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The Salamón gold deposit (León, Spain)
Journal of Geochemical Exploration 71 (2000) 191–208 www.elsevier.nl/locate/jgeoexp
The Salamo´n gold deposit (Leo´n, Spain) J.L. Crespo a,*, M.C. Moro b, O. Fado´n b, R. Cabrera a, A. Ferna´ndez b a SIEMCALSA, Incas, 5, 47008 Valladolid, Spain Departamento de Geologı´a, Universidad de Salamanca, 37008 Salamanca, Spain
b
Abstract Located 55 km NE of the provincial capital Leo´n, Salamo´n deposit, discovered in 1985, is located on the southern slope of the Cantabrian Mountains, in the north of the Iberian Peninsula. The deposit is located on the Leo´n fault, which is a lateVariscan, E–W trending, deep structure extending for more than 100 km. The Leo´n fault has a complex history, and many mines and occurrences are located near it. The deposit is also close to small stocks and dykes of igneous rocks with intermediate to basic composition to which the mineralisation is related. The mineralisation is hosted mainly by the limestones and bituminous shales of the Lena Group (Namurian–Westphalian). There is also some mineralisation in other stratigraphic units of the Upper Carboniferous, such as the Maran˜a Group or the Stephanian B sediments. Apart from local and regional exploration, a detailed mineralogical and metallogenic research has been carried out. The epithermal mineralisation of Salamo´n was developed in two phases: an early dominant and extensive stage, with very fine crystalline gold-bearing sulphides, mainly pyrite, arsenic-bearing pyrite and arsenopyrite, in a matrix of quartz–chalcedony (jasperoid) and dolomite, and a later stage, of a larger crystal size, which occurs replacing the early stage or in pockets and veins, with greater mineralogical variety. Last of all there is a stage of supergene mineralisation, a product of the oxidant action of meteoric waters over the previous minerals. The hydrothermal alterations of the host rocks related to the orebodies are fundamentally decarbonatisation–dolomitisation, silicification and argillitisation. The early stages of mineralisation were produced in a temperature of 148–241⬚C, while that in the later stages occurred at 86–123⬚C. The early stage has been dated as 269 ^ 5 Ma; and this agrees with the ages of the other deposits of the district, which lay between 292 and 263 Ma, and the igneous rocks of the Pen˜a Prieta stock 277 ^ 1 Ma; all which are of Permian age. The results of the studies carried out until now lead to the conclusion that Salamo´n is a Carlin-type gold deposit. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: gold deposit; Carlin type; Salamo´n; Cantabrian Zone
1. Introduction The Salamo´n gold deposit is located within the Cantabrian Zone, in an area where four of its main tectonic units converge: the Esla-Valsurvio Unit, Sobia-Bodo´n Unit, the Central Carboniferous Basin and the Pisuerga-Carrio´n Unit. Furthermore, the * Corresponding author. E-mail addresses: [email protected] (J.L. Crespo), cmoro@ gugu.usal.es (M.C. Moro).
observed mineralisation is associated with an E–W fracture belonging to the Leo´n Fault system, which is an important structure extending over 100 km and marked by a series of Stephanian B intramontane basins. Other significant high-angle faults in the vicinity include the Porma Fault (NE–SW) and the Ventaniella Fault (NW–SE) (Fig. 1). Although igneous rocks are relatively rare in the Cantabrian Zone, Salamo´n lies on the western edge of the Pisuerga-Carrio´n Unit, where the intrusion of several stocks and dykes ranging from basic to intermediate
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00152-7
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Fig. 1. Regional geology of the Rian˜o area around the Salamo´n Project, with location of mining occurrences (After SIEMCALSA, 1997, simplified). Coordinates of Salamo´n are 42⬚56 0 47 00 N and 5⬚7 0 52 00 W, located at 1066 m.a.s.l.
composition, is related to the major regional lineations (Corretge´ et al., 1987; Corretge´ and Sua´rez, 1990). Salamo´n is located in the As–Sb–Au Rian˜o-Estalaya district (SIEMCALSA, 1997). The presence of gold at Salamo´n was discovered by BP Minera Espan˜a in 1985, during a regional precious metal exploration programme that took place in the Cantabrian Zone. In 1988 BP invited SIEMCALSA to join the project. SIEMCALSA, in its role as operator, began its programme in 1990, subsequent to the signing of a new agreement with the RTZ Group, who had acquired BP’s metal division in early 1989. This paper describes the work carried out and the main results obtained. Since January 1998 SIEMCALSA is the only titular of the project (Crespo, 1998).
Parallel to the exploration programme, research has been undertaken into the mineralogical and metallogenic aspects of the project and has been summarised in three recent papers (Paniagua et al., 1996, 1997; Paniagua, 1998). Further work is now being carried out by Salamanca University, in collaboration with SIEMCALSA, in the framework of the metallogenic research on gold-bearing hydrothermal deposits at Castilla-Leo´n Autonomy Community (Fado´n et al., 1998).
2. Local geology The units related to the mineralisation are all
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Fig. 2. Simplified geology of the Salamo´n prospect area. A–A 0 is a cross section of the deposit shown in Fig. 4.
Carboniferous, from old to young: Lena Group, Maran˜a Group and the Stephanian B sediments. The Esla-Valsurvio region, south of the Leo´n Fault and the Stephanian B basin, includes sequences ranging from the Cambrian to the Namurian (Figs. 1 and 2). 2.1. The Lena Group The Lena Group outcrops to the north of the Leo´n Fault and belongs to the so-called Lois-Ciguera Sector in the Central Carboniferous Basin. In this sector the Lena Group is characterised by an alternation of limestones and shales (Heredia et al., 1990; Barba et al., 1991) have defined a series comprising the following lithostratigraphical units, from bottom to top: Yordas limestones, Lois shales, Bachende limestones, Duen˜as lutites and sandstones, Ciguera limestones, and Anciles lutites and limestones. The lowest strata belong to the Bashkirian, in all likelihood the Lower Bashkirian, whereas the uppermost strata correspond
to the Lower Myachovsky (the highest horizon of the Moscovian). The total thickness of the sequence is less than 2000 m. Mineralisation occurs mainly in the Bachende limestones and the Duen˜as lutites and sandstones. The limestones are frequently thick bedded, are oolitic and occasionally bioclastic. Both the carbonate and the detrital units are often bituminous. 2.2. The Maran˜a Group The Maran˜a Group belongs to the Pisuerga-Carrio´n Unit, and unconformably overlies the Lena Group. It occurs in the core of the Ciguera Syncline, just to the north of Salamo´n, and to the east of the frontal thrusts of the Ponga Nappe and the Central Carboniferous Basin. This group consists primarily of lutites, with calcareous breccias, conglomerates and occasional very large calcareous olistolites (measuring up to 1 km). This group represents a clastic wedge generated in the front of the nappes, in a foredeep depression
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related to the emplacement of the Ponga Nappe and the Central Carboniferous Basin. It has been dated as Upper Westphalian D–Stephanian A (Rodrı´guez Ferna´ndez and Heredia, 1987). 2.3. Stephanian B sediments These sediments are made up by polymict conglomerates, sandstones, shales and coal seams. The detritus in the eastern sector, which is called Reyero-Salamo´n, originated from the north, and the sediments are discordant on those of the Central Carboniferous Basin or the Maran˜a Group. They are thrust on the southern side. Their age has been established as Stephanian B. They are exposed in a narrow carboniferous intramontane basin known as the Canseco–Rucayo–Reyero–Salamo´n basin, which extends along the Leo´n fault (Navarro et al., 1987; Martı´nez Garcı´a, 1990). 2.4. Igneous rocks A varied group of igneous rocks has been described to the east of Salamo´n, than are intrusive to the Pisuerga-Carrio´n Unit. These igneous rocks are related to late-hercynian deep faults of considerable importance (Corretge´ et al., 1987; Corretge´ and Sua´rez, 1990). The intrusives consist of small stocks, dykes and sills, with a composition ranging from gabbro to quartz–diorite. One of these dykes outcrops in the project area. In spite of being only 2–5 m thick, the dyke displays considerable lateral continuity and has been intersected by numerous drill holes (Figs. 2, 4 and 5). Since it is not always possible to see the dyke at the surface, Fig. 2 shows the position of this dyke as determined from drilling. It can be deduced that this dyke is relatively late, since it cuts through all the stratigraphic units and structures, and is sub-parallel to the principal mineralised structure. Moreover, it features a high degree of superimposed hydrothermal alteration (Paniagua et al., 1996). It is a microgabbro or a microdiorite, and furthermore is carbonised and silicified. The dyke contains a high concentration of the opaque minerals, pyrite, arsenic-bearing pyrite and arsenopyrite. These minerals are the same that constitute the early mineralisation in the sedimentary rocks.
2.5. Structure Salamo´n is located at the intersection of several major tectonic units in the Cantabrian Zone. The Pisuerga-Carrio´n Unit, a relatively autochthonous unit situated east of Salamo´n, is overthrusted by different nappes coming from the south (Esla thrust Unit) and the west (Central Carboniferous Basin and Ponga Unit) (Pe´rez Estaun and Bastida, 1990) (Fig. 1). In addition to the tangential tectonic structure of thrust and associated folds, there is a major tectonic structure characterised by large strike-slip faults that often reactivate earlier fractures (Rodriguez Ferna´ndez and Heredia, 1987) and control the location of the igneous activity described above. The mineralising process appears to be related to this late or postHercynian activity. The Leo´n Fault, which is directly linked to the mineralisation, is one of the larger fractures in the area. This fracture has a long history, commencing as a lateral tear fault structure during the development of the Central Carboniferous Basin and the Ponga Nappe emplacement, and later as a reverse fault, in relation to the N–S compression. Subsequently it was reactivated, possibly during the Alpine orogeny, as reflected by the Sabero-Gordo´n Fault or the Ventaniella Fault, which displace the Tertiary sequence (Heredia et al., 1990). The structure of the Esla Unit, located to the south of Salamo´n, was studied by Alonso (1985), while the tectonic structure to the north of the Leo´n Fault, was researched by Heredia (1991).
