Abstract:Significant economic sediment-hosted base metal deposits occur in Lanping, Changdu, Yushu and Tuotuohe areas of Sanjiang region in southwest China. The authors hold that they were formed during Indian-Asian continental collision and developed within the fold-thrust belt combined with thrust and/or strike-slip-related Cenozoic basins in the interior of the collsional zone. Ore bodies are hosted by carbonate and/or clastic rocks as hanging wall of the thrust fault, controlled by thrust-related salt diapir, second-order faults of the thrust fault, syn-ore hydrothermal karst, dolomitization, thrust-related breccias zones in carbonate, pre-ore karst, and fold-related intense cleavages/faults. The metals have Pb+Zn, Pb+Zn(+Cu+Ag), Cu+Co, or Cu associations in individual ore deposits. Sphalerite, galena, pyrite, ±marcasite, calcite/dolomite, and bitumen are common in the Pb-Zn deposits whereas sulfates and fluorite are abundant only in some of these ore deposits. The ores exhibit replacement and open-space filling forms. The main minerals in the Pb-Zn(-Cu-Ag) deposits include sphalerite, jordanite, galena, tetrahedrite, pyrite, calcite, and dolomite. They occur in veins or in hydrothermal karst as replacements and open-space fillings. The Cu deposits are composed of quartz + carbonate minerals + Cu sulfides (chalcopyrite, tetrahedrate, bornite and chalcocite) veins. The ore-forming fluids in the Pb-Zn and Pb-Zn(-Cu-Ag) deposits are dominated by basin brines with low temperatures (<210℃) and higher salinities . The ore-forming fluids are characterized by CO₂-rich fluids of metamorphic origin having relatively higher temperatures (180～230℃) and lower salinities in the Cu veins. Biogenic sulfate reduction±thermochemical reduction of sulfate by organic matter might have provided reduced sulfur for most Pb-Zn and Pb-Zn(-Cu-Ag) deposits whereas reduced sulfur in the Cu veins probably originated from volcanic rocks and sedimentary rocks. Upper crustal rocks contributed lead to all ore deposits. The base metal deposits in the Sanjiang region had no apparent affinity with magmatic activity. The authors tend to classify the Jinding, the Zhaofayong, the Dongmozhazhua, the Mohailaheng, and the Chaqupacha Pb-Zn deposits and the Pb-Zn(-Cu-Ag) deposits in eastern Baiyangping ore belt as MVT-like deposits, and to classify Pb-Zn(-Cu-Ag) veins in western Baiyangping ore belt and the Cu veins as polymetallic veins. Therefore, the Jinding MVT-like deposit, characterized by the tectonic setting of the interior of a continental collisional zone and thrust-controlling, can not be explained by the existing MVT genetic model.

Abstract:Based on an analysis of sedimentary sequence, regional unconformity, lithologic character and distribution of Paleogene-Neogene sediments, the authors hold that the Paleogene-Neogene sediments can be divided into four upward-shallowing sequences of Tuotuohe Formation, Yaxicuo Formation, Wudaoliang Formation and Quguo Formation, which constitute two complete continental orogenic molasses formation sequences. Basin analysis shows that the Paleogene-Neogene Tuotuohe basin has experienced such evolutionary stages as foreland basin evolution stage (56.5～45.0 Ma)→strike-slip pull-apart basin stage(45.0～30.0 Ma)→overall uplift and intermountain residual basin stage (23.5～16.0 Ma)→foreland basin-confined basin-intermountain residual basin stage (16～2.6？ Ma). According to tectonic, lithofacies and paleographic evolution, it is considered that at the early stage of the deposition of Yaxicuo Formation (45 Ma or so), regional geotectonic background experienced remarkable transformation from the regional compression-thickening stage to the domination of inter-plate lateral strike-slip action, thus entering into the intracontinental plate convergence evolution stage. Viewed from Paleogene-Neogene sedimentary evolution of the Tuotuohe basin, the collision between the Indian plate and the Eurasian plate was of pulsation nature, the four regional surfaces of unconformity and two molasses formation seq uences in the whole Paleogene-Neogene sedimentation seemed to be sedimentary response to pulsating orogenic process, and the initial collision probably took place at the time when Cretaceous changed into Paleogene before 56.5 Ma.

