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Journal of Geosciences, 56 (2011), 81–104 DOI: 10.3190/jgeosci.090

Original paper

Epithermal gold mineralization in Costa Rica, Cordillera de Tilarán – exploration geochemistry and genesis of gold deposits

Petr MIXA1*, Petr DOBeŠ1, VlADIMír ŽÁČeK1, Petr lUKeŠ1, enrIqUe M. qUIntAnIllA2

1 Czech Geological Survey, Klárov 3, 118 21 Prague 1, Czech Republic; [email protected] MINAET, Dirección de Geología y Minas, Apdo. 10104, San José, Costa Rica* Corresponding author

Epithermal gold mineralization in quartz veins forms part of a large ore belt extending in the NW–SE direction paral-lel to the Cordillera de Tilarán, Costa Rica. It is confined to Miocene–Pliocene andesites and basalts of the Aguacate Group volcanic arc. Gold-bearing quartz veins are related to faults and fractures of steep inclinations, accompanied by pronounced hydrothermal alteration. The key tectonic zones strike NW–SE but the majority of the ore veins are control-led by local extensional structures of Riedel shear type in the NE–SW, N–S to NNW–SSE directions. The brecciation, mylonitization and healing of deformed structures suggest that three main pulses of mineralization took place during the hydrothermal process. The gold is present as electrum (30 and 42 wt. % Ag) tiny inclusions up to 25 µm in size enclosed in quartz, pyrite and arsenopyrite. The other ore minerals are chalcopyrite, galena, sphalerite and marcasite and less abundant to scarce acanthite, pyrargyrite, greenockite, covellite, bornite and cassiterite. The principal elements exhibit-ing significant positive correlations with Au are Ag, Sb, As, Pb and Hg. Fluid inclusions of the H2O type were found in quartz and sphalerite from several Au-bearing occurrences. Temperatures of homogenization of fluid inclusions from several quartz generations and sphalerite vary generally between 150 and 290 °C; the salinity of the aqueous solution was very low, not exceeding 5 wt. % NaCl equiv. The age of the mineralization is estimated in the period between the intrusion of the Guacimal Pluton and effusions of the discordant volcanic Monteverde Formation, which is barren (i.e. between c. 6.0 and 2.1 Ma). Geochemical study indicated altogether 14 promising gold-bearing areas in the Montes del Aguacate and Cordillera de Tilarán, of which four can be recommended for further exploration.

Keywords: gold, epithermal, mineralization, fluid inclusions, alteration, Cordillera de Tilarán, Costa RicaReceived: 14 January 2011; accepted: 7 April 2011; handling editor: M. Štemprok

1. Introduction

A joint project of the Czech Geological Survey (CGS) and Dirección de Geologia y Minas (DGM), Department of the Ministry of Environment and Telecommunica-tions of Costa Rica (MINAET) was implemented in the framework of Czech Foreign Aid in 2006–2009. The area studied covered three map sheets on a scale of 1 : 50 000, specifically Miramar, Juntas and Chaper-nal, located in the NW of Costa Rica in the provinces of Puntarenas, Guanacaste and Alajuela. Moreover, the project was intended to survey and subsequently to compile three basic geological maps on a scale of 1 : 50 000, to study natural hazards in relation to the geological structure of the area under consideration and to investigate the mineral potential of the area, including assessment of the impacts of mining and mineral pro-cessing on the local environment. With regard to intense mining of gold in the past (summarized by Muñoz 1997 and USGS et al. 1987) the chief objective of this part of the project was the evaluation of the gold potential of new localities. Nevertheless, appropriate attention was also paid to other mineral deposits, to industrial rocks

and minerals and, in particular, to construction materi-als. The reason for inclusion of prospecting works in the scope of the current project consisted in the mining operations within the above-mentioned three map sheets that lasted for almost two hundred years (1815–2007). The mining of gold confined to quartz veins took place in several ore districts and hundreds of small mines and deposits, which were mostly discovered by prospec-tors. In contrast to these extensive mining operations, no systematic exploration geochemistry combined with geological mapping and structural geological investiga-tion on a regional scale have been undertaken so far to establish the mineral potential of the area. This is rather unfortunate, as it is essential for land use planning and economic development of the whole region.

The results of new geochemical exploration using, in particular, heavy mineral and stream sediment surveys enabled us to identify and outline a number of localities and areas that are promising for the presence of economic gold deposits. Some of them were suggested for follow-up detailed exploration and investigation.

All the field and laboratory data obtained in course of the project were, together with individual maps and

Petr Mixa, Petr Dobeš, Vladimír Žáček, Petr lukeš, enrique M. quintanilla

82

relevant project outputs including the GIS database, sum-marized in the Final Report (Kycl et al. 2010) and stored in the archives of CGS Prague, Czech Republic and DGM San José, Costa Rica. The present study is focused chiefly on the definition of prospecting criteria, the investigation of gold deposits and the likely mode of their origin.

2. Geological setting

The area covered by the three map sheets of Miramar, Juntas and Chapernal lies virtually in the axis of a vol-canic arc extending above the Central American subduc-tion zone. An important regional transcurrent zone of sinistral character (Marshall et al. 2000) is believed to have significantly affected the development of brittle deformations (faults and fracture systems) in volcanic formations in the Late Cenozoic times.

The region in which the gold deposits are concentrated consists of two major geological units, the Aguacate Group (including Guacimal Pluton) and the Monteverde Formation. Recent geological studies dealing with the area of interest and adjacent regions include Alvarado (2000) and Denyer et al. (2003). A new geological survey on a scale of 1 : 50 000 (Žáček et al. 2010a, b, c; Kycl et al. 2010) contributed substantially to better knowledge of the geological structure of the area.

The Aguacate Group is the main geological unit in the studied area. It consists of tholeiite basalt to basaltic andesite lavas accompanied by abundant pyroclastics and breccias of andesite composition. This group of effusive rocks is of Mio-cene to Pliocene age (2.1 to 23.0 Ma: Bellon and Tournon 1978; Amos and Rogers 1983). The rocks are hydrother-mally altered on a regional scale, having been affected by carbonatization and silicification (Laguna 1983, 1984).

The Aguacate Group is discordantly overlain by the Monteverde Formation which, according to radiometric dating, is Pleistocene in age (2.2–1.0 Ma – Kussmaul and Spechmann 1982; Alvarado et al. 1992). In contrast to the older Aguacate Group, the Monteverde Formation is characterized by the occurrence of more acid, calc-alkaline volcanism with dominant andesites and by the absence of regional hydrothermal alteration.

Volcanites of the Aguacate Group were intruded by granitoids of the Guacimal Pluton exposed over an area of c. 15 × 6 km, and elongated in the NW–SE direction. Gray porphyric biotite granite is the dominant rock type of the Pluton, whereas more mafic varieties (monzodio-rites to gabbros) are much less abundant (Žáček et al. this volume). The K–Ar dating carried out by Alvarado et al. (1992) yielded ages of 7.2–3.9 Ma, whereas laser ablation ICP-MS dating of zircons from two granite samples gave statistically undistinguishable U–Pb ages of 6.3 ± 0.5 and 6.0 ± 0.4 Ma (Žáček et al. this volume).

A large contact metasomatic aureole up to 1.5 km wide developed around the intrusion; here the volcanites were converted to hornfelses and epidote schists. Although the Pluton exposed on the surface is relatively small, the gravity anomaly interpreted by Ponce and Case (USGS et al. 1987) indicates a large body of relatively light crust extending below the Cordillera de Tilarán and Montes del Aguacate. It is believed to represent a huge hidden gra-nitic intrusion extending in the NW–SE direction over a distance of c. 100 km, of which the southern edge follows the line of the towns of Esparza – Pozo Azul – Juntas and continues as far as Liberia. A similar negative anomaly of almost identical direction, intensity and size was identi-fied in the SE sector of Costa Rica. It corresponds to the large granitic mass of Cordillera de Talamanca which, due to deep erosion level, is exposed on the surface. Related epithermal gold deposits in Talamanca area are abundant and well known.

3. Mining history

Numerous ore districts exploited in the past lie in the area of the three map sheets investigated (see Fig. 1). Gold deposits in the Cordillera de Tilarán and Montes del Aguacate were discovered as early as in 1815 by Nicolás Garcia (1756–1825), a catholic bishop sta-tioned in Costa Rica and Nicaragua. Mining operations on an industrial scale began in the middle of the 19th century. The largest mine Mina Tres Hermanos in the Abangares area was developed in 1870 and during the main mining boom, when the shaft reached a depth of 500 m, as many as 1 500 miners extracted ore of about 8 g/t Au grade. A system of ropeways several kilometres long was developed to transport the ore to the central ore dressing plant in the village of Sierra near the town of Juntas. Following a catastrophic min-ing accident, during which as many as 120 miners died, a decline in mining took place and mining has never recovered.

Attempts were made in the last few decades to reopen mines in historical ore districts. The most meaningful achievement was the development of the Bellavista open-cast mine and ore dressing plant by a Canadian mining company. However, after several years of op-eration, a landslide destroyed the ore dump, including the ore dressing plant using heap leaching technology of gold extraction, resulting in complete abandonment of the mine. Similar attempts lasting for a few years were made by foreign investors in the mining districts of Veta Vargas, Recio and Tres Hermanos. Currently, no legal mining for gold takes place in the area under consideration. Only illegal hand picking of gold by lo-cal miners is under way (Fig. 5). Brief information on

Gold mineralization in Cordillera de tilarán, Costa rica

83

the most important mining districts is given below (see also Fig. 1).

3.1. Bellavista

Until 2007, Bellavista (NE of the city of Miramar) was the only and so far the last gold mine in operation on the territory of Costa Rica. It was abandoned following a catastrophic landslide triggered by torrential rains. The ore zone consists of stockwork and quartz veins inclined at angles of 30–90° to the Falla Liz fault line trending N–S. The main strikes of local quartz veins run NE, less frequently NNE. The host rocks are volcaniclastic brec-cias and basaltic lavas of the Aguacate Group overlain by sterile andesite lava flows and lahars of the Monteverde Formation, which were removed as overburden. The gold occurs both in quartz veins and stockwork with max. grade 50 g/t and also in altered wall rocks with a grade of 0.5–1 g/t. Recalculation of ore reserves adjusted to a workable grade of 1.54 g/t (Alán 1990) gave 13.6 Mt of ore reserves. Underground drilling penetrated dykes of diorite to granodiorite which probably belong to the Guacimal Pluton.

