Abstract Microanalysis can provide rapid, quantitative characterization of mineral systems that complements the field- and core-scale observations traditionally made in ore deposits. We review recent innovations in microanalytical procedures and their application to studies of ore deposits. Case studies are presented examining how microanalysis can provide constraints on macroscopic processes within mineral systems. Synchrotron X-ray fluorescence shows centimetre-scale chemical variations associated with proximity to mineralization in samples from Sunrise Dam Gold Mine, Western Australia. Pseudomorphs of igneous plagioclase and chemically driven recrystallization interpreted from electron backscatter diffraction suggest that the system was dominated by fluid-driven brecciation with very little shearing. Both the fluid chemistry and fluid pressure evolved during a protracted sequence of vein formation and alteration accompanying gold mineralization. A second case study of sulphide mineralogy at the Mt Keith nickel sulphide deposit, Western Australia demonstrates how X-ray computed tomography combined with trace element mapping can constrain the chemistry and dynamics of magmatic systems. Large-scale interaction between silicate and sulphide melts, shown by homogenous palladium enrichment in pentlandite, leads to a large proportion of globular ores with a high nickel content. Increasing use of microanalysis in ore deposit geology is resulting in the constant reassessment of established models for ore genesis though a combination of micro- and macroscale datasets.

Abstract Extreme grain-size reduction due to cataclasis, neocrystallization or phase change results in a switch to diffusion creep and dramatic weakening in deforming rocks. Grain growth increases strength until dislocation creep becomes a significant deformation mechanism. We quantify the ‘lifetime’ of diffusion creep by substituting the normal grain growth law into the diffusion creep flow law to calculate the time taken for dislocation creep to become significant. Stress-temperature and strain-rate-temperature space is outlined where diffusion creep may accommodate significant strain: these regions have an upper temperature limit beyond which grain growth is fast enough to move the rock quickly into the dislocation creep field. For plagioclase the limit lies in the amphibolite facies. Rocks in a mantle upwelling experience grain-size reduction during phase changes. Pressure-dependent grain growth limits the deformation that can be accommodated by diffusion creep. This time limit and associated strain limit is independent of starting grain size with a small dependence on upwelling rate and plume width. In both these tectonic environments, second phases are likely to play a role in the maximum achievable grain size due to grain-boundary pinning. Hence we predict the minimum lifetimes of diffusion-creep-dominated deformation following extreme grain-size reduction.