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TitleStructural controls on orogenic Au mineralization during transpression: Lupa Goldfield, southwestern Tanzania
AuthorLawley, C; Imber, J; Selby, D
SourceEconomic geology and the bulletin of the Society of Economic Geologists vol. 108, 2013 p. 1615-1640,
Alt SeriesEarth Sciences Sector, Contribution Series 20120448
PublisherSociety of Economic Geologists
Mediapaper; on-line; digital
File formatpdf; html
AreaTanzania, United Republic of
Lat/Long WENS 30.0000 33.0000 -6.0000 -9.0000
Subjectsstructural geology; igneous and metamorphic petrology; economic geology; gold; mineralization; orogenesis; shear zones; deformation; vein deposits; veins; Lupa Goldfield
Illustrationslocation maps; plots; photomicrographs; schematic diagrams; schematic models; stereonets
ProgramTargeted Geoscience Initiative (TGI-4), Gold Ore Systems
Released2013 09 30
AbstractAu 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.
Summary(Plain Language Summary, not published)
Previous studies have shown that ancient faults play an important role in focusing metal-rich fluids through the earth. However, the development of fault networks and their role in forming mineral deposits remains poorly understood. In this contribution we report new evidence from poorly understood mineral deposits in southwest Tanzania that emphasize the role of fluid-rock interaction in the development of ancient fault networks.