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 800m). 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 K1 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 zone is composed by fine-grained equigranular Fe-

bearing dolomite and less abundant calcite, displaying a As mentioned above, each sample shows distinct mineral texture which may resemble to that of the surrounding assemblages. According to XRD, sample LISV-1 ultramafic rocks, and suggesting that the carbonates are consists of chlorite (59 %), and minor calcite (23 %) and pseudomorphs after the primary ferromagnesian silicates

198 MINERAL RESOURCES IN A SUSTAINABLE WORLD • 13th SGA Biennial Meeting 2015. Proceedings, Volume 1 (olivine, pyroxene and serpentine) (fig. 1c). This assemblage is crosscut by later calcite veinlets. Various sulphides, sulphide-arsenides and oxides are found disseminated in the carbonate matrix. They mostly are euhedral arsenopyrite and anhedral chalcopyrite, pyrrhotite, ilmenite, rutile, with minor amounts of nickeline (fig. 3c).

 



   

Figure 2. Textural features of a. fine-grained and oriented, Figure 3. Au, Fe-Cu-sulphide, Fe-Ni-arsenide and Fe-Ti-Cr- coarse-grained Fe-clinochlore in the Au-bearing chlorite zone oxide assemblages in the studied samples from Mina (crossed polars), b. Fe-Ti-oxides surrounded by Fe-clinochlore Descanso: a. gold (Au) associated with galena (Gn) and pseudomorphs and fine-grained matrix in the Au-free chlorite nickeline (Nc) within clinochlore (Clc); b. arsenopyrite (Apy), zone (crossed polars on the left, reflected light on the right), ilmenite (Ilm) and rutile (Rt) with chromian spinel (Chr), and c. Fe-dolomite and calcite grains with disseminated oxides surrounded by clinochlore and crosscut by calcite veins (Cc) in and sulphides in the carbonatised zone (plane polarised light). the Au-free chlorite zone; c. Chalcopyrite (Cpy), pyrrhotite (Po), nickeline (Nc), arsenopyrite (Apy) and ilmenite (Ilm) in Mg-Fe-carbonates, (Mg-Fe)Cc, in the carbonatised zone.