3. Exploration work The Salamo´n deposit is generally divided into two zones, called Salamo´n East and West, on each side of the village that gave the deposit its name (Fig. 2). Below, the major exploration activities carried out in the project are enumerated in a brief discussion. 3.1. Stream sediment geochemical survey This technique has been used on a regional scale at a density of about 2 samples per km 2. As, Sb, Hg and Cu are locally anomalous. Also, gold was analysed and reaches 1220 ppb in Salamo´n East. This Au anomaly continues about 3 km downstream along
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Fig. 3. Distribution of gold and arsenic in soil samples in the area of Salamo´n project.
the Duen˜as River, until it meets the Esla River, with values up to several tens of ppb Au. 3.2. Soil geochemistry The entire area 2:000 × 1:000 m is covered with a 100 × 25 m grid, although the more interesting areas have a 50 × 25 m; and even a 25 × 12:5 m grid. The orientation of the grid lines is about N 5⬚E. The samples were analysed for Au (aqua regia digest/ atomic absorption; fire assay fusion/direct coupled
plasma) and multi-elements (aqua regia digest/ induced coupled plasma). Au, Ag, As, Sb, Hg, Cu, Zn and Pb are anomalous in Salamo´n East and West. The Au and As anomalies are particularly striking (Fig. 3). The gold anomaly in Salamo´n East (⬎1,000 ppb Au) is 500 m long. There are eight samples of more than 10,000 ppb, the maximum value being 18,800 ppb. Analogous anomalous areas are shown for As, although the higher values are in Salamo´n West, with values up to 12,500 ppm.
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3.3. Outcrop sampling Of the 110 outcrop rock samples taken within the project area during different campaigns, 35 have values of more than 1 ppm for Au, and 15 have more than 10 ppm. The maximum value is 155 ppm. The correlation of the values for gold with several elements, primarily As, Ag, Tl, Sb, Pb and Hg, is remarkable. 3.4. Geophysics Different techniques have been applied: magnetometry, very low frequency (VLF), induced polarisation (IP), radiometry, etc. Detailed magnetics, VLF resistivity and radiometry appeared to be useful for mapping purposes. In some cases the IP chargeability response is a product of both a lithological marker horizon (a zone of fossiliferous black shales) and mineralisation. 3.5. Drilling There are 35 drill holes, totalling 6500 m in length in Salamo´n. Fig. 4 shows three sections of Salamo´n East, which includes drill hole 24, the deepest in the project area, with a length of 377 m (Section A–A 0 ). The most remarkable results were obtained from drill hole 31 (Section 5137 E, B–B 0 ), in which 29 m (22.2 m of true width) of rock were intersected grading 20.5 ppm Au. This result is the average of 28 samples of about 1 m each, of which only three had values less than 10 ppm Au. Fig. 5 is the plan at 1100 m.a.s.l. of Salamo´n East compiled from the results of the drill holes. 876 drill hole samples have been analysed for several elements, with their correlation coefficients established. The positive correlation coefficients of gold with silver, arsenic, antimony and mercury is remarkable, all of which are greater than 0.5 (e.g. Fig. 6). 3.6. Resource estimate In Salamo´n East, in a lens with a maximum width of 22 m, intersected by drill hole 31 in its centre, is estimated a resource of 450,000 t of mineralised material with an average grade of 7.86 ppm Au. The
mineralisation can continue laterally and at depth and more work needs to be done. Only four drill holes have been made in Salamo´n West, and it is impossible until now to define a mineralised body and estimate a mining resource. 3.7. Metallurgical testwork The gold at Salamo´n is refractory, hosted by arsenic-bearing pyrite and the arsenopyrite, which is found in crystals measuring a few dozen microns. The final conclusion of the metallurgical testwork is that more than 90% of the gold can be recovered by roasting under special conditions (Garcı´a Frutos and Alvarez, 1999). 4. Mineralisation Two surface occurrences follow a N110 E subvertical fracture. Also two mines are located along this fracture. They are the Rosa Mine in Salamo´n East, where the presence of gold was first discovered, which consists of one small extraction gallery at a height of 1120 m and Valle Mine in Salamo´n West, which consists of two small extraction galleries at a height of 1140 and 1150 m (Fig. 2). 4.1. Morphology The mineralised bodies appear, in general, as elongate shaped lenses with a subvertical outline that are more or less parallel to the fracture to which they are related (Figs. 4 and 5). These mineralised bodies are made up with mineralised tectonic breccias, veins and pockets, and disseminated sulphides in a quartz–carbonate gangue. 4.2. Characteristics of the host rocks The host rocks consist primarily of carbonate rocks with very bituminous pelitic intercalations from the Lena Group and, to a lesser extent, of the clastic component of the Maran˜a Group, Stephanian B sediments and igneous rocks. The carbonate rocks from the Lena Group occur in thick layers with a compact structure and a greyish colour that is related to the carbonaceous content. Microscopic observations of these rocks show micritic and biomicritic types with abundant disseminated
J.L. Crespo et al. / Journal of Geochemical Exploration 71 (2000) 191–208 Fig. 4. Geological cross sections A–A 0 , B–B 0 and C–C 0 through the Salamo´n project area (see Fig. 5). The black area corresponds to the silicified strata, with more that 1 ppm Au. The white zone is mostly dolomitised, with some silicification and some gold. The dyke is shown, which crosses into the different stratigraphic units.
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Fig. 5. Geological plan 1100 m.a.s.l. of Salamo´n East based on the results of the drill holes. In black, the silicified area is shown with more that 1 ppm Au. In white, the dolomitised zone with some silification is shown, which bears some gold.
fossil relics (coral fragments, echinoderms, bivalve shells, brachiopods and foraminifera) and occasional facies of sandy fossil-rich pelmicrites and intramicrites. Relics of silicified fossils (principally calcispheres) can sometimes be differentiated, without any evidence of other processes of alteration. It is
therefore probable that this replacement has a diagenetic origin. The mineralogical composition of these rocks (from drill holes) has been determined by X-ray diffraction and chemical analysis. The rock constituents are: 85–95% calcite, 4–8% dolomite, 2–4%
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Fig. 6. Correlation between gold content of mineralised samples and their arsenic (A) and antimony (B) content for 876 samples from drill holes. The best-fit lines for data points along with correlation coefficients (r) are also indicated.
quartz and 0.5–1% of illite, with traces of pyrite, carbonaceous matter and iron oxyhydroxides. The very bituminous pelitic rocks from the Lena Group consist of interbedded, strongly laminated, argillaceous–arenaceous dolomite or lime mudstone and weakly laminated fine-grained sandstones (Fig. 9a). These rock types appear physically similar. However they differ in composition and texture. In all the beds, especially in those with coarser grain size, a micritic and calcareous–dolomitic matrix of early diagenetic origin can be differentiated together with the remains of bioclasts, ooids and pellets and a very fine detrital phase made up of small grains of quartz, micas and clay minerals. Occasionally, in some of these beds, an incipient silicification and argillisation is observed. The carbonaceous matter and the illite are commonly admixed in some laminated pelitic beds, where they are very abundant and occur in a disseminated form or are concentrated in 0.5 to 2 mm thick current-induced laminations. The pyrite in these beds is very abundant and occurs as euhedral–subhedral crystals and as framboids. Inclusions of chalcopyrite, probably of diagenetic origin, have been identified in some of these pyrite forms. The chemical composition of some laminated pelitic beds was determined through a chemical analysis of 30 samples collected from different drill holes. The results (Table 1) emphasise the low contents of
trace elements (such as gold, arsenic, copper, mercury and nickel), Corg (1.14 and 1.65%) and S (1.84 and 2.30%), and the high contents of CaO (2.63 and 6.43%) and Cinorg (1.12 and 2.01%). In the correlation matrix of the analysed variables it can be seen that the Corg displays some negative correlation coefficients with the majority of the metallic elements; however, the S is positively correlated with Fe, As, Sb and Pb, reflecting the presence of pyrite and other sulphides in these rocks. The clastic lithologies of the Maran˜a Group are made up of mudstones, conglomerates, calcareous breccias and isolated olistolites from the limestone of the Lena Group. The Stephanian B sediments consist of fairly coarse heterometric polymict conglomerates, in which clasts of limestones and sandstones–quartzites of the Ordovician and Devonian formations from the Esla Unit and Lena Group are found. A carbonate or ferroan cement and/or a sandy siliceous matrix is present in these lithologies. The igneous rocks that have been found in the area correspond to a dyke that has a thickness of less than 4 m, but good lateral continuity, and has been intersected in the majority of the holes drilled in the zone. This dyke cuts through all the stratigraphic units and seems to be located along an E–W subvertical fracture dipping steeply to the S, sub-parallel to the mineralised structure.
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Table 1 Average chemical composition of the unalterated-host rocks from the Lena Group (Salamo´n). Analytical techniques: major and trace elements by ICP-ES, INAA and FUSION-ICP, and Corg and Cinorg using a LECO䉸 CS-125 carbon–sulphur determinator (Krom and Berner, 1983). Analysts: Activation Laboratories Ltd of Ontario (Canada) and Geology Department of the University of Salamanca (Spain), respectively. n: number of samples. S: standard deviation. L.O.I.: loss on ignition at 950⬚C Siltstones Mean n 18 SiO2 (%) Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 Corg Cinorg S L.O.I.
