Hydrogeology of thermal waters in Viterbo area, central Italy

A conceptual hydrogeological model of the Viterbo thermal area (central Italy) has been developed. Though numerous studies have been conducted on its geological, geochemical and geothermal features, there is no generalized picture defining the origin and yield of the hydrothermal system. These latter aspects have therefore become the objectives of this research, which is based on new hydrogeological and geochemical investigations. The geological setting results in the coexistence of overlapped interacting aquifers. The shallow volcanic aquifer, characterized by fresh waters, is fed from the area around the Cimini Mountains and is limited at its base by the semiconfining marly-calcareous-arenaceous complex and low-permeability clays. To the west of Viterbo, vertical upflows of thermal waters of the sulphate-chloride-alkaline-earth type with higher gas contents, are due to the locally uplifted carbonate reservoir, the reduced thickness of the semiconfining layer and the high local geothermal gradient. The hot waters (30–60°C) are the result of deep circulation within the carbonate rocks (0.5–1.8 km) and have the same recharge area as the volcanic aquifer. The upward flow in the Viterbo thermal area is at least 0.1 m³/s. This flow feeds springs and deep wells, also recharging the volcanic aquifer from below.     

Distribution of cadmium, chromium, copper, lead and zinc in marine sediments in Hong Kong waters

Partitioning of heavy metals (Cd, Cr, Cu, Pb, Zn) in marine sediments collected from various sites in Hong Kong waters were determined using sequential extraction method. Sediments from Kellette Bank, located in Victoria Harbour, had higher metal concentrations especially Cu and Zn than most other sites. Slightly over 20% of total Cu and Cr existed as readily available forms in Peng Chau and Kellette Bank. At most sampling sites, over 15% of the Cu existed as the exchangeable form indicating that Cu could be readily released into the aqueous phase from sediments. A significantly higher percentage of Pb and Zn was associated with the three non-residual fractions. Hence, there is a greater environmental concern for remobilization of Pb and Zn compared with Cr. The high amount of residual Cd (>50%) and the relatively lower Cd content indicate that little environmental concern is warranted for the remobilization of Cd. Distribution of metals in sediments collected from different depth at Kellette Bank shows that metal concentrations decreased with profile depth. The levels of Pb and Zn associated with the two readily available fractions increased sharply in the surface sediment. These metals represented the pollutants, which were introduced into the area in the mid-eighties through early nineties as a result of rapid economic and industrial development in the territory. As significant portions of these metals were bound to the readily available phases in the surface sediments, metal remobilization could be a concern. An erratum to this article can be found at     

Geochemistry and formation conditions of rare-metal granites with various fluorine-bearing minerals (fluorite,topaz, and cryolite)

A comparative geochemical study of rare-metal granitoids with various fluorine-bearing minerals (fluorite, topaz, and cryolite) was carried out. It was shown that these rocks correspond to both plumasitic and agpaitic geochemical types. The fluorite-, topaz-, and cryolite-bearing granites of these geochemical types are distinctly different in geochemical parameters and the character of magmatic evolution. These differences are related to the composition of initial magmas and their sources. Rare-metal granitoids with fluorine-bearing minerals compose small massifs, stocks, and dike swarms. Their formation is independent of the composition and age of the country rocks or geologic structures where they occur. Original Russian Text ? V.S. Antipin, E.A. Savina, M.A. Mitichkin, 2006, published in Geokhimiya, 2006, No. 10, pp. 1040–1052.     

An iterative method for the construction of equilibrium N-body models for stellar disks

One widely used technique for the construction of equilibrium models of stellar disks is based on the Jeans equations and the moments of velocity distribution functions computed using these equations. Stellar disks constructed using this technique are shown to be “not entirely” in equilibrium. Our attempt to abandon the epicyclic approximation and the approximation of infinite isothermal layers, which are commonly adopted in this technique, failed to improve the situation substantially. We conclude that the main drawback of techniques based on the Jeans equations is that the system of equations employed is not closed and, therefore, requires adopting an essentially ad hoc additional closure condition. A new iterative approach to constructing equilibrium N-body models with a given density distribution is proposed. The main idea behind this approach is that a model is first constructed using some approximation method, and is then allowed to adjust to an equilibrium state with the specified density distribution—if necessary, with the required parameters of the velocity distribution remaining fixed in the process. This iterative approach was used to construct isotropic, spherically symmetric models and models of stellar disks embedded in an external potential. The numerical models constructed prove to be close to equilibrium. It is shown that the commonly adopted assumption that the profile of the radial-velocity dispersion is exponential may be wrong. The technique proposed can be applied to a wide range of problems involving the construction of models of stellar systems with various geometries.     

