Lianxing Gu

Petrogenesis of the Palaeoproterozoic Xishankou pluton, northern Tarim block, northwest China: implications for assembly of the supercontinent Columbia

The Tarim block, one of the largest cratons in China, records an important part of the Proterozoic crustal evolution of the Earth. Many previous studies have focused on the Neoproterozoic magmatism and tectonic evolution of this block in relation to the break-up of Rodinia, although relatively little is known about its earlier tectono-magmatic history. In this article, we present detailed petrographic, geochronologic, whole-rock geochemical, and in situ zircon Hf isotope data for the pre-Neoproterozoic Xishankou granitoid pluton (XBP), one of several blue quartz-bearing granitoid intrusions well exposed in the Quruqtagh area, and discuss these intrusions in terms of their tectonic environment. Zircon LA-ICP-MS dating indicates that gneissic quartz diorite and granodiorite of the XBP crystallized at 1934 ± 13 and 1944 ± 19 Ma, respectively. Both underwent metamorphism essentially coeval with emplacement, a time that is compatible with the globally distributed 2.1–1.8 Ga crustal amalgamation during formation of the supercontinent Columbia. Petrographic and geochemical evidence suggest that the XBP is a continental-arc-type granite and may have been generated by the partial melting of Archaean thickened lower crust; this would suggest that the northern Tarim block was a continental-type arc at ca. 1940 Ma. Our new data, together with previous regional geological studies, indicate that a series of Palaeoproterozoic (ca. 2.0–1.8 Ga) tectono-magmatic events occurred in the northern Tarim attending the assembly of Columbia.

Formation mechanisms of hydrocarbon reservoirs associated with volcanic and subvolcanic intrusive rocks: Examples in Mesozoic–Cenozoic basins of eastern China

Numerous petroleum-producing Mesozoic–Cenozoic basins are present in the coastal areas of eastern China. Voluminous volcanic and subvolcanic intrusive rocks, dominantly basaltic and, to a lesser degree, trachytic in composition, are intercalated or intruded in the sedimentary sequences. These magmatic rocks can serve as cap rocks as well as reservoir beds for hydrocarbons. Hydrocarbon reservoirs related to volcanic rocks can be classified into the volcanic-trapped type, the volcanic-sealed type, and the weathering crust type, and those related to subvolcanic intrusions include the doming-derived fracture type, the cryptoexplosive breccia type, the primary fracture type, the alteration zone type, the contact zone type, and the laterally sealed type. A generalized model for the formation of hydrocarbon reservoirs related to volcanic and subvolcanic rocks is proposed. We call for more attention to volcanic- and subvolcanic-related reservoirs during the exploration for hydrocarbons.

Geology and formation mechanism of late Quaternary shallow biogenic gas reservoirs in the Hangzhou Bay area, eastern China

Late Quaternary shallow biogenic gas reservoirs have recently been discovered and exploited in the coastal Hangzhou Bay area, northern Zhejiang Province, eastern China. The river in this area strongly incised the underlying old beds during a period of glacial maximum, which resulted in the formation of the Qiantangjiang and the Taihu incised valleys. These incised valleys were filled with fluvial sediments and buried by marine sediments during the postglacial period. Late Quaternary strata of the incised-valley area are composed of four sedimentary facies in ascending order: fluvial floor facies (IV), flood-plain facies (III), sublittoral-marine bay facies (II), and estuarine facies (I).All commercial gas fields occur in flood-plain sand bodies of incised valleys. The bodies are buried 3060 m (98197 ft) deep and are 3.07.0 m (9.823 ft) thick, with a maximum thickness of more than 10 m (33 ft). They are surrounded by impermeable clays. Rapid deposition of overlying sublittoral-marine bay sediments supplied not only abundant gas sources, but also good preservation conditions. The main hydrocarbon sources are dark gray clays of the flood-plain facies and gray muds of the sublittoral-marine bay facies. Sediments of both facies have organic carbon content generally more than 0.4%. Shallow biogenic gas fields and deep gas fields require vastly different drilling and completion techniques. Drilling and completion costs are much lower for the biogenic gas fields.Quaternary incised valleys and flood plains other than Hongzhou Bay in coastal areas of eastern China are promising targets of further exploration for shallow biogenic gas.

Hydrocarbon reservoirs in a trachyte porphyry intrusion in the Eastern depression of the Liaohe basin, northeast China

Hydrocarbon reservoirs in the Oulituozi area of the Liaohe basin, northeastern China, occur within a subvolcanic trachyte porphyry intrusion. This intrusion was emplaced at a depth of less than 1 km during the Paleogene. The prevalent reservoir porosity is in explosive breccias, fracture networks, steeply dipping cracks, and dilational microfractures in the apical zone of the intrusion. These voids were formed by underground explosion during magma solidification and by postsolidification volume contraction. Minor oil and gas volumes are trapped also in fractures in the overlying brittle beds. The massive rocks in the lower part of the porphyry body serve as hydrocarbon seals for reservoir beds lateral to the intrusion. We propose a model for reservoir formation related to a subvolcanic intrusion that includes three types of reservoirs: (1) reservoirs in the apical zone of the intrusion, (2) reservoirs in the upper wall of the intrusion, and (3) reservoirs lateral to the intrusion. Attention should be paid to all the three reservoir types during exploration around a subvolcanic intrusion.

