Maokou Formation Research Articles

Discovery of SEDEX gold in the Youjiang basin, SW China: Implications for a new type Au mineralization

Carlin-type gold mineralization represents one of the world’s most important Au resources. Despite the many studies conducted in the past three decades, the gold sources and metallogenic dynamics remain enigmatic. Here, we report actively forming auriferous mounds and sulfide concretions from the Middle-Upper Permian basalt-dominated submarine volcanic-sedimentary rocks within the Carlin-style gold deposits, Youjiang basin, China. The volcanic-sedimentary rocks consist of basalt, tuff, scoria, black ash and metalliferous sediments in an unaltered sequence, which overlies the Middle Permian Maokou Formation and underlines by the Upper Permian strata. In this paper, we investigate the native gold nanoparticles in the volcanic-sedimentary rocks, and provide new insight into a new type of Au mineralization in the Youjiang basin. We unravel the mineralogy, trace elements and sulfur isotope compositions of these native gold and associated sulfides (e.g., marcasite and pyrite) via EPMA, in-situ LA-ICP-MS and NanoSIMS analyses. Petrographic obsevations show that native gold grains are present in marcasite microfractures, with sizes of 1 to 5 μm. The marcasites is interpreted to be of sedimentary exhalative (SEDEX) origin, as they are hosted by sulfide concretions comprising of sulfide-rich aggregates, quartz, barite, and basaltic tuff fragments. The EPMA elemental mapping suggests that the native gold grains have high contents of Hg, Cu, Zn and Sb, an ore elements assemblage common in typical Carlin-type Au mineralization (Au-As-Hg-Sb±Cu±Zn). LA-ICP-MS analysis shows the Au0-bearing marcasite is featured by lower Co/Ni ratios (avg. = 0.12), relatively high Ti (mostly 39–890 ppm, some > 10000 ppm) and low Cu (1–32 ppm), As (1–920 ppm), Zn (0–5 ppm) and Au (0–0.53 ppm) contents. The low Co/Ni ratios and elevated Ti contents suggest that the marcasite was likely sedimentary and/or diagenetic process, which associated with high-Ti Emeishan basalt eruption in Middle-Late Permian. NanoSIMS analysis shows that the marcasite δ 34S varies from 20.30‰ to 34.29‰ (avg. = 25.86‰), indicating that thermochemical sulfate reduction (TSR) may have provided the sulfur under open system for sulfate but close for H2S. Overall, this paper proposed that (1) the Middle-Upper Permian volcanic-sedimentary rocks enriched in Au-Sb-Hg±Cu±Zn probably formed the source-bed for the Carlin-gold mineralization in Yanshanian (~140 Ma); (2) significant amounts of gold can occur in the basalt-dominated SEDEX environment. In addition, the genetic relationship between Emeishan plume and Carlin-type gold mineralization is also discussed in this paper. Based on previous studies, the juvenile lower crust beneath the eastern Emeishan large igneous province (LIP) may have been a potential Au source, and the associated mantle underplating may have contributed to the formation of Carlin-style Au deposits. Therefore, we propose a possible genetic link between the Carlin-type mineralization and Emeishan LIP magmatism in the Youjiang basin. Our findings offer a new exploration target which is unaltered volcanic-sedimentary rocks in the Youjing basin, may have been a source of gold for the large-scale Carlin-style gold mineralization in Yanshanian (~140 Ma), and demonstrate that significant amounts of gold can occur in a basalt-dominated SEDEX environment.

Mixed seawater and hydrothermal sources of nodular chert in Middle Permian limestone on the eastern Paleo-Tethys margin (South China)

Middle Permian limestone and marl of the Maokou Formation deposited along the eastern Paleo-Tethys margin (South China) host abundant chert nodules that formed in association with the global Permian Chert Event (PCE). An enhanced understanding of the origin of the chert may help elucidate Middle Permian oceanic and diagenetic conditions. However, the mechanisms responsible for the formation of chert, especially the source of silica, remain controversial. Maokou chert nodules, which are made up of quartz and subordinate calcite, hematite, pyrite, and fluorite, and rimmed by F-bearing sepiolite laminae, appear to have formed mainly by the replacement of precursor nodular limestone by silica. The nodular limestone may have formed by differential compaction of interbedded limestone and marl, after which carbonate was replaced gradually outward from nodule centers by dissolved silica released from the dissolution of siliceous skeletal grains (including spicules and radiolarians) in host marl during early diagenesis. The introduction of high-T hydrothermal fluids into ocean water is evidenced by the presence of F-bearing minerals (fluorite and sepiolite), a hybrid rare earth element (REE) + Y pattern, and a wide range of δ30Si values (−0.4‰ to +0.7‰) documented from the Maokou nodular chert. Conservative estimation indicated that Middle Permian seawater may have had a δ30Si composition (+0.7‰ to +1.8‰) similar to that of modern open ocean water, suggesting that seawater was the dominant silica source of nodular chert. Mixing calculations based on REE + Y geochemistry suggest additional contributions of ca. 0.05–0.2% high-T hydrothermal fluid and ca. 1–10% of continental-derived freshwater to ocean water. Such increased influx of silica to seawater may have been related to the early rifting induced by Emeishan volcanism and intensified chemical weathering during an interglacial period, which played an important role in promoting the flourishing of siliceous organisms and subsequent chert enrichment during the Middle Permian.

