西秦岭铀成矿带中志留系硅岩灰岩的成因研究

通过大量的剖面测制和室内分析，从岩石的地质产状、岩石学及地球化学特别是稀土元素地球化学方面研究了西秦岭铀成矿带中硅、灰岩的成因。原始沉积的泥晶灰岩和球粒泥晶灰岩沉积于水下高地较低能环境，球粒主要为粪球粒及少量硅质生物体。硅岩具生物化学沉积（Ｉ类）和由灰岩硅化（交代）成因（Ｉ类）两种成因类型，它们在地质产状、结构构造、常量元素及稀土元素地球化学等方面都存在显著差异。在综合分析的基础上建立了西秦岭铀成矿带早志留世沉积环境模式     

石灰岩地区砼灌注桩抽芯检测实例

以灰岩地区砼灌注桩两个抽芯工程为例，介绍灰岩地区砼灌注桩抽芯检查过程中应注意的问题，为砼灌注桩抽芯检查积累经验。     

云南遮放地区柳湾组灰岩岩石强度测试方法研究

分别采用回弹、点荷载和室内单轴抗压强度三种测试方法,对云南遮放地区中侏罗统柳湾组(J2l)灰岩岩石强度进行测试,结果显示该灰岩岩石强度值在47~56Mpa之间。通过对三种测试成果的对比研究,发现三者呈多项式关系,可通过经验公式相互换算。因此岩石强度研究中,将上述三种测试方法有机的结合,可获取较可靠的资料。     

Mesogenetic dissolution of the middle Ordovician limestone in the Tahe oilfield of Tarim basin,NW China

The mesogenetic dissolution is well developed in the middle Ordovician Yijianfang formation (O₂yj) limestone, and the dissolution pores are very important for petroleum accumulation in the south slope area of the Tahe oilfield which lies in the north of the Tarim basin, northwestern China. Mottled, dotted or laminar dissolution can be observed in the O₂yj limestone. Under microscope, the grains, lime matrix and all stages of calcite cements (including oil-inclusion-bearing blocky calcite cements) can all be found dissolved ubiquitously. The stylolites in the limestone were enlarged and rounded because of dissolution. Some dolomite rhombs, precipitated along stylolites in burial environment, were found dissolved as well. The dissolution of the blocky calcite cements and dolomite rhombs and the enlarging of stylolites demonstrate that the dissolution took place in the mesogenetic environment. Concentration of trace elements, including REEs, of the eroded part of the O₂yj limestone is intermediate between that of the uneroded part and that of the underlying lower Ordovician limestone hydrocarbon source rocks. Both δ¹³C_PDB and δ¹⁸O_PDB values of the eroded part are less than those of the uneroded part, respectively. The geochemical characteristics indicate that the eroding fluids are hydrocarbon-bearing fluids coming from the underlying hydrocarbon source rocks.     

Palæohydrology of the calcsilicate aureole of the Beinn an Dubhaich granite, Skye, Scotland: a stable isotopic study

The contact aureole developed in siliceous carbonates surrounding the Beinn an Dubhaich granite, Skye, shows textural and stable isotope evidence for infiltration of aqueous fluids during both prograde and retrograde metamorphism. Strongly depleted isotope compositions of reaction-product calcite correlate with high silica and fluorine contents, demonstrating a strong link between isotopic alteration and metasomatism by fluids with a significant magmatic component, even at the margins of the aureole. The oxygen and carbon isotope compositions of the carbonates form a linear cluster with a positive slope of about five, consistent with the depletion of isotope compositions by the infiltration of magmatic and/or meteoric fluids. Rayleigh fractionation during devolatilization played a minor role in determining the final isotope composition. Stable isotope compositions of coexisting calcite–dolomite pairs show varying amounts of isotopic disequilibrium, which correlate with the inferred fluid infiltration mechanism. Much of the calcite in dolostones is the product of infiltration-driven reactions along fractures, and is greatly depleted isotopically relative to the host dolomite, especially at talc grade. At higher grades the calcite–dolomite fractionation is smaller, probably due to both increased fluid–rock interaction and a greater tendency for fluid infiltration to be pervasive on the grain-scale. Limestones generally show near-equilibrium fractionation of oxygen and carbon owing to the overwhelming compositional influence of the host calcite. Veins formed during late-stage hydrothermal circulation have strongly ¹⁸O-depleted compositions relative to the host rock. No small-scale spatial patterns to the isotopic depletion were observed, but the extent of fluid infiltration was greatest in the west of the aureole. Fluid infiltration was clearly highly heterogeneous, with no evidence of a consistent flow direction. It is not possible to determine fluid fluxes or flow directions from one-dimensional flow models based on continuum flow in the Beinn an Dubhaich aureole.     

Evidence for the silicate source of relict soils on the Edwards Plateau, central Texas

Relict soils provide insights into Quaternary soil formation and erosion on the Edwards Plateau of central Texas and into soil-forming processes in karst terranes. Late Quaternary climate-driven soil erosion produced a mosaic of thick and thin soils on the Edwards Plateau landscape. Thick soils on uplands of the Edwards Plateau are interpreted to be relicts of a formerly more extensive soil cover that was eroded during the late Pleistocene to middle Holocene. The relict, thick soils are silicate-rich and most commonly overlie the relatively silicate-poor Cretaceous Edwards Limestone, which supports the idea that the thick soils did not form from weathering of the underlying limestone. Other potential sources of silicates for the relict soils include dust, alluvial sediments, and the Del Rio Clay, a stratigraphically higher but locally eroded clay-rich unit. Here we investigate the geographic distribution, texture, clay-sized mineralogy, rare earth element geochemistry, and neodymium isotope composition of the relict soils. We have found that the relict, thick soils are deeply weathered soils that occur dominantly over the Edwards Limestone and have a high clay content and a neodymium isotope composition that is similar to that of the Del Rio Clay. Thus, we propose that in situ weathering of the Del Rio Clay, along with partial weathering of the upper portion of the underlying Edwards Limestone produced thick chert- and clay-rich soils over resistant limestone. In areas like the Edwards Plateau, where pure limestones are interbedded with clay-rich strata, the overlying clay-rich strata must be considered as a possible silicate source to soils on pure limestone bedrock.     

