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Mineralogical characterization of sulfates in Fortuna de Cobre mine. Lomas Bayas, II Region of Antofagasta, Chile. Andrea Rojas Pardo1*, Darwin Werthessen2, Johann Díaz1 and Mauricio Rubio1. 1Compañía Minera Lomas Bayas, Gerencia Minería, Superintendencia de Geología. Camino Minsal Km 36 S/N, Comuna Sierra Gorda, Antofagasta, Chile.

2New México School of Mines. Socorro, New México, U.S.A. * Andrea.Rojas@Glencore.cl Abstract. This document presents the results obtained from the mineralogical analysis performed to samples of Fortuna, which show that it is mainly composed of sulfates, especially hydrated iron sulfates, related to the decomposition of pyrite and containing copper. Keywords: Oxidation, sulfates, efflorescence. 1 Introduction Fortuna de Cobre is located in the Atacama Desert in Northern Chile, 1,700 m.a.s.l. and 110 km northeast of the city of Antofagasta (Figure 1).

Figure 1. Location and access routes to Fortuna de Cobre (Taken and modified from Sanguinetti, 1999).

2 Geological information The deposit is located in a plutonic system formed by rocks from the San Cristóbal Pluton of age Rb/Sr of 80,3±2,3 Ma y 73,7±1,3 Ma (Pichowiak, 1994) and Ar/Ar 78,6±0,4 Ma (Maksaev et al., 1988), and minor porphyritic bodies with ages within the Lower Tertiary (Paleocene), according to Ar-Ar and K-Ar datings, measures in samples of granodiorite with hydrothermal alteration associated with the location of the porphyry and mineralization (Boric et al., 1990).

Prior studies made using x-ray diffraction and geochemical analysis in samples of pluton, determined that they correspond to granodiorite with compositional variations

of monzodiorites and tonalities, and calc-alkaline in character and sub-alkaline affinity, according to AFM y SiO2 v/s Alkalis discrimination diagrams (González & Olivares, 1996).

The host rock has different porphyritic phases of tonalitic composition (Tobar, 1966) and breccias. The main porphyritic is called Pórfico Fortuna and the most important breccia body is called Brecha Fortuna, formed by the porphyry intrusion, which is distributed in the central part of the deposit and has the higher concentration of copper and sulfates (Vega et al., 2015).

The hydrothermal alteration has a concentric distribution with remnant potassium alteration, a phyllic core, a phyllic-chloritic middle halo and a chloritic outer halo. There are also important argilized and/or leached areas located between the phyllic core and intermediate halo (Sanguitetti, 1999).

The hypogene mineralization is formed by pyrite >> chalcopyrite, characterized by the occurrence of veins and veinlets rich of quartz, sericite and chlorite, and by a moderate dissemination associated with the porphyry or vein system boundaries.

The structural systems, mainly of EW directions and subvertical attitudes, strongly controlled the emplacement of porphyry and breccias, and the mineralization and alteration processes (Sanguinetti, 1999).

3 Mineralogical characterization In the deposit has been recognized, at a macroscopic scale, the presence of brocantite, chrysocolla, atacamite and chalcanthite, as the main copper minerals oxidized. However, there are minerals which are hard to distinguish with the naked eye. They are identified after performing the iron nail in water test and with HCL at 10%, in which copper without apparent ore mineralogy is precipitated.

Also, the emergence of a large amount of post-drilling/mining efflorescence or sulfate generated when they are exposed to weathering and/or humidity, provided information about the instability of the mineralogy when weather conditions change, in which the rock changes its color, expands and shows cohesion (Rojas Pardo et al.,

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2015a).

As Fortuna is a low grade ore, it is important to recognize its mineralogy. Therefore, we performed mineralogical studies to indicate the characteristics and occurrence of minerals that form the ore.

The visual study identified minerals corresponding mainly to ferrous and ferric sulfate (Figure 2: A, B, C, D, E), atacamite (Figure 2: F and G), and Fe>>Cu sulphides (Figure 2: H).

Sulfates detected were observed mainly in breccias, both in fragments and the matrix, as irregular aggregates, associated mainly to decomposition of pyrite to sulfates with hydration or dehydration products of them (e.g. Szomolnokite: FeSO4×H2O - Melanterite: FeSO4×7H2O).

We also observed sulfates associated with tourmaline and secondary interstitial quartz. Sulfates were observed in lower quantities in the granodiorite and porphyry, as fracture-filling>>disseminates minerals.

Atacamite was mainly observed in the porphyry and granodiorite as fracture-filling, disseminated in fine-grained aggregates on biotite phenocrysts mainly and in undifferentiated limonite boundaries.

Pyrite>>chalcopyrite is found as main matrix components in the breccias and pyrite>>covelline disseminated in fragments. Pyrite>>chalcopyrite in veins>>disseminated, occasionally together with magnetite can be found in the porphyry and granodiorite.

X-ray diffraction (DRX) analysis in whole rock was performed on 27 samples and other 34 samples to which a manual separation was applied to avoid incorporating rock-forming and alteration mineralogy, focusing on green, orange, dark yellow, red, black and russet colors (Figure 2).

A wide range of sulfates was mainly interpreted and copper ores in sulfates, essentially, chloride, silicates and phosphate. Table 1 shows the mineralogy determined for diffractions, where the wide spectrum of sulfate detected can be observed, which comprise sulfates insoluble in water, such as brochantite-antlerite, jarosite, hydronium jarosite, alunite, and other water soluble, such as chalcanthite, szomolnokite, voltaite, among others.

The QEMSCAN analysis made for the identification of copper ores show that hydrated iron sulfates contained copper (Fe, Fe-Cu, Mg-Cu sulfates, among others), being the szomolnokite the main source of copper, followed by the voltaite (Campos et al., 2015).