New constraints on the onset age of the Emeishan LIP volcanism and implications for the Guadalupian mass extinction

The onset of the Middle Permian Emeishan Large Igneous Province (ELIP), is commonly implicated in the Guadalupian mass extinction, one of the largest Phanerozoic extinction events, yet the contribution of the ELIP to this biotic event remains unresolved because of a lack of appropriate age constraints. In this study, we examined claystone beds overlying the cherty limestone of the Maokou Formation and underlying the flood basalts of the ELIP that were emplaced around the Guadalupian-Lopingian Boundary. Integrated petrographic, mineralogical, elemental, and isotopic features indicate that the studied claystones contain significant components from mafic volcanic ash that was likely associated with the emplacement of the ELIP. Zircons from the claystones yield UPb ages of ~262.5 Ma, which predates the subaerial eruptive phase of the ELIP (259.1–260.4 Ma), but is comparable to the J. altudaensis conodont biostratigraphic zone (~263 Ma, Middle Capitanian Stage) which is contemporaneous with the emplacement of early-stage Emeishan subaqueous pillow basalts and mafic volcaniclastics. Our data provide a crucial new constraint on the lower age limit for the onset of the ELIP, which was likely associated with violent submarine eruptions and thus could have contributed to the loss of marine fauna during the Guadalupian mass extinction.

Genesis for Rare Earth Elements Enrichment in the Permian Manganese Deposits in Zunyi, Guizhou Province, SW China

AbstractThe Zunyi manganese deposits, which formed during the Middle to Late Permian period and are located in northern Guizhou and adjacent areas, are the core area of a series of large‐medium scale manganese enrichment minerogenesis in the southern margin and interior of the Yangtze platform, Southern China. This study reports the universal enrichment of rare earth elements (REEs) in Zunyi manganese deposits and examines the enrichment characteristics, metallogenic environment and genesis of REEs. The manganese ore bodies present stratiform or stratoid in shape, hosted in the silicon–mud–limestones of the Late Permian Maokou Formation. The manganese ores generally present lamellar, massive, banded and brecciated structures, and mainly consist of rhodochrosite, ropperite, tetalite, capillitite, as well as contains paragenetic gangue minerals including pyrite, chalcopyrite, rutile, barite, tuffaceous clay rock, etc. The manganese ores have higher ΣREE contents range from 158 to 1138.9 ppm (average 509.54 ppm). In addition, the ΣREE contents of tuffaceous clay rock in ore beds vary from 1032.2 to 1824.5 ppm (average 1396.42 ppm). The REEs from manganese deposits are characterized by La, Ce, Nd and Y enriched, and existing in the form of independent minerals (e.g., monazite and xenotime), indicating Zunyi manganese deposits enriched in light rare earth elements (LREE). The Ceanom ratios (average –0.13) and lithofacies and paleogeography characteristics indicate that Zunyi manganese deposits were formed in a weak oxidation‐reduction environment. The (La/Yb)ch, Y/Ho, (La/Nd)N, (Dy/Yb)N, Ce/Ce* and Eu/Eu* values of samples from the Zunyi manganese deposits are 5.53–56.92, 18–39, 1.42–3.15, 0.55–2.20, 0.21–1.76 and 0.48–0.86, respectively, indicating a hydrothermal origin for the manganese mineralization and REEs enrichment. The (δ13CV‐PDB (−0.54 to −18.1%) and (δ18OSMOW (21.6 to 26.0%) characteristics of manganese ores reveal a mixed source of magmatic and organic matter. Moreover, the manganese ore, tuffaceous clay rock and Emeishan basalt have extremely similar REE fractionation characteristic, suggesting REEs enrichment and manganese mineralization have been mainly origin from hydrothermal fluids.

