A New Type of Rare Metal Deposit:The Yushishan Leptynite-Type Nb-Ta Deposit in Eastern Altun,Gansu Province,NW China

The Yusishan deposit is a newly found leptynite-type niobium and tantalum(Nb-Ta)deposit,which is located in the Eastern Altun of Gansu Province.The leptynite of the Neoproterozoic Aoyougou Formation occurs more than 10 km long and 3 km wide.In this paper,we carried out a detailed study of the leptynite and related mineralization.The U-Pb dating of zircon reveals ages of 831±5 Ma and 790±5 Ma for the unmineralized leptynite and 491±4 Ma and 455±4 Ma for the mineralized leptynite.The petrographic and geochemical evidence identified the protolith of the leptynite as alkalin e volcanic rocks that erupted during Neoproterozoic at ca.830 Ma.At ca.490 Ma,the tectono-magmatic and metamorphic event triggered remobilization and enrichment of Nb-Ta as well as other critical metals(REE,Zr,Hf etc)with the formation of industry orebodies in the leptynite strata.The Yushishan deposit presents many similarities with the alkaline volcanic rocks related rare metal deposits in the south Qinling of China and the Brochman,Toogni and Southern Peak Range deposits in Australian.We therefore propose that the Yushishan deposit is a new type(sub-type)of Nb-Ta deposit and termed as leptynite type that represents the metamorphic counterpart of the alkaline volcanic rocks related rare metal deposits.

Geochronology and Geochemistry of the Xingxingxia Triassic A-type Granites in Central Tianshan,NW China:Petrogenesis and Tectonic Implications

The Triassic granitoids in Central Tianshan play a key role in determining the petrogenesis and tectonic evolution on the southern margin of the Central Asian orogenic belt.In this study,we present SHRIMP zircon U-Pb ages,Hf isotopic and geochemical data on the Xingxingxia biotite granite,amazonite granite and granitic pegmatite in Central Tianshan,NW China.Zircon U-Pb dating yielded formation ages of 242 Ma for the biotite granite and 240 Ma for the amazonite granite.These granitoid rocks have high K_(2)O with low MgO and CaO contents.They are enriched in Nb,Ta,Hf and Y,while being depleted in Ba and Sr,showing flat HREE patterns and negative Eu anomalies.They have typical A-type granite geochemical signatures with high Ga/A_(1)(8–13)and TFeO/(TFeO+MgO)ratios,showing an A_(2) affinity for biotite granite and an A_(1) affinity for amazonite granite and granitic pegmatite.Zircon ε_(Hf)(t)values of the granitoids are 0.45–2.66,with Hf model ages of 0.99–1.17 Ga.This suggests that these A-type granites originated from partial melting of the lower crust.We propose that Xingxingxia Triassic A-type granites formed under lithospheric extension from post-orogenic to anorogenic intraplate settings and NE-trending regional strike-slip fault-controlled magma emplacement in the upper crust.

Metallogenic Epoch and Ore-forming Environment of the Lamasu Skarn-porphyritic Cu-Zn deposit,western Tianshan,Xinjiang,NW China

Granitic rocks, widely developed in the Lamasu copper ore region, western Tianshan were formed at 390.5±7.7 Ma according to the SHRIMP zircon U-Pb dating of the plagioclase granite porphyry. Based on the regional tectonic evolution and published chronological data of both diagenesis and mineralization, the Biezhentao- Kokirqin region was rolled into the orogen associated with the closure of Yili Ocean during early Devonian. The N-S-trending thrust faults were formed during this period and accompanied by the intrusion of granitic rocks. On this stage, the paleo-Asian Ocean Plate entered into the early collision orogenic phase and the plagioclase granite porphyry intruded （390.5±7.7 Ma） and replaced with limestone of the Mesoproterozoic Kusongmuqieke Group, Jixianian System and formed the early phase of skarn-type copper mineralization in the Lamasu region. Furthermore, the subduction-melting of Bayingou Ocean Plate during Carboniferous generated a deep-seated magmatic chamber in the Lamasu copper ore region which located in the northwestern part of the Paleozoic Biezhentao-Kokirqin island arc. The magmatic chamber segregated Cu-bearing magmas, which transported upward to the shallow earth crust along the faults or fractures and formed the Cu-hosting porphyry. According to the research on the characteristics of the ore deposit and the ore-forming environment as mentioned above, the Lamasu Cu-Zn deposit was characterized by the superposing of mineralization at different geological settings and it was skarn-porphyritic type.