3 Discussion and comments The occurrence of massive pyrite, the mineralogy and rocks and alteration, the neutralizing capacity of rocks, the permeability generated by the structures, breccias and circulation of meteoric waters helped to the development

of significant amounts of sulfates. The circulation of meteoric waters generated the oxidation of the ore and primary mineralogy, releasing elements (major, minor, trace), ions and acid solutions, causing the destabilization, transformation and crystallization of new secondary minerals such as sulfates (Rojas Pardo et al., 2015b). The numerous elements released have been attached to insoluble phases or formed part of a complex chain in which transitions from one phase to the other occurred for a change in the oxidation or hydration grade or for the partial substitution between elements in structure (Arana & Gillén, 1997). This explains the wide range of sulfates. The appearance of efflorescence or post-drilling/mining sulfates show the metastability of sulfates when the environment conditions change (Rojas et al., 2015b).

References Arana, R., Guillén Mondéjar, F., 1997. Mineralogía de sulfatos en

Siena Almagrera (Almería). Recursos naturales y medio ambiente en el sureste peninsular, 1997-01-01, ISBN 84-81080121-3, pags. 283-290.

Boric, R., Díaz, F. y Maksaev, V., 1990. Geología y yacimientos metalíferos de la Región de Antofagasta. Servicio de Geología y Minería, N°40, 246 p. Santiago, Chile.

Campos, J., Rubio, M., Domic, T. 2015. Chemical and Physical Characterization of the Fortuna de Cobre Ore. Lomas Bayas, Antofagasta – Chile. Actas XIV Congreso Geológico Chileno, La Serena - Coquimbo, Geología Económica y Recursos Naturales, en prensa.

González, J. & Olivares, J., 1996. Petrografía y Petrología de Fortuna de Cobre. Informe interno Compañía Minera Gibraltar Ltda.

Maksaev, V., Boric, R., Zentilli, M., Reynolds, P.H., 1988. Significado Metalogénico de dataciones K-Ar, Ar40-Ar39 y trazas de fisión de zonas mineralizadas en el Norte Grande de Chile. V Congreso Geológico Chileno, Vol. 1, B65 – B86. Santiago, Chile.

Pichowiak, S., 1994. Early Jurassic to Early Cretaceous magmatism in the Coastal Cordillera and the Central Depression of Noth Chile. In Tectonic of the Southern Central Andes. Reutter, Scheuber, Wigger (Eds.). Springer-Verlag.

Rojas Pardo, A., Ausburn, K., Aguilera, M., Rubio, M., 2015b. Implications of expansive mineralogy on the productive process in the Fortuna de Cobre mine. Lomas Bayas, II Region of Antofagasta, Chile. Actas XIV Congreso Geológico Chileno, La Serena - Coquimbo, Geología Económica y Recursos Naturales, en prensa.

Rojas Pardo, A., Chávez, W., M., Rubio, M., 2015b. Conditions of formation and stability of sulfates in Fortuna de Cobre mine. Lomas Bayas, II Region of Antofagasta, Chile. Actas XIV Congreso Geológico Chileno, La Serena - Coquimbo, Geología Económica y Recursos Naturales, en prensa.

Sanguinetti, A., 1999. Geología del yacimiento Fortuna de Cobre, Región de Antofagasta, Chile. Memoria de Título. Universidad Católica del Norte, Departamento de Ciencias Geológicas, Antofagasta. 31 p.

Tobar, A., 1966. Estratigrafía del Area de Baquedano – Rencoret. Provincia de Antofagasta. Memoria de Título. Universidad de Chile, Departamento de Geología, Santiago. 69 p.

Vega, D., Díaz, J., Marey, A., 2015. Geological Events at the Fortuna de Cobre Deposit, Lomas Bayas District, Antofagasta Region. Actas XIV Congreso Geológico Chileno, La Serena - Coquimbo, Geología Económica y Recursos Naturales, en prensa.

Werthessen, D. 2015. Preliminary Mineralogical Study of the Fortuna de Cobre Copper Oxide Mine, Chile. Informe interno Compañía Minera Lomas Bayas. 2p.

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Figure 1. (A) Fragments of szomolnokite (Szo) rehydrated to melanterite (Mel) in matrix of voltaite (Vol) and jarosite (Jar). (B) Matrix composed of opaque (Op), voltaite (Vol) and szomolnokite (Szo). (C) Fragments of szomolnokite (Szo) with bloedite (Bloed) and jarosite (Jar) in matrix of voltaite (Vol). (D) Fragments with copiapite (Cop) in aggregates and covered by jarosite (Jar). (E) Granitic fragments (Gran) in matrix of alunite associated with clay (Alun+Arc) cut by ore of coquimbite (Coq). a tte l t e l at . (F) Chlorite (Chl) replacing biotite crystal and with mineralization related to Atacamite (Atac) in Granodiorite. a tte l t a allel

l at . (G) Ore composed of atacamite (Atac), opaques (Op) and tourmaline (Turm) with halo of Sericite (Ser) in porphyry. a tte l t a allel l at . (H) Disseminated Pyrite (Py) with replacement in caries by Chalcopyrite (Cpy) and in contact

with Magnetite (Mgt). e le te l t a allel l at .

A B

C D

E

G H

F

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Figure 2. Hand samples separated manually to perform DRX for detecting sulfates in the mass: (A) Sample with jarosite and hydronium jarosite. (B) Sample with pyrite, szomolnokite and copiapite. (C) Sample with chalcanthite, szomolnokite and jarosite (D) Sample with szomolnokite, poitevinite, anhydrite, muscovite and quartz (Werthessen, 2015).

Table 1. Sulfates and other mineralogy recognized for X-ray diffraction.

A B

C D