Application of Multi-well Subsidence Analysis of the Beryl Embayment,Viking Graben, Northern North Sea

1 Introduction A series of studies have indicated that there were two extensional phases in the North Sea (Fig. 1). An earlier period (Late Permian-Early Triassic) of rifting occurred widely in these areas, with predominant extension direction of W-E (F?rseth, 1996; F?rseth et al., 1997). In contrast to the widely distributed Permo-Triassic extension, Jurassic extension in the North Sea were generally much more localized into the three main rift arms (Fig. 1): the Viking Graben, Moray…     

Detailed bed topography and sediment load measurements for two stepdown flows in a laboratory flume

Streams and rivers, particularly smaller ones, often do not maintain steady flow rates for long enough to reach equilibrium conditions for sediment transport and bed topography. In particular, streams in small watersheds may be subject to rapidly changing hydrographs, and relict bedforms from previous high flows can cause further disequilibrium that complicates the prediction of sediment transport rates. In order to advance the understanding of how bedforms respond to rapid changes in flow rate,...     

Depositional record of historic lahars in the upper Whangaehu Valley, Mt. Ruapehu, New Zealand: implications for trigger mechanisms, flow dynamics and lahar hazards

Mt. Ruapehu, in the central North Island of New Zealand, is one of the most lahar-prone volcanoes in the world. Since historic observations began in 1861 AD, more than 50 individual lahars have been recorded in the Whangaehu valley alone, the natural outlet to the summit Crater Lake. These lahars have been triggered by a variety of mechanisms, including explosive eruptions that displaced Crater Lake water over the outlet or ejected it onto the snow-clad summit area of the volcano; rain-remobilisation of tephra deposits on steep slopes; displacement over the outlet as a result of syn-eruptive changes in lake bathymetry; and lake break-outs from Crater Lake following impoundment of excess water behind temporary barriers of tephra and/or ice emplaced over the outlet. However, only 9 lahar deposits can be distinguished in the upper Whangaehu valley on sedimentological, stratigraphic, geomorphic and petrological grounds, and these are skewed towards either the largest or the most recent flows. In some cases magnitude can be reconstructed from deposit geometry, with the largest lahars producing the highest level terraces, the coarsest deposits, and crossing drainage divides into normally inactive channels. This under-representation of historic events reflects the low preservation potential of unconsolidated deposits in a steep alpine environment, and the overprinting and recycling effect of large magnitude lahars that rework material down to bedrock and effectively reset the stratigraphic record. Development of magnitude-frequency relationships for Ruapehu lahars therefore requires the identification of lahar deposits in proximal, medial and distal settings in order to ensure that the full range of events is represented.     

Granite-hosted gold mineralization at Timbarra, northern New South Wales, Australia

The Timbarra gold deposits, located in the southern New England Fold Belt of New South Wales, Australia, represent an economically significant and distinctive member of the intrusion-related class of gold deposits. The five known deposits possess a total identified mineral resource of 16.8 Mt at 0.73 g/t gold, for a total of 396,800 contained ounces. The granites in the Timbarra region form a texturally complex, zoned pluton. The gold deposits are found within the Stanthorpe leucomonzogranite (242 to 238 Ma), which intrudes and forms a core to the more mafic, barren, Bungulla monzogranite (248 to 243 Ma). Gold is disseminated in the roof zone (upper 240 m) of a fractionated, magnetite- and ilmenite-bearing, I-type leucomonzogranite phase of the Stanthorpe body. The entire gold resource occurs in the areally extensive main leucomonzogranite pluton and is hosted by a medium- to coarse-grained granite. Disseminated ore is present in all five deposits, comprises >95% of the overall resource at Timbarra, and occurs predominantly as gently dipping, tabular to lenticular bodies that are conformably constrained beneath a fine-grained aplite carapace and internal aplite layers. The disseminated ore consists of gold-bearing muscovite-chlorite-carbonate alteration and infill of primary miarolitic cavities within massive leucomonzogranite or microgranite, and contains no discernable vein, joint, or fracture control at the outcrop or hand specimen scale. Structurally controlled mineralization forms the remaining 5% of the Timbarra resource, and comprises minor, low-density (0.02 to 0.25 per meter), vein-dikes and quartz-molybdenite veins emplaced along steeply dipping east-southeast, east-northeast, and north-northeast striking cooling joints. Both mineralization styles and alteration share a common paragenetic sequence of mineral precipitation. Quartz, perthitic K-feldspar, minor biotite, and albite are the earliest and most abundant infill minerals and commonly line primary cavities and vein-dikes. Subsequent minerals include coeval arsenopyrite, pyrite, fluorite, and molybdenite. The latest minerals include muscovite, chlorite, gold, calcite, silver-bismuth telluride, lead-bismuth telluride, and rare galena and chalcopyrite. The gold ore has a low total sulfide mineral concentration (Б%). Ore contains elevated concentrations of Bi, Ag, Te, As, Mo, and Sb; gold is strongly correlated with Bi, Ag, and Te, but only weakly with Mo, As, and Sb. Gold grains are generally <1 to 50 µm in size, but rarer grains as large as 1 mm in diameter have been observed. Gold fineness ranges from 950 to 600, and varies both within and between individual grains for a given deposit. The moderately oxidized I-type host granite, low-sulfide (Б%) ores, Au-Bi-Ag-Te geochemical signature, muscovite-chlorite-carbonate alteration assemblage, and low-salinity aqueous and carbonic fluids suggest that Timbarra is part of the newly recognized intrusion-related gold deposit class. Timbarra is distinguished from other intrusion-related gold deposits by the disseminated mineralization style within pervasively altered granite, forming gently dipping, tabular to lenticular ore zones.     