Carboniferous Alaskan-type complex along the Sino–Mongolian boundary,southern margin of the Central Asian Orogenic Belt

We present zircon ages and geochemical data for the Hongshishan Carboniferous Alaskan-type mafic–ultramafic complex exposed in the Beishan area along the Sino–Mongolian boundary, southern margin of the Central Asian Orogenic Belt. This complex mainly consists of dunite,harzburgite, lherzolite, wehrlite, and gabbro, which intrudes Early Carboniferous volcanic rocks and reveals a zoned structure. Zircons of a gabbro sample yielded a 206Pb/238 U age of 357 ± 4 Ma, reflecting the time of Early Carboniferous magmatism. Zircon ages were also obtained for an andesite(322 ± 3 Ma) and a basaltic andesite(304 ± 2 Ma).High initial Nd isotope whole-rock values suggest that the Hongshishan gabbro [e_(Nd(t))= +9.6-+10.2] and basalt[eNd(t)= +10.0-+10.8] were derived from a depleted mantle source. Slightly lower eNd(t)values for the ultramafic rocks [eNd(t)= +8.5-+8.7] suggest some interaction of the parental magma with the continental crust. In contrast, the Late Carboniferous Quershan samples in this area represent subduction-related arc volcanic rocks with Adakite-like compositions. The early Carboniferous Hongshishan Alaskan-type complex was interpreted to represent the remnants of a magma chamber that crystallized at the base of a mature island arc, whereas the Quershan island arc volcanic rockssuggest the resurrection of the subduction process after arccontinent collision and uplift of the roots of the arc.

Genesis of the Changba–Lijiagou Giant Pb-Zn Deposit, West Qinling, Central China: Constraints from S-Pb-C-O isotopes

The extensive Changba-Lijiagou Pb-Zn deposit is located in the north of the Xihe–Chengxian ore cluster in West Qinling. The ore bodies are mainly hosted in the marble, dolomitic marble and biotite-calcite-quartz schist of the Middle Devonian Anjiacha Formation, and are structurally controlled by the fault and anticline. The ore-forming process can be divided into three main stages, based on field geological features and mineral assemblages. The mineral assemblages of hydrothermal stage I are pale-yellow coarse grain, low Fe sphalerite, pyrite with pits, barite and biotite. The mineral assemblages of hydrothermal stage II are black-brown cryptocrystalline, high Fe shalerite, pyrite without pits, marcasite or arsenopyrite replace the pyrite with pits, K-feldspar. The features of hydrothermal stage III are calcite-quartz-sulfide vein cutting the laminated, banded ore body. Forty-two sulfur isotope analyses, twenty-five lead isotope analyses and nineteen carbon and oxygen isotope analyses were determined on sphalerite, pyrite, galena and calcite. The δ34 S values of stage I(20.3 to 29.0‰) are consistent with the δ34 S of sulfate(barite) in the stratum. Combined with geological feature, inclusion characteristics and EPMA data, we propose that TSR has played a key role in the formation of the sulfides in stage I. The δ34 S values of stage II sphalerite and pyrite(15.1 to 23.0‰) are between sulfides in the host rock, magmatic sulfur and the sulfate(barite) in the stratum. This result suggests that multiple S reservoirs were the sources for S2-in stage II. The δ34 S values of stage III(13.1 to 22‰) combined with the structure of the geological and mineral features suggest a magmatic hydrothermal origin of the mineralization. The lead isotope compositions of the sulfides have 206 Pb/204 Pb ranging from 17.9480 to 17.9782, 207 Pb/204 Pb ranging from 15.611 to 15.622, and 208 Pb/204 Pb ranging from 38.1368 to 38.1691 in the three ore-forming stages. The narrow and symmetric distributions of the lead isotope values reflect homogenization of granite and mantle sources before the Pb-Zn mineralization. The δ13 CPDB and δ18 OSMOW values of stage I range from-0.1 to 2.4‰ and from 18.8 to 21.7‰. The values and inclusion data indicate that the source of fluids in stage I was the dissolution of marine carbonate. The δ13 CPDB and δ18 OSMOW values of stage II range from-4 to 1‰ and from 12.3 to 20.3‰, suggesting multiple C-O reservoirs in the Changba deposit and the addition of mantle-source fluid to the system. The values in stage III are-3.1‰ and 19.7‰, respectively. We infer that the process of mineralization involved evaporitic salt and sedimentary organic-bearing units interacting through thermochemical sulfate reduction through the isotopic, mineralogy and inclusion evidences. Subsequently, the geology feature, mineral assemblages, EPMA data and isotopic values support the conclusion that the ore-forming hydrothermal fluids were mixed with magmatic hydrothermal fluids and forming the massive dark sphalerite, then yielding the calcite-quartz-sulfide vein ore type at the last stage. The genesis of this ore deposit was epigenetic rather than the previously-proposed sedimentary-exhalative(SEDEX) type.

