Short-term changes in water chemistry in the Baccu Locci stream (Sardinia,Italy) affected by past mining     

Franco Frau  Rosa CiduCarla Ardau 《Applied Geochemistry》2012

Short-term changes in water chemistry, and especially in dissolved trace element concentrations, associated with diel cycles during base-flow conditions at a specific sampling station in the Baccu Locci stream draining the homonymous old mine area in Sardinia (Italy) were investigated. Diel fluctuations in pH and alkalinity were correlated with the temperature-dependent CO₂ solubility and the biologically-induced CO₂ production, both of which were higher during the night. Adsorption/desorption to/from streambed material, in particular ferrihydrite, is believed to be the main in-stream mechanism causing the observed diel variations in dissolved concentrations of As and Zn. Arsenic was mainly affected by the dual action of temperature and competing carbonate ions, whereas pH seemed less important. Temperature acted in accordance with the exothermic feature of anion sorption onto hydrous metal oxide surfaces; aqueous carbonate species exerted their competitive effect in relation to alkalinity variation. Zinc was primarily affected by temperature, in accordance with the endothermic feature of metal cation sorption onto hydrous metal oxide surfaces, and secondly by pH. Co-precipitation of As and Zn with calcite is another possible mechanism, which requires further investigation involving examination of inorganic and biological materials coating the streambed. All these processes potentially controlling the diel cycles of trace elements should be carefully considered to assess the effectiveness of remediation actions currently in progress at Baccu Locci. A normalization method for data from asynchronous sampling has been developed and proposed in order to eliminate or at least attenuate the effect of sampling time and provide an additional tool to identify the processes/mechanisms involved in trace element concentration fluctuations observed along a contaminated stream during base-flow conditions.  相似文献   

Geochemistry and geochronology of the volcanic rocks associated with the Dong'an adularia–sericite epithermal gold deposit,Lesser Hinggan Range,Heilongjiang province,NE China: Constraints on the metallogenesis     

Zhaochong Zhang  Jingwen Mao  Yanbin Wang  Franco Pirajno  Junlai Liu  Zhidan Zhao 《Ore Geology Reviews》2010,37(3-4):158-174

Late Mesozoic volcanism is widespread throughout NE China. On the basis of lithological associations and spatial relationships, the volcanic rocks in the Lesser Hinggan Range can be divided into two formations, i.e., felsic-dominant Fuminghe Formation and overlying mafic-dominant Ganhe Formation. The Dong'an gold deposit, a typical adularia–sericite epithermal system, is spatially closely associated with rhyolitic porphyry, which is a subvolcanic intrusion of the Fuminghe Formation. Total measured, indicated, and inferred resources for the Dong'an deposit are 70 tonnes (2.25 Moz) of gold with the grade of 5.04 g/t Au, making it one of the largest epithermal gold deposits in China.SHRIMP U–Pb zircon and ⁴⁰Ar/³⁹Ar geochronology applied to one rhyolitic porphyry sample and sericite separated from auriferous quartz veins of the main mineralization stage were carried out to constrain magmatic and hydrothermal events. The results suggest that the mineralization age of 107.2 ± 0.6 Ma overlaps with the age of the rhyolitic porphyry 108.1 ± 2.4 Ma. Our new age data indicate that there was a previously unrecognized mineralization event in NE China at 107–108 Ma.Systematic geochemical investigations on the volcanic rocks in the Lesser Hinggan Range show that both Fuminghe and Ganhe Formations are characterized by significant large ion lithophile elements (LILE) and light rare earth elements (LREE) enrichment coupled with high field strength elements (HFSE) depletion, but they have distinct Sr and Nd isotopic compositions. The Fuminghe Formation has relative high ⁸⁷Sr/⁸⁶Sr ratios of 0.707253 to 0.707373, and negative ε_Nd(t) values of ?2.78 to ?3.05 (t = 108 Ma), whereas the Ganhe Formation displays slightly lower ⁸⁷Sr/⁸⁶Sr range of 0.705434–0.705763 and positive ε_Nd(t) values of + 0.76 to +1.83. These geochemical data suggest that the rhyolitic magmas of the Fuminghe Formation probably represent the final differentiates of parental andesitic magmas, resulted from the partial melting of mafic lower crust, whereas the volcanic rocks of the Ganhe Formation were produced by fractionation of basaltic magmas generated from partial melting of a mixture of an incompatible element depleted anhydrous lherzolite asthenospheric mantle source and a hydrous enriched lithospheric mantle source in an extensional tectonic setting, in response to upwelling of asthenospheric mantle. The rhyolite porphyries of the Fuminghe Formation are inferred to have supplied heat that drove the convective hydrothermal system at Dong'an deposit, but also provided some of the fluid sources responsible for the development of the Dong'an epithermal system.  相似文献