3.2. Mina Chassoul (Prospecto Corinto)

The Mina Chassoul is located on the Miamar map sheet. The deposit was exploited in the past but its old stopes were re-opened in 2008. The mine was developed on the Veta Cajeta and Veta Virgilio veins trending N–S to NE–SW. The veins are intensely affected by tectonics and filled with mylonite (Fig. 6). The average grade in the Cajeto vein corresponds to 9.67 g/t Au, locally exceeding 100 g/t Au. The average grade in the Virgilio vein corresponds to 7.34 g/t Au.

3.3. Mina Moncada

Occasionally recently exploited gold mine (on the SE edge of the Miramar map sheet) with ore dressing plant using cyanide leaching technology. The mine was devel-oped in a max. 1.6 m thick, N–S trending quartz–carbon-ate vein, accompanied by numerous stringers. Local ore is uncommonly rich in base-metal sulphides (chalcopy-rite, sphalerite and galena) with an average grade of 8.91 g/t Ag, locally exceeding 100 g/t Ag. The steep vein was extracted by a system of headings. The ore was trans-ported by ropeway to the ore dressing plant.

Puntarenas

map sheet Juntas

map sheet Miramarmap sheet Chapernal

Guacimal

La Union

Bellavista

Chassoul

Buena Suerte

Moncada

Miramar

Veta Vargas

Juntas

Sierra Alta

La Luz

Tres Hermanos

a) b)

10 km

10°20'N

10°00 N'

84°30

W'

85°00

W'

85°00'

09°00'

08°00'

10°00'

11°00'

84°00' 83°00'

Pacific Ocean

Caribbean

Sea

Nicaragua

Panam

á

50 kmstudied area

Santa Rosa

Fig. 1 Position of the major ore districts within the zone Cordillera de Tilarán – Montes del Aguacate. Following Schulz et al. (1987).


84

3.4. Abangares mining district

The Abangares mining district (Juntas map sheet) used to be the most important mining centre in the whole region (see also Fig. 2). The ore is confined to numer-ous thick veins running N–S to NNE–SSW with steep inclinations (60–85° to NW). The ore district is divided, from the west to the east, into sub-districts: La Luz, Tres Hermanos and El Recio located to the west, whereas San Martin, Sierra Alta, Boston and Gongolona are situated in the east.

3.4.1. la luz Mine

This mine was developed on the parallel La Pita and La Olga veins oriented 10° NNE having a thickness of 1.2–2 m with average grade 12.5–37 g/t Au (OEA 1978). An ore dressing plant was erected in place of historical stopes, using cyanide leaching technology, and process-ing material extracted by illegal mining and dumped near the plant.

3.4.2. el recio Mine

El Recio Mine (known also as Silencio) is located about 3 km N of the city of Juntas. It exploits sub-vertical veins of El Silencio (which splits southwards into the Guayacán and Santa Anna veins), El Recio and Villalobos running in general N–S. El Recio vein dips 80° to the west with a thickness of as much as 5 m. The grade is estimated to have ranged between 7.8 and 15.6 g/t Au. The veins

were exploited by a system of levels, raises and winzes about 30 m apart down to a drainage tunnel, about 150 m below the surface.

3.4.3. tres Hermanos Mine

This mine was developed on a namesake vein and also on the neighbouring Limón, Pedernal, Palo Negro, Nispero and Balsa veins. The Tres Hermanos vein was the major object of exploitation in the past. It is 3 m thick and runs 20–30° NNE, steeply dipping. In its northern part, it was exploited to a depth of 200 m, whereas its southern sec-tion was mined to a depth of 150 m. Data on historical production indicate the grade to have been around 16 g/t Au. The gold is rich in silver (electrum) accompanied by sphalerite, galena and stibnite.

3.4.4. Boston, Sierra Alta (also named tres Amigos) and Gongolona mines

These three mines used to exploit a system at maximum 5 m thick, of steeply inclined veins oriented 30° NNE to 70° ENE (Boston, Año Nuevo, San Martín, San Rafael, La Fortuna Gongolona and others). Millaflor (1979) re-ported an average grade of 7.41 to 12.6 g/t Au.

3.5. Guacimal

The Guacimal ore district in the eastern sector of the Juntas map sheet consists of a system of quartz veins with Pb–Zn–Cu–Au–Ag mineralization. The veins are as

Fig. 2 A scheme of economically most important ore veins in the Abangares mining district (LLM – La Luz Mine, Rec – El Recio Mine, RGB – casa de Rigober-to). Following Castillo (1997) and Kycl et al. (2010).


85

much as 1.5 thick and run NNW–SSE, inclined 66–70° to ENE.

4. Exploration geochemistry

4.1. Methods of study – sampling and laboratory treatment of samples

Exploration geochemistry, specifically the heavy mineral and stream sediment survey, were chosen as the most efficient methods for assessing the ore potential of the area under consideration. Mineral occurrences found and documented during geological mapping were also investigated for the economic potential of the local min-eralization. Lastly, the collected samples were used for the evaluation of possible environmental pollution with respect to ecological burdens left after earlier mining and mineral processing. Particular attention was paid to mercury that was formerly used for amalgamation and extraction of gold and also to anomalous contents of heavy metals in dumped waste rock or to their enhanced background values in wall rocks.

Altogether 237 heavy mineral concentrates were collected using panning of 10 litres (about 20 kg) of sandy material. The density of the sampling sites was 1 sample per 4 km2 depending on the drainage pattern and accessibility. All the samples were taken from ac-tive river beds, mostly with flowing water. The heavy mineral survey on the Juntas and Chapernal map sheets concentrated on the Aguacate Group. Areas covered by young and supposedly sterile volcanic rocks of the Monteverde Formation and also sedimentary formations or areas that are poorly accessible or are a subject of environmental protection were excluded from the heavy mineral survey.

The development of alluvial sediments varied substan-tially depending on the topography and stream gradients, which are steep, particularly in the central and northern sectors of the Miramar and Juntas map sheets. Each sam-ple was documented using a standard procedure recording 17 parameters, and inserted in a database. The samples were sieved on the spot using a 2 mm sieve and panned to obtain a rough concentrate. These were then analyzed in the mineralogical laboratory of GEOMIN Co. Jihlava, Czech Republic. The concentrates were first observed under a binocular microscope and some 16 minerals were selected to be studied in more detail and to establish their relative proportions. The heavy fraction was sieved on a 0.16 mm screen and, after magnetic separation of ferro-magnetic fraction, the above-screen fraction was studied under a binocular microscope to identify and count gold particles. The below-screen fraction was pulverized for chemical analyses (see below).

With respect to the aforementioned character of the mineralization, the heavy mineral survey was supple-mented by a stream sediment survey, within which a total of 296 samples were collected. Despite the rough topography, reflected in the steep river gradients, it was still possible to take reasonable and representative mate-rial for analyses. Stream sediment samples were dried at the camp sites, sieved on 0.18 mm mesh sieve (~80 mesh), subsequently homogenized in the laboratories of the Czech Geological Survey in Prague.

These powders, together with the below-screen frac-tion of heavy mineral concentrates, were analyzed in ACME Laboratories in Vancouver, Canada using the ICP MS method (Group ADX analytical procedure), enabling determination of 46 elements, including Au with a detection limit of 0.2 ppb. Chemical analysis of the below-screen fraction of heavy mineral concentrates was used to identify Au, which is extremely fine-grained or even of sub-microscopic dimensions. Alternatively it may be bound in pyrite, arsenopyrite and/or other base metal sulphides.

4.2. Heavy mineral survey

Gold was found in 55 samples of heavy mineral con-centrates of a total of 237 samples. In samples in which gold was optically identified, its particles overwhelmingly occur in the above-screen fraction (>0.16 mm). However, gold was successfully optically detected in the below-screen fraction. The number of gold particles identified in the individual samples varied between 1 and 10, only scarcely reaching tens of particles in a single sample. The richest heavy mineral concentrates included the follow-ing samples (presented with above-screen/below-screen numbers of gold particles): Ju-Tl-3 (Juntas map sheet, 45/66), Ju-TM-23 (Juntas map sheet, locality Quebrada Gongolona, 29/53) and Mi-TM-25 (Miramar map sheet – dextral tributary of the Quebrada Zamora, Río Seco 40/16). The size of the gold particles ranged from 0.1 up to 1 mm. The largest “nugget” was identified in sample Ju-Tl-3 from the Río Abangares river, reaching 2 mm in diameter (Fig. 3a).

The crystal form (habit) of the gold particles often indicates their short transport (Fig. 3d–h). They mostly occur as tiny plates or packed plates with sharp edges or tiny wires; rare is dendritic gold. Gold particles often enclose rock-forming minerals – quartz and K-feldspar in particular. Gold from larger rivers (e.g. Río Abangares, Río Congo, Río Boston) also occurs in the form of well-rounded tiny grains or nuggets with indication of longer transport (Fig. 3a–c). These particles are often coated with iron oxides and clay minerals. Silver contained in gold (in the form of a solid solution) is usually leached out from the surface of the gold (electrum) grains. In sample JU-


86

TL-42 (Río Tres Amigos), the identified tiny gold grains contain surface coatings of clay minerals, which seem to absorb anthropogenic mercury (due to contamination of the stream by Hg from amalgamation in the past).

Pyrite, arsenopyrite, chalcopyrite, to lesser extent galena, sphalerite and cinnabar are primary sulphides accompanying gold in heavy mineral concentrates; cerus-site is probably the most abundant secondary mineral. Although the majority of anomalous contents of mercury detected in analyses of heavy mineral concentrates and stream sediments are ascribed to contamination during extraction of the gold through amalgamation, the occur-

rence of cinnabar argues for the presence of primary Hg mineralization in the studied districts.

4.3. relationships between elements detected in stream sediments and in the below-screen fraction of heavy mineral concentrates

Positive correlations between Au and other elements emerge from chemical analyses of stream sediments and the below-screen fraction of heavy mineral con-centrates.

