Geological setting

The study area is located in Santander Massif, which consists of Precambrian, metamorphic rocks, Jurassic intrusive igneous rocks, and Tertiary subvolcanic rocks (Miocene), also with colluvial deposits (Figure 1).

Bucaramanga gneiss rocks

In this area are outcrops paragneisses (quartz-feldspathic, quartz-biotitic, quartz feldspathic, and biotitic and silimanithic), quartzites and amphibolites. These rocks present retrograde metamorphism for biotite, evidenced by the presence of chlorite (^{Urueña & Zuluaga, 2011}). These rocks have a strong hydrothermal alteration, as evidenced by the presence of chlorite, epidote, feldspar, sericite, carbonates, and silicification associated with plagioclase and aluminosilicates, and veins.

Jurassic intrusive igneous rocks

Petrographically, these rocks are holocrystalline, fine-to-medium grained with a very light pink and gray color; their composition varies from granite, diorite, and tonalitic igneous rocks, and correspond to different bodies but contemporaneous (^{Ward et al., 1973}); which lie in the north and east of the study area (Figure 1). These rocks, close to mineralization zone have a moderate and zoned hydrothermal alteration, characterized by silicification and pyritization on the periphery and argilic alteration near the mineralized veins. The argilic alteration is strong and pervasive masking of the rock's original texture. These rocks are strongly fractured, helping the formation of stockwork mineralization.

Miocene intrusive rocks

These rocks occur as intrusives and dikes outcropping in the northern part of the study area, in the road to Reina de Oro Mine and within a few tunnels. The rocks are porphyritic quartzmonzonites, light pinkish to gray and composed of quartz, plagioclase (oligoclase) and biotite, zircon as accessories.

In some tunnels some dikes of this unit were observed cutting veins. In the La Reina de Oro tunnel, a sample of dacite was taken, where fine disseminated pyrite, arsenious pyrite, chalcopyrite and magnetite (Figure 2) was identified.

Figure 2 Argillic dacite dike cutting by some mineralized veinlets, and the same time the dike cut other veins too. Reina de Oro Mine 

Structural Control setting

The Vetas mineralizations have a structural control related to the Bucaramanga (NW direction), Cucutilla (NE direction) faults, and their associated structures, which create a distensive tectonically originating spaces that served as conduits where the bodies were emplaced (^{Mantilla et al, 2009} and ^{Mantilla et al, 2011}).

Structurally reefs are related to fractures set with N55°W and N30°-60° strikes, with veins strikes of N30° to 50°E, N60°to 88°W and EW, and with dips between 30° to 90°. Dip variations are observed in places that vein thickness change (^{Mantilla and Mogollón, 1991}), that situation is caused for little faults that cut the reefs.

The El Volcán Fault has a sinistral (left-lateral) movement, moving veins from 10 to 15 meters. Other smaller faults with a NE strike late to mineralization are significant because they cause truncations and wedge veins (Figure 2 and 3).

Figure 3 Vein wedged by faulting, La Peter mine 

Ore deposit mineralogy

The mineralization occurs mainly in veins with thicknesses ranging from 30 cm to 1 meter. The most of the veins are banded, where the central strip correspond to recrystallized quartz and sulphides, occasionally tourmaline and sericite, this area contains higher gold concentrations. On both sides of these bands of quarts are presented primary and recrystallized quartz, and sulfides, also a significant increase in the intensity of sericite alteration. The most outer part of the vein corresponds to an argillic and mineralized zone, which is characterized for a bluish green color, and also presents significant gold concentrations.

Gangue Minerals

Quartz is the most abundant mineral, and was identified in all mineralizing stages (4 mineralizing stages and the post mineral event). Quartz occur with different textures (^{Dong et al., 1995}) that were used as a guide for recognizing the paragenetic sequence. These textures are:

Figure 4 a. Quartz (Qz) textures, a. Quartz with "Comb" texture (center of the picture), Quartz with mosaic texture and tourmaline (left) of the third event, and microcrystalline quartz (right), b. Fragment of massive quartz with tourmaline (To) in a matrix of cryptocrystalline quartz (brecciated texture). 