Abstract:With the uplifting and large-scale thrusting and striking of Qinghai-Tibet Plateau, a lot of Tertiary intracontinental red basins were formed on the basis of Palaeo-Tethys multi-island ocean framework around Yushu area in the northeast of the Plateau. In these basins a set of such fluvial-lacustrine sediments as red clasolite, marlite and gypsum were deposited, accompanied by magmatism in some basins, with Nangqen basin being most typical in this aspect. Based on field petrologic and stratigraphic study, the authors divided the Gonjo Formation of Nangqen Paleogene basin in northeastern Qinghai-Tibet Plateau into five sections of different sedimentary environments such as alluvial fans, fluvialite, lake and delta facies, which formed two sequences characterized by coarse grains in the lower part and fine grains in the upper part. Different sections have different rock assemblages, suggesting that they were formed in different sedimentary environments. The first section has a limited distribution and is only exposed on the west side of the middle part of the basin; it consists of siltstone, mudstone, argillaceous limestone, shale and slate, implying lake facie deposition. The second section exposed on the north part of the basin shows a proximal and rapid deposition environment composed of red conglomerate, glutenite and sandstone. The third section has a large distribution area and is exposed all over the basin; it is composed of dark purple mudstone, silty mudstone and gypsum deposited in alluvial and fluviatile fans under the condition of dry and hot weather. The fourth and fifth sections only exposed in the middle and south parts of the basin are composed of fluvial-lacustrine sediments composed of red conglomerate, glutenite and sandstone deposited in a proximal and rapid deposition environment. An analysis of sediment sources has revealed that the material sources evolved from stable interior of craton through debris recyclic orogenic zone to mixed recyclic orogenic zone. Owing to the asymmetry of the basin and dissimilar depositional environments, different lithologic sections are exposed in different places of the basin. These data suggest that the basin has experienced the evolution history of early stage extrusion thrust foreland basin, middle stage strike-slip pull-apart basin, and late stage extrusion strike-slip foreland basin.

Abstract:Nangqen basin is a typical representative of the medium-mini-type Paleogene basins controlled by local NNW-treading faults distributed along Jinsha River-Red River belt on the northeastern margin of Qinghai-Tibet plateau. Gonjo Formation is perfectly developed and dominated by a set of thick-bedded, purplish red, red terrigenous clastic rock formation containing gypsum rock in association with large-scale high potassium rocks in Nangqen basin. Combined with results of field geological investigation and analysis and determination of samples as well as quantitative studies of the sandstone framework detrital composition of the Paleogene strata, the authors systematically analyzed petrologic characteristics of the sandstones of the Gonjo Formation in Nangqen basin in the framework of the basin evolution. According to the analysis of geochemical features of sandstones of the Gonjo Formation, the characteristics of paleoclimate, paleoenvironment and provenance in the study area were revealed. For sandstones in different strata of Gonjo Formation, systemic sampling and determination were conducted, the detrital composition of sandstone was observed under a microscope, and the sandstone framework detrital composition was calculated and analyzed using the line-count method. The quantitative data of the clastic constituents not only constitute the nomenclature foundation of sandstones but also provide an important basis for understanding the nature of the sedimentary basin and estimating the provenance region. Meanwhile, the data processing methods included the genetic classification of the clastic constituents and the triangular graphical analysis. According to the theoretical analysis and the results of Dickinson W.R, the authors analyzed and discussed the relationship between the provenance and the plate tectonics of clastic rocks of Gonjo Formation. The results indicate that the sandstones of Gonjo Formation belong to recycling orogenic belt area, and only the samples of Eg² came from polycyclic orogenic belt area and cratonic interior. From the viewpoint of geological features as well as the position of composition plots, it is considered that the provenance evolution began from stable cratonic interior to lithic recycling orogenic belt and polycyclic orogenic belt area. The trend of the composition maturity in the sedimentary successions from Eg¹ to Eg⁵ was low-high-low. Provenance evolution analysis shows that the tectonic movement was very intense in the northern and northwest parts of the basin during the early deposition, which provided the period of the 2nd member (Eg²) with low maturity detritus. From Eg¹ sequence to Eg² sequence, with the increasing strength of the tectonic movement, the content of feldspar had a decreasing tendency while that of lithoclast had an increasing tendency. And the alluvia fans, i.e., the fluvial facies were at the primary stage of development. In terms of Eg³, the tectonic movement began to taper off, the content of feldspar increased, and lithoclast decreased gradually, which however brought about the end of lake sequence development, with strong uplifting and erosion in northwest and north parts of the basin, the deposition heaped up in the depressions, and the existence of igneous activities in east and south parts of the basin. As for Eg⁴ and Eg⁵, the provenance changed into a transitional recycled orogen, and the content of feldspar presented an increase-decrease tendency, while the tendency of lithoclast content was on the contrary. Through an analysis of the changes of the detrital composition of sandstone, Nangqen basin underwent two stages of tectonic evolution in which the early stage belonged to the reverse-compression and the late stage to strike slipping-compressional deformation. The evolution trend of tectonic evolution combined with the detrital composition changes in each sequence was studied. An analysis of the major elements of sandstones indicates that felsic rocks were dominant in the provenance, accompanied by mafic rocks. Varying tendency of chemical index of weathering (CIW) is similar to that of the chemical index of alteration (CIA) from Eg¹ to Eg⁴,and this index is used to reflect the weathering intensity from weak to strong and to weak again. Variation of the index of compositional variability indicates the circle of lower compositional maturity-higher compositional maturity-lower compositional maturity from Eg¹ to Eg⁴ in the section, suggesting that geological conditions of the provenance region had significant differences. The paleo-climatic environment changed from warm-moisture through drought-hot to moisture in upward succession during the depositional period. The provenance of the sediments of Gonjo Formation was based on felsic rocks, with the provenance of Eg³ sequence having a small quantity of mafic rocks, probably due to the movement of shear-strike slip faulting that resulted in gradual extension to the upper mantle and induced partial melting of basement materials which erupted or intruded into Eg³ sequence.