Lewis Ponds, a hybrid carbonate and volcanic-hosted polymetallic massive sulphide deposit, New South Wales, Australia

The Lewis Ponds Zn–Pb–Cu–Ag–Au deposit, located in the eastern Lachlan Fold Belt, central western New South Wales, exhibits the characteristics of both volcanic-hosted massive sulphide and carbonate-hosted replacement deposits. Two stratabound massive to disseminated sulphide zones, Main and Toms, occur in a tightly folded Upper Silurian sequence of marine felsic volcanic and sedimentary rocks. They have a combined indicated resource of 5.7 Mt grading 3.5% Zn, 2.0% Pb, 0.19% Cu, 97 g/t Ag and 1.9 g/t Au. Main Zone is hosted by a thick unit of poorly sorted mixed provenance breccia, limestone-clast breccia and quartz crystal-rich sandstone, whereas Toms Zone occurs in the overlying siltstone. Pretectonic carbonate–chalcopyrite–pyrite and quartz–pyrite stringer veins occur in the footwall porphyritic dacite, south of Toms Zone. Strongly sheared dolomite–chalcopyrite–pyrrhotite veins directly underlie the Toms massive sulphide lens. The mineralized zones consist predominantly of pyrite, sphalerite and galena. Paragenetically early framboidal, dendritic and botryoidal pyrite aggregates and tabular pyrrhotite pseudomorphs of sulphate occur throughout the breccia and sandstone beds that host Main Zone, but are rarely preserved in the annealed massive sulphide in Toms Zone. Main and Toms zones are associated with a semi-conformable hydrothermal alteration envelope, characterized by texturally destructive chlorite-, dolomite- and quartz-rich assemblages. Dolomite, chlorite, quartz, calcite and sulphides have selectively replaced breccia and sandstone beds in the Main Zone host sequence, whereas the underlying porphyritic dacite is weakly sericite altered. Vuggy and botryoidal textures resulted from partial dissolution of the dolomite-altered sedimentary rocks and unimpeded growth of base metal sulphides, carbonate and quartz into open cavities. The intense chlorite-rich alteration assemblage, underlying Toms Zone, grades outward into a weak pervasive sericite–quartz assemblage with distance from the massive sulphide lens. Limestone clasts and hydrothermal dolomite at Lewis Ponds are enriched in light carbon and oxygen isotopes. The dolomite yielded ¹³C_VPDB values of –11 to +1 and ¹⁸O_VSMOW values of 6 to 16. Liquid–vapour fluid inclusions in the dolomite have low salinities (1.4–7.7 equiv. wt% NaCl) and homogenization temperatures (166–232°C for 1,000 m water depth). Dolomitization probably involved fluid mixing or fluid–rock interactions between evolved heated seawater and the limestone-bearing facies, prior to and during mineralization. ³⁴S_VCDT values range from 2.0 to 5.0 in the massive sulphide and 3.9 to 7.4 in the footwall carbonate–chalcopyrite–pyrite stringer veins, indicating that the hydrothermal fluid may have contained mamgatic sulphur and a component of partially reduced seawater. The sulphide mineral assemblages at Lewis Ponds are consistent with moderate to strongly reduced conditions during diagenesis and mineralization. Low temperature dolomitization of limestone-bearing facies in the Main Zone host sequence created secondary porosity and provided a reactive host for fluid-rock interactions. Main Zone formed by lateral fluid flow and sub-seafloor replacement of the poorly sorted breccia and sandstone beds. Base metal sulphide deposition probably resulted from dissolution of dolomite, fluid mixing and increased fluid pH. Pyrite, sphalerite and galena precipitated from a relatively low temperature, 150–250°C hydrothermal fluid. In contrast, Toms Zone was emplaced into fine-grained sediment at or near the seafloor, above a zone of focused up-flowing hydrothermal fluids. Copper-rich assemblages were deposited in the Toms Zone footwall and massive sulphide lenses in Main and Toms zones as the hydrothermal system intensified. During the D₁ deformation, fracture-controlled fluids within the Lewis Ponds fault zone and adjacent footwall volcanic succession remobilized sulphides into syntectonic quartz veins. Lewis Ponds is a rare example of a synvolcanic sub-seafloor hydrothermal system developed within fossiliferous limestone-bearing facies. The close spatial association between limestone, hydrothermal dolomite, massive sulphide and dacite provides a basis for new exploration targets elsewhere in New South Wales.Editorial handling: D. Lentz     

Discussion of Jamaican Cenozoic ichnology: review and prospectus: (v. 50, pp. 364–382)

We make two comments on the paper ‘Jamaican Cenozoic ichnology: review and prospectus’ by Donovan et al. (2015). Based on biostratigraphy and new radiometric dates, we recommend the separation of the Richmond Formation (deposited in the Wagwater Graben and onlapping onto the edges of the Clarendon Block and Blue Mountains Block) and the Moore Town Formation (deposited in the John Crow Mountain Graben) which are of early Eocene and early Paleocene age, respectively. The ichnofossils of the Scolicia ichnocoenesis are different in the two basins. We also point out that extensive borings recorded from the late Eocene Somerset Formation of the White Limestone Group most probably came from the late Oligocene Walderston Formation. Copyright © 2015 John Wiley & Sons, Ltd.