The Magmatism And Mineralization Related To The Subduction Of Neo-Tethys In The Southern Part Of Lhasa Block In Tibet

There are many studies on the porphyry metallogenic system in the background of continental collision in the Tethyan metallogenic belt,while the study of porphyry mineralization system in the subduction background is relatively few.In this study,the southern part of the Lhasa block in the east of the Tethys metallogenic area is taken as the study area,which is intended to reveal the diagenesis and mineralization during the subduction of Neo-Tethys through detailed field geological survey and laboratory analysis.Specifically divided into the following three aspects:First of all,we should know the start-up and end time of the Neo-Tethys ocean.Scholars basically agree with?210Ma(late Triassic)as the start-up time of the Neo-Tethys subduction,which is marked by the appearance of arc magma.The Controversy of the end time of subduction(e.g.the collision time of the India-Asian continent)is very large because of the different data.this.paper uses the theory of petrology,ore deposits to support the collision occurred in?56Ma,(e.g.the Paleocene-Eocene time line).After the start-up and end time is determined,the authors have studied the two important diagenetic and mineralizing processes during the subduction of Neo-Tethys,which is the Late Triassic-Cretaceous porphyry Cu-Au mineralization in the southern margin of the Lhasa block that related to the intra-oceanic subduction of Neo-Tethys oceanic slab,and the Paleocene epithermal Ag polymetallic mineralization in the southern part of the Lhasa block that related to the magmatism of continental margin arc.The paper gets the following achievements:(1)For the collision time of the India-Asian continent,the results of this study support the idea of?56Ma(e.g.the Paleocene-Eocene time line).The evidence is as follows:Firstly,the mantle-derived magma is difficult to invade and form a small amount of crust derived granite during the strong crush of India-Asian continent,the relatively small number of volcanic rocks(Nianbo formation:56?52Ma)in the Linzizong volcanic group,and the?54Ma S-type granite discovered in the Nanmulin basin,suggesting that 56?52Ma may be the initial collision time of India-Asian continent.Secondly,the Linzizong volcanic rocks record the whole process of the India-Asian continent collision,the geochemical characteristics affinity between Dianzhong volcanic rocks(>56Ma)and continental margin volcanic rocks,and the affinity between Pana volcanic rocks(<52Ma)and intraplate volcanic rocks(the Nianbo volcanic rocks show transitional characteristics),suggesting a transition of the tectonic setting.Finally,The magma-hydrothermal deposits is not easy to form in the background of continent collision,the 56?51 Ma metallogenic stagnation period in the Paleocene-Eocene deposits in the southern part of Lhasa block may be caused by the strong squeeze background at the initial stage of the collision.In addition,the epithermal deposit is often formed in the volcanic are background,and the zircon U-Pb age of the ore bearing surrounding rock in Sinongduo epithermal deposits is?61Ma(besides,the Ar-Ar of illite during mineralization is?63 Ma),indicating that there may have been no collision before?61Ma.(2)The late-Triassic to Cretaceous magmatic rocks in the southern margin of Lhasa block have potential for Cu-Au mineralization.The diagenetic age of the Xietongmen hornblende gabbro,granodiorite,and the Nymo gabbrodiorite are 170,180,88 Ma.These rocks show medium-to high-Kcalc-alkaline,metaluminous,enriched in the large ion lithophile elements and depleted in the high field strength elements,indicating that the magmas generated in a subduction-related tectonic setting.The intrusive rocks in the Xietongmen and Nymo areas have similarly high and positive ?Hf(t)values(Xietongmen hornblende gabbro:13.3-18.7;Xietongmen granodiorite:14.2-17.6;Nymo diorite:12.3-17.8)that yield Hf model ages younger than 312 Ma.Combining these new data with the results of previous research suggests that the magmas that formed the late-Triassic to Cretaceous igneous rocks in the southern margin of Lhasa block were derived from a metasomatized region of an asthenospheric mantle wedge during northward intra-oceanic subduction of the Neo-Tethys oceanic slab.The presence of the large Jurassic Xiongcun porphyry-epithermal Cu-Au and Cretaceous Sambujiala-Kelu skarn Cu-Au deposits in this area suggests that the late-Triassic to Cretaceous magmatism in the southern margin of Lhasa block was an important control on the genesis of Cu-Au mineralization.Combined with the achievements of geology,exploration geophysics,exploration geochemistry,wo suggested that the further exploration for coeval deposits in the southern margin of Lhasa block should ideally focus on regions with exposed arc volcanic rocks(i.e.,the volcanic Sangri Group)with strongly depleted isotopic compositions.(3)The Paleocene Dianzhong volcanic rocks in the southern part of Lhasa block have potential for epithermal deposit.The zircon U-Pb age of the host volcanic rocks in Sinongduo cryptoexplosive breccias type,hydrothermal vein-type Ag(Au)-Pb-Zn deposit is 61 Ma,which should belong to the Dianzhong formation,rather than the Pana formation confirmed by the former researchers.The age of sericite Ar-Ar is?63 Ma,which is consistent with the northern part of the Nyainqentanglha metallogenic belt.According to the characteristics of the ore mineral assemblage,gangue mineral assemblage,ore structure and ore-forming fluid,wo determined that Sinongduo Ag(Au)-Pb-Zn deposit is low-sulfidation epithermal deposit.There is obvious concentration zoning and higher peak values of Pb,Sb,Au,Ag,As,Hg elements in mining district,which is a potential exploration area for low-sulfidation epithennal precious metal deposits.At the same time,the Lu-Hf isotopic analysis show that the Dianzhong volcanic rocks(?Hf(t)ranged from-5.2 to + 3.5)have the mantle-derived components and are well fitted with the geochemical anomaly of Sb-As-Hg which suggest the potential for Au exploration.