53.89 18.02 7.64 0.07 1.70 2.63 0.35 3.64 0.54 0.15 1.14 1.12 1.84 10.84
Marly siltstones Mean n 12
S 2.03 1.22 1.34 0.03 0.36 0.90 0.13 0.37 0.04 0.01 0.62 0.38 2.05 0.76
51.81 15.22 7.34 0.07 1.58 6.43 0.31 2.97 0.50 0.15 1.65 2.01 2.30 13.13
S 3.45 2.76 0.64 0.02 0.57 2.10 0.06 0.50 0.07 0.02 1.50 0.73 1.88 1.91
Cu (ppm) Cr Zn Y V Sc Ba Ni Co U Th Pb Rb Cd Bi As Cs Hf Sb
32 104 59 18 129 18 529 48 19 3 15 28 256 0.6 0.4 142 11 6 3.3
3 9 37 1 10 2 103 14 5 1 2 12 375 0.1 0.1 236 3 2 1.7
26 94 59 21 114 15 418 39 18 4 13 40 127 0.8 0.3 89 9 7 4.3
7 13 53 2 20 3 105 7 3 2 2 21 29 0.2 0.1 94 2 2 5.0
Au (ppb)
5
2
5
2
4.3. Hydrothermal alteration and zonation The types of hydrothermal alterations at the Salamo´n deposit (Paniagua et al., 1996) are similar to those documented in Carlin-type deposits in Nevada (Radtke, 1985; Kuehn and Rose, 1992, 1995; Arehart,
1996). More detailed studies on the zonation of alteration, mineralogy, distribution and its relation to gold and to other ore minerals and alteration minerals are in progress and are briefly summarised below. The hydrothermal alterations of the host rocks related to the mineralised bodies are fundamentally decarbonatisation–dolomitisation, silicification and argillisation, and vary according to the type of host rock. These alterations appear on the surface to be distributed irregularly around the mineralised bodies (Fig. 2). The alteration is zoned outwards from the mineralised structures. This will be discussed below based on observation from the abandoned Valle Mine (Fig. 7) and drilling information at Salamo´n, specially in hole 31 (Fig. 8). 4.3.1. The abandoned Valle Mine To define the mineralogical and chemical characteristics of the alteration zones, several rock samples were collected from N–S trending galleries, perpendicular to the strike of the mineralised system of E–W trending high-angle normal faults in the abandoned Valle Mine (Fig. 7). According to the characteristics observed in the galleries, five hydrothermal alteration phenomena have been distinguished in the Lena Group limestones, namely: dolomitisation front, dolomitised zone, silicification front, silicified zone and calcite veins (Fig.7). Dolomitisation front: This consists of a weakly dolomitised grey limestone. Field and petrographic observations indicate that pre-ore forming fluids moved along a complex of high-angle normal faults (E–W) and spread out laterally due to the porosity of the limestone. Dolomitisation caused minor dissolution of the matrix and of allochems. This gave rise to the formation of a medium-crystalline dolomitic limestone poor in carbonaceous material. In this zone some euhedral and subhedral crystals of pyrite are found, isolated or present as aggregates. Dolomitised zone: This zone consists of a massive dolomite that has a grey colour when it is fresh or beige when it is weathered. This equigranular medium-crystalline dolomite contains a large amount of non-carbonate impurities (Fig. 9b). Very small, disseminated euhedral to subhedral crystals of pyrite and arsenopyrite, sometimes partially oxidised, have also been identified.
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Fig. 7. Schematic plan showing mine-scale spatial relationships of zonation of alteration centred around the brecciated and silicified fault zone at the Valle Mine in the western part of Salamo´n Project.
Silicification front: This front is characterised by a grey coloured dolomite with abundant veins and geodes filled with white dolomite. In this zone the alteration ranges from weak fracture-controlled
boxworks to complete replacement (jasperoid) and, therefore, several subzones with varying intensity of silicification can be distinguished. Some of them correspond to a dolomitic rock, very porous and
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Fig. 8. Detail on the silicified feeder structure within drill hole 31 (Salamo´n east). Major Au mineralisation is developed in these structures.
partially silicified. In other more silicified dolomitised rock, only the remains of the dolomite is preserved (Fig. 9d). In both cases there is weak argillisation and sericitisation, as evidenced by dickite and sericite/ illite as accessory components in the studied samples. Petrographic evidence suggests that most of the hydrothermal pyrite and arsenopyrite from early mineralisation fill the porosity of these rocks (Fig. 9b). Hydrothermal pyrite occurs mainly as cubic crystals and occasionally as spherical or sub-spherical agregate of granules or microcrysts (framboidal). Both varieties are closely associated with hydrothermal silica (Fig.10a). Silicified zone (Jasperoids): This zone is formed as the result of progressive replacement of altered carbonate rocks by hydrothermal silica along structures of the complex sets of high-angle normal faults E–W (Fig. 7), and also along the more permeable stratigraphic horizons of the laminated argillaceous– arenaceous dolomite with interbedded bituminous pelites of the Lena Group (Fig. 9a). Generally, hydrothermal-related silicification is less extensive than dolomitisation (Fig. 2). This zone is characterised by its coincidence with the strongest mineralisation at ore scale. All the samples taken at different points of the gallery are strongly mineralised, silicified (jasperoids) and occasionally argillitised. They consist of a microto cryptocrystalline quartz aggregates with abundant disseminated carbonaceous matter and sporadic dickite and sericite (Fig. 9d). The intergrowths between these minerals frequently occur in fractures and cavities as a consequence of the removilisation processes that affect them (Fig. 9f). Mineralogical data and textures observed in these siliceous rocks confirm that very fine crystalline disseminated arsenopyrite and pyrite are the most common ore minerals. Calcite veins: They occur as open space filling along mineralised faults, fractures and breccia zones (Fig. 7). The studied veins consist of relatively clean white calcite as the essential mineral, and sulphide minerals, especially realgar–orpiment, sphalerite, galena and locally arsenic-rich sulphosalt minerals. In some places these veins contain enriched pockets in the removilised sulphide minerals and hydrocarbons (tucholites) (Fig. 10d–f). Field relations and mineralogical data indicate that the formation of many of the shallow calcite veins probably resulted from a very late hydrothermal process, and others may be
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Fig. 9. a. Thin-bedded laminated hand specimen of argillaceous–arenaceous limestone from Lena Group. Salamo´n Project area (SS-31-28). b. Photomicrograph showing initial penetration of dolomite (Dol) rhombs by hydrothermal quartz associated with pyritic gold ore from Salamo´n west (Valle Mine) (SS–12–11). Transmitted light, crossed nicols. c. Hand specimen of mineralised jasperoid from east part of Salamo´n Project (SS-31-31). d. Photomicrograph of strongly silicified dolomite rock (SS–31–1). Transmitted light, crossed nicols. e. Photograph showing the fabrics of mineralised jasperoid breccia from Salamo´n east (SS–31–32). f. Photomicrograph details showing dickite (Dk) (hydrothermal clay) and quartz (Qtz) filling spaces of jasperoid breccia (Au–W–28). Transmitted light, crossed nicols.
post-hydrothermal and have formed during supergene oxidation. 4.3.2. Drill hole no. 31 A zoned pattern of the hydrothermal alteration of
the host rocks was also observed in drill hole no. 31 (Fig. 8), which is one of the key-drill holes in the area. The mineralogical and chemical characteristics of the studied samples confirm that the samples closest to the mineralised fractures, which are considered to be
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Fig. 10. Photographs illustrating features of the first stage of hydrothermal mineralisation from the Salamo´n deposit. a. Note the fine-crystalline subhedral to euhedral habit of pyrite and arsenopyrite (SS–31–10). Reflected light, parallel nicols. b. and c. Arsenic-bearing pyrite forming overgrowth rims on pre-existing pyrite, which may exhibit compositional zoning either simple or oscillatory (Au–E–4) and (Os–4–B.2). Reflected light, parallel nicols. d. and e. Inclusions of tennantite (tnn) and grains of uraninite (Ur) in tucholite (TI) (Au–W–14.5) and (Os–4– B.4). Reflected light, parallel nicols. f. Inclusions of an octahedral of vaesite (Va) followed by a cubic crystal of gersdorffite (Gf) and surrounded by uraninite (Ur) in massive gersdorffite groundman (Os–5–2). Reflected light, parallel nicols.
the feeders for the hydrothermal fluids, have the highest dickite/sericite–illite and dolomite/calcite ratio. In contrast, in the more distal samples the opposite occurs since K and the K/K ⫹ Al ratio increase with the distance from the fracture. Regarding the gold content, it has been observed in this case that the
highest values are found in the more proximal silicified (jasperoids) zones. 4.4. General paragenesis at Salamo´n According to Paniagua et al. (1996) and our studies,
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two hypogene phases in the mineralisation can be differentiated. The early stage is made up of very fine crystalline disseminated sulphides in a matrix of quartz–chalcedony (jasperoid) and dolomite. The later stage consists of a coarser crystalline size and mineralogy that is found either replacing the early mineralisations or occurring in pockets and late veins with quartz and quartz–carbonate gangue. A final phase of supergene mineralisation is composed of alteration products of previous mineralisation through the action of oxidizing meteoric water. The early hypogene mineralisation consists predominantly of pyrite, arsenic-bearing pyrite and arsenopyrite, with sphalerite, cobaltite–gersdorffite, Ni–Co–Cu–Fe disulphides of the pyrite group (Paniagua, 1993), graphite, linnaeites and uraninite as accessory minerals and, occasionally, apatite and florencite. The common characteristic of all the minerals is their very fine crystal size, normally less than 10 mm (Fig. 10a and b). The late hypogene mineralisation presents more pronounced local mineralogical variations than in the previous case. This consists of pyrite, chalcopyrite, sphalerite, tennantite with galene, cinnabar, realgar–orpiment and stibnite as accessory minerals, and locally livingstonite, bournonite, Pb–Sb sulphosalts (boulangerite, jamesonite, plagionite, zinkenite) and Pb–As sulphosalts (jordanite, leveingita y rathite). Tennantite is more abundant than chalcopyrite at Salamo´n West. The supergene mineralisation consists of a paragenesis of oxyhydroxides of Fe, Cu, Ni and Co, and carbonates, arsenates and sulphates of Cu, Co, Ni and U. These are all products of meteoric oxidation of the early and later hypogene phases. Fluid inclusion studies on quartz and carbonate show two differentiated phases: a main early phase of higher TH and lower salinity, and a late phase of lower TH and higher salinity (Paniagua et al., 1996, 1997). Primary fluid inclusions in quartz, which are cogenetic with the arsenopyrite and pyrite from the early phase show TH from 148 to 241⬚C, with a mode at 218⬚C. Freezing point depression varies from ⫺1.0 up to ⫺7.7⬚C, indicating low to moderate salinities (mode 9.2% NaCl equiv.). Primary fluid inclusions of the late phase and secondary fluid inclusion have TH in the range of 86–123⬚C and Tm from ⫺10.0 to ⫺15.0⬚C, and show evidence of the presence of ions such as Ca ⫹2 or Mg ⫹2 in the solution.