Abstract:Based on field observation and indoor integrated analysis of some 20 outcrop profiles in Lanping basin in combination with such division criteria as regional sedimentary framework and sedimentary characteristics, the authors analyzed Palaeogene sedimentary environment, sequence stratigraphic features, sedimentary facies distribution and evolutionary features. Some tentative conclusions have been reached: In Palaeogene, alluvial fan, river, delta and lacustrine environments were dominantly developed in the Lanping basin. Sedimentary facies experienced regular concerted evolution in the course of evolution: In the lowstand period, the subduction and compressive action of Indian plate towards Eurasian plate caused the formation of Lanping strike-slip pull-apart basin and the deposition of a set of confined lake basin facies red gypsum fine detrital rock formation; in the lake expansion period, the conversion of compressional stress field into pull-apart stress field resulted in the expansion of the lake basin in early Eocene and the extensive deposition of a suite of littoral shallow lucustrine facies purplish red and brick red interbeds of sandstone and mudstone, reflecting the gradual rising of the lake surface and the gradual expansion of sedimentary area; in the lake contraction period, influenced by late Eocene Himalayan movement, the basin and its periphery experienced the conversion of pull-apart stress field into intense compression, and the orogenic belts on both east and west sides jointly expanded towards the interior of the basin through napping, leading to the beginning of the differential rapid uplift of the basin and the further contraction of the basin; as a result, only a series of small-sized piedmont continental foreland basins were formed at the front of the nappe whih were later filled with continental clastic molasses sediments. Stratigraphic sequence paleographic mapping has revealed that sedimentary ore resources (evaporates) had obvious stratabound characteristics and were developed in lowstand tracts in relation to sedimentary filling features and climate of the lowstand period. This period was the basin-forming period, during which the water body of the basin was shallower than that in the lake expansion period characterized by rapid tectonic subsidence and fast rising of the lake surface, and the continental clastic material supply was less than that in the lake contraction period characterized by formation of molasses formation, thus favorable for the preservation of a large-area shallow water lacustrine environment. Besides, this period had a hot and dry climate, and hence the evaporation quantity of the water body was larger than the recharge quantity. These integrated factors resulted in the deposition of stably distributed evaporate formation in the lake basin, which, of course, causes the basin to have extremely promising vista in search for potash salt.

Abstract:The Yushu ophiolite mélange, which has preserved important record concerning the evolution of the Palo-Tethys Ocean, is mainly composed of pillow basalts, diabases, gabrros and ultramafic rocks. The zircon SHRIMP U-Pb age of Rangnianggongba gabbro in this mélange is 258.4±2.9 Ma, suggesting that the emplacement age of the rock is Late Permian. Petrologic and geochemical studies suggest that the gabbro is alkaline and has characteristics of high Ti and Fe. All the rock samples have relatively high REE abundances, and are enriched in LREE . The rock is enriched in large ion lithophile elements (LILE: Rb,Ba,Th,K) and some high field strength elements (HFSE: Nb, Ta, Zr, Hf). These data indicate that the Rangnianggongba gabbro was derived mainly from the enriched mantle. According to geochemical characteristics, the authors have concluded that the gabbro was formed in an oceanic island setting. The age dating and geochemical data indicate that an oceanic crust was already existent at least in the Late Permian. Moreover, the existence of large-scale Paleozoic Plume activities on the southwest margin of Yangtze block has been extensively reported. It is considered that the emplacement of the Rangnianggongba gabbro was related probably to activities of the Plume.

