Detrital gold particles in paleoplacer deposits develop recrystallised rims, with associated expulsion of Ag, leading to the formation of Ag-poor rims which have been recognised in most placer gold particles around the world. Recrystallisation is facilitated by accumulation of strain energy as the gold particles are deformed, particularly on particle margins, during transportation in a fluvial system. The recrystallisation process ensues after sedimentary deposition and can occur at low temperatures (<40 degrees C) over long geological time scales (millions of years). In the Otago placer goldfield of southern New Zealand, paleoplacers of varying ages contain gold with varying transport distances and these display differing degrees of rim formation. Narrow (1-10 mu m) recrystallised rims with 0-3 wt% Ag formed on gold particles that had been transported <10 km from their source and preserved in Eocene sediments. Relict, coarse grained (similar to 100 mu m) gold particle cores have 3-10 wt% Ag, which is representative of the source gold in nearby basement rocks. Gold in the Miocene paleoplacers was recycled from the Eocene deposits and transported >20 km from their source. The gold particles now have wider recrystallised rims (up to 100 mu m), so that some particles have essentially no relict cores preserved. Gold in Cretaceous paleoplacers have wide (similar to 100 mu m) recrystallised low-Ag rims, even in locally-derived particles, partly as a result of diagenetic effects not seen in the younger placers. Gold particles in all the paleoplacers have delicate gold overgrowths that are readily removed during recycling, but are replaced by groundwater dissolution and reprecipitation on a time scale of <1 Ma. The recrystallisation that leads to Ag-poor rim formation is primarily related to the amount of deformation imposed on particles during sedimentary transport, and is therefore broadly linked to transport distance, but is also partly controlled by the age of the paleoplacer on time scales of tens of millions of years. Gold particles that have been derived directly from basement sources can retain their original composition for long distances (tens to hundreds of kilometres) in a river system, with only minor recrystallised rim development. Gold particles that have been recycled through paleoplacer deposits can lose this link to source composition after relatively short transport distances because of extensive recrystallisation. Crown Copyright (C) 2017 Published by Elsevier B.V. All rights reserved.

Fractures play an important role as fluid flow pathways in geothermal resources hosted in indurated greywacke basement of the Taupo Volcanic Zone, New Zealand, including the Kawerau Geothermal Field. Over time, the permeability of such geothermal reservoirs can be degraded by fracture sealing as minerals deposit out of transported geothermal fluids. Calcite is one such fracture sealing mineral. This study, for the first time, utilises combined data from electron backscatter diffraction and chemical mapping to characterise calcite vein fill morphologies, and gain insight into the mechanisms of calcite fracture sealing in the Kawerau Geothermal Field. Two calcite sealing mechanisms are identified 1) asymmetrical syntaxial growth of calcite, inferred by the presence of single, twinned, calcite crystals spanning the entire fracture width, and 2) 3D, interlocking growth of bladed vein calcite into free space as determined from chemical and crystallographic orientation mapping. This study also identifies other potential uses of combined EBSD and chemical mapping to understand geothermal field evolution including, potentially informing on levels of fluid supersaturation from the study of calcite lattice distortion, and providing information on a reservoir's history of stress, strain, and deformation through investigation of calcite crystal deformation and twinning patterns.

Abstract The lattice preferred orientation (LPO) of both muscovite and biotite were measured by electron backscatter diffraction (EBSD) and these data, together with the LPOs of the other main constituent minerals, were used to produce models of the seismic velocity anisotropy of the Alpine Fault Zone. Numerical experiments examine the effects of varying modal percentages of mica within the fault rocks. These models suggest that when the mica modal proportions approach 20% in quartzofeldspathic mylonites the intrinsic seismic anisotropy of the studied fault zone is dominated by mica, with the direction of the fastest P and S wave velocities strongly dependent on the mica LPOs. The LPOs show that micas produce three distinct patterns within mylonitic fault zones: C-fabric, S-fabric and a composite S–C fabric. The asymmetry of the LPOs can be used as kinematic indicators for the deformation within mylonites. Kinematic data from the micas matches the kinematic interpretation of quartz LPOs and field data. The modelling of velocities and velocity anisotropies from sample LPOs is consistent with geophysical data from the crust under the Southern Alps. The Alpine Fault mylonites and parallel Alpine schists have intrinsic P-wave velocity anisotropies of 12% and S-wave anisotropies of 10%.

This special publication of the Geological Society of London presents recent advances in the study of deformation mechanisms and rheology and their application to tectonics. We have subdivided the papers into two themed sections. The inference of deformation processes, conditions and rheology at depth in active tectonic settings is of fundamental importance to a quantitative geodynamic understanding of deformation in the Earth. The papers in the section on Lattice Preferred Orientations and Anisotropy are extremely important as they underpin our ability to make such geodynamic interpretations from global seismic data. These papers reflect the growing emphasis on the determination of elastic properties from microstructures, from which acoustic properties can be computed for comparison with in situ seismic measurements. The component of the microstructure that receives most attention is the lattice preferred orientation (LPO), otherwise known as the crystallographic preferred orientation (CPO) or the texture (the term used in material science and metallurgy). The papers include new LPO measurements (made almost exclusively by the relatively new technique of electron backscatter diffraction or EBSD), exploration of the significance of these data for seismic properties of both the crust and the mantle and modelling of LPO generation. An invited contribution from Mainprice and colleagues introduces a computational toolbox to help researchers calculate anisotropic physical properties from their LPO data. Rock microstructures evolve during deformation and rock physical properties, including both elastic properties and creep rheology, evolve with the microstructures as a function of strain and time. The section on Microstructures, Mechanisms and Rheology reflects the fundamental importance of understanding microstructural evolution to our ability to estimate deformation processes and conditions from recovered samples or geophysical data and to our modelling of tectonics. An invited contribution from Austin focuses on some of the key issues from the last few decades: how different mechanisms (grain size sensitive and grain size insensitive) compete and interact to control the evolution of grain size and LPO. Many of the other papers touch on these issues and make use of combinations of laboratory experiments, field studies and computational methods to explore the controls on microstructural evolution and to relate microstructural evolution to rheology and largescale tectonic processes. It is clear from this collection of papers that resolution of the controls on microstructural evolution in rocks remains at the cutting edge of Earth sciences.

