2019 Vol. 30, No. 1
The paper presents an overview of the relationships between the interior structures of tectonic terranes and the distribution of tectonic-metallogenic zones in Southeast Asia. Episodic tectonic activities occurred in this archipelagic area, generating metallogenic belts in multi-terranes. Since the Late Paleozoic, opening and closure of the Paleotethys and Neotethys led to multiple suture zones between different blocks, mainly between the Indochina terrane, the Nambung terrane, the Sibumasu terrane and the West Myanmar terrane. During the Mesozoic to Cenozoic, the formation of accreted terranes and their related islands was caused by subduction and collision processes between the Pacific and Australian plates toward the Eurasian Continent, forming Sundaland and its affiliated islands, the Philippines and its subsidiary islands, the Papua New Guinea terrane and its related islands and the Sunda epicontinental arc system. Within the margin of terranes resulted in the structural transfer zones, their secondary tectonic units can be divided into island arc belts, back-arc basins, suture zones, marginal fold belts and orogenic belts. The metallogenic assemblages are mainly distributed within these structural zones of the terranes. According to the relationship between these tectonic units and the distribution of mineral resources, the tectonic-metallogenic belts can be divided into 24 metallogenic belts in Southeast Asia. They are characterized by a diversity and frequency of metallogenic material combination which is likely to reflect the complexity of the material distribution during mineralization processes, mostly by the structural transformation during the dissociation-convergence process between multiple terranes. Therefore, the formation of ore deposits was not only restricted by the evolution (opening and closure) of Paleo-and Neotethys, but may also be controlled by the interaction of the terranes with different tectonic attributes which provided multiple sources of metallogenic material.
The Payangazu complex in the central Myanmar is composed mainly of quartz diorite, granodiorite, and some synplutonic mafic dikes. The quartz diorite and granodiorite have zircon U-Pb ages of 130.5±4.0 (MSWD=3.5) and 118.4±2.5 Ma (MSWD=2.4), respectively. Rock samples of the quartz diorite and granodiorite are metaluminous, enriched in large-ion lithophile elements like LREE, Rb, Th, and U, and depleted in high field-strength elements such as HREE, Nb, Ta, P, and Ti, indicative of arc-type magmatic affinities. Whole rock samples of the quartz diorite have εHf(t) value of +0.6, initial 87Sr/86Sr ratios of 0.708 6 to 0.710 0, and εNd(t) values of -4.8 to -4.9; whereas rocks of the granodiorite are relatively isotopically enriched, with εHf(t) values of -5.1 to -7.2, initial 87Sr/86Sr ratios of 0.711 7 to 0.711 8, and εNd(t) values of -8.7 to -8.8. The isotopic data together with the high Mg# (both the quartz diorite and granodiorite have Mg# values of >40) suggest a strong involvement of mantle materials in the genesis of the parent magmas. The possible petrogenetic process may be that the ascending of melts from partial melting of metasomatized mantle wedge triggered by dehydration of subducted slab resulted in partial melting of the lower crust and mixed with the latter. These Early Cretaceous intrusions from the complex are older than those found in the eastern Wuntho-Popa arc in western Myanmar, eastern Himalaya, and western Yunnan which are interpreted to be related to the Neo-Tethyan subduction, and have εNd(t), εHf(t) values lower than the latter. On the contrary, the ages and geochemical characteristics of the Payangazu complex are consistent with some of the intrusions in the northern magmatic belt in Tibet, eastern Himalaya, and western Yunnan which are believed to be associated with the subduction of the Bangong-Nujiang Ocean crust. Thus, we propose that the Early Cretaceous intrusions in the central Myanmar are most likely related to the southward subduction of an ocean slab that was possibly an extension of the Bangong-Nujiang Ocean.