Abstract:The northern Sanjiang porphyry Mo(-Cu) ore belt is located in the southern Qinghai Province, where a series of NE-SE trending Cenozoic Mo-Cu mineralized rock bodies occur. In recent years, lots of large-size porphyry-skarn Mo(-Cu) oren deposits, such as the Narigongma porphyry-type Mo(-Cu) deposit and the Lurige and Dagugongka porphyry-skarn-type Mo(-Cu) lead-zinc deposits have been discovered. Based on analyzing petrologic characteristics of element geochemical data and Sr-Nd-Pb istopic data of representative porphyries in Narigongma, Lurige and Dagugongka, this paper discusses the metallogenic geological background and the origin and evolution of porphyries in the study area, with the purpose of contributing to future mineral exploration. Ore-bearing porphyries in the study area are mainly biotite granitic porphyries and leucogranitic porphyries, with SiO₂ content of 64.87%～76.30%, K₂O+Na₂O 3.43%～8.90%, and K₂O+Na₂O 0.79%～47.24%, belonging to the calc-alkaline-shoshonitic series and being similar to things of the Yulong mineralization belt. All the ore-bearing porphyries in the study area have similar distribution of rare earths and trace elements, with right-oblique curve, characterized by LILE (large-ion lithophile element) concentration, high field strength elements depletion, and significant Ti-Nb-Sr trough. The porphyries show weak Eu-depleted anomaly and adakite affinity. The ²⁰⁶Pb/²⁰⁴Pb ratio ranges from 18.410 0 to 19.429 0, ²⁰⁷Pb/²⁰⁴Pb from 15.609 0 to 15.685 0, ²⁰⁸Pb/²⁰⁴Pb from 38.577 0 to 39.522 8, ⁸⁷Ru/⁸⁶Sr from 1.901?0 to 8.722, ⁸⁷Sr/⁸⁶Sr from 0.705 291 to 0.710 307, (⁸⁷Sr/⁸⁶Sr)_i from 0.702 873 to 0.705 859, and ¹⁴³Nd/¹⁴⁴Nd from 0.512 573 to 0.513 204, and the depleted mantle age (t_DM) is 0.60～0.85 Ga. These data are basically consistent in all the ore deposits. Most of ²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb are distributed in enriched mantle (EMⅡ). (⁸⁷Sr/⁸⁶Sr)_i-εNd(t)values are on the evolution curve of the depleted mantle (MORB) and enriched mantle (EMⅡ). Sr-Nd-Pb isotopic data indicate that the enriched mantle (EMⅡ) metamorphosed by subducted plate fluid might have been the source of the porphyries. In comparison with things of the Yulong ore belt, the depletion of Sr-Nd-Pb isotopes is more obvious in the northern Sanjiang ore belt, and the injection rate of asthenospheric material caused the geochemical differences between the northern Sanjiang ore belt and the Yulong ore belt. The establishment of the molybdenum-copper metallogenic belt in northern Sanjiang Region, whose center is the Narigongma-Lurige large-size molybdenum-copper deposit, suggests that there still exists a great potential for finding large porphyritic and postmagmatic hydrothermal ore deposits in this region.

Abstract:Mafic rocks, especially those with low crystallization degree such as basalt and diabase, generally contain very rare zircon. It is thus not very easy to determine their rock-forming epochs by in-situ zircon U/Pb analysis. However, this kind of magmatic rock is likely to entrap numerous zircon xenocrysts in the rising process of magma, whose geochronology may provide valuable constrains on the tectonic evolution of the penetrated geological units. Such zircon-bearing mafic rocks play a role just like the role of lithosphere probe. This paper reports a case study of such zircon xenocrysts. The Yushu tectonic mélange in north-central Tibet is a Late Paleozoic one consisting of metamorphosed and deformed continent-derived clastics, metabasalt, diabase dykes, and numerous gabbroic plutons. Its tectonic affinity and tectonic implications remain in controversy mainly due to the absence of precise geochronological data. In order to get the precise time of the mélange, the authors sampled a diabase dyke for zircon separation, which cuts across the basalts with pillow structures. Approximately 130 zircon grains were collected from the diabase sample, 12 and 60 of which were analyzed for U, Th, and Pb isotopic content by employing SHRIMP and LA-ICP-MS techniques, respectively. The results reveal that the zircon U/Pb ages are in a large range of 400 to 2 900 Ma, obviously older than the time of magmatism, suggesting that all the analyzed zircons are xenocrystals entrapped from the country rocks of the diabase. Based on the field relationship and the petrographic nature of the diabase, the authors hold that these xenocrystic zircons came from a crystalline basement at the earlier stage of the magma rising process instead of being captured from the meta-clastics of the Yushu mélange. Thus, the mafic rocks of the Yushu mélange were developed on a continental crystalline basement, probably in a rifting basin. Furthermore, the geochronology of the xenocrystic zircons provides valuable constraint on the thermo-tectonic evolution of the penetrated crystalline basement. The 72 analyses form an age distribution spectrum that exhibits three major age-peaks: 450 to 750 Ma, 850 to 1 400 Ma, and 1 500 to 1?800 Ma, respectively, which is quite comparable with the age distribution spectrum of the Yangtze block. For the zircon grains of each age-peak, internal zoning structure in the zircons with older ages was commonly clearer than those with younger ages. This fact implies that the thermo-tectonic events of the penetrated crystalline basement were all composed of magmatism and associated metamorphism. Although further studies are required, the new data presented here demonstrate that the Yushu mélange was a continent marginal one along the northern margin of the eastern Qiangtang block of Cathaysian affinity instead of a remnant oceanic slice.

