根据地球化学特征在古火山岩区寻找金矿

介绍了在湖南益阳古火山岩区依据地球化学特征寻找金矿及其地质效果。针对古火山岩区成矿元素的分布规律 ,按元素组合 ,区分不同含金地质体 。

赣东北张村岩群古火山岩地球化学特征及形成构造环境

赣东北张村岩群所含之中基性变火山岩的地球化学研究表明 :其主元素具低钾高钠 ;稀土元素具Eu亏损 ,Pr、Tb、Tm富集 ;微量元素具Rb、Zr、Ce微弱亏损 ,部分样品Ta富集 ,其总体特征类似于岛弧拉斑玄武岩 。

石家庄市西部古元古代火山岩地球化学特征

石家庄西部古元古代火山活动形成一套火山—沉积岩系列,间歇明显,前期火山活动较弱,呈薄层状、透镜状产出,后期火山活动增强,火山岩呈厚层状产出。根据地质特征、岩石学特征研究,以及地球化学样品分析,石家庄西部古元古代火山岩大离子亲石元素(LILE)Rb、Ba明显富集,Sr表现出正异常,Nb和Ta相对亏损,指示其岩浆可能来源于地壳重熔,或在岩浆上升过程中受到了壳源物质的混染,所有样品中轻重稀土分异非常弱,没有明显的Nb、Ta、Zr、Hf负异常,与岛弧或者大陆边缘弧中的基性火山岩有明显的差异,类似于陆内玄武岩。通过研究系列的地球化学图解,古元古代甘陶河群碎屑岩夹基性火山岩沉积建造形成于板内拉张的构造环境。

Geology and geochemistry of Huanggangliang stratabound skarn deposit in Inner Mongolia,China

Over the recent three decades, exploration of a large-size Sn-Fe polymetallic ore deposit at the Huanggangliang, the Da Hinggan Mountains, Inner Mongolia, China, has been largely focused on the premise that the mineralization represents epigenetic magmatic hydrothermal ore deposit in genetic connection with the Mesozoic magmatism. The Huanggangliang Sn-Fe polymetallic ore deposits occurred in Permian strata, with silt- stone/marble of the Zhesi Formation and spilite/andesite/tuff of the Dashizhai Formation. The characteristics of geological and geochemical data demonstrated that sedimentary hydrothermal mineralization occurred during the basin evolution at the Permian, rather than representing epigenetie magmatic hydrothermal genesis with the following evidences. The ore-bearing skarus are stratiform with underlying metasedimentary rocks and overlying volcanic sedimentary rocks. Sedimentary hydrothermal textures and structures are observed in the stratabound ore-bearing skarn such as bedded-laminated skarn and magnetite ores with small-size folding or soft deforma- tion, synsedimentary breccia of skarn and magnetite ores and concentric shell structure of magnetite ores. So the stratabound ore-bearing skarns associated with magnetite ore and micro-disseminated tin, are peculiar examples of exhalite. The REE contents of the stratabound ore-bearing skarns display ∑LREE-rich （36.91×10^-6） but EHREE-depletion （6. 42 ×10^-6） , with positive Eu anomaly （Eu/Eu ＊ 1.28） and negative Ce anomaly （Ce/ Ce ＊ 0.88 ） , which is totally different from REE pattern of the Huanggang magmatic rocks（ with ∑REE 277.73 ×10^-6, ELREE 220.24 ×10^-6, ∑HREE 57.49 ×10^-6, Eu/Eu＊ 0.06, Ce/Ce＊ 1.52） , which is comparable with modern sea-floor hydrothermal fluid,sedimentary hydrothermal ore deposit and associated hydrothermal sedimentary rocks. Calcite samples with δ13CPDB from -5. 400 ‰ to -4. 397 ‰ and δ18SOSMOW from 9. 095 ‰ to 9. 364 ‰ in the stratabound ore-bearing skarns show sedimentary hydrothermal genesis of the Huanggangliang deposit. This proposition is useful not only for interpretation of the genesis of the Huanggangliang large Sn-Fe polymetallic ore deposit but also significant for mineral exploration in the area especially for finding large deposits caused by sedimentary exhalative mineralization processes.

Peperites in the Permian Tarim large igneous province in Northwest China and their constraints on the local eruption environments

