New constraints from structural data and U-Pb calcite geochronology on La Collada fluorspar ore body (Asturian basin, NW Spain)

The Permian–Mesozoic Asturian (NW Spain) hosts a series of world-class fluorspar deposits - a critical raw material for the EU - with more than 15 Mt. extracted to date. Asturian fluorspar deposits are hosted in silicified Paleozoic and Triassic carbonate rocks, spatially associated with the fault-bound margins of the basin. The Asturian fluorspar deposits show an important structural and lithological control, being hosted in fault-fill veins, jogs and breccias associated with steeply-dipping extensional faults and related folds, and strata-bound bodies replacing carbonate rocks. The general paragenesis comprises fluorite, calcite, quartz, (±) barite, and minor sulfides including pyrite, sphalerite, chalcopyrite, (±) galena. Formation of these fluorspar deposits has been assigned to the Permian volcanic activity during the extensional event following the Variscan orogeny, or the Mesozoic opening of the Atlantic realm.

For this study, we have sampled and measured the orientation of the main ore body at La Collada underground works. The ore body is hosted in a fault-fill vein moderately dipping to the SSW (mean plane is 214/47; n=41), ranging in thickness from a few meters to 15m. Observed sub-vertical veins of calcite are kinematically compatible with the main SSW-dipping fault-fill vein, and suggest concomitant ore deposition and extensional faulting. Laminated textures and localized high-dilation breccias in the main fault-fill vein suggests multi episodic fault activity associated with fluid overpressures (fault-valving?). S/C structures and fault slickensides (n=6) indicate oblique right lateral reactivation. Three representative samples from the main fault-fill vein were taken for U–Pb dating by LA-ICP-MS. Calcite spot analysis yielded two age populations (155Ma, 131Ma.) thus suggesting multiepisodic precipitation of calcite, and a potential open system. Obtained radiometric ages are consistent with rift initiation in the Asturian basin, and opening of the Bay of Biscay since Upper Jurassic times. Coupled with structural geology and textural analysis, LA-ICP-MS U-Pb dating of hydrothermal calcite aids in constraining the timing of fluid-flow events and fault activity responsible for ore deposit formation and re-mobilization, and refines the regional context of the Asturian fluorspar deposits.