Geological Characteristics And Genesis Of The Lala Fe-Cu Deposit, Sichuan Province

Lala Iron-Copper Deposit,one of the largest deposits in SW China,is hosted in the Late Paleoproterozoic of lower strata of the Hekou Group.Though in the past Lala Iron-Copper Deposit had been once considered as sub Marine volcanic sedimentary mineralization,now most of the scholars agreed that it was one of the largest IOCG deposits in Kang Dian copper belt.However,there are still a lot of questions for the case study of typical IOCG deposits,such as the controversy over the widely distributed albitites and their relationship to mineralization in Lala area,little detailed report of the cryptoexplosive breccia and the origin of ore-forming fluid has not been solved.The questions implications for explain ore-forming age and mineralization processes as well as for mineral resources exploration.In this doctoral dissertation,the following investigations have been undertaken:(1)this article takes on the analysis of geology and petrography of the Xiaolutian and Dalutian pit,exploring the potassium,sodium and carbonate rock of the Lala,providing evidence of their mineralized composition,and comparing with classic IOCG deposits in the world;(2)published breccia CSD data of the Lala deposits,which used to compare with other countries,then evaluated these types of breccia and discusses relationship between cryptoexplosive breccia and mineralization;(3)published U-Pb geochronological data of Magmatic zircons from the cryptoexplosive breccia and rock spilite-keratophyre formation,which closely related to the mineralization,it can provide constraints for the metallogenic epoch;(4)a case study on the geology and geochemistry of the spilite-keratophyre formation has provided a chance to discuss their characteristics of fluid transformation,source region and deep processes,which could also provide the complete chain of evidence for the IOCG nature of Lala.Based on the above results,the main conclusions can be drawn as follows:(1)Our petrography data showed that both primary albite phenocrysts and secondary albites were in the keratophyre matrix;secondary albites replaced primary albites with small anhedral granular albites;and ore-bearing capacity of keratophyre was low;(2)Geochemical composition revealed that spilite-keratophyre formation was significantly enriched in Nb,Ta,Zr and Hf,and depleted greatly in Ba,Pb,Sr and Ti;?REE changed largely;and the ore-forming fluid had lower Na but higher K,Al,CO2 and H2 O contents.The initial 87Sr/86 Sr ratio lied between 0.70126~0.76241,indicating it has been affected by crustal contamination.Furthermore,five keratophyre samples yielded a ?Nd(t)value of-1.09~6.75,consistent with a mantel-crust process,such that the ore-forming fluid formed initially in the deep magma chambers,ascended then to the crust and intruded into the weak zone or foliated rock;(3)Fractal dimension of the breccia illustrates the cause of its mechanical fragmentation in comparison with fault breccia.The spatial density of population reveals that it is mechanically compressed and can be distinguished from the fault breccia in the alignment factor and roundness data;(4)Experiments data yield the age of 1720±11Ma of spilite keratophyre formation.Zircon crystals of cryptoexplosive breccia showed significant characteristics of late hydrothermal alteration,nevertheless,it still deny the theory of sub Marine volcanic sedimentary mineralization.The discovery of cryptoexplosive breccia provides an important basis for which Lala is IOCG deposit;(5)It was the fluid rich in iron or copper that added into the fluid reservoir,which resulted fluid overpressure internally,then caused cryptoexplosion.After that,fluid in the lower part intruded the weak zone of strata,which forms the main host of the ores.The spilite,keratophyre and cryptoexplosive breccia infilled by hydrothermal minerals derived from the upper fluid.The process was like the transmagmatic fluid model;(6)The ore-forming fluid formed initially in the deep magma chambers,ascended then to the crust and intruded into the weak zone or foliated rock.This process is similar to the transmagmatic fluid model.