Ore Minerals

The paragenesis includes gold, sulfides like pirrotite, pyrite, tennantite, pyrrotite, galena sphalerite and traces ofmolybdenite and oxides like, magnetite-ilmenite, the oxides being the most abundant. These minerals occur in veinlets or disseminated in intergrowth textures, replacement and filling spaces in the veins and host rocks.

Gold: It was identified in the third mineralizing stage, included in tennantite or fractures of low iron sphalerite (figure 5), and massive quartz associated with tourmaline and quartz with flamboyant and massive texture. Gold is irregularly shaped, its size varies between 26 and 41 urn. Other authors reported free gold associated with quartz and euhedral pyrite (^{Cortés, 2002} and ^{Garcia y Uribe, 2003}).

Figure 5 Gold (Au) include in tennantite (TNT), in paragenetic association with sphalerite and galena early. 

Silver: it has been reported in other works (^{Cortés 2002}, ^{Garcia y Uribe, 2003}), and others) associated with pyrite and galena. Within the Electron probe micro-analyzer (EMPA) analysis show, between 10% and 15%. Ag in the tennantite.

Pyrite: Three generations were identified: 1. Fine pyrite, 2. Coarse-grained (>100 microns) pyrite associated with As- pyrite (lighter and anisotropic) and 3. Fine-grained pyrite (±50 microns, Figures 6 and 7). Some crystals of pyrite are product of recrystallization of pyrrhotite (Figure 6).

Figure 6 Ore minerals. a. Chalcopyrite crystals in the contact with molybdenite, b. Magnetite intergwith ilmenite exsolution, c. Tennantite in paragenesis with sphalerite and chalcopyrite, pyrite later or in paragenesis with sphalerite, d. Pyrrhotite and pyrite, before sphalerite , e. Ass- pyrite intergrown with pyrite (porous maybe product of pyrrhotite recrystallization) f. Chalcopyrite in paragenesis with pyrrhotite before As pyrite. 

Galena: It has coarse particle size (>100 microns) and is associated with quartz with flamboyant and massive texture; in some cases, it is being replaced by sphalerite, so this indicates that was formed during first mineralizing event (figure 5 and 6).

Sphalerite: two species were identified, one clear and the other dark related to variations in the iron content possibly (Table 1). The first one crystallized during the first stage associated with carbonate when is disseminate in the host rock close to the veins and sericite, also appears as islands in pyrite, sometimes like inclusions and other times filling pores or associated .with tennantite (Figure 5 and 6).

The clear (visible with transmitted light) sphalerite is found mainly in the central part of the veins, as coarse-grained, euhedral crystals. It can be found inter-grown with tennantite, replacing galena and pyrite and filling spaces in pyrite and As-pyrite.

Tennantite (Cu₁₂AgZn₃AsSb₄S₁₆-Cu₅Ag₂ZnAsSb₂S₈): occurs in coarsegrained aggregate, with clear sphalerite. Its composition was determined by EMPA analysis (figure 5 and 6).

Molybdenite: is pinkish dirty pale gray color, with a fibrous and pseudohexagonal habit, presents a strong anisotropy and pleochroism. It occurs in the second stage of mineralization (Figure 6).

Chalcopyrite: occurs as pyrite islands in "atoll" textures or in contact (Figure 6) with fibrous cristal of molybdenite, indicating that they were precipitated on first to early second mineralization stages, even though some crystal of chalcopyrite occur in the three and four mineralizing event. According to some EMPA analysis, they contains arsenic in their composition (Figure 6).

Pyrrhotite: occurs as fine-grained, pink crystals, they are presented in the first mineralization event associated with chalcopyrite and dark sphalerite (Figure 6).

Magnetite: the magnetite have exsolutions of ilmenite, this indicate Ti in the structure of magnetite. This mineral appear in the second mineralization stage (Figure 6).

Alteration minerals and hydrothermal alterations

Sericite andillite (figure 7): main minerals alteration has good crystallinity and its size varies from fine to very coarse-grained (> 100 microns, possibly muscovite). It is presented as alteration in sillimanite and plagioclase alteration from the host rock, and is associated with sulfide and quartz veinlet in the first and second mineral stage, when the granulometry of sericite is fine-grained is presented with illite (identified by DRX).