Peperites are special kinds of volcaniclastic materials generated by mingling of magma and unconsolidated sediments. They directly demonstrate the contemporaneity of volcanism and sedimentation, and hence they can be used to constrain the local paleoenvironments during volcanic eruptions. We identified peperites in the lower sequence of the northwest outcrops（Inggan-Kalpin area） of Permian Tarim large igneous province（TLIP）, Northwest China. In Inggan, blocky peperites were observed at the base of lava flows generated in the second eruption phase. This kind of peperites is generated by quenching of magma in a brittle fragmentation mechanism. While in Kalpin, both the second and the fourth eruption phases preserved peperites in the base of lava flows. Not only blocky but also fluidal peperites can be observed in Kalpin. The fluidal peperites were generated in vapor films, which insulated the magmas from cold sediments and avoided direct thermal shock, and therefore kept the fluidal forms of magma. All of these peperites are hosted by submarine carbonates. In lava sequences generated in the same eruption phases but located in Kaipaizileike, ~15 km east to Inggan, terrestrial flood basalts developed while peperites are absent, implying a paleoenvironmental transition between Kaipaizileike and Inggan-Kalpin area. Gathering information from observed peperites, TLIP lava flows, and the Lower Permian sedimentary strata, we precisely constrained the spatial distribution and temporal evolution of sedimentary facies of the early stage of TLIP. As a result, two marine transgressions were identified. The first transgression occurred contemporaneous with the second eruption phase. The transition from submarine to subaerial is located between Kaipaizileike and Inggan. The second transgression occurred contemporaneous with the forth eruption phase, and the transition from submarine to subaerial occurred between Inggan and Kalpin.

Paleomagnetism of the Late Cretaceous volcanic rocks of the Shimaoshan Group in Yongtai County, Fujian Province

It has been debated whether there was southward movement of the South China Block （SCB） during the Cretaceous. In this study, a paleomagnetic investigation was carried out on the Late Cretaceous volcanic rocks （-88 Ma） of the Shimaoshan Group in Yongtai County, Fujian Province. Rock magnetic experiments showed that magnetite in pseudo-single-domain and multi-domain grain and hematite were predominant magnetic phases. Stepwise thermal demagnetization successfully isolated characteristic directional components at high-temperature interval （〉 500℃） from 383 specimens in 19 sites, which yielded a paleomagnetic pole for the studied section at 83.1°N, 152.6°E （N = 19, A95 = 3.9°）, and the scatter SB = 9.0. The Fisher distri- bution of virtual geomagnetic poles （VGPs） and the consistence of S8 with the expected value at the 95% confidence level in- dicate that the yielded paleomagnetic pole is free of paleomagnetic secular variation influence. The new pole, which is well consistent with that from the Eurasian apparent polar wander path （APWP） curve, suggests no obvious southward movement of the sampling site during the Cretaceous.

The discovery of Palaeoproterozoic volcanic rocks in the Bulunkuoler Group from the Tianshuihai Massif in Xinjiang of Northwest China and its geological significance

In the Dabudaer region of the Tianshuihai Massif （Xinjiang, Northwest China）, metavolcanic rocks within the mainly meta- sedimentary Bulunkuoler Group are basalt, basaltic andesite, and rhyolite. In situ zircon U-Pb LA-ICP-MS dating on rhyolite magmatic zircons yielded an age of 2481±14 Ma, interpreted as the eruption age of the Bulunkuoler Group volcanic rocks. The basalt and basaltic andesite are low-A1 tholeiite series and the rhyolite is low-Al calc-alkaline series. All of rocks are charac- terized by enrichment of LREE and LILE （Rb, Th and Ba）, depletion of P, Nb, Ta and Ti, and absence of Eu anomalies. The Cnd（t） of basalts ranges from 3.14 to 4.88 indicating a depleted mantle source. The trace element signatures show that these magmas experienced intense crustal contamination during their ascent. Direct evidence for crustal contamination is the xenocrystic zircons in the ryholite, with ages back to ca. 3300 Ma. The primitive-mantle normalized and Zr/Y-Zr diagrams for the basic volcanic rocks show that they formed in an intra-continental tectonic setting. Combined with the previous studies and compared with North China Craton Neoarchean basic volcanic rocks, it is supposed that the Bulunkuoler Group volcanic rocks reflect the Palaeoproterozoic mantle magma underplating and interaction with felsic crust.

Thermal history of the Sichuan Basin,SW China:Evidence from deep boreholes

The Sichuan Basin is a superimposition basin composed of terrestrial and marine sediments that is well known for its abundant petroleum resources. Thermal history reconstruction using paleogeothermal indicators, including vitrinite reflectahoe and thermochronological data, shows that different structural subsections of the Sichuan Basin have experienced various paleogeothermal episodes since the Paleozoic. The lower structural subsection comprising the Lower Paleozoic to Middle Permian （Pz-P2） successions experienced a high paleogeothermal gradient （23.0-42.6℃/km） at the end of the Middle Permian （P2）, whereas the upper structural subsection comprising Late Permian to Mesozoic strata underwent a relatively lower paleogeothermal gradient （13.2-26.9℃/km） at the beginning of the denudation （Late Cretaceous or Paleocene in the different regions）. During the denudation period, the Sichuan Basin experienced a successive cooling episode. The high paleogeothermal gradient resulted from an intensive thermal event correlated to the Emeishan mantle plume. The heat flow value reached 124.0 mW/m2 in the southwestern basin near the center of the Emeishan large igneous province. The low geothermal gradient episode with heat flow ranging from 31.2 to 70.0 mW/m2 may be related to the foreland basin evolution. The cooling event is a result of the continuous uplift and denudation of the basin.