200 �ma)

50 �mb)

100 �mc)

100 �md)

50 �me)

200 �mf)

�mg)

h)

10 �mi)

2 m�0

Fig. 3 Morphology, shape and intergrowths of gold particles with gangue minerals. a – JU-TL-3 (Rio Abangares) – the largest tiny and well roun-ded nugget ever found in studied samples indicating long transport from the primary source; b – JU-TL-42 (Río Tres Amigos) worn tiny nugget containing more than 50 wt. % Ag, accompanied by a grain of alkaline feldspar; c – CH-TM-10 (Quebrada Barrantes) worn tiny nugget with quartz grains, 20 wt. % Ag; d – JU-TL-42 (Río Tres Amigos) dendritic gold with Ag < 10 wt. % accompanied by clay minerals concentrated on surface Hg and pure gold (product of amalgamation); e – CH-TL-2 (Quebrada Vueltas) wrinkled tiny leaf of gold intergrown with quartz, showing marks of transport; f – JU-TM-4 (Quebrada Castillo) gold wire with 20 wt. % Ag enclosing grains of barite and quartz (BSE image), g – JU-TL-104 tiny gold nugget with quartz–feldspar gangue, Ag < 30 wt. %, locally coatings of clay minerals and Fe-oxides; h – JU-TL-3 (Río Abangares) close up photo of Au in quartz gangue, 20 wt. % Ag; i – JU-TL-39 (Río Boston) close up photo of porous surface of worn tiny gold nugget with clay mi-nerals, 20 wt. % Ag.


87

The closest positive correlations were found between Au on the one hand and Ag, Sb with As on the other, locally was also correlated Au with Hg. These relation-ships are also an important criterion in exploration geochemistry, enabling pinpointing of anomalous and/or promising areas. Other elements such as W, Mo, Tl and perhaps even Pb may be used as additional criteria in the search for Au mineralization (Tabs 1–2). In contrast, no positive correlation was found between Au and base metals (Cu, Zn), probably reflecting a simple mineral assemblage of gold accompanied with minimum base metal sulphides in the ore. On the other hand, the min-eralogy of epithermal veins and their vertical zoning in ore elements may differ in various areas depending on the erosion surface at which they are exposed (cf. Mosier and Sato 1986).

4.4. results of geochemical exploration and their interpretation

The following features were used for demarcation of anomalous areas and deserve further attention: a) geologi-cal structure characterized by the occurrence of volcanic rocks of the Aguacate Group, b) tectonics characterized by combination of NW–SE and N–S trending faults and crushed zones, c) occurrence of hydrothermal alterations, particularly silicification, carbonatization and propylitiza-tion, d) results of heavy mineral survey presented in the form of mono-mineral maps and mono-element maps of chemical analyses of the below-screen fraction of heavy mineral concentrates (Fig. 4) and e) the results of the stream sediment survey.

Altogether 14 areas prospective for the occurrence of gold mineralization were identified based on geo-chemical exploration. Among them, four were inter-preted as particularly promising for the occurrence of gold mineralization and were suggested for follow-up investigation. Although another ten areas show posi-tive indication of gold mineralization, they may be disqualified for follow-up exploration because of their small areal extent or problematic origin of the Au mineralization (possible long transport from primary

sources). One area (No. IV – Baranquilla, NNE sector of the Miramar map sheet – in Kycl et al. 2010) shows indications of base-metal mineral assemblage accompa-nied by a broad spectrum of anomalous elements – Cu, Mo, As, La, V, Se, Ga, Bi and Sc with no relation to any gold mineralization. This locality is linked to the nearby Pleistocene (sub-) volcanic dome of Cerro la Cruz formed of amphibole–biotite rhyodacites.

The remaining four promising areas all exhibit posi-tive prospecting criteria, including sufficient areal extent (more on the subject in Kycl et al. 2010).

4.4.1. Area II +XI (Zamora and estrella)

This anomaly is defined at the intersection of the Juntas, Miramar and Chapernal map sheets covering an area of c. 40 km2 in the NW extension of the historical mining district of La Union. The area is formed of andesites and pyroclastics of the Aguacate Group. Zones of intense alteration, characterized chiefly by pyritization, propy-litization, silicification and argillization, can be locally observed. Stream sediments and below-screen fraction of heavy mineral concentrates showed highly enhanced con-tents of Au, anomalous concentrations of Ag, Sb and Hg (thirty times the background values) as well as elevated higher contents of Cu, Pb, As and Cd. The rod-like shape of gold particles indicates that the primary source of the gold is not too far away.

4.4.2. Area IX (Pozo Azul)

The Area IX occurs in the central part of the bound-ary between the map sheets of Juntas and Chapernal, c. 40 km2 in size, being formed by thirteen catchments. The Aguacate Group represents the chief lithology of the area. A large, hydrothermally altered zone extending in the NW–SE direction occurs at the southern edge of the complex. In this area, silicification and propylitization are the major types of alteration, with which the miner-alizations are believed to be linked. Only four samples of heavy mineral concentrates, representing the catchments, contained gold, but the contents of elements positively correlated with gold in other catchments indicate that the mineralization may cover a much larger area than that of the above-mentioned four catchments. Some heavy mineral concentrates also contained cinnabar (Ju TM 4, Ju-TL 3, Ch-TL-6, Ch-TM 10). Their below-screen frac-tion and also the stream sediments exhibited anomalous contents of Hg, Bi, Sb, Cu, Pb, Zn, Ba and Cd. The Veta Vargas deposit near the village of Pozo Azul, exploited in the past (Alán and Castillo 1983; Mora 1984), is part of area IX. However, it is notable that the highest contents of gold identified during the heavy mineral survey sur-

Tab. 1 Positive correlations of elements detected in stream sediments and in the below-screen fraction of heavy mineral concentrates (HMC)

Stream sediments (296 samples)

Undersize fraction of BSF (237 samples)

Au, As, Ag, Sb, Tl, Cd, Hg, W Au, As, Ag, Sb, Pb, Hg

U, Th, Bi, Se, Ga Zn, Fe, Ga, Sc

Ni, Co, Cr Ni, Co, Cr

Mo, V, Se Se, Bi, S

La, U, Th, Al, Ti La, U, Th, P


88

Tab.

2 C

orre

latio

n co

effic

ient

s of

ele

men

ts e

ssen

tial f

or m

iner

al p

rosp

ectin

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rived

from

ana

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s of

the

belo

w-s

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n fr

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n of

hea

vy m

iner

al c

once

ntra

tes

As

Ag

SbH

gT

lC

dM

oC

uPb

Zn

W

Au

0.51

0 0.

892

0.47

4 0.

398

0.43

7 0.

063

0.42

1 0.

100

0.50

7 −0

.022

0.

405

(0.4

09, 0

.598

) (0

.862

, 0.9

15)

(0.3

69, 0

.567

) (0

.286

, 0.5

00)

(0.3

28, 0

.534

) (−

0.06

4, 0

.189

)(0

.310

, 0.5

20)

(−0.

027,

0.2

25)

(0.4

06, 0

.596

) (−

0.14

9, 0

.106

) (0

.293

, 0.5

06)

As

10.

572

0.92

0 0.

181

0.90

4 0.

098

0.89

5 0.

151

0.91

5 0.

231

0.85

0

1(0

.480

, 0.6

52)

(0.8

98, 0

.938

) (0

.055

, 0.3

01)

(0.8

78, 0

.925

)(−

0.03

0, 0

.222

)(0

.867

, 0.9

18)

(0.0

24, 0

.273

)(0

.892

, 0.9

34)

(0.1

07, 0

.348

) (0

.811

, 0.8

82)

Ag

0.57

2 1

0.54

2 0.

281

0.52

1 0.

055

0.49

3 0.

070

0.56

3 −0

.041

0.

495

(0.4

80, 0

.652

) 1

(0.4

46, 0

.626

) (0

.159

, 0.3

94)

(0.4

22, 0

.608

) (−

0.07

3, 0

.181

)(0

.390

, 0.5

83)

(−0.

058,

0.1

95)

(0.4

69, 0

.644

) (−

0.16

8, 0

.086

) (0

.392

, 0.5

85)

Sb0.

920

0.54

2 1

0.05

50.

991

0.11

00.

972

0.10

30.

983

0.34

2 0.

933

(0.8

98, 0

.938

) (0

.446

, 0.6

26)

1(−

0.07

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90

prisingly occur west and northwest of the known deposit, not in its surroundings.

4.4.3. Area X (Santa rosa)

The area X is located on the Juntas map sheet, comprising nine source areas and covering c. 20 km2. Its northeast-ern edge borders on granitoids of the Guacimal Pluton. Basalts and andesites of the Aguacate Group, accompa-nied by silicification, cover the investigated area. Heavy mineral concentrates from six catchments contained one to six particles of gold per sample. The undersize frac-tion was characterized by enhanced contents of Au, As and Sb; locally were also found Cu, Pb, Zn and Hg. The small and poorly documented ore district of Santa Rosa (Alán 1981) occurs in this area. Possibly only one vein was exploited there but the anomalous area following the contact aureole with the Guacimal Pluton is much larger.

4.4.4. Area VII (Agua Agria – río Jesús)

This area lies in the SE sector of the Miramar map sheet. The anomaly covering 20 km2 consists of five catchments, of which three were found to be positive or promising for the occurrence of gold. Greatly enhanced contents of As, Au, Sb and Ag were detected particularly in the stream sediments. The geological structure is varied, the pre-dominant basalt and andesite lava flows and pyroclastics of the Aguacate Group were penetrated by several small rhyolite intrusions. Numerous NE–SW fault zones form horst structures in local volcanites. Abundant alteration zones characterized by pyritization and silicification can be observed with anomalies of the above-mentioned elements. No historical mine is known in the region but the abandoned mines Magallanes, Quarenta Leones and San Gerardo are located to the NW of the surveyed area.

5. Mineralization

5.1. Geological background – veins, structures and mineralization

Epithermal gold deposits exploited in the past are con-centrated in a NW–SE trending zone called the Golden Belt or Cinturón de Oro (Amos and Rogers 1983; Schulz et al. 1987), almost 100 km long. This zone is located in the SW foothills of the Cordillera de Tilarán and also topographically belongs to the Montes del Aguacate and Serranías de Abangares subregions.

Epithermal gold-bearing mineralization corresponding to the SADO type (Mosier and Sato 1986) occurs exclu-sively in the Aguacate Group. The veins are developed

in fault and fracture zones, are steeply inclined and of varying strikes. They often suffered from brecciation and remobilization. The principal tectonic zones run mostly NW–SE, but the direction most of the veins indicates that the major structures running NE–SW, N–S to NNW–SSE were affected by local extension tectonics rather than formed in a homogenous strain field. Even though the stockwork with two main veins (each of the two as much as 5 m thick) in the Veta Vargas (Juntas) deposit follow the prevailing strike fluctuating between NE–SW to NW–SE, the two veins within the stockwork strike E–W. Although the main stress field linked with subduction gave rise to significant NW–SE faults following the main Cordillera, the quartz veins seem to originate in an en echelon system of Riedel shears striking N–S to NE–SW. It is noteworthy that this system does not propagate into the younger Monteverde Formation.