205
The aforementioned authors also present stable isotope data. The d 34S values range from ⫹6.3 to ⫹9.4‰ for early stage sulphides and from ⫹2.45 to ⫺9,4‰ for late stage sulphides. The d 13C and d 18O values in carbonates dolomite and Mg–calcite) range from ⫹0.9 to ⫹4.1‰ for d 13C and from ⫹17.6 to ⫹26.1‰ for d 18O. These data, according to Paniagua et al. (1996), suggest a reduction of sulphur from marine evaporites, the existence of a later hypogene oxidation and buffer of carbon in the fluid by the host carbonates. Making use of the abundance of uranium, along with the absence of thorium, the early stage mineralisation at Salamo´n has been dated using the U–Pb method at 269 ^ 5 My (Paniagua et al., 1996). These data are similar to those obtained from the Providencia 271 ^ 11 My and Profunda 273 ^ 11 My mines, which are epithermal deposits of Cu, Co, Ni with Au, in carbonate rocks located near the Leo´n fault, 40 km west of Salamo´n (Paniagua et al., 1993). All these deposits therefore formed during the Permian. These ages also correlate with the age determinations of intrusive rocks spatially associated to several ore deposits in the area (Gallastegui et al., 1990). 4.5. Relation of gold to sulphide minerals Macroscopic and microscopic visible gold from the Salamo´n gold deposit have not been observed. It is a clear example of an “invisible” gold deposit, with pyrite and arsenopyrite being the gold-bearing minerals. Pyrite is As-rich and studies by EPMA show that Au is positively correlated with As in pyrite and arsenopyrite (Paniagua et al., 1997). In general, textures observed in the mineralised samples studied from the Salamo´n deposit, in addition to the chemical composition of the iron sulphides obtained by microprobe, suggest that there are two types of iron sulphides present. The first type is interpreted as a diagenetic pyrite because of its presence in mineralised and unmineralised rocks. It is characterised by the lack of Au or As (Table 2). The second type forms overgrowths on pre-existing pyrite and is interpreted as pyrite deposited from the hydrothermal solution that also carried As and Au. It exclusively occurs in the mineralised rocks (Table 2). These overgrowths vary in thickness from less than 1 mm (Fig. 10a and b) to 25–30 mm (Fig. 10c). The As
642 103 1809 1103 796 476 1533 60 2282 1240 1017 318 604 145 256 382 528 252 353 102 270 541 673 223 226 1333 1486 1209 384 481 183 870 1227 622 797 210 303 176 116 1130 577 233 349 101 103 981 1104 130 7418 7013 812 356 811 351 5401 3471 630 340 1219 370 364 307 188 688 791 122 595 220 183 322 978 50 242 454 1190 320 176 747 423 827 330 477 n.d.
53.12 45.96 0.24 47.02 46.73 5.60 53.35 45.50 0.10 52.07 47.47 0.27
329 137 606 727 530 280
S(%) Fe As
Cu (ppm) Zn Ni Se Au Ag
1.19 1.07 0.34
Au–E–5 S Mean n 13
0.30 0.41 0.09
Au–E–6 S Mean n 6 Au–E–4 S Mean n 9
2.11 0.90 2.47
47.94 43.85 6.40
2.64 1.46 2.35
52.86 45.84 0.70
0.98 0.52 0.33
53.01 46.02 0.38
0.51 0.69 0.89
SS–31–12.1 S Mean n 9 SS–31–1 S Mean n 6 Au–W–31-1 S Mean n 5 Arsenic pyrite Pyrite Sample
251 179 382 907 365 257
22.60 35.04 41.73
Au–E–5 S Mean n 5 Os–24.1 S Mean n 7
0.45 0.32 0.37
Arsenopyrite Marcasite–pyrite
1.43 0.72 1.77
J.L. Crespo et al. / Journal of Geochemical Exploration 71 (2000) 191–208 Table 2 Microprobe analyses of pyrite, arsenic-bearing pyrite, marcasite–pyrite and arsenopyrite from Salamo´n gold deposit. n: number of analyses. n.d.: not determined. S: standard deviation
206
content of these overgrowths commonly varies as a function of proximity to the sulphide mineral cores (Fig. 11a). Marcasite is also a common constituent of the studied mineralisation and is closely associated with arsenic-bearing pyrite (Table 2 and Fig. 11b). Arsenopyrite is also very fine-crystalline, and appears in the form of needle-like to curve-face crystals (Fig. 10a). It also contains Au-traces (Table 2). The correlation between Au and As on a microscopic scale is positive (Table 2) and is consistent with the commonly observed geochemical association between As and Au in soil geochemistry (Fig. 3) and rocks (Fig. 6). The association of As and Au is so common that the former is commonly used as a pathfinder element for hidden gold deposits (Boyle, 1979). All these features are very similar to those in Carlin-type deposits, where the majority of the Au and As were deposited in arsenic-bearing pyrite overgrowths on pre-existing pyrite surfaces, as a function of redox reactions involving oxidation of Au and reduction of As (Fleet et al., 1988; Cathelineau et al., 1988; Chryssoulis, 1990; Arehart et al., 1993a,b).
5. Conclusions The geological, mineralogical and geochemical features of the Salamo´n deposit are similar to those defined for Carlin-type deposits (Berger, 1986; Berger and Bagby, 1991; Kuehn and Rose, 1992; Arehart, 1996). This conclusion is based on the following arguments: high-angle fault zones; selective replacement with silica of carbonaceous carbonate rocks along the high-angle faults, regional thrust and bedding; very fine sulphides dissemination in carbonaceous calcareous rocks and the associated jasperoids; general scarcity of fine crystalline sulphides (less than 1%); presence of igneous rocks; mineralisation of pyrite, realgar, orpiment with arsenopyrite, cinnabar, fluorite and stibnite in a gangue of quartz, calcite and organic matter. Other data can support this interpretation, such as the type and characteristics of the associated alteration; the temperatures and formation pressures; the presence of heavy sulphur, possibly of an evaporitic origin in the early stages and the existence of a later hypogene oxidation (Paniagua et al., 1996). All of these characteristic provide a framework for establishing a model for a similar genesis.
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207
Fig. 11. Sketch of several crystals of diagenetic pyrite with arsenic-bearing pyrite overgrowths (a) and/or marcasite (b).
Acknowledgements We would like to thank the Journal of Geochemical Exploration reviewers for their patience and their constructive suggestions. We thank Kerr Anderson for his help with improving the English at the last revision of the text and his useful comments. This work was supported by the Comunidad Auto´noma de Castilla y Leo´n (Research Project SA 54/97); by the Agencia de Desarrollo Econo´mico de Castilla y Leo´n, through a grant for training of the investigators, and by a I ⫹ D Project, FEDER Program (Ref. 1FD97-0235).
References Alonso, J.L., 1985. Estructura y evolucio´n tectonoestratigra´fica de la regio´n del Manto del Esla (Zona Canta´brica, NW de Espan˜a). PhD thesis. Diputacio´n Provincial de Leo´n, 276pp. Arehart, G.B., 1996. Characteristics and origin of sediment-hosted disseminated gold deposits: a review. Ore Geol. Rev. 11, 383– 403. Arehart, G.B., Chryssoulis, S.L., Kesler, S.E., 1993a. Gold and arsenic in iron sulphides from sediment-hosted disseminated gold deposits: implication for depositional processes. Econ. Geol. 88, 171–185. Arehart, G.B., Eldridge, C.S., Chryssoulis, S.L., Kesler, S.E., 1993b. Ion microprobe determination of sulfur isotope variations in iron sulphides from the Post/Betze sediment-hosted
disseminated gold deposit, Nevada, USA. Geochim. Cosmochim. Acta 57, 1505–1519. Barba, P., Heredia, N., Villa, E., 1991. Estratigrafı´a y edad del Grupo Lena en el sector de Lois-Ciguera (Cuenca Carbonı´fera Central, NO de Espan˜a). Rev. Soc. Geol. Esp. 4 (1-2), 61–77. Berger, B.R., 1986. Descriptive model of carbonate-hosted Au–Ag. In: Cox, D.P., Singer, D.A. (Eds.), Mineral Deposit Models. US Geol. Surv. Bull., 1693. 379pp. Berger, B.R., Bagby, W.C., 1991. The geology and origin of Carlintype gold deposit. In: Foster, R.P. (Ed.). Gold Metallogeny and Exploration, Blackie and Son, pp. 210–248. Boyle, R.W., 1979. The geochemistry of gold and its deposits. Geol. Surv. Can. Bull., 280, 584pp. Cathelineau, M., Boiron, M., Holliger, P., Marion, P., Denis, M., 1988. Gold in arsenopyrites: crystal chemistry, location and state, physical and chemical conditions of deposition. Econ. Geol. Mon. 6, 328–341. Chryssoulis, S.L., 1990. Detection and quantification of invisible gold by microprobe techniques. In: Hausen, D.M. (Ed.). Gold’90, Society of Mining Engineers, New York, pp. 323–332. Corretge´, L.G., Sua´rez, O., 1990. Cantabrian and Palentian zones: igneous rocks. In: Dallmeyer, R.D., Martı´nez Garcı´a, E. (Eds.). Pre-Mesozoic Geology of Iberia, Springer, pp. 72–79. Corretge´, L.G., Cienfuegos, I., Cuesta, A., Gala´n, G., Montero, P., Rodrı´guez Pevida. S., Sua´rez, O., Villa, L., 1987. Granitoides de la Regio´n Palentina (Cordillera Canta´brica, Espan˜a). XI Reun. Geol. Oeste Penins., Porto, 1985, Mem. 1: 469–501 Crespo, J.L., 1998. Salamo´n gold project (Leo´n, Spain). In: Arias, D., Martı´n-Izard, A., Paniagua, A., (Eds.), International Meeting of Gold Exploration and Mining in NW Spain, Oviedo, Facultad de Geologı´a, Universidad de Oviedo, pp. 86–95. Fado´n, O., Moro, M.C., Cabrera, R., Ferna´ndez, A., 1998. Preliminary results on the morphology and mineral distribution of the
