New structural and mechanical insights on natural hydrofractures, Panasqueira Mines, Portugal.

Abstract:

Hydrofractures are mostly tensile fractures like barren joints or mineral veins created by relatively high pore fluid pressures but under relatively low differential stresses. They can retain significant porosity at geological time-scales, providing preferential pathways for crustal fluids and often host mineral deposits.

The mines of Panasqueira, central Portugal, represent the largest W-Sn deposit currently mined in western Europe and constitutes a fossil record of natural hydrofracturing at presumably high levels of pore pressure in the upper crust. The ore deposit comprises an extensive sub-horizontal vein network hosted by a regional metasedimentary sequence of Palaeozoic age, which was folded, foliated and metamorphosed to greenschist facies during the Hercynian Orogeny. The vein system is connected to a greisenized cupola of the underlying granitic intrusion, evidenced by the pronounced contact metamorphism.

The extensive network of underground excavations allows for exceptional exposure of the geological structures, and gives the possibility to use the mines of Panasqueira as a natural laboratory in order to understand the mechanisms driving its formation and propagation.

Paleostress analyses using vein data, complemented with fluid inclusion and geothermometry/geobarometry studies were conducted. Preliminary results of vein attitudes indicate a cluster distribution of the poles with σ₃ vertical. The other stress axes were oriented NW-SE (σ₁) and NE-SW (σ₂), respectively. Microthermometry of fluid inclusions in quartz related to the main opening stages of the veins evidence low salinity fluids belonging to the system H₂O-NaCl-CO₂-(CH₄)-(N₂). The respective homogenization temperatures fall in the range of 250-320˚C. Coeval arsenopyrite contains 33,5–34,0 atomic % As, corresponding to mineralisation temperatures of ~450 ˚C. Fluid isochores together with the formation temperatures derived from arsenopyrite compositions suggest fluid paleopressures of ~300 MPa contemporaneous to vein opening. Similar pressure values are corroborated by Mole % FeS compositions obtained on coeval sphalerite.

Further work will include the calculation of the “full” paleostress state associated to the formation of the quartz veins.