Genesis of the hydrothermal gold deposits in the ... - Springer Link

Int J Earth Sci (Geol Rundsch) (2014) 103:901–928 DOI 10.1007/s00531-013-0987-0

Original Paper

Genesis of the hydrothermal gold deposits in the Canan area, Lepaguare District, Honduras Michele Mattioli · Marco Menichetti · Alberto Renzulli · Lorenzo Toscani · Emma Salvioli‑Mariani · Pedro Suarez · Alessandro Murroni

Received: 25 March 2013 / Accepted: 13 November 2013 / Published online: 18 December 2013 © Springer-Verlag Berlin Heidelberg 2013

Abstract The Canan area (Honduras) is characterized by a gold-bearing ore deposit that is associated with quartzveined shear zones. Gold mineralization occurs in low-to medium-grade metamorphic host-rocks (graphitic and sericitic schists). Hydrothermal fluids, which are associated with the emplacement of Cretaceous-Tertiary granodioritic intrusions, are responsible for the formation of quartz veins and the hydrothermal alteration of wall-rocks. Three main altered zones have been detected in the wall-rocks as far as 150 cm from the quartz veins. The distal zone (up to 50-cm thick) contains quartz, chlorite and illite. The intermediate zone is the thickest (up to 80 cm) and is marked by quartz, muscovite, sulphides, kaolinite and native elements such as Au and Ag. The proximal zone, which is close to the quartz veins, is rather thin (up to 25 cm) and contains clay minerals, Al-oxides-hydroxides and sulphides. The transition from the distal to the proximal zone is accompanied by the enrichment of SiO2 and the depletion of all other major elements, except for Fe2O3(tot). Precious metals occur in the highest concentrations in the intermediate zone (Au up to 7.6 ppm and Ag up to 11 ppm). We suggest that gold was transported as a reduced sulphur complex M. Mattioli (*) · M. Menichetti · A. Renzulli Dipartimento di Scienze della Terra, della Vita e dell’Ambiente, Università di Urbino “Carlo Bo”, Campus Scientifico “Enrico Mattei”, 61029 Urbino, Italy e-mail: [email protected] L. Toscani · E. Salvioli‑Mariani Dipartimento di Fisica e Scienze della Terra “Macedonio Melloni”, Università di Parma, Parco Area delle Scienze, 43100 Parma, Italy P. Suarez · A. Murroni Goldlake Group, Colonia Palmira, Av. Republica del Brasil casa N 2401, Tegucigalpa, MDC, Honduras CA

and was precipitated from the hydrothermal solution by the reaction of the sulphur complexes with Fe2+ from the alteration of the mafic minerals of the host-rock. Fluid– wall-rock interactions seem to be the main cause of gold mineralization. Genetic relationships with a strike-slip fault system, hydrothermal alteration zones within the metamorphic wall-rocks, and an entire set of geochemical anomalies are consistent with orogenic-type gold deposits of the epizonal class. Keywords Hydrothermal alteration · Geochemistry · Gold · Canan · Honduras · Central America

Introduction In the past 25 years, many studies have demonstrated the close association of vein-hosted gold deposits with alteration halos due to hydrothermal fluid infiltration (e.g. Böhlke 1989; Boiron et al. 1991; Eilu and Mikucki 1998; Garofalo et al. 2002; Garofalo 2004a, b; Yang et al. 2006; Phillips and Powell 2009; Esmaeily et al. 2012). Fluids flow through fault-fracture systems that have developed along mixed brittle-ductile shear zones, which are mainly active in the mesozonal environment. During the faulting process, large volumes of fluid are injected along the shear zones, forming mesh structures. These structures include hydraulic extensional veins that are interconnected with shear fractures, and subsequent mineral precipitation (Sibson and Scott 1998; Cox 1995; Zoheir 2008a, b; Bark and Weihed 2012). Hydrothermal alteration is often associated with various types of quartz-vein mineralization, and some altered wall-rocks can contain high concentrations of metals (e.g. Vallance et al. 2003; Zhu et al. 2011; Andrada de Palomera