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Salamo´n gold deposits (Leo´n, Spain). In: Arias, D., Martı´nIzard, A., Paniagua, A., (Eds.), International Meeting of Gold Exploration and Mining in NW Spain, Oviedo, Facultad de Geologı´a, Universidad de Oviedo. Fleet, M.E., MacLean, P.J., Barbier, J., 1988. Oscillatory-zoned Asbearing pyrite from stratabound and stratiform gold deposits: an indicator of ore fluid evolution. Econ. Geol. Mon. 6, 356–362. Gallastegui, G., Heredia, N., Rodrı´guez Ferna´ndez, L.R., Cuesta, A., 1990. El stock de Pen˜a Prieta en el contexto del magmatismo de la Unidad del Pisuerga-Carrio´n (zona Canta´brica, N de Espan˜a). Cuad. Lab. Xeol. Laxe 15, 203–217. Garcı´a Frutos, F.J., Alvarez, R., 1999. Influencia de la mineralogı´a y liberacio´n en los posibles procesos de tratamiento de menas aurı´feras refractarias de Salamo´n, en la provincia de Leo´n. Bol. Geol. Min. 110–2, 159–168. Heredia, N., 1991. Estructura geolo´gica de la regio´n del Mampodre y a´reas adyacentes (zona Canta´brica). Unpublished PhD thesis, University of Oviedo, 320pp. Heredia, N., Alonso, J.L., Rodrı´guez Ferna´ndez, L.R., 1990. Mapa Geolo´gico de Espan˜a, E. 1:50.000, n⬚ 105 (Rian˜o). Segunda serie (MAGNA), Primera edicio´n. ITGE. Madrid. Krom, M.D., Berner, R.A., 1983. A rapid method for the determination of organic and carbonate carbon in geological samples. J. Sed. Petrol. 53, 660–663. Kuehn, C.A., Rose, A.W., 1992. Geology and geochemistry of wallrock alteration at the Carlin gold deposit, Nevada. Econ. Geol. 87-7, 1697–1721. Kuehn, C.A., Rose, A.W., 1995. Carlin gold deposits, Nevada: origin in a deep zone of mixing between normally pressured and overpressured fluids. Econ. Geol. 90-1, 17–36. Martı´nez Garcı´a, E., 1990. Cantabrian and Palentian zones: Stephanian and Permian basins. In: Dallmeyer, R.D., Martı´nez Garcı´a, E. (Eds.). Pre-mesozoic Geology of Iberia, Springer, pp. 39–54. Navarro Va´zquez, D., Mun˜oz, J.L., Santos, J.A., 1987. Invetigacio´n geolo´gico-minera del Estefaniense de los sectores Canseco–
Rucayo y Reyero–Salamon (Leo´n), 2 fase. IGME, Internal Report n⬚ 11,193. Paniagua, A., 1993. Mineralizaciones asociadas a fracturas tardihercı´nicas en la rama Sur de la zona Canta´brica. Unpublished PhD thesis, University of Oviedo, 337pp. Paniagua, A., 1998. Gold showings and deposits of the palentian zone (NW Spain). In: Arias, D., Martı´n-Izard, A., Paniagua, A., (Eds.), International Meeting of Gold Exploration and Mining in NW Spain, Oviedo, Facultad de Geologı´a, Universidad de Oviedo, pp. 112–123. Paniagua, A., Fontbote´, L., Fenoll, P., Fallick, A.E., Moreiras, D.B., Corretge´, L.G., 1993. Tectonic setting, mineralogical characteristics, geochemical signatures and age dating of a new type of epithermal carbonate-hosted, precious metal-five element deposits: the Villamanin area (Cantabrian Zone, northern Spain). In: Fenoll, P., Torres-Ru´iz, J.A., Gervilla, F. (Eds.). Current Research in Geology Applied to Ore Deposits, , pp. 531–534. Paniagua, A., Rodrı´guez Pevida, L.S., Loredo, J., Fontbote´, L., Fenoll Hach-Alı´, P., 1996. Un yacimiento de Au en carbonatos del Oro´geno Hercı´nico: el a´rea de Salamo´n (N Leo´n). Geogaceta 20 (7), 1605–1608. Paniagua, A., Loredo, J., Fenoll Hach-Alı´ P., Rodrı´guez Pevida, L.S., 1997. Extrapolation at low temperatures of the arsenopyrite geothermometer con contrasting with fluid inclusion data: the example of Salamo´n (Spain). XIV ECROFI, Nancy, pp. 255–256. Pe´rez Estaun, A., Bastida, F., 1990. Cantabrian zone: structure. In: Dallmeyer, R.D., Martı´nez Garcı´a, E. (Eds.). Pre-mesozoic Geology of Iberia, Springer, pp. 55–69. Radtke, A.S., 1985. Geology of the Carlin Gold Deposit, Nevada. US Geol. Surv. Prof. Paper, (1267), 124pp. Rodrı´guez Ferna´ndez, L.R., Heredia, N., 1987. La estratigrafı´a del Carbonı´fero y la estructura de la Unidad del Pisuerga-Carrio´n. NO de Espan˜a. Cuad. Lab. Xeol. Laxe 12, 207–229. SIEMCALSA, 1997. Mapa Geolo´gico y Minero de Castilla y Leo´n E. 1:400,000. Macrolibro, S.A. Valladolid, 459pp.
The Salamo´n gold deposit (Leo´n, Spain) J.L. Crespo a,*, M.C. Moro b, O. Fado´n b, R. Cabrera a, A. Ferna´ndez b a SIEMCALSA, Incas, 5, 47008 Valladolid, Spain Departamento de Geologı´a, Universidad de Salamanca, 37008 Salamanca, Spain
b
Abstract Located 55 km NE of the provincial capital Leo´n, Salamo´n deposit, discovered in 1985, is located on the southern slope of the Cantabrian Mountains, in the north of the Iberian Peninsula. The deposit is located on the Leo´n fault, which is a lateVariscan, E–W trending, deep structure extending for more than 100 km. The Leo´n fault has a complex history, and many mines and occurrences are located near it. The deposit is also close to small stocks and dykes of igneous rocks with intermediate to basic composition to which the mineralisation is related. The mineralisation is hosted mainly by the limestones and bituminous shales of the Lena Group (Namurian–Westphalian). There is also some mineralisation in other stratigraphic units of the Upper Carboniferous, such as the Maran˜a Group or the Stephanian B sediments. Apart from local and regional exploration, a detailed mineralogical and metallogenic research has been carried out. The epithermal mineralisation of Salamo´n was developed in two phases: an early dominant and extensive stage, with very fine crystalline gold-bearing sulphides, mainly pyrite, arsenic-bearing pyrite and arsenopyrite, in a matrix of quartz–chalcedony (jasperoid) and dolomite, and a later stage, of a larger crystal size, which occurs replacing the early stage or in pockets and veins, with greater mineralogical variety. Last of all there is a stage of supergene mineralisation, a product of the oxidant action of meteoric waters over the previous minerals. The hydrothermal alterations of the host rocks related to the orebodies are fundamentally decarbonatisation–dolomitisation, silicification and argillitisation. The early stages of mineralisation were produced in a temperature of 148–241⬚C, while that in the later stages occurred at 86–123⬚C. The early stage has been dated as 269 ^ 5 Ma; and this agrees with the ages of the other deposits of the district, which lay between 292 and 263 Ma, and the igneous rocks of the Pen˜a Prieta stock 277 ^ 1 Ma; all which are of Permian age. The results of the studies carried out until now lead to the conclusion that Salamo´n is a Carlin-type gold deposit. 䉷 2000 Elsevier Science B.V. All rights reserved. Keywords: gold deposit; Carlin type; Salamo´n; Cantabrian Zone
1. Introduction The Salamo´n gold deposit is located within the Cantabrian Zone, in an area where four of its main tectonic units converge: the Esla-Valsurvio Unit, Sobia-Bodo´n Unit, the Central Carboniferous Basin and the Pisuerga-Carrio´n Unit. Furthermore, the * Corresponding author. E-mail addresses: [email protected] (J.L. Crespo), cmoro@ gugu.usal.es (M.C. Moro).
observed mineralisation is associated with an E–W fracture belonging to the Leo´n Fault system, which is an important structure extending over 100 km and marked by a series of Stephanian B intramontane basins. Other significant high-angle faults in the vicinity include the Porma Fault (NE–SW) and the Ventaniella Fault (NW–SE) (Fig. 1). Although igneous rocks are relatively rare in the Cantabrian Zone, Salamo´n lies on the western edge of the Pisuerga-Carrio´n Unit, where the intrusion of several stocks and dykes ranging from basic to intermediate
0375-6742/00/$ - see front matter 䉷 2000 Elsevier Science B.V. All rights reserved. PII: S0375-674 2(00)00152-7
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Fig. 1. Regional geology of the Rian˜o area around the Salamo´n Project, with location of mining occurrences (After SIEMCALSA, 1997, simplified). Coordinates of Salamo´n are 42⬚56 0 47 00 N and 5⬚7 0 52 00 W, located at 1066 m.a.s.l.
composition, is related to the major regional lineations (Corretge´ et al., 1987; Corretge´ and Sua´rez, 1990). Salamo´n is located in the As–Sb–Au Rian˜o-Estalaya district (SIEMCALSA, 1997). The presence of gold at Salamo´n was discovered by BP Minera Espan˜a in 1985, during a regional precious metal exploration programme that took place in the Cantabrian Zone. In 1988 BP invited SIEMCALSA to join the project. SIEMCALSA, in its role as operator, began its programme in 1990, subsequent to the signing of a new agreement with the RTZ Group, who had acquired BP’s metal division in early 1989. This paper describes the work carried out and the main results obtained. Since January 1998 SIEMCALSA is the only titular of the project (Crespo, 1998).
Parallel to the exploration programme, research has been undertaken into the mineralogical and metallogenic aspects of the project and has been summarised in three recent papers (Paniagua et al., 1996, 1997; Paniagua, 1998). Further work is now being carried out by Salamanca University, in collaboration with SIEMCALSA, in the framework of the metallogenic research on gold-bearing hydrothermal deposits at Castilla-Leo´n Autonomy Community (Fado´n et al., 1998).
2. Local geology The units related to the mineralisation are all
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193
Fig. 2. Simplified geology of the Salamo´n prospect area. A–A 0 is a cross section of the deposit shown in Fig. 4.
