Paleozoic Tectono-magmatic Evolution And Its Control On Lead-zinc Mineralization In Xitieshan, North Qaidam

The North Qaidam ultrahigh-pressure metamorphic?UHPM?belt is located at the northeastern margin of the Tibet Plateau in NW China.It extends in NWW direction for about 400 km and is composed of the Yuka eclogite-bearing terrane,the Lüliangshan garnet peridotite-bearing terrane,the Xitieshan eclogite-bearing terrane and the Dulan eclogite-bearing terrane from northwest to southeast.The North Qaidam UHPM belt is a typical Alpine-type UHPM terrane resulted from the early Paleozoic subduction of the Qaidam Block beneath the Qilian Block.It is generally believed to record the whole evolution of a continental orogen from early seafloor subduction,to continental subduction and collision,to the exhumation of the subducted slab and the final orogen collapse.The Xitieshan terrane,located in the central segment of the North Qaidam UHPM belt,contains many Paleozoic intrusions related to the orogeny.In addition,the Xitieshan lead-zinc deposit in this terrane is one of the largest base metal deposits in NW China.This dissertation reports integrated study of LA-ICP-MS zircon U-Pb ages and Hf isotopic compositions,as well as whole rock major and trace elements and Sr-Nd isotopic compositions of Xitieshan syn-collisional and post-collisional magmatism with the aims of:?1?constraining the tectonic setting of the magmatism;?2?researching the origin and petrogenesis of the granites;and?3?understanding the mechanisms of crust growth in response to continental collision.In the meanwhile,we employ detailed petrographic observations on the sulfides and conbernates of Xitieshan deposit to understand the characteristics of defroamed and metamorphosed textures,and the effects of deformation and metamorphism on the deposit.Combined with results of LA-MC-ICPMS sulfide in-situ S isotope analyses and C-O-Sr isotopic analyses on carbonates,this dissertation discusses the origin of sulfur and ore-forming fluids,studies the classification and geodynamic setting of this deposit,and explain how the Paleozoic tectonic evolution control the magmatim and mineralization in this region.The main results achieved and understanding are as follows.?1?The Xitieshan Pb-Zn deposit in the North Qaidam orogenic belt,northeastern margin of the Tibetan Plateau,contains a geologic resource of 64 million tons grading4.86 percent Zn,4.16 percent Pb,58 g/t Ag and 0.68 g/t Au.The deposit is hosted in a succession of Middle to Late Ordovician metamorphic marble-bearing volcanic rocks,that are interpreted to have formed in a back-arc basin.The deposit was intensely deformed and metamorphosed during the exhumation of the subducted Qaidam slab at ca.400 Ma.According to the field observation,automated minerology and electron microprobe analyses,the primary morphology of orebodies and most of the original sulfide textures have been destroyed or highly modified.A few primary mineral textures and assemblages survived deformation because they were totally enclosed within marble.Pyrite tends to be brittle at high temperature and stress environments and exhibits cataclastic deformation textures.Local recrystallization of pyrite led to annealing aggregations and large cubic crystals.Pyrrhotite is most likely the product of the metamorphic breakdown of pyrite.Most sphalerite,galena,pyrrhotite and chalcopyrite show ductile deformation textures that include filling cracks and fractures in brittle pyrite and host rocks,and laminated textures with transported clasts of brittle sulfides and host rocks.Ankerite and siderite are the main alteration minerals associated with mineralization and they show textures that confirm the replacement of the carbonate protolith of the host marble.?2?LA-ICPMS zircon U-Pb dating of the Xitieshan granite yields magmatic crystallization ages of 441±2 Ma and 442±2 Ma,which are consistent with the peak age of ultrahigh-pressure eclogite-facies metamorphism in the Xiteishan terrane.The temporal correlation between them confirms that the Xitieshan granite was products of syn-collisional magmatism during the continental collision between Qaidam and Qilian Blocks.These granites show high-K calc-alkaline and slightly peraluminous signature,low zirconium saturation temperatures and high contents of K₂O with the S-type characteristics.They have rare earth element and trace element patterns resembling those of bulk continental crust,with strong fractionation of light and heavy rare earth elements??La/Yb?_N=19–26?,moderately Eu negative anomalies??Eu=0.65–0.71?and the obviously Nb,Ta,P and Ti negative anomalies.The Xitieshan granite also exhibits remarkable Sr-Nd isotopic differences(initial ⁸⁷Sr/⁸⁶Sr=0.70920–0.71080 and?_Nd?t?=-4.54–4.11)from the contemporaneous granites within North Qaidam UHPM belt.Combined with the positive?