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Aguablanca

Spain

Main commodities: Ni Cu PGE PGM
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The Aguablanca magmatic Ni-Cu-(PGE) deposit is hosted by a subvertical magmatic breccia and is located near Seville, within the Iberian Massif of south-western Spain.

The deposit is regarded as unusual in that it occurs as a steeply punging magmatic breccia within a continental magmatic arc setting rather than the more common a layered mafic intrusion in a rift environment.

The ore deposit at Aguablanca is hosted by the Aguablanca intrusion, a mafic body composed gabbro-norite with minor quartz-diorite, gabbro and norite. The sub-circular Aguablanca intrusion covers an area of approximately 3 sq. km and is located in the southern limb of a WNW­ESE trending longitudinal Hercynian structure, the Olivenza-Monesterio antiform, which is in turn situated on the southern boundary of the Ossa-Morena zone, one of the five parts of the Iberian Massif, the westernmost belt of the European pre-Mesozoic Variscan chain.

The Ossa-Morena zone initially evolved as a continental magmatic arc characterised by calc-alkaline magmatism and the formation of dioritic-tonalitic rocks and anatectic granites throughout the northern and central sectors of the zone, respectively. Subsequent Cambrian and Ordovician intra-continental rifting resulted in the formation of new oceanic crust accompanied by extensive bimodal alkaline magmatism. The zone then evolved as a passive margin with the deposition of Cambrian to Lower Permian volcano-sedimentary, terrigenous and carbonate sequences that were deformed by the Hercynian orogeny (390­300 Ma). During this period of orogenesis, an Andean-type magmatic arc was developed reflected by the emplacement of a large volume of calc-alkaline plutonic rocks, including the ore-bearing Aguablanca intrusion which has been dated at 338.6 ±0.8 Ma (U-Pb in zircon) and 338 ±3 Ma (40 Ar-39 Ar in phlogopite).

The Aguablanca pluton intrudes the Neoproterozoic Tentudia succession of interlayered volcanic greywackes and pyrite-bearing black slates, which are the upper section of the 3000 m thick Serie Negra which predominantly comprise black slates and quartzites. The Tentudia succession is unconformably overlain by late Neoproterozoic to Lower Cambrian rhyolitic and dacitic volcanic and volcaniclastic rocks which pass up into carbonate rocks. The Aguablanca intrusion is bounded to the south by the Santa Olalla pluton, a calc-alkaline intrusive body dated at 341 ±3 Ma.

The dominant lithology of the Aguablanca pluton is a hornblende-bearing gabbro-norite which comprises a medium- to coarse-grained meso- and ortho-cumulate containing variable amounts of orthopyroxene (27­48 modal %), plagioclase (23­47%), clinopyroxene (4­11%), amphibole (10­21%), phlogopite(<5%) and minor interstitial quartz (<1%). Locally, orthopyroxene or clinopyroxene are very minor, producing gabbro and norite, respectively. Orthopyroxene and clinopyroxene are invariably cumulus minerals. The Gabbro-norites exhibit sub-horizontal layering on a decimeter scale, reflecting variations in the plagioclase contents.

The magmatic breccia hosting the Ni-Cu sulfide mineralisation at Aguablanca is located near the northern margin of the Aguablanca intrusion and is developed from a few meters below the surface to more than 600 m in depth. It has the shape of a subvertical (70­80° N dipping) downward tapering cone shaped magmatic breccia. It has a width of approximately 250 to 300 m (north-south) and is 600 m long (east-west). Within the breccia, the mineralisation is mainly concentrated in subvertical orebodies which are truncated by northeast trending post-mineral faults. Where exposed, the sulphide mineralisation is reflected by an 8 to 10 m thick gossan.

Disseminated and semi-massive Ni-Cu-Fe sulphide mineralisation is concentrated within the gabbronorite matrix of a the magmatic breccia that also contains barren to weakly mineralised ultramafic-mafic cumulate fragments. These fragments are more abundant in the semi-massive ore and represent a wide variety of rock types, including peridotite (hornblende-rich werhlite, dunite, and hornblende-rich harzburgite), pyroxenite (ortho- and clino-pyroxenite), gabbro (gabbro, gabbronorite and hornblende gabbro) and anorthosite.

The semi-massive ore consists of "leopard-textured" sulphides (commonly between 20 and 70% sulphides), comprising black spots of silicates (mostly pyroxene, olivine, and/or plagioclase) enclosed in a yellowish sulphide groundmass. The more abundant disseminated sulphide ores (with around 20% sulphides) occur as polymineralic aggregates of variable grain size interstitial to the silicate framework. In general, the semi-massive ore occurs in the core of the breccia surrounded by the disseminated ore.

The ore mineralogy has been interpreted to comprise two assemblages, namely:  i), a subsolidus, re-equilibrated magmatic assemblage, comprising pyrrhotite, pentlandite, chalcopyrite, and minor PGE (merenskyite, michenerite, palladian melonite, moncheite and sperrylite), magnetite, ilmenite, Bi-, Ag-tellurides (tellurobismuthite, volinskyite, and hessite) and native gold; and  ii), a hydrothermal assemblage, consisting of several textural types of pyrite.

The primary silicate assemblage consists of olivine, orthopyroxene, clinopyroxene, plagioclase, amphibole and phlogopite. The wide range of rock types and the Fe-enrichment trends in the primary ferromagnesian silicates has been interpreted to suggest magmatic differentiation processes in the parent melts, with the fragments representing different stages of cumulate formation.

Ore textures of both the semi-massive and disseminated sulphides in the gabbro-norite matrix vary between meso- and ortho-cumulate. The rock forming magmatic silicates comprise orthopyroxene (Mg no. 0.83­0.74), clinopyroxene (Mg no. 0.89­0.78), plagioclase (An50-An77) and intercumulus amphibole (Mg no. 0.86­0.70), phlogopite (0.84­0.69) and minor quartz. The gabbronorite in the matrix of the breccia is petrographically and chemically very similar to that of the unmineralised parts of the main Aguablanca intrusion and has a similar differentiation trend.

Reserves have been quoted at 15.7 Mt @ 0.66% Ni, 0.46% Cu, 0.47 g/t PGE, 0.13 g/t Au .

The most recent source geological information used to prepare this summary was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Pina R, Lunar R, Ortega L, Gervilla F, Alapieti T and Martinez C,  2006 - Petrology and Geochemistry of Mafic-Ultramafic Fragments from the Aguablanca Ni-Cu Ore Breccia, Southwest Spain: in    Econ. Geol.   v101 pp 865-881
Tornos F, Galindo C, Casquet C, Pevida L R, Martinez C, Martinez E, Velasco F and Iriondo A,   2006 - The Aguablanca Ni-(Cu) sulfide deposit, SW Spain: geologic and geochemical controls and the relationship with a midcrustal layered mafic complex : in    Mineralium Deposita   v41 pp 737-769


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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