Jorge Loredo

Geological, mineralogical and geochemical characteristics of the Carles gold mineralization, Asturias, Spain

In the western part of the Cantabrian Zone, Carlés gold mineralization is related to an igneous stock of granodioritic composition which intruded into devonian limestones, forming a well-developed exo-skarn and quantitatively less important endo-skarn. The skarn is characterized by the presence of garnet, pyroxenes, amphiboles, layers of hornfels corresponding to pelitic intercalations, and a number of opaque oxides and sulfides. Gold inclusions in arsenic minerals are present in quartzveins occurring in the skarn and in the granodiorite, but gold-rich zones are limited to the skarn. The crystallization temperature of arsenopyrite is coherent with T-P conditions (T = 460°C, P = 2200 bars) deduced from intersections of isochores of aqueous and carbonic fluid inclusions studied in samples of cogenetic quartz.

Fluid inclusions in quartz from a gold-mineralized granodioritic intrusion at Carlés, Asturias, Spain

Abstract The granodioritic stock of Carles, located near Ovideo (Asturias, Spain) is intruded in Devonian limestones which are transformed by contact metamorphism. Small inclusions of native gold are associated with arsenopyrite in the granodiorite and metamorphic aureole, either in quartz veins or in fractured rocks. Heating and freezing measurements carried out on the fluid inclusions in the quartz veins reveal the presence of four types of fluid inclusions: aqueous two-phase inclusions (type A); carbonic two-phase inclusions (type B); aqueous three-phase inclusions containing a halite cube (type C); and CO 2 -rich three-phase inclusions (type D). Aqueous inclusions (types A and C) show two generations, the homogenization of the whole occurring in a wide temperature interval (100–380°C). The carbonic and hydrocarbonic inclusions (types B and D, respectively) show each as a single generation, with the homogenization temperature of the CO 2 component occurring — either in the liquid or in the vapor phase — from 21° to 26°C, and the total homogenization temperature for type D varies between 280° and 380°C. Estimated pressures and temperatures from the stated inclusions would have maximum values of 2.4 kbar and 550°C. Two very important aspects of the fluids associated with the mineralization are: (1) the presence of CO 2 as a factor which could help the Au mobilization; and (2) the occurrence of important differences in density and salinity for apparently contemporaneous fluid inclusions, which could be related to boiling phenomena — by pressure drap — facilitating the Au precipitation.

The potential for arsenic mobilisation in the Caudal River catchment, north-west Spain

Abstract Asturias has numerous derelict Hg mine sites as a legacy of a past intensive mining activity, and the most important of these are located in the Caudal River catchment. These mines have been abandoned without remediation, and the legacy of these mine works remains currently in the form of mine wastes stocked in piles on the surface. In the exploited ore deposits, As is quite abundant in the form of sulphides (As-rich pyrite), sulpho-arsenides (arsenopyrite, realgar and orpiment), and supergenic minerals (scorodite). Significant quantities of mine and metallurgical wastes stocked at surface in spoil heaps of different age and size, and old galleries and transversals acting as pseudokarstic aquifers, are potential pollution pathways for the spreading of As to the environment. As a consequence of the weathering of As-rich minerals, As contents are high in mine effluents and spoil heap leachates. In the spoil heaps, waste materials are subjected to the action of a humid environment; in consequence, the instability of sulphides and sulpho-arsenides leads to the production of acid- and metal-rich leachates which pass incorporated into surface water or infiltrate through the land reaching groundwater. Electron microprobe analyses of pyrites confirm its high content of As (up to 10·11% As). Mining wastes stocked in the spoil heap present total As contents reaching up to 6·2%, and mine effluents and spoil heap leachates show As concentrations that can range from 1·4 to 9·2 mg/l. High concentrations of As have been detected too in stream sediments (up to 3·2%). In this context and on the basis of the risk assessment, proposals for remediation strategies are considered with the aim to reduce the leaching and transport of dissolved As from mining wastes.