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In the Palai-Islica deposit, in southeastern Spain, gold is found associated with sulfide mineralization (particularly Fe sulfides).Grains of Au-Ag alloy (gold of types A and B) occur with pyrite in mineralized veins, and grains of native gold (type C) are associated with areas of massive silicification at the uppermost levels of the deposit. The content of "invisible" gold in the various Fe sulfides is practically nil. Of all the textural varieties of pyrite studied, unzoned medium-to coarse-grained pyrite is the only one bearing gold. A clear distinction can be established between three types of gold (A, B and C), each with a different genesis. Grains of type-A alloy deposited as a result of variations in the thermodynamic parameters of the system, mainly a decrease in sulfur activity, whereas the appearance of the type-B alloy was mainly controlled by electrochemical factors. Type-C gold may have been produced from colloidal solutions in significantly different geochemical conditions. The chemical evolution of the alloys is characterized by Ag enrichment as precipitation continued. Type-A grains [mean Au/(Au + Ag) = 0.861] were the first to form, encased in the pyrite, with a relatively low Ag content and barely any zonation, followed by gold of type B, overgrowing pyrite, with a higher average Ag content [Au/(Au + Ag) = 0.756] and commonly zoned, with later zones richer in Ag. Finally, type-C native gold has practically no Ag [on average, Au/(Au + Ag) = 0.988].Keywords: gold, gold-silver alloy, epithermal deposit, Palai-Islica, Spain. SOMMAIREDans le gisement de Palai-Islica, de la partie sud-est de l'Espagne, l'or est associé avec la minéralisation en sulfures, de fer surtout. Les grains d'un alliage Au-Ag (or de types A et B) sont associés à la pyrite dans les veines minéralisées, et les grains d'or natif (dits de type C) sont associés aux zones de silicification massive dans les parties supérieures du gisement. La teneur en or "invisible" des divers sulfures de fer est quasiment nulle. De toutes les variétés texturales de pyrite étudiées, seule la génération de pyrite en grains non zonés et à granulométrie moyenne à grossière est aurifère. On peut établir une distinction nette entre les trois types d'or (A, B et C), chacune ayant une différente genèse. Les grains d'alliage de type A ont été déposés suite à des variations des paramètres thermodynamiques du système, surtout une diminution de l'activité du soufre, tandis que la formation des grains d'alliage de type B était surtout régie par des facteurs électrochimiques. L'or de type C pourrait avoir été produit à partir d'une solution colloïdale dans des conditions géochimiques nettement différentes. L'évolution de l'alliage Au-Ag est marquée par un enrichissement en Ag au fur et à mesure que la précipitation progressait. Les grains de type A [en moyenne, Au/ (Au + Ag) = 0.861] ont été les premiers à se former, étant encastrés dans la pyrite, avec une teneur relativement faible en Ag, sans zonation importante, et ont été suivis par les grains de ty...

The Las Aguilas Ni-Cu-PGE deposit is associated with a sequence of basic-ultrabasic rocks made up of dunite, harzurgite, norite and amphibolite. These igneous (partially metamorphosed) rocks, and their host granulites, gneisses and migmatites of probable Precambrian age, are highly folded. The sulphide ore, consisting of pyrrhotite, pentlandite and chalcopyrite, occurs in the cores of both antiform and synform structures, within dunite, harzburgite and mainly along shear zones in bronzitite, replacing small mylonitic subgrains. The platinum-group mineral assemblage is dominated by Pd bismuthotellurides (Pt-free merenskyite, palladian bismuthian melonite and michenerite), with minor sperrylite, and PGE-sulpharsenides. The latter often occur as single, zoned crystals frequently showing cores of irarsite; outside these are concentric zones of cobaltian hollingworthite, rhodian nickelian cobaltite and Fe-rich nickelian cobaltite.Mineralogical, textural and chemical evidence indicate that the sperrylite and platinum-group element sulpharsenides were formed during a primary magmatic event associated with the fractionation of a basaltic melt, which was contaminated by the assimilation of metamorphic crustal rocks. PGE sulpharsenides crystallized from As-bearing, residual magmatic liquids that collected PGE and segregated after the crystallization of the monosulfide solid solution. During high-grade metamorphism, sulpharsenides were remobilized as solid crystals in the liquated sulfides suffering partial dissolution and fracturing. On the other hand, there is no evidence of a primary concentration of Pd-bismuthotelluride minerals, and their present spatial distribution is only the consequence of their formation under high- to medium-grade metamorphism, down to temperatures of below 500°C. Pd bismuthotellurides crystallize even in fractures of sulpharsenides, attached to the boundaries of highly dissolved sulpharsenide crystals, and intergrown with molybdenite.