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Rapid identification and large-scale mapping of salt-affected lands will help improve salinity management in watersheds and ecosystems. This study was conducted to examine spectral reflectance of soils treated with saline solutions containing NaCl, NaHCO3, Na2SO4, and CaSO4.2H2O. Spectral reflectance was measured upon salt crusts formed on two soils (Torrifluvents) subirrigated with saline solutions of 500, 1000, and 1500 mmolc L-1 with a spectroradiometer in the visible and near-infrared region (400-2500 nm). Spectral analyses revealed that samples of gypsum crusts have diagnostic absorption features near 1023, 1225, 1457, 1757, 1800, and 2336 nm, whereas halite crusts have diagnostic absorption features near 1442, 1851, 1958, and 2226 nm. Several broad absorption features were seen in the spectra of the crusts of sodium bicarbonate at 1243, 1498, 1790, 1988, and 2356 nm. The spectrum of soils treated with sodium sulfate exhibited absorption features at 1243, 1472, 1677, 1774, 1851, 1968, and 2245 nm. Crystal size or salt concentrations did not affect the positions of the absorption bands of the salt crusts. However, reflectance increased as particle sizes decreased or with increasing presence of salt crusts. Spectroscopy can be used under certain conditions to identify the presence of primary diagnostic spectral features of gypsum, nahcolite, thenardite, and halite crusts.

The Nopal I site in the Peña Blanca uranium district has a number of geologic and hydrologic similarities to the proposed high-level radioactive waste repository at Yucca Mountain, making it a useful analogue to evaluate process models for radionuclide transport. The PB-1 well was drilled in 2003 at the Nopal I uranium deposit as part of a DOE-sponsored natural analogue study to constrain processes affecting radionuclide transport. The well penetrates through the Tertiary volcanic section down to Cretaceous limestone and intersects the regional aquifer system. The well, drilled along the margin of the Nopal I ore body, was continuously cored to a depth of 250 m, thus providing an opportunity to document the local stratigraphy. Detailed observations of these units were afforded through petrographic description and rock-property measurements of the core, together with geophysical logs of the well. The uppermost unit encountered in the PB-1 well is the Nopal Formation, a densely welded, crystal-rich, rhyolitic ash-flow tuff. This cored section is highly altered and devitrified, with kaolinite, quartz, chlorite, and montmorillonite replacing feldspars and much of the groundmass. Breccia zones within the tuff contain fracture fillings of hematite, limonite, goethite, jarosite, and opal. A zone of intense clay alteration encountered in the depth interval 17.45-22.30 m was interpreted to represent the basal vitrophyre of this unit. Underlying the Nopal Formation is the Coloradas Formation, which consists of a welded lithic-rich rhyolitic ash-flow tuff. The cored section of this unit has undergone devitrification and oxidation, and has a similar alteration mineralogy to that observed in the Nopal tuff. A sharp contact between the Coloradas tuff and the underlying Pozos Formation was observed at a depth of 136.38 m. The Pozos Formation consists of poorly sorted conglomerate containing clasts of subangular to subrounded fragments of volcanic rocks, limestone, and chert. Three thin (2-6 m) intervals of intercalated pumiceous tuffs were observed within this unit. The contact between the Pozos Formation and the underlying Cretaceous limestone basement was observed at a depth of 244.40 m. The water table is located at a depth of ~223 m. Several zones with elevated radioactivity in the PB-1 core are located above the current water table. These zones may be associated with changes in redox conditions that could have resulted in the precipitation of uraninite from downward flowing waters transporting U from the overlying Nopal deposit. All of the intersected units have low (typically submillidarcy) matrix permeability, thus fluid flow in this area is dominated by fracture flow. These stratigraphic and rock-property observations can be used to constrain flow and transport models for the Peña Blanca natural analogue.

The sulphide ores from the Julcani mining district (Peru) display many retrograde reactions that may be attributed to solid-state processes accompanying cooling. Fahlores [˜(Cu,Ag)10(Zn,Fe)2(Sb,As)4S13] from the Herminia mine exhibit pronounced downstream enrichments in molar Ag/(Ag+Cu) ratios that are strongly correlated with the abundance of PbS-AgSbS2-AgBiS2phases. These correlations, discontinuous core to rim Sb/(Sb+As) enrichments in bournonites, and prominent reaction textures involving fahlores, bournonites and galenas provide strong evidence that the fahlores in these ores have been enriched in Ag by the Ag–Cu exchange reactionwhich occurred during cooling following mineralization of a PbS-AgSbS2-AgBiS2galena and has been documented elsewhere. Secondary PbS-AgSbS2-AgBiS2minerals aramayoite, bismuthian diaphorite [Pb2Ag3(Bi,Sb)3S8], and diaphorite were produced from primary galenas with cooling of ores with high Pb/Cu and Bi/Sb; pyrargyrite formed from the breakdown of the Ag10Zn2Sb4S13component in the most Ag-rich fahlores, as an exsolution product of galena, and from replacement of aramayoite and galena with the evolution of semimetal sulphides. Based on mineral compositions, phase equilibria, a thermochemical database for sulphides and sulphosalts, and the reintegrated composition for primary grains of Ag-rich PbS-AgSbS2-AgBiS2phases, we estimate that the primary temperature of hydrothermal mineralization was >320±10°C, that these reactions ceased to affect fahlore Ag/(Ag+Cu) ratios and Bi/(Bi+Sb) ratios of aramayoite and miargyrite after cooling through 220±10°C. Galenas, however, appear to have continued to adjust their compositions to reflect even lower temperatures by continued exsolution of semimetals and production a diverse suite of sulphosalts that occur in fine intergrowths with galena.

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