微生物参与前寒武纪条带状铁建造沉积的研究进展

地球演化早期太古代和早元古代大规模的条带状铁建造(BIF)是目前世界上最重要的铁矿资源。已有的稳定同位素组成、分子化石以及岩石磁学性质等证据支持早期微生物广泛参与了BIF的形成。本文评述了微生物参与BIF形成过程中铁搬运和沉淀及其同位素分馏、生物标志物和岩石磁学证据。深入地研究BIF成矿中的微生物矿化贡献,有助于解释BIF形成机制,反演前寒武纪大气-海洋环境演化,以及理解地球早期生命的过程。     

Origin and formational history of some Pb-Zn deposits from Alborz and Central Iran: Pb isotope constraints

Several Pb-Zn deposits and occurrences within Iran are hosted by Mesozoic–Tertiary-aged sedimentary and igneous rocks. This study reports new Pb isotope analyses for galena from 14 Pb-Zn deposits in the Alborz and Central Iran structural zones. In general, Pb isotope ratios are extremely variable with data plotting between the upper crustal and orogenic curves in a plumbotectonic diagram. The latter may be attributed to Pb inputs from crustal and mantle end-members. Most of the galena samples are characterized by high ²⁰⁷Pb/²⁰⁴Pb ratios, suggesting significant input of Pb from old continental crust or pelagic sediment. Pb isotope data also indicate that some of the deposits, which are hosted by sedimentary rocks in Central Iran and Alborz, have similar Pb isotopic compositions and hence suggest similar source regions. Most of the galenas yield Pb model ‘ages’ that vary between ~140 and ~250 Ma, indicating that mineralization resulted from the extraction of ore-bearing fluids from Upper Triassic–Lower Jurassic sequences. The similarity in Pb isotope ratios for the Pb-Zn deposits located within these zones suggests analogous crustal evolution histories. Our preferred interpretation is that Pb-Zn mineralization within the sedimentary and igneous rocks of the Central Iran and Alborz tectonic regions occurred following a Late Cretaceous–Tertiary accretionary stage of crustal thickening in Iran.     

渭河中游全新世黄土剖面光释光测年及记录的古洪水事件

通过对渭河流域进行广泛的考察,在中游咸阳附近一处阶地发现保存完好的全新世黄土-古土壤剖面里夹有古洪水滞流沉积层,对其进行了年代学和沉积学研究。对于采集的全新世地层样品,进行粒度、磁化率测量分析,证明所夹沉积物是典型的古洪水滞流沉积物,该层记录了古洪水事件发生的气候水文信息。应用红外后蓝光的SAR光释光测年技术,获得该剖面9个OSL年龄值,确定渭河在距今3.2～2.8ka之间为一个洪水多发时期,并建立了渭河古洪水事件的年代序列。这一结果揭示了古洪水的频发与全新世中期向晚期过渡的转折时期,气候由温湿向干旱化发展,大气系统失稳,气候变化剧烈,降水量异常变化,变率增大,是导致渭河流域特大古洪水多发的主要原因。     

Mineral compositions and fluid evolution of the Tonglushan skarn Cu–Fe deposit,SE Hubei,east-central China