Carboniferous, from old to young: Lena Group, Maran˜a Group and the Stephanian B sediments. The Esla-Valsurvio region, south of the Leo´n Fault and the Stephanian B basin, includes sequences ranging from the Cambrian to the Namurian (Figs. 1 and 2). 2.1. The Lena Group The Lena Group outcrops to the north of the Leo´n Fault and belongs to the so-called Lois-Ciguera Sector in the Central Carboniferous Basin. In this sector the Lena Group is characterised by an alternation of limestones and shales (Heredia et al., 1990; Barba et al., 1991) have defined a series comprising the following lithostratigraphical units, from bottom to top: Yordas limestones, Lois shales, Bachende limestones, Duen˜as lutites and sandstones, Ciguera limestones, and Anciles lutites and limestones. The lowest strata belong to the Bashkirian, in all likelihood the Lower Bashkirian, whereas the uppermost strata correspond
to the Lower Myachovsky (the highest horizon of the Moscovian). The total thickness of the sequence is less than 2000 m. Mineralisation occurs mainly in the Bachende limestones and the Duen˜as lutites and sandstones. The limestones are frequently thick bedded, are oolitic and occasionally bioclastic. Both the carbonate and the detrital units are often bituminous. 2.2. The Maran˜a Group The Maran˜a Group belongs to the Pisuerga-Carrio´n Unit, and unconformably overlies the Lena Group. It occurs in the core of the Ciguera Syncline, just to the north of Salamo´n, and to the east of the frontal thrusts of the Ponga Nappe and the Central Carboniferous Basin. This group consists primarily of lutites, with calcareous breccias, conglomerates and occasional very large calcareous olistolites (measuring up to 1 km). This group represents a clastic wedge generated in the front of the nappes, in a foredeep depression
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related to the emplacement of the Ponga Nappe and the Central Carboniferous Basin. It has been dated as Upper Westphalian D–Stephanian A (Rodrı´guez Ferna´ndez and Heredia, 1987). 2.3. Stephanian B sediments These sediments are made up by polymict conglomerates, sandstones, shales and coal seams. The detritus in the eastern sector, which is called Reyero-Salamo´n, originated from the north, and the sediments are discordant on those of the Central Carboniferous Basin or the Maran˜a Group. They are thrust on the southern side. Their age has been established as Stephanian B. They are exposed in a narrow carboniferous intramontane basin known as the Canseco–Rucayo–Reyero–Salamo´n basin, which extends along the Leo´n fault (Navarro et al., 1987; Martı´nez Garcı´a, 1990). 2.4. Igneous rocks A varied group of igneous rocks has been described to the east of Salamo´n, than are intrusive to the Pisuerga-Carrio´n Unit. These igneous rocks are related to late-hercynian deep faults of considerable importance (Corretge´ et al., 1987; Corretge´ and Sua´rez, 1990). The intrusives consist of small stocks, dykes and sills, with a composition ranging from gabbro to quartz–diorite. One of these dykes outcrops in the project area. In spite of being only 2–5 m thick, the dyke displays considerable lateral continuity and has been intersected by numerous drill holes (Figs. 2, 4 and 5). Since it is not always possible to see the dyke at the surface, Fig. 2 shows the position of this dyke as determined from drilling. It can be deduced that this dyke is relatively late, since it cuts through all the stratigraphic units and structures, and is sub-parallel to the principal mineralised structure. Moreover, it features a high degree of superimposed hydrothermal alteration (Paniagua et al., 1996). It is a microgabbro or a microdiorite, and furthermore is carbonised and silicified. The dyke contains a high concentration of the opaque minerals, pyrite, arsenic-bearing pyrite and arsenopyrite. These minerals are the same that constitute the early mineralisation in the sedimentary rocks.
2.5. Structure Salamo´n is located at the intersection of several major tectonic units in the Cantabrian Zone. The Pisuerga-Carrio´n Unit, a relatively autochthonous unit situated east of Salamo´n, is overthrusted by different nappes coming from the south (Esla thrust Unit) and the west (Central Carboniferous Basin and Ponga Unit) (Pe´rez Estaun and Bastida, 1990) (Fig. 1). In addition to the tangential tectonic structure of thrust and associated folds, there is a major tectonic structure characterised by large strike-slip faults that often reactivate earlier fractures (Rodriguez Ferna´ndez and Heredia, 1987) and control the location of the igneous activity described above. The mineralising process appears to be related to this late or postHercynian activity. The Leo´n Fault, which is directly linked to the mineralisation, is one of the larger fractures in the area. This fracture has a long history, commencing as a lateral tear fault structure during the development of the Central Carboniferous Basin and the Ponga Nappe emplacement, and later as a reverse fault, in relation to the N–S compression. Subsequently it was reactivated, possibly during the Alpine orogeny, as reflected by the Sabero-Gordo´n Fault or the Ventaniella Fault, which displace the Tertiary sequence (Heredia et al., 1990). The structure of the Esla Unit, located to the south of Salamo´n, was studied by Alonso (1985), while the tectonic structure to the north of the Leo´n Fault, was researched by Heredia (1991).
3. Exploration work The Salamo´n deposit is generally divided into two zones, called Salamo´n East and West, on each side of the village that gave the deposit its name (Fig. 2). Below, the major exploration activities carried out in the project are enumerated in a brief discussion. 3.1. Stream sediment geochemical survey This technique has been used on a regional scale at a density of about 2 samples per km 2. As, Sb, Hg and Cu are locally anomalous. Also, gold was analysed and reaches 1220 ppb in Salamo´n East. This Au anomaly continues about 3 km downstream along
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195
Fig. 3. Distribution of gold and arsenic in soil samples in the area of Salamo´n project.
the Duen˜as River, until it meets the Esla River, with values up to several tens of ppb Au. 3.2. Soil geochemistry The entire area 2:000 × 1:000 m is covered with a 100 × 25 m grid, although the more interesting areas have a 50 × 25 m; and even a 25 × 12:5 m grid. The orientation of the grid lines is about N 5⬚E. The samples were analysed for Au (aqua regia digest/ atomic absorption; fire assay fusion/direct coupled
plasma) and multi-elements (aqua regia digest/ induced coupled plasma). Au, Ag, As, Sb, Hg, Cu, Zn and Pb are anomalous in Salamo´n East and West. The Au and As anomalies are particularly striking (Fig. 3). The gold anomaly in Salamo´n East (⬎1,000 ppb Au) is 500 m long. There are eight samples of more than 10,000 ppb, the maximum value being 18,800 ppb. Analogous anomalous areas are shown for As, although the higher values are in Salamo´n West, with values up to 12,500 ppm.
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3.3. Outcrop sampling Of the 110 outcrop rock samples taken within the project area during different campaigns, 35 have values of more than 1 ppm for Au, and 15 have more than 10 ppm. The maximum value is 155 ppm. The correlation of the values for gold with several elements, primarily As, Ag, Tl, Sb, Pb and Hg, is remarkable. 3.4. Geophysics Different techniques have been applied: magnetometry, very low frequency (VLF), induced polarisation (IP), radiometry, etc. Detailed magnetics, VLF resistivity and radiometry appeared to be useful for mapping purposes. In some cases the IP chargeability response is a product of both a lithological marker horizon (a zone of fossiliferous black shales) and mineralisation. 3.5. Drilling There are 35 drill holes, totalling 6500 m in length in Salamo´n. Fig. 4 shows three sections of Salamo´n East, which includes drill hole 24, the deepest in the project area, with a length of 377 m (Section A–A 0 ). The most remarkable results were obtained from drill hole 31 (Section 5137 E, B–B 0 ), in which 29 m (22.2 m of true width) of rock were intersected grading 20.5 ppm Au. This result is the average of 28 samples of about 1 m each, of which only three had values less than 10 ppm Au. Fig. 5 is the plan at 1100 m.a.s.l. of Salamo´n East compiled from the results of the drill holes. 876 drill hole samples have been analysed for several elements, with their correlation coefficients established. The positive correlation coefficients of gold with silver, arsenic, antimony and mercury is remarkable, all of which are greater than 0.5 (e.g. Fig. 6). 3.6. Resource estimate In Salamo´n East, in a lens with a maximum width of 22 m, intersected by drill hole 31 in its centre, is estimated a resource of 450,000 t of mineralised material with an average grade of 7.86 ppm Au. The
mineralisation can continue laterally and at depth and more work needs to be done. Only four drill holes have been made in Salamo´n West, and it is impossible until now to define a mineralised body and estimate a mining resource. 3.7. Metallurgical testwork The gold at Salamo´n is refractory, hosted by arsenic-bearing pyrite and the arsenopyrite, which is found in crystals measuring a few dozen microns. The final conclusion of the metallurgical testwork is that more than 90% of the gold can be recovered by roasting under special conditions (Garcı´a Frutos and Alvarez, 1999). 4. Mineralisation Two surface occurrences follow a N110 E subvertical fracture. Also two mines are located along this fracture. They are the Rosa Mine in Salamo´n East, where the presence of gold was first discovered, which consists of one small extraction gallery at a height of 1120 m and Valle Mine in Salamo´n West, which consists of two small extraction galleries at a height of 1140 and 1150 m (Fig. 2). 4.1. Morphology The mineralised bodies appear, in general, as elongate shaped lenses with a subvertical outline that are more or less parallel to the fracture to which they are related (Figs. 4 and 5). These mineralised bodies are made up with mineralised tectonic breccias, veins and pockets, and disseminated sulphides in a quartz–carbonate gangue. 4.2. Characteristics of the host rocks The host rocks consist primarily of carbonate rocks with very bituminous pelitic intercalations from the Lena Group and, to a lesser extent, of the clastic component of the Maran˜a Group, Stephanian B sediments and igneous rocks. The carbonate rocks from the Lena Group occur in thick layers with a compact structure and a greyish colour that is related to the carbonaceous content. Microscopic observations of these rocks show micritic and biomicritic types with abundant disseminated
J.L. Crespo et al. / Journal of Geochemical Exploration 71 (2000) 191–208 Fig. 4. Geological cross sections A–A 0 , B–B 0 and C–C 0 through the Salamo´n project area (see Fig. 5). The black area corresponds to the silicified strata, with more that 1 ppm Au. The white zone is mostly dolomitised, with some silicification and some gold. The dyke is shown, which crosses into the different stratigraphic units.
