Geology and Genesis of the Post-collisional Porphyry-skarn Mo-Cu-Pb-Zn Deposit at Bangpu,Tibet

Author:Zhao Xiaoyan

Supervisor:Zhao Zhidan, hou Zengqian


Degree Year:2015





Bangpu Mo-Cu-Pb-Zn deposit in Tibet is a large but poorly studied Mo-rich(0.089%),and Cu-poor(0.32%)porphyry-skarn deposit that located in the northeast of the Gangdese porphyry copper belt,which is composed by porphyry Mo-Cu and skarn Pb-Zn mineralizations.Porphyry Mo-Cu mineralization in the deposit is generally associated with a mid-Miocene porphyritic monzogranite rock,whereas skarn Pb–Zn mineralization is hosted by lower Permian limestone-clastic sequences.The account of garnet decreases and the account of both calcite and other carbonate minerals increase with distance from the porphyritic monzogranite,suggesting a decrease temperature trend.Coprecipitated pyrite and sphalerite from the Bangpu skarn yield a Rb–Sr isochron age of 13.93?0.87 Ma.The similar diagenetic and mineralization ages between the porphyry and skarn mineralization events and also their close spatial positions indicate the same porphyry-skarn metallogenic system they share.The Bangpu deposit is formed(15.32 Ma)at the same time as other deposits in the Gangdese porphyry copper belt(1218Ma),but it has a different ore assemblage that is dominated by Mo-Pb-Zn.Five main magmatic units are associated with the Bangpu deposit:(1)Paleogene biotite monzogranite,which belongs to the calc-alkaline and peraluminous acid rock.It has zircon U–Pb age of 62.24±0.32,?Hf(t)=5.59.8,(87Sr/86Sr)i=0.7099660.710369,?Nd(t)=-4.79-3.11,which was originated from the partial melting of the mantle wedge triggered by the subducted Tethyan oceanic slab,also involving the process of strong fractional crystallization and crustal material contamination.(2)The diabase belongs to the sub-alkaline mafic rock.It has zircon U–Pb age of 14.46±0.38Ma,?Hf(t)=3.17.0,(87Sr/86Sr)i=0.7057930.705887,?Nd(t)=-1.60-1.15,which was formed by partial melting of upwelling asthenospheric mantle.(3)Porphyritic monzogranite,which belongs to the high K calc-alkaline and weak metaluminous acid rock,also with the character of adakite rock.It has zircon U–Pb age of 14.63±0.25Ma,?Hf(t)=2.28.7,(87Sr/86Sr)i=0.7075040.710012,?Nd(t)=-3.96-3.56,which was generated by partial melting of a thickened new-born lower crust with some mantle components.(4)The diorite porphyry belongs to the high K calc-alkaline and metaluminous neutral rock whose zircon U–Pb age is14.84±0.35Ma.Its Sr/Y value and Y content are between the porphyritic monzogranite and diabase,indicating the mixing endmembers of the new-born lower crust and asthenospheric mantle.(5)Fine-grained diorite,whose zircon U–Pb age is13.237±0.036Ma.The Bangpu porphyry Mo-Cu mineralization has annular alteration zonation,ranging from an inner zone of biotite alteration through silicified and phyllic alteration zones to an outer propylitic alteration zone,similar to typical porphyry deposits,but the K-feldspar alteration at Bangpu is dispersed.Skarn alteration is identified by the presence of typical skarn minerals within the surrounding rocks,such as epidote,actinolite,epidote,chlorite.Fluid inclusion microthermometry indicates a change in ore-forming fluids from high-temperature(320°C–550°C)and high-salinity(17 wt.%–67.2 wt.%)fluids to medium-temperature(213°C–450°C)and medium-low-salinity(7.3 wt.%–11.6 wt.%)fluids.All the C-H-O isotope data demonstrate two processes of fluid evolution in the Bangpu porphyry-skarn deposit,which are dehydration-degassing process and atmospheric precipitation addition process.Sulfides at Bangpu yield?34SV-CDT-CDT values of-3.8‰+1.1‰.δ34SΣSS of sudfides in the porphyry mining area,skarn mining areas that close and far away from the monzogranite porphyry are-0.22‰、-0.56‰and-1.22‰respectively which indicative of mantle-derived S.The Pb isotopic compositions of these three mining areas are as followings:206Pb/204Pb=18.7919.13、19.0819.28、18.7318.86,207Pb/204Pb=15.6415.93、15.8215.89、15.6915.76,208Pb/204Pb=39.1640.45、39.8740.34、39.2639.60,demonstrating high radiogenic Pb.The mantle-derived S imply that coeval mantle-derived mafic magma(e.g.,diabase)simultaneously supplied S and Cu to the porphyry system at Bangpu whereas the Mo,Pb,Zn were likely derived from an ancient crustal source.The formation of the Bangpu deposit can be explained as follows:A change in tectonism from compression to extension in the Gangdese Range was caused by the delamination of the thickened lithosphere or breaking-off of underthrusted Indian continent,which resulted in upwelling of asthenospheric mantle and generation of mafic magmas(e.g.,diabase),with the latter supplying heat to trigger melting of a thickened new-born lower crust with large amounts of ancient crustal materials,which formed the porphyritic monzogranite magma bearing Mo,Pb,Zn.The presense of crust-mantle magma mixture(e.g.,diorite porphyry)indicate that the mantle-devided magma provide Cu and S to the resulting porphyry system at Bangpu.The ore-forming magma migrated upwards which resulted in the porphyry Mo-Cu within the porphyritic intrusion and the skarn Pb-Zn in the the Permian limestone–clastic sequences,thus forming the porphyry-skarn Mo-Cu-Pb-Zn deposit.