Andiñuela Gold Project
The 58 km2 Andiñuela Gold Project consists of two investigation permits, P.I. Caia and P.I. Gala (Figures 1 and 2) and is located in the north-central portion of Leon Province, within the municipalities of Santa Colomba de Somoza, Lucillo, Los Barrios de Salas and Molinaseca. It is approximately 350km northwest of Madrid and a 4 hour drive by four lane highway and all weather tarmac roads.
The area underwent extensive gold exploitation by the Romans as early as the year 19 BC onward. Evidence remains of over 20 Roman era pits and workings (Figure 3) within and around the Andiñuela Project. Major faulting occurs within the central part of the project and divides the area into two distinct geological formations. The trends of the Roman workings are consistently oblique or sub-parallel to this major structure. Gold mineralization at Andiñuela is associated with quartz veins and breccias in greenschist facies metamorphosed and hydrothermally altered Cambrian to Ordovician sediments. Recent grab samples have yielded up to 21 g/t Au. A two phase soil sampling program has delineated several significant gold in soil anomalies. The population density is low in the area, with higher elevations more conducive to forestry plantations.
The Romans extracted over 7 million cubic metres of bedrock material from the major pits and workings around Andiñuela, between 19 BC and the 3rd century. They are attributed to having worked over 34 million cubic metres of Tertiary and Quaternary alluvial material 10 to 20 kilometres downstream from the Andiñuela area. In the late 18th and early 19th centuries, British and German companies organized small scale artisanal mining in northwestern Spain. It wasn't until the late 20th century when a portion of the Andiñuela area was explored by Rio Tinto, Sevelar and BRGM. Stream sediments, soil sampling and 4 drill holes comprise the known recent work in the area. Anomalous gold and coincident arsenic anomalies were located by BRGM in the central part of the existing project. These targets were not drilled.
GEOLOGY AND MINERALIZATION
Andiñuela is situated within that portion of the Hercynian Massif covering most of the western half of the Iberian Peninsula where it forms the so-called Hesperian or Iberian Massif. This massif is sub-divided into five zones, elongated following the strike of the Hercynian structures: Cantabrian Zone, Western Asturian-Leonede Zone, Central Iberian Zone, Ossa Morena Zone and South Portuguese Zone. Andiñuela is located in the Western Asturian Zone (Figure 4), specifically within the Somoza anticlinorium in the Mondonedo Thrust domain, characterized by a thick Cambrian-Ordovician sequence composed of quartzites, slates and sandstones, locally with marls and limestone, of the Los Cabos Series, all affected by intensive hydrothermal activity. This is the same structural belt and sequence that the Salave Project (Measured - 2.2Mt @ 3.88g/t Au; Indicated – 15.8Mt @ 2.79g/t Au; and Inferred – 3.8Mt @ 2.8g/t Au (total M+I + INF 2.0M oz Au)) is located within.
A salient WNW-ESE structure, the Compludo Thrust, divides Andiñuela in two sectors with important differences:
- In the northern sector, where the base of sequence is constituted by the Compludo quartzites followed by a thick sequence of slates with quartzite and sandstone beds; infrequently thin tuffites may be present; and
- The southern sector is characterized by a monotonous sequence of slates and siltstones with intercalated diabase sills.
The rocks outcropping at Andiñuela show a low-grade metamorphism reaching the green-schist facies. In the NW portion of the Somoza anticlinorium, to the north of the area, the Los Cabos series materials show evidences of thermal metamorphism that is attributed to the presence of non-outcropping Late Hercynian intrusions.
The above sequences have been affected by Hercynian Orogeny in three compressive phases and a late extensional one:
- A compressive phase represented by asymmetric folds with subhorizontal axis N110-120° and axial planes dipping to the south; an associated axial plane schistosity (S) is well developed;
- A second phase giving rise to thrusts and shearing, subparallel to the pre-existing structures, indicating compression from the south; it has an associated fracture cleavage (S2);
- A third phase is characterized by open folds with axes parallel to the first phase ones with subvertical axial planes; a crenulations cleavage (S3) is well developed in the subhorizontal flanks of the first phase folds; and
- A late extensional phase pertaining to the longitudinal faults and radial folds and faults; the radial open folds strike NNE-SSW roughly perpendicular to the previous structures; the radial fault are subvertical with a strike-slip component. This phase also caused reactivation of thrust and inverse faults planes.
The primary mineralization occurs as very fine to millimetre-sized dissemination of pyrite, arsenopyrite (native gold is also reported in literature) and in cracks in both hydrothermal quartz and in the host rock. In the case of the cracks, these are filled with arsenopyrite, pyrite, native gold, goethite, scorodite and sometimes minor galena and sphalerite. Gold mineralization appears to also be associated with supergene argilization. Pyrrhotite and chalcopyrite is also reported in one Roman pit; pyrrhotite and scheelite in a second pit and tourmaline, ankerite and marcasite in a third pit.
