Applied Geochemistry (v.69, #C)

Experimental simulation of element mass transfer and primary halo zone on water-rock interaction by Qingjie Gong; Taotao Yan; Jinzhe Li; Mu Zhang; Ningqiang Liu (1-11).
Primary halo zone is an important theory for prospecting the deep blind ore body and predicting resource potential. The common description of primary halo zone of multiple elements is based on their behaviors during hydrothermal alteration or mineralization. A geofluid with multiple elements, which are Bi, Cu, Pb, Zn, Cd, Ag, and As (bearing Ni and Mo), is prepared to simulate the dispersive transfer of elements in a hydrothermal fluid–granite reaction at 15 MPa, 100 °C using a new-constructed core flow-through pressure vessel reactor. Core granite samples with 5 cm long after water-rock reaction are cut into ten sections to analyze their major oxides and trace elements. According to experiment results and the method of immobile plateau, we determine that Al and Ti are immobile elements during the water-rock reaction and the hydrothermal alteration of granite is a mass gained process. The mobility index (MI) of each of experimental elements is calculated based on the immobile component Al2O3. The sequence of experimental elements is from As to Ag, Pb, Zn, Cu, Bi, Ni, Cd, and Mo, which are sorted on their values of MI in these reacted granite cores from near to far end respecting to the reaction fluid. This result is consistent with the empirical sequence of zonation of primary halo elements which is derived from the statistical results from 58 gold deposits in China. The denudation coefficient of primary halo zone (Zf or Zfe) is proposed to assess the degree of approaching to ore body on these experiments, which is the averaged MI ratio of front halo elements to ore-body halo elements. The denudation coefficient of primary halo zone is applied on the cross section of gold ore-body hosted in the granite of the Linglong gold deposit located in the Jiaodong peninsula of eastern China. The degree of approaching to the gold ore body is clearly illustrated by the denudation coefficient of primary halo zone.
Keywords: Water-rock interaction; Mass transfer of elements; Denudation coefficient of primary halo zone; Core flow-through pressure vessel reactor; Linglong gold deposit;

Fluid-rock interactions in a geothermal Rotliegend/Permo-Carboniferous reservoir (North German Basin) by Simona Regenspurg; Elvira Feldbusch; Ben Norden; Marion Tichomirowa (12-27).
Comprehensive data on the chemical composition of reservoir rocks and geothermal brines from the geothermal well doublet Groβ Schönebeck (North German Basin) drilled into a Rotliegend sedimentary and Permo-Carboniferous volcanic rock reservoir were sampled over the past years. They were characterized with respect to their major and minor elemental composition including various isotope ratios. The study considered the impact of drilling and reservoir operations on fluid composition and aimed at determining fluid–rock interactions to gain information on fluid origin and hydraulic pathways.The highly saline fluids (up to 265 g/L TDS) show δ 18O and δD of water (2.7–5.6 and −3.1–15, respectively) as well as δ 34S of sulfate (3.6–5), and 87Sr/86Sr ratios (0.715–0.716) that resemble Rotliegend brines from an area located around 200 km in the west (the Altmark). Halogen ratios indicated that brines developed predominantly by evaporation of meteoric water (primary brine) together with halite dissolution brine (secondary brine). Indication for mixing with Zechstein brine or with younger meteoric water was not found.No geochemical distinction was possible between fluids deriving from different rock formations (dacites or sedimentary rocks, respectively). This is due to the evolution of the sediments from the effusive rocks resulting in a similar mineralogical and chemical composition and due to a hydraulic connectivity between the two types of rock. This connection existed probably already before reservoir stimulation as indicated by a set of faults identified in the area that could connect the Rotliegend formation with both, the volcanic rocks and the lower units of the Zechstein. Additional geochemical indication for a hydraulic connectivity is given by (1) the very high heavy metal contents (mainly Cu and Pb) in fluids and scaling that derive from the volcanic rocks and were that were also found in increased amounts up at the Zechstein border (Kupferschiefer formation). (2) The 87Sr/86Sr isotope ratios of fluid samples correspond to the ratios determined for the sedimentary rocks indicating that initially the fluids developed in the sedimentary rocks and circulated later, when faults structures were created by tectonic events into the volcanic rocks.Display Omitted
Keywords: Geothermal fluid; Groβ Schönebeck; Water rock interaction; Origin of brine; Isotopes; Strontium; Rotliegend; Permo-Carboniferous; Hydraulic connectivity;