Gold is bound in quartz veins and stockworks of varying thickness ranging from a few centimetres up to 7 m (La Unión, Los Angeles vein, Abangares – Tres Her-manos, Recio, Sierra Alta, Veta Vargas, Buena Suerte and Mina Chassoul; Figs 5–6). The greatest gold enrichment in quartz veins can be observed in places of intersection of the main NE–SW striking faults and associated N–S to NW–SE shear zones. Gold anomalies also occur in host rocks altered by hydrothermal fluids immediately adjacent to the quartz veins.

Gangue minerals in gold-bearing veins as well as in base-metal veins are mostly represented by milky brec-ciated quartz. While chlorite and sericite are also com-mon, barite, calcite, adularia and ankerite are much less abundant.

Three main generations of quartz suggest a multi-stage hydrothermal process. The first includes fine-grained massive quartz of gray-white, gray and gray-black colour exhibiting greasy to glassy lustre. This type of quartz forms several metres-thick veins, which are the richest in gold. The Au contents commonly range in tens ppm and exceptionally even exceed 100 ppm (Tab. 3). The second generation is represented by coarse prismatic quartz, of-ten forming druses filled with crystals max. 1 cm in size. This quartz type is frequently accompanied by base-metal sulphides exhibiting increased gold contents. The third generation represents the final stage of the hydrothermal ore-forming process, producing fine-grained snow-white quartz of sugary appearance. This generation occurs in the form of thin (X–X0 cm thick) veins and veinlets, which are gold-bearing, but the Au contents do not ex-ceed 10 ppm (e.g. Abangares region, Sierra Alta mine – 0.5–6.8 g/t Au or SE part of the Miramar map sheet, Moncada area – 0.3–5.2 g/t Au; see Tab. 3).

The mineral assemblage is simple. The most abundant is pyrite, which is also a common admixture in altered volcanites, but without any relationship to gold minerali-


91

zation. Gold is always accompanied by pyrite, which is often rich in arsenic. Other sulphides include common ar-senopyrite and marcasite, whereas chalcopyrite, sphaler-ite and galena are minor to scarce. Anglesite, cerussite,

acanthite, pyrargyrite, greenockite (primary), covellite, bornite and cassiterite, mostly in trace amounts, were also identified. Although cinnabar was identified in several heavy mineral concentrates, this mineral was not found in the samples of primary ores. Of earlier investigations in the Bellavista ore district, Shawe (1910) described proustite, pyrargyrite, miargyrite and freibergite, while Roberts and Irving (1957) identified realgar and stibnite in the Montezuma Mine. USGS et al. (1987) reported stibnite from the El Recio and Tres Hermanos deposits.

The Cu–Pb–Zn sulphides, which occasionally form massive vein and nest-like ores (Tab. 3), prevail in the Guaria–Guacimal district and mina Moncada. The gold content in this type of mineralization is low or negligible.

The gangue and ore textures are often comb-like, banded or crustiform, whereas colloform textures are rare. Many features like brecciation or fracture fillings suggest that multistage ore-forming processes took place. Strongly altered and pyritized relics of initial andesite, forming lenses in the fault zone filled with mylonite, can also be observed (e.g., at Mina Chassoul adit or Veta Cajeta – Fig. 6).

5.2. Methods

The electron microprobe study was performed on (i) samples rich in a variety of sulphides identified by ore microscopy and (ii) samples which showed high concen-trations of gold and other metallic elements established by chemical analyses. The electron microprobe analyses (EPMA) were performed using the Cameca SX-100 elec-tron microprobe (Joint Laboratory of Masaryk University and CGS, Brno, R. Škoda, analyst) in the WDS mode. The minerals were analyzed with beam diameter of 1–4 µm at 25 kV accelerating voltage and 10–20 nA beam current. The following standards were used: Ag, Sb, Au, Bi, Cu, Co, Mn – native elements, S – chalcopyrite, Pb – PbS, Hg – HgTe, Fe – FeS2, Ni, As – pararammelsber-gite, Se – PbSe, Zn – ZnS, Cl – PbCl2. Some minerals (quartz, calcite, K-feldspar, barite, cassiterite and prob-able bornite) were not analyzed but only identified in the energy-dispersive mode (EDX) carried with PGT prism 2000 with LN2 cooled Si : Li detector.

Microthermometric analyses of fluid inclusions en-trapped in quartz and sphalerite were undertaken on doubly polished (200–300 μm thick) sections. The Chaix-meca apparatus (Poty et al. 1976) at the CGS, Prague was calibrated in the range of –100 °C and +400 °C using Merck chemical standards, the melting point of distilled water, and phase transitions in natural pure CO2 inclu-sions.

The homogenization temperature (Th), temperature of the first melting (Te) and of the final melting of ice (Tm) were routinely measured (Roedder 1984). The salinity of

Fig. 5 An old stope in the Boston vein from which sections rich in gold are still occasionally illegally extracted by local miners. Sierra Alta ore district on the Juntas map sheet. Photo by P. Mixa.

Fig. 6 Mina Chassoul – strongly tectonized and altered zone filled with mylonite clay, brecciated andesite and limonitized rod-like lenses of quartz. Quartz gangue with pyrite and gold contains over 100 g/t Au. Photo by P. Mixa.


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the fluids was calculated according to Bodnar and Vityk (1995) and the salt composition determined following Borisenko (1977). The reproducibility of the Th values was ± 3 °C, of Tm values ± 0.2 °C.

The powder XRD data of altered volcanics were acquired by the Philips X’Pert System difractometer; secondary monochromator producing CuKα, radiation 40 kV/40 mA, graphite monochromator, angle interval 3–65° 2α, step 0.05 2α and 3 s exposure/step were used. The XRD record of some samples was made in their natural state after saturation with ethylene glycol.

5.3. Ore mineralogy

Mineralogical samples from three main ore veins in the Juntas District were studied in detail: Recio, Tres Herma-nos and Sierra Alta – San Martín veins. Three samples from the much smaller and less important ore district of Guacimal were also investigated. They were found on the dump of an abandoned adit of the Guacimal mine (Tab. 4).

Pyrite is, in general, the most abundant sulphide in the studied samples. The mineral in sample TH 5b exhibits concentric oscillatory zoning with zones that contain up to 2.2 wt. % As that are lighter in the BSE image (see Fig. 7a). Sample SAP 1 contains older pyrite poor in As, which is overgrown by younger pyrite rich in As (1.8–8.8

wt %, Fig. 7b). Pyrite from the Guacimal district is poor in As, whereas some pyrites have enhanced contents of Cu (up to 0.25 wt. %). The concentrations of Ni and Co were mostly below their detection limits (Tab. 5).

Gold (virtually electrum) was identified in two polished sections of gangue from the Tres Hermanos (TH 5b) and Recio (Qtz 4b) veins. Gold forms anhedral, mostly rounded inclusions in As-bearing pyrite but it is also found in quartz. Gold inclusions do not exceed 25 µm (Fig. 7a); however, in some samples, regardless of their high Au content, no native gold was found in polished sections, most likely due to the small size (be-low 1 µm).

The chemistry of gold was found to vary only slightly, with major Ag and traces of Cu.

Other elements like Bi, Sb, Ni, Mn and Hg were also analysed but not detected (Tab. 9). The empirical formu-lae are Au0.43–0.45Ag0.54–0.57Cu0.00–0.01 (TH5b) and Au0.47–0.51Ag0.49–0.53 (Qtz4b). The gold from the Juntas ore district corresponds to electrum with Ag content mostly slightly exceeding that of Au: Au0.43–0.51Ag0.47–0.57Cu0.00–0.01.

Sphalerite is generally minor or accessory in the studied samples, but was found to be abundant in the Guacimal district, whereas it is rare in the Juntas district. It mostly occurs as anhedral grains and ag-gregates, but tiny inclusions of sphalerite in pyrite can

Tab. 3 Contents of selected elements in ore samples from the investigated localities

Locality Sample Quartz generation Au As Ag Sb Hg Tl Mo Cd Cu Pb Zn Ba S

ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm ppm wt. %

El Recio REC 5 II 1.372 116.9 19.3 9.9 0.04 0.1 107.5 44.3 377.1 2 925.7 7 289 24 3.88

El Recio REC P2 I 7.885 40.9 18.8 11.5 0.10 0.1 185 0.2 3.9 6.8 4 15 0.05

Sierra Alta SAN 4 I 1.466 98.6 1.6 2.2 0.02 0.2 27.9 0.1 35.3 11.0 42 68 0.71

Sierra Alta SAN 6B I 7.826 1 225.2 1.6 39.9 2.14 3.2 425.5 1.3 57.2 61.8 57 62 2.28

Sierra Alta SAP 1 III 65.083 611.8 48.1 54.8 0.29 0.6 194.8 5.9 95.1 497.7 177 282 0.42

Sierra Alta SAP 2 III 5.869 42.8 4.2 10.6 0.08 0.1 68.4 0.1 8.2 14.9 13 29 0.05

Tres Hermanos TH 5a I 5.781 104.2 4.6 40.1 0.19 0.5 77.0 0.1 8.9 22.2 16 439 0.39

Tres Hermanos TH 5b I 61.113 25.1 57.3 9.8 0.75 0.2 83.1 1.2 224.2 41.0 77 293 0.26

Mina Moncada L 2 II 4.900 421.5 20.9 8.8 0.24 <0.1 3.0 207.5 6 980.5 >10000 >10000 16 >10.00

Mina Moncada L 4F II 0.587 73.0 7.6 1.2 0.04 <0.1 <0.1 131.4 2 183.5 459.3 >10000 5 5.96

Buena Suerte L 9 I 0.210 361.4 <0.1 11.7 2.55 0.4 0.5 <0.1 30.6 5.5 1 66 <0.05

Mina Macacona L 17 B III 3.980 429.4 1.0 19.8 3.78 <0.1 0.1 <0.1 10.7 24.8 9 1 781 0.05

Mina Macacona L 17 C III 1.119 325.6 0.2 11.6 3.20 <0.1 0.1 <0.1 8.8 13.9 11 3 499 <0.05

Mina Macacona L 17 G III 5.265 406.0 1.4 35.6 6.30 <0.1 0.1 <0.1 26.6 42.0 22 1 632 <0.05

Mina Chassoul L 18 BA I 5.660 952.9 2.3 19.6 0.20 <0.1 3.6 0.3 31.1 36.9 56 66 <0.05

Mina Chassoul L 18 BB I >100.00 2 269.6 >100.0 36.6 0.33 <0.1 7.8 1.0 77.0 118.3 113 192 <0.05


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be euhedral or rounded (Fig. 7c). Its chemistry is rather simple, being poor in both Fe (0.74–2.21 wt. %) and Cd (0.45–1.04 wt. %), but a single analysis yielded even 3.34 wt. % Cd. The Mn contents range between 0.02 and 0.18 wt. %. Brown sphalerite from the Guacimal mine was slightly richer in Fe and Cd and poorer in Mn than the green variety from the same locality. Tiny inclusions of rare dark Fe-rich and Cd-poor sphalerite were found to contain 8.42 wt. % Fe and 0.22 wt. % Cd. This sphalerite grows on pyrite grains in sample Qtz 7. The EMPA of sphalerite are summarized in Tab. 6.

