PYRRHOTITE 4M IN MASSIVE SULFIDES DEPOSIT OF LA COLORADA MINE, SALTA-ARGENTINA

ORTEGA PÉREZ, MARÍA DE LAS MERCEDES; LOPEZ, VANINA; GIONCADA, ANA

Santa Fe

Congreso; XVI Reunión Anual de la Asociación Argentina de Cristalografía; 2021

AACr

La Colorada mine is an example of massive sulfides deposit in Cobres range (Salta, Argentina), at an average altitude of 3.900 m a.s.l. Based on stratigraphic data, this deposit is estimated to be of Ordovician age. The ore mineralogy includes dominant pyrite-pyrrhotine, accompanied by galena, chalcopyrite, sphalerite and minor amounts of electrum-associated arsenopyrite. This contribution aims to provide data on the composition of the pyrrhotite found in drill cores samples and its geological meaning. Pyrrhotite is a mineral from the group of sulfides and sulfosalts according to the Nickel and Strunz classification. Depending on the type of structure, it crystallizes in the monoclinic (bellow 250°C) or hexagonal (above 300°C) crystal system with the chemical composition FeS to Fe11S12, respectively. The chemical formula Fe(1–x)S given with x between 0 to 0.17 is often used. From a chemical point of view, the mineral is therefore an iron (II) sulfide with a slight iron unsaturation. Pyrrhotite is opaque in all its forms and develops mainly tabular, pyramidal or prismatic crystals, but also massive aggregates of bronze yellow to brown colour with blackish gray stripes. In air, pyrrhotite quickly turns dull brown, rarely also iridescent in color. The analyzed crystal sample taken comes from 38 m-depth drill hole located at geographical coordinates X: 3470584 and Y: 7385427 (plunge 50° and azimuth 335°). The sample was obtained and processed by crushing and grinding in an agate mortar, for the study by means of X-ray Diffraction (XRD) with the Philips PW 1710 equipment at Universidad Nacional del Sur. The results are shown in Figure 1. The XRD analysis indicates the presence of 4M pyrrhotite, which reveals that it is monoclinic, with the monoclinic peak centered at 2.0581 dÅ, similar to other pyrrothites from sulfide ore deposits and its crystalline structure is based on the NiAs unit cell, where the metal occurs in octahedral coordination and the anions follow a trigonal prismatic arrangement. An important characteristic of this structure is its ability to omit some metal atoms in a total fraction of up to 1/8, by creating vacant gaps in iron. One such structure is pyrrhotin-4C or “4M” (in reference to monoclinic system of crystallization) (Fe7S8). Here the number 4 indicates that the vacant holes in iron form a superlattice that is 4 times larger than the unit cell. The crystallographic direction is conventionally chosen parallel to the main axis of symmetry of the crystal; this address usually corresponds to the largest network spacing [1]. A chemical composition with less than 47% atomic Fe and a temperature bellow 250°C can be interpreted for the analyzed monoclinic pyrrothite.