Geodynamics, Orogenic cycles and mineral systems 199 5 Concluding remarks Buckman S, Ashley P (2010) Silica-carbonate (listwanites) related fold mineralisation associated with epithermal alteration of The listvenite mineralisation in Mina Descanso is hosted serpentinite bodies. New England Orogen, Armidale, Australia 94-105 by serpentinised peridotites coexisting with porphyric Buisson G, Leblanc M (1986) Gold-bearing listwaenites diorites. The preliminary investigations on this deposit (carbonatized ultramafic rocks) from ophiolite complexes. In: show that gold is found in chloritites (not in carbonates) Gallagher JJ, Ixer RA, Neary CR, Prichard HM (eds) and is mostly associated with galena. Zoheir and Metallogeny of basic and ultrabasic rocks, The Institution of Lehman (2011) also stated that gold and galena form Mining and Metallurgy Publ, London pp 121-132 Cabrera R, Kramer JL, Dobrovolskaya M, Catá A (1986) La during the same stage of mineralisation. It is worth formación menífera auro-listvenítica del yacimiento Descanso noting that chloritisation is pervasive, and the only en Villa Clara (Cuba). Revista de Ciencias de la Tierra y del remaining remnant of the original mineralogy of the Espacio 11:59-66 ultramafic rock is the chrome spinel. Escayola M, Proenza JA, van Staal C, Rogers N, Skulski T (2009) In general, the main mineralogy and the ore The Point Rousse listvenites, Baie Verte, Newfoundland: assemblages of the studied samples are similar to those altered ultramafic rocks with potential for gold mineralization? Newfoundland and Labrador Department of Natural Resources reported in other listvenite occurrences worldwide (e.g. Geological Survey Report 09-1:1-12 Buisson and Leblanc 1986; Halls and Zhao 1995). Gahlan HA, Arai S, Almadani SA (2015) Petrogenesis of However, according to the optical microscopy carbonated meta-ultramafic lenses from the Neoproterozoic observations and SEM-EDS analyses, the studied Heiani ophiolite, South Eastern Desert, Egypt: A natural analogue to CO2 sequestration. J African Earth Sci 102:102- The Hira-Buddini gold deposit is located on the samples are characterised by a lack of silicification, and eastern part of the Neo-Archean Hutti-Maski Schist Belt in other typical listvenite-forming minerals such as Cr-mica 115 García-Casco A, Torres Roldan RL, Millán Trujillo G, Monié P, Dharwar Craton of Peninsular India. The auriferous lode (fuchsite) were not detected either. Although previous Schneider J (2002) Oscillatory zoning in eclogitic garnet and (mineralized vein and wall rock) in the deposit is almost works define Mina Descanso as a listvenite s.s. amphibole, northern serpentinite mélange, Cuba: A record of parallel to the sheared contact of amphibolite and felsic occurrence (Cabrera et al. 1986; Proenza and Melgarejo tectonic instability during subduction? J Metam Geol 20:581- volcanic rock. Geological studies in the underground 1998; López Kramer et al. 2009; Nelson et al. 2011), our 598 levels and observations along the shear zone hosted auriferous lode have indicated the presence of five types preliminary data may suggest that this deposit is not an García-Casco A, Torres-Roldan RL, Iturralde-Vinent M, Millán G, Nuñez Cambra K, Lazaro C, Rodríguez Vega A (2006) High of veins such as fault-, flat-, conjugate-, irregular- and ordinary listvenite. pressure metamorphism of ophiolites in Cuba. Geol Acta 4:63- youngest calcite- veins. The fault- and flat- veins are Finally, the exact age of the mineralisation is unkown 88 laminated and brecciated containing slivers of altered wall but may be related to the deposits formed during the García-Casco A, Iturralde-Vinent MA, Pindell J (2008) Latest rock indicating multiple episodes of filling. Quartz is the collision/accretion process related to closure of the Cretaceous collision/ accretion between the Caribbean Plate dominant mineral in the auriferous vein with subordinate proto-Caribbean ocean basin (Nelson et al. 2011). and Caribeana: Origin of metamorphic terranes in the Greater amount of plagioclase, calcite and tourmaline. Gold and Emplacement of ophiolite onto the Bahamian Antilles. Int Geol Rev 50:781-809 associated sulfide minerals occur in the vein as well as in Halls C, Zhao R (1995) Listvenite and related rocks: perspectives the altered wall rock. Host rocks proximal to the vein have borderlands took place during the Paleocene to early on terminology and mineralogy with reference to an undergone intense alteration showing an assemblage of Late Eocene (Iturralde- Vinent 1998). occurrence at Cregganbaun, Co. Mayo, Republic of Ireland. chlorite-biotite (± calcite± tourmaline) and sericite-calcite- Nevertheless, further work is needed on the mineral Miner Deposita 30:303-313 chlorite. Based on the current investigations, three phases chemistry of the Mina Descanso listvenites. For Hansen LD, Dipple GM, Gordon TM, Kellett DA (2005) of deformation (D1, D2 and D3) and related geological example, Buisson and Leblanc (1986) report a positive Carbonated serpentinite (listwanite) at Atlin, British Columbia: processes have been inferred: i) D1 with regional correlation between gold and arsenic from whole-rock a geological analogue to carbon dioxide sequestration. Can amphibolite facies metamorphism, ii) Ductile-brittle Mineral 43:225-239 analyses, and therefore the presence of structural gold deformation (D2) coeval with auriferous vein formation and Iturralde-Vinent MA (1996) Geología de las ofiolitas de Cuba. In: iii) Brittle deformation (D3) forming fractures and calcite and/or gold inclusions in the arsenopyrite of Mina Iturralde-Vinent M (ed) Ofiolitas y arcos volcánicos de Cuba: veins. Descanso should be considered and investigated. In Miami, USA, IGCP Project 364, pp 83-120 addition, a deeper study of the silicates associated with Iturralde-Vinent MA (1998) Sinopsis de la constitución geológica Dharwar Craton, Hutti-Maski Schist Belt, gold is required in order to constrain the conditions in de Cuba. Acta Geológica Hispánica 33:9-56 Metamorphism, Gold, Auriferous Vein, Wall rock Lefebure D (1984) New mineral deposit models of the Cordillera. which gold is concentrated in the chloritites, by means of Geoscience Canada 24:143-149 using chlorite thermometry, for instance. Lewis JF, Draper G, Proenza JA, Espaillat J, Jiménez J (2006) Ophiolite-related ultramafic rocks (serpentinites) in the Acknowledgements Caribbean region: a review of their occurrence, composition, origin, emplacement and Ni-laterite soil formation. Geol Acta This research has been financially supported by the 4:237-263 López Kramer JM, Pimentel H, Redwood S, Gandarillas Hevia J, Spanish project CGL2012-36263, sponsored by the Pérez Vázquez RG (2009) Depósitos primarios de oro y plata Ministerio de Educación (Spain). del archipiélago Cubano. Geominas 37(48):11-19 Nelson CE, Proenza JA, Lewis JF, López-Kramer J (2011) The References metallogenic evolution of the Greater Antilles. 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200 MINERAL RESOURCES IN A SUSTAINABLE WORLD • 13th SGA Biennial Meeting 2015. Proceedings, Volume 1