Figure 7 a. Veinlet of adularia in gneiss quartz feldspathic and sericitization on plagioclase, b. Plagioclase with sericitic (coarse sericite) alteration. c. Veilent of adularia. d. Ankerite and calcite of alteration in quartzmonzonite. 

Adularia: is the second alteration mineral in abundance, is found as coarse-grained (100 to 500 microns), euhedral crystals in veinlets, sometimes associated with albite and sericite. It represents in the first stage of mineralization.

Ankerite (Identified by SEM) is restricted to the areas close to veins, where the rock alteration is not strong (figure 8).

Figure 8 Analysis of ankerite with electronic microscopy was identified calcium, iron and magnesium. 

Tourmaline: It is characterized by having prismatic habit, thin and elongated crystals with brownish green color. It is associated with the ore-rich area of the veins, accompanied by quartz with flamboyant and massive texture between second and third stage.

Other identified alteration minerals include chlorite and epidote, in remote areas of the mineralization; kaolin, affecting feldspars from the host rock and alunite (Identified by SEM and DRX), possibly product of supergene alteration as well as iron oxides and hydroxides are also recognized.

Paragenetic sequence

Four mineralizing events and one post-mineral stages separated by fracturing, brecciation and mineralogical changes were identified. Such associations are mentioned below and are represented in Figure 9:

Figure 9 Paragenetic sequence 

Stage 1: association of sericite, carbonate (ankerite and calcite?), massive and microcrystalline quartz (possibly recrystallized ^{Demoustier, Castroviejo, t Charlet, 1998}), sphalerite, adularia, albite, galena, thin pyrite, pyrrhotite, chalcopyrite.

Mineralogical change is produced by a temperature increase and enrichment of fluids.

Stage 2: Molybdenite, magnetite with exsolution of ilmenite, As-pyrite, sphalerite, fine-grained pyrite and little chalcopyrite quartz with huge, feathery, fine mosaic, flamboyant (product of recrystallization of massive quartz first event or from chalcedony ^{Demoustier, Castroviejo, t Charlet, (1998)}) and microcrystalline textures quartz and, tourmaline and sericite.

Brittle deformation, which produced conduits where new minerals were mobilized and precipitated.

Stage 3: Gold and tennantite associated with sphalerite, fine-grained pyrite, coarse-grained pyrite and As-pyrite, sometimes with chalcopyrite like inclusions, and quartz with flamboyant, mosaic, massive and "comb" textures, and tourmaline.

Stage 4: Fine-grained pyrite microcrystalline quartz and sericite. A generation of fine-grained pyrite with microcrystalline quartz that produces brecciated textures.

Post-mineral stage: This event happens in open spaces result of brittle deformation or remaining space of crystallization, which where was formation of small quartz geodes with "comb" texture, ferrum hydroxide and alunite, halloysite and kaolinite.

Fluid inclusions mycrothermometry

The study of fluid inclusions provides information related to changes in the temperature and composition of the mineralizing fluids.

The measured fluid inclusions correspond to 1, 2, 3 mineralizing stage and the post-mineral event (Figure 10).

Figure 10 Fluid inclusions in quartz with massive texture of the second mineralizing stage. 

The research was carried out in five polished thin sections chosen because of their mineralogical characteristics and characterized by their textures and abundance of fluid inclusions that distinguish the different mineralizing pulses. The measured fluid inclusions correspond to 1, 2, 3 mineralizing events and the paragenetic post-mineral sequence (Figure 9). During the analysis, the following was discovered: biphasic inclusions (the most abundant), very small monophasic and three phasic (only two examples, with crystals of opaque minerals).

