La Silla Gold Project
COMMODITY: Gold and Silver
DEPOSIT TYPE: Epithermal Vein Systems
LOCATION: Sinaloa, Mexico
SIZE: 2,070.78 has
STATUS: Available for Option
The La Silla epithermal Au-Ag project is similar to the San Dimas (Primero Mining Corp.) and El Gallo (McEwen Mining) vein systems being mined to the south and north of La Silla. The project is located 60 kms northeast of Mazatlan and is accessed from the town of San Ignacio via paved and dirt roads.
The La Silla Property consists of 2,070 has hectares, contained on five exploitation concessions 100% owned by Riverside Resources Inc. and is available for option.
GEOLOGY, MINERALIZATION AND ALTERATION
The La Silla property is situated in the western foothills of the northwesterly trending Sierra Madre Occidental mountain range that extends through northwestern Mexico. The oldest rocks found in the district are Late Cretaceous to Early Tertiary plutonic rocks ranging in composition from diorite to granodiorite and generally occurring at elevations of 200 m or less. Overlying the intrusive rocks is a sequence of andesite to rhyolite volcanic rocks considered to be broadly coeval with the underlying plutonic rocks. This volcanic sequence is conventionally referred to as the Lower Volcanic Series (LVS) and on the La Silla property it is represented by two main rock packages: a lower andesite package that is unconformably overlain by a thick sequence of variably welded dacite tuff and dacitic lithic tuff. The gold-bearing breccia structures and epithermal veins that have been drill-tested on the property are generally localized along the contact between the lower andesite package and the upper dacite sequence of rocks as at Ciruelos.
Following the deposition of the LVS, the region hosting the Ollitas mineral district underwent extensional tectonism that resulted in these volcanic rocks being faulted into blocks that rotated to the north and to the southwest. It is this tectonic event that generated the prominent E-W to ENE-WSW structural trend which is reflected by the district’s vein systems and intrusive dikes. Later, in the Late Tertiary, the region was affected by a second extensional event that formed numerous NW-striking, moderately to steeply dipping normal and strike-slip faults which offset certain veins by distances of 10’s of meters to a few hundred meters.
During this extension quartz veining is developed within dilatant structures with features that are characteristic of an epithermal low-sulphidation environment. Textures include thin chalcedonic banding, sugary quartz, clear or amethystine cockscomb and green quartz are regularly present. Within a vein system identifying the elevation of the boiling point, the zone where mineral bearing fluids produces these textures is key to any exploration project.
A vein paragenesis at La Silla comprises the following:
- Deposition of one or multiple stages of massive to banded, pale yellow to dark green or grey quartz containing trace to minor amounts of disseminated black (Ag-rich) sulphides, native gold and hematite;
- Multiple crystallizations of white, sugary to chalcedonic quartz;
III. Deposition of amethyst or clear cockscomb quartz in open spaces; and
- Deposition of reddish, hematite-stained cherty silica that seals brecciated zones in earlier formed veining.
Stage I veining, particularly that which is marked by greenish quartz, hosts significant gold and silver mineralization. Structures that have relatively high concentrations of Stage II, III or IV silica typically show low Au and Ag grades due to the dilution effect of these later stages of veining. As is typical for low-sulphidation vein systems, hydrothermal alteration of the La Silla wall-rocks tends to be limited to vein ‘halos’ that rarely extend more than a few meters from the vein margins and are commonly only tens of centimeters wide. Silicification is the most prominent alteration type at La Silla and tends to be more strongly developed in the hanging-wall which consists of dacite lithic tuff unit. Argillic alteration of the dacite lithic tuff is more widespread in the area of the property, but its relationship to the auriferous quartz veining is unclear. It is possible that much of the argillic alteration displayed by the dacitic tuff is simply a product of supergene processes including weathering due to its porous nature.
