MeV Ion Micro-Beam Applications in the Geosciences Jamie S Laird 1,2,3 and Chris G Ryan 1,2,3 EARTH SCIENCE AND RESOURCE ENGINEERING/MINERALS DOWN UNDER 1 CSIRO Earth Science and Resource Engineering, Clayton, Victoria, Australia 2 CODES, University of Tasmania, Hobart, Tasmania, Australia 3 School of Physics, University of Melbourne, Parkville, Victoria, Australia Accelerator School 2012, Australian Synchrotron

MeV Nuclear or Ion Microprobe  Micron to sub-micron beam spots  Assorted techniques making use of the various interactions between MeV ions and materials:  Particle Induced X-ray Emission (PIXE)  Rutherford Backscattering Spectroscopy (RBS)  Particle Induced Gamma-ray Emission (PIGE)  Scanning Transmission Ion Microscopy (STIM)  Ionoluminescence (IL)  Ion Beam Induced Current (IBIC and TRIBIC)  Particle Beam Writing (PBW)

University of Melbourne 5MV Pelletron

PIXE Elemental Mapping of Minerals Non-invasive, ppm level mapping  Proton induced excitation followed by return to ground state via x-ray emission  Normally use 3MeV protons since the x-ray yield (cross-section) is a maximum for elements of interest for this proton velocity.

PIXE Elemental Mapping of Minerals Non-invasive, ppm level mapping

Example Images for Apatite Assemblage

Example: Electrical Micro-Junctions in Metal Sulfides crack Conchoidal fracture Au complex (Hydrosulphide or chloride)  Neighboring p and n-type regions with a shared boundary most commonly seen in zoned pyrites and their relationship to late gold ore-genesis under epithermal to hydrothermal settings. “Late” Gold Ore-genesis (Moller model) { { n-type p-type p n 0.85eV

Elemental and Phase Heterogeneity PIXE Measurements on Arsenian pyrite from Otago, NZ MAJORS > wt%MINORS/TRACE << wt% ~ ppm detection limit

Elemental and Phase Heterogeneity PIXE Measurements Junctions should be at the highest spatial derivatives Predicted Junction Locations  Twin subhedral plus ancillary euhedral grains. Central collision fracture F-F’  As and Ni overprint  Cp phases are invariably n-type cp py

 -Junction Mapping: Laser Beam Induced Current S(f m ) S L (f m +  ) ( x ) X Y LBIC   Short absorption length means junction must be < 100 nm of the surface. We miss any below this or those electrically shorted.  Measure short circuit current (similar to galvanic mode) ZOOM Ideally: planarize from a 3D to 2D circuit by section thinning. Photons > 0.85eV

LBIC Confirmation of Junctions Due to Cp-Py This is a first for natural minerals !  Characteristic bipolar signature  Complex Structure Due to As-Ni and Mixed Phase Heterojunctions  Central regions correlate with cp-py junctions and give by far the strongest LBIC response.  Undulations elsewhere are due to As-Ni gradients but complex interplay is difficult to interpret.  Peaks in LBIC represent shifts in Eh for gold complexes due to the surface fields.

Mixed Sulfide Heterojunction Interpretation of Isolated Junctions  Junction Under Thermal Equilibrium

Back to PIXE: Where’s the gold ? Bimodal Gold Distributions  Gold associated with Ni-As gradients (~1000 ppm hot spots)  Gold associated with Cp-Py gradients (~500 ppm hot spots)  ~20-50 ppm invisible component

Summary of Galvanic adsorption of Gold/Silver  Gold deposited on p-type (As rich py) acting as a cathode in a galvanic cell. The n-type cp anode dissolves releasing electrons which injects a diffusion current pushing an electron from a pyrite surface state which then reduces gold (or a compound) onto the As rich area. Anodic Dissolution Gold Reduction  Pyrite rest potential largest of all sulphides.  same surface= mixed corrosion Fe Surface States

Jamie S Laird wwww.csiro.au/esre EARTH SCIENCE AND RESOURCE ENGINEERING/MINERALS DOWN UNDER Thanks for listening