Research On Metamorphism And Mineralization Geochemistry Of The Hongnipo Ore Block Of Lala Copper Deposit At Huili,Sichuan Province

The Hongnipo ore block of the Lala copper deposit was located at the middle part of the Xikang-Yunnan axis of the Yangtze platform,where was a junction of the Jinshajiang fault fold belt and the structural zone of the Sichuan-Yunnan-Panzhihua rift valley.Geological structure was complex in the ore block and metamorphic rocks were both wall rocks and ore-boring rocks.Then,it was controversial whether the metamorphism and mineralization were identical processes.On the basis of systematic collecting and summarizing previous work achievements,this thesis focuses on metallogenic and geochemical characteristics of the metamorphic sedimentary rocks in the Hongnipo ore block,by means of petrology,elements geochemistry and isotope geochemistry.The aim of the thesis is to deepen understanding of genesis of the deposit and to provide references for peripheral and depth prospecting.1)In the Hongnipo ore block,the ore-controlling structure is the Hongnipo compound syncline and ore bodies occur in the Pre-Sinian Tianshengba Formation of the Hekou Group(Pt₁t).The ore formation is a set of metamorphic volcanic sedimentary rocks and ore bodies occur in layers and lenses.Ore minerals are mainly chalcopyrite,pyrite and bornite,whereas main gangue minerals are quartz,albite,muscovite,biotite and calcite.Wall-rock alterations are dominated by carbonation,sodium alteration,sericitization and so on.Metamorphic rocks are mainly muscovite-albite schist,biotite-albite schist and mica-albite schist.There are obvious characteristics of hydrothermal alteration and multi-stage mineralization under microscope.2)The protoliths of the metamorphic wall-rocks in the Hongnipo ore block are identified as normal clastic sedimentary rocks such as argillaceous calcareous siltstone and iron shales,some other volcanic sedimentary rocks.3)In the standardized diagrams of post-Archean Australian shale(PAAS),SiO₂,Al₂O₃ and Mg O curves of the non-mineralized metamorphic wall-rocks are close to PAAS values,Na₂O and K₂O values show significant losses,and CaO,MnO,P₂O₅and TiO₂ values wave strongly.In contrast,the mineralized metamorphic rocks are rich in Fe₂O₃,MnO and CaO,with other oxides depleted to varying degrees.4)In the standardized diagrams of trace element chondrites,Sr curves of the non-mineralized metamorphic wall-rocks are close to chondrite values,P values are depleted obviously,with other elements enriched in varying degrees.By comparison,Rb and K curves of the mineralized sedimentary rocks fluctuate between loss and enrichment around chondrite values,high-field strong element contentsare relatively stable,Ti curves are close to chondrite values,but P values are depleted strongly,and other elements are enriched in different degrees.5)In the standard diagrams of rare earth element(REE)chondrites,the total amount of REE of the non-mineralized metamorphic wall-rocks change greatly.The average value of(La/Yb)_N is 21.65,showing an apparent right-leaning type,?Eu shows an obviously positive anomaly.In contrast,the mineralized metamorphic rocks are more enriched in REE,with(La/Yb)_N is 25.92,showing an obvious right-leaning type,?Eu showing an obvious negative anomaly.The results show that the non-mineralized metamorphic wall-rocks mainly undergo regional metamorphism with the sodium-rich deep-source high-temperature fluids,while the mineralized metamorphic rocks mainly undergo the mineralization of ore-bearing crust-mantle fluids,and the process of action is relatively low temperature and slow with the characteristics of component differentiation.6)According to the structure discriminant diagram of K₂O/Na₂O-SiO₂,Th-Sc-Zr/10 and Th/Sc-?_Nd(t),it is indicated that the protoliths of the metamorphic rocks were formed in a back-arc sedimentary basin environment under a continental island arc system.Through analysis and comparison of a series of parameters such as chemical alteration index(CIA),composition change index(ICV),K₂O/Al₂O₃,Al₂O₃/TiO₂,Cr/Zr,LREE/HREE,and Th/Sc,it is concluded that the source area of metamorphic rocks in the Hongnipo ore block wasin an unstable tectonic environment with low material maturity,very few clay-bearing minerals and medium chemical weathering.Theprotoliths are mainly composed of upper crustal felsic,with a small amount of alkaline feldspar,ferromagnesian constituents and possible ancientcrustal materials.7)By comparing and analyzing element geochemical characteristics of the non-mineralized and mineralized metamorphic rocks in the Hongnipo ore block,combined with discussion of metamorphic diagenesis and metallogenic ages and S isotope geochemistry,it is shown that the relation between metamorphic diagenesis and metallogenesis is not a cause-result process.The formerage is about1747?1656Ma.After formation of the original volcanic-sedimentary rocks,theywere transformed into regional metamorphic rocks,and volcanic rocks were transformed into albite rocks or metamorphic spilite-keratophyre rocks under combined action of regional metamorphism and sodium-rich high-temperaturefluid.However,the metallogenic age is divided into two stages,1200?1000Ma and 1000?800Ma,respectively.In the metallogenic epoch,ore-bearing Crust-mantle hydrothermal fluid carrying ore minerals itself and extracting ore elements from source bed rise along fault channelsand act on metamorphic rocks to form superimposed ore deposits,which are mainly composed of disseminated and vein-like ores.