Ignacio Romeo

U-Pb age constraints on Variscan magmatism and Ni-Cu-PGE metallogeny in the Ossa-Morena Zone (SW Iberia)

New U–Pb zircon ages from the Santa Olalla Igneous Complex have been obtained, which improve the knowledge of the precise timing of Variscan magmatism in the Ossa–Morena Zone, SW Iberia. This complex has a special relevance as it hosts the most important Ni–Cu–platinum group element (PGE) mineralization in Europe: the Aguablanca deposit. U–Pb zircon ages have been obtained for seven samples belonging to the Santa Olalla Igneous Complex and spatially related granites. With the exception of the Cala granite (352 ± 4 Ma), which represents an older intrusion, the bulk of samples yield ages that cluster around 340 ± 3 Ma: the Santa Olalla tonalite (341.5 ± 3 Ma), the Sultana hornblende tonalite (341 ± 3 Ma), a mingling area at the contact between the Aguablanca and Santa Olalla stocks (341 ± 1.5 Ma), the Garrote granite (339 ± 3 Ma), the Teuler granite (338 ± 2 Ma), and dioritic dykes from the Aguablanca stock (338.6 ± 0.8 Ma). The Bodonal–Cala porphyry, which has also been dated (530 ± 3 Ma), comprises a group of sub-volcanic rhyolitic intrusions belonging to the Bodonal–Cala volcano-sedimentary complex, which hosts the igneous rocks. The knowledge that emplacement of the Aguablanca deposit was related to episodic transtensional tectonic stages during the Variscan orogeny will be fundamental in future mineral exploration in the Ossa–Morena Zone.

Origin and emplacement of the Aguablanca magmatic Ni-Cu-(PGE) sulfide deposit, SW Iberia: A multidisciplinary approach

A model is proposed for the origin and emplacement of the ca. 341 Ma Aguablanca magmatic Ni-Cu-(platinum group element [PGE]) sulfi de deposit (SW Iberia) integrating petrological, geochemical, structural, and geophysical data. The Aguablanca deposit occurs in an unusual geodynamic context for this ore type (an active plate margin) as an exotic , magmatic subvertical breccia located at the northern part of the coeval gabbronorite Aguablanca stock (341 ± 1.5 Ma). Structural and gravity data show that mineralized breccia occurs inside the inferred feeder zone for the stock adjacent to the Cherneca ductile shear zone, a Variscan sinistral transpressional structure. The orientation of the feeder zone corresponds to that expected for tensional fractures formed within the strain fi eld of the adjacent Cherneca ductile shear. Two distinctive stages are established for the origin and emplacement of the deposit: (1) initially, the ore-forming processes are attributed to magma emplacement in the crust, assimilation of crustal S, and segregation and gravitational settling of sulfi de melt (a scenario similar to most plutonic Ni-Cu sulfi de ores), and (2) fi nal emplacement of the Ni-Cu sulfi de-bearing rocks by multiple melt injections controlled by successive opening events of tensional fractures related to the Cherneca ductile shear zone.

Crystallographic preferred orientations and microstructure of a Variscan marble mylonite in the Ossa-Morena Zone (SW Iberia)

The mylonitic marbles of the Cherneca shear zone, a Variscan sinistral strike-slip structure developed in the Ossa-Morena Zone (SW Iberia) related to the emplacement of the Santa Olalla Igneous Complex, were studied in detail. Different mylonites (from protomylonites to ultramylonites) were analyzed by Electron Back-Scattering Diffraction (EBSD) in order to determine the crystallographic preferred orientation (CPO) of calcite. The CPOs show very good agreement with experimental torsion work giving suitable interpretations of the operative slip systems and strain quantifications, thus the obtained natural CPOs validate the experimental approaches. A wide range of observed textures and microstructures are mainly controlled by the amount of shear strain, the presence of secondary phases, and the development of antithetic subsidiary shears. The effect of secondary phases and the nucleation of antithetic shear bands on texture development are discussed.

Magma emplacement in transpression: The Santa Olalla Igneous Complex (Ossa-Morena Zone, SW Iberia)

The Santa Olalla Igneous Complex, a late-Variscan group of intrusions located in the Ossa-Morena Zone (SW Iberia), has been the focus of a gravity and structural study. The structure outlined by the foliation map is complex, showing two different structural domains: one characterized by vertical, and the other by horizontal, magmatic foliations. The vertical fabrics are restricted to the NE half of the complex, which is in direct contact with a Variscan sinistral strike-slip fault (Cherneca fault) whereas the horizontal fabrics are developed in the SW half of the complex, which is characterized by a horizontal tabular geometry. Gravity modeling indicates that the deeper floor of the plutons is closely related to the NE margin and the Cherneca fault. An emplacement and structural evolution model for this igneous complex is proposed following these structural and gravity results: (1) magma ascent was favored by releasing bends in the trace of the Cherneca fault; (2) when magma reached the present level it intruded to the SW with a horizontal sheet geometry generating the subhorizontal foliation domain; (3) after emplacement, the NE half of the complex suffered the external tectonic stress field provoked by sinistral motion along the Cherneca fault, subsequently generating the subvertical magmatic foliation domain.