Abstract:The Jianchuan picritic porphyrite consists predominantly of olivine, clinopyroxene and plagioclase. In this paper, the authors determined chemical composition of clinopyroxene from the picritic porphyrite and picritic cumulate from Erlong and Manyuan areas in detail by electron microprobe analysis. The results show that most clinopyroxenes in Jianchuan area are composed of augite,which contains relatively low TiO₂(0.62%～2.84%), Al₂O₃(1.86%～5.15%) and Na₂O(0.23%～0.61%). Compared with those in the matrix, clinopyxenes of phenocrysts from Erlong and Manyuan areas are enriched in more magnesium but have poorer iron. The parental magma of the Jianchuan picritic porphyrite should belong to tholeiitic series, characterized by low alkali content, high CaO content and very low oxygen fugacity. During the formation of the clinopyroxene, cationic isomorphic substitution commonly occurs, and the main equations are Mg+Si=Al^Ⅳ + Al^Ⅵ and Mg+Si=Al^Ⅳ+Fe³⁺, in which Ti and Na tend to be incorporated into the titanic augite. The crystallization temperature and pressure of clinopyroxene in Jianchuan picritic porphyrite were estimated by the clinopyroxene_melt equilibrium. The results show that the crystallization temperature (1 324～1?428℃) and pressure (1.5～2.9 GPa) of clinopyroxenes in Erlong and Manyuan picritic porphyrite were predominantly lower than the crystallization temperature (1?439～1 462℃) and pressure (3.0～3.2 GPa) of clinopyroxenes in Manyuan picritic cumulate, and the formation depth of clinopyroxene in Manyuan picritic cumulate might have been larger than that in Erlong and Manyuan picritic porphyrite. Generally, the formation temperature and pressure of the Jianchuan picritic porphyrite were high, suggesting the derivation of materials from a relatively great depth. One possibility is that the rocks were formed by partial melting of lithospheric mantle resulting from upwelling of asthenosphere along the deep faults, whereas the other possibility is the involvement of mantle plume,i.e., the Permian Emeishan mantle plume was reactivated, probably resulting in large_scale magmatic activities. The evidence is as follows: the temperature of the clinopyroxene in Jianchuan picritic porphyrite is moderately higher than that of asthenosphere and the clinopyroxene in Lijiang picrite; the Mg^# both in bulk_rock and in clinopyroxene of Jianchuan picritic porphyrite is close to that in Lijiang picrite, suggesting that Jianchuan picritic porphyrite might have shared a common source region with the Lijiang picrite, or the characteristics of their sources were largely inherited from the Permian plume.

Abstract:Located about 60 km west of the Tuotuohe Town in central Tibet and occurring in the hinterland of Tibetan continent-continent collisional orogen, the Chaqupacha lead-zinc deposit has developed into a major ore deposit in the northern segment of the Sanjiang polymetallic metallogenic belt. In this paper, the authors collated the structures of the ore district on field and microscopic scales, and then analyzed ore-controlling structures. Detailed geological mapping reveals two stages of folding that caused the deformation of the Late Palaeozoic-Early Mesozoic strata, with two Cenozoic thrust nappe systems, i.e., the Tanggula Thrust System (TTS) and the Fenghuoshan-Nangqian Thrust System, superimposed on the folds. Thrust in Chaqupacha has a northward, northwestward, or northeastward dipping plane, dependent on its location, characterized by the thrusting of middle Permian limestone onto late Permian clastic rocks. Miocene Wudaoliang Formation muddy limestone and mudstone occur in the north of Permian limestone, and the strata dip gently or subhorizontally over vast areas, implying that no evident crustal shortening has occurred since Miocene. Field observation and drill data indicate that the contact relationship between Permian and Miocene is unconformity. Besides, normal faults commonly have zigzag geometry in map. These extensional fractures are locally filled with marl of Wudaoliang Formation, where Pb-Zn minerals are well developed. Drilling data reveal that main ore bodies are located at ca. 20 m below the surface and is strictly confined in breccia belts. All Pb-Zn minerals are deposited in open space of the breccia belts, coexistent with marl of Wudaoliang Formation where horizontal stratiform structure is preserved. Such phenomenon suggests that lead-zinc mineralization occurred during the period when the Wudaoliang Formation was deposited.

Abstract:Located in the northeast margin of Qiangtang terrane between Jinshajiang suture zone and Bangonghu-Nujiang suture zone, the Dongmozhazhua and Mohailaheng Pb-Zn deposits in Yushu area of Qinghai Province are typical Pb-Zn deposits in the Cu-Pb-Zn polymetallic mineralization belt within the northern part of the Nujiang-Lancangjiang-Jinshajiang area. Although these Pb-Zn deposits are in the front belt of Yushu thrust nappe system, the connection between Pb-Zn ore deposits and thrust nappe system remain unclear. Field investigations show that calcite is milk white in color, macrocrystalline, irregular and sack-like in form and is intersected by late mineralization. It was formed during the transitional stage from the compression stage (thrust nappe stage) to the extension stage (mineralization stage) and can be regarded as the link between Pb-Zn ore deposits and the thrust nappe system. Formation ages of the transitional stage in the Dongmozhazhua and Mohailaheng ore districts, determined by both Rb-Sr and Sm-Nd isochron methods for calcite, are 35.2～35.5 Ma with an average of 35.4 Ma and 34.0～34.6 Ma with an average of 34.3 Ma, respectively. Formation ages of such a transitional stage are synchronous with Pb-Zn mineralization epoch (35 Ma and 33 Ma, respectively). In combination with regional mineralization geological setting, the authors established a tentative tectonic model for metallogeny of the Pb-Zn deposits from the thrust nappe stage to the mineralization stage in Yushu area, i.e., India-Asian collision led to the lifting of the orogenic belt relative to the Tertiary foreland basins in the northern margins of the Qinghai-Tibetan plateau, and strong compression resulted in the thrusting of these basins (37～40 Ma). During the transitional stage from the compression stage to the extension stage (34～35 Ma), the mixing of early orogenic fluid and basin fluid resulted in the formation of macrocrystalline calcite. At last, fluids generated from the orogen migrated laterally along gently-dipping detachment faults of the thrust systems toward the basins and evolved into Pb-Zn ore-forming fluids through the interaction with country rocks and the mixture of early orogenic fluid. The ore-forming fluids migrated vertically along major thrust faults and finally accumulated in the back thrust induced by thrust-nappe structures at shallow levels to form Pb-Zn ore deposits.