The Tonglushan Cu–Fe deposit (1.12 Mt at 1.61% Cu, 5.68 Mt at 41% Fe) is located in the westernmost district of the Middle–Lower Yangtze River metallogenic belt. As a typical polymetal skarn metallogenic region, it consists of 13 skarn orebodies, mainly hosted in the contact zone between the Tonglushan quartz-diorite pluton (140 Ma) and Lower Triassic marine carbonate rocks of the Daye Formation. Four stages of mineralization and alterations can be identified: i.e. prograde skarn formation, retrograde hydrothermal alteration, quartz-sulphide followed by carbonate vein formation. Electron microprobe analysis (EMPA) indicates garnets vary from grossular (Ad_20.2–41.6Gr_49.7–74.1) to pure andradite (Ad_47.4–70.7Gr_23.9–45.9) in composition, and pyroxenes are represented by diopsides. Fluid inclusions identify three major types of fluids involved during formation of the deposit within the H₂O–NaCl system, i.e. liquid-rich inclusions (Type I), halite-bearing inclusions (Type II), and vapour-rich inclusions (Type III). Measurements of fluid inclusions reveal that the prograde skarn minerals formed at high temperatures (>550°C) in equilibrium with high-saline fluids (>66.57 wt.% NaCl equivalent). Oxygen and hydrogen stable isotopes of fluid inclusions from garnets and pyroxenes indicate that ore-formation fluids are mainly of magmatic-hydrothermal origin (δ¹⁸O = 6.68‰ to 9.67‰, δD = –67‰ to –92‰), whereas some meteoric water was incorporated into fluids of the retrograde alteration stage judging from compositions of epidote (δ¹⁸O = 2.26‰ to 3.74‰, δD= –31‰ to –73‰). Continuing depressurization and cooling to 405–567°C may have resulted in both a decrease in salinity (to 48.43–55.36 wt.% NaCl equivalent) and the deposition of abundant magnetite. During the quartz-sulphide stage, boiling produced sulphide assemblage precipitated from primary magmatic-hydrothermal fluids (δ¹⁸O = 4.98‰, δD = –66‰, δ³⁴S values of sulphides: 0.71–3.8‰) with an extensive range of salinities (4.96–50.75 wt.% NaCl equivalent), temperatures (240–350°C), and pressures (11.6–22.2 MPa). Carbonate veins formed at relatively low temperatures (174–284°C) from fluids of low salinity (1.57–4.03 wt.% NaCl equivalent), possibly reflecting the mixing of early magmatic fluids with abundant meteoric water. Boiling and fluid mixing played important roles for Cu precipitation in the Tonglushan deposit.     

Large-scale gold mineralization in eastern China induced by an Early Cretaceous clockwise change in Pacific plate motions

Orogenic gold (Au) deposits are the most important type, accounting for more than half of the world's proven Au reserves. They are mainly controlled by three key factors: (1) abundant andesitic rocks (SiO₂ of 55–60 wt.%) at depth, which have systematically higher Au contents than other rock types; (2) a pervasive transition from greenschist facies to amphibolite facies metamorphism within a short period, which releases S^2?-rich fluids that may scavenge Au from host rocks; and (3) deformation and fracturing under a compressive/transpressive tectonic regime. Orogenic belts at convergent margins are the best places for such mineralization because convergent margins are rich in andesites; the transition from greenschist to amphibolite facies recrystallization commonly occurs as a result of collision, compression, and thickening at convergent margins, forming large amounts of Au-rich fluids within a short period of time; and strong deformation and fracturing during orogenic processes provide channels for fluid transportation. Moreover, the overlying plate is injected and enriched by auriferous fluids released during amphibolite facies metamorphism of the subducting plate. The Pacific plate changed course by ～80° (from SW to NW) at approximately 125–122 Ma, reflecting an altered thermal structure and the elevation of the South Pacific plate attending the appearance of the plume head that formed the Ontong Java large igneous province. Consequently, the tectonic regime changed from extension to compressive/transpressive in eastern China, causing deformation, thickening, and metamorphism of the overriding plate, especially along weak crustal belts (e.g. overlying plates of palaeosutures), which resulted in world-class mineralization of orogenic Au deposits. During this process, pyrite changed to pyrrhotite during the transition from greenschist to amphibolite facies, releasing sulphur. Sulphur mobilized and scavenged Au and other chalcophile elements into metamorphic ore-forming fluids. A series of NE-trending compressive faults were formed at ?120 Ma as a result of continuous compression of the subducting Pacific plate, releasing these ore-forming fluids. Auriferous carbonate-rich quartz veins and/or metasomatized Au-bearing wall rocks were formed due to the decompression of the ascending ore-forming fluids. Orogenic belts along the margins of the North China craton and the Jiangnan block were the most favourable regions for mineralization. Compared with the former, the latter has much smaller proven Au reserves. However, more exploration is needed along the margins of the Jiangnan block. Promising targets include accessory faults and kink points of large, NE-trending Cretaceous faults that transect greenschist facies metamorphic rocks of the Niuwu and Jingtan Groups, etc.     