Galena is, in general, minor to rare or accessory but was found to be abundant in the Guacimal ore district, where it forms aggregates a few cm in size. This mineral in the Juntas district is also common but tiny, occurring

as euhedral to subhedral grains ~200 µm in size. It is often intimately intergrown with pyrite and sphalerite (Fig. 7c). Tiny blebs in pyrite are only a few µm across. Galena from the Juntas district (samples SAP 1, Qtz 7) was found to contain 0.18–0.85 wt. % Fe and varying concentrations of Se (0.0–0.84 wt. %). The contents of Bi, Ag, Sb, Hg and As are mostly below the respective detection limits. Only one tiny grain (sample Ju100c) of galena 100 µm in size was found to have higher contents of Bi (3.37 wt. %), Ag (1.63 wt. %) and Se (1.84 wt. %). The higher zinc contents (up to 1.18 wt. %) are ascribed to intergrowths of galena with sphalerite (Tab. 7)

Chalcopyrite is a common accessory mineral that is less abundant than sphalerite and galena. It occurs as tiny grains less than 1 mm in size and was found only in sam-

Tab. 4 Localization and mineral assemblages of the mineralogical samples

locality sample gangue ore minerals noticeRacio vein Qtz 3 Qtz, Cal, Py, Ccp, Cas Cas in inclusions in Py

Qtz 4 Qtz, Kfs Au, Py, Acn, Prg, Pst, Ag–Sb–Au min bonanza-like mineralizationTres Hermanos TH 5b Qtz Au, Py, Ccp, Bn Au in tiny inclusions Sierra Alta SAN 6b Qtz Py, Bn

SAP 1 Qtz Py, Sp, Gn, Grn, Ccp content of Au 65 ppmQtz 7 Qtz Py, Py, Gn, Sp, Fe-Sp, Mrc

Guacimal Gu 3 Qtz Ag-Py, Sp, Gn Qtz II dominantJu100b Qtz Py, Sp, Gn, Ccp, Ang Qtz II dominantJu100c Qtz Sp, Py, Gn, Ccp Qtz II dominant

Qtz – quartz, Cal – calcite, Kfs – K-feldspar, Py – pyrite, Ccp – chalcopyrite, Gn – galena, Apy – arsenopyrite, Sp – sphalerite, Cas – cassiterite, Acn – acanthite, Prg – pyrargirite, Pst – proustite, Ang – anglesite, Mrc – marcasite, Grn – greenockite, Bn – bornite

Tab. 5 Chemical compositions of pyrite and marcasite* (wt. % and apfu)

sample TH 5B TH 5B SAP1 SAP1 SAP1 SAP1 Ju100b Ju100b Ju100c Qtz7 Qtz7*Fe 46.48 45.94 45.46 42.43 44.11 44.15 45.90 45.78 46.22 45.80 45.28Co 0.00 0.00 0.08 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Ni 0.00 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Cu 0.00 0.04 0.08 0.25 0.15 0.18 0.00 0.00 0.15 0.00 0.03Zn 0.01 0.01 0.00 0.01 0.97 0.02 0.00 0.00 0.00 0.02 0.00Cd 0.04 0.00 0.01 0.05 0.06 0.00 0.04 0.02 0.04 0.04 0.02As 0.00 2.20 0.16 8.81 0.21 1.75 0.00 0.44 0.00 0.00 0.00S 53.99 52.01 53.83 47.43 53.75 52.13 53.77 53.32 54.08 53.64 53.47Se 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 100.51 100.22 99.62 98.97 99.24 98.22 99.73 99.56 100.50 99.50 98.79

Sb 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Fe 0.992 0.997 0.978 0.965 0.953 0.971 0.986 0.988 0.986 0.986 0.981Co 0.000 0.000 0.002 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Ni 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Cu 0.000 0.001 0.002 0.005 0.003 0.003 0.000 0.000 0.003 0.000 0.001As 0.000 0.036 0.003 0.149 0.003 0.029 0.000 0.007 0.000 0.000 0.000S 2.007 1.966 2.017 1.880 2.022 1.997 2.013 2.005 2.010 2.013 2.018Se 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Total 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000 3.000

calculated on the basis of 3 atoms per formula unit


94

Py

Py

Py

Py

Py

Py

Py

Py-I

Py-IPy-I

PyI

Py-II

Py-II

Py-II

PyII

Aspy

Gal

Gal

Gn

Gn

Gn

Sph

Sph

Sph

Sp

Sp

Sp

Sp

Grn

Grn

Grn

Ang

Ccp

Ccp

Mrc

Mrc

Mrc

Au

Au

Au

Au

Au

Mrc

Apy

Aspy

Apy

m

m

m

m

m

m

m100

100

100

50

100

100

200

a b

c d

e f

gFig. 7 Back-scattered electron (BSE) photomicrographs of ore minerals from the Juntas district. a – Gold inclusions (Au) in zoned As-bearing pyrite (Py). Lighter zones in pyrite contain up to 2.2 wt. % As. Small aggregates of anhedral chalco-pyrite (Ccp) also occur in the rim of pyrite and elsewhere in the matrix. Sample Th 5b, Tres Hermanos vein. b – Subhedral pyrite I (Py-I) is overgrown by a thin irregular rim of As-bearing pyrite II (Py-II). Other minerals are galena (Gn) and sphalerite (Sp). Sample SAP 1, Sierra Alta. c –Subhedral pyrite (Py) grain with inclusions of sphalerite (Sp) and galena (Gn) in quartz (black). Sample Qtz 7, Si-erra Alta. d – Pyrite (Py) rimmed by skeletal arsenopyrite (Apy). The same sam-ple. e – Cluster of prismatic crystals of probable marcasite (Mrc). Tiny arseno-pyrite (Apy) grains occur nearby. The same sample. f – Mode of occurrence of primary greenockite (Grn) in the sample Sap1. Greenockite forms intimate inter-growths with galena (Gn) and also subhedral grains enclosed in pyrite (Py) part-ly transformed for marcasite (Mrc). Other minerals on the picture are sphalerite (Sp) and tiny anglesite (Ang). Sample SAP 1, Sierra Alta. g – Aggregates of un-named Ag–Sb–Au sulphide (of acanthite affiliation) grown in the quartz. Sample Qtz 4b, Recio vein. All BSE photomicrographs by R. Škoda.


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ples from the Guacimal mine. Its chemistry corresponds to a pure formula with no detectable admixtures (Tab. 8).

Arsenopyrite is much less abundant than pyrite, mostly forming skeletal build-ups on pyrite or isolated subhedral to euhedral grains or aggregates up to 100 µm in size enclosed in quartz gangue (Fig. 7d). The mineral was identified in samples from Sierra Alta (Qtz 7) ex-hibiting slightly higher Cu content (0.18 wt. %, Tab. 8).

Marcasite is rare, identified in samples Qtz 7 and SAP 1 from Sierra Alta (Alto Bochinche Hill) as clus-ters of prismatic or irregular crystals max. 0.3 mm long embedded in quartz, mostly isolated from other sulphides (Fig. 7e). Textures found in sample SAP1 indicate that arsenopyrite sporadically replaced marcasite (Fig. 7f). Its chemistry is simple (Tab. 5).

Acanthite is rare but was found in sample Qtz 4b as relatively abundant anhedral aggregates 10–30 µm in size, closely associated with pyrite. Its chemistry is close to the ideal formula (Ag1.84Fe0.06As0.01)(S1.04Se0.05) with slightly enhanced contents of Fe (1.31 wt. %), As (0.39 wt. %) and Se (1.57 wt. %) (Tab. 7).

Pyrargyrite (Ag3SbS3) or its monoclinic polymorph pyrostilpnite is a very rare mineral in the studied samples, identified only in sample Qtz 4b as anhedral aggregates 30–50 µm in size associated with acanthite and Ag–Sb–Au sulphide. It exhibits higher contents of As (0.41 wt. %) and Se (0.52 wt. %, Tab. 7). Its formula corre-sponds to Ag3.00Sb0.99(S2.95Se0.04).

Covellite is again a very rare mineral that was actu-ally identified only in sample Ju100b from the Guacimal mine. It occurs as a single grain 50 µm across embedded in quartz. It exhibits a higher content of Ag (0.93 wt. %) and its formula, based on EMPA, is Cu1.02Ag0.08S0.97 (Tab. 8).

Bornite (possible) was found as an isolated single grain in sample SAN 6b (Recio vein) at a contact of banded wall rock consisting of fine-grained silicified mylonite with quartz gangue containing randomly dis-seminated tiny blebs of pyrite. The grain of supposed bornite is anhedral to subhedral and 50 µm in size, with an inclusion of pyrite (10 µm) in its centre.

Greenockite (or cubic polymorph of CdS hawleite) is rare, being associated with other sulphides in sample SAP1 from Sierra Alta. The mineral occurs in graphic intergrowths with galena, forming an aggregate 70 × 30 µm in size, or occurs as subhedral grains enclosed in pyrite–marcasite aggregate. The structural criteria clearly indicate that greenockite is a primary ore mineral. Other minerals associated with greenockite include sphalerite and anglesite (Tab. 6, Fig. 7f).

Cassiterite was found in sample Qtz 3 from the Recio vein as a single anhedral inclusion 15 × 8 µm in size, identified using the EDX mode.