Abstract:The Dongmozhazhua Pb-Zn ore deposit in Yushu area of Qinghai Province is one of the important ore deposits located in the "Sanjiang" sediment-hosted base metal ore belt. In the ore deposit, several types of altera-tion and hydrothermal minerals were recognized in previous studies. However, the spatial distribution of the alterations, the characteristics of element content in the minerals, and the spatial and temporal relationships of different minerals in the orefield remain ambiguous, thus the data available couldn't provide clear ore-searching direction. In view of such a situation, the authors made systematic thin section observations of ore and host rocks, systematic electron-probe micro-analysis (EPMA) of the most important sulfides pyrite and sphalerite, and ICP-MS analysis of important metallic compositions of typical ore rocks, with the purpose of finding detailed scientific information on the above problems. Four hydrothermal alteration types were recognized in the ore deposit, namely dolomitization, pyritization, silicification and baritization. Dolomitization is almost distributed in the whole orefield and occurred from S1 to S4 ore-forming stages, whereas pyritization occurred from S3 to S4 ore-forming stages; in both alterations considerable lead and zinc minerals precipitated, and the pyritization was closely accompanied by Pb-Zn mineralization. Silicification occurred in the same time with pyritization and is mainly distributed to the south of the main thrust fault in the ore deposit. Barite precipitated before and after S3 polymetallic sulfide ore-forming stage, and baritization is mainly developed along the faults south of the main thrust fault. Relatively high content of Cd was observed from the typical ore rocks by ICP-MS. According to EPMA, during S3 ore-forming stage, S and Fe content and atomic ratios of S/Fe in pyrite were higher than their theoretical values, some As and Cu were observed, and the ratio of Co/Ni was usually higher than 1; in sphalerite the S content and the atomic ratios of S/Zn were higher than their theoretical values while the Zn content was lower than their theoretical values, and some Cd and Fe were observed. The atomic ratios of S/Fe and S/Zn in pyrite and sphalerite crystallized from most active hydrothermal fluids were obviously higher than their theoretical values; Micro-sections were studied for spherical pyrite and strawberry and crustal sphalerite in this ore-forming stage and some changes of As, Fe, S and atomic ratios of S/Fe in pyrite and Cd, S and Zn content and S/Zn atomic ratios in sphalerite were observed. During S4 ore-forming stage, the sphalerite had similar element content to that in S3 ore-forming stage, but it had lighter color, lower Zn content and higher atomic ratios of S/Zn. Combining the petrographic characteristics and the EPMA results of pyrite and sphalerite, the authors have reached three conclusions: ① in the S3 ore-forming stage, relative to metal ions, S was enough for sulfide crystallization, and the hydrothermal environment was stable in the early time of this stage;with the precipitation of sphalerite, however, the temperature and activity of S and Ph of the ore-forming fluid decreased while the Cd/Zn ratio rose; ② there were two active centers of hydrothermal fluids in the above ore-forming stage, which were located near Line 0 in the MI ore belt and in the north of MIV belt, respectively; ③ the properties of fluids at S3 and S4 ore-forming stages were different, and the latter stage was characterized by lower temperature and higher sulfur fugacity. Based on all these data, the authors give detailed suggestions on the prospecting for similar Pb-Zn mineralization: ① searching for dolomitization and silicification regionally; ② searching for pyritization in the dolomitization and silicification areas; ③ searching for sphalerite and galena at the positions with most pyrite; ④ searching for small-size but high-grade Pb-Zn ore bodies in the baritization areas along faults, and ⑤ searching for Zn-Pb mineralization in limestones with high content of Cd. Four Pb-Zn mineralization locations worthy of detailed work for further prospecting in the Dongmozhazhua Pb-Zn ore deposit are delineated.