The genetic relationship between hydrocarbon systems and Mississippi Valley-type Zn–Pb deposits along the SW margin of Sichuan Basin,China

The Chipu Mississippi Valley-type (MVT) deposit is located on the southwest (SW) margin of the Sichuan Basin. Occurrence of plentiful organic matter (bitumen) at this deposit and abundant hydrocarbon reservoirs in the SW Sichuan Basin implies a link between lead–zinc mineralization and hydrocarbon systems in this area. The high δ³⁴S values of most metal sulphides from the different ore stages suggest that H₂S-bearing gases and/or thermochemical sulphate reduction (TSR) by organic matter could have been the source of reduced sulphur involved in ore formation. Sulphides with small positive to negative δ³⁴S values can be attributed to organically bound sulphur at the Chipu deposit. Carbon and oxygen isotopic compositions from sparry carbonates suggest mixing of organic carbon with seawater-derived carbon in the mineralization process. From the early to the later ore stages, δ¹³C_PDB values of ore-hosting carbonates are increasingly more negative, which indicates strengthening of the TSR role during mineralization. Hydrogen and oxygen isotopes in fluid inclusions in the quartz gangue indicate the provenance of the ore-forming fluids in the hydrocarbons. Moreover, some extremely low hydrogen isotope values suggest the addition of hydrogen from the same source. The low H/C ratios and high non-hydrocarbon component of the bitumen (Zhang et al. 2010 Zhang, C.Q., Yu, J.J., Mao, J.W., Yu, H. and Li, H.M. 2010. Research on the biomarker from Chipu Pb-Zn Deposit, Sichuan. Acta Sedimentologica Sinica, 28: 832–844. v.p.in Chinese with English abstract [Google Scholar]) also suggest that the organic matter may have been involved in TSR and subjected to a strong oxidation by ore-bearing fluids. This study attempts to explain the lead–zinc mineralization process and the role of organic matter in it. As there is a demonstrable relationship between the evolution of the hydrocarbons and regional lead–zinc mineralization along the SW edge of the Sichuan Basin, we propose a possible model in which the MVT mineralization coincided with the degradation of hydrocarbon reservoirs due to the large-scale migration of basinal fluids, most likely driven by the late Indosinian orogeny in response to the closure of the Palaeo-Tethys Ocean.     

Two geodynamic–metallogenic events in the Balkhash (Kazakhstan) and the West Junggar (China): Carboniferous porphyry Cu and Permian greisen W-Mo mineralization

This study focuses on the geochronology and elemental and Nd isotopic geochemistry of the Baogutu Cu deposit and the newly discovered Suyunhe W-Mo deposit in the southern West Junggar ore belt (Xinjiang, China), as well as the geology of the newly discovered Hongyuan Mo deposit in the southern West Junggar ore belt and the Kounrad, Borly, and Aktogai Cu deposits and the East Kounrad, Zhanet, and Akshatau W-Mo deposits in the North Balkhash ore belt (Kazakhstan). The aim is to compare their petrogenesis, tectonic setting, and mineralization and to determine the relationship between the southern West Junggar and North Balkhash ore belts. Based on our newly acquired results, we propose that the Kounrad, Borly, Aktogai, and Baogutu deposits are typical porphyry Cu deposits associated with calc-alkaline magmas and formed in a Carboniferous (327–312 Ma) subduction-related setting. In contrast, the East Kounrad, Zhanet, Akshatau, Suyunhe, and Hongyuan deposits are quartz-vein greisen or greisen W-Mo or Mo deposits associated with alkaline magmas and formed in an early Permian (289–306 Ma) collision-related setting. Therefore, two geodynamic–metallogenic events can be distinguished in the southern West Junggar and North Balkhash ore belts: (1) Carboniferous subduction-related calc-alkaline magma – a porphyry Cu metallogenic event – and (2) early Permian collision-related alkaline magma – a greisen W-Mo metallogenic event. The North Balkhash ore belt is part of the Kazakhstan metallogenic zone, which can be extended eastward to the southern West Junggar in China.     

