The Omitiomire copper deposit is a relatively recent discovery in the Pan-African Damara Belt of central Namibia. The deposit is situated in Mesoproterozoic gneisses and amphibolites of the Ekuja Dome overlain by amphibolite-grade metaturbidites of the Southern Zone accretionary prism that formed during northward subduction of the Kalahari Craton below the Congo Craton between ca. 580–520 Ma. Copper mineralisation is confined to an anastomosing system of shallowly-dipping, retrograde mylonitic shear zones within the Ekuja Dome. The shear zones are centred around a lithologically heterogeneous amphibolite-gneiss sequence. Mylonitisation and copper mineralisation are closely associated with the retrogression of particularly amphibolites and the partial or complete replacement of amphibolites by biotite–epidote and biotite–chlorite–epidote schists that host the chalcocite-dominated mineralisation.Deformation and mineralisation in the heterogeneous shear-zone system can be shown to describe a progression. Initial strain localization is confined to lithological (amphibolite-gneiss) contacts and associated quartz veining and fluid flow are preferentially developed around the margins of competent amphibolite units. Fluid infiltration and the retrogression of amphibolites to biotite–epidote schists leads to strain localization into the marginal schists that envelop amphibolites. Further veining and fluid flow are localised into the central parts of amphibolite units leading to the pervasive retrogression to biotite–epidote schists that dominate the central parts of the shear-zone system. Earlier quartz-vein generations appear as isoclinally folded and dismembered ribbons or boudins in mineralised schists. The clearly syntectonic introduction of the copper mineralisation is underlined by the intergrowth of chalcocite with the retrograde assemblages and chalcocite forming part of the mylonitic shear-zone fabric.3D modelling of drillhole data combined with limited surface exposure delineates a shallow east dipping, gently undulating ore body parallel to the regional gneissosity of the Ekuja Dome. The ore body comprises several mineralised lenses varying in thickness from 10 m to > 100 m. Prominent ore shoots are gently doubly plunging to the N and S and parallel to the regionally developed L > S fabric in the gneisses. Kinematic indicators in the mineralised shear zone system point to a top-to-the S sense of shear, parallel to the regional L fabric and parallel to the southverging transport recorded in the structurally overlying prism metasediments.The regional setting of the Omitiomire deposit, kinematics, and retrograde, but high-temperature overprint of original mineral assemblages in the mineralised shear zones indicate deformation and fluid flow during the expulsion of the basement gneisses during N-ward direction subduction of the Kalahari Craton below the Congo Craton. Lithological, geochronological, structural and P–T data suggest numerous similarities and, indeed, correlations between the Omitiomire-style copper mineralisation of the Damara Belt with the large copper deposits hosted by basement gneisses in the Domes Region of the Lufilian Arc in Zambia. 相似文献
Baihua meta-igneous complex consists mainly of pyroxenite-gabbro(diorite)-diorite-quartz diorite. They form a complete comagmatic evolutionary series. The geochemical characteristics of basic-intermediate basic igneous rocks indicate that they belong to a tholeiite suite. The REE distribution pattern is nearly flat type and LREE is slightly enriched type, and their primitive mantle-normalized and MORB-normalized trace element spider diagrams are generally similar; the LIL elements (LILE) Cs, Ba, Sr, Th and U are enriched, but Rb, K and the HFSEs Nb, P, Zr, Sm, Ti and Y are relatively depleted. All these show comagmatic evolution and origin characteristics. The tectonics environment discrimination of trace element reveals that these igneous complexes formed in an island-arc setting. The LA-ICP-MS single-zircons U-Pb age of Baihua basic igneous complex is 434.6±1.5 Ma (MSWD = 1.3), which proves that the formation time of the island-arc type magmatite in the northern zone of West Qinling is Late Ordovician or Early Silurian, also reveals that the timing of subduction of paleo-ocean basin represented by the Guanzizhen ophiolite and resulting island-arc-type magmatic activities is probably Middle-Late Ordovician to Early Silurian.
In 2009, the Gulf of Mexico (GOM) Gas Hydrates Joint-Industry-Project (JIP) Leg II drilling program confirmed that gas hydrate occurs at high saturations within reservoir-quality sands in the GOM. A comprehensive logging-while-drilling dataset was collected from seven wells at three sites, including two wells at the Walker Ridge 313 site. By constraining the saturations and thicknesses of hydrate-bearing sands using logging-while-drilling data, two-dimensional (2D), cylindrical, r-z and three-dimensional (3D) reservoir models were simulated. The gas hydrate occurrences inferred from seismic analysis are used to delineate the areal extent of the 3D reservoir models. Numerical simulations of gas production from the Walker Ridge reservoirs were conducted using the depressurization method at a constant bottomhole pressure. Results of these simulations indicate that these hydrate deposits are readily produced, owing to high intrinsic reservoir-quality and their proximity to the base of hydrate stability. The elevated in situ reservoir temperatures contribute to high (5–40 MMscf/day) predicted production rates. The production rates obtained from the 2D and 3D models are in close agreement. To evaluate the effect of spatial dimensions, the 2D reservoir domains were simulated at two outer radii. The results showed increased potential for formation of secondary hydrate and appearance of lag time for production rates as reservoir size increases. Similar phenomena were observed in the 3D reservoir models. The results also suggest that interbedded gas hydrate accumulations might be preferable targets for gas production in comparison with massive deposits. Hydrate in such accumulations can be readily dissociated due to heat supply from surrounding hydrate-free zones. Special cases were considered to evaluate the effect of overburden and underburden permeability on production. The obtained data show that production can be significantly degraded in comparison with a case using impermeable boundaries. The main reason for the reduced productivity is water influx from the surrounding strata; a secondary cause is gas escape into the overburden. The results dictate that in order to reliably estimate production potential, permeability of the surroundings has to be included in a model. 相似文献
This paper systematically compares modeled rates of change provided by global integrated assessment models aiming for the 2 °C objective to historically observed rates of change. Such a comparison can provide insights into the difficulty of achieving such stringent climate stabilization scenarios. The analysis focuses specifically on the rates of change for technology expansion and diffusion, emissions and energy supply investments. The associated indicators vary in terms of system focus (technology-specific or energy system wide), temporal scale (timescale or lifetime), spatial scale (regional or global) and normalization (accounting for entire system growth or not). Although none of the indicators provide conclusive insights as to the achievability of scenarios, this study finds that indicators that look into absolute change remain within the range of historical growth frontiers for the next decade, but increase to unprecedented levels before mid-century. Indicators that take into account or normalize for overall system growth find future change to be broadly within historical ranges. This is particularly the case for monetary-based normalization metrics like GDP compared to energy-based normalization metrics like primary energy. By applying a diverse set of indicators alternative, complementary insights into how scenarios compare with historical observations are acquired but they do not provide further insights on the possibility of achieving rates of change that are beyond current day practice. 相似文献