Abstract:The Baiyangping lead-zinc-copper-silver polymetallic deposit in Lanping basin is composed of a few ore veins and can be divided into several ore blocks. Researches on ore geological features and mineralization characteristics of different ore blocks of the ore deposit have been very insufficient. Based on an analysis of data available in combination with field geological survey, it can be concluded that the strata in the ore district comprise mainly Mesozoic sedimentary rocks. Three main cross-cutting fault systems are observed in the ore district, namely nearly north-south-striking faults, NWW-striking faults and NE-SW-striking faults. Ore bodies are evidently controlled by faults in different ore blocks and have no obvious affinity with any special strata and lithology; they primarily occur in the fault zone, with some in fractures, cleavages and space of wall rock collapse. There are breccia ore, massive ore, veinlike ore and disseminated ore, suggesting they were formed by filling epigenetic mineralization. In different ore blocks, mineral composition and association of elements are somewhat different: in Liziping and Wudichang ore blocks, the main minerals include sphalerite, gratonite, galena, jordanite, realgar and orpiment, comprising the Pb-Zn-As-Sb-Ag element assemblage; in Fulongchang ore blocks, the main minerals include sphalerite, jordanite, galena, tetrahedrite series minerals, bournonite, argentite and kongsbergite, composing the Pb-Zn-Cu-Ag element assemblage; in Fulongchang ore blocks, the main minerals include tetrahedrite series minerals, chalcocite, chalcopyrite, jordanite, cobaltine, siegenite, cobalt-bearing arsenopyrite, galena and sphalerite, composing the Cu-Co-As-Zn-Pb element assemblage. A comprehensive analysis reveals that there were two mineralization events in the Baiyangping ore district, one responsible for the main copper mineralization between late Paleocene and early Eocene, the other for main lead and zinc polymetallic mineralization in late Eocene-early Oligocene period. The lead and zinc ore-forming fluids activated early copper element and formed the Pb-Zn-Cu polymetallic assemblage in Fulongchang and Baiyangping ore blocks. The mineralization probably occurred at the stage of short-term stress relaxation after strong extrusion when the wall rocks supplied open space, and ore-forming fluids poured into the space to form ore deposit.

Abstract:Based on the geological and mineral geo-database, the authors analyzed the metallogenic conditions of porphyry molybdenum-copper deposits in northern Sanjiang region by means of GIS. A prognostic model for regional ore deposits was established, and the factors of mineral prediction were extracted and assigned values by Graded-WOE. The favorable prospecting areas were delineated, and the quantitative forecast of regional resources were finally realized. The research included several aspects, and the weighing of prediction included strata, magmatite, structure, mineralized anomalies and other related quantitative factors. Firstly, the geological and mineral geo-database based on the platform of Mapgis and Arcgis of northern Sanjiang region was established, which included regional geological data of the geological map, gravity, air magnetic force measurement, geochemistry, remote sensing and mineral data. This database constitutes the basis of regional mineral prediction. Secondly, the research probed into the metallogenic conditions of porphyry molybdenum-copper deposits in northern Sanjiang region by comprehensive handling of mineral information and analysis of the conditions of mineral deposits and the ore-forming indicators. The research suggested first-level factors of mineral prediction which included strata, magmatite, structure and mineralized anomalies, 19 related factors and 24 relevant characteristic variables. The authors analyzed characteristic variables by Graded-WOE, and then converted them into the values of posterior probability of strata, structure, magmatite and mineralized anomalies. Two favorable areas, i.e., Narigongma-Dongmozhazhua and Aduo-Jieduo, were delineated by the values of posterior probability. There are 5 target areas of Grade-A molybdenum-copper deposits, 4 target areas of Grade-B molybdenum-copper deposits and 4 target areas of Grade-C molybdenum-copper deposits. Using the bundance model, it is estimated that the quantity of copper in this region is 7 412 730 t and that of molybdenum is 8 566 180 t.