197
198
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Fig. 5. Geological plan 1100 m.a.s.l. of Salamo´n East based on the results of the drill holes. In black, the silicified area is shown with more that 1 ppm Au. In white, the dolomitised zone with some silification is shown, which bears some gold.
fossil relics (coral fragments, echinoderms, bivalve shells, brachiopods and foraminifera) and occasional facies of sandy fossil-rich pelmicrites and intramicrites. Relics of silicified fossils (principally calcispheres) can sometimes be differentiated, without any evidence of other processes of alteration. It is
therefore probable that this replacement has a diagenetic origin. The mineralogical composition of these rocks (from drill holes) has been determined by X-ray diffraction and chemical analysis. The rock constituents are: 85–95% calcite, 4–8% dolomite, 2–4%
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Fig. 6. Correlation between gold content of mineralised samples and their arsenic (A) and antimony (B) content for 876 samples from drill holes. The best-fit lines for data points along with correlation coefficients (r) are also indicated.
quartz and 0.5–1% of illite, with traces of pyrite, carbonaceous matter and iron oxyhydroxides. The very bituminous pelitic rocks from the Lena Group consist of interbedded, strongly laminated, argillaceous–arenaceous dolomite or lime mudstone and weakly laminated fine-grained sandstones (Fig. 9a). These rock types appear physically similar. However they differ in composition and texture. In all the beds, especially in those with coarser grain size, a micritic and calcareous–dolomitic matrix of early diagenetic origin can be differentiated together with the remains of bioclasts, ooids and pellets and a very fine detrital phase made up of small grains of quartz, micas and clay minerals. Occasionally, in some of these beds, an incipient silicification and argillisation is observed. The carbonaceous matter and the illite are commonly admixed in some laminated pelitic beds, where they are very abundant and occur in a disseminated form or are concentrated in 0.5 to 2 mm thick current-induced laminations. The pyrite in these beds is very abundant and occurs as euhedral–subhedral crystals and as framboids. Inclusions of chalcopyrite, probably of diagenetic origin, have been identified in some of these pyrite forms. The chemical composition of some laminated pelitic beds was determined through a chemical analysis of 30 samples collected from different drill holes. The results (Table 1) emphasise the low contents of
trace elements (such as gold, arsenic, copper, mercury and nickel), Corg (1.14 and 1.65%) and S (1.84 and 2.30%), and the high contents of CaO (2.63 and 6.43%) and Cinorg (1.12 and 2.01%). In the correlation matrix of the analysed variables it can be seen that the Corg displays some negative correlation coefficients with the majority of the metallic elements; however, the S is positively correlated with Fe, As, Sb and Pb, reflecting the presence of pyrite and other sulphides in these rocks. The clastic lithologies of the Maran˜a Group are made up of mudstones, conglomerates, calcareous breccias and isolated olistolites from the limestone of the Lena Group. The Stephanian B sediments consist of fairly coarse heterometric polymict conglomerates, in which clasts of limestones and sandstones–quartzites of the Ordovician and Devonian formations from the Esla Unit and Lena Group are found. A carbonate or ferroan cement and/or a sandy siliceous matrix is present in these lithologies. The igneous rocks that have been found in the area correspond to a dyke that has a thickness of less than 4 m, but good lateral continuity, and has been intersected in the majority of the holes drilled in the zone. This dyke cuts through all the stratigraphic units and seems to be located along an E–W subvertical fracture dipping steeply to the S, sub-parallel to the mineralised structure.
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Table 1 Average chemical composition of the unalterated-host rocks from the Lena Group (Salamo´n). Analytical techniques: major and trace elements by ICP-ES, INAA and FUSION-ICP, and Corg and Cinorg using a LECO䉸 CS-125 carbon–sulphur determinator (Krom and Berner, 1983). Analysts: Activation Laboratories Ltd of Ontario (Canada) and Geology Department of the University of Salamanca (Spain), respectively. n: number of samples. S: standard deviation. L.O.I.: loss on ignition at 950⬚C Siltstones Mean n 18 SiO2 (%) Al2O3 Fe2O3 MnO MgO CaO Na2O K2O TiO2 P2O5 Corg Cinorg S L.O.I.
53.89 18.02 7.64 0.07 1.70 2.63 0.35 3.64 0.54 0.15 1.14 1.12 1.84 10.84
Marly siltstones Mean n 12
S 2.03 1.22 1.34 0.03 0.36 0.90 0.13 0.37 0.04 0.01 0.62 0.38 2.05 0.76
51.81 15.22 7.34 0.07 1.58 6.43 0.31 2.97 0.50 0.15 1.65 2.01 2.30 13.13
S 3.45 2.76 0.64 0.02 0.57 2.10 0.06 0.50 0.07 0.02 1.50 0.73 1.88 1.91
Cu (ppm) Cr Zn Y V Sc Ba Ni Co U Th Pb Rb Cd Bi As Cs Hf Sb
32 104 59 18 129 18 529 48 19 3 15 28 256 0.6 0.4 142 11 6 3.3
3 9 37 1 10 2 103 14 5 1 2 12 375 0.1 0.1 236 3 2 1.7
26 94 59 21 114 15 418 39 18 4 13 40 127 0.8 0.3 89 9 7 4.3
7 13 53 2 20 3 105 7 3 2 2 21 29 0.2 0.1 94 2 2 5.0
Au (ppb)
5
2
5
2
4.3. Hydrothermal alteration and zonation The types of hydrothermal alterations at the Salamo´n deposit (Paniagua et al., 1996) are similar to those documented in Carlin-type deposits in Nevada (Radtke, 1985; Kuehn and Rose, 1992, 1995; Arehart,
1996). More detailed studies on the zonation of alteration, mineralogy, distribution and its relation to gold and to other ore minerals and alteration minerals are in progress and are briefly summarised below. The hydrothermal alterations of the host rocks related to the mineralised bodies are fundamentally decarbonatisation–dolomitisation, silicification and argillisation, and vary according to the type of host rock. These alterations appear on the surface to be distributed irregularly around the mineralised bodies (Fig. 2). The alteration is zoned outwards from the mineralised structures. This will be discussed below based on observation from the abandoned Valle Mine (Fig. 7) and drilling information at Salamo´n, specially in hole 31 (Fig. 8). 4.3.1. The abandoned Valle Mine To define the mineralogical and chemical characteristics of the alteration zones, several rock samples were collected from N–S trending galleries, perpendicular to the strike of the mineralised system of E–W trending high-angle normal faults in the abandoned Valle Mine (Fig. 7). According to the characteristics observed in the galleries, five hydrothermal alteration phenomena have been distinguished in the Lena Group limestones, namely: dolomitisation front, dolomitised zone, silicification front, silicified zone and calcite veins (Fig.7). Dolomitisation front: This consists of a weakly dolomitised grey limestone. Field and petrographic observations indicate that pre-ore forming fluids moved along a complex of high-angle normal faults (E–W) and spread out laterally due to the porosity of the limestone. Dolomitisation caused minor dissolution of the matrix and of allochems. This gave rise to the formation of a medium-crystalline dolomitic limestone poor in carbonaceous material. In this zone some euhedral and subhedral crystals of pyrite are found, isolated or present as aggregates. Dolomitised zone: This zone consists of a massive dolomite that has a grey colour when it is fresh or beige when it is weathered. This equigranular medium-crystalline dolomite contains a large amount of non-carbonate impurities (Fig. 9b). Very small, disseminated euhedral to subhedral crystals of pyrite and arsenopyrite, sometimes partially oxidised, have also been identified.
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201
Fig. 7. Schematic plan showing mine-scale spatial relationships of zonation of alteration centred around the brecciated and silicified fault zone at the Valle Mine in the western part of Salamo´n Project.
Silicification front: This front is characterised by a grey coloured dolomite with abundant veins and geodes filled with white dolomite. In this zone the alteration ranges from weak fracture-controlled
boxworks to complete replacement (jasperoid) and, therefore, several subzones with varying intensity of silicification can be distinguished. Some of them correspond to a dolomitic rock, very porous and
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Fig. 8. Detail on the silicified feeder structure within drill hole 31 (Salamo´n east). Major Au mineralisation is developed in these structures.
partially silicified. In other more silicified dolomitised rock, only the remains of the dolomite is preserved (Fig. 9d). In both cases there is weak argillisation and sericitisation, as evidenced by dickite and sericite/ illite as accessory components in the studied samples. Petrographic evidence suggests that most of the hydrothermal pyrite and arsenopyrite from early mineralisation fill the porosity of these rocks (Fig. 9b). Hydrothermal pyrite occurs mainly as cubic crystals and occasionally as spherical or sub-spherical agregate of granules or microcrysts (framboidal). Both varieties are closely associated with hydrothermal silica (Fig.10a). Silicified zone (Jasperoids): This zone is formed as the result of progressive replacement of altered carbonate rocks by hydrothermal silica along structures of the complex sets of high-angle normal faults E–W (Fig. 7), and also along the more permeable stratigraphic horizons of the laminated argillaceous– arenaceous dolomite with interbedded bituminous pelites of the Lena Group (Fig. 9a). Generally, hydrothermal-related silicification is less extensive than dolomitisation (Fig. 2). This zone is characterised by its coincidence with the strongest mineralisation at ore scale. All the samples taken at different points of the gallery are strongly mineralised, silicified (jasperoids) and occasionally argillitised. They consist of a microto cryptocrystalline quartz aggregates with abundant disseminated carbonaceous matter and sporadic dickite and sericite (Fig. 9d). The intergrowths between these minerals frequently occur in fractures and cavities as a consequence of the removilisation processes that affect them (Fig. 9f). Mineralogical data and textures observed in these siliceous rocks confirm that very fine crystalline disseminated arsenopyrite and pyrite are the most common ore minerals. Calcite veins: They occur as open space filling along mineralised faults, fractures and breccia zones (Fig. 7). The studied veins consist of relatively clean white calcite as the essential mineral, and sulphide minerals, especially realgar–orpiment, sphalerite, galena and locally arsenic-rich sulphosalt minerals. In some places these veins contain enriched pockets in the removilised sulphide minerals and hydrocarbons (tucholites) (Fig. 10d–f). Field relations and mineralogical data indicate that the formation of many of the shallow calcite veins probably resulted from a very late hydrothermal process, and others may be
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203
Fig. 9. a. Thin-bedded laminated hand specimen of argillaceous–arenaceous limestone from Lena Group. Salamo´n Project area (SS-31-28). b. Photomicrograph showing initial penetration of dolomite (Dol) rhombs by hydrothermal quartz associated with pyritic gold ore from Salamo´n west (Valle Mine) (SS–12–11). Transmitted light, crossed nicols. c. Hand specimen of mineralised jasperoid from east part of Salamo´n Project (SS-31-31). d. Photomicrograph of strongly silicified dolomite rock (SS–31–1). Transmitted light, crossed nicols. e. Photograph showing the fabrics of mineralised jasperoid breccia from Salamo´n east (SS–31–32). f. Photomicrograph details showing dickite (Dk) (hydrothermal clay) and quartz (Qtz) filling spaces of jasperoid breccia (Au–W–28). Transmitted light, crossed nicols.