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et al. 2012; Cepedal et al. 2013 and references therein). Intensive host-rock alteration around the quartz vein reveals distinct mineralogical changes that are indicative of metasomatic processes as a result of the interaction with external hydrothermal fluids. Chemical variations around mineralized rocks in hydrothermal gold deposits are critical both when it comes to constraining genetic models and, ultimately, as a tool for exploration. The value of geochemical methods is evident in the extensive literature documenting the alteration chemistry associated with a wide range of hydrothermal ore deposits (e.g. Phillips 1986; Robert and Brown 1986; Bierlein et al. 1998; Deksissa and Koeberl 2004; Henley and Berger 2011; Zhu et al. 2011; Cepedal et al. 2013; Zachariáš et al. 2013). The amount of precipitation of native elements and the type of hydrothermal alteration minerals are strongly dependent on the physical–chemical nature of the mineralizing fluids and the host-rock composition. Several gold deposit studies have described in detail the mineral associations of the different wall-rock alteration zones, as well as the chemical features of the fluids associated with the alteration processes (e.g. Lowell and Guilbert 1970; Klein et al. 2002; Yang et al. 2006; Su et al. 2008; Modabberi and Moore 2004). Some authors point out that wall-rock alteration is strictly related to the host-rock mineralogy (e.g. Callaghan 2001; Miur 2002; Botros 2004; Deksissa and Koeberl 2004; Kister et al. 2006). Nevertheless, very few attempts have been made to define the complete evolutionary path of the alteration with the quantification of mass changes in different alteration zones during reactions with hydrothermal fluids (e.g. Kolb et al. 2005; Phillips and Powell 2009; Andrada de Palomera et al. 2012; Esmaeily et al. 2012). Similarly, studies on detailed-scale variations in element concentrations in the hydrothermally altered wallrocks with progressive distances to the mineralized veins are rare (e.g. Silberman and Berger 1985; Warren et al. 2007). In a recent paper on the epithermal gold deposit of La Josefina, Andrada de Palomera et al. (2012) described well the variations of chemical elements in terms of distance to Au-rich veins, evidencing how some of them can be used as potential geochemical indicators of proximity to gold deposits. The Honduras region (Central America) is a well-known metallogenic Province, which is represented by several areas of hydrothermally altered metamorphic rocks that are related to the Late Cretaceous-Early Tertiary magmatic systems extending from Guatemala to Costa Rica (e.g. Roberts and Irving 1957; Kesler 1978; Sundblad et al. 1991; Samson et al. 2008; Williams-Jones et al. 2010). However, except for the gold deposits of the Rosario Mining District (San Juancito Mts.; Carpenter 1954) and the Minas de Oro District (Central Highlands; Drobe and Cann 2000), very

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Int J Earth Sci (Geol Rundsch) (2014) 103:901–928

little is known about the genesis of the gold deposits in this region. Recent investigations in Central-Eastern Honduras (Menichetti et al. 2007; Bersani et al. 2009) were focused on the gold deposits in the Canan area (Lepaguare District, Olancho Department) where in the past, mining activity was made through tunnels in the rocks adjacent to the quartz veins. In this location, gold mineralizations occur in low-grade metamorphic rocks consisting of graphitic and sericitic schists and quartzites, which can be related to the pre-Mesozoic metamorphic basement (Carpenter 1954). Gold is found in both quartz veins and disseminated (with sulphides) in the altered host-rock. The quartz of the veins is rich in fluid inclusions that were entrapped during hydrothermal growth or because of the healing of the fractures (Bersani et al. 2009). The fracturing and shearing of the host-rocks are the main factors controlling the formation of the vein network and the flow of the mineralizing fluid (Menichetti et al. 2007; Mattioli et al. 2008). The aim of this study is to describe the structural geology, mineralogy, petrography and geochemistry of the hydrothermal veins and the adjacent alteration zones of the Canan ore deposit. The purpose is to infer the origins and style of the gold mineralizations. Mass changes in the major and trace elements that are induced by the hydrothermal fluids are evaluated quantitatively.

