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71.
The Polaris deposit is one of the largest Mississippi Valley-type deposits in the world, with 22 million tonnes of ore at
14% Zn and 4% Pb contained in a single, compact orebody surrounded by dolomitized host rocks. Using detailed sampling of carbonates
in the orebody and the dolostone halo, this paper aims to characterize the temporal and spatial evolution of the mineralizing
system, and to understand the mechanisms that controlled the accumulation of this large, compact Zn–Pb deposit. Five types
of dolomite have been distinguished, including three replacement (RD) and two pore-filling dolomites (PD). The paragenetic
order is RD1, RD2, RD3, PD1, and PD2. Pore-filling calcite (PC) postdates all other minerals. In most cases, sulfides and
dolomite did not co-precipitate, but sphalerite and galena largely overlap with RD3 and PD1. Various dolomites are dissolved
or replaced by sulfide-precipitating fluids; sulfides in turn can be overgrown by dolomites. Colloform texture in sphalerite
is widespread. Fluid inclusions were studied in RD3, PD1, PD2, sphalerite, and PC. The overall ranges of homogenization temperatures
(T
h) and last ice-melting temperatures (T
m-ice) for fluid inclusions in dolomites and sphalerite are from 67 to 141 °C and from −46.7 to −27.0 °C, respectively, consistent
with warm basinal brines with high salinities and Ca/Na ratios. Gas chromatographic analysis of these fluid inclusions indicates
low concentrations of hydrocarbons (<0.06 mol%). C, O, and Sr isotopes were analyzed for all dolomites and PC, as well as
for the fine-grained host limestone and early diagenetic calcite (SC–RC). The isotopic values of RD2, RD3, PD1, and PD2 cluster
tightly and form largely overlapping domains. With respect to the host limestone, they are depleted in 18O, similar in δ13C, and slightly enriched in 87Sr. There are no regular spatial variations for fluid inclusion and isotope data, indicating an overall geochemical homogeneity
in the hydrothermal system. However, certain samples close to the fracture zones in the orebody with slightly elevated T
h and 87Sr/86Sr values and depleted δ18O values suggest that the fracture zone was the conduit for the hot brines. Based on the geological and geochemical characteristics
of the deposit, we propose that sulfide precipitation at Polaris was caused by mixing of a reduced, metal-rich, sulfur-poor
fluid with a reduced, metal-poor, sulfur-rich fluid at the site of mineralization. The metal-carrying fluid ascended along
fractures from below the deposit and was hotter than the host rocks, whereas the reduced sulfur-carrying fluid was delivered
to the site of mineralization laterally and was in thermal equilibrium with the host rocks. This model can readily explain
the dissolution of dolomite during sulfide precipitation and the abundance of colloform sphalerite, as well as the low concentrations
of hydrocarbons in fluid inclusions.
Accepted: 20 December 1999 相似文献
72.
Abd el-aziz Khairy Abd el-aal Mahmoud Sami Soliman 《Pure and Applied Geophysics》2013,170(11):1849-1857
It has been two decades since the last comprehensive standard model of ambient earth noise was published Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The PETERSON model was updated by analyzing the absolute quietest conditions for stations within the GSN (Berger et al. in J Geophys Res 109, 2005; Mcnamara and Buland in Bull Seism Soc Am 94:1517–1527, 2004; Ringler et al. in Seismol Res Lett 81(4) doi:10.1785/gssrl.81.4.605, 2010). Unfortunately, both the original model and the updated models did not include any deployed station in North Africa and Middle East, which reflects the noise levels within the desert environment of those regions. In this study, a survey was conducted to create a new seismic noise model from very broadband stations which recently deployed in North Africa. For this purpose, 1 year of continuous recording of seismic noise data of the Egyptian National Seismic Network (ENSN) was analyzed in order to create a new noise model. Seasonal and diurnal variations in noise spectra were recorded at each station. Moreover, we constructed a new noise model for each individual station. Finally, we obtained a new cumulative noise model for all the stations. We compared the new high-noise model (EHNM) and new low-noise model (ELNM) with both the high-noise model (NHNM) and low-noise model (NLNM) of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The obtained noise levels are considerably lower than low-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993) at ultra long period band (ULP band), but they are still below the high-noise model of Peterson (Observations and modelling of seismic background noise, US Geological Survey, open-file report 93–322, 1993). The results of this study could be considered as a first step to create permanent seismic noise models for North Africa and Middle East regions. 相似文献
73.
The variation of rotation with latitude is poorly known on stars other than the Sun. Several indirect techniques, photometric
and spectroscopic, have been used to search for departure from rigid rotation for sufficiently fast rotators. Here we investigate
the possibility of measuring stellar differential rotation for solar-type stars through asteroseismology. Rotationally split
frequencies of global oscillation provide information about rotation at different latitudes depending on the azimuthal order,
m, of the mode of pulsation. We present a method to estimate differential rotation based on the realization that the m = ±1 and m = ±2 components of quadrupole oscillations can be observed simultaneously in asteroseismology. Rotational frequency splittings
can be inverted to provide an estimate of the difference in stellar angular velocity between the equator and 45° latitude.
The precision of the method, assessed through Monte Carlo simulations, depends on the value of the mean rotation and on the
inclination angle between the rotation axis and the line of sight. 相似文献
74.
Solar variability and climate change: is there a link? 总被引:1,自引:0,他引:1
Sami K Solanki 《Astronomy& Geophysics》2002,43(5):5.09-5.13
75.