post-hydrothermal and have formed during supergene oxidation. 4.3.2. Drill hole no. 31 A zoned pattern of the hydrothermal alteration of
the host rocks was also observed in drill hole no. 31 (Fig. 8), which is one of the key-drill holes in the area. The mineralogical and chemical characteristics of the studied samples confirm that the samples closest to the mineralised fractures, which are considered to be
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Fig. 10. Photographs illustrating features of the first stage of hydrothermal mineralisation from the Salamo´n deposit. a. Note the fine-crystalline subhedral to euhedral habit of pyrite and arsenopyrite (SS–31–10). Reflected light, parallel nicols. b. and c. Arsenic-bearing pyrite forming overgrowth rims on pre-existing pyrite, which may exhibit compositional zoning either simple or oscillatory (Au–E–4) and (Os–4–B.2). Reflected light, parallel nicols. d. and e. Inclusions of tennantite (tnn) and grains of uraninite (Ur) in tucholite (TI) (Au–W–14.5) and (Os–4– B.4). Reflected light, parallel nicols. f. Inclusions of an octahedral of vaesite (Va) followed by a cubic crystal of gersdorffite (Gf) and surrounded by uraninite (Ur) in massive gersdorffite groundman (Os–5–2). Reflected light, parallel nicols.
the feeders for the hydrothermal fluids, have the highest dickite/sericite–illite and dolomite/calcite ratio. In contrast, in the more distal samples the opposite occurs since K and the K/K ⫹ Al ratio increase with the distance from the fracture. Regarding the gold content, it has been observed in this case that the
highest values are found in the more proximal silicified (jasperoids) zones. 4.4. General paragenesis at Salamo´n According to Paniagua et al. (1996) and our studies,
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two hypogene phases in the mineralisation can be differentiated. The early stage is made up of very fine crystalline disseminated sulphides in a matrix of quartz–chalcedony (jasperoid) and dolomite. The later stage consists of a coarser crystalline size and mineralogy that is found either replacing the early mineralisations or occurring in pockets and late veins with quartz and quartz–carbonate gangue. A final phase of supergene mineralisation is composed of alteration products of previous mineralisation through the action of oxidizing meteoric water. The early hypogene mineralisation consists predominantly of pyrite, arsenic-bearing pyrite and arsenopyrite, with sphalerite, cobaltite–gersdorffite, Ni–Co–Cu–Fe disulphides of the pyrite group (Paniagua, 1993), graphite, linnaeites and uraninite as accessory minerals and, occasionally, apatite and florencite. The common characteristic of all the minerals is their very fine crystal size, normally less than 10 mm (Fig. 10a and b). The late hypogene mineralisation presents more pronounced local mineralogical variations than in the previous case. This consists of pyrite, chalcopyrite, sphalerite, tennantite with galene, cinnabar, realgar–orpiment and stibnite as accessory minerals, and locally livingstonite, bournonite, Pb–Sb sulphosalts (boulangerite, jamesonite, plagionite, zinkenite) and Pb–As sulphosalts (jordanite, leveingita y rathite). Tennantite is more abundant than chalcopyrite at Salamo´n West. The supergene mineralisation consists of a paragenesis of oxyhydroxides of Fe, Cu, Ni and Co, and carbonates, arsenates and sulphates of Cu, Co, Ni and U. These are all products of meteoric oxidation of the early and later hypogene phases. Fluid inclusion studies on quartz and carbonate show two differentiated phases: a main early phase of higher TH and lower salinity, and a late phase of lower TH and higher salinity (Paniagua et al., 1996, 1997). Primary fluid inclusions in quartz, which are cogenetic with the arsenopyrite and pyrite from the early phase show TH from 148 to 241⬚C, with a mode at 218⬚C. Freezing point depression varies from ⫺1.0 up to ⫺7.7⬚C, indicating low to moderate salinities (mode 9.2% NaCl equiv.). Primary fluid inclusions of the late phase and secondary fluid inclusion have TH in the range of 86–123⬚C and Tm from ⫺10.0 to ⫺15.0⬚C, and show evidence of the presence of ions such as Ca ⫹2 or Mg ⫹2 in the solution.
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The aforementioned authors also present stable isotope data. The d 34S values range from ⫹6.3 to ⫹9.4‰ for early stage sulphides and from ⫹2.45 to ⫺9,4‰ for late stage sulphides. The d 13C and d 18O values in carbonates dolomite and Mg–calcite) range from ⫹0.9 to ⫹4.1‰ for d 13C and from ⫹17.6 to ⫹26.1‰ for d 18O. These data, according to Paniagua et al. (1996), suggest a reduction of sulphur from marine evaporites, the existence of a later hypogene oxidation and buffer of carbon in the fluid by the host carbonates. Making use of the abundance of uranium, along with the absence of thorium, the early stage mineralisation at Salamo´n has been dated using the U–Pb method at 269 ^ 5 My (Paniagua et al., 1996). These data are similar to those obtained from the Providencia 271 ^ 11 My and Profunda 273 ^ 11 My mines, which are epithermal deposits of Cu, Co, Ni with Au, in carbonate rocks located near the Leo´n fault, 40 km west of Salamo´n (Paniagua et al., 1993). All these deposits therefore formed during the Permian. These ages also correlate with the age determinations of intrusive rocks spatially associated to several ore deposits in the area (Gallastegui et al., 1990). 4.5. Relation of gold to sulphide minerals Macroscopic and microscopic visible gold from the Salamo´n gold deposit have not been observed. It is a clear example of an “invisible” gold deposit, with pyrite and arsenopyrite being the gold-bearing minerals. Pyrite is As-rich and studies by EPMA show that Au is positively correlated with As in pyrite and arsenopyrite (Paniagua et al., 1997). In general, textures observed in the mineralised samples studied from the Salamo´n deposit, in addition to the chemical composition of the iron sulphides obtained by microprobe, suggest that there are two types of iron sulphides present. The first type is interpreted as a diagenetic pyrite because of its presence in mineralised and unmineralised rocks. It is characterised by the lack of Au or As (Table 2). The second type forms overgrowths on pre-existing pyrite and is interpreted as pyrite deposited from the hydrothermal solution that also carried As and Au. It exclusively occurs in the mineralised rocks (Table 2). These overgrowths vary in thickness from less than 1 mm (Fig. 10a and b) to 25–30 mm (Fig. 10c). The As
642 103 1809 1103 796 476 1533 60 2282 1240 1017 318 604 145 256 382 528 252 353 102 270 541 673 223 226 1333 1486 1209 384 481 183 870 1227 622 797 210 303 176 116 1130 577 233 349 101 103 981 1104 130 7418 7013 812 356 811 351 5401 3471 630 340 1219 370 364 307 188 688 791 122 595 220 183 322 978 50 242 454 1190 320 176 747 423 827 330 477 n.d.
53.12 45.96 0.24 47.02 46.73 5.60 53.35 45.50 0.10 52.07 47.47 0.27
329 137 606 727 530 280
S(%) Fe As
Cu (ppm) Zn Ni Se Au Ag
1.19 1.07 0.34
Au–E–5 S Mean n 13
0.30 0.41 0.09
Au–E–6 S Mean n 6 Au–E–4 S Mean n 9
2.11 0.90 2.47
47.94 43.85 6.40
2.64 1.46 2.35
52.86 45.84 0.70
0.98 0.52 0.33
53.01 46.02 0.38
0.51 0.69 0.89
SS–31–12.1 S Mean n 9 SS–31–1 S Mean n 6 Au–W–31-1 S Mean n 5 Arsenic pyrite Pyrite Sample
251 179 382 907 365 257
22.60 35.04 41.73
Au–E–5 S Mean n 5 Os–24.1 S Mean n 7
0.45 0.32 0.37
Arsenopyrite Marcasite–pyrite
1.43 0.72 1.77
J.L. Crespo et al. / Journal of Geochemical Exploration 71 (2000) 191–208 Table 2 Microprobe analyses of pyrite, arsenic-bearing pyrite, marcasite–pyrite and arsenopyrite from Salamo´n gold deposit. n: number of analyses. n.d.: not determined. S: standard deviation
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content of these overgrowths commonly varies as a function of proximity to the sulphide mineral cores (Fig. 11a). Marcasite is also a common constituent of the studied mineralisation and is closely associated with arsenic-bearing pyrite (Table 2 and Fig. 11b). Arsenopyrite is also very fine-crystalline, and appears in the form of needle-like to curve-face crystals (Fig. 10a). It also contains Au-traces (Table 2). The correlation between Au and As on a microscopic scale is positive (Table 2) and is consistent with the commonly observed geochemical association between As and Au in soil geochemistry (Fig. 3) and rocks (Fig. 6). The association of As and Au is so common that the former is commonly used as a pathfinder element for hidden gold deposits (Boyle, 1979). All these features are very similar to those in Carlin-type deposits, where the majority of the Au and As were deposited in arsenic-bearing pyrite overgrowths on pre-existing pyrite surfaces, as a function of redox reactions involving oxidation of Au and reduction of As (Fleet et al., 1988; Cathelineau et al., 1988; Chryssoulis, 1990; Arehart et al., 1993a,b).
5. Conclusions The geological, mineralogical and geochemical features of the Salamo´n deposit are similar to those defined for Carlin-type deposits (Berger, 1986; Berger and Bagby, 1991; Kuehn and Rose, 1992; Arehart, 1996). This conclusion is based on the following arguments: high-angle fault zones; selective replacement with silica of carbonaceous carbonate rocks along the high-angle faults, regional thrust and bedding; very fine sulphides dissemination in carbonaceous calcareous rocks and the associated jasperoids; general scarcity of fine crystalline sulphides (less than 1%); presence of igneous rocks; mineralisation of pyrite, realgar, orpiment with arsenopyrite, cinnabar, fluorite and stibnite in a gangue of quartz, calcite and organic matter. Other data can support this interpretation, such as the type and characteristics of the associated alteration; the temperatures and formation pressures; the presence of heavy sulphur, possibly of an evaporitic origin in the early stages and the existence of a later hypogene oxidation (Paniagua et al., 1996). All of these characteristic provide a framework for establishing a model for a similar genesis.
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Fig. 11. Sketch of several crystals of diagenetic pyrite with arsenic-bearing pyrite overgrowths (a) and/or marcasite (b).
Acknowledgements We would like to thank the Journal of Geochemical Exploration reviewers for their patience and their constructive suggestions. We thank Kerr Anderson for his help with improving the English at the last revision of the text and his useful comments. This work was supported by the Comunidad Auto´noma de Castilla y Leo´n (Research Project SA 54/97); by the Agencia de Desarrollo Econo´mico de Castilla y Leo´n, through a grant for training of the investigators, and by a I ⫹ D Project, FEDER Program (Ref. 1FD97-0235).
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