Emeishan mantle plume activity and carbon isotope responses in the Middle Permian, South China

During the Permian, Emeishan mantle plume activity created a large igneous province and deeply impacted carbon isotope records in marine carbonate strata in South China. To evaluate the isotopic variations caused by this catastrophic event, 140 carbonate samples from four sections were collected and analyzed. The δ13Ccarb values in the upper section of the Qixia Formation gradually increased, stabilized at ~4.0‰ in the lower-middle sections of the Maokou Formation, and then abruptly decreased in the upper section of the Maokou Formation. This isotopic evolution trend is likely related to mantle plume geochemical processes occurring during these respective stages. Large amounts of upwelling heat from the emplacement of the mantle plume triggered large-scale decarbonation reactions in the continental crust resulting in CO2 production enriched in 13C. Substantial amounts of 13C-enriched CO2 directly affected the isotope compositions of marine carbonates during this time. Temporal and spatial fluctuations in carbon isotopes were also controlled by terrestrial materials as a result of regression and doming caused by plume emplacement. The abrupt decrease in δ13Ccarb in the upper section of the Maokou Formation reflects the onset of volcanism, resulting in the breakdown of organic materials. These findings emphasize the contribution of a carbon reservoir (with a thermal decarbonation origin) in the upper crust, particularly against the Permian tectono-thermal background. This mechanism, which is similar to biological isotope fractionation in the carbon cycle, should not be overlooked.

Type and distribution of Mid-Permian Maokou Formation karst reservoirs in southern Sichuan Basin, SW China

Abstract Based on the analysis of the responses of conventional logs such as natural gamma (GR), density (DEN), acoustic interval transit time (AC), compensated neutron (CNL), dual lateral resistivity (Rlld, Rlls), and caliper log (CAL), combined with drilling data, cores, thin section and productivity of 65 wells, the reservoirs in the Mid-Permian Maokou Formation of southern Sichuan Basin were divided into four types, fractured-vuggy, pore-vuggy, fractured and fractured-cavity. The main reservoirs in high productivity wells are fractured-vuggy and pore-vuggy. The reservoirs of Maokou Formation are generally thin, and can be divided into the upper reservoir segment (layer a of the second member to the third member of Maokou Formation, P2m2a-P2m3) and the lower segment (layer b of the second member of Maokou Formation, P2m2b). The two reservoir segments are mainly controlled by two grain beaches during the sedimentation of P2m2a-P2m3 and P2m2b, the vertical zonation of karst, and the fractures. The upper reservoir segment is generally better than the lower one in development degree and single well productivity, and is much thicker than the lower one. It is thicker in the Yibin-Zigong-Weiyuan-Dazu area, the southwestern area of Chongqing and the southeastern area of Luzhou, while the lower segment is thicker in the Neijiang-Zigong-Luzhou area and the Dazu-Luzhou area. The areas with big reservoir thickness at tectonic slope or syncline parts are the favorable exploration areas.

Geochemical features of the Middle–Upper Permian cherts and implications for origin, depositional environment in the Sichuan Basin, SW China

Permian cherts are widely developed in the Sichuan Basin and carry significant information about the evolution of the sedimentary environment. To understand the origins and depositional environments of the Middle–Upper Permian cherts in the Sichuan Basin, we studied the lithological and geochemical characteristics of the cherts. Based on field observations and thin section analysis, Permian cherts occur mainly in the form of nodular and bedded cherts. Nodular cherts present irregular lumps or tuberculous patterns and are composed of mostly cryptocrystalline and microcrystalline quartz with small amounts of carbonate mineral. Bedded cherts contain organic matter, and radiolarians can be found in Late Permian bedded cherts. The geochemical analysis of Permian cherts shows that the chert sources are complex; nodular Middle Permian cherts show hydrothermal and upwelling influences, with less terrestrial influence. Nodular chert is the product of diagenetic metasomatism, formed when silica‐rich fluids entered carbonate that was partially dissolved by H+ released from the oxidation of H2S. Bedded cherts of the Maokou Formation have the same source as nodular cherts but were deposited in a slope or depression of the platform, which was below the CaCO3 compensation depth (CCD) interface, so that bedded chert could be deposited directly. Bedded cherts of the Wujiaping Formation show influence from hydrothermal, biogenetic, and terrestrial sources and were deposited in a deeper water environment, such as basin or the bottom of slope. The Permian cherts in the Sichuan Basin are the response to the global Permian Chert Event (PCE), which was closely related to the Late Palaeozoic Ice Age (LPIA) and the global active volcanism during the Permian.