The El Roble vein zone can be traced on surface for approximately 350 meters, following a N25 west trend. The two main veins dip 70°NE on the surface. An alternative interpretation from drill hole intercepts suggest the main vein may be dipping as shallow as 35°NE, making it a potential open-pit target. The El Roble vein is hosted both within andesite and a porphyritic diorite dike or sill. A second less defined vein zone, El Roble Oeste, outcrops approximately 30-50 meters west of the El Roble vein, following the same general trend. Still another vein, El Roble 2, following a northeast trend dips southeast and appears to intercept the El Roble vein.
The El Roble Vein has been explored by a number of surface pits, one collapsed shaft (reportedly 40 meters deep) and one crosscut tunnel (also collapsed) are also present on the vein. No production is recorded, although the remnants of a mill site have been preserved with at least two “tounas” for the crushing of ore. The character of the El Roble Vein at surface is fairly distinct for the district, being composed of mostly white-leached, vuggy, granular quartz, containing locally 20-30% goethite in voids, and locally abundant with quartz-after-calcite replacement pseudomorphs. Some massive to weakly banded pale green quartz has been in El Roble 2 Vein and on dumps as well as in drill hole intercepts.
Drilling: El Roble
Holes ER1 to ER6 (totaling 451 meters) were drilled to test a 400-meter long segment of the El Roble vein zone. All six holes intersected zones of quartz veining. The strongest veining was encountered in holes ER1 and ER4, located adjacent to the El Roble Mine. Starting at the surface, both holes intersected over 15 meters of multi-stage white/tan to green quartz, with abundant iron oxides replacing sulfides. Mineralization appears to be strongest where the vein zone forms in a porphyritic intermediate dike or sill. Drill intercepts above 1.0 g/t are summarized below.
|TARGET||HOLE||FROM (feet)||TO (feet)||INTERVAL (feet)||INTERVAL (metres)||Au g/t||Ag g/t|
Table 1: Summary of El Roble Drill Results (Meridian Gold Company, April 16, 2004)
Starting at the surface, hole ER1 intersected a zone grading 15.2 meters @ 6.59 g/t Au and 34.4 g/t Ag (although the upper 3-6 meters of the zone may represent mine dump material filling a small draw). A second, deeper vein zone in ER1 returned 3.0 m @ 1.0 g/t Au; this zone may correlate to the El Roble Oeste Vein. Hole ER4 was drilled to get a better idea of the true dip and width of the vein/stockwork zone in ER1. The hole intersected 9.1 meters @ 2.06 g/t Au and 21.3 g/t Ag. Hole ER3, drilled 110 meters southeast and along strike, intersected 1.5 meters @ 5.33 g/t Au 26.0 g/t Ag. Away from the mine area down dip and further northwest on trend, Holes ER2, 5 and 6 intersected weakly anomalous to 1.0 g/t AuEq mineralization in quartz-calcite veins.
Future exploration should test northwest along strike where the ore zone may be preserved in a downthrown block beneath felsic tuffs of the Upper volcanic series on the north side of the Hornitos Fault. The number of high-grade samples seen in the El Roble area suggests this is a bonanza grade vein system. The continuation of these veins could be tested along strike to the northwest.
The Ciruelos Vein zone was mapped along a strike length of approximately 1.5 km and roughly following the contact between Cretaceous andesite and overlying dacite lithic tuff flows. The mineralization is exposed as a series of stockwork/sheeted veins in a belt 5-30 meters wide rather than one continuous vein. The zone trends generally N80W and dips at 60-80N. Previous mining focused on several 0.5 to 3.0-meter-wide vein zones displaying apple green, white sugary to granular quartz, with local banding and traces of disseminated sulfides. The vein zone consists of a diffuse zone of primarily white, quartz stockworks and stringers in a zone up to 80 meters wide hosted within the younger overlying dacite lithic tuff. The Ciruelos Vein has been historically developed by one drift, several small crosscuts and several surface pits; historic production has been minimal.