Density, (ρ), speed of sound, (W), and viscosity, (η) of natural geothermal fluids from south Russia Geothermal Fields (Dagestan, Caspian seashore) have been measured over the temperature range from (277–353) K at atmospheric pressure. The measurements were made using the Anton Paar DMA4500 densimeter and Stabinger SVM3000 viscodensimeter for four geothermal fluid samples from the various hot-wells Izberbas (No. 68 and 129), Ternair (No. 27T and No. 38T). A sound-speed analyzer (Anton Paar DSA 5000) was used for simultaneously measurement of the speed of sound and density of the same geothermal fluid samples. The average differences between the measured geothermal fluids densities and viscosities and pure water values (IAPWS formulation) are within (0.1–1.77) % and (0.13–2.1) %, respectively, which are considerably higher than their experimental uncertainties. This differences are caused by the high concentrations of some type of ion species, such as (Na+: 7.7 g/l (#38T); Cl−1:7.7 g/l (#38T); SO 4 − 2 : 0.75 g/l (#68); S+: 0.24 g/l (#68); K+:0.15 g/l (# 27T); Ca+2: 0.074 g/l (#27T); B+:0.06 g/l (#38T); and Mg +2: 0.033 g/l (#38T)), in the geothermal fluids, which strongly effect on salt concentration dependence of the measured properties. Measured values of density and speed of sound were used to calculate other derived thermodynamic properties such as adiabatic coefficient of bulk compressibility (β S), coefficient of thermal expansion (αP ), thermal pressure coefficient (γ V), isothermal coefficient of bulk compressibility (βT ), isochoric heat capacity (C V), isobaric heat capacity ( C P ), enthalpy difference ( Δ H ), partial pressure derivative of enthalpy ( ∂ H ∂ P ) T , and partial derivatives of internal energy (internal pressure) ( ∂ U ∂ V ) T , of the geothermal fluid samples. Measured values of density, viscosity, and speed of sound were used to develop correlation models for the temperature and ion species concentration dependences, which reproduced the measured values within 0.03% (density), 2.47% (viscosity), and 0.20% (speed of sound). To confirm the accuracy and predictive capability of the developed correlation models for density, speed of sound, and viscosity, we have applied the models to well-studied binary aqueous salt solutions (H2O + NaCl). The prediction of the density and viscosity of aqueous sodium chloride solutions based on the developed models were very close to their experimental uncertainties (within 0.03% for density and 1.56% for viscosity). The measured properties at atmospheric pressure have been used as a reference data for prediction of the high-pressure thermodynamic behavior. The predictive capability of the model has been checked on reliable experimental data for binary aqueous NaCl solutions at high pressures reported by Kestin and Shankland (1984) and Rogers and Pitzer (1982). The prediction for density and viscosity is within 0.03% and 1.57%, respectively.
Keywords: Geothermal fluids; Geochemistry; Density; Vibrating tube densimeter; Speed of sound; Viscosity; Water;

Filtered subglacial meltwater samples were collected daily during the onset of melt (May) and peak melt (July) over the 2011 melt season at the Athabasca Glacier (Alberta, Canada) and analyzed for strontium-87/strontium-86 (87Sr/86Sr) isotopic composition to infer the evolution of subglacial weathering processes. Both the underlying bedrock composition and subglacial water–rock interaction time are the primary influences on meltwater 87Sr/86Sr. The Athabasca Glacier is situated atop Middle Cambrian carbonate bedrock that also contains silicate minerals. The length of time that subglacial meltwater interacts with the underlying bedrock and substrate is a predominant determining factor in solute concentration. Over the course of the melt season, increasing trends in Ca/K and Ca/Mg correspond to overall decreasing trends in 87Sr/86Sr, which indicate a shift in weathering processes from the presence of silicate weathering to primarily carbonate weathering.Early in the melt season, rates of carbonate dissolution slow as meltwater approaches saturation with respect to calcite and dolomite, corresponding to an increase in silicate weathering that includes Sr-rich silicate minerals, and an increase in meltwater 87Sr/86Sr. However, carbonate minerals are preferentially weathered in unsaturated waters. During the warmest part of a melt season the discharged meltwater is under saturated, causing an increase in carbonate weathering and a decrease in the radiogenic Sr signal. Likewise, larger fraction contributions of meltwater from glacial ice corresponds to lower 87Sr/86Sr values, as the meltwater has lower water–rock interaction times in the subglacial system. These results indicate that although weathering of Sr-containing silicate minerals occurs in carbonate dominated glaciated terrains, the continual contribution of new meltwater permits the carbonate weathering signal to dominate.
Keywords: Radiogenic strontium; Oxygen; Isotopes; Athabasca Glacier; Subglacial weathering; Meltwater fraction contribution;

Crystallization behavior of Na2SO4–MgSO4 salt mixtures in sandstone and comparison to single salt behavior by Nadine Lindström; Tanya Talreja; Kirsten Linnow; Amelie Stahlbuhk; Michael Steiger (50-70).
We report on the crystallization behavior and the salt weathering potential of Na2SO4, MgSO4 and an equimolar mixture of these salts in natural rock and porous stone. Geochemical modeling of the phase diagram of the ternary Na2SO4–MgSO4–H2O system was used to determine the equilibrium pathways during wetting (or deliquescence) of incongruently soluble minerals and evaporation of mixed electrolyte solutions. Model calculations include stable and metastable solubilities of the various hydrated states of the single salts and the double salts Na2Mg(SO4)2·4H2O (bloedite), Na2Mg(SO4)2·5H2O (konyaite), Na12Mg7(SO4)13·15H2O (loeweite) and Na6Mg(SO4)4 (vanthoffite). In situ Raman spectroscopy was used to study the phase transformations during wetting of pure MgSO4·H2O (kieserite) and of the incongruently soluble salts bloedite and konyaite. Dissolution of kieserite leads to high supersaturation resulting in crystallization of higher hydrated phases, i.e. MgSO4·7H2O (epsomite) and MgSO4·6H2O (hexahydrite). This confirms the high damage potential of magnesium sulfate in salt damage of building materials. The dissolution of the incongruently soluble double salts leads to supersaturation with respect to Na2SO4·10H2O (mirabilite). However, the supersaturation was insufficient for mirabilite nucleation. The damage potential of the two single salts and an equimolar salt mixture was tested in wetting–drying experiments with porous sandstone. While the high damage potential of the single salts is confirmed, it appears that the supersaturation achieved during wetting of the double salts at room temperature is not sufficient to generate high crystallization pressures. In contrast, very high damage potentials of the double salts were found in experiments at low temperature under high salt load.1
Keywords: Salt weathering; Incongruently soluble double salts; Bloedite; Konyaite; Loeweite; Vanthoffite; Geochemical modeling; Raman microscopy; Crystallization pressure;