Distribution and exploration direction of medium- and large-sized marine carbonate gas fields in Sichuan Basin, SW China

Abstract Based on the analysis of the basic characteristics of medium- and large-sized marine gas fields in Sichuan Basin, combined with the division of major reservoir forming geological units in the marine craton stage and their control on key hydrocarbon accumulation factors, the distribution law of medium- and large-sized marine carbonate gas fields in the basin was examined and the exploration direction was pointed out. Through the analysis of the periodic stretching-uplifting background, it is concluded that five large scale paleo-rifts, three large scale paleo-uplifts, five large scale paleo erosion surfaces were formed in the marine craton stage of Sichuan Basin, and these geological units control the key reservoir forming factors of medium and large sized gas fields: (1) Large-scale paleo-rifts control the distribution of high-quality hydrocarbon generation centers. (2) The margin of large-scale paleo-rifts, high position of paleo-uplifts and paleo erosion surfaces control the distribution of high-quality reservoirs. (3) Large-scale paleo-rifts, paleo-uplifts, paleo erosion surfaces and present tectonic setting jointly control the formation of many types of large and medium-sized traps. (4) Natural gas accumulation is controlled by the inheritance evolution of traps in large geological units. Based on the comparative analysis of the distribution characteristics of medium- and large-sized gas fields and large geological units, it is proposed that the superimposition relationship between single or multiple geological units and the present structure controls the distribution of medium- and large-sized gas fields, and the “three paleo” superimposed area is the most advantageous. According to the above rules, the main exploration fields and directions of medium- and large-sized marine carbonate gas fields in Sichuan Basin include periphery of Deyang-Anyue paleo-rift, eastern margin of Longmenshan paleo-rift, margins of Kaijiang-Liangping oceanic trough and Chengkou-western Hubei oceanic trough, the high part of the subaqueous paleo-uplifts around Central Sichuan, paleo erosion surfaces of the top boundary of Maokou Formation in eastern and southern Sichuan Basin, paleo erosion surfaces of the top boundary of the Leikoupo Formation in central and western Sichuan Basin.

Stratigraphic and Structural Control on Hydrothermal Dolomitization in the Middle Permian Carbonates, Southwestern Sichuan Basin (China)

The Qixia Formation and Maokou Formation of Middle Permian in the southwestern Sichuan Basin were pervasively dolomitized during the diagenetic history. Petrographically, four types of dolomites, namely three replacive dolomites (Rd1, Rd2, and Rd3) and one dolomite cement (Cd), were distinguished. Rd1 dolomite occurs as very fine (<50 µm), planar-s to nonplanar crystals; Rd2 dolomite shows planar-e to planar-s crystal shapes with fine crystal sizes (50–250 µm) and is characterized by center-frog and margin-clear; Rd3 dolomite occurs as medium to coarse (250 µm–2 mm), nonplanar crystals; and Cd dolomite is characterized by saddle crystals filling dissolution pores and/or fractures, translucent white color in the hand samples, and strong sweeping extinction under cross-polarized light. In areas close to reactivated basement faults (Zhangcun outcrop and well Hanshen1), Rd3 (~65% by abundance) was the dominant type of replacement dolomite and minor amounts of Rd1 and Rd2 (~10%) were found in this area. Cd (~25%) was extensively developed in fractures and dissolution pores, whereas, in areas far away from the fault zones (Xinjigu outcrop), Rd1 (~20%) and Rd2 (~55%) were dominant replacement dolomites, and only a small portion of them were recrystallized to form Rd3 (~20%), with minor Cd (~5%) dolomite occurring in some dissolution pores. The δ13CV-PDB (−0.37‰ to 4.32‰) and δ18OV-PDB values (−7.41‰ to −5.19‰), 87Sr/86Sr ratios (0.707085 to 0.707795), and rare earth elements (REE) patterns (flat REE patterns with slight light rare earth element (LREE) enrichment and slight negative Ce anomalies) suggest that Rd1 dolomite was formed penecontemporaneously in an evaporitic tidal flat evaporation environment with salinities higher than seawater. The Rd2 dolomite, characterized by δ13CV-PDB (−0.18‰ to 4.89‰) and δ18OV-PDB values from −6.6‰ to −5.5‰, 87Sr/86Sr ratios from 0.707292 to 0.707951, and LREE enrichment and slight negative Ce anomalies, was interpreted as forming from the recrystallization of Rd1 at shallow burial. The δ18OV-PDB values (−12.01‰ to −8.23‰), the prominent positive anomaly of Eu, high 87Sr/86Sr ratios (0.7081–0.7198) and high fluid inclusion homogenization temperatures (149–255 °C) suggest that Rd3 and Cd dolomite were formed from hot fluids. Based on regional stratigraphic data, the Rd3 and Cd were likely formed at depths less than 1500 m; thus, the ambient burial temperature would be lower than 85 °C. The high fluid temperatures recorded by fluid inclusions, thus, indicate that the dolomitization was of hydrothermal nature. The δ18OV-SMOW values, homogenization temperatures, and salinities of the fluid inclusions of Rd3 and Cd in proximal areas were systematically higher than those in distal areas, suggesting that the hydrothermal fluid ascended along faults in proximal areas and then migrated laterally along the strata to distal areas. The dolomites of the Middle Permian carbonates in the southwestern Sichuan Basin, thus, resulted from different dolomitization phases, and the latter hydrothermal dolomitization was controlled by a combination of strata and structures.

