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The massive sulphide deposit of Aznalcóllar, Spain, Iberian Pyrite Belt: review of geology and mineralogy

  • Josefina Sierra

Synopsis

Aznalcóllar is a strabound deposit located within the so-called Iberian Pyrite Belt. The deposit has reserves of 44 × 106 t of pyrite ore grading 0.44% Cu, 1.76% Pb, 3.33% Zn, 67 g/t Ag, 1 g/t Au, and 34 × 106 t of sulphides grading 0.58% Cu, 0.40% Zn and 10 g/t Ag, the latter belonging to the so-called “cupriferous pyroclastic”. The pyrite-Cu-Zn-Pb orebodies strike E-W and dip 45° towards the north. The pit is 1,500 m long, 700 m wide and has a depth of 300 m.

The orebodies occur within a lower Carboniferous volcano-sedimentary unit named “Complejo Volcánico Sedimentario”, in which the volcanic rocks account for most of the sequence. Three episodes of acid volcanism (V1, V2, V3) and one of basic volcanism (Vb, which ended the first acid volcanic episode) are recognized within this unit. The massive sulphide mineralization is genetically related to V1; V2 is thought to be responsible for some minor jasperoid-Mn mineralizations, and V3 is “barren”. The most important rock types at Aznalcóllar are tuffs, ashes, rhyolites, volcanic breccias and shales. These rocks were affected by the Hercynian compressions, which resulted in folding, penetrative flow schistosity and partial obliteration of So planes.

Alteration is characterized by an early stage of silicification as a consequence of submarine fumarolic activity. This was followed in time by the formation of new minerals (quartz, plagioclase, chlorite, sericite) due to hydrothermal alteration and devitrification. The massive mineralization is mostly formed by pyrite with minor sphalerite, galena, chalcopyrite, and lead sulfosalts. A 3D statistical analysis of 576 samples taken from the mine showed no zonality of metal distribution and a reverse behaviour of Cu versus the pair Pb-Zn.

The origin of Aznalcéllar is time-related to the initial stage of the first acid volcanic episode (V1), during which exhalative activity occurred, thus leading to metal leaching from the volcanosedimentary complex, expulsion of brines from vents, and eventually, to precipitation of the metal load onto the seafloor. Metal transport was probably in the form of halide complexes in weakly acid brines, which shortly after expulsion reacted with H2S to form colloidal sulphide precipitates. Either gravitational instability or earthquake activity ultimately led to the generation of turbidite flows with rhythmic layering of black shales and sulphide layers.

The acid volcanics and sulphide mineralization are here thought to be a consequence of the emplacement of a subvolcanic plug locally known as the “felsite”. Since the ore is spatially related to this subvolcanic body, Aznalcóllar is here considered to be a “proximal”, although a non-rooted, volcanosedimentary deposit.

Keywords

Black Shale Massive Sulphide Gravitational Instability Massive Sulphide Deposit Iberian Pyrite Belt 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

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Copyright information

© The Institution of Mining and Metallurgy 1990

Authors and Affiliations

  • Josefina Sierra
    • 1
  1. 1.Departamento de Cristalografía y Mineralogía, Facultad de CC. GeológicasUniversidad ComplutenseMadridSpain

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