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| Titel |
A structural analysis of the Minas da Panasqueira vein network and related fracture generations |
| VerfasserIn |
Dominique Jacques, Romeu Vieira, Philippe Muchez, Manuel Sintubin |
| Konferenz |
EGU General Assembly 2014
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 16 (2014) |
| Datensatznummer |
250089337
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| Publikation (Nr.) |
 EGU/EGU2014-3537.pdf |
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| Zusammenfassung |
| The Minas da Panasqueira is a world-class W-Cu-Sn vein-type deposit, situated within the
Central Iberian Zone of the Palaeozoic Iberian Massif (Portugal). The deposit consists of a
network of subhorizontal, sill-like massive quartz veins situated above the southwestern
extremity of a greisen cupola, within regionally metamorphosed, isoclinally folded,
lower-greenschist slates and greywackes. The greisen cupola is part of a larger intrusive
complex, emplaced during the late- to post-tectonic stage of the Variscan orogeny. The
late-Variscan granitoid(s) underlying the Panasqueira deposit is considered to have served as
a major metal source.
The structure of the network of subhorizontal extension veins, consists of numerous
planar vein lobes that are separated by host-rock bridges and merge at branch-points. A
structural analysis demonstrates that not only within the Panasqueira mine, but also on a
more regional scale, one or more generations of flat-lying fractures are present.
The veins clearly exploited these pre-existing discontinuities, as confirmed by (1)
the vein geometry being directly influenced by variations in the orientation of the
initial fracture sets and (2) the geometry of the rock bridges and overlapping vein
morphologies, consistently showing straight-line propagating crack tips. If veining
is governed by a preferential, strongly developed anisotropy in the host rock, the
hypothesis of vein lobes and rock bridges forming during propagation of the parent crack
by tip-line bifurcation and confinement processes (Foxford et al., 2000) does not
seem plausible. Instead, we propose that the rock bridges formed from several,
initially separate and small veinlets that eventually overlapped in an en echelon
arrangement during progressive propagation and inflation. Bending of the rock bridges
and incipient vein rotation indicate that veining occurred near the brittle-ductile
transition.
Using a quantitative analysis of bridge orientations, vein aspect ratios and tip lines, we try
to sort out if a dominant Ïă2 propagation direction, typical for hydrofractures, exists
within the vein network. By doing so, we can evaluate whether the subhorizontal
vein network formed under a compressive stress regime, or was mainly dictated
by the strength anisotropy of the rocks under near-isotropic stress conditions of
Ïăhmax - Ïăhmin.
The regional dominance of subhorizontal aplites, pegmatites and hydrothermal veins,
exploiting subhorizontal fracture networks, occurs over a wide area of more than 100 km2
along the Serra de Estrela granitic massif (Derré et al., 1986). This orientation contrasts with
the more common vertical attitude of granite-related hydrothermal veins, observed
throughout the Iberian massif. A detailed orientation analysis of the fracture sets should allow
to explore the possible causes of this particular late orogenic, flat-lying fracture network
related to the granitic intrusion.
References
Derré, C., Lecolle, M., Roger, G., Tavares de Freitas Carvalho, J., 1986. Tectonics,
magmatism, hydrothermalism and sets of flat joints locally filled by Sn-W, aplite-pegmatite
and quartz veins, southeastern border of the Serra de Estrela granitic massif (Beira Baixa,
Portugal). Ore Geology Reviews 1, 43-56.
Foxford, K. A., Nicholson, R., Polya, D. A., and Hebblethwaite, R. P. B., 2000.
Extensional failure and hydraulic valving at Minas da Panasqueira, Portugal; evidence from
vein spatial distributions, displacements and geometries. Journal of Structural Geology 22,
1065–1086. |
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