Ag–Sb–Au sulphide, most likely a new mineral spe-cies, was found in the Recio vein (sample Qtz4) as a few anhedral corroded aggregates up to 70 × 25 µm in

Tab. 6 Chemical compositions of sphalerite (Sp) and greenockite (Grn) (wt. % and apfu)

mineral Sp Sp Sp Sp Sp brown Sp brown Sp green Sp green Sp Grn Grnsample SAP1 SAP1 Qtz7 Qtz7 Ju100b Ju100b Ju100b Ju100c Ju100c SAP1 SAP1comment Fe-rich centre rim centre rimFe 2.01 0.85 1.60 8.42 1.68 1.21 1.55 0.74 0.78 2.06 0.42Co 0.00 0.00 0.00 0.00 0.00 0.02 0.03 0.02 0.02 0.00 0.00Cu 0.07 0.00 0.04 0.00 0.00 0.09 0.02 0.00 0.00 0.25 0.06Mn 0.05 0.02 0.05 0.10 0.10 0.05 0.08 0.18 0.13 0.00 0.00Zn 63.89 64.73 61.00 56.72 64.70 63.97 63.88 65.02 65.10 0.99 1.08Cd 0.94 1.04 3.34 0.22 0.72 0.74 0.84 0.45 0.46 72.28 75.04As 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.53 0.18S 32.78 32.80 33.01 34.01 33.37 33.60 33.49 33.82 33.92 22.74 22.47Total 99.73 99.43 99.03 99.48 100.56 99.67 99.89 100.23 100.40 98.85 99.25Fe 0.035 0.015 0.028 0.145 0.029 0.021 0.027 0.013 0.013 0.052 0.011Co 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000Cu 0.001 0.000 0.001 0.000 0.000 0.001 0.000 0.000 0.000 0.006 0.001Mn 0.001 0.000 0.001 0.002 0.002 0.001 0.001 0.003 0.002 0.000 0.000Zn 0.955 0.972 0.923 0.833 0.957 0.951 0.949 0.961 0.960 0.021 0.024Cd 0.008 0.009 0.029 0.002 0.006 0.006 0.007 0.004 0.004 0.902 0.957As 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.010 0.003S 0.999 1.004 1.018 1.019 1.006 1.019 1.015 1.019 1.020 0.995 1.004Total 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000

Ni, In, Se were also analysed but not detected; calculated on the basis of 2 atoms per formula unit


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Tab. 7 Chemical compositions of galena (Gn), acanthite (Acn) and pyrargyrite (wt. % and apfu)

mineral Gn Gn Gn Gn Gn Gn Acn *pyrargyritesample SAP1 SAP1 SAP1 SAP1 Ju100c Qtz7 Qtz4b Qtz4bPb 86.69 86.11 85.65 85.57 78.70 85.97 0.00 0.00Bi 0.00 0.00 0.00 0.00 3.37 0.00 0.00 0.00Ag 0.00 0.00 0.00 0.00 1.63 0.00 82.14 58.68Sb 0.00 0.00 0.00 0.00 0.00 0.00 0.00 21.81Fe 0.18 0.34 0.66 0.85 0.00 0.27 1.31 0.00Zn 0.03 0.50 0.00 0.00 1.18 0.38 0.00 0.03Cd 0.01 0.05 0.00 0.00 0.00 0.00 0.00 0.00Hg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00As 0.00 0.20 0.00 0.00 0.00 0.00 0.39 0.42S 13.51 13.56 13.44 13.21 12.93 13.49 13.84 17.11Se 0.00 0.00 0.69 0.84 1.84 0.00 1.57 0.61Total 100.42 100.76 100.44 100.46 99.65 100.12 99.24 98.65Pb 0.992 0.970 0.969 0.971 0.888 0.980 0.000 0.000Bi 0.000 0.000 0.000 0.000 0.038 0.000 0.000 0.000Ag 0.000 0.000 0.000 0.000 0.035 0.000 1.839 2.997Sb 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.987Fe 0.008 0.014 0.028 0.036 0.000 0.012 0.057 0.000Zn 0.001 0.018 0.000 0.000 0.042 0.014 0.000 0.003Cd 0.000 0.001 0.000 0.000 0.000 0.000 0.000 0.000Hg 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000As 0.000 0.006 0.000 0.000 0.000 0.000 0.012 0.031S 0.999 0.988 0.983 0.968 0.943 0.994 1.043 2.941Se 0.000 0.000 0.020 0.025 0.054 0.000 0.048 0.042Total 2.000 2.000 2.000 2.000 2.000 2.000 2.999 7.000

*or pyrostilpnite

Tab. 8 Chemical composition of chalcopyrite (Ccp), covellite (Cov) and arsenopyrite (Apy) (wt. % and apfu)

mineral Ccp Ccp Ccp Cov Apy Apy Apysample TH5B Ju100b Ju100c Ju100b Qtz7 Qtz7 Qtz7Pb 0.00 0.07 0.00 0.00 n.a. n.a. n.a.Ag 0.00 0.00 0.00 0.93 0.00 0.00 0.00Fe 28.97 30.17 30.15 0.00 34.94 33.27 33.70Co 0.00 0.00 0.00 0.00 0.00 0.02 0.00Ni 0.00 0.03 0.00 0.00 0.00 0.02 0.00Cu 36.05 34.27 34.41 66.18 0.10 0.18 0.08Zn 0.02 0.32 0.03 0.04 0.01 0.01 0.00Cd 0.00 0.00 0.06 0.00 0.03 0.02 0.02As 0.00 0.00 0.00 0.00 40.52 43.87 44.57S 35.20 34.96 34.86 31.84 23.90 21.91 22.04Total 100.24 99.83 99.52 98.98 99.49 99.28 100.42Pb 0.000 0.001 0.000 0.000 – – –Ag 0.000 0.000 0.000 0.008 0.000 0.000 0.000Fe 0.950 0.993 0.995 0.000 0.981 0.956 0.959Co 0.000 0.000 0.000 0.000 0.000 0.000 0.000Ni 0.000 0.001 0.000 0.000 0.000 0.000 0.000Cu 1.039 0.992 0.998 1.019 0.002 0.005 0.002Zn 0.000 0.009 0.001 0.001 0.000 0.000 0.000Cd 0.000 0.000 0.001 0.000 0.000 0.000 0.000As 0.000 0.000 0.000 0.000 0.848 0.940 0.946S 2.010 2.005 2.004 0.972 1.169 1.097 1.093Total 4.000 4.000 4.000 2.000 3.000 3.000 3.000

Bi, Sb, Mn and Se were also analysed but not detectedNumbers of atoms calculated on the basis of a total of 4 for chalcopyrite, 2 for covellite and 3 for arsenopyrite


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size, enclosed in quartz (Fig. 7g). The mineral appears to be homogeneous and the microprobe analyses yielded 72.52–73.33 wt. % Ag, 3.46–3.64 wt. % Au, 6.22–8.05 wt. % Sb, 1.10–2.13 wt. % As, 12.76–13.46 wt. % S and 1.69–1.79 wt. % Se. The Ag/Au ratios in the empirical formula range between 39 and 40. Alternative formulae derived from microprobe analyses correspond to

Ag38.9–40.0Au1.03–1.07Sb2.99–3.83As0.85–1.67S23.15–23.95Se1.25–1.31 (total 70 atoms) or

Ag1.67–1.71Au0.04–0.05Sb0.13–0.16As0.04–0.07S1.00–1.04Se0.05–0.06 (total 3 atoms),

which are close to the formula of acanthite, Ag2S (Tab. 10). The most likely formula is: (Ag39Au1)40(SbAs)5(S24Se1)25 or Ag1.7Au0.05Sb0.15As0.05S1.0Se0.05.

Anglesite is rare and was only found in samples from Sierra Alta (SAP 1) and Guacimal (Ju100b), forming sub-hedral to euhedral grains max. 60 µm across, associated with pyrite and galena. It seems to form pseudomorphs after galena.

Barite is rare and was found as inclusions up to 100 µm in sample SAP 1 (Sierra Alta) and 10 µm in TH 5b (Tres Hermanos vein).

5.4. Fluid inclusion study

Fluid inclusions of the H2O type were found in vari-ous generations of quartz at several gold occurrences, Moncada base-metal deposit and also in sphalerite from the Guacimal mine. Fluid inclusions with a gaseous phase, such as CO2 or CH4, were not found. The fluid inclusions show different patterns in individual genera-tions of quartz and also in sphalerite, but are more or less uniform at any given locality.

Quartz I is found in numerous crystal forms and habits. It is colloform or comb-like or occurs as small euhedral crystals rimming pieces of rock (breccia-type texture) and passes into coarse-grained quartz, in which the growth

zones are filled with dark inclusions forming chevron-like textures (Fig. 8c–d). Drusy quartz can also be observed in the investigated samples. Dark-coloured “empty voids” were often seen in the intergranular space of the quartz crystals (Fig. 9a). Inclusions in quartz I are of very ir-regular shape, less than 20 µm in diameter, and exhibit very variable liquid/vapour ratios (LVR). Varying LVR are believed to reflect a long maturation of fluid inclusion at relatively lower temperatures and nucleation of the vapour phase (Bodnar et al. 1985) rather than be due to the boil-ing of the fluid. Measurable primary and pseudosecond-ary inclusions occur mostly in the apical parts of small euhedral quartz crystals. These inclusions are of oval to irregular shape, from 5 to 40 µm in diameter, and show variable LVR ranging from 0.5 to 0.95. Liquid-only or vapour-only inclusions were also observed in some of the studied samples. Due to the variable LVR homogenization temperatures (Th), the inclusions in clusters with LVR of

Tab. 9 Chemical compositions of gold (electrum) (wt. % and apfu)

sample TH 5B TH 5B TH 5B TH 5B TH 5B Qtz4b Qtz4b Qtz4b

Ag 41.32 42.15 39.30 40.04 39.61 34.44 37.62 37.86

Au 59.61 57.87 60.16 59.59 60.27 66.00 61.14 61.48

Cu 0.38 0.20 0.00 0.05 0.00 0.00 0.00 0.00

Total 101.31 100.22 99.47 99.67 99.88 100.45 98.75 99.34

Ag 0.554 0.568 0.544 0.550 0.545 0.488 0.529 0.529

Au 0.438 0.427 0.456 0.449 0.455 0.512 0.471 0.471

Cu 0.009 0.005 0.000 0.001 0.000 0.000 0.000 0.000

Total 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000

Bi, Sb, Ni, Mn, Hg were also analysed but not detected; calculated on the basis of 1 atom per formula unit

Tab. 10 Chemical composition of Ag–Sb–Au–S phase from the sample Qtz4b (wt. % and apfu).