Geodynamics and geological background The geodynamic evolution of Honduras is the result of the complex interaction along a triple junction among the North American plate, the Cocos plate and the Caribbean plate (e.g. Mann and Burke 1984, 1988; Heubeck and Mann 1991; Guzman-Speziale 2001; Rogers and Mann 2007; Silva-Romo 2008; Fig. 1). This interaction is responsible for the Central American subduction system, which consists of a north-eastwards slab subduction under the Caribbean and the North American plates. The subduction is oblique to the plate boundary, and both transpressional and transtensional tectonics seem to be active (Corti et al. 2005). The Caribbean plate includes minor crustal blocks like the Chortis block, which has traditionally been referred to the Precambrian–Paleozoic continental nucleus of northern Central America (Rogers et al. 2007). The Chortis block is separated from the North American plate by the Motagua-Polochíc sinistral transform fault (northern side) and from the Cocos plate by the Middle American Trench (Fig. 1). The southern margin of the Chortis block may be represented by the Hess escarpment suture, while the eastern Chortis block margin is still undefined (Rogers and Mann 2007; Baumgartner et al. 2008). The Chortis


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Fig. 1 Tectonic sketch of the Caribbean region. NAP North American Plate, CAP Caribbean Plate, SAP South American Plate, CP Cocos Plate, MPF Motagua-Polochic Fault system, GF Guayape Fault system, PFZ Panama Fracture Zone, CB Colombia Basin, HE Hess Escarpment, EPGF EnriquilloPlantain Gaden Fault. Arrows indicate predicted plate velocity vectors in mm/yr relative to the Caribbean plate (DeMets 2001; DeMets et al. 2010; Mann et al. 2002)

block has experienced rotation, shearing and stretching following the Late Cretaceous-Early Cenozoic collision with the Maya block to the north. Recent studies (e.g. Rogers and Mann 2007; Rogers et al. 2007; Lodolo et al. 2009) show that the Chortis block is not geophysically homogeneous and can be divided into different tectonic terranes. Its Paleozoic metamorphic basement consists of well-foliated, graphitic and sericitic schists containing pods of milky white quartz (Cacaguapa Schist; Carpenter 1954; Horne et al. 1976) that extensively crop out in the study area (Fig. 2). Recent radiometric dating by Ratschbacher et al. (2009) suggests a Devonian age for the metamorphism. The Paleozoic basement is overlain by a fairly thick sequence of sedimentary rocks from roughly the Jurassic (Agua Fria Formation) to the Cretaceous age, which is common throughout much of Central Honduras. In the area surrounding Canan, this sequence is comprised of two main units. The lower unit consists of cliff-forming, massive, dark-grey to black, strongly fractured limestones belonging to the Lower Cretaceous age (Atima Formation, Yojoa Group; Mills et al. 1967; Rogers et al. 2007), which crop out mainly in the mountains north of the Lepaguare valley and in the southern area of Rio Jalan (Fig. 2). The upper unit is formed by Upper Cretaceous redbed siliciclastic strata, which contain quartz-rich conglomerates, limestones and minor evaporite lithosomes (Valle de Angeles Formation; Mills et al. 1967; Rogers et al. 2007) that crop out in the upper stream of Rio Guayape and in the eastern area of S. Nicolas (Fig. 2). During the Upper Cretaceous-Lower Tertiary (Ratschbacher et al. 2009), the Paleozoic basement and its Mesozoic sedimentary cover were intruded by granodiorite

plutons which crop out a few kilometres south and in the north-northwest region of Lepaguare village (Fig. 2). The plutons are represented by a medium-grained, equigranular, biotite-hornblende granodiorite, which varies locally from granite to quartz monzonite (Carpenter 1954; Simonson 1977; Drobe and Cann 2000).

Methods Field data and sampling The Canan area is located in the Lepaguare District of the Olancho Department between the southern margin of the Lepaguare Valley and Rio Guayape, about 100 km northeast of Tegucigalpa (Figs. 2, 3). Our fieldwork activities in the studied area included surface geological mapping with structural analysis, early selected rock-chip geochemistry, and systematic trench and channel geochemistry in the final stage. All of the existing outcrops in the Canan area have been documented in detail by geological and structural investigations at both the district- and deposit-scale. District-scale structures refer to structural elements that extend throughout an area of about 100 km2 around the Canan focus area, whereas deposit-scale structures (which are included in the District-scale structures) refer to those structures located within the limits of the Canan ore deposit, covering an area of about 8 km2. The geometry (dip and inclination) of the discontinuity surfaces (joints, cleavage, veins, faults) has been measured in all of the surface exposures, and the collected data have been analysed with structural statistical tools.