Abstract:The podiform chromite deposit constitutes the main source of metallurgical-grade chromite in industry, but its genesis remains one of the problems for which geologists all over the world show great concern. This paper outlined the status and latest progress in the study of podiform chromite deposits. The latest studies show that podiform deposits mainly occur in a certain layer of the mantle peridotite, which has been the lowest part of ophiolite (CMB, crust-mantle boundary) since Phanerozoic. The podiform chromite deposit-bearing mantle peridotites in the world usually exhibit vertical zoning of melting, with more basic material in the upper part and more acid material in the lower part. With the increasing of partial melting from the bottom upward, there appears lherzolite, harzburgite and dunite in turn. Alpine-type podiform chromite deposits generally have a fairly thick dunitic "envelop". After the mineralization of the podiform chromite deposit, the gravity of itself caused the sinking and dragging of some of the dunite around the ore body downward into the harzburegite facies side under the contact interface with the upside dunite facies, thus forming the typical dunitic "envelop" of the Alpine-type chromite deposit. The authors have thus reached the conclusion that the harzburgite facies under the contact interface is the best target area in search for large chromite deposits. Podiform deposits have a very close genetic relationship with harzburgite and dunite; however, it is really rare to find chromite deposits in lherzolite. The abundance of chromium in primitive mantle is much higher than that in the crust, the chromium element of chromitite was derived from primitive mantle itself, mainly coming from the alteration of associated spinel and the incongruent melting of the two pyroxene (chromium diopside and enstatite). With the increase of partial melting, the mantle peridotite gradually evolved in the magnesium-rich direction, and the mineralization of spinel evolved in the magnesium-rich and chromium-rich direction. With the evolution from lherzolite to harzburgite and dunite, the Mg^# values increased significantly. With the decrease of the fusible components of SiO₂, Al₂O₃, CaO in rocks, the refractory components of MgO, NiO, Cr₂O₃ content increased relatively. Meanwhile, with the increase of partial melting, the silicate minerals in mantle peridotite gradually evolved in the magnesium-rich direction, while the mineralization of spinel evolved in the magnesium-rich and chromium-rich direction. The discovery of such ultra-pressure minerals as diamonds, SiC and many other "abnormal mineral groups" in mantle peridotite and podiform Cr-deposits suggests that mantle peridotite was mainly derived from deep mantle (transition zone, or even lower mantle). As Cr has a very priority in occupying the octahedral site in the deep mantle, the authors infer that the primitive Cr-rich minerals in the mantle might have been the Cr-rich octahedral silicate minerals which were not conducive to the formation of Cr-spinel, but Cr was easily released from Cr-bearing minerals to from Cr-spinel which became abundant in the shallow upper mantle environment. The ore bodies of the podiform deposits are very complex in form, with most of them mainly having lenticular and tabular forms, and the host rocks extensively developed cleavages always parallel to the long axis of the ore body, suggesting that podiform Cr-deposits were enriched by the mantle plastic deformation under the plastic condition. The mineralization of podiform chromite deposits is characterized by multi-stage, polygenesis and a variety of tectonic settings. Based on a study of the correlation between Cr^# and Mg^# of Cr-spinel and petromineralogy, the authors hold that the formation of the Cr-deposits underwent the process of preconcentration at the early expansion stage of MOR and the subsequent transformation to the subduction setting, and the expansion of the fore-arc basin above the subduction zone (SSZ) in the intra-oceanic island arc was the most favorable structural environment for the formation of metallurgical-grade chromite ore. Podiform chromite deposits, dunite and boninite are products of the highest melting degree of depleted mantle peridotite. Obviously, boninite could not provide the chromium element for chromite deposits; nevertheless, the more boninitic magma existed outside the mantle peridotite, the more Cr-rich chromite deposits could be formed. Therefore, the size of the mantle peridotite, the degree of partial melting and the existence or nonexistence of boninite can serve as a recognition indicator as to whether the mantle peridotite has the potential of podiform chromite mineralization. As we all know, boninite, which has a very close genetic relation with podiform chromite deposit, is one of the typical rocks that always occur in the forearc basin, so we can reach the conclusion that the podiform chromite deposit is also a typical product in the forarc basin.

Abstract:The Dongqiyishan alkali feldspar granite is the largest rare metal mineralized granite in Beishan area, Iinner Mongolia, and is of remarkable representative significance in similar rock masses. Based on previous research result of petrologic and metallogenetic characteristics, the authors performed a lot of detailed work on geochemistry, isotope geochemistry and geochronology. Petrochemical research shows that the granite is rich in silicon and alkali, and poor in Fe, Ti, Mg and Mn, thus belonging to high-k calc-alkaline series. Petrochemical research shows that the granite is strongly depleted in Eu(δEu＜0.1), Ba, Sr, P, Ti and obviously enriched in Rb, Nb+Ta, W, Mo, Bi, Li, etc. The Rb-Sr isochron age is 128.2±1.1 Ma, with I_Sr being 0.709 79, suggesting that the original rock had obvious crustal characteristics. Relative to values of the depleted mantle, the two types of rocks have somewhat higher ¹⁴⁷Sm/¹⁴⁴Nd values and lower ¹⁴³Nd/ ¹⁴⁴Nd values, and the Nd isotopes of granites are similar to things of Beishan area granites. According to lithological characteristics and formation environment, the authors hold that the Dongqiyishan alkali feldspar granite was formed by the magma derived from partial melting of the upper crust through violent differentiation evolution in the late Yanshanian extrusion-tension environment. First, the alkali-feldspar granite porphyry was formed and constituted the exterior facies of the porphyraceous alkali feldspar granite. The formation environment of the alkali-feldspar porphyraceous granite was relatively closed; it was formed through the further enrichment of the volatile components and rare elements and a slow process of crystallization differentiation. Therefore, the self-metasomatism produced by the residual fluid became more developed, resulted in more close connection of lepidolite mineralization and secondary albitization and silicification to niobium-tantalum, tungsten, tin, rubidium and other rare metals mineralization The results obtained by the authors have some guiding significance in the study of Mesozoic granite-related metal ores of Yanshanian period, especially in the prospecting for niobium-tantalum, tungsten, tin, rubidium and other rare metal deposits.