G. R. Almodóvar

Geological constraints on massive sulphide genesis in the Iberian Pyrite Belt

Abstract The Iberian Pyrite Belt (IPB), SW Iberian Peninsula, Spain and Portugal, one of the most famous and oldest mining districts in the world, includes several major concentrations of massive sulphides, unique on Earth (e.g., Riotinto, Neves Corvo), as well as a large number of smaller deposits of this same type. All these deposits, in spite of their general similarities, show significant differences in geological setting, age, relations to country rocks, hydrothermal alteration, mineralogy and geochemistry. As a consequence of a review of the available data in the IPB, together with new findings on volcanism, hydrothermal alteration and ore mineralogy, we propose a modified genetic scenario, that can account particularly for the diversity of the geological situations in which sulphide deposits occur, as well as for their mineralogical and petrological diversity. It is concluded that there is no direct genetic relationship between felsic volcanic activity and massive sulphide deposition in the IPB, and that most of the massive sulphide bodies, including all of the giant ones, are closely related to hydrothermal vents, being therefore proximal. The available isotopic data yield additional genetic information: (a) Homogeneous lead isotope values indicate a single (or homogenized) metal source; (b) sea and connate water are the fluid reservoirs for hydrothermal input, and (c) bacterial reduction of sulphur is the most probable cause of differences in δ 34 S between stockwork and massive sulphide mineralizations. Finally, current geodynamic models suggested for the IPB are discussed. It is suggested that an intracontinental, ensialic rift or pull-apart environment is the most probable genetic environment for the IPB mineralizations.

U–Pb dating of stockwork zircons from the eastern Iberian Pyrite Belt

Zircons found in the stockwork zones of the massive sulphide Los Frailes deposit, Iberian Pyrite Belt, are interpreted to have grown during the hydro-thermal alteration of host felsic volcanic rocks. Ion microprobe (SHRIMP) dating gives a206Pb/238 U age of 345.7 ± 4.6 Ma (2σ) and together with published spore data from the deposit suggest an age of uppermost Devonian to lowermost Carboniferous. Stockwork zircons offer the possibility of precisely defining the emplacement ages of these giant sulphide accumulations over the whole Pyrite Belt and from this will emerge estimates of the thermal budget necessary to generate the deposits.

Mineralogy of the hardpan formation processes in the interface between sulfide-rich sludge and fly ash: Applications for acid mine drainage mitigation

Abstract In the present study, experiments in non-saturated leaching columns were conducted to characterize the neoformed phases that precipitate at the interface between two waste residues having different chemical characteristics: an acid mine drainage producer residue (i.e., pyritic sludge) and an acidity neutralizer residue (i.e., coal combustion fly ash). A heating source was placed on top of one of the columns to accelerate oxidation and precipitation of newly formed phases, and thus, to observe longer-scale processes. When both residues are deposited together, the resulting leachates are characterized by alkaline pH, and low sulfate and metal concentrations. Two mechanisms help to improve the quality of the leachates. Over short-time scales, the leaching of pyrite at high pH (as a consequence of fly ash addition) favors the precipitation of ferrihydrite, encapsulating the pyrite grains and attenuating the oxidation process. Over longer time scales, a hardpan is promoted at the interface between both residues due to the precipitation of ferrihydrite, jarosite, and a Ca phase-gypsum or aragonite, depending on carbonate ion activity. Geochemical modeling of leachates using PHREEQC software predicted supersaturation in the observed minerals. The development of a relatively rigid crust at the interface favors the isolation of the mining waste from weathering processes, helped by the cementation of fly ash owing to aragonite precipitation, which ensures total isolation and neutralization of the mine residues.

La estructura sísmica de la corteza de la Zona de Ossa Morena y su interpretación geológica

El experimento de sismica de reflexion profunda IBERSEIS ha proporcionado una imagen de la corteza del Orogeno Varisco en el sudoeste de Iberia. Este articulo se centra en la descripcion de la corteza de la Zona de Ossa Morena (OMZ), que esta claramente dividida en una corteza superior, con reflectividad de buzamiento al NE, y una corteza inferior de pobre reflectividad. Las estructuras geologicas cartografiadas en superficie se correlacionan bien con la reflectividad de la corteza superior, y en la imagen sismica se ven enraizar en la corteza media. Esta esta constituida por un cuerpo muy reflectivo, interpretado como una gran intrusion de rocas basicas. La imagen de las suturas que limitan la OMZ muestra el caracter fuertemente transpresivo de la colision orogenica varisca registrada en el sudoeste de Iberia. La Moho actual es plana y, en consecuencia, no se observa la raiz del orogeno.

ACID NEUTRALIZATION AND METALS RETENTION IN SULFIDE-RICH MINING WASTES INDUCED BY THE ADDITION OF FLY AS

In mining environments with sulfide-rich wastes, the aqueous oxidation of pyrite and other metallic sulfides (arsenopyrite, chalcopyrite, sphalerite, galena, etc.) originates an acid drainage with high contents of sulfate and metals called Acid Mine Drainage (AMD). In this work, the oxidative dissolution of a sulfide-rich sludge sample (71.6% pyrite) from the Iberian Pyrite Belt (IPB), the generation of acid mine drainage (AMD), and the potential use of fly ash (a residue of coal combustion) to neutralize the acidity and reduce the metal content of the drainage, have been studied in column experiments. Obtained results show that: (1) a non-saturated column experiment filled with pyrite-rich sludge with artificial irrigation leached an acid drainage (pH approx. 2) with high concentrations of sulfate, iron and heavy metals; (2) non-saturated columns filled with sulfide sludge and fly ash leached a drainage characterized by high pH values (pH  10), very low sulfate content, and lack of iron and other metals in solution; (3) inside columns with fly ash, the pyrite oxidative dissolution at high pH (as a consequence of the leaching of fly ash) favours metals precipitation (mainly iron), the coating of pyrite grains and the oxidation attenuation; (4) in addition to ferric hydroxide coating, the precipitation of other minerals in the interface between pyrite-rich sludge and fly ash developed a rigid crust, or hardpan, which isolates mining waste from the weathering processes. Definitely, the addition of fly ash to a pyrite-rich sludge showed an improvement of the quality of the acid drainages and the development of mechanisms (iron coating and hardpan formation) that prevent the AMD production in a long term. Additional