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Fig. 2 Geological map of the Lepaguare District. Inset: GUA Guatemala, B Belize, HON Honduras, ES El Salvador, NIC Nicaragua, T Tegucigalpa

Based on geological mapping and the structural data, the Canan area has been lithologically subdivided into three main units: (1) unaltered metamorphic host-rocks, (2) quartz veins and (3) hydrothermally altered wall-rocks. The main mineralized zones (quartz veins and hydrothermally altered wall-rocks) were sampled for a selected rockchip geochemical analysis. Finally, these data were used to plan a systematic surface trench and underground channel geochemistry. Surface trenches were excavated across the mineralized zones. Typically, trenches vary between 10 and 50 m in length and reach depths ranging between 1 and 3 m, depending on whether they were located over bedrock, the exposed soil profile or colluvial material. Underground channels (up to 10 m in length) were dug around the quartz veins, particularly where access to the hydrothermally

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altered wall-rocks was easily achieved. All of the trench and channel assay samples were collected underneath the soil cap. The intervals where the sulphides have been completely replaced by Fe-oxides or hydroxides were considered to be intervals of strong oxidation. As oxidation could be a proxy for the degree of chemical weathering, the samples from these intervals were discarded. Based on various criteria (i.e. Au content, types of rock, degree of hydrothermal alteration, position relative to the quartz veins), a total of about 200 samples were collected from the entire Canan area. From these, a selection of 36 samples of metamorphic host-rocks and quartz veins were used for petrographic thin section analyses, 55 samples of hydrothermally altered wall-rocks were utilized for mineralogical investigations, and 48 rock-chip


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Petrography and mineralogy

Fig. 3 Geological map of the investigated Canan area showing the geometry of the main quartz veins (see Fig. 2 for its location in the Lepaguare District). DB Dos Bocas, EC El Cafetal, VB Veta Blanca, CR El Crucero, LM Los Mochos

For each collected sample, horizontal and vertical sections were prepared for macroscopic observations of the structures. From these, orientated blocks and polished thin section samples were prepared and studied in reflected and transmitted light with a polarizing microscope. Samples for the thin section petrographic study were selected to cover all of the lithologies observed in the entire Canan area, including the unaltered metamorphic host-rocks, the quartz veins and the adjacent hydrothermally altered wall-rocks. In order to define their morphology and to verify the qualitative elemental composition, the main ore minerals were examined by Scanning Electron Microscopy (SEM) and Energy Dispersion Spectroscopy (EDS) using a Philips 515 equipped with EDAX 9900 at the University of Urbino (Italy). The operating conditions applied were a 15 kV accelerating potential and a 2–15 nA beam current. The mineralogy of the samples was determined using a Philips X’Change PW 1830 X-ray diffractometer (Philips X’PERT; Cu Kα radiation). The samples were run between 2° and 70° 2θ. The analytical conditions were a 35 kV accelerating potential, a 30 mA filament current, a 0.02° step, and a counting time of 1 s/step. All of the powder samples were prepared by side loading an aluminium holder to obtain a quasi-random orientation. Geochemistry

samples from the entire Canan ore deposit were used for the geochemical analysis of 22 selected elements and precious metals. Finally, the trenches and channels with the higher gold grades were selected for detailed-scale (few metres) geochemical investigations (major- and trace elements). For each trench and channel, the length of the sampled intervals (1–3 m) was adjusted to obtain a complete section from the quartz vein to the unaltered rocks, crossing the entire hydrothermally altered interval. One of these sites (Dos Bocas, Fig. 3) has been chosen as representative of the entire deposit. This is because it satisfies three criteria: (1) it has the highest gold grade in the studied area, (2) the hydrothermal alteration associated with the quartz veins is representative of the entire Canan ore deposit, and (3) the outcrops are large (and accessible) enough to enable us to capture possible small-scale variations within the deposit. In this channel, the sampled interval was 2.5 m in the eastern side of the quartz vein, and the samples were systematically collected at intervals of 25 cm, with the aim being to identify the overall differences in the element concentrations from the quartz vein to the adjacent wall-rocks throughout the hydrothermally altered zone.

Whole-rock analyses were performed at Activation Laboratories Ltd., Ancaster, Canada. The rock samples were crushed in a low-blank agate mortar to obtain a fine rock powder of 1 and Ki 0 and Ki − 1 σKi. Finally, the validity of the Ki values implies that the hydrothermal alteration occurred at a constant volume. Such a condition should be satisfied for the

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studied area, as the hydrothermal alterations occurred at a depth where the system was confined between rocks with rigid behaviour.

