The Zn–Pb±Ag±Cu San Cristobal district is located 100 km east of Lima in the western cordillera of Peru. It is centred around the Chumpe intrusion and is composed of vein and carbonate replacement ore types. The main San Cristobal vein presents a paragenesis that can be divided into three stages: (a) an early wolframite–quartz–pyrite stage, (b) a quartz–base metal stage, and (c) a late quartz–carbonate–barite stage.
Fluid inclusions in quartz from the tungsten stage are biphase (LV) at room temperature and homogenise to the liquid phase between 146 and 257 °C. Their salinities range between 2.1 and 5.1 wt.% NaCl equiv. Rare inclusions contain an additional crystal of halite and have salinities of 46–54 wt.% NaCl equiv. Data of the first two stages show a decrease in homogenisation temperatures concomitant with a salinity decline. Fluid inclusions in quartz from the late stage homogenise at higher temperatures, between 252 and 323 °C, with salinities ranging between 4.6 and 6.7 wt.% NaCl equiv.
Hydrogen and oxygen isotope data indicate a two-stage evolution. Isotopic compositions of the fluid associated with the first two stages define a trend with constant δ18O values and decreasing δD values (δ18O=3.2‰ to 5.0‰ V-SMOW and δD=−60‰ to −112‰ V-SMOW), which is interpreted as mixing of a dominantly magmatic component with minor meteoric water that had equilibrated with the host rocks. This interpretation is supported by sulphur and lead isotopic data from previous studies. By contrast, the quartz–carbonate–barite stage bears isotopic characteristics defining a trend with a coupled decrease of δ18O and δD (δ18O=−8.1‰ to 2.5‰ V-SMOW and δD=−57‰ to −91‰ V-SMOW) and is explained by addition of meteoric water to the system and subsequent mixing with a less important magmatic component.
Different fluid origins are confirmed by laser ablation ICP-MS analyses of the triphase (LVH) and biphase (LV) primary inclusions. The concentrations of the major ore elements, i.e., W, Cu, Zn and Pb, decrease throughout the paragenesis; W, and to a lesser extent Cu, show significant variations, associated with a steep decrease in their concentration. The decreasing concentrations can be explained by mineral deposition and dilution by the meteoric fluid; differences in the rate of decrease indicate selective precipitation of W. Fluid inclusions of the quartz–carbonate stages show an abrupt increase in Ba and Sr concentrations. This is interpreted to reflect a higher volume of host rock silicate alteration, probably due to the increasing size of the fluid flow cell and is explained by the input of a third fluid of unknown origin. LA-ICP-MS analyses show that the fluids were already depleted in W and Cu before reaching the emplacement of carbonate replacement ore type, whereas Zn and Pb were still present in considerable amounts. This is again due to selective precipitation and is consistent with the interpretation that the economically interesting metals were dominantly introduced by magmatic fluids. 相似文献
With the use of a sonic anemometer, vertical heat and momentum fluxes were measured at three different levels in an oak forest canopy. A quadrant analysis of the resulting data shows that approximately half of the transport occurs in extreme events lasting about 5 to 10% of the time. The partition of transport into momentum sweeps, bursts and interactions shows good agreement with existing data. The heat flux is analysed by observing the fluxes during the different momentum events and considering concurrent momentum and heat flux intensities by means of conditional probabilities. While low intensity (normal) events show similar probability distributions throughout the canopy, different structures appear at the three measurement heights for high intensity (extreme) events that can tentatively be explained by taking the temperature profile into account. This supports the idea that these events are coherent motions with scales comparable to the canopy height. 相似文献
Summary A interpolation method is given for the case in which a function consisting of a systematic and a random part is to be estimated
from measurements affected by errors. This is a combined problem of parameter estimation, filtering and prediction, which
has applications in different fields of geodesy and gravimetry. The solution of the problem is derived from a least-squares
principle; it is formally very similar to a general case of least-squares adjustment.
Address: Hardenbergstr. 34, 1 Berlin 12. 相似文献
The paper treats the problem of averaging two independent determinations of the same function (curve, surface, etc.). Starting
from the simple arithmetic mean, the problem is successively generalized to scalar and vector functions, arriving at an integral
equation of Wiener-Hopf type. Aspects of least-squares collocation are also included. Finally the problem of determining transformation
parameters and other systematic deviations is treated, furnishing a simple example of continuous least-squares adjustment. 相似文献
The fluorite deposits of the Valle de Tena, Central Pyrenees, include stratabound (Portalet) and vein (Lanuza and Tebarray)
deposits the formation of which are linked to a Namurian-Westfalian emersion episode and to post-Hercynian hydrothermal systems
similar to those occurring elsewhere in Hercynian Europe. In this study, strontium isotopes were used to determine the source(s)
of strontium, and by inference calcium, of the fluorite mineralizations, as well as the nature of the ore-forming fluids.
