Geography
The Río Tinto is river about 100 km long with a basin area of 1700 km2, located in the southwestern parts of Spain in the Andalucia region. It is flowing south-southwesterly from Sierra Morena Mountains to the Gulf of Cádiz in the Mediterranean sea(Rodríguez-Tovar and Martín-Peinado, 2014).
The river are distinguishably reddish in color, both in the deposited sediments and in the flowing water and is therefore called “The River of Fire” (Martin, 2013). The water is highly polluted and notable acidic with extremely low pH (< 3), and contains a lot of iron and heavy metals in solution (Rodríguez-Tovar and Martín-Peinado, 2014).
Geological Background
For about 350 million years ago, in the Paleozoic era, a volcanic-sedimentary complex was taking form, the so called Iberian Pyrite Belt (IPB). Three large stratigraphical units are present within the IPB; (Cáceras et al., 2013)
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The Phyllite-Quartz Group
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The Carboniferous Volcanic-sedimentary complex (VSC)
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The Culm Group
The VSC rocks has undergone an intense postmagmatic hydrothermal alteration. They are of geochemical and economical interest, because of the rich sulfide bodies that are present in the complex. (Cáceras et al., 2013)
It is believed that during an exhumation of the the massive sulfide ore mineralisation, it was brought up to surface. The sulfides began to oxidize, and that created gossan formations that covers most of the ore bodies. Scientists are at odds about when this natural oxidation process started to form in the area. Statements are widely spread, evidence indicates that the gossans already was existing for about 300 000 years ago (Rodríguez-Tovar and Martín-Peinado, 2014). Other studies shows that the gossans could be older, 6 million years by the end Miocene (Fernández-Remolar et al., 2005) or even as old as 24 million years (Cáceras et al., 2013).
The IPB comprise ⅔ of the drainage basin of the Río Tinto. The rest ⅓ of the basin located in the southernmost parts is composed of younger Neogene and Quaternary sediments, and are in order marine sediments. Through time the environment changed, and as cause of a continentalization that was ongoing during Pliocene, the deposited sediments in Quaternary turned to be predominate of fluvial gravels, sands and conglomerate. (Cáceras et al., 2013)