Results Structural data District‑scale tectonic features The Lepaguare District is dominated by a structurally controlled valley (Lepaguare valley) that is bounded on the northern side by a north-verging fold-and-thrust belt mountain region (Lower Cretaceous limestone of the Atima Formation), and on the southern part by the Cacaguapa Schists of the Paleozoic basement (Fig. 2). This region represents an important boundary between extensional and strike-slip structures. In the north-western sector of the Lepaguare district, a number of NW–SE orientated extensional faults move down the southern blocks with an offset of 100 s of metres and can be related to the main graben system that crosses Central Honduras. These extensional faults seem to be the most recent structures crosscutting all of the transcurrent and compressive faults, as well as the Quaternary deposits. In contrast, the entire south-eastern sector is intersected by NE-SW strike-slip faults that are related to the Guayape fault system (Fig. 2), which is one of the major tectonic structures within the Chortis block of the Caribbean Plate (GF, Fig. 1). The Guayape fault system is the longest and most continuous morphological and tectonic feature in Honduras. It is defined as a complex zone of several faults ranging from 2 to 25 km in width, and trends N30°– 35°E for 290 km from the Honduras-Nicaragua border to the Caribbean coast (Ritchie and Finch 1984; Finch and Ritchie 1991; Gordon and Muehlberger 1994). Sub-vertical faults with horizontal and sub-horizontal slickensides, which are associated with vertical drag-folds, shutter ridges and extensional basins generated by strike-slip displacement, suggest a left-lateral transtension of the Guayape fault system (Menichetti et al. 2007). In particular, the Canan mining area is located between the La Rosa-Campamento and Juticalpa-Rio Jalán fault systems (Fig. 2), which represent different strands of the main Guayape fault system and have main left-lateral transtensional kinematics. In the southern side of Juticalpa and along Rio Jalán, NE-SW fault scarps that are related to the Guayape fault system show a left-lateral strike-slip component with an extension striking NW–SE. Along Rio Jalán, an important strike-slip fault marks the contact between the Cacaguapa Schists and the Agua Fria Formation (Fig. 2). At present, the Guayape fault system seems to be inactive (no seismic activity, no


movements along faults from the geodetic GPS measurements), and the observed deformations in the Quaternary outcrops along the eastern valley could be indicative of the presence of a silent fault (Lyon-Caen et al. 2006; Rodriguez et al. 2009). Deposit‑scale structural analysis In the Cacaguapa Schists of the Canan area, intense ENEWSW strong stretching lineations, which are sub-horizontal to gently dipping, have been distinguished from depositscale mapping (Figs. 3, 4a–d). Their spatial arrangement shows a normal distribution density for all of the population

Fig. 4 Metamorphic host-rocks cropping out in the Canan area, represented by well-foliated Cacaguapa graphitic and sericitic schists. a Relationships between schistosity (S) and stretching lineations (L); lower hemisphere equal area plot with Gaussian k = 100 contours of the poles of the schistosity (n. 131) and of the stretching lineation (n. 241). b Disharmonic folds with the hinge parallel to the stretch-

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data (n. 241), with a cluster close to N65°E and an inclination of 25°. The lineations are homogeneous, with a small rotation northwards (Fig. 3). The plane perpendicular to the lineations contains millimetric, anastomized bands of lenticular mica surrounding elliptical quartz (Fig. 4d). In some outcrops, this plane has a disharmonic folding with a wavelength of a few centimetres and variable amplitude (Fig. 4b). The fold axes are parallel to the stretching lineation, with the axial plane dipping to the south-west. The plane of schistosity is well developed, but less than the lineation (Fig. 4a). The planes generally dip to the southeast, with variable inclinations. The density distribution of the structural data in the area is unimodal, with a cluster

ing lineation and quartz crystals. c Systematic fractures that cross the schistosity and the stretching lineation; lower hemisphere equal area plot of the fracture planes and Gaussian k = 100 contours of the poles of the stretching lineation (n. 241). d Sericitic schists with elliptical quartz dipping SE

