ranges from-0.807 to-0.121(average-0.562,all less than-0.1).Multiple redox indicators consistently indicate that the phosphorus-bearing rock series was formed in an oxidizing sedimentary environment.Both the SiO2-Al2O3 diagram and the lg w(U)-lg w(Th)diagram show that the samples of the phosphorus-bearing rocks mainly fall into the hydrothermal sedimentary field.This,combined with the U/Th ratios ranging from 1.44 to 102(average 30.5,all greater than 1),confirms that significant hydrothermal activity was involved in the mineralization process.In summary,this paper clarifies that the main ore-hosting horizon of the Meishucun Formation in the Yangchang phosphate deposit was formed in an epicontinental sea environment with an arid and hot climate and saline oxidizing water,and that hydrothermal activity is a key controlling factor for phosphorus enrichment and mineralization.The vertical evolution sequence of the sedimentary environment from the Meishucun Formation to the Niutitang Formation is reconstructed.From bottom to top,it gradually evolved from the weakly oxidizing to weakly reducing environment of the Xiaowaitoushan member,to the strongly oxidizing environment of the main metallogenic period of the Zhongyicun member,and finally transitioned to the weakly oxidizing to weakly reducing environment at the bottom of the Niutitang Formation.Both the aridity of the paleoclimate and the salinity of the paleo-water show a synergistic evolution trend of first increasing and then decreasing.The research results not only deepen the understanding of the metallogenic mechanism of Early Cambrian phosphate deposits on the Upper Yangtze Platform and enrich the regional phosphate metallogenic theory,but also provide solid theoretical support regarding the metallogenic environment for deep exploration,resource potential evaluation and subsequent development planning of the Yangchang phosphate deposit.