支持向量机在回归预测有机碳含量中的应用研究——以川东南地区为例

支持向量机在回归预测有机碳含量中的应用研究——以川东南地区为例

Orbitally forced glacio-eustatic origin of third-order sequences and parasequences in the Middle Permian Maokou Formation, South China

Abstract Astronomical forcing on the development of depositional sequences has been of particular interest to researchers in recent years. To investigate the forming mechanism of depositional sequences in the Middle Permian Maokou Formation, outcrop sequence stratigraphic analysis and cyclostratigraphic analysis were conducted on one outcrop section and one drilling well in South China. Up to ninety-two meter-scale 6th-order parasequences, thirty-one 5th-order parasequences, eight 4th-order parasequences and two 3rd-order sequences were recognized based on vertical facies changes and stratal stacking patterns. Astronomical cycles (i.e., eccentricity, obliquity and precession) were detected by spectral analysis. An ∼3.1 Myr temporal framework and a high-resolution sedimentation rate curve were constructed by cyclostratigraphic analysis. The similar ratios of average thickness for the high-frequency sequences and the spectral periodicities suggest an astronomical forcing origin for the parasequences. The 4th-, 5th-, and 6th-order high-frequency parasequences correlate well with long eccentricity, short eccentricity and obliquity cycles, respectively. The durations of local and global 3rd-order sea-level sequences match the s4-s3 obliquity modulation cycles. Our results suggest that tectonic and autocyclic processes did not exert a major influence on the development of depositional sequences and that in the Middle Permian Maokou Formation the development of 3rd-order sequences and 4th-, 5th- and 6th-order parasequences was controlled by orbitally-driven glacio-eustatic sea-level changes. The coevolution trends in obliquity power, sea-level change and sedimentation rate hold important implications for stratigraphic correlation and sequence stratigraphic subdivision on a regional scale.

Strike-slip faults and their significance for hydrocarbon accumulation in Gaoshiti–Moxi area, Sichuan Basin, SW China

Abstract Based on the three dimensional seismic data and drilling data in Gaoshiti–Moxi area, the structural characteristics and evolution process of strike-slip faults in the study area and their significance for gas accumulation were examined using the method of fault structure analysis. The results show that: (1) Transtensional strike-slip faults are developed in the Paleozoic in study area. On the profile, there are three types of structural styles: steep and erect, flower structures, “Y” and reversed “Y” type faults. On the plane, strike-slip faults developed in the Cambrian extending linearly and along the nearly EW, NW and NE direction. The faults are composed of multiple secondary faults in en-echelon arrangement; faults in the Permian are mainly along nearly EW and NW direction, and the Permian faults are distributed in the middle east part of the study area and they are more in the north than in the south. (2) The nearly EW and NW trending faults are dextral strike-slip faults, and the NE trending faults are sinistral strike-slip faults. The strike-slip strength of the nearly EW trending faults are the strongest and the maximum horizontal displacement is 550 m, while the strike-slip strength of the NE trending faults is the weakest. The Cambrian faults had stronger activities than the Permian faults. (3) The transtensional strike-slip faults were active in two periods, the early Caledonian period and the late Hercynian period, and were the products of oblique extension of pre-existing weak zones in Xingkai and Emei taphrogenesis, with certain inheritance in the main faults. (4) The strike-slip faults and surrounding fractures enhance the porosity and permeability of the reservoir in Cambrian Longwangmiao Formation and control the distribution of karst reservoirs in the Permian Qixia Formation–Maokou Formation, bringing about the situation of multiple gases bearing series in this area.