Biphasic inclusions have sizes ranging from less than 5 µm (the most abundant), 10µm - 30µm and the biggest range from 80µm to over 100µm. A lot of varieties of shapes are observed: Regular, irregular, ovoid and tabular. The primary inclusions in quartz with massive texture are the biggest, usually they appear lonely or as isolated groups and in the center of the crystals. Some pseudo-secondary inclusions are found as smaller aligned sets along microfractures that do not touch the edges of the crystals

Secondary inclusions were observed in the recrystallized edges of the quartz with flamboyant and feathery textures or in fractures that cross a lot of crystals (Figure 11).

Figure 11 Secondary fluid Inclusions in quartz with flamboyant texture of the third mineralizing event. 

Fluid Inclusions that have an evidence of strangulation or fluid leakage was not measured.

Four temperature changes were visualized:

These temperature changes allowed the authors to not only determine the chemical system but also the minimum trapping temperature, which allowed to determine important variations in temperature during deposit formation.

First melting temperatures are quite low, indicating the presence of salts in the water, and can be divided into three groups of temperature: -40°C to -35°C, -30°C to -25°C and -23°C to -20°C. This range of temperatures show that the composition of the mineralizing fluids was quite complicate, where possibly a saline mixture of NaCl, CaCl₂ and MgCl₂ is presented (table 1).

In some of the fluid inclusions changes at temperatures between -25°C and -20°C corresponding to the final hydro-halite melting point (table 2). Low temperatures of the first melting and final melting hydro-halite, in most of the measurements in Events 1, 2 and 3 indicate that the chemical system is complex, with at least three different kinds of solutes presented: H₂O - NaCl, H₂O - NaCl - CaCl₂ y H₂O - NaCl - MgCl₂, of which NaCl is the predominant, while the MgCl₂ is the lowest in abundance. To determine the presence of these salt systems the following programs were used: AqSo1e, AqSo2e and AqSo3e, Software Package Fluids, Version 1, for aqueous fluid inclusions developed by Ronald Bakker of the University of Leoben in Austria (Table 2).

Table 2 Measurements and calculated parameters from fluid inclusions mycrothermometry. The analyses of mycrothermometry was doing with the Bakker - FLUID, software AqSole AqSo2e y AqSo3e y Loner 38, for the watery inclusions. (T ff: first fusion, Thh: final fusion of hydrohalite, Tfl: Final fusion of ice, Th: homogenization temperature, L: Liquid, V: Vapor). 

First melting temperatures are also variable, and generally low, being samples between -6°C to 0°C and others between -12°C to -8 ° C, that in terms of salt concentration correspond to approximate salinities of 16 wt%eqNaCl to 11 wt%eqNaCl and 10 wt% eqNaCl to 0 wt%eqNaCl respectively. Inclusions related to the early mineralizing events are a little more saline, for example, for the first and second stage the salinity may be above 10 wt%eq NaCl, for the case of second stage can be up 18 wt "/oeqNaCl (for the first stage 12 wt %eqNaCl), meanwhile for the third and post mineral stage only achieve to 10 %eqNaCl (figure 12).

Figure 12 Histogram showing the salinity of fluid for the stage measured. The salinity is similar for the all satage, but for the second event is showing some pikes of salinity, and the most dates are between 6 and 10 %wt NaCl eq. 

Fluid inclusions of the first mineralizing stage show that temperatures are between 210°C and 250°C, with an average of 220°C and minimum trapping pressure (calculated using the Software Package Fluids, version 1. of ^{Zhang and Frantz, 1987}), varies from 1 to 2 MPa and depths calculated in general under 1000 m, showing that the conditions were propitious for the formation of epithermal deposits (^{Camprubi & Albinson, 2006}).

The second mineralizing stage show a significant increase in the homogenization temperatures, reaching values above 350°C (the range the temperatures is from 227 to 387°C), with an average of 279°C. These high temperatures are consistent with the presence of minerals such as tourmaline, and molybdenite in this event. Values calculated show a minimum pressure range from 3 to 9 Mpa, these being higher values compared to the first mineralizing event. This change in conditions could be explained as an increase in pressure associated with the emplacement of igneous bodies that heated the system contemporaneous with the second mineralizing stage like was observed in the Reina de Oro Tunnel.

