Abstract
AbstractGraphitization in fault zones is associated both with fault weakening and orogenic gold mineralization. We examine processes of graphitic carbon emplacement and deformation in the active Alpine Fault Zone, New Zealand by analysing samples obtained from Deep Fault Drilling Project (DFDP) boreholes. Optical and scanning electron microscopy reveal a microtextural record of graphite mobilization as a function of temperature and ductile then brittle shear strain. Raman spectroscopy allowed interpretation of the degree of graphite crystallinity, which reflects both thermal and mechanical processes. In the amphibolite-facies Alpine Schist, highly crystalline graphite, indicating peak metamorphic temperatures up to 640°C, occurs mainly on grain boundaries within quartzo-feldspathic domains. The subsequent mylonitization process resulted in the reworking of graphite under lower temperature conditions (500–600°C), resulting in clustered (in protomylonites) and foliation-aligned graphite (in mylonites). In cataclasites, derived from the mylonitized schists, graphite is most abundant (<50% as opposed to <10% elsewhere), and has two different habits: inherited mylonitic graphite and less mature patches of potentially hydrothermal graphitic carbon. Tectonic–hydrothermal fluid flow was probably important in graphite deposition throughout the examined rock sequences. The increasing abundance of graphite towards the fault zone core may be a significant source of strain localization, allowing fault weakening.
Highlights
Martina Kirilova1*, Virginia Toy1, Nick Timms2, Timothy Little3, Angela Halfpenny4, Catriona Menzies5, Dave Craw1, DFDP-1 Science Team, DFDP-2 Science Team
The subsequent mylonitization of the schist rock resulted in partial recrystallisation of the rock volume under slightly lower temperatures (500 – 600◦ C from Raman Spectrometry of Carbonaceous Material (RSCM) thermometry), which coincide with previous temperature estimates in the Alpine Fault mylonites (Toy, et al, 2010)
Our study demonstrates that ongoing deformation processes and hydrothermal fluid flow resulted in deposition of graphite with different textural and structural characteristics throughout the examined rock sequences
Summary
Martina Kirilova1*, Virginia Toy, Nick Timms, Timothy Little, Angela Halfpenny, Catriona Menzies, Dave Craw, DFDP-1 Science Team, DFDP-2 Science Team. Tectonic-hydrothermal fluid flow was probably important in deposition of graphite throughout the examined rock sequences. Graphite is a common component of orogenic and Carlin style gold deposits around the world, where it is intimately associated with hydrothermal deposits (Bierlein et al 2001; Kribeck et al.2008; Large et al 2007, 2011). Graphite may be inherited from the host rocks, in which case it acts as a chemical reductant that facilitates precipitation of gold and associated sulphide minerals from younger hydrothermal fluids (Cox et al 1995; Bierlein et al 2001; Large et al.2007). Hydrothermal gold deposits are commonly intimately associated with zones of focussed deformation E. the deposits are “structurally controlled”) and graphite derived from both these origins commonly becomes involved in later deformation including fault zone inception and evolution. Its presence affects fault mechanics because it can preferrentially accommodate localized shear and result in further structurally-controlled mineralization (Binu et al 2003; Upton & Craw 2008; Oohashi et al 2011; Kuo et al 2014; Craw & Upton 2014)
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