3D-permeability structures in an orogenic setting with the emphasis on the lateral continuity in associated vein systems.

Granite-related W-Sn deposits typically occur in close association with highly specialised S-type granites, which are especially numerous in late- to post-orogenic settings. The formation of W-Sn hydrothermal ore deposits has been extensively studied from a geochemical-mineralogical viewpoint. Few studies, however, tend to focus on the geometry and kinematics of the deposits and their overall structural context. The latter is especially relevant for W-Sn vein-type deposits, whose geometry is highly dependent on array of brittle-ductile deformation processes and represent the main global W-Sn source. Nonetheless, the emplacement of W-Sn bearing vein systems is too often stated to hinge on a granite-controlled or post-orogenic extensional stress regime. Detailed structural analyses to support such a kinematic hypothesis and highlight the relationship with the orogenic framework are typically lacking.

To fill this hiatus, we have performed an integrated structural analysis of different vein-type deposits within the metallogenetic province of northern Portugal (Central Iberian Zone). These deposits consist of (i) hydrothermal quartz vein systems (e.g. Panasqueira, Rio de Frades, Queiriga, Argemela) and (ii) late-magmatic aplite-pegmatites (e.g. Lagares). The objective of the dissertation is to formulate a metallogenetic model for the structural emplacement of Iberian W-Sn mineralisation, highlighting the relationship between the regionally observed vein and fracture geometry, the Variscan deformation structure and the late- to post-orogenic geodynamic context.

The studied vein-type deposits can broadly be subdivided in two groups. A first, major group consists of subhorizontal quartz and aplite-pegmatite vein systems that are exploiting a regional cross-fold joint system within the metasedimentary host rock. This cross-fold joint system is oriented perpendicular to the late-Variscan F3 fold generation, which developed simultaneously with oroclinal buckling. The F3 fold generation is characterised by subvertical fold axes, and is associated with dextral, pure-shear dominated transpression. A phyllosilicate texture analysis, in conjunction with other mesoscale observations, has indicated that fold-axis parallel stretching during transpression lies at the mechanic origin of cross-fold jointing. Veining reactivated the cross-fold joints through hydraulic fracturing under low differential stresses. The consistent perpendicular orientation of the veins relative to the non-cylindrical F3 fold axes, also when moderately-plunging, indicates that this reactivation did not occur during far-field horizontal compression. Instead, the vein geometry is dictated by a local stress state with the minimum principal stress consistently subparallel to the F3 fold axis. Hence, the veins developed in a similar stress regime as the cross-fold joints, either broadly cogenetic or due to the post-tectonic release of residual elastic strain.

A second, minor group consists of vein systems whose geometry is controlled by a local stress regime induced by granite emplacement and expansion of the underlying magma body. Granite emplacement was coupled to doming and an extensional stress regime within the overlying metasedimentary host rock. This stress regime is reflected in (i) the occurrence of concentric, granite-hosted quartz veins, and (ii) subvertical veins hosted by the neighbouring metasedimentary host rock.

Combining the above observations with a detailed literature study, demonstrated that the structural emplacement of Iberian W-Sn mineralisation is controlled by the specific late-Variscan deformation style, i.e. vertical-axis oroclinal buckling. This intimate relationship with the geodynamic context is manifested by different W-Sn vein systems showing (i) a consistent perpendicular to the F3 fold generation through the exploitation of cross-fold joints, (ii) a control by widespread granite emplacement simultaneous with oroclinal buckling and crustal thickening, (iii) a subvertical orientation in agreement with the transpressional stress regime, and (iv) a temporal proximity to the tectonic inversion, triggering structural permeability. Future mineral exploration efforts will need to keep in mind that the geometry of W-Sn vein-type mineralisation appears to be controlled principally by (i) the nature of the host rock, and (ii) the geometry of the late-Variscan fold generation and the associated strike-slip fault systems. A prime objective of an exploration campaign should thus always be a detailed assessment of the local deformation structure.

This study has not only highlighted the necessity of detailed geometric and kinematic analyses to fully understand the structural emplacement of W-Sn vein-type deposits. In addition, several of the interpretations have proven to be significant in the general understanding of the mechanics of cross-fold jointing, the structure of the Iberian Variscides, the kinematics of oroclinal buckling and the geodynamic framework of the Variscan orogeny.