The fluid inclusions in the third mineralizing event measurements show a decrease in temperature, ranging between 200°C and 240°C, this change is also evident in the mineral assemblages presented in this event, where high-temperature minerals were not identified. The calculated pressures are from 1 to 2,8MPa, which are similar to the first mineralizing event.

For the fourth mineralizing event it was not possible to make any microthermometry measurement, because the quartz type present in this event is very fine-grained, cryptocrystalline quartz. However according to the mineralogical scarcity could be inferred that fluids were becoming more impoverished, and the luid continuous with the cooling process.

In the post-mineral mineralizing event few measurements were made because most inclusions were found liquid-rich, biphasic or with a very small size. Of the few that could be measured, low homogenization temperatures were obtained from 125°C to 190°C (Figure 13).

Figure 13 Histogram showing the homogenization Temperature.  

Stable isotopes

Around 25 sulfur isotope analysis (δ³⁴S) in pyrite and galena, 7 isotopes of O (δ ¹⁸O) and 6 isotopes H (δ ²H) were performed.

These analyzes were carried out in the laboratory of the United States Geological Survey USGS for stable isotopes in Denver, Colorado (δ ³⁴S, δ ¹⁸O and δ ²H) and University of Salamanca in Spain (d¹⁸O and δ ²H). The results of isotopie analysis δ ³⁴S are reported, based on the standard values of troilite from Diablo's Canyon; for δ ²H and δ ¹⁸O isotopes values are given according to the isotopie signature of seawater.

Deuterium and oxygen analyzes were performed into sericite and quartz. Sericite was separated from samples of mineralized veins and backups, which were crumpled and pulverized, and quartz was obtained from mineralized veins.

Sulfur isotopes

Isotope values for δ ³⁴S show a bimodal trend with values between 1 %o to -6 %o and -17.6 %o to -20 %o (Figure 14), indicating two different sources of sulfur. The closer to zero values are associated with felsic igneous rocks (^{Campbell & Larson, 1998}), sulfur was possibly leached from the sulfides presented in igneous rocks (^{Rye & Ohmoto, 1974}). The most negative δ ³⁴S values are the result of leached of metamorphic rocks (from sedimentary) for reactive fluids during hydrothermal process, and imposed their isotopic signature to mineralization (Tang et al., 1998 from Y. ^{Zhang et al., 2017})

Figure 14 Sulfur isotope analysis (δ ³⁴S) 

Deuterium and oxygen isotopes

Analyses were performed on samples obtained from quartz veinlets and sericite from the alteration zone of the host rocks in the vein zone.

The isotopic values of δ ¹⁸O_SMOW%0 are close to 10 indicating that fluid have strong magmatic affinity (^{Campbel & Larson, 1998}), in the case of δD_{SMOW %o} the situation is the same the most values have magmatic signature (^{Campbel & Larson, 1998}).

When deuterium and oxygen results are compared it (Figure 15), a strong magmatic affinity is observed. This affinity is illustrated by the fact that most of the points fall very close to the field of magmatic waters and even two of the values are within the field. The points that are outside of this field tend to move toward the line of meteoric water, indicating that they are also related.

Figure 15 Diagram of δ ¹⁸O vs δD. Modified from Taylor, 1979 in Barnes, 1997 

According to the previous facts it can be interpreted that the mineralizing fluids were generated due to a mixture of meteoric and magmatic waters, being magmatic waters dominant.

When deuterium and oxygen results are compared it (Figure 15), a strong magmatic affinity is observed. This affinity is illustrated by the fact that most of the points fall very close to the field of magmatic waters and even two of the values fall within the field. The points that are outside of this field tend to move toward the line of meteoric water, indicating that they are also related.

According to the previous facts it can be interpreted that the mineralizing fluids were generated due to a mixture of meteoric and magmatic waters, being magmatic waters dominant.

Ore deposit age

Four rock samples were taken from three different veinlets; these were chosen according with their high content of sericite.

The rock samples were pulverized, sieved and clays were separated by decanting. In the case of thicker sericite these were separated by handpicking method under stereomicroscope. The samples were analyzed in the laboratories of the University of Oregon.

