Au Mineralisation in “Listvenites” from Mina Descanso, Central Cuba: Preliminary Results Daniel Rodríguez García, Cristina Villanova-de-Benavent, Lidia Butjosa, Thomas Aiglsperger, Joan Carles Melgarejo, Joaquín A Proenza Departament de Cristal·lografia, Mineralogia i Dipòsits Minerals, Facultat de Geologia, Universitat de Barcelona (UB), Martí i Franquès s/n, 08028 Barcelona, Spain Manuel Iturralde-Vinent Museo Nacional de Historia Natural, Ciudad Habana, Cuba Antonio García-Casco Departamento de Mineralogía y Petrología, Universidad de Granada, Spain Abstract. Listvenites are carbonatised serpentinites been widely studied worldwide (e.g. Buisson and occurring along faults in ophiolite complexes, which are Leblanc 1986; Ash and Arksey 1989; Halls and Zhao commonly associated with gold, and sulphide-arsenide 1995; Akbulut et al. 2006; Escayola et al. 2009; mineralisations. The origin of the ores is probably due to Buckman and Ashley 2010; Zoheir and Lehmann 2011; the metasomatism of the previous ultramafic rocks by Aftabi and Zarrinkoub 2013), but the source of gold in CO2-rich hydrothermal fluids during obduction, but the these deposits is still unclear. processes leading to the formation of these deposits are not yet defined. This contribution presents a The present study focuses on the “listvenite” occur- mineralogical and textural study of the gold-bearing rence in Mina Descanso (Central Cuba), which is curren- chloritite and the gold-free chlorite-carbonated rock as tly being mined and reaches grades of up to 280 ppm Au well in Mina Descanso (Central Cuba). The preliminary (Cabrera et al., 1986; López Kramer 2009 and references results state that gold is found as individual grains and is therein). The aim is to characterise the gold and related also associated with galena, nickeline and cuproaurite in sulphide mineralisations associated with the carbona- chloritites. In contrast, arsenopyrite, Fe-Cu-sulphides and tisation of serpentinites in the listvenites of this deposit. Fe-Ti-oxides are typical of the Au-free mineral associations in chlorite-carbonated rock. 2 Geological setting Keywords. Au, listvenites, galena, Ni-Cu sulphide- arsenides, Cuba The Cuban orogenic belt formed during the Cretaceous– Tertiary convergence of the Caribbean oceanic plate and 1 Introduction the North American margin. Ophiolite-related ultramafic rocks from Villa Clara (Central Cuba) are part of the Listvenite (or listwänite) is the name given to the Northern Ophiolite Belt of Cuba (Upper Jurassic− carbonate rocks occurring along ophiolite complexes, Cretaceous; Iturralde-Vinent 1996, 1998). The collision formed as a result of the carbonatisation of ultramafic in latest Cretaceous–Tertiary times triggered the tectonic rocks (Buisson and Leblanc 1986). Since Rose (1837) emplacement of ophiolitic units (including oceanic introduced this term, several carbonatised ultramafic volcanic arc, and subduction mélange units) onto the rocks were included in the definition of “listvenite” with margin of North America (García-Casco et al. 2008). different phyllosilicate associations: serpentine, talc, Ophiolites in Cuba constitute about 7 per cent of the chlorite and/or mica (Halls and Zhao 1995). land surface of the island, and occur as seven separate The formation of listvenites is linked to the intense massifs exposed along its northern coast (Iturralde- carbonatisation and silicification by CO2-rich fluids Vinent 1996, 1998). The ultramafic rocks were originally penetrating through highly sheared ultramafic rocks peridotites formed in the upper mantle and were partially during obduction. The fluids react with the primary or completely altered to serpentinites during their ferromagnesian silicates and this leads to the passage to their present tectonic position (Lewis et al. precipitation of metals (Nelson et al. 2011 and references 2006). therein). Buisson and Leblanc (1986) already suggested One of the aforementioned seven ophiolite massifs is that gold was leached from the opaque minerals and the Villa Clara serpentinite-matrix mélange in Central transported by CO2- fluids related to serpentinisation. Cuba (fig. 1a). This mélange comprises a set of tectonic Listvenites frequently host gold mineralisations. bodies of an ophiolite suite, bodies formed at depth in a These deposits have been described as “ophiolite-hosted subduction channel (eclogite, garnet amphibolite, mesothermal gold-quartz veins” by Lefebure (1998), as amphibolite, blueschist, greenschist, quartzite, metapelite collision-related deposits by Sawkins (1984), and based and antigoritite), bodies of volcanic-arc sequences and of on the processes which form the mineralisation, they fit unrelated platform-derived sediments. The ophiolitic in the classification of “orogenic