From back-arc rifting to arc accretion: the Late Jurassic–Early Cretaceous evolution of the Guerrero terrane recorded by a major provenance change in sandstones from the Sierra de los Cuarzos area,central Mexico

The Guerrero terrane is composed of Middle Jurassic–Lower Cretaceous arc assemblages that were rifted from the North American continental mainland during Late Jurassic–Early Cretaceous back-arc spreading within the Arperos Basin, and subsequently accreted back to the continental margin in the late Aptian. The Sierra de los Cuarzos area is located just 50 km east of the Guerrero terrane suture belt and, therefore, its stratigraphic record should be highly sensitive to first-order tectonic changes. Two Upper Jurassic–Lower Cretaceous clastic units were recognized in the Sierra de los Cuarzos area. The Sierra de los Cuarzos Formation is the lowermost exposed stratigraphic unit. Petrographic data and U-Pb zircon ages suggest that the Sierra de los Cuarzos Formation was derived from quartz-rich sedimentary and igneous sources within the North American continental mainland. The Sierra de los Cuarzos Formation is overlain by the Pelones Formation, which is composed of volcanoclastic sandstones derived from a mix of sources that include the mafic arc assemblages of the Guerrero terrane and quartz-rich sedimentary and volcanic rocks exposed in the continental mainland. The provenance change documented in the Sierra de los Cuarzos area suggests that the Pelones Formation was deposited when the Arperos Basin was closed and the Guerrero terrane was colliding with the North American continental mainland. Based on these data, we interpret the Pelones Formation as the syn-tectonic stratigraphic record associated with the accretion of the Guerrero terrane.     

Geochemical Features and Metallogenic Prediction of the Caixiashan‐Weiquan Area in the East Tianshan Region

The Caixiashan-Weiquan area is an important ore concentration area in the eastern Tianshan metallogenic belt. Firstly, this paper studies geochemical features of 1564 samples of 1:200000 stream sediments of the Matoutan mapsheet, where the Caixiashan and Weiquan deposits are located. Processing, analysis and explanation of exploration geochemical data play an important role in the procedure of finding the ore, which are related to whether the measured elements content of geochemical samples can effectively guide the work of mineral exploration. As a highly nonlinear dynamical system, the neural network is more analogous to the human brains in terms of principles and features compared with conventional geochemical approaches. It can adapt itself to the environment, sum up laws, complete pattern recognition. Secondly, the authors used the Kohonen neural network to classify all samples based on 10 mineralization elements of stream sediment samples in order to determine possible mineral ores, reduce the scope of ore targets and study indicator elements of the ninth group of samples, which is the mostly closest to the deposit. The results show that the neural network can delineate metallogenic prospective areas and is effective in the discovery of deep geochemical information.     

Geological Characteristics and Zircon U‐Pb Dating of Volcanic Rocks from the Beizhan Iron Deposit in Western Tianshan Mountains,Xinjiang, NW China

The Beizhan large iron deposit located in the east part of the Awulale metallogenic belt in the western Tianshan Mountains is hosted in the Unit 2 of the Dahalajunshan Formation as lens, veinlets and stratoid, and both of the hanging wall and footwall are quartz-monzonite; the dip is to the north with thick and high-grade ore bodies downwards. Ore minerals are mainly magnetite with minor sulfides, such as pyrite, pyrrhotite, chalcopyrite and sphalerite. Skarnization is widespread around the ore bodies, and garnet, diopside, wollastonite, actinolite, epidote, uralite, tourmaline sericite and calcite are ubiquitous as gangues. Radiating outwards from the center of the ore body the deposit can be classified into skarn, calcite, serpentinite and marble zones. LA-ICP-MS zircon U-Pb dating of the rhyolite and dacite from the Dahalajunshan Formation indicates that they were formed at 301.3±0.8 Ma and 303.7±0.9 Ma, respectively, which might have been related to the continental arc magmatism during the late stage of subduction in the western Tianshan Mountains. Iron formation is genetically related with volcanic eruption during this interval. The Dahalajunshan Formation and the quartz-monzonite intrusion jointly control the distribution of ore bodies. Both ore textures and wall rock alteration indicate that the Beizhan iron deposit is probably skarn type.