Abstract: Field analysis shows that fault cores of brittle, extensional faults at a medium to mature stage of development are commonly dominated by lozenge-shaped horses (fault-core lenses) characterized by a variety of lithologies, including intact, mildly to strongly deformed country rock derived from the footwalls and hanging walls, various types of fault rocks of the protocatalasite and breccia series, breccia, fault gouge and clay smear. The lenses are sometimes stacked to form complex duplexes. These structures are commonly separated by high-strain zones of sheared cataclasite, and/or clay smear/clay gouge. The geometry and distribution of clay gouge in high-strain zones sometimes display evidence of intrusion, indicating high fluid pressure. Although the sizes of the horses vary over several orders of magnitude, they frequently display a length:thickness (a:c) ratio of between 1:4 and 1:15. The high-strain zones of fault rocks commonly constitute unbroken, 3D membranes that are likely to constrain fluid communication both across and along the fault zone. There are significant contrasts in fault core architecture that are probably related to processes associated with contrasting fluid pressure, strain intensity and strain hardening/strain softening. Faults associated with strain softening are characterized by less abundant brittle deformation products and are less likely to be conduits for fluid flow compared to those that are affected by strain hardening.

Abstract The Neoarchaean Sittampundi Anorthosite Complex located in central Tamil Nadu represents layered sequence of chromitite and meta-anorthosite with discontinuous lenses of eclogite/garnet-pyroxene granulite. The chromitite hosting the PGE mineralisation occurs as discontinuous bands and lenses within the meta-anorthosite with width of the individual band / layer varies from a few centimeter to 5 m. It is made up of 30 to 70% chromite/chrome-spinel, clino-amphibole (magnesio-hornblende/tschermakite) with minor clino-pyroxene (augite). Rutile occurs as main accessory along with magnetite and ilmenite. Chromite/ chrome-spinel occurs in two generations – one as larger grains with inclusions of rutile and amphibole and the other as tiny interstitial grains amidst amphibole. The PGE mineralisation is mostly confined to the chromitite bands in the eastern part of the complex, while it is found both in chromitite and in the adjoining sulphide-bearing anorthosite in the western part. The PGE concentration in the chromitite ranges from 0.5 to 2.0 ppm (Pt + Pd) with occasional high values up to 25 ppm. The chromitite and the associated anorthosite show moderate to rich disseminations of sulphides viz. chalcopyrite, pyrite, pyrrhotite, pentlandite, polydymite and millerite in the western part around Tasampalaiyam. An array of PGM phases both as native metal as well as alloys in the form of sulphides, arsenides, tellurides and antimonides have been identified within the 3.6 km long PGE mineralised zone delineated in this complex.