An integrated study of zircon U-Pb geochronology and petrochemistry, together with zircon Lu-Hf isotopes, has been carried out on the basaltic-andesitic tuff and volcanic breccia from the Nam Hang Formation and andesitic tuff from the Muang-Nan Formation in the Xaignabouli area, which had been mapped as the Permian-Early Triassic on the 1:1 000 000 geological map or Late Carboniferous on the 1:200 000 geological maps. Zircon U-Pb dating of three samples yielded weighted mean ages of 235±2.6, 232±1.4 and 278±2.8 Ma, respectively, suggesting a Late Triassic origin for the Nam Hang Formation and an Early Permian origin for the Muang-Nan Formation. Geochemically, they are characterized by depletions in HFSEs (e.g., Nb, Ta, Ti) and high LILE/HFSE ratios, and they have positive zircon εHf(t) values of 8.7-15.9, which exhibits the continental arc volcanic affinity and partial melting of subducting oceanic slab in the magma source. Combined with spatial occurrence of the volcanic rock and existing geochronological and geochemical data, we suggest that the Xaignabouli-Luang Prabang volcanic belt can be linked to the Loei-Phetchabun belt. The Permian-Triassic volcanic rocks in this belt might be a product of the Nan back-arc basin eastward subduction.
Piaoac granites exposed in the Cao Bang region, northern Vietnam, are S-type granite, which are associated with W-Sn-Mo-Be-F mineralization. Zircon U-Pb ages, major and trace elements, mineral chemical and Hf isotopic compositions of the W-Sn-bearing granites from the Piaoac District have been investigated in detail. LA-ICP-MS U-Pb dating of zircon grains from these granites yielded ages of 82.5±2.3 and 82±1.8 Ma, representing an episode of Late Cretaceous magmatic event. These granites are characterized by high peraluminous and have typical S-type geochemical signatures with high SiO2 (72.37 wt.%-73.07 wt.%), high A/CNK values (1.61-1.65) and Al2O3 (14.4 wt.%-15 wt.%). They are enriched in Rb, U, K, Th, Ta and Pb and display pronounced negative Ba, Sr, Nb, Ti and Eu (Eu/Eu*=0.19-0.24) anomalies. The high degree of fractional crystallization is characterized by low Rb, Sr, Ba and Eu concentrations with high ratios of La/Sm and Eu/Eu*. Zircon grains show εHf(t) values from -9.69 to -0.9 and the corresponding TDM2 range from 1.2 to 1.7 Ga, indicating that these granites could be derived from the Proterozoic basement rocks with minor input from mantle-derived magmas. The calculation of Fe3+ and Fe2+ of biotites indicates a low oxygen fugacity condition (log fO2 ranging from 10-17 to 10-18 bars, below MH), which is favorable for the W-Sn mineralization. Tungsten and tin have been enriched in granitic magmas through fractionation, and low oxygen fugacity conditions have promoted the accumulation and transportation of W-Sn in the hydrothermal fluids, leading to deposition of mineral phases. The geochemical data suggest that Piaoac granites formed in an extensional setting related with the Late Cretaceous magmatism occurring large-scale lithospheric extensional in South China Block.
The Hermyingyi W-Sn deposit, situated in southern Myanmar, SE Asia, is a typical quartz-vein type W-Sn deposit. The ore-bearing quartz veins are mainly hosted by the Hermyingyi monzogranite which intruded into the Carboniferous metasedimentary rocks of Mergui Series. According to mineral assemblages and crosscutting relationships, four ore-forming stages are recognized:(1) silicate-oxide stage; (2) quartz-sulfide stage; (3) barren quartz vein stage; (4) supergene stage. Five molybdenite samples from the deposit yield Re-Os model ages ranging from 67.8±1.6 to 69.2±1.6 Ma (weighted mean age of 68.7±1.2 Ma), and a well-defined isochron age of 68.4±2.5 Ma (MSWD=0.18, 2σ). This Re-Os age is consistent with the previously published zircon U-Pb age of the Hermyingyi monzogranite (70.0±0.4 Ma) (MSWD=0.9, 2σ) within errors, which indicates a genetic link between the monzogranitic magmatism and W-Sn mineralization. The new high-precision geochronological data reveal that the granitic magmatism and associated W-Sn mineralization in southern Myanmar took place during the Late Cretaceous (70–68 Ma). The extremely low Re contents (22.9 ppb to 299 ppb) in molybdenite, coupled with sulfide δ34S values in the range of +1.9‰ to +5.6‰ suggest that ore-forming metals were predominately sourced from the crustal-derived granitic magma.
