Composite Metallogenic System And Exploration Research Of The Geza Porphyry Belt,SW China

The Geza porphyry belt is the southern portion of the Yidun Terrane resulted from the evolution of the Sanjiang Tethys,consisted by the Zhongza block,the Geza arc,the Ganzi-Litang suture,and the western Yangtze block.The belt has experienced successively Paleo-,Meso-to Neo-Tethys tectonic evolution,resulting in complicated tectono-magmatic and mineralization events with the characteristics of multiple stages,types and metals.Previous studies mainly focuses on the genesis of individual deposit,e.g.mineralogy,petrology or fluid inclusion of some deposits,rarely having studies focus on the regional or belt-scale metallogenesis and ore prospecting and exploration,which makes the belt in the bottleneck of prospecting at present.The difficulties in the genesis study of the Geza porphyry belt composite metallogenic system lie in(i)knowing well the mineralization-controlling factors unique to the metallogenic system in each stage,(ii)recognizing the ore source-transport-storage process for every metallogenic system,and(iii)ascertaining the superimposition mechanisms among different metallogenic systems.Based on this and combined with the geophysical,geochemical,remote-sensing and heavy-sand data,it would be possible to extract the ore-causing anomaly information,establish the ore-prospecting model for the belt,and propose a new ore-finding direction.This will not only provide practical guidance basis for further ore prospecting and exploration,but also supply informative typical case to enriching the composite metallogenic theory.Based on the ideas and methods of regional metallogeny and metallogenetic regularity,this study concentrates on the Hongshan-Hongniu,Xiuwacu,Tongchanggou,Donglufang,Pulang,and Songnuo deposits in the Geza porphyry belt and studies including ore geology,petrology,mineralogy,geochronology,isotopes,and fluid inclusion.The Hongshan-Hongniu Cu-Mo and Xiuwacu Mo polymetallic deposits are classified as typical composite origin.At least two sets of porphyry metallogenic systems are recognized in the Geza porphyry belt,i.e.,the Indosinian porphyry Cu polymetallic metallogenic system superimposed by the Yanshanian Mo polymetallic metallogenic system constitutes a typical composite metallogenic system.It is characterized by “five different composite metallogenic mechanisms”—?metallogenic epochs,characteristics and origins of metallogenic magmas,sources of ore-forming materials,ore-forming fluids and metallogenic geodynamic settings are all different between two sets of systems.However,the tectonic evolution history is complete and metallogenic structure has inheritance.The Indosinian Cu polymetallic mineralization mainly occurred at 220–210 Ma,whereas the Yanshanian Mo polymetallic mineralization mainly formed at 85–75 Ma.The whole-rock Sr-Nd-Pb,zircon Hf-O and sulfides S-Pb-Re isotope geochemistry show that(i)the Indosinian porphyry is mainly originated from enriched mantle and mixed with small amounts of continental crustal materials by crust-mantle interaction,and it is a typical continental-arc I-type granite;(ii)the Yanshanian porphyry is principally derived from partial melting of the ancient thickened intermediate-mafic lower crustal materials,and it is post-collision high-fractionated I-type granite characterized by crust-sourced adakitic rocks.Moreover,the Indosinian Cu polymetallic ore-forming materials are mainly sourced from mantle-related island-arc I-type granitic magma formed by partial melting of lithospheric mantle due to oceanic crustal subduction,while the Yanshanian Mo polymetallic ore-forming materials are mainly formed under the intracontinental post-collision extensional tectonic transformational setting and derived from high-fractionated I-type granitic magma formed by partial melting of the thickened intermediate-mafic lower crustal materials.The late Indosinian magmatism develops middle-high temperature and high salinity ore-forming hydrothermal solutions of NaCl-CO2-H2 O system,and the late Yanshanian magmatism forms middle-high temperature and high salinity NaCl-CO2-H2 O ore-forming solutions in the early stage,but involved into middle-low temperature and low salinity NaCl-H2 O ore-forming solutions in the late stage.The Indosinian tectono-magmatic metallogenic system wasformed by the westward subduction of the Garzê-Litang oceanic crust,developing volcanic arc and I-type porphyries belt associated with Cu polymetallic mineralization.The Yanshanian tectono-magmatic metallogenic system occurred at the stage of intra-plate post-collisional extension environment.The tectonic regime transformed from compressional to relatively tensional environment.Duo to such intensive intraplate stretching-fracturing,the Yanshanian Mo polymetallic mineralization formed in the Geza area,namely intracontinental tectonic transformation mineralization.The Indosinian and Yanshanian tectono-magmatic metallogenic systems represent the superposed and composite mineralization footprint by accretionary and collisional orogenesis within the same district.Furthermore,two stages of composite porphyries developed in the same deposit become a representative case for composite mineralization.The Yanshanian felsic magma intruded along the early-formed deep faults or the tectonic weak zone by the Indosinian subdction and superimposed on the Indosinian porphyry.The later-stage secondary faulting reactivates and enriches the ore-forming metallic elements such as Cu and Mo,thus forming thick and large ore bodies.The metallogenic structures are marked by inheritance.Based upon the study on the characteristics of the typical deposits and genesis of composite metallogenic systems,the present thesis summarizes and discusses the regularity of the Cu and Mo metallization in the Geza belt.In combination with the multiple information including geophysics,geochemistry,remote sensing and heavy sand to extract the mineralization anomaly,the metallogenic factors are revealed and the ore-prospecting model for the Geza porphyry belt is built.The paper also puts forward that the Songnuo and Donglufang is the most potential prospecting and exploration area for finding the Indosinian porphyry Cu and Yanshanian porphyry Mo deposits in the belt,respectively.In addition,it has made new major progress in the Donglufang prospecting and exploration under the guidance of the new achievements and understandings from this study of the regularity of the composite metallogenic system in the Geza porphyry belt.