Andrés Paniagua

PLATINUM-GROUP ELEMENT SULPHARSENIDES AND PD BISMUTHOTELLURIDES IN THE METAMORPHOSED NI-CU DEPOSIT AT LAS AGUILAS (PROVINCE OF SAN LUIS, ARGENTINA)

Abstract 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-ffee 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℃. Pd bismuthotellurides crystallize even in fractures of sulpharsenides, attached to the boundaries of highly dissolved sulpharsenide crystals, and intergrown with molybdenite.

Polymetallic VMS deposits of the Andes Fueguinos (southernmost Argentina): Preliminary report

The polymetallic volcanic-hosted massive sulphide deposits of the Andes Fueguios occur in a highly deformed but coherent stratigraphic succession of Late Jurassic metavolcanic and metasedimentary rocks. Local stratigraphy consists of porphyries along with rhyolitic lava, acidic tuff, breccias, conglomerates, radiolarian lutites and basaltic rocks: Mineralization occurs as massive stratiform, massive replacement and sulphide stringer veins. Sulphides are typically massive, fine-grained, layered and locally brecciated andconsists of pyrite and sphalerite, with lesser pyrrhotite, chalcopyrite, galena, tetrahedrite-freibergite, bourmonite, cobaltite and arsenopyrite. Styles of hydrothermal alteration identified in the host rocks include proximal silicification and more distal chloritization, sericitization and, in places, carbonatization. Future research will be focused on identifying the salient physico-chemical controls on the mineralization process and their implications for volcanic-hosted massive sulphide exploration in the district.

Simple deposition versus replacement and re-equilibration at the Crescencia Ni-(Co-U) deposit (Central Pyrenees, Spain)

The mineral assemblage of the Crescencia mine contains Ni sulfarsenides, arsenides and diarsenides, uraninite and pyrite. Three stages of mineral deposition can be distinguished: Stage I: nickeline, gersdorffite I, and rammelsbergite; Stage II: gersdorffite II; Stage III: uraninite. Gersdorffite II has been subdivided into six groups on the basis of textural and compositional criteria. Some of these gersdorffite types are clearly pertained to disequilibrium processes. Gersdorffite B, C, E and F represent intermediate steps during the nickeline replacement and progressive re-equilibration with residual fluids, representing by gersdorffite D. Thus, compositional variations found in gersdorffite are the result of nickeline replacement and reequilibration processes rather than a direct precipitation under different conditions of the ore-forming fluids.

Unusual PGE concentration in early disulfides of a low-temperature hydrothermal Cu-Ni-Co-Au deposit at Villamanin (Leon, northern Spain)

The early mineral assemblage of the Providencia mine in northern Spain includes metastable Cu-Ni pyrite-type disulfides with economic concentrations of Au and subeconomic concentrations of PGE in solid solution. High chondrite-normalized relations of Ir, Rh and Pt versus Pd, and the lack of Ru also characterize the ore. Our studies show that the ore-forming fluids were quasi-hydrostatic, low-temperature hydrothermal, and had a high salinity, and a high sulfur fugacity. The source of PGE remains unknown.