The Phapon gold deposit, located in northern Laos, is a unique large-scale gold deposit in Luang Prabang-Loei metallogenic belt. It is hosted in the Lower Permian limestone and controlled by a NE-trending ductile-brittle fault system. There are three types of primary ore including auriferous calcite vein type, disseminated type, and breccia type, and the first two are important in the Phapon gold deposit. Based on fluid inclusion petrography and microthermometry, three types of primary fluid inclusions including type 1 liquid-rich aqueous, type 2 vapor-rich aqueous and type 3 daughter mineral-bearing aqueous were identified in hydrothermal calcite grains. The ore-forming fluids are normally homogeneous, as indicated by the widespread type 1 inclusions with identical composition. The coexistence of type 1 and type 2 inclusions, showing similar final homogenization temperature but different compositions, indicate that fluid immiscibility did locally take place in both two types of ores. The results of microthermometry and H-O isotopes geochemistry indicate that there are little differences on ore-fluid geochemistry between the auriferous calcite vein-type and disseminated type ores. The ore-forming fluids are characterized by medium-low temperatures (157-268℃) and low salinity (1.6 wt.%-9.9 wt.% NaCl eq.). It is likely to have a metamorphic-dominant mixed source, which could be associated with dehydration and decarbonisation of Lower Permian limestone and Middle-Upper Triassic sandstones during the dynamic metamorphism. The fluid-wallrock interaction played a major role, and the locally occurred fluid-immiscible processes played a subordinate role in gold precipitation. Combined with the regional and ore deposit geology, and ore-fluid geochemistry, we suggest that the Phapon gold deposit is best considered to be a member of the epizonal orogenic deposit class.
Xinzhai sandstone-type copper deposit located in northern Laos lies in the Jiangcheng-Phongsaly-Phrae Mesozoic basin (JPMB), which is regarded as southern extension of the Lanping-Simao Mesozoic basin in China. The copper deposit belt is bounded by the Ailaoshan-Heishui River fault and the Dian Bien Phu-Luang Prabang fault at the east and Lancang River-Bannankan faults at the west. Two types of orebodies are identified in the Xinzhai area based on geological investigation. One is lamellar copper orebody hosted by the fine lithic feldspar sandstones and feldspar lithic sandstones; another is vein-type orebody. The sulfur isotopic compositions of the chalcopyrite and tetrahedrite are from -11.6‰ to -1.8‰, indicating that sulfur is derived from bacterial sulfate reduction (BSR). δD values of fluid inclusions in ore-bearing quartz samples are from -99‰ to -78‰. The calculated δ18OH2O values of ore-forming fluid vary from -2.3‰ to 0.4‰ using the quartz-water fractionation equations and the mineralization temperature. Oxygen and hydrogen isotopic compositions show that the ore-forming fluid was derived from basin fluid. Rock-mineral identifications show that both of the mineral grain maturity and the structural maturity are high in the Jurassic Huakaizuo Formation, reflecting a far-source accumulation and lake facies sedimentatary environment. Based on tectonic determination diagram of the Al2O3/SiO2-TFe+MgO, the sandstone samples collected from the Huakaizuo Formation were plotted in the passive continental margin. The collision of the Indian and Eurasian blocks during the Cenozoic formed large-scale strike-slips and thrust nappe structures in margin of the basin. With the tectonic movement, Cu-rich basin fluid from the basement of basin migrated upward along the contemporaneous fault and into the high porosity strata. At the same time, in organic matter-riched condition, bacterial sulfate reduction (BSR) has been triggered, forming a large number of S2- ions, and then precipitation of sulfide started. This mechanism describes the process of copper mineral deposition in the Xinzhai deposit.