Application of CCSEM to heavy mineral deposits: Source of high-Ti ilmenite sand deposits of South Kerala beaches, SW India

For the characterization of sediments, Computer Controlled Scanning Electron Microscopy (CCSEM) is a powerful method in obtaining chemical data on individual mineral grains and modal analysis of the heavy mineral fraction of sediment samples. Here we show how the CCSEM method can be used to evaluate ilmenite ore grade as well as a tool to investigate the source of heavy mineral deposits.The heavy mineral rich deposits in beach sands around the town of Chavara in SW India are characterized by ilmenite with elevated TiO₂ contents, often exceeding 60 wt.%. In order to determine the origin of these high-TiO₂ ilmenite deposits, we collected a series of beach sediment samples (22) from a c. 800 km long stretch of coastline from northern Kerala state to well within the Tamil Nadu state. A set (7) of river sediments was also taken, roughly covering the catchment area to the beach samples. The data show that the sediments in the Chavara high-Ti ilmenite deposit are distinguished by minor elements in ilmenite, garnet chemistry and heavy mineral assemblage: Chavara ilmenite has high MgO and low MnO contents; garnets have low grossular components and the heavy mineral assemblage is dominated by sillimanite–kyanite in addition to ilmenite. These features correlate with basement geology in the hinterland, and with sediments from rivers, draining the basement. Based on these observations we conclude that high-Ti ilmenite from Chavara beaches originates in the khondalite belt of high-grade metasediments. Our study demonstrates rapid mineral analyses in sediments by CCSEM to be efficient in the characterization of mineral compositions and assemblages in sediments, in the identification of possible source regions and thus ultimately in exploration for industrial mineral resources.     

The partitioning of trace transition elements in crystal structures: a provocative review with applications to mantle geochemistry

Interrelationships between chemical compositions and crystal structures of minerals pioneered by Goldschmidt have been overlooked by modern geochemists. While analytical techniques and data for trace elements have become more sophisticated, progress has been slow in relating abundance data of individual elements to their relative enrichments in specific sites in mineral structures. The concept of diadochy has degenerated into an analytical relationship between major and trace elements, instead of its original crystallographic basis of replacement of one atom by another at a specific site in a crystal structure. Future interpretations of trace element data must consider the partitioning of atoms between different coordination sites in multisite mineral structures.The well-known partitioning of nickel and chromium into the earliest minerals during magmatic crystallization is extended to magma evolution in the mantle. The strong preference of Ni²⁺ and Cr³⁺ for octahedral sites in minerals leads to their enrichment in certain Iherzolites which are refractory residua during partial fusion of the mantle.     

NEW SET OF LOW CONCENTRATION STANDARDS FOR LA, CE, SM, YB, LU, Y, SC, V, NB AND TA IN SILICATES

We have developed a set of four synthetic standards for the rare earth and high field strength elements designed for use in the determination of those elements in silicates. The base material is a glass nominally at the eutectic of the MgO-Al₂O₃-SiO₂ ternary system. The nominal doping level was 2 wt% of the elements as oxides. To avoid problems associated with peak interference, the elements were doped in four separate glasses.