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TitreStructural controls on orogenic Au mineralization during transpression: Lupa Goldfield, southwestern Tanzania
AuteurLawley, C; Imber, J; Selby, D
SourceEconomic geology and the bulletin of the Society of Economic Geologists vol. 108, 2013 p. 1615-1640, https://doi.org/10.2113/econgeo.108.7.1615
Année2013
Séries alt.Secteur des sciences de la Terre, Contribution externe 20120448
ÉditeurSociety of Economic Geologists
Documentpublication en série
Lang.anglais
DOIhttps://doi.org/10.2113/econgeo.108.7.1615
Mediapapier; en ligne; numérique
Formatspdf; html
Lat/Long OENS 30.0000 33.0000 -6.0000 -9.0000
Sujetsor; minéralisation; orogenèse; zones de cisaillement; déformation; gisements filoniens; filons; géologie structurale; pétrologie ignée et métamorphique; géologie économique
Illustrationslocation maps; plots; photomicrographs; schematic diagrams; schematic models; stereonets
ProgrammeÉtude des gîtes d'or, Initiative géoscientifique ciblée (IGC-4)
Résumé(disponible en anglais seulement)
Au mineralization in the western Lupa goldfield, southwestern Tanzania was associated with transpression and reverse sinistral slip along a network of steeply S dipping shear zones with non-Andersonian geometries. Slip was accommodated by: (1) frictional failure and sliding during emplacement of quartz ± Au-bearing veins; and (2) crystal plasticity and fluid-assisted diffusive mass transfer. The Kenge mineral system is situated along a NW-SE-trending shear zone and is characterized by ?10-m thick, Au-bearing fault-fill veins hosted by welldeveloped phyllosilicate-rich mylonites. The broadly contemporaneous Porcupine mineral system is situated
along an ENE-WSW to E-W trending shear zone, which is characterized by narrow, discontinuous mylonitic shear zone within a silicified and nonfoliated granitoid protolith. Au mineralization at Porcupine occurs within steeply dipping fault-fill and subhorizontal extension/oblique-extension veins. Three-dimensional frictional reactivation theory provides a self-consistent explanation for the different vein styles at Kenge and Porcupine and extends the classic fault valve model to the general case of oblique slip along multiple, arbitrarily oriented shear zones. Analysis of the differential stress required for frictional reactivation suggests the following: (1) the Kenge shear zone was intrinsically weaker than the Porcupine shear zone, consistent with the lack of welldeveloped mylonites at Porcupine; and (2) frictional reactivation of the Kenge shear zone occurred under suprahydrostatic but sublithostatic pore fluid pressures, whereas frictional reactivation of the Porcupine shear zone occurred under near-lithostatic fluid pressures. We hypothesize that near-lithostatic pore fluid pressuresrelieved effective normal stresses at grain-grain contacts, helping to preserve intragranular and fracture porosity
at the Porcupine orebody. As such, these pore spaces may be important microstructural sites for Au mineralization. Low effective normal stresses can also explain the poorly developed phyllosilicate-rich mylonites and limited degree of shear zone weakening at Porcupine.
Résumé(Résumé en langage clair et simple, non publié)
Des études antérieures ont montré que les failles anciennes jouent un rôle important dans la canalisation des fluides riches en métaux à travers la Terre. Toutefois, le développement des réseaux de failles et leur rôle dans la formation des gisements minéraux demeurent mal connus. Dans le cadre de cette contribution, nous présentons de nouvelles données sur des gisements minéraux mal connus du sud-ouest de la Tanzanie qui mettent l'accent sur le rôle de l'interaction fluide-roche dans le développement des réseaux de failles anciennes.
GEOSCAN ID292367