The Truong Son metallogenic belt in central Laos and Vietnam is an important Fe-Cu-Sn-Au polymetallic ore district. The Pha Lek Fe deposit is closely related to Late Carboniferous-Early Permian Ⅰ-type granitic magmatism, and contains >50 Mt@45% to 50% of Fe ore. Ore minerals occur mainly as magnetite and hematite in the skarn alteration zone between a granitic pluton and metamorphosed Middle-Upper Devonian carbonates. The granitic pluton comprises granodiorite and granite, with zircon U-Pb dating indicating synchronous emplacement at 288.2±1.3 and 284.9±1.2 Ma, respectively. Zircons from these granitoids have εHf(t) values of 2.9-11.2 and relatively young TDM2 ages (< 1.0 Ga), indicating an origin by partial melting of depleted mafic crust or magma mixing. Previous studies have shown that these granitoids have high Y, Yb, and K2O contents, and low Sr and Na2O contents, which are interpreted as the melting of mafic continental crust. Pyrite of the main mineralization stage yields an 187Re/188Os-187Os/188Os isochron age of 287±17 Ma, indicating that mineralization is associated with Pha Lek granitic magmatism. A Late Carboniferous-Early Permian subduction-related skarn-type Fe mineralization model is proposed for the Pha Lek deposit. More evidence is needed to verify a hypothesis of volcanic overprinting during Late Triassic post-collisional extension.
The Boloven bauxite deposit occurs either in the weathered basalt (alkali basalt and tholeiite), or in the Cretaceous sandstone. It is generally agreed that the bauxite deposits/laterites overlying the alkali basalt and tholeiite were derived from weathering of underlying basalt, however, the origin of bauxite deposit overlying the sandstone remains controversial. Chondrite-normalized REE patterns show that the bauxite ores/laterites overlying the sandstone exhibit quite similar chondrite-normalized REE patterns to those overlying the alkali basalt. Diagram of Al-Ti-Zr shows that the bauxite ores/laterites overlying the sandstone, tholeiite and alkali basalt are close to each other and to the calc-alkaline suite, however, significantly different from the sandstone and shales. Binary diagram of log Nb/Y vs log Zr/Ti further indicates that the parent rocks of bauxite ores/laterites overlying the sandstone belong to the suit of ultra-alkali to alkali basalt. Multivariate statistical analysis of geochemical data exhibit that the geochemical characteristics of HREE, Y, LREE and Al2O3 for bauxite ores/laterites overlying the sandstone are similar to those overlying the alkali basalt (15.7 Ma), obviously different from those overlying the tholeiite (1.2 to 0.5 Ma). Consequently, it can be inferred that the bauxite deposits/laterites overlying the sandstone were derived from the alkali basalt.
The Anjing Hitam Pb-Zn deposit in northern Sumatra (Indonesia) is one of the largest Pb-Zn deposits in the region. The stratiform orebodies are mainly hosted in the middle member of the Carboniferous-Permian Kluet Formation of the Tapanuli Group. Mineral paragenesis and crosscutting relationships suggest a two-stage Pb-Zn mineralization:(Ⅰ) sedimentary and (Ⅱ) hydrothermal mineralization. Ore-related calcite from both stages Ⅰ and Ⅱ contains mainly liquid-and gas-liquid two-phase-type fluid inclusions (FI). For stage Ⅰ ore-forming fluids, FI homogenization temperatures (Th) are 105 to 199℃, and the salinities are 9.6 wt.% to 16.6 wt.% NaCleqiv, reflecting low temperature and medium-low salinity; whereas in stage Ⅱ, the Th (206 to 267℃) and salinity (19.0 wt.% to 22.5 wt.% NaCleqiv) are considerably higher. Fluid inclusion and C-O isotope characteristics suggest that the stage Ⅰ ore-forming fluids were mainly derived from a mixture of seawater and magmatic fluids (probably from deep-lying plutons), whereas the stage Ⅱ ore-forming fluids were likely magmatic-derived with wall rock input. We propose that the Anjing Hitam deposit was a Carboniferous exhalative sedimentary (SEDEX) deposit overprinted by the Pleistocene vein-style magmatic-hydrothermal mineralization.
