There are no reported experimental data on hydrogen isotope fractionation between muscovite and water at low temperatures (< 400 °C). A fractionation curve derived from extrapolation of the high temperature calibration of Suzuoki and Epstein (1976) yields 20 to 40%. higher D values than the empirical graphical calibration of Bowers and Taylor (1985) at temperatures of about 300 °C. Data from natural hydrothermal systems formed at approximately 300 °C, where D analyses are available both from fluid inclusions and alteration muscovite/sericite, support the Bowers and Taylor (1985) calibration, thus indicating smaller fractionation factors at these temperatures than suggested by extrapolations from high-temperature experimental results. 相似文献
From rocket and radar-meteor wind observations, annual and semi-annual components of the zonal flow are derived for latitudes N at heights between 60 and 130 km. Height regions of maximum and minimum amplitude are described with reference to changes in phase. The annual components decrease with height throughout the mesosphere and, after a reversal of phase, enhance to 25 m/sec at 100 ± 5 km. The semi-annual components have maximum amplitudes of 25 m/sec over a wide range of latitude in two height regions at 90 and 120 km and in a limited range of latitude (near 50°) at 65 km.
Calculated temperatures and log densities are discussed in terms of amplitude and phase as functions of height and latitude. Below 100 km a comparison is made with temperature amplitudes derived from independent temperature data. Above 100 km the annual temperature variation maximizes at 115 km and is particularly large at high latitudes (exceeding 50°K). On the other hand, the semi-annual component increases rapidly with height between 110 and 120 km at all latitudes maximizing at the 120 km level, where amplitudes exceed 25°K at high and low latitudes and 10°K at mid-latitudes. The annual component of log density, like the temperature variation, is largest at high latitudes up to 125 km. The semi-annual variation has a minimum at 110–115 km, above which amplitudes increase with height, reaching 5–12 per cent at 130 km according to latitude. The phases at and near 130 km for the annual and semi-annual density variations are very close to those found at greater heights from satellite orbits and amplitudes could be readily extrapolated to agree with those in the satellite region. 相似文献
Stratabound, uraniferous diagenetic xenotime cements provide a minimum depositional age of 1,632±3 Ma for the sedimentary
Birrindudu Group in the Killi Killi Hills, Tanami Region in northern Australia. The age of xenotime formation is broadly coeval
with that recently proposed (1,650–1,600 Ma) for uranium mineralisation in the unconformity-associated deposits of the Pine
Creek Inlier, northern Australia, and Athabasca Basin, Canada. The geological setting and formation model for the uraniferous
xenotime crystals are similar to those widely proposed for unconformity-associated uranium deposits, suggesting a genetic
link between the two. However, xenotime formation in the Birrindudu Group occurred during an apparently earlier stage of diagenesis,
compared to late diagenetic formation of unconformity-associated uranium deposits. This could be explained by variations in
the thickness of sediment cover and diachronous diagenesis across the basin, at the time of the basin-wide uranium mobilisation
event, herein dated at ca. 1,630 Ma. In such a scenario, stratabound uraniferous xenotime cements could represent the remote
distal zones of a more deeply buried, uranium mineralising system. Alternatively, the xenotime layer represents a precursor
to, or a source for, later unconformity-associated ore deposition. In this case, the presence of diagenetic uraniferous xenotime
in an area prospective for unconformity-associated uranium mineralisation would be an indication of, and still provide an
approximate age for, uranium mobilisation within the cover sequence. Xenotime is a far more robust mineral than uraninite
for U–Pb geochronology and can potentially provide a more reliable and precise timeframe for uranium mineralisation and subsequent
recrystallisation events if present in the immediate uranium-ore environment. 相似文献
Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit “trends.” Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the trends. Interestingly, the large Cretaceous Alaska–Yukon intrusion-related gold deposits (e.g., Fort Knox) are associated with the northern extension of the same lithospheric margin in the Selwyn basin, which experienced an analogous series of geologic events. 相似文献
The Racetrack Au−Ag deposit, in the Archaean Yilgarn Block, Western Australia, is hosted by a porphyritic basalt in a low
greenschist facies setting and is associated with a brittle strike-slip fault system. Three distinct and successive stages
of hydrothermal activity and late quartz-carbonate veining resulted in multiple veining and/or brecciation: Stages I and II
are Au-bearing, whereas Stage III and late veins are barren. The ore shows features of both classic epithermal and mesothermal
deposits. Alteration assemblages, typified by sericitization, carbonization, silicification and chloritization, are similar
to those of mesothermal gold deposits, wheras the quartz vein-textures including comb, rosette, plumose and banded, ore mineralogyof
arsenopyrite, pyrite, chalcopyrite, sphalerite, galena, freibergite, tetrahedrite, tennantite, fahlore, electrum and gold,
and metal associations (Cu, As, Ag, Sn, Sb, W, Au and Pb) are more characteristics of epithermal deposits. Fluid inclusions
related to Stage II are two phase and aqueous with 1–8 (average 4) wt. % NaCl equiv. and CO2 content of <0.85 molal. Pressure-corrected homogenisation temperatures range from 190°C to 260°C. Mineral assemblages indicate
that ore fluid pH ranged between 4.2 and 5.3, fO2 between 10−38.8 and 10−39.6 bars, and mΣs between 10−3.2 and 10−3.6. Calculated chemical and stable isotope compositions require a component of surface water in the ore fluid depositing the
mineralisation, but evidence for deep crustal Pb indicates that deeply sourced fluids were also involved. The deposit is interpreted
to have formed in a shallow environment via mixing of deeply sourced fluids, from at least as deep as the base of the greenstone
belt, with surface waters. It therefore represents the upper crustal end-member of the crustal depth spectrum of Archaean