Transgranular microcracking is fundamental for the initiation and propagation of all fractures in rocks. The geometry of these microcracks is primarily controlled by the interaction of the imposed stress field with the mineral elastic properties. However, the effects of anisotropic elastic properties of minerals on brittle fracture are not well understood. This study examines the effects of elastic anisotropy of quartz on the geometry of brittle fracture and related acoustic emissions (AE) developed during indentation experiments on single crystals at ambient pressure and temperature. A Hertzian cone crack developed during blunt indentation of a single crystal of flawless Brazilian quartz parallel to the c axis shows geometric deviation away from predictions based on the isotropic case, consistent with trigonal symmetry. The visible cone crack penetration depth varies from 3 to 5 mm and apical angle from 53 degrees to 40 degrees. Electron backscatter diffraction (EBSD) mapping of the crack tip shows that fracturing initiates along a similar to 40 mu m wide process zone, comprising damage along overlapping en echelon high-index crystallographic planes, shown by discrete bands of reduced electron backscatter pattern (EBSP) quality (band contrast). Coalescence of these surfaces results in a stepped fracture morphology. Monitoring of AE during indentation reveals that the elastic anisotropy of quartz has a significant effect on AE location and focal mechanisms. Ninety-four AE events were recorded during indentation and show an increasing frequency with increasing load. They correspond to the development of subsidiary concentric cracks peripheral to the main cone crack. The strong and complex anisotropy in seismic velocity (similar to 28% V-p, similar to 43% V-s with trigonal symmetry) resulted in inaccurate and high uncertainty in AE locations using Geiger location routine with an isotropic velocity model. This problem was overcome by using a relative (master event) location algorithm that only requires a priori knowledge of the velocity structure within the source volume. The AE location results correlate reasonably well to the extent of the observed cone crack. Decomposition of AE source mechanisms of the Geiger relocated events shows dominantly end-member behavior between tensile and compressive vector dipole events, with some double-couple dominated events and no purely tensile or compressive events. The same events located by the master event algorithm yield greater percentage of vector dipole components and no double-couple events, indicating that AE source mechanism solutions can depend on AE location accuracy, and therefore, relocation routine that is utilized. Calculations show that the crystallographic anisotropy of quartz causes apparent deviation of the moment tensors away from double-couple and pure tensile/compressive sources consistent with the observations. Preliminary modeling of calcite anisotropy shows a response distinct from quartz, indicating that the effects of anisotropy on interpreting AE are complex and require detailed further study.

Meteorites such as Allende - type III carbonaceous chondrites - contain strange dark clasts that would have been formed as part of early planet-forming processes in the early solar system. Opinion has been divided for years on the provenance of these components. Formation of source materials in the protoplanetary disc, impact-induced fragmentation of the parent asteroid or complex aqueous and thermal histories have all been suggested. We describe new analytical approaches that have cast light on these dark objects and their origins.

1 Summary The mid-crust is the locus of several fundamental geological and geophysical phenomena: these include the transitions from brittle to ductile behaviour and from unstable to stable frictional sliding; earthquake nucleation and predominant moment release; the peak in the crustal stress envelope; the transition from predominantly cataclastic to mylonitic fault rocks; and mineralisation associated with fracture permeability. Our current understanding of faulting and seismogenesis in this tectonically important zone is largely based on remote geophysical observations of active faults and direct geological observations of fossil faults. The Alpine Fault, New Zealand, is a globally significant dextral-reverse fault that is thought to fail in large earthquakes (c. Mw 7.9) every 200-400 years and last ruptured in 1717 AD. Ongoing uplift has rapidly exhumed a crustal section from c. 20 km, providing a young (<1 Myr), well-preserved sample of mid-crustal structures currently active at depth. Here we outline a novel experiment in which we drill, sample, and monitor the Alpine Fault to address fault zone evolution via brittle and ductile processes operating in the upper and mid-crust. The remarkable along-strike homogeneity of the Alpine Fault's hanging wall, the rapid rate of slip, and the dextral-reverse kinematics that progressively exhume the fault's own faulting products together enable us to examine the progressive evolution of fault zone materials by effectively targeting a single rock mass at two points on its exhumation trajectory. We will do this by placing a borehole at a measurable distance tectonically upstream (i.e. back along the exhumation trajectory) from a well-exposed and thoroughly documented surface outcrop of the fully evolved suite of fault rocks. By comparing fault rocks exposed at the surface with their correlatives at depth along a single exhumation trajectory, we can take advantage of a rare window into the physical character of the seismologically expressed brittle-ductile transition zone in a fault that is active today and which can be geophysically monitored in the coming decades. We propose to hold a five-day international workshop in early 2009 at which we bring together scientists with expertise in a broad range of fields to discuss and plan a program of site characterisation and deep fault drilling into the central Alpine Fault.