Carbonaceous debris (CD) is widely distributed in the sandstone of the Daying Uranium Deposit, northern Ordos Basin, and coexists with uranium minerals, which provides a favorable case for studying their relationship. Vitrinite reflectance (VR), macerals, moisture, volatile matter, ash, total sulfur (St) and uranium concentration of CD within the sandstone were studied. The results show that VR ranges from 0.372%Ro to 0.510% Ro with an average value of 0.438%Ro, indicating that CD is in the stage of lignite. The contents of vitrinite (V), inertinite (I) and minerals range from 83.18%-99.48%, 0-7.70%, and 0.34%-15.72%, respectively, with the corresponding average value of 95.51%, 1.34%, and 3.15%, respectively which indicates that V is the major maceral. Moisture on air dried basis (Mad), volatile matter yield on dry, ash-free basis (Vdaf), ash yield on dried basis (Ad) and St mostly range from 7.95%-16.09%, 44.70%-66.54%, 4.84%-26.24% and 0.24%-1.12%, respectively, while their average values are 12.43%, 53.41%, 16.57% and 0.77%, respectively. It suggests that CD is of medium-high moisture, super-high volatile matter, low-medium ash and low sulfur. Uranium concentration ranges from 29 ppm to 92 ppm with an average value of 50 ppm, and uranium concentration increases with the decreased distance to CD. On the whole, Mad and Vdaf decrease with increasing burial depth, which indicates that CD experienced the burial metamorphism. However, Mad and Vdaf obviously decrease in uranium-rich areas whereas Ad and St noticeably increase. Comprehensive studies suggest that there is a certain relationship between uranium enrichment and CD. CD in the stage of lignite helps the adsorption of uranium. On one hand, radioactivity uranium enrichment makes organic matter maturation increase with a decrease in moisture and volatile matter. On the other hand, an increase in organic matter maturation, caused by radioactivity uranium enrichment, results in an increase in uranium minerals, which is instructive in the study of regional uranium mineralization and metallogenic regularity.
Studies on basin fills have provided significant insights into reservoir distribution and prediction in petroliferous basins, however, the effect of basin fills on source rock properties has been underexplored. This paper documents basin filling characteristics and their implications for depositional processes and heterogeneity of source rock in the Qingnan subsag of the Jiuquan Basin, by using subsurface geological data from recent hydrocarbon exploration efforts in this area. Drill core data reveals that the basin fill of the Qingnan subsag was dominated by fan delta-lacustrine systems, in which deposition of the fan deltas along the basin margin was mainly through gravity flows. The temporal and spatial evolution of the depositional systems indicates that the basin fill was characterized by a continuously retrogradational process, with decreasing extent of fan deltas in vertical succession. Weakening of tectonic activities and climate change from humid to semi-arid are interpreted to be the main control factors that were responsible for the retrogradational basin fill. The different depositional environments in the early stage and late stage of the retrogradational basin filling history resulted in the different depositional processes and properties of source rocks. This study suggests that source rock heterogeneity associated with basin fills in lacustrine basins should be considered in hydrocarbon exploration.
Detailed petrographic, geochemical (O-C-Sr isotopes) and fluid inclusion studies of the deeply buried Cambrian carbonates in the West-central Tarim Basin revealed three types of crystalline dolomites (fine-crystalline, nonplanar-a(s), dolomite (RD1), fine-to medium-crystalline, planar-e(s) dolomite (RD2), and medium-to coarse-crystalline, nonplanar-a dolomite (RD3)), medium-to coarse-crystalline, nonplanar-a saddle dolomite cement (CD) and early and later-stage calcite cement. The occurrence of RD1 along low-amplitude stylolites points to link with pressure dissolution by which minor Mg ions were likely released for replacive dolomitization during early-to intermediate-burial seawater dolomitization. The increasing crystal sizes of RD2 and RD3 with irregular overgrowth rims suggests intense recrystallization and replacement upon the RD1 or remaining precursor limestones by dolomitizing fluids during late intermediate burial dolomitization. The overlap of δ18O, δ13C and 87Sr/86Sr values of RD1-RD3 and CD dolomite with coeval seawater values, suggests that the principal dolomitizing fluids that precipitated these dolomites was connate (Cambrian) seawater preserved in the host limestones/dolomites. Their high 87Sr/86Sr ratios suggest influx of radiogenic strontium into the Cambrian seawater. Two regimes of fluid flow are recognized in the study area:firstly, influx of magnesium-rich higher-temperature basinal brines along deep-seated faults/fractures, resulting in cementation by CD dolomite. Secondly, the incursion of meteoric waters, mixing with ascending higher-temperature basinal brines, and an increase in Ca2+/Mg2+ ratio in the fluids probably results in the precipitation of calcite cement in vugs and fractures.