Ag 73.33 72.84 72.52Au 3.46 3.65 3.64Sb 6.22 7.08 8.05Ni 0.00 0.02 0.02As 2.13 1.69 1.10S 12.76 13.46 13.24Se 1.69 1.72 1.79Total 99.59 100.45 100.34Ag 39.781 38.860 38.981 1.705 1.665 1.671Au 1.028 1.066 1.070 0.044 0.046 0.046Sb 2.988 3.347 3.833 0.128 0.143 0.164Ni 0.000 0.022 0.018 0.000 0.001 0.001As 1.666 1.301 0.848 0.071 0.056 0.036S 23.286 24.153 23.936 0.998 1.035 1.026Se 1.251 1.252 1.314 0.054 0.054 0.056Total 70.000 70.000 70.000 3.000 3.000 3.000

Pb, Bi, Fe, Co, Mn, Zn, Hg were also analysed but not detectednumbers of atoms calculated assuming 70 and 3 atoms per formula unit


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0.9 were measured, and the values of Th fluctuated from 156 to 248 °C (Tab. 11, Figs 10, 12). The salinity of an aqueous solution varied from 0.2 to 2.9 wt. % NaCl equiv. (Figs 11–12). NaCl is assumed to be the major compound of aqueous solutions (Te = –22.1 °C).

Coarse-grained zoned prismatic quartz II (Figs. 8d) from gold deposits contains several types of fluid in-clusions. Some quartz crystals enclose primary fibrous inclusions of very dark colour up to a few hundreds of µm long. It was difficult to obtain any reasonable fluid inclusion parameters of these. The growth zones are also characterized by primary and pseudosecondary H2O inclusions. The inclusions are of negative crystal to oval shape, from 5 to 80 µm in diameter, and mostly with consistent LVR varying from 0.7 to 0.8. Rare accidental solids can be found in these inclusions. The temperatures of homogenization (Th) range between 182 and 288 °C, and the salinity of an aqueous solution is very low, not exceeding 2.1 wt. % NaCl equiv. The eutectic tempera-ture (Te = –23.4 to –31.0 °C) indicates an H2O–NaCl

type of solutions with possible small admixture of K ± Fe ± Mg chlorides.

Fine-grained euhedral crystals of quartz III rim the coarse-grained quartz II (Fig. 8d). The primary and pseudosecondary inclusions of H2O type are of oval to irregular shape, from 5 to 50 µm in diameter, and show variable LVR (0.5–0.95). Liquid-only or vapour-only inclusions were also found in the investigated samples. Due to the variable LVR homogenization temperatures (Th) the inclusions in clusters with LVR = 0.80 to 0.95 were measured. The obtained Th values range from 146 to 248 °C, and the salinity of an aqueous solution from 0.2 to 2.7 wt. % NaCl equiv.

Round to irregular grains of sphalerite from the Guaci-mal gold deposit are enclosed in quartz I. Secondary H2O inclusions (Fig. 9f) with consistent LVR (0.7–0.8) were observed along healed microfractures in sphalerite. The Th values of these inclusions range from 214 to 282 °C. The salinity of an aqueous solution is low (0.7–3.1 wt. % NaCl equiv.), but slightly higher than that in inclusions

Fig. 8 Scans of selected doubly polished sections used for fluid inclusion study. a – Breccia-type structure of quartz I, sample TH 5b; b – Col-loform and fine-grained quartz I which overgrows drusy coarse-grained chevron quartz II, sample REC 6; c – Fine-grained quartz I on rock pie-ces overgrowths to chevron quartz, sample BPA 2; d Prismatic quartz II which is coated by later fine-grained crystals of quartz III, sample BV 1. Photos by Petr Dobeš.

dc

ba

1 cm 1 cm

1 cm1 cm


99

a b

c d

e f

Fig. 9 Photographs of fluid inclusions in various quartz types and sphalerite. a – Dark-coloured “empty voids” in the intergranular spaces, samp-le REC 6; b – Primary H2O inclusions with variable LVR in quartz I, sample REC 2; c – Growth zones decorated by primary fluid inclusions vs. micro cracks and trails of secondary fluid inclusions in quartz II, sample BV 1; d – Primary H2O inclusions with consistent LVR in quartz II, sam-ple BV 1; e – Primary H2O inclusions with variable LVR along growth zones in quartz III, sample BV 1; f – Secondary vapour-rich H2O inclusi-ons in sphalerite, sample Gu 7. Photos by Petr Dobeš.


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trapped in quartz. The homogenization temperatures of secondary inclusions correspond to those of the primary inclusions measured in quartz II.

Coarse-grained prismatic quartz and quartz aggregates from the Moncada base-metal mine were also found to contain H2O-type inclusions. Primary H2O inclusions occur in the growth zones of quartz. They are of oval shape, up to 80 µm in diameter, and exhibit persistent LVR around 0.7. The Th values range between 248 and 278 °C, and the salinity of an aqueous solution is slightly higher, ranging from 0.5 to 4.3 wt. % NaCl equiv. The eutectic tempera-tures (Te = –25.5 to –29.4 °C) indicate H2O–NaCl type of solution, with a small admixture of K ± Fe ± Mg chlorides.

5.5. Hydrothermal alteration

All the studied epithermal gold mineralizations are accom-panied by a variety of intense and characteristic hypogene alterations (Laguna 1983, 1984), which affect the wall

rocks to a distance of a few metres from the ore veins (Tres Hermanos c. 5–6 m, Sierra Alta as much as 10 m). Fragments of strongly altered volcanic rocks are often an integral and common part of quartz veins, due to multi-stage syn-mineralization brecciation and also build-up of quartz veins through hydraulic fracturing of volcanites. Besides, effects of regional-scale alterations with no direct relation to the ore mineralization, are also observed.

The main alteration processes included:1) Silicification, carbonatization – resulting in the forma-

tion of quartz veinlets and small stockworks sometimes without any mineralization. These are processes accom-panying the formation of gold-bearing quartz veins, and represent a useful tool in mineral exploration. The quartz usually occurs in several generations, of which the older are of gray-black colour, whereas the younger are light-coloured to white. Drusy quartz can be found in the central parts of the ore veins. Carbonates (calci-te, less abundant dolomite and rhodochrosite) are also

Tab. 11 Fluid inclusion data from various quartz types and sphalerite from the Au-bearing and base-metal veins of the Aguacate Group volcanic arc

Locality Sample Generation of quartz FIA Th (°C) Tm (°C) Salinity Te (°C)

(wt. % NaCl eq.)

Sierra Alta REC 1 Quartz II primary 212–268 –0.2 to –0.9 0.4–1.6 –24.5

REC 2 Quartz I primary 174–246 –0.1 to –0.9 0.2–1.6

REC 5 Quartz I primary 156–172 –0.1 to –0.5 0.2–0.9 –22.1

REC 6 Quartz I pseudosecondary 172–202 –0.1 to –0.7 0.2–1.2

REC P-2 Quartz I primary 188–210 –0.6 to –1.7 1.1–2.9

SAP 1 Quartz I pseudosecondary 174–224 –0.1 to –1.4 0.2–2.4

Tres Hermanos TH 5 Quartz I pseudosecondary 219–232 –0.2 to –0.7 0.4–1.2

TH 5b Quartz I pseudosecondary 182–240 –0.4 to –0.6 0.7–1.1

Veta Vargas BV 1 Quartz II (root) primary 218–250 –0.4 to –0.6 0.7–1.1

Quartz II (central part) primary 218–247 –0.1 to –0.6 0.2–1.1 –23.4

Quartz II (peripheral p.) primary 182–275 –0.2 to –0.4 0.4–0.7

Quartz III primary 192–248 –0.1 to –1.6 0.2–2.7

Quartz III pseudosecondary 146–178 –0.1 to –0.4 0.2–0.7 –22.7

BV 1B Quartz II primary 236–274 –0.1 to –0.7 0.2–1.2

Quartz II pseudosecondary 236–276 –0.1 to –0.3 0.2–0.5 –31.0

BV 1C Quartz II primary 254–288 –0.1 to –0.8 0.2–1.4

BV 4 Quartz I primary 192–248 –0.3 to –0.5 0.3–0.9

Veta Pozo BPA 2 Quartz II primary 246–276 –0.1 to –1.2 0.2–2.1 –24.2

Guacimal Gu 7 Sphalerite secondary 214–262 –0.4 to –1.8 0.7–3.1

Gu PL-1 Sphalerite secondary 262–282 –0.6 to –1.6 1.1–2.7

Juntas TM 40A Quartz II pseudosecondary 232–274 –0.2 to –0.6 0.4–1.1

TM 40B Quartz II primary 245–278 –0.2 to –1.0 0.4–1.7

Moncada 1 Quartz II primary 248–262 –0.3 to –0.6 0.5–1.1 –29.4

4B Quartz II primary 264–278 –1.4 to –2.6 2.4–4.3 –25.5


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common alteration zones on a regional scale. Carbona-tes form abundant tiny veinlets of millimetre dimensi-ons, little stockworks, but mostly occur as several-cen-timetres-thick schlieren in the andesites of the Agua-

cate Group. Although this type of alteration is not lin-ked to gold mineralization, it is still an important pro-specting tool to distinguish, on a regional scale, ande-sites of the Aguacate Group from those of the Monte-verde Formation, the latter of which are barren.

2) Pyritization – always accompanies gold mineralizati-on, in which pyrite occurs in quartz veins. However, massive pyrite ores or disseminated pyrite crystals in volcanic rocks also form separate extensive alteration zones, which show no relationship to gold mineraliza-tion, so that this kind of alteration has no straight for-ward use in prospecting for gold.

