A case study of the internal structures of gossans and weathering processes in the Iberian Pyrite Belt using magnetic fabrics and paleomagnetic dating |
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Authors: | Mourad Essalhi Stanislas Sizaret Luc Barbanson Yan Chen France Lagroix François Demory José M Nieto Reinaldo Sáez M Ángeles Capitán |
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Institution: | 1.Université d’Orléans, CNRS/INSU, Institut des Sciences de la Terre d’Orléans-UMR 6113 Campus Géosciences 1A,Orléans cedex 2,France;2.Laboratoire de Paléomagnétisme, Institut de physique du Globe de Paris,Paris cedex 05,France;3.CEREGE, Université Aix Marseille-CNRS,Aix En Provence,France;4.Departamento de Geologia,Universidad de Huelva,Huelva,Spain |
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Abstract: | In the Rio Tinto district of the Iberian Pryrite Belt of South Spain, the weathering of massive sulfide bodies form iron caps,
i.e., true gossans and their subsequent alteration and re-sedimentation has resulted in iron terraces, i.e., displaced gossans.
To study the stucture and evolution of both types of gossans, magnetic investigations have been carried out with two foci:
(1) the characterisation and spatial distribution of magnetic fabrics in different mineralised settings, including massive
sulfides, gossans, and terraces, and (2) paleomagnetic dating. Hematite has been identified as the suceptibility carrier in
all sites and magnetic fabric investigation of four gossans reveals a vertical variation from top to bottom, with: (1) a horizontal
foliation refered to as “mature” fabric in the uppermost part of the primary gossans, (2) highly inclined or vertical foliation
interpreted as “immature” fabric between the uppermost and lowermost parts, and (3) a vertical foliation interpreted to be
inherited from Hercynian deformation in the lowermost part of the profiles. In terraces, a horizontal foliation dominates
and is interpreted to be a “sedimentary” fabric. Rock magnetic studies of gossan samples have identified goethite as the magnetic
remanence carrier for the low-temperature component, showing either a single direction close to the present Earth field (PEF)
direction or random directions. Maghemite, hematite, and occasionally magnetite are the remanence carriers for the stable
high-temperature component that is characterized by non PEF directions with both normal and reversed magnetic polarities.
No reliable conclusion can be yet be drawn on the timing of terrace magnetization due to the small number of samples. In gossans,
the polarity is reversed in the upper part and normal in the lower part. This vertical distribution with a negative reversal
test suggests remanence formation during two distinct periods. Remanence in the upper parts of the gossans is older than in
the lower parts, indicating that the alteration proceeded from top to bottom of the profiles. In the upper part, the older
age and the horizontal “mature” fabric is interpreted to be a high maturation stage of massive sulfides’ alteration. In the
lower part, the age is younger and the inherited “imature” vertical Hercynian fabric indicates a weak maturation stage. These
two distinct periods may reflect changes of paleoclimate, erosion, and/or tectonic motion. |
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