Genesis of Permian sedimentary manganese deposits in Zunyi, Guizhou Province, SW China: Constraints from geology and elemental geochemistry

The Permian was a key period of manganese deposition in Southern China, as evidenced by a series of large-to medium-scale manganese deposits along the margin of the Yangtze Block. This study characterizes representative examples that occur in late Middle Permian silicon–mud–limestones in Zunyi and adjacent areas and examines their implications for manganese deposit genesis and sedimentary environment. The manganese deposits are typical carbonates deposits, with the main manganese-bearing mineral being rhodochrosite, followed by calcimangite, capillitite, manganocalcite, and kutnohorite in decreasing abundance. The ore bodies, hosted in the black shale series of the Late Permian Maokou Formation, are mainly stratiform or stratoid in shape, usually have lamellar, massive, banded, fragmental, or pisolitic structures, and contain paragenetic gangue minerals including rutile, barite, pyrite, and chalcopyrite, etc. These features are characteristic of hydrothermal exhalative sedimentary deposits, implying that this was their mode of genesis. The Mn-ores have higher Co, V, Ni, Cu, Sr and total REE content, lower Y/Ho ratios, and characterized by the Fe–Mn–(Cu + Co + Ni) × 10, lgTh vs. lgU, P2O5vs. Y and Fe/Ti vs. Al/(Al + Fe + Mn) diagrams strongly suggest hydrothermal activity during manganese mineralization. The Th/U, V/Cr and V/(V + Ni) ratios of samples from the Zunyi manganese deposits are 0.38–3.27, 3.11–24.00 and 0.38–0.99, respectively, indicating the sedimentary environment was weakly oxidizing to reducing condition.

A new shear rheological model for a soft interlayer with varying water content

The rheological behavior of a soft interlayer is critical to understanding slope stability, which is closely related to the water content of the soft interlayer. This study used the soft interlayer of the Permian Maokou Formation in Southwest China as an example to perform ring shear creep tests with different water content amounts. The effect of water content on the creep properties of the soft interlayer was analyzed, and a new shear rheological model was established. This research produced several findings. First, the ring shear creep deformation of the soft interlayer samples varied with the water content and the maximum instantaneous shear strain increment occurred near the saturated water content. As the water content increased, the cumulative creep increment of the samples increased. Second, the water content significantly affected the long-term strength of the soft interlayer, which decreased with the increase of water content, exhibiting a negative linear correlation. Third, a constitutive equation for the new rheological model was derived, and through fitting of the ring shear creep test data, the validity and applicability of the constitutive equation were proven. This study has developed an important foundation for studying the long-term deformation characteristics of a soft interlayer with varying water content.

Gold mineralization associated with Emeishan basaltic rocks: Mineralogical, geochemical, and isotopic evidences from the Lianhuashan ore field, southwestern Guizhou Province, China

A number of gold deposits are newly discovered in the Lianhuashan ore field, southwestern Guizhou Province, China. The gold orebodies are restricted to the Emeishan basaltic rocks and the interlayer fractures between the Emeishan basaltic rocks and the limestone of the Maokou Formation. Three generations of pyrite (Py0, Py1, and Py2) have been recognized in these deposits based on morphology and paragenetic relationships. Electron microprobe (EMP), laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS) analyses were carried out to characterize their chemical and sulfur isotopic compositions.The framboidal Py0 is enriched in As but depleted in most of the other trace elements. It has negative δ34S values of −14.5–−1.9‰, which are typical of diagenetic pyrites resulted from bacterial reduction of marine sulfate. Py1 and arsenopyrite exhibit high concentrations of Au and As, making them the most dominant auriferous minerals. They yield comparable δ34S values of −2.8– + 2.1‰, consistent with the derivation of sulfur from the Emeishan basaltic rocks. Py2 is characterized by well developed oscillatory-zoned textures, that are interpreted in terms of internal self-organization processes. It contains low concentrations of Au and highly variable concentrations of As, Sb, Co, and Ni. The Py2 grains show slightly positive δ34S values of + 0.4– + 7.0‰, indicating that the sulfur was mainly sourced from the Emeishan basaltic rocks with variable amounts of contamination from reduced marine sulfate.The Emeishan basaltic rocks may have provided not only ore-hosting space but also Au, S, and other associated elements for gold mineralization in the Lianhuashan ore field. Similar gold deposits are expected to be discovered in the areas of Emeishan basaltic rocks with high Au backgrounds and small thicknesses.

