Purificación Fenoll Hach-Alí

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.

Textural and chemical features of sphalerite from the Palai-Islica deposit (SE Spain): implications for ore genesis and color

Este articulo ha sido publicado en la revista Neues Jahrbuch fur Mineralogie – Abhandlungen, correspondiendo la presente version a un preprint

SIGNIFICANCE OF PHYLLOSILICATE MINERALOGY AND MINERAL CHEMISTRY IN AN EPITHERMAL ENVIRONMENT. INSIGHTS FROM THE PALAI-ISLICA Au-Cu DEPOSIT (ALMERÍA, SE SPAIN)

Phyllosilicate mineralogy is key to understanding hydrothermal processes within accepted epithermal deposit models but little information has been published about the mineral chemistry of epithermal deposits. X-ray diffraction, optical and electronic microscopy (scanning and transmitted), electron microprobe, and Fourier transform infrared spectroscopy were used in this work to study phyllosilicates in the Palai-Islica Au-Cu epithermal, volcanic-hosted deposit, in order to link phyllosilicate mineralogy and mineral chemistry to ore genesis. Different phyllosilicate assemblages are characteristic of two types of mineralization, and related hydrothermal alteration. Chlorite and mica appear in polymetallic quartz veins with sulfides, and in the related chloritic and sericitic hydrothermal alteration. These minerals have notable textural and chemical differences (i.e. Fe/(Fe+Mg), Si and Al in chlorite and illitic and phengitic components in mica) amongst veins and altered rocks, revealing different genetic conditions. These chemical features also distinguish propylitic and regional, non ore-related, low-temperature alteration. Hot hydrothermal fluids of near-neutral pH are responsible for vein mineralization and alteration. Illite, interstratified illite-smectite, kaolinite, and pyrophyllite are characteristic, with a distribution pattern by zones, of the intermediate argillic and advanced argillic alteration around areas of silicification. In the latter, native gold appears associated with interstratified illite-smectite, suggesting a relatively low-temperature formation. Hot, low-pH fluids are responsible for this mineralization and alteration assemblage. The present study contributes to epithermal models showing the co-existence of two different alteration styles in the same hydrothermal system.

Mineralogical and chemical features of gangue phases in relation to hydrothermal mineralization and their host rocks

Gangue minerals from hydrothermal deposits (apatite and carbonates) and their host rocks from three different volcanic areas (SE Spain, La Serena and Melipilla in Coastal Range of Chile) have been studied for broad elemental compositions. Carbonate units at the Au-Cu Palai-Islica epithermal deposit are Fe-Mn-bearing, with a slightly higher concentration of these elements in the orebody than in adjoining hydrothermal alteration zones. Apatite has a composition that correlates with its origin and with hydrothermal processes. Thus, volcanic apatite is Cl-rich, whereas apatite from the associated orebody is almost pure fluorapatite. Furthermore, apatite from hydrothermally altered volcanic rocks has a transitional composition between volcanic and ore-related apatite. Samples of carbonate from Mn, Cu(-Ag) and Ba-Ag deposits in the La Serena area are all Mn-bearing calcite. In addition, Mn(Fe)-poor and Mg-rich calcite is common in low-grade Mn areas. Similar features have been found at the Melipilla Cu (Cu-Ag) deposit where epithermal calcite is also enriched in Mn(+Fe) with respect to non-mineralized veins, carbonate host rock, and metamorphic mineralization. In summary, F in apatite and Mn have been introduced in to carbonate from the La Serena area by hydrothermal fluids and could provide an index of hydrothermal ore-forming activity.