_Hf?t??0.5–5.3?and ages of inherited zircons?475–518 Ma?,the magmatism is best explained as resulting from melting of subducted oceanic and continental crust during continental collision.Isotopic mixing calculations suggest that ca.28–35%ocean crust and ca.65–72%continental materials contribute to the origin of the Xitieshan granite.Thus,the syn-collisional felsic magmatism represents juvenile crust with input of oceanic crustal materials derived from the depleted mantle and the hypothesis“continental collision zones are primary sites for net continental crust growth”is applicable in the North Qaidam.?3?The Xitieshan gabbros and granitoids formed coevally at ca.372 Ma?Late Devonian?,about 50 million years after the continental collision between the Qaidam and Qilian blocks.The gabbro samples have low-to medium-K tholeiitic composition,with high FeO^T and TiO₂ contents.They are enriched in large ion lithophile elements,depleted in Nb and Ta,similar to the lithospheric mantle source,which is also indicated by their Zr/Nb,La/Nb and Nb/Ta values.In addition,these rocks have relatively enriched isotopic signatures compared to the MORB.This implies that the mantle source may have been metasomatized by subduction related melts,which is further confirmed by the signatures of trace elements.The coeval felsic rocks are high-K calc-alkaline,slightly peraluminous I-type granites.They are characterized by high ratios of light rare earth elements/heavy rare earth elements with a negative Eu anomaly,strong depletion in Ba,Sr and high-field-strength elements?e.g.Nb,Ta,Ti and P?and enrichment in Rb,Th,U and Pb,resembling the upper continental crust?UCC?.The Sr-Nd-Hf isotopic signatures of these granitoids show remarkable differences with UCC-derived melts and North Qaidam basements,but are generally consistent with previously emplaced syn-collisional granite in Xitieshan terrane.Therefore,the granitoids are best interpreted as the products of melting of the syn-collisional granite.Meanwhile,the involvement of coeval mantle-derived melts is suggested by the relatively depleted isotopic signatures and mafic magmatic enclaves hosted by the granitoids.Additionally,some of the inherited zircons in the granitoids display negative?_Hf?t?values and Paleoproterozoic T_DM2 ages,suggesting that the source also involved ancient crustal materials.Based on the temporal and spatial associations of the gabbro–granitoid suite,in conjunction with their petrogenetic features,we propose a geodynamic model involving convective removal of the lithosphere and resultant asthenospheric upwelling for explaining the crustal reworking,crust-mantle interaction and related post-collisional geodynamic processes in North Qaidam UHPM belt.?4?In situ analysis of sulfur isotope by laser ablation in combination with multiple collector inductively coupled plasma mass spectrometry yielded values of?³⁴S ranging-15.4‰to+25.7‰for pyrite,+2.2‰to+4.9‰for sphalerite,-0.3‰to+3.3‰for galena,+1.4‰to+3.7‰for chalcopyrite and+2.0‰to+4.1‰for pyrrhotite.The?³⁴S values display retention of the isotopic variability,which is not supportive of a deformational homogenization.The dominant source of sulfur was determined to be leached from the host volcanic rocks or directly derived from a deep-seated magma.Seawater sulfate reduction also contributed minor component.The lack of isotopic equilibrium may reflect rapid crystallization due to mixing of hot hydrothermal fluids with ambient seawater.Carbon,oxygen and strontium isotope compositions of ankerite and siderite that are closely associated with original mineralization,are consistent with metasomatic replacement of the host marble by hydrothermal fluids related to the volcanism.While the host marble originated from debris flow or turbidities of shallow marine carbonates that underwent hydrothermal alteration.Several lines of evidence suggest that the Xitieshan deposit resulted from the replacement of marble.Although laminated sulfides in the host schist is an important ore type,these apparently stratiform ores were produced mainly through the ductile deformation and plastic flow of the original ore bodies.The preserved textures,host volcanic sequence,minor sulfur from seawater sulfate reduction and high Au concentration argue against a clastic-dominated sediment-hosted deposit or an intrusion-related carbonate replacement deposit.The Xitieshan deposit is more likely to have formed by carbonate sediments replacement in a volcanogenic hydrothermal system.Thus,the Xitieshan deposit is an unusual volcanic-hosted sulfide deposit that formed below the sea floor in a deep marine basin by replacing carbonate rocks from debris flow or turbidities off a carbonate platform of shallow marine environment.?5?Based the previous study of the Tanjianshan Group metamorphosed volcanic-sedimentary rocks,the Tanjianshan Group and lead-zinc orebodies formed in back-arc basin.This contribution summarized the mechanism and sequence of magmatism and mineralization in response to the orogeny of North Qaidam,and illuminated how the regional geodynamic evolution control the magmatism,mineralization and the preservation and changes of orebodies with a coupled tectono-magmatism-mineralization model.Furthermore,this contribution analyzed the prospecting potentiality of Xitieshan deposit and proposed three ares that have potential to find more ores according to the studies of tectonic analysis,distribution of orebodies and tends of ore-forming elements.