The Guadalupe y Calvo Project occurs within an erosional window through the UVS, exposing a LVS package of andesites that are cut by a quartz monzonite pluton. The LVS is reported to be Paleocene in age (SGM, 2008). No older basement rocks are exposed in the district. The LVS package has been cut by a northwest-striking fault system which was mineralized to form the Rosario vein.
The LVS andesite is predominantly a dark green unit with porphyritic-aphanitic texture and locally aligned phenocrysts suggesting a flow origin. Phenocrysts consist of plagioclase, hornblende and augite that are variably altered to clays, epidote, chlorite and oxides. The andesites have a weakly developed propyllitic alteration that is strongest near the veins. This alteration consists of epidote, chlorite, pyrite and calcite.
Where the andesites are more propylitized, they are commonly also more deeply oxidized. The andesites have been intruded by a complex stock of granodiorite to quartz monzonite (predominantly the latter) that is believed to be Laramide in age, based on it being cut by the veining, and being in clear angular unconformity with the UVS tuffs. The quartz monzonite grades from slightly porphyritic (with large phenocrysts up to 1.5 cm in the longest dimension), to hypidiomorphic-granular.
It appears to have been emplaced at a moderate depth and is not believed to be a hypabyssal stock nor directly related to the mineralization. It is brittle and thus forms good open fractures for veining. This pluton has many complex emanations of dikes and sills in the andesite that are difficult to correlate from section to section. The quartz monzonite appears to be equally well-mineralized as the andesite in the drilling completed to date. All of the LVS units are locally cut by small aphanitic andesite/diorite dikes and minor sills that appear to be post-mineral but pre-UVS. These are unrelated to mineralization.
Guadalupe y Calvo Property Geology
Note: The wide hydrothermal breccia of the Rosario vein system is covered by the unconformable post mineral volcanics to the southeast, and is down-thrown by the Bufa Fault on the order of 400m vertically to the northwest.
The UVS rocks in the Project area consist of tuffs and flows of predominantly rhyolite composition, grading to rhyodacite and dacite. Most units have some quartz phenocyrsts and generally have abundant glass, variably devitrified. The tuffs commonly have large fiame typical of ignimbrites of the UVS, and are generally moderately to strongly welded, forming prominent cliffs and mountains. Minor flow units have been noted, but not subdivided, in this unit.
There appear to have been three phases of brittle extensional faulting in the district:
- Early north-northwest-striking and east-west normal faulting that is pre- and syn-mineralization, which may be semi-continuous with number 2), below.
- Later post-mineral north-northwest-striking faulting.
- Latest north-northeast-striking normal faulting.
The structural geology of the Project is dominated by northwest-striking normal faults that dip moderately to steeply to the southwest. The SGM regional geologic map shows a major normal fault on the southwest side of the district (Falla Guadalupe) that down-drops UVS rocks and is thus post-mineral.
The major vein structure (Rosario vein) in the district is subparallel to this fault and forms part of a series of conjugate fracture systems related to this regional system. The Rosario vein is clearly a fault-related feature, with abundant gouge and breccia related to the mineralization. No good marker units have been recognized in the LVS to date, so the actual offset on these older faults is unknown.
The coincidence in attitude between the pre-mineral and post-mineral faults suggests that the structural system here was long-lived. These structures are visible as linear features on satellite imagery, and the SGM has mapped a series of northwest-striking faults sub-parallel to the Guadalupe-Rosario system on its regional geologic map.
Along with this prominent northeast trend, there were also east-northeast to east-striking vein/faults, such as the Nankin vein in the main Project area, that are mineralized and part of the oldest structural system.
The youngest recognized deformation is a north-northeast-striking fault “Bufa Fault” that Down-drops the UVS volcanics against the LVS and the Rosario vein, and is thus post-mineral. Based on surface mapping and contouring of the basal UVS contact from drill data, the Bufa Fault is believed to have on the order of 400 m of apparent vertical displacement.
The low-sulfidation epithermal mineralization at Guadalupe y Calvo occurs in structurally controlled, semi-tabular-shaped veins that appear to be vertically zoned in mineral content.
The mineralization consists predominantly of bands and irregular lenses of hydrothermal breccias and banded quartz-calcite-adularia veins that form wide zones (over 60 metres locally) of veins, sheeted veinlets and stockwork veinlets. The gangue is primarily quartz that ranges in colour from white to light grey to green to beige-green and is locally amethystine.
There are lesser amounts of calcite locally, and rare evidence of calcite replaced by quartz (a typical boiling feature in epithermal veins). Rare pink bands within the vein suggest the presence of adularia, and light green quartz-chlorite veining is observed locally. Sulphides are generally pyrite with rare argentite, and locally galena-sphalerite-chalcopyrite, and total sulphide content is generally less than 5%.
In the oxide zone, the sulphides