Ioan V. Sanislav

The structural history and mineralization controls of the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania

The Geita Hill gold deposit is located in the Archean Geita Greenstone Belt and is one of the largest gold deposits in East Africa. The Geita Greenstone Belt experienced a complex deformation and intrusive history that is well illustrated and preserved in and around the Geita Hill gold deposit. Deformation involved early stages of ductile shearing and folding (D1 to D5), during which episodic emplacement of large diorite intrusive complexes, sills, and dykes occurred. These ductile deformation phases were followed by the development of brittle-ductile shear zones and faults (D6 to D8). The last stages of deformation were accompanied by voluminous felsic magmatism involving the intrusion of felsic porphyry dykes, within the greenstone belt, and the emplacement of large granitic bodies now forming the margins of the greenstone belt. Early, folded lamprophyre dykes, and later lamprophyre dykes, crosscutting the folded sequence are common, although volumetrically insignificant. The gold deposit formed late during the tectonic history of the greenstone belt, post-dating ductile deformation and synchronous with the development of brittle-ductile shear zones that overprinted earlier structural elements. The main mineralizing process involved sulfide replacement of magnetite-rich layers in ironstone and locally the replacement of ferromagnesian phases and magnetite in the diorite intrusions. The intersection between the brittle-ductile (D6) Geita Hill Shear Zone and different structural elements of ductile origin (e.g., fold hinges), and the contact between banded ironstone and folded diorite dykes and sills provided the optimal sites for gold mineralization.

Porphyroblast rotation and strain localization: Debate settled!: COMMENT

This comment addresses two inferences used by Johnson (2009) to argue for porphyroblast rotation during bulk coaxial shortening. Firstly, he interprets that porphyroblast inclusion trails that are inclined (his figure 1) became inclined because the porphyroblast rotated during growth. He used orientation data from millipede microstructures from one hand sample and concluded on the basis of the total spread of inclusion trail orientations and porphyroblast axial ratios that porphyroblasts had rotated relative to one another during ductile deformation. Secondly, he presents a numerical model that indicates that asymmetrically shaped porphyroblasts rotated during coaxial deformation. However, I will show that the porphyroblasts did not rotate after they grew, in spite of localized shearing along the developing S3 that rotated visible matrix S2 in Johnsons figure 1.

The control of deformation partitioning and strain localization on porphyroblast behaviour in rocks and experiments

Multiple generations of sub-vertical and sub-horizontal foliations preserved as inclusion trails in garnet in mylonitic rocks from the hanging wall of the Main Central Thrust in the Himalayas indicate that these porphyroblasts did not rotate during thrusting. This result is predicated by (i) a consistent succession of 5 changes in FIA trend (foliation inflection/intersection axes in porphyroblasts) for samples where the orientation changes from porphyroblast cores to rims; (ii) sub-vertical and sub-horizontal foliations occur as inclusion trails around each of the 5 FIAs in the succession, which would not be the case if the garnet porphyroblasts rotated during subsequent phases of deformation as tectonism continued; (iii) a change in inclusion trail asymmetry immediately prior to the commencement of mylonitzation indicates top to the south thrusting only if the porphyroblasts had not rotated as they grew; (iv) the latter asymmetry matches truncated crenulation relics preserved within the mylonitic matrix foliation that indicate top to the south thrusting as the latter foliation formed. Partitioning of deformation into shortening and shearing components stops rotation of porphyroblasts during their growth and during following periods of ductile tectonism. This can be replicated via computer modelling by duplicating the crenulation-hinge-like coaxial environment in which porphyroblasts nucleate and grow before the strain intensifies. This was done using Drucker-Prager constitutive models with temperature-dependent strain softening behaviour and resulted in no porphyroblast rotation when followed by non-coaxial deformation no matter how intense. Furthermore, strain localization in the model containing competent objects of variable size, shape and orientation, produced no rotation during deformation involving components of shortening and shearing. These approaches to modelling mechanically resolve the sub-vertical/sub-horizontal foliations defined by inclusion trails and consistent FIA trend successions obtained from the Main Central Thrust rocks as well as in orogens elsewhere.

Foliation intersection/inflection axes within porphyroblasts (FIAs): a review of advanced applications and significance

Foliation Intersection/Inflection Axes within porphyroblasts (FIAs) provide a quantitative approach to microstructural and metamorphic analysis that can be used to integrate multiple deformation and metamorphic events and develop lengthy PT path histories during orogenesis. This paper reviews applications of FIAs for determining (1) porphyroblast rotation vs. nonrotation; (2) inclusion trail orientations and orogenic processes involving changes in the direction of bulk horizontal shortening; (3) in situ age determinations for multiple deformation episodes; (4) P-T-t-d paths of poly-metamorphism in an orogenic belt; and (5) constraining relative plate movement paths within ancient orogens. Quantitative structural and metamorphic data correlated directly to relative plate motions, are currently allowing the tectonic reconstruction of multiply deformed and metamorphosed terrains in orogens around the world.