lode-gold mineralisation. 相似文献
The Archaean greenstone terrane between Menzies and Kambalda exhibits a coherent, although deformed, stratigraphic sequence
intruded by granitoids and bounded by major NNW-trending shear and/or fault zones. The greenstone terrane hosts a large number
of lode gold prospects and deposits, including the giant Kalgoorlie deposits. The initial Pb isotope compositions of lode
gold deposits, as determined from ore related galena and pyrite, vary systematically in a linear trend on a207Pb/204Pb versus206Pb/204Pb diagram which reflects crustal heterogeneity at the time of mineralisation. Deposits hosted within a 90 km section of the
Menzies-Boorara Shear Zone have a uniform, radiogenic initial Pb isotope composition irrespective of temperature of mineralisation
and proximity to granitoid-gneiss in plan view. The Pb in these deposits is considered to be derived largely from older felsic
crust underlying the greenstone belt and was accessed via this major shear-zone system. Deposits in a transect unrelated to
a major shear zone show a systematic correlation between initial Pb isotope compositions and proximity to granitoid-gneiss
and/or to mineralisation temperature. These compositions are less radiogenic than those within the Menzies-Boorara Shear Zone,
but trend on a207Pb/204Pb versus606Pb/204Pb diagram between this isotope signature and the uniform Pb isotope signature which characterises the >100 km greenstone
transect from the Mt Pleasant area through Kalgoorlie to Kambalda. These data are interpreted to reflect Pb derivation from
discrete crustal segments within and below the greenstones, and require that mineralisation was related to crustal-scale hydrothermal
systems that accessed both sialic mid- to lower-crust and the greenstone succession. 相似文献
The Camaquã Cu (Au, Ag) and Santa Maria Pb-Zn (Cu, Ag) deposits are the largest base-metal deposits discovered so far in the sedimentary clastic sequences of the Neoproterozoic-age Camaquã Basin. The origin of the Camaquã-Santa Maria deposits has been the centre of dispute, with three alternative genetic hypotheses proposed: a syngenetic model, a diagenetic model, and a magmatic hydrothermal model. In detail, this mineralization has been suggested to be related to sedimentary clastic-diagenetic processes, volcanic-related processes, or deep granitic magmatism.Reevaluation of previous data and new studies in the area yield the following conclusions: (1) mineralization is fracture-controlled and magmatic-hydrothermal in origin rather than stratiform syngenetic or diagenetic; (2) the temperature of deposition of the main ore minerals was 210 to 300°C; (3) the ∂34S of sulphides of around 0% indicates an external magmatic-hydrothermal source of sulphur; (4) Pb isotope ratios of sulphides indicate that metals were derived at the end of the Brasiliano Cycle from a large crustal source with very primitive Pb and that (5) the age of mineralization is 594 Ma, as constrained by U/Pb SHRIMP (Sensitive High Resolution Ion Microprobe) determinations on zircons of the Lavras Granite.Thus, the Camaquã and Santa Maria deposits are interpreted to be of magmatic-hydrothermal origin, with the metals derived from an old crustal-basement source during the end of Dom Feliciano Collisional Orogeny, at 594 Ma, late in the Brasiliano Cycle.The interpretation above is critical for base-metal exploration in the Sul Riograndense Shield. Previous exploration methodologies mainly followed models based on a sedimentary hypothesis for the origin of the deposits. However, the occurrence of mineralization along fractures within specific wall-rocks requires consideration of alternative exploration parameters. These include: (1) ancient EW- and NW-trending regional fractures and their intersections, which are potential structural sites for base-metal mineralization, and (2) preferential wall-rock sites, either rocks with high initial porosity or secondary fracture-induced porosity or soluble rocks which are susceptible to replacement processes. Post-collisional plutonism of the Dom Feliciano Orogeny was the most likely heat source, and also the source of sulphur and base metals. Gravity surveys and alteration studies would be useful to determine the presence of intrusive bodies at depth which may have the potential to host porphyry-style Cu-Au deposits. 相似文献
A comparison between tin-bearing granitoids in an anorogenic setting (Bushveld Complex) and an orogenic setting (Blue Tier Batholith, Tasmania) reveals a number of genetically important similarities. These include: in situ fractional crystallization characterised by marked decrease in Ba and Sr and increase in Rb; the accumulation of late melt in a sheet-like form near the roof zone; the association of barren pegmatites overlying the ore; and of aplites; and the occurrence of conformable tin-bearing sheets, often exhibiting greisenization. These features allow the formulation of the following genetic model. A crustally-derived granitoid magma is emplaced and undergoes fractional crystallization from the margins inwards, with bottom crystallization dominating. Disruption of earlier formed solids by rest liquid commonly occurs. Continued fractional crystallization causes enrichment in volatiles and incompatible elements in the late rest melts, which have a sheet-like habit. The efficiency of enrichment of incompatible elements is critically dependant on the degree of separation of melt from solids throughout crystallization. An early, tin-poor vapour may separate after initial water-saturation of the magma is achieved, and this collects under the roof, commonly forming an impermeable barrier to later tin-bearing fluids. Continued fractional crystallization on the floor further enriches incompatible elements, and at a very late stage a Sn-rich vapour separates within the intercumulus phase and becomes concentrated by progressive crystallization of the intercumulus melt. At a late stage of solidification, this vapour loses equilibrium with the earlier formed feldspars and greisenization ensues, accompanied by the crystallization of cassiterite and other ore minerals. The nature of the mineralization changes if through-going fractures tap the late fluids. This model predicts systematic changes in trace element geochemistry with crystallization which provide useful tools for assessing the tin potential of a granitoid, and for indicating the direction of crystallization of the magma, and hence the location of possible ore. 相似文献