Abiotic and biotic responses to Milankovitch-forced megamonsoon and glacial cycles recorded in South China at the end of the Late Paleozoic Ice Age

Abstract At the end of the Late Paleozoic Ice Age (LPIA) from late Early Permian to early Late Permian, the global climate was impacted by a prevailing megamonsoon and Gondwanan deglaciation. To better understand the abiotic and biotic responses to Milankovitch-forced climate changes during this time period, multi-element X-ray fluorescence (XRF) geochemistry analyses were conducted on 948 samples from the late Early−late Middle Permian Maokou Formation at Shangsi, South China. The Fe/Ti, S/Ti, Ba/Ti and Ca time series, which were calibrated with an existing “floating” astronomical time scale (ATS), show the entire suite of Milankovitch rhythms including 405 kyr long eccentricity, 128 and 95 kyr short eccentricity, 33 kyr obliquity and 20 kyr precession. Spectral coherency and cross-phase analysis reveals that chemical weathering (monitored by Fe/Ti) and upwelling (captured by S/Ti and Ba/Ti) are nearly antiphase in the precession band, which suggests a contrast between summer and winter monsoon intensities. Strong obliquity signal in the Ba/Ti series is proposed to derive from changes in thermohaline circulation intensity from glaciation dynamics in southern Gondwana. The abundance of foraminifer, brachiopod and ostracod faunas within the Maokou Formation were mainly controlled by the ~1.1 Myr obliquity modulation cycle. The obliquity-forced high-nutrient and oxygen-depleted conditions generally produced a benthic foraminifer bloom, but threatened the brachiopod and ostracod faunas.

Petrology and geochemical characteristics of dolomite in the Middle Permian Maoukou Formation, central Sichuan

Dolomites of the Maokou Formation in the central Sichuan were mainly developed in the middle-upper part of Member 2 and the lower part of Member 3 of Maokou Formation, it could be divided into fine-to medium-grained calcareous dolomite, fine-to medium-grained dolomite and breccia dolomite according to petrology characteristic, and the latter two were dominant. Through contrast of geochemical characteristic between micritic limestone and different types of dolomite, the dolomite was characterized by low Fe content, high Mn content and low Sr content relative to the micritc limestone. The micritic limestone and different types of dolomite had similar rare earth element (REE) distribution patterns which were characterized by depletion of light REE, weak positive anomaly of La and negative anomaly of Ce, and the dolomite also had weak positive anomaly of Eu. Compared with the micritic limestone, the fine-to medium-grained dolomite and breccia dolomite had low values of δ18O and obvious high ratios of 87Sr/86Sr. REE distribution patterns of different types of dolomite indicated that some geochemical characteristics of primitive limestone were preserved during the dolomitization process in the Maokou Formation, while weak Eu positive anomaly of dolomite and isotopic difference in dolomite and limestone might be caused by high-temperature thermal fluids during the dolomitization process. The dolomitization of the Maokou Formation was controlled by some factors: (1) strata residual seawater and hydrothermal fluids derived from magmatism were major sources of Mg during the dolomitization process; (2) heating effect of Emeishan large igneous province provided abnormal high paleo-geotemperature for dolomitization; (3) fault systems offered rapid migration channels for abnormal geothermal convection and dolomitization fluid; (4) limestone of grain-shoal facies with good porosity and permeability was more prone to dolomitization than micritic limestone. Abnormal geotemperature and abundant Mg supply derived from Emeishan Large Igneous Province were key factors for dolomitization in the Maokou Formation, and different types of dolomites were formed in different periods of the dolomitization process.

Evolution of sedimentary facies and fossil communities in the Middle Permian Maokou Formation in Zigui County, Hubei Province, South China

A geological event that happened at the end of the Middle Permian resulted in different levels of erosion in a shallow-water platform of South China, which led to diverse geological records in different sections. The Lianziya section in western Hubei Province of South China has a well-exposed stratigraphic sequence of the late Middle Permian rocks with abundant fossils and sedimentary facies types, providing new evidences for understanding the evolution of marine biota and environment in the late Middle Permian. Our study shows that four fossil communities can be recognized with the change of sedimentary facies in the late Middle Permian: Foraminiferal–algal community, phylloid algal community, bryozoan community and Ungdarella community. The foraminiferal–algal community is dominated by fusulinids and calcareous algae, but was soon replaced by the phylloid algal community. With an increase of terrestrial input, the phylloid algal community was again replaced by the bryozoan community. Near the end of the Middle Permian, with a decrease of terrestrial input, the bryozoan fossil community disappeared while the Ungdarella community became dominant. A 10 cm-thick weathered crust occurred at the top of the Middle Permian limestone, marking a large amplitude fall in sea-level and the beginning of a terrestrial erosion stage. The sedimentary facies and fossil community changes in the Lianziya section indicates that the Middle Permian crisis was a gradual process while the sea-level fall mainly occurred in the latest Middle Permian.