The most frequent occurrence of gold is in irregular veins and boudins sub-parallel in an E-W band coincident with the hinge of the Somoza Anticlinorium and developed commonly with directions of N 080° E to N 130° E in the south flanks of the minor folds. The veins also exist in the hinges of the folds and in north flanks. The veins are often brecciated and sometimes they are folded and affected by younger cleavage. Milky and gray quartz are frequently coeval as zebra-like texture in the veins. The thickness of the veins ranges from a few millimetres to up to 5 metres. The breccia zones can be up to 8m thick and sometimes they exhibit fragments of the host rock affected by silicification and chloritization. Other veins of smaller thickness (few centimetres to 40 cm) and less deformed, can be observed with directions N-S, 30°- 45°, 60°- 80°, 160°- 180°.
Gold is also reported in alteration zones along fractures that can be followed for 2 to 3 km along strike. The extent of the mineralization in the host rock, adjacent to the veins, is unknown. The alteration consists of silicification and locally brecciation of the host rock. Local supergene argilization along fractures, down to depth of 2-3 m, is observed in some pits. Ferruginous goethite crusts and iron oxides and scorodite coating and staining are frequent in fractures. Goethite is also forming part of the cement in some breccia.
A particular case in the mineralization has been described in the literature at one pit. It is described as a 4x8 m working containing in a sinistral shear zone with irregular veins of up to 4 m thick, trending N 025° E and dipping 67° E. The same author indicates that the mineralized structure can be followed for about 1 km in both directions from aerial photographs.
Au mineralization in the veins and host rocks has been confirmed, but the presence of gold away from the veins has not been established. The presence of gold mineralization controlled by sedimentary or technically generated porosity and gold-trapping components is worth investigating. Gold within a band of several kilometres lends support to this possibility.
2011 and 2012 EXPLORATION BY AUROPEAN
During 2011, a total of 1474 soil samples were collected over that portion of the Project marked by the highest concentration of Roman workings. The samples were collected at 50m intervals on lines spaced 100m apart; a total of 80 hectares was systematically sampled. Coincidentally bedrock mapping and prospecting was completed. During 2012 a phase 2 program of soil sampling and bedrock sampling was completed. The total sampling to date is 3986 soil samples and 221 rock samples. See results for gold in soil (Figure 5) and arsenic in soil (Figure 6).
Results from this 2-phase program are extremely promising and include:
- A newly expanded 6.5 km plus long gold in soil anomaly (values to 1805 ppb Au) paralleling the Compludo structure and encompassing numerous Roman workings (in rock). The anomaly is open along strike. Numerous grab samples returned values in excess of 1.0g Au/t and up to 26.1g Au/t and channel samples returned 2.9g Au/t over 4.7m including 8.5g Au/t over 1.5m (see Press Release dated February 8, 2012). Numerous mineralization styles were noted in the Roman pits and the gold - arsenic (Au-As) relationship noted in the soil and rock samples has similarities to the mineralization as described for the non-refractory gold deposit at the Corcoesto Gold Project in NW Spain.
- A newly discovered 5 km plus long gold in soil anomaly (values to 402ppb Au) coinciding with the regional WNW trending Compludo Thrust Fault. The anomaly is mainly confined to the lower plate under the thrust fault, suggesting that the anomaly may extend under the overlying thrust plate. The anomaly is also open along strike to the WNW and ESE Numerous rock samples have yielded in excess of 1g Au/t and up to 26.1g Au/t, although to date only a limited number of grab samples have been assayed.
Between November 2013 and January 2014, three diamond drill holes totaling 572 metres were drilled to follow up on gold in soil anomalies, channel samples showing significant gold values and Roman pits and trenches (Figure 7 and Table 1).
The results were exceptional having hit both wide consistent mineralization in silicified and mineralized sandstone as well as high grade gold values in narrow vein structures. Table 1 shows the highlights of our results. Cross sections of each hole are shown in Figures 8, 9 and 10.
Hole AND-001 (Figure 8) intersect a wide zone of silicified sandstone with abundant quartz veining. Pyrite and scorodite were evident in the mineralized area. In Hole AND-002 (Figure 9) a narrow section of silicified material with a quartz vein containing arsenopyrite and scorodite accounted for the gold mineralization. In Hole AND-003 (Figure 10), a wide zone of finely disseminated arsenopyrite was responsible for the gold mineralization from 222m to 257.5m. A 2.6cm wide quartz vein at 263m showed visible gold. The sample returned 24.1 g Au/t over 1.2m.
The drilling indicates that there are at least 2 styles of mineralization:
A second phase of drilling is recommended to follow up on the intersections in holes AND-001 and AND-003, it is also recommended that detailed mapping be done at Quaberca el Agua, to see if the intersection in Hole AND-002 reflects the mineralization associated with the wide channel samples at surface; hole AND-002 may not have been positioned to have it intersect the zone of mineralization indicated on surface.
It is also recommended that more exploration be completed on the areas of high gold in soil anomalism. To refine the drill targeting within the multi-kilometre long anomaly ground geophysics including magnetics and Induced Polarization would help prioritize drill targets.
The positive results from the soil sampling programs and drilling provide evidence of a robust vein gold system within a low-grade envelope. Its surface is well defined by strong cohesive gold and arsenic anomalies. The highly background values for gold and arsenic in the mineralized belt combined with the nature of the mineralization noted to date suggest that Andiñuela could host a high grade gold with lower tonnage, a lower grade gold with large tonnage or a combination of both.