As shown in the Table 5 and figure 16, mineralization originated from Miocene to Pliocene, where 3 different hydrothermal pulses were identified, the oldest at 10.78±0,23 Ma, an intermediate between 7,68±0,15 Ma and 6,89±0,41 Ma and the last one of 5,24±0,10Ma. The first pulse could be contemporaneous with intrusion bodies, one of them a granodiorite with age is (U/Pb on zircon) 10.9±0.2 Ma, located in the Meseton sector (^{Mantilla et al, 2011}) 1 to 3 Km at NW of mineralizations visited on Vetas, indicating that body like that could be source of fluids and temperature for the first stage of the mineralization.

Figure 16 Age plateau and inverse isochron. The graphics show satisfactory results because the plateaus are almost perfect, with exception to the samples 1002 and 1018 (duplicate), where the 10% of the released gas is below the average age and the sample and the sample 1022 where 10% of the last released gas is above of the average age. The ages calculate with the inverse isochron are very similar to the plateau values, even though in some case there are small variations but those values are into the margin of error, only in the case of sample 1018 the values are more scattered, for this reason the assay was repeated. 

The others ages 7,68±0,15Ma and 6,89±0,41Ma could be with the second and possibly third stage, because the petrography in that samples corresponding with that stages, and the 5,24±0,10Ma corresponding to fourth stage.

Also, there are some intrusive rhyodacitics bodies younger in the area with age from 8.4 ±0.2 y 9.0 ± 0.2 Ma (^{Mantilla et al, 2009}), that indicate that the magmatism was continue for a long time, and contribute to warm and elements to mineralizing fluids. It's therefore possible there are intrusive bodies younger no date in the area

Metallogenic model

The low to intermediate sulfidation epithermal ore deposit located in Vetas, Santander, originated in a tectonic framework associated with The Bucaramanga fault (main structure) and crossings structures with strikes of NE and NW, which helped as a duct for the magmas ascending, product of Caribbean Plate depth subduction under the South American Plate (^{Mantilla et al. 2011}) .

These magmas with ryodacitic to quartz-monzonites composition were important in the ore deposit formation; because they were initially the main source of mineralizing fluids. The first one of 10, 9± 0.2 Ma (^{Mantilla et al, 2011}), was the source of first stage possibly, and 8.4 ±0.2 and 9.0 ± 0.2 Ma (^{Mantilla et al, 2009}) of second stage, even though other bodies more recent no dated.

Between the Miocene - Pliocene, magmatic fluids, reacted with Bucaramanga gneiss rocks and intrusive igneous rocks (Jurassic and Tertiary) altering it. During the first stage (10.78 ±0.23) these fluids were mixed with meteoric water; they were neutralized and the metals were transported by bisulphides complexes because according with the alteration mineralogy (illite - sericite - carbonates), and temperature (200 to 240°C in the first stage) was the most appropriated mechanism, on shallower depth the enviroments is most oxidant and the metals deposited on open spaces in an low sulfidation environment.

A magmatic pulse between first and second stage of 8.4 ±0.2 and 9.0 ± 0.2 Ma was identified to 8 km NW approx. this pulse could be related to the second stage (7,58±0,15Ma and 6,89±0,41Ma ). The difference between ages is almost 1 Ma, but is possible the presence of others bodies no dated but identified in the area (Figure 1). The mechanism of transported to the second stage its considered similar to the first stage, but the mineralogy, the fluid inclusions show more temperature and stable isotopes more magmatic affinity, this aspects allow suggest that overprinted a system of intermediate sulfidation to almost marginal porphyritic conditions on low sulfidation environment.

The third stage is considered like a progression of second stage, but with the coolest and poorest (in metals) fluid, was separated of first event because the minerals were deposited in new spaces opened, product offracturing event. The four stage dated with (5.24 ±0.10 Ma), could be representing a progression on the luid evolution, with more interaction of meteoric luids, produce a hydraulic brecciating (^{Mciver, 1997}). The post mineral stage is the continuation of evolution of luid colder and poor in metals, possibly on areas close to surface with a strong oxidation activity and an acid ph (alunite, kaolinite and halloysite).