The stability and solubility of natural arsenopyrite (FeAsS) in pure water and moderately acid to slightly basic aqueous solutions buffered or not with H2 and/or H2S were studied at temperatures from 300 to 450°C and pressures from 100 to 1000 bar. The solubilities of FeAsS in pure water and dilute HCl/NaOH solutions without buffering are consistent with the formation of the As(OH)30(aq) species and precipitation of magnetite. At more acid pH (pH ≤2), arsenopyrite dissolves either stoichiometrically or with formation of the As-FeAsS assemblage. In H2S-rich and H2-rich aqueous solutions, arsenopyrite dissolution results in the formation of pyrrhotite (±pyrite) and iron arsenide(s), respectively, which form stable assemblages with arsenopyrite.Arsenic concentrations measured in equilibrium with FeAsS in slightly acid to neutral aqueous solutions with H2 and H2S fugacities buffered by the pyrite-pyrrhotite-magnetite assemblage are 0.0006 ± 0.0002, 0.0055 ± 0.0010, 0.07 ± 0.01, and 0.32 ± 0.03 mol/kg H2O at 300°C/400 bar, 350°C/500 bar, 400°C/500 bar, and 450°C/500 bar, respectively. These values were combined with the available thermodynamic data on As(OH)30(aq) (Pokrovski et al., 1996) to derive the Gibbs free energy of FeAsS at each corresponding temperature and pressure. Extrapolation of these values to 25°C and 1 bar, using the available heat capacity and entropy data for FeAsS (Pashinkin et al., 1989), yields a value of −141.6 ± 6.0 kJ/mol for the standard Gibbs free energy of formation of arsenopyrite. This value implies a higher stability of FeAsS in hydrothermal environments than was widely assumed.Calculations carried out using the new thermodynamic properties of FeAsS demonstrate that this mineral controls As transport and deposition by high-temperature (>∼300°C) crustal fluids during the formation of magmatic-hydrothermal Sn-W-Cu-(Au) deposits. The equilibrium between As-bearing pyrite and the fluid is likely to account for the As concentrations measured in modern high- and moderate-temperature (150 ≤ T ≤ 350°C) hydrothermal systems. Calculations indicate that the local dissolution of arsenopyrite creates more reducing conditions than in the bulk fluid, which is likely to be an effective mechanism for precipitating gold from hydrothermal solutions. This could be a possible explanation for the gold-arsenopyrite association commonly observed in many hydrothermal gold deposits. 相似文献
76.
Kayhomayoon Zahra Ghordoyee Milan Sami Arya Azar Naser Kardan Moghaddam Hamid 《Natural Resources Research》2021,30(6):4165-4185
Natural Resources Research - In this study, a new 3-stage approach that consists of clustering, simulation, and optimization stages is proposed for the simulation of groundwater level (GWL) in an... 相似文献
77.
The experimental results have so far shown that when a wave breaks on a vertical wall with an almost vertical front face at the instant of impact that is called perfect breaking or perfect impact, the greatest impact forces are produced on the wall. Therefore, the configuration of breaking waves is important in the design considerations of coastal structures. The present study is concerned with determining the geometrical properties of oscillatory waves that break perfectly on the vertical wall of composite-type breakwaters. The laboratory tests for perfect breaking waves on composite breakwaters are conducted with base slopes of 1/2, 1/4 and 1/6, and with berm widths of 0.00, 0.10, 0.20, 0.30 and 0.40 m. The shape and the dimensions of waves at the instant of perfect breaking on the wall are determined using a video camera. The experimental results for the geometrical properties of the breakers are presented non-dimensionally. Within the range of present experimental conditions, it is found that the dimensionless breaker crest height, hb/dw, and dimensionless breaker height, Hb/dw, decrease; and, dimensionless breaker depth, dw/H0, increases with increasing relative berm width, B/D. The breaker height index, Hb/H0, is almost unaffected by B/D. The deep-water wave steepness and the base slope of the breakwater do not seem to influence the geometrical properties of the breakers at wall systematically. 相似文献
78.
79.
Jixiong Zhang Qiang Sun Nan Zhou Jiang Haiqiang Deon Germain Sami Abro 《Arabian Journal of Geosciences》2016,9(10):558
In China’s western coal mining area, the traditional room mining technology is facing coal pillar instability, mine earthquake, large-area roof subsidence in the goaf, surface subsidence, water and soil loss, vegetation deterioration, and other environmental problems. To solve the aforementioned problems and to improve coal recovery, the roadway backfill coal mining (RBCM) method was proposed as a solution and its technical principle and key equipment were presented in this paper. In addition, the microstructure and mechanical behavior (strain-stress relation in confined compressive test) of aeolian sand and loess backfill materials were studied for a rational backfill design for underground mines. Further, coal pillar stress, plastic zone change, and surface deformation of the RBCM schemes were studied using the FLAC3D numerical simulation software, and a reasonable mining scheme of “mining 7 m and leaving 3 m” was determined. The engineering application in Changxing Coal Mine shows that the RBCM method with loess and aeolian sand as backfill materials allows a stable recovery of coal pillars with a recovery ratio of more than 70 %. The maximum accumulated surface subsidence and the maximum horizontal deformation were measured to be 15 mm and 0.8 mm/m respectively, indicating that the targeted backfilling effect can help protect the environment and also control surface subsidence. 相似文献
80.