Fluorite and calcite from each deposit have similar 87Sr/86Sr ratios (Portalet 0.7085–0.7108; Lanuza 0.7086–0.7104 and Tebarray 0.7086–0.7101). In all deposits, the Sr isotope composition
of most of the Ca-minerals is more radiogenic than that of the host limestones. This indicates that the Ca-minerals contain
a mixture of Sr derived locally from the host limestones and 87 Sr-enriched Sr leached from silicate minerals in the siliciclastic portion of the basement sequence and in granites from
the study area. Volcanic rocks are ruled out as a significant Sr source for the fluorite deposits. The observed trend in 87Sr/86Sr versus 1/Sr support a fluid-rock interaction model which satisfactorily reproduces the marked 87Sr-enrichment in the fluorites and calcites from the deposits.
Received: 19 February 1997 / Accepted: 22 July 1997 相似文献
Previous research revealed that severe overgrazing and resultant land degradation in the semiarid areas of northern Mexico created significantly higher temperatures in the border area. In this investigation, the temperature and precipitation records from ten homogeneous stations are identified in the arid and hyperarid areas of northwest Sonora and are compared with the records from ten stations in southwestern Arizona. Our data show that the Mexican stations are again consistently warmer than the Arizona stations when statistical controls are applied to correct for the linear or non-linear effects of latitude and/or elevation. The stations in Sonora warm at a statistically significantly faster pace than the stations in Arizona during the study period. Furthermore, and consistent with other dryland areas undergoing land degradation, the stations in Sonora reveal a significant increase in the diurnal temperature range during the summer season. Local precipitation reduces the temperature differential between nations on the time scale of days, but enhances the differential on the time scale of months and seasons. Among other findings, the results show how land degradation in dryland areas appears to influence local historical temperature records. 相似文献
Comparisons between high-degree models of the Earth’s topographic and gravitational potential may give insight into the quality and resolution of the source data sets, provide feedback on the modelling techniques and help to better understand the gravity field composition. Degree correlations (cross-correlation coefficients) or reduction rates (quantifying the amount of topographic signal contained in the gravitational potential) are indicators used in a number of contemporary studies. However, depending on the modelling techniques and underlying levels of approximation, the correlation at high degrees may vary significantly, as do the conclusions drawn. The present paper addresses this problem by attempting to provide a guide on global correlation measures with particular emphasis on approximation effects and variants of topographic potential modelling. We investigate and discuss the impact of different effects (e.g., truncation of series expansions of the topographic potential, mass compression, ellipsoidal versus spherical approximation, ellipsoidal harmonic coefficient versus spherical harmonic coefficient (SHC) representation) on correlation measures. Our study demonstrates that the correlation coefficients are realistic only when the model’s harmonic coefficients of a given degree are largely independent of the coefficients of other degrees, permitting degree-wise evaluations. This is the case, e.g., when both models are represented in terms of SHCs and spherical approximation (i.e. spherical arrangement of field-generating masses). Alternatively, a representation in ellipsoidal harmonics can be combined with ellipsoidal approximation. The usual ellipsoidal approximation level (i.e. ellipsoidal mass arrangement) is shown to bias correlation coefficients when SHCs are used. Importantly, gravity models from the International Centre for Global Earth Models (ICGEM) are inherently based on this approximation level. A transformation is presented that enables a transformation of ICGEM geopotential models from ellipsoidal to spherical approximation. The transformation is applied to generate a spherical transform of EGM2008 (sphEGM2008) that can meaningfully be correlated degree-wise with the topographic potential. We exploit this new technique and compare a number of models of topographic potential constituents (e.g., potential implied by land topography, ocean water masses) based on the Earth2014 global relief model and a mass-layer forward modelling technique with sphEGM2008. Different to previous findings, our results show very significant short-scale correlation between Earth’s gravitational potential and the potential generated by Earth’s land topography (correlation +0.92, and 60% of EGM2008 signals are delivered through the forward modelling). Our tests reveal that the potential generated by Earth’s oceans water masses is largely unrelated to the geopotential at short scales, suggesting that altimetry-derived gravity and/or bathymetric data sets are significantly underpowered at 5 arc-min scales. We further decompose the topographic potential into the Bouguer shell and terrain correction and show that they are responsible for about 20 and 25% of EGM2008 short-scale signals, respectively. As a general conclusion, the paper shows the importance of using compatible models in topographic/gravitational potential comparisons and recommends the use of SHCs together with spherical approximation or EHCs with ellipsoidal approximation in order to avoid biases in the correlation measures. 相似文献