gold deposits” by bodies are mainly composed of hydrated ultramafic Groves et al. (1998). In addition, listvenites also recalled rocks (serpentinised mantle peridotites and serpentinites some interest lately as natural analogues for CO2 s.s.), though fragments of layered and isotropic gabbros, sequestration (Hansen et al. 2005; Gahlan et al. 2015). diabase, basalt and pelagic sediments are also present The occurrence of gold associated to listvenites has (García-Casco et al. 2002; 2006). Geodynamics, Orogenic cycles and mineral systems 197 serpentine (18 %), whereas sample LISV-2 is mostly composed by dolomite (56 %), calcite (31 %) and chlorite (12 %). Representative textures of each zone are shown in the optical photomicrographs in figure 2 and the associated gold, sulphide, sulphide-arsenide and oxide assemblages are presented in the SEM-BSE images in figure 3. As seen under optical microscope and SEM, sample LISV-1 is a gold-bearing chloritite, and will be hereafter referred to as “Au-bearing chlorite zone”. In contrast, in sample LISV-2 (“chlorite-carbonated rock”), two different areas are identified, one consisting mainly of chlorite (namely “Au-free chlorite zone”) and one formed by carbonates (“carbonatised zone”), separated by a transition zone. a. 4.1 Au-bearing chlorite zone Figure 1. Map showing the location of the three listvenite occurrences in Cuba (modified from Iturralde-Vinent et al. The dominant mineral phase in this zone is Fe-bearing (1998), and b. schematic cross-section indicating the position of listvenites in Mina Descanso deposit (modified from clinochlore. Slight variations in Fe content are detected. Cabrera et al. 1986). This Fe-clinochlore is either coarse grained and strongly foliated, or fine grained and unoriented, crosscut by tiny Three listvenite occurrences have been reported in calcite veinlets (fig. 2a). Villa Clara: Descanso, Melonera and La Mascota Gold is found as individual micron-sized grains (Proenza and Melgarejo 1998; fig. 1a). The present study (usually around 20 m and up to 200 m) or intergrown focuses on the ore mineralogy and associated with galena, all disseminated in the chlorite matrix (fig. assemblages of Mina Descanso. According to Cabrera et 3a). The Au-galena assemblage is frequently al. (1986) the mineralised bodies display irregular accompanied by nickeline and/or cuproaurite (Au-Cu shapes, with thicknesses between 3 and 4 metres, and are alloy) and may reach sizes of several hundreds of distributed along fractured and sheared zones. These microns (up to 800m). bodies are hosted by hydrothermally altered, strongly serpentinised peridotites, which are crosscut by 4.2 Au-free chlorite zone porphyric diorites (fig. 1b). The main mineralogy is practically identical to that of 3 Sampling and analytical techniques the Au-bearing chlorite zone, but the textures are different. Fe-bearing clinochlore occurs as Two samples from Mina Descanso were selected for the pseudomorphs after previous minerals, possibly primary present study: a) LISV-1, a gold-bearing chloritite from olivine, pyroxene and serpentine, and as a fine-grained 110 m; and b) LISV-2, representing the transition matrix surrounding these pseudomorphs (fig. 2b, left). between a gold-free chloritite and chlorite-carbonated However, gold, galena and nickeline are absent, and rocks, from 126 m. Firstly, the samples were carefully the only opaque minerals present are altered rutile and hand-ground with agate mortar and pestle to be analysed ilmenite (fig. 2b, right). by powder X-ray Diffraction (XRD) in the Centres Científics i Tecnològics of the Universitat de Barcelona 4.3 Transition zone (CCiT-UB). The instrument used is a PANalytical X’Pert PRO MPD Alpha1 powder diffractometer in Bragg- This zone is less than one millimetre thick and separates Brentano /2 geometry, using Cu K1 radiation ( = the Au-free chlorite zone from the carbonatised zone. 1.5406 Å) and working power of 45 kV – 40 mA. Under the optical microscope, a sharp contact between Secondly, these samples were prepared as polished thin fine-grained calcite and Fe-clinochlore is observed. In sections, and examined under an optical microscope addition, SEM-EDS allowed detecting the local presence using both transmitted and reflected light. These thin of Cr-bearing clinochlore. sections were investigated in the Environmental In this zone it is worth noting the abundance of Scanning Electron Microscope Quanta 200 FEI, XTE euhedral arsenopyrite, crosscut by calcite veins, 325/D8395, equipped with an INCA Energy 250 EDS coexisting with minor phases such as ilmenite, rutile and microanalysis system (SEM-EDS), with the operating relicts of chromian spinel (fig. 3b). conditions of 20 kV and 5 nA, in the CCiT-UB. 4.4 Carbonatised zone 4 Results This