To improve the success rate of locating hydrocarbon reservoirs in pre-Cenozoic inland compressional basins, taking the Early Yanshanian succession of eastern Yihezhuang salient as an example, this paper studied the sedimentary facies and sequence stratigraphic patterns. First, through seismic profiles, well logs, cores and outcrops, the sequence framework was established and internal sedimentary facies were identified. Further, according to analysis of single-wells and connecting-wells, the vertical evolution and horizontal distribution of sedimentation inside the sequence frameworks were discussed. The following results were acquired:(1) meandering river characterized by dual structures superposing each other was developed, and the dual structures can be further divided into three kinds; (2) the entire Early Yanshanian succession was interpreted as one first-order sequence, composed of three third-order sequences, including SQ-Fz1, SQ-Fz2 and SQ-St from bottom to top. Each third-order sequence can be further divided into three system tracts; (3) in different system tracts, different types of dual structures developed separately, and sedimentary bodies showed different horizontal distribution scales and vertical superposition patterns. Finally, the model of sequence stratigraphic patterns was established. This study enhanced the use of sequence stratigraphy to inland tectonically active basins, and would be helpful to predict reservoirs in pre-Cenozoic residual basins.
An accurate and detailed seismic landslide inventory is essential to better understand the landslide mechanism and susceptibility. The 8th August 2017 MW 6.5 Jiuzhaigou Earthquake of China initiated a large number of coseismic landslides. The results of the post-seismic survey show the actual landslide number might be underestimated in previous publications. Coupled with field investigation and visual interpretation on high-resolution remote sensing images before and after the main shock, we established a detailed inventory of landslides triggered by the earthquake. Results show that this event caused at least 4 834 individual landslides with a total area of 9.64 km2. They are concentrated in an elliptical area of 434 km2, dominated by medium-and small-scale rock falls and debris slides. Statistics indicate that, except for slope aspect that seems not significantly correlated with the landsliding, these landslides are most common in the places with following features:elevation of 2 800-3 400 m, slope angle greater than 30°, slope positions of upper, middle and flat slopes, and Carboniferous limestone and dolomite. Besides, the landslide area percentage (LAP) and landslide number density (LND) values decrease with the increasing distance to river channels and roads, implying a positive correlation. Instead of centering around the epicenter, most of these coseismic landslides are distributed along the inferred seismogenic fault, which means that the seismogenic structure played a more important role than the location of the epicenter. Remarkable differences in landslide densities along the fault indicate the varied landslide susceptibility which may be attributed to other varied controls along the fault such as the rock mass strength. In sum, this study presents a more detailed inventory of the landslides triggered by the 2017 MW 6.5 Jiuzhaigou Earthquake, describes their distribution pattern and analyzes its control factors, which would be helpful to understand the genesis of the coseismic landslides and further study their long-term impact on the environment of the affected area.
Lherzolite is one of the most important components of the subcontinental mantle lithosphere, and the study of its heat transfer properties aids in understanding the thermal structure of the continental mantle lithosphere. Currently, few studies have examined the heat transfer properties of lherzolite, and the experimental results remain controversial. This experiment utilized a pulse method to measure the thermal diffusivity of lherzolite at pressures ranging from 1.0 to 4.0 GPa and temperatures from 300 to 1 073 K on a cubic press apparatus. We obtained a thermal diffusivity for lherzolite of approximately 2.10 mm2s-1 at ambient condition. The experimental pressure derivative of the thermal conductivity of lherzolite decreased with temperature, reaching approximately 10% at high temperature, a value higher than the previously reported 4%, which indicates that the temperature gradient of the upper mantle lithosphere is smaller than previously thought. Therefore, concerning calculation of the lithosphere thickness using the thermal conductivity of the lherzolite, the previous calculation using pressure derivative of the thermal conductivity of 4% may cause an underestimation of the upper mantle lithosphere thickness by approximately 6% in a first approximation.