3) Propylitization – pervasive alteration characterized by the formation of chlorite, epidote, calcite, sericite, kao-linite, illite, beidellite and pyrite. It is the most com-mon type of alteration affecting volcanic rocks, giving them characteristic greenish to ochre colour. The most intense propylitization affected host volcanites up to a few metres away from the quartz veins with gold mineralization. The XRD analyses of propylitized samples from the historical stopes of the Recio, Tres Hermanos, Guaria–Guacimal and Sierra Alta mineral deposits (see Tab. 12) revealed that the original ande-sites were altered into mixtures consisting of quartz, kaolinite, illite/smectite, pyrite and epidote, where also dickite, anatase, gypsum, jarosite and diaspore were identified. Relics of plagioclase, K-feldspar and muscovite are preserved. Although this type of altera-tion accompanies the gold mineralization, it can also be observed in areas lacking it. Similar to pyritization, propylitization cannot be considered as a direct pro-specting criterion in search for gold deposits.

4) Argillization – an alteration producing kaolinite, illite, beidellite, alunite and pyrophyllite was found in only one sample from the Bellavista deposit.

5) Sericitization of feldspars in volcanic rocks combi-ned with silicification is a common alteration accom-panying mineralizing processes.

0

5

10

15

20

140 160 180 200 220 240 260 280 300

Temperature of homogenization (°C)

Freq

uen

cy

Quartz I

0

5

10

15

20

25Quartz II

0

5

10Quartz III

0

5

10Sphalerite

0

5

10Quartz II-Moncada

Fig. 10 Histogram of homogenization temperatures for fluid inclusions in various types of quartz and sphalerite of the Au-bearing and base-me-tal veins of the Aguacate Group volcanic arc.

0

20

40

60

80

1 2 3 4 5

Salinity (wt. % NaCl equiv.)

Freq

uen

cy

Quartz I Quartz II Quartz III Sphalerite Quartz II-Moncada

Fig. 11 Histogram of aqueous solution salinities for fluid inclusions in various types of quartz and sphalerite.

100

150

200

250

300

0 1 2 3 4 5

Salinity (wt. % NaCl equiv.)

Th

(°C

)

Quartz I Quartz II Quartz III Sphalerite Quartz II-Moncada

Fig. 12 Salinity vs. homogenization temperatures of fluid inclusions in various types of quartz and sphalerite of the Au-bearing and base-metal veins of the Aguacate Group volcanic arc.


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A distinct metasomatic aureole can be seen around the Guacimal Pluton, but these alterations are not spatially related to metallic mineral assemblages.

5.6. Genesis of the gold mineralization

The gold mineralization of the SADO type (Mosier and Sato 1986) currently studied in the Cordillera de Tilarán, Costa Rica, is linked with volcanic rocks of the older, now extinct Central American volcanic arc. The gold deposits investigated are located in a NW–SE-oriented zone at the NW foothills of the Cordillera de Tilarán. The mineralization is confined to Miocene–Pliocene andesites of the Aguacate Group intruded by the mainly monzogranitic–granodioritic Guacimal Pluton. New LA ICP-MS dating of zircons from two granite samples yielded U–Pb ages of 6.3 ± 0.5 and 6.0 ± 0.4 Ma (Žáček et al. this volume). This broadly agrees with the pre-existing K–Ar ages on a monzonite (biotite: 3.9 ± 1.0 Ma, alkali feldspar: 5.0 ± 0.2 Ma; Schulz et al. 1987) and a quartz diorite (whole rock: 7.2 ± 1.4 Ma; Alvarado et al. 1992). The exposed part of the pluton covers an area of c. 70 km2 but, judging from the results of a gravimetric survey carried out by USGS et al. (1987), the hidden part of the pluton is elongated in the NW–SE direction and ~100 km long. Based on the hornblende igneous barom-etry (Žáček et al. this volume), the pluton intruded the Aguacate Group at shallow depths of ~3 km.

The gold mineralization as well as the whole Aguacate Group is discordantly covered by an Early Pleistocene Monteverde andesite lava sheet up to 500 m thick, whose age was established at 1.1–2.1 Ma (K–Ar method, mostly whole rock, e.g. Alvarado et al. 1992). Lavas of the

Monteverde Formation show no signs of hydrothermal alteration or ore mineralization, contact metamorphism or structural patterns similar to those observed in volcanites of the Aguacate Group.

The gold mineralization is linked only to young N–S to NE–SW tectonic movements, which agree with the final stage of Neogene tectonic development of the mag-matic arc prior to relatively fast uplift of the area dur-ing the Quaternary (Marshall et al. 2000). Brittle shear failures originated during this stage of tectonic develop-ment, giving rise to major faults and conjugated fracture systems of NNW–SSE to NW–SE strikes associated with abundant N–S to ENE–SSW to NE–SW running systems of faults and fractures. This tectonic system can be linked with extensive ENE–WSW to E–W-oriented transten-sional tectonics (Grygar in Kycl et al. 2010).

The gangue and ore textures are often comb-like, banded or crustiform, whereas colloform textures are rare. Many features, such as brecciation or fracture fill-ings, suggest that multistage ore-formation processes took place. The mineralogy and geochemistry of the ore deposits document the presence of at least three hydro-thermal pulses defined by distinct generations of quartz and also by separation of gold mineralization from the base-metal mineral assemblage.

According to the fluid inclusions study, gold and base-metal mineralization events took place at temperatures ranging from 150 to 290 °C from H2O–NaCl fluids of low salinities (0.2 and 4.3 wt. % NaCl equiv.). These data confirmed the epithermal nature of the studied mineral-ization. Assuming that the trapping temperatures of in-clusions were close to the homogenization temperatures, then the depth can be estimated from the course of the

two-phase equilibrium curves. In this case, the depth of mineral precipita-tion is thought to have varied between 500 and 1200 m below the surface (see Haas 1971).

As follows from the above observa-tions, the gold-bearing quartz origi-nated as a product of shallow hydro-thermal circulation of meteoric waters whose motion was most probably trig-gered by the thermal gradient around Guacimal Pluton granitic intrusion. Mineralizations occurred, including large regional alterations, in the Late Pliocene–Early Pleistocene in an in-terval between c. 6.0 ± 0.4 Ma (U–Pb age of the Guacimal Pluton, Žáček et al. this volume) and the 2.1 Ma (the oldest of the K–Ar whole-rock ages from the barren Monteverde Formation, Alvarado et al. 1992).

Tab. 12 Qualitative XRD analysis of altered volcanics adjacent to the quartz veins of selected gold deposits

Sierra Alta, Nivel 8 Tres Hermanos El Recio Guaría – Guacimal

major minerals quartz quartz quartz quartzplagioclase kaolinite mica/smectite mica/smectite

K-feldspar chloriteillite/smectite dickite

K-feldsparminor minerals albite pyrite alunite jarosite

chlorite anatase anatase diasporeK-feldspar hematite

trace minerals pyrite gypsum pyrite pyritegypsum anataseanatase goethitemarkasite Fe-dolomitemicas


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6. Conclusions

Exploration geochemistry (chiefly heavy mineral and stream sediment surveys) was carried out in the map sheets of Juntas, Chapernal and Miramar, Costa Rica, on a scale of 1 : 50 000. The study was intended to identify and outline new areas promising for the occurrence of epithermal gold mineralization; samples from historical mining districts were also studied in order to asses its likely genesis. The field-work (2006–2009) was focused predominantly on the area formed by the volcanic rocks of the Aguacate Group. The results can be summarized as follows:• Altogether 237 heavy mineral concentrates, 297 stre-

am sediments and 98 lithogeochemical samples were collected over an area of c. 950 km2.

• The gold mineralization is confined exclusively to the Pliocene Aguacate Group and is always accompanied by intense hydrothermal alteration.

• A positive correlation in heavy mineral concentrates was found between the presence of gold and arseno-pyrite, cerussite, cinnabar and base-metal sulphides.

• A positive correlation was also found in stream sedi-ments and the below-screen fraction of heavy mineral concentrates between Au + Ag on the one hand, and As, Sb, Hg, Pb, Tl, Mo with W on the other.

• The shapes of gold chips in sediments (sharp plates, wires and dendritic forms) indicate that the primary source for most of the defined anomalies is very close to the sampling sites. The gold occurs in the form of fine particles mostly several tenths of µm across, oc-casionally attaining a size of 1–2 mm.

• On three map sheets, 14 areas promising for the occur-rence of gold mineralization were defined. Of them, four were suggested for follow-up exploration.

• Recent mineralogical investigation identified the fol-lowing ore minerals: gold (electrum, 30–42 wt. % Ag), pyrite, sphalerite, galena, chalcopyrite, arsenopyrite, marcasite, acanthite, freibergite, pyrargyrite, greenoc-kite, covellite, cassiterite, cerussite and an unidentified Ag–Sb–Au sulphide. Gold in primary ores is always very fine-grained (< 1–25 µm) and mostly enclosed in pyrite and arsenopyrite.

• Fluid inclusions study showed that Au-mineralizati-on originated during a multi-stage hydrothermal pro-cess at 150 to 290 °C from H2O–NaCl fluids with very low salinity (0.2–4.3 wt. % NaCl equiv). The epither-mal nature of these veins was ascertained. The depth of the mineral precipitation is estimated to have vari-ed between 500 and 1 200 m below the palaeosurface.

• Gold mineralization is interpreted as being the pro-duct of shallow hydrothermal circulation of domi-nantly meteoric waters, whose motion was triggered by the thermal gradient around the Guacimal Pluton.

At least three pulses of ascending ore fluids gave rise to quartz veins with rich gold and base-metal mine-ralization, which are confined to brittle structures of N–S to ENE–WSW strikes. The age of mineralization and intense regional hydrothermal alteration falls in the Late Pliocene–Early Pleistocene (c. 6.0–2.1 Ma).

Acknowledgements. This study was made possible thanks to the Program of Development and Cooperation (project No. RP/6/2006) between the Czech Republic and Costa Rica, specifically between the Ministry of Environment of the Czech Republic and the Department of Geology and Mines (DGM) of MINAET, Costa Rica, carried out during 2006–2009. Petr Hradecký and Petr Kycl were in charge of the project and the authors are obliged to them and the whole team for their assistance in the field. We are very grateful to Mr. José Francisco Castro Mu-ñoz, Ms. Marlene Salazar Alvarado and Ms. Sofia Hu-apaya for their support, cooperation and friendship. We are also indebted to Radek Škoda of Masaryk Universi-ty Brno for carrying out microprobe analyses, František Laufek and Irena Haladová for XRD analyses, Jan Ma-lec for photomicrographs of gold particles and Zdeněk Táborský (all CGS) for valuable suggestions regarding ore microscopy. The reviewers, Jiří Zachariáš and Peter Koděra, as well as handling editor Miroslav Štemprok, provided helpful comments on the manuscript that im-proved its quality.

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