The results of research in the use of Au grain morphological and compositional properties applied in primary Au ore exploration are presented here. Two different and independent topics are discussed: (1) morphological characteristics of Au grains from active stream sediments for use as a distance-to-source indicator; (2) compositional signature of Au grains from various deposit types for use as a discrimination tool for source type and present deposit erosion level determination. The purpose of this study is to improve and integrate these two approaches as an exploration tool for Andean covered areas. Au grain morphology for over 1500 nuggets recovered from 60 active stream sediment samples in the Coastal Cordillera of Central Chile shows morphological variations (general shape, outline, surface, primary crystal imprints, associated minerals, flatness index) characteristic of three distance ranges (0–50 m; 50–300 m; >300 m) from source. Comparison with results from other similar studies of Au morphology characteristics in different climatic and/or sedimentological environments (arid, semi-arid, wet, lateritic, fluvial, fluvio-glacial and glacial) resulted in the determination of the recommended parameters (outline, surface, associated minerals, flatness index) to be used as distance-to-source indicator, independent of the climatic and/or sedimentological environment. Au grain morphological characteristics may assist in location of target but are not indicators of source type. Study of Au composition via electron microprobe analysis of Au grain cores from epithermal, Au-rich porphyry and Au-rich porphyry Cu systems indicated Au–Ag–Cu contents to be the best discrimination tool for these different types of Au-bearing deposits. In addition, such analysis of grains recovered at different vertical levels from the Cerro Casale Au-rich porphyry provides evidence that the Au compositional signature for a single type of deposit can also aid in the determination of vertical position. This may provide an estimate of the current level of erosion and remaining potential of the source. Some limitations of the proposed techniques are: (1) Au liberated from rock fragments already distant from source would be common in cordilleran and glacial environments, although this would be a detectable feature; (2) these techniques are applicable only for coarse-Au sources; (3) estimate of erosion level of liberated Au is limited to the case here presented.

To decipher the petrogenesis of chromitites from the MohoTransition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of $500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1^50 mm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr þAl) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, f O2 , of $3 log units below the quartz^fayalite^magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 9508C at subsolidus conditions, were overgrown by more oxidized host chromite (f O2 % QFM) in association with microcrystal inclusions of silicates (plagioclase An 86 , clinopyroxene, and pargasite) that were formed between 950 and 10508C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to $0·18C a À1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional

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