Basin-scale structure control of Carlin-style gold deposits in central Southwestern Guizhou, China: Insights from seismic reflection profiles and gravity data

Carlin-style gold deposits are widely distributed in Southwestern Guizhou, China. Research has dominantly focused on deposit-scale geology and structural control of mineralization. The relationship between the gold deposits and regional structures, in particular their control on the formation and distribution of those deposits is less well understood. Here we use seismic survey and gravity data to reveal the basin-scale structure control of the gold deposits in central Southwestern Guizhou. The seismic reflection profiles show strong reflection horizons including Upper Permian coal measures, and Dongwu and Guangxi unconformities. The seismic data provide important evidence of the basinal structural style and the relationship between fault-related folding and the Carlin-style gold deposits. Regional gravity data also revealed several paleo-uplifts, which appear to control the distribution of Carlin-style gold deposits.The Carlin-style gold deposits are spatially and temporally associated with fault-related folds. Based on their location, we classify the gold deposits into two types, (1) low-angle thrust-controlled deposits; and (2) high-angle reverse fault-related deposits. In the low-angle thrust-controlled deposits, main thrusts are listric, strike NW-SE and dip southwest. These thrusts detach into coal measures, tuffs and volcanic rocks and are bedding-parallel in the vicinity of the Dongwu unconformity. Gold ore bodies are found in the thrusts and characterized by cutting-layer veins near the surface and bedding-parallel veins at depth. The high-angle reverse fault-related deposits are located in the hinge of northeast-southwest-striking fault-related folds, with high-angle reverse faults near the hinge of folds crosscutting the Upper Permian coal measures, volcanic rocks and Dongwu unconformity, and then into the limestone of Middle Permian Maokou Formation. Ore bodies are bedding-parallel in anticline culminations where interlayer fracture zones in Upper Permian coal measures and volcanic rocks are generated by high-angle reverse faulting.We consider that the formation of the gold deposits is part of the tectonic evolution in the area. The two sets of NW-SE- and NE-SW-striking fault-related folds were successively formed during the closure of eastern Tethys Ocean in Late Triassic, and major gold deposits formed in the fault-related folds. The Dongwu unconformity and faults provided conduits for ore fluid flow, and the coal measures and the tuffs of Dachang Member acted as detachment layers and hosts for ore deposition.

The origin and geochemical characteristics of Permian chert in the Eastern Sichuan Basin, China

Chert is widespread in Permian Maokou Formation and Wujiaping Formation of the eastern Sichuan Basin, China. Field observations in conjunction with major element, REE, and isotope analysis suggest that the chert is both biogenic and hydrothermal in origin. Further analysis indicates that hydrothermal features are more prominent in chert of the Maokou Formation, whereas chert of the Wujiaping Formation contains stronger evidence of biogenic origin. The SiO2 content of chert of the Maokou and Wujiaping formations are 80.09–97.91 and 65.52–97.76 wt%, respectively. The average Al/(Al + Fe + Mn) values are 0.35 (Maokou Formation) and 0.38 (Wujiaping Formation), and most samples are plotted in the Fe-rich section of the Al–Fe–Mn diagram. SiO2 abundance and Al/(Al + Fe + Mn) ratio analysis of chert of the Maokou and Wujiaping formations suggest a hydrothermal origin, although several samples were more indicative of biogenic origin. The average REE content of chert in both formations is low, and average REE is lower in chert of the Maokou Formation than in chert of the Wujiaping Formation. The Ce anomaly is weakly negative in the chert of the Maokou Formation (0.62) and Wujiaping Formation (0.71), whereas the average Eu value is greater in chert of the Maokou Formation than in chert of the Wujiaping Formation. The δ30Si values of chert are 0.4–1.2‰ in the Maokou Formation and 0.7–1.4‰ in the Wujiaping Formation, and these values are close to the δ30Si values of modern radiolarians. The average palaeoseawater temperatures under which the chert formed were calculated as 66 °C for chert of the Maokou Formation and 62 °C for chert of the Wujiaping Formation. These calculated temperatures are significantly higher than estimated ancient sea temperatures. These data and interpretations suggest that the chert of the Permian Maokou and Wujiaping formations were affected by volcanism and fracturing during the Permian. Seawater seeped through fractures and interacted with upwelling hot magma along basement faults, resulting in silica dissolution and the enrichment of waters from hot spots along fractures as well as the enrichment of seawater. This enrichment resulted in the mass reproduction of siliceous organisms, such as radiolarians and sponges, which were eventually deposited to form biosiliceous rocks.