P. Shand

Groundwater evolution in the Continental Intercalaire aquifer of southern Algeria and Tunisia: trace element and isotopic indicators

Abstract The geochemical processes taking place along an 800 km flow line in the non-carbonate Continental Intercalaire aquifer (CI) aquifer in North Africa are described using chemical (major and trace element) and isotopic indicators. The aquifer is hydraulically continuous from the Atlas Mountains in Algeria to the Chotts of Tunisia and the geochemical evidence corroborates this. The highest discharge temperature is 73xa0°C but silica geothermometry indicates a maximum temperature of 94xa0°C at depth. Chloride concentrations increase from 200 to 800 mg l−1 and the Br/Cl ratios confirm the dissolution of non-marine evaporites or interstitial waters as the main source of salinity. Fluoride concentrations are low and are likely to be derived from rainfall, recording oscillations in source. Radiocarbon ages, except near outcrop, are at or near detection limits and the δ18O and δ2H values indicate a cooler recharge regime with rainfall having lower primary evaporation than today. This is shown by the fact that mean isotope ratios of CI waters are around 3‰ lighter than the present-day weighted mean value for rain. Major ion ratios and most trace elements indicate that despite the complex structure and stratigraphy, uniform evolution with continuous water-rock interaction takes place along the flow lines, which are only disturbed near the Tunisian Chotts by groundwater converging from additional flow lines. The ageing of the water can also be followed by the smooth increase in several indicator elements such as Li, K and Mn which are least affected by solubility controls. Similarly the influence of marine facies in the Tunisian sector may be recognised by the changing Mg/Ca and higher Br/Cl as well as trace element indicators. The groundwaters are oxidising up to 300 km from outcrop (dissolved O2 has persisted for at least 20 ka) and within this zone the concentrations of several elements forming oxy-anions, such as U and Cr, increase and NO3 remains conservative. Beyond 300 km from outcrop, the groundwaters are reducing and contain high Fe concentrations. The basin contains huge reserves of fresh/brackish waters but these need careful development due to the limiting high salinity and scaling tendency resulting from the high temperatures and mineral super-saturation caused during abstraction as well as high concentrations of some harmful elements such as Cr in the oxidising section.

Hydrogeochemical exploration for gold in the Osilo area, Sardinia, Italy

A study to test the use of hydrogeochemical methods for gold prospecting was carried out in the Osilo area, northern Sardinia. The study area, covering about 30 km2 is characterised by Tertiary andesitic rocks. Gold concentrations up to several ppm, associated with abundant pyrite, arsenopyrite, stibnite, tetrahedrite and electrum, and subordinate galena, sphalerite and chalcopyrite, are present in quartz veins associated with a polyphase, incipient and pervasive alteration of the andesitic rocks. n nForty-eight water samples (17 streams, 29 springs and 2 boreholes) were analysed for Au and a wide range of major and trace elements, both in solution (< 0.4 μm) and in suspension. Background values for dissolved Au were below the detection limit of the methods used (between 0.3 and 0.5 ng L−1 Au). Gold concentrations in solution up to 3 ng L−1 were found in waters draining the mineralised vein system. The observed dispersion of Au in surface waters was restricted to about 500 m from the auriferous veins. Dissolved Au anomalies do not vary significantly in water samples, taken monthly over a one year period, suggesting that the dispersion of Au is unaffected by seasonal conditions in the Osilo area. For samples where Au was detected both in solution and in suspension, the Au content of the suspended matter was usually lower than that in solution. n nThe best indicators of Au mineralisation, apart from Au itself, both in solution and in suspension, were As and Sb which showed a dispersion clearly related to the known auriferous veins.