Laser ablation ICP-MS imaging of gold and other trace elements in pyrite from four different sedimenthosted\ud gold-arsenic deposits has revealed two distinct episodes of gold enrichment in each deposit: an early\ud synsedimentary stage where invisible gold is concentrated in arsenian diagenetic pyrite along with other trace\ud elements, in particular, As, Ni, Pb, Zn, Ag, Mo, Te, V, and Se; and a later hydrothermal stage where gold forms\ud as either free gold grains in cracks in overgrowth metamorphic and/or hydrothermal pyrite or as narrow goldarsenic\ud rims on the outermost parts of the overgrowth hydrothermal pyrite. Compared to the diagenetic\ud pyrites, the hydrothermal pyrites are commonly depleted in Ni, V, Zn, Pb, and Ag with cyclic zones of Co, Ni,\ud and As concentration. The outermost hydrothermal pyrite rims are either As-Au rich, as in moderate- to highgrade\ud deposits such as Carlin and Bendigo, or Co-Ni rich and As-Au poor as in moderate- to low-grade deposits\ud such as Sukhoi Log and Spanish Mountain. The early enrichment of gold in arsenic-bearing syngenetic to diagenetic\ud pyrite, within black shale facies of sedimentary basins, is proposed as a critical requirement for the later\ud development of Carlin-style and orogenic gold deposits in sedimentary environments. The best grade sediment-\ud hosted deposits appear to have the gold climax event, toward the final stages of deformation-related\ud hydrothermal pyrite growth and fluid flow

Compositions of ∼2500 spinel–olivine pairs and 400 melt inclusion–spinel pairs have been analysed from 36 igneous suites from oceanic, arc and intraplate tectonic settings. Our data confirm that Cr-spinel mg-number is largely controlled by melt composition, but also influenced by octahedral site substitutions, and rate of cooling. Lavas quenched in submarine environments tend to have higher mg-number at a given cr-number than slowly cooled subaerial lavas and peridotites. Unlike mg-number, Cr-spinel Al₂O₃ and TiO₂ contents show good correlations with melt composition, with only limited post-entrapment modifications. Our data suggest that increased activity of Al₂O₃ decreases the partitioning of TiO₂ into spinels. The Al₂O₃ content of Cr-spinel is a useful guide to the degree of partial melting of mantle peridotites; however, this same relationship is obscured in volcanic rocks. Al₂O₃ contents of volcanic Cr-spinels are mostly determined by melt composition rather than mantle source composition. The data also suggest that most spinels from residual mantle peridotites can be readily differentiated from those hosted in volcanic rocks. Mantle peridotite spinel tend to have lower TiO₂ and higher Fe²⁺/Fe³⁺ ratios than spinel from volcanic rocks. The spinel compositions in our database can be subdivided on the basis of tectonic setting and mode of occurrence using an Al₂O₃ vs TiO₂ diagram. A total of seven fields can be distinguished with varying degrees of overlap. This diagram can then be used to determine the tectonic setting of spinel from altered mafic igneous rocks such as serpentinites or meta-basalts, or detrital spinel in sandstones.