Petrogenesis and tectonic significance of the early Paleozoic Delenuoer ophiolite in the Central Qilian Shan, northeastern Tibetan Plateau

The newly discovered early Paleozoic Delenuoer ophiolite,in the western margin of the Central Qilian Shan,is composed of serpentinized peridotite,cumulate gabbro,diabase,massive basalt,and pillow basalt.This study presents geochronological and geochemical data for the cumulate gabbro and basalt.LA-ICP-MS U-Pb dating of zircons from the cumulate gabbro yielded a magmatic crystallization age of 472±4 Ma.The basalts have normal mid-ocean ridge basalt(N-MORB)compositions and a narrow range ofεNd(t)values(+4.5 to+5.3),which indicates they were derived from a depleted mantle source.On the basis of regional geological constraints,it is proposed that the Delenuoer ophiolite is a westward extension of the South Ophiolite Belt(Yushigou-Youhulugou-Donggou-Dongcaohe Ophiolite Belt)in the North Qilian Shan.The Delenuoer ophiolite,along with the Gulangxia-Delenuoer fault,defines the westernmost part of the tectonic boundary between the North and Central Qilian Shan.This ophiolite may have formed during southward subduction of the Qilian Ocean slab during the early Paleozoic.

The Effects of Applying the Association of Al,K,and Sn in Stream Sediments for Gold Exploration in Different Landscapes

Geochemical data of altered wall rocks are important for the metallogenic prognosis of hydrothermal gold deposits. Indicator elements of altered wall rocks such as K, AI, As, Sb, and Hg have been successfully used to assess gold anomalies in lithogeochemicai survey. However, such researches have rarely been done in stream sediment survey for the exploration of gold from various landscapes. On the basis of the geochemical analysis of altered wall rocks of gold deposits in the cold desert areas of Gansu （甘肃） Province in Northwest China, it is found that the combination of AI, K, and Sn could serve as an important indicator of hydrothermal gold deposits and can be used to evaluate the metallogenic prospective of gold anomalies in stream sediments. More studies performed in the cold grassland areas and the moderate-low relief mountainous areas showed that, both weak and strong geochemical anomalies can be extracted, if strictly abiding by the ways of calculation and addition of the binary values of the indicator elements with equal weight, and this provides the sound delineation of metallogenic perspective areas.

The quantitative classification of granites and their metallogenetic relations in West Qinling,Gansu Province,China

The quantitative classification of granite and their metallogenetic relations have never been discussed.The Q-system clustering analysis and discriminant analysis methods were alternately used to quantitatively analyze the 11 oxide data in granite samples from the West Qinling area of Gansu Province,and then to construct the quantitative classification series models of granite(oxide).The granites samples are divided into three categories and eight subcategories.The classification of granites is biased toward prospecting.According to the spatial correlation between eight types of granites and copper deposits,lead and zinc deposits,gold deposits,etc.(within 3 km of the intrusion)in the West Qinling area in Gansu Province,the“metallogenic related intrusions”are sought,and the prospecting target areas are defined.Furthermore,they provide reliable basis for regional geological prospecting.

The first discovery of Permian conodont fauna from peri-Gondwana cool water facies in Tibet,China

The Angjie Formation and Xiala Formation,present in the Shiquanhe area of Gar County in the western part of Gangdise,Tibet,belong to the Gangdise stratigraphic subregion. Conodonts have been found in the Angjie Formation,and they permit an age determination of Early to Middle Permian for that formation. The age of the Xiala Formation could be late Middle Permian. Whether the Late Permian marine deposits are present in this area still needs to be determined,but it is possible that the lower part of the Xiala Formation overlaps partly the upper part of the Angjie Formation. More importantly,the study has brought about a finding of typical peri-Gondwana cool water facies conodonts,namely,Vjalovognathus sp. nov. x. This is the first report and brief description of a conodont fauna from peri-Gondowana cool water facies in China. It indicates that the Gangdise stratigraphic subregion can be subdivided;the western part belongs to peri-Gondwana cool water facies,and the eastern part belongs to Tethys.

Blooming of bacteria and algae is a biokiller for mass-extinction of Devonian coral-stromatoporoid reef ecosystems

Studies of rock slices showed that there were many kinds of symbioses between bacteria and algae and corals-stromatoporoids in the coral-stromatoporoid reefs from the Devonian Ganxi section of Sichuan and the Dushan section of Guizhou in South China. They included encrustations, microborings, bioclaustration, etc. In the host corals-stromatoporoids invaded by bacteria and algae were many residues of dead soft issue in the infected parts, where the skeletal structures were injured. Therefore, we considered there were direct interactions between corals-stromatoporoids and bacteria and algae in coral-stromatoporoid reefs, which included that bacteria and algae blocked growth of corals-stromatoporoids and the latter had the ability of self-healing. And the bacteria and algae usually was the active side. In the Middle Devonian with normal seawater condition, corals and stromatoporoids had the ability to resist the invasion of bacteria and algae, and the host coral-stromatoporoids would not be killed; but in the Late Devonian with deterioration of seawater quality, the ability of bacteria and algae infection increased while corals-stromatoporoids' ability to resist infection declined, and therefore the host corals-stromatoporoids would be dead. Hence we suggested that the invading of bacteria and algae was a possible biokiller for mass-extinction of the Devonian coral-stromatoporoid reefs ecosystem. Beyond that, blooming of bacteria and algae and its triggering cumulative environmental effects played an important role in the reduction and extinction of metazoan in the Late Devonian. Furthermore, it can be used as a useful example to learn the trend and the reasons for the disease and decrease of modern coral reefs.