The Petrogenesis And Its Implications For Mineralization Of Composite Sill In Lala Ore District,Western Sichuan,China

The formation and relationship between mineralization of the layered intrusion cause many queries.Some geologists considered that the layered intrusion formed by multi-period pulsation through the study about the mafic ore-bearing intrusions and discuss the mineralization through magma conduit model.We first report mafic ore-bearing layered intrusions in Lala ore district which will help us to solve the problem.Based on detailed petrographical study,the layered intrusion can be divided into nine lithofacies zone,which is characteristic by mirror image and the adjacent lithofacies zone is intrusive contact.It can be divided into rich spherulitic texture diabase lithofacies zone?YWS-1?,weak spherulitic texture diabase lithofacies zone?YWS-2?,rich pyroxene phenocryst diabase lithofacies zone?YWS-3?,weak pyroxene phenocryst diabase lithofacies zone?YWS-4?and muscovite-bearing iron ore lithofacies zone?YWS-5?from inside to outside which indicated that the magma of mafic ore-bearing continues intrusive,the former un-consolidation magam became the channel of the magma and the rich ore magma uplifted at the last.Based on the observation by microscope,X-ray diffraction analyses and CSD method,each lithofacies zone is characteristic by porphyritic texture which indicated that each lithofacies zone was formed by 2 times pulsed invasions at least.The porphyritic mineral association is similar in each thofacies zone and is consisted of mica,hornblende,pyroxene,Ti-Fe oxide,plagioclase and so on.But the major phenocryst and size of phenocryst is different:the major phenocryst in YWS1-2 lithofacies zone consist of mica,hornblende and Ti-Fe oxide,the major phenocryst in YWS3 lithofacies zone consist of big pyroxene except for mica,hornblende;the major phenocryst in YWS-4 lithofacies zone consist of small pyroxene and calcite;the major phenocryst in YWS-5 lithofacies zone consist of muscovite,potash feldspar and quartz.The hydrous phenocryst indicated that each face belts was produced by magma with saturated volatile?H₂O+CO₂?or supersaturated volatile,but the magma of each lithofacies zone was different.The analysis of the EMPA profile of the crystals show that the hornblende in each lithofacies zone is kaersutite and the composition is stable in the process of crystallization;the mica can be divided into two crystal population:the mica of the YWS1-4 lithofacies zone is biotite and the mica of the YWS-5 lithofacies zone is muscovite and they derived from different magma chamber;The pyroxene is derived from cycle-crystal of magma in depth and can be divided into three crystal population which represent three period different magmatic process;the apatite can be divided into two different crystal population:YWS-5 lithofacies zone characteristic by granular,high P₂O₅,high F and other lithofacies zone is characteristic by acicular,low P₂O₅,low F;Ti-Fe oxide mainly distribute in YWS-5 lithofacies zone show highest Ti-Fe oxide components,higher Fe components than other lithofacies zone.All of this indicated that the source of the metallic mineral in YWS-5 lithofacies zone which is formed by rich ore fluid is different from other lithofacies zone.The geochemistry show that the intrusive belong to gabbro rocks of alkaline series.Sr-Nd isotopic data show that they derived from lithosphere mantle.The Sr-Nd isotopic data of the sample of YMS1-4 ploted on the PM area which indicated they derived from lithosphere mantle and The Sr-Nd isotopic data of the sample of YMS-5 ploted on the EM?area combined with its water-rich characteristics,which indicated that it was caused by the mantle-source flow injected into the crust source in the later magma pulsation process.The U-Pb dating on the apatite separated by LA-ICP-MS yields concordant age at 278±28 Ma.All of this show that mineral in layered intrusions is belonged to different crystal population which is consist of melt-crystal population and fluid-crystal population with minor solid-crystal population.The mineral crystallize in different magma chamber and formed each lithofacies by the magam uplifted,intrude and consolidated rapidly through magmatic pulsation.These magma chambers were packed by the evolved magma with different composition,which maybe enriched the corresponding ore-forming metal.When the deep metallogenetic fluid input this magmatic system,maybe cause a“domino effect”,lead to all kinds of metallogenetic fluid mass release.The sill in LaLa ore district may be played the shielding role,which drives the migration of ore-bearing fluid and formed superlarge deposit.The metallogenic model of LaLa is similar to to the Panzhihua-type iron deposit.