Metamorphism of sulfides and gangue quartz at the Degdekan and Gol’tsovsky gold deposits, Magadan oblast

The Degdekan and Gol’tsovsky gold-quartz deposits are located in the southeastern Yana-Kolyma gold belt. The orebodies occur as quartz veins hosted in metaterrigenous rocks and cut by postmineral basic-intermediate dikes. It was established that metamorphism of sulfides and gangue quartz was restricted to a few centimeters off the dike contact. According to sulfide geothermometers, the metamorphic temperatures close to the contact of dikes attained 700°C at the Degdekan deposit and were no higher than 491°C at the Gol’tsovsky deposit. The formation of the forbidden assemblage of quartz and loellingite and its fine-grained texture indicate that the thermal effect on the Degdekan ore was short-term. The prolonged heating of the ore at the Gol’tsovsky deposit gave rise to the aggradation recrystallization of quartz and the formation of equilibrium sulfide aggregates that show only insignificant differences in composition from the primary phases. The average homogenization temperature of primary and pseudosecondary fluid inclusions is 206 ± 40°C in the unmetamorphosed veins and 257 ± 33°C in the metamorphosed veins. The salinity of fluids in the primary and pseudosecondary inclusions in quartz veins of both types varies from 0.5 to 14.0 wt % NaCl equiv. The melting temperature of liquid CO₂ in the carbon dioxide inclusions, ranging from ?57.0 to ?60.8°C, suggests an admixture of CH₄ and/or N₂. The unmetamorphosed quartz veins were formed at a fluid pressure varying from 0.7 to 1.3 kbar, while quartz veins at the contact with dikes crystallized at a pressure of 0.8–1.5 kbar. The results of gas chromatography showed the presence of CO₂ and H₂O, as well as N₂ and CH₄. The average bulk of volatiles contained in the fluid inclusions in quartz from the metamorphosed veins is 1.5–2 times lower than in the unmetamorphosed veins; this proportion is consistent with the occurrence of decrepitated gas inclusions in the heated quartz.     

Metal sources of the Navan carbonate-hosted base metal deposit, Ireland: Nd and Sr isotope evidence for deep hydrothermal convection

Nd and Sr isotope analyses are presented for gangue mineral samples from the giant carbonate-hosted Navan Zn–Pb deposit, Ireland, and for rocks from which Navan metals may have been derived. Analysis of gangue minerals spanning the Navan paragenetic sequence reveals systematic evolution in the composition of the mineralising fluid. Early fluid represented by replacive dolomite exhibits the lowest initial ⁸⁷Sr/⁸⁶Sr ratio (0.7083–0.7086), closest to that of the host limestone and to Lower Carboniferous seawater, and the highest ¹⁴³Nd/¹⁴⁴Nd ratio (0.51161–0.51176). Later generations of dolomite, barite and calcite, which encompass sulphide precipitation, have higher initial ⁸⁷Sr/⁸⁶Sr ratios (maximum 0.7105) and lower initial ¹⁴³Nd/¹⁴⁴Nd ratios (minimum 0.51157). All samples have initial Nd isotope ratios that are too low to have been acquired only from the host limestone. Drill core samples of presumed Ordovician volcanic and sedimentary rocks from beneath the Navan orebody have ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr ratios at the time of mineralisation of 0.51184–0.51217 and 0.7086–0.7138, respectively. The data are interpreted to indicate mixing of sulphide-rich, limestone-buffered brine, with a metal-bearing hydrothermal fluid, which had passed through sub-Carboniferous rocks, consistent with published fluid inclusion and S isotope data. The ¹⁴³Nd/¹⁴⁴Nd ratio of this basement-derived fluid is too low to have been imparted by flow through the Devonian Old Red Sandstone, as required in models of regional fluid flow in response to Hercynian uplift. Irrespective of whether such regional fluid flow occurred, the hydrothermal Nd must have been derived from sub-Devonian rocks. These conclusions broadly support the hydrothermal convection cell model in which brines, ultimately of surface origin, penetrated to a depth of several kilometres, leaching metals from the rocks through which they passed. The data also support increasing depth of penetration of convection cells with time. Metals were subsequently precipitated in carbonate rocks at sites of mixing with cooler, sulphide-rich fluids. However, comparison of the Navan hydrothermal gangue Nd–Sr isotope data with data from Lower Palaeozoic rocks strongly suggests that the latter cannot alone account for the “basement” signature. As the Navan deposit lies immediately north of the Iapetus Suture, this suggests that the Laurentian margin includes Precambrian basement.