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around N125°E and an inclination of 40°. Systematic and conjugate fractures striking NNW-SSE are also very common (Fig. 4c). The spatial statistical distribution of the data (n. 351) shows a cluster of sub-vertical planes striking NNW and dipping to the SW, with a few scattered data in other directions. In the Canan area, the patterns of such lineations are controlled by two main groups of structural elements. The first group has a general direction of N60°E, and is represented by two shear zones that are located in the Quebrada El Naranjal-El Nance and Dos Bocas-Quebrada Los Mochos areas (Fig. 3). These zones have subvertical fault planes consisting of a metric-thick shear zone where the S and C surfaces are well developed, while kinematic indicators show a prevalent left-lateral transtension. These shear zones appear to be controlled by the major Guayape fault system. The second group of faults is N–S striking, and is located on the western side of the Canan area, in correspondence with Dos Bocas (Fig. 3). The fault planes are sub-vertical, dipping to the west with a left-lateral transtensional kinematic and a metric shear zone. Vein systems The major mineralized rocks occur just where the northern shear zone (Dos Bocas-Quebrada Los Mochos) intersects the N–S striking faults at Dos Bocas (Fig. 3). In this area, the principal ore deposit is associated with two main quartz veins striking N20°E and developing along a distance of a few hundred metres. The quartz veins are hosted in the Cacaguapa Schists and form, with their foliation, an acute angle along sharp contacts. The veins are sigmoidal lenses that are located in the host-rock blocks and are bounded by shear planes; these planes have moderate to steep dips and form a pattern of fault-fracture meshes that interlink shear and extensional veins. Striations on slip surfaces are common, and are consistent with the slip direction of the host left-lateral strike-slip shear zone. The veins have a variable thickness, ranging from 2 to 30 cm, with an average of 15 cm; it is only at the intersections of the two fracture systems that the thickness of the quartz veins significantly increases by up to 1 m. The veins are generally sub-vertical, dipping to the NW and striking N20°E; the inclination tends to decrease in the lower part of the outcrops to 50°. South-east of the Dos Bocas area, the main veins are displaced a few tens of metres by a N60°E strike-slip fault with an extensional component to the SW. Two types of breccia occur locally along or within fault veins and form narrow elongated lenses within some small veins. The first type is a fault breccia composed of angular and rotated fragments of host-rocks with

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a millimetre to centimetre grain size, whereas the second type is a hydrothermal breccia consisting of angular wallrock fragments set in a siliceous cement. The fault breccias are located between the host schists and the hydrothermally altered wall-rocks around the quartz veins. They also suffer modifications from subsequent alteration events and are crosscut by the quartz veins. These observations suggest that the brecciation represents an early stage in vein development. Petrography and mineralogy A summary of the fundamental rock types and key mineral assemblages of the three main units of the Canan ore deposit are reported in Table 1, whereas the mineral associations of the hydrothermally altered wall-rocks are illustrated in Fig. 5. Their main features are described below. Unaltered metamorphic host‑rocks The unaltered metamorphic host-rocks are mainly represented by well-foliated graphitic and sericitic schists. The graphitic schists are fine to medium (0.5–5 mm) grained and are essentially composed of quartz, biotite, chlorite, white mica, graphite and subordinate K-feldspar (Fig. 5). Biotite is commonly transformed into chlorite, and some crystals reveal an oblique orientation to cleavage in the rock. Quartzitic mylonites with evident stretching lineation, small quartz lenses with circular to elliptical shapes (1–4 cm), and bedding defined by diffuse graphite layering in quartz-rich areas are also present. Quartz is dynamically recrystallized by sub-grain recovering. At least three phases of deformation occur in the rock and can also be observed in the thin section. The rock colour varies from white-yellowish to blue-grey. In places, the intense weathering of the rocks gives them a strong red/ochre oxidant colour. The sericitic schists are very fine (1 g/t, and almost all of the observed gold grains occurred within the intermediate zone of the hydrothermally altered wall-rocks, which are variably associated with clays (illite, illite/smectite, kaolinite). Other native elements (Ag and PGEs) are rarely associated with type-1 gold, and occur as small grains (from 100 μm to 1 mm; Fig. 7g, h). Type-2 gold is 5-10 μm in size, displays euhedral-subhedral morphologies and is directly attached to (or included in) pyrite and arsenopyrite (Fig. 7b, c). This type of gold is generally present in the quartz veins. Type-3 gold occurs as sub-microscopic (