Strukturelle und mechanische Charakterisierung naturlichen hydraulischer Risse in 3D (HyFract3D)
Zusammenfassung der Projektergebnisse
Hydrofractures are mostly tensile fractures created by relatively high pore fluid pressures but under relatively low differential stresses and may originate barren joints and mineral veins. 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 quartz vein network hosted by a regional metasedimentary sequence of Palaeozoic age, which was previously 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 to understand the mechanisms pertaining to natural hydrofracture. Paleostress analyses using vein data, complemented by fluid inclusion and geothermometry studies were conducted. Preliminary results of vein attitudes indicate a cluster distribution of the poles with σ3 vertical, as expected. The other stress axes were oriented NW-SE (σ1) and NE-SW (σ2), respectively. The main outcome of the study consists in the determination of the full paleostress tensor. Our results suggest stress magnitudes of ~ 300 MPa and depths for the crystallisation of the mineralised veins of ~10 km. These results add new insights on the formation of the ore deposit, which is currently operated, and should benefit further research to understand what mechanisms control the deposition of economic deposits. In addition, our study represents a step forward to quantify tectonic forces in space and time. We used the arsenopyrite geothermometer together with Fluid Inclusion studies to constrain paleopressure. Unexpectedly, our samples showed two arsenopyrite generations, whereas a single one was anticipated. However, this “surprise” turned to be an advantage to better constrain the uncertainty domain pertaining to our determinations.
Projektbezogene Publikationen (Auswahl)
-
2015. New structural and mechanical insights on natural hydrofractures, Panasqueira Mines, Portugal. American Geophysical Union, Fall Meeting 2015
Jaques, L. M., Pascal, C., Yamaji, A. and Pinto, F.
-
2015. Structural and mechanical characterisation of natural hydrofractures: example from the mines of Panasqueira, Portugal. Geophysical Research Abstracts Vol. 17, EGU2015-12114, EGU General Assembly 2015
Jaques, L. M., Pascal, C., Yamaji, A. and Vieira, R.
-
2017. Full paleostress tensor reconstruction using quartz veins of Panasqueira Mine, central Portugal; part I: paleopressure determination, accepted for Journal of Structural Geology
Luís Jaques and Christophe Pascal