Comparative study of ore-forming fluids of hydrothermal copper–gold deposits in the lower Yangtze River Valley, China

Fluid-inclusion and stable isotope studies were carried out on five types of Mesozoic (Yanshanian) hydrothermal copper–gold deposits in the lower Yangtze River Valley. Deposits include (1) copper in cryptoexplosive breccia pipes, (2) skarn copper, (3) porphyry copper, (4) high-temperature quartz vein-type copper and gold, and (5) medium–lower temperature fracture zone gold. This research has allowed a comparison between various types of ore-forming fluids. Melt-fluid inclusions in garnet from the matrix of the breccia pipe at the Shizishan copper deposit reveal the existence of a water-rich magma. In all deposit types, fluid temperatures and salinities were higher at early stages and generally decreased with time. Magmatic water is dominant in the high-temperature ore-forming fluids, whereas meteoric water was involved only in the medium–lower temperature Xiaomiaoshan gold deposit and in the post-mineralization stage of the Shaxi porphyry copper deposit. Fluid boiling played an important role in the mineralization of most deposits, particularly at Shizishan, where multi-stage boiling was associated with the formation of cryptoexplosive breccia, skarn, quartz-sulphide, and quartz-carbonate-sulphide stages. Boiling of an aqueous magmatic fluid system at high temperatures reflects the release of crystallization heat and increase of total volume of the magma–fluid system, and hence it can be referred to as active boiling. On the contrary, boiling of a fluid at lower temperatures is typically triggered by pressure release due to fracturing or dilation in the surrounding rocks, and is thus referred to as passive boiling. In general, passive boiling occurs more commonly at the higher levels of a hydrothermal mineral system and at later stages of the ore-forming process.

Transition from platemargin to intraplate environment: Geochemistry of basalts in Paleogene Liaohe basin, northeastern China

Paleogene basalts from the Liaohe basin, northeastern China, are dominated by alkaline olivine basalts and olivine basalts. These basalts are generally enriched in high field strength elements (HFSE), depleted in large ion lithophile elements (LILE) and comparable to those of typical ocean island basalts (OIB). Positive anomalies of Ba, Sr and Zr with high Nb/U, U/Pb, Ce/Pb and Zr/Hf ratios imply that materials from an oceanic crust had been added to the mantle sources of the basalts. In addition, the basalts are generally depleted in Sr, Nd and Pb isotopes, indicating that an enriched mantle (EMI) and a depleted mantle (DM) sources were added to the OIB-like resource.Comprehensive research on lithosphere evolution and tectonic setting of the Liaohe basin and surrounding areas suggests that these basalts were derived by variable degrees of partial melting from an upwelling asthenosphere mantle. Materials from an oceanic lithosphere were added to the source in company with Paleogene tectonic transition from platemargin to intraplate environment. Retreating and steepening of the subducting Pacific oceanic plate could be the main cause for the tectonic environment transition.

Geology, geochemistry and genesis of the Mazhuangshan gold deposit in Hami, East Tianshan, Xinjiang, China

AbstractThe Mazhuangshan area is located in the east of the Aqikekuduke Island Arc, where there are distributed intermediate-acid magmatic rocks emplaced during the Middle-Late Carboniferous. There are more than 20 orebodies in the area with an average gold grade 6.4 × 10−6 at present. The dominant metallic minerals are natural gold, auriferous silver, natural silver, pyrite and galena. Pyrite is the key gold carrier, high in Fe and low in S. Wall-rock alterations mainly include pyritization, silicification, and sericitization. Carbonation alteration was extensive at the late stage, often resulting in a high-grade orebody. Three mineralization stages may be distinguished.The peak homogenization temperatures of primary fluid inclusions range from 240 to 260°C. Mineralization pressures and depths are 47.2–68.8 MPa and 1.6–2.3 km respectively, showing the ore-forming features of hypergene gold deposit. The average salinity is 15 wt% NaCl equivalent. Fluid inclusion geochemistry data show that Ca2+ is far higher than Mg2+, and mK+/mNa+, mNa2+/mCa2+ (0.001∼0.338), mΣCl/mΣS and mΣC/mΣS ratios change with the reduction parameter [R = (CO + CH4)/CO2] and temperature. And the gold contents of ores and gangues are positively correlated withR. The pH values of inclusion water in quartz range from 5.8 to 7.4. Oxygen fugacity

Zircon U–Pb chronology and Hf isotope of the Xingxingxia granodiorite from the Central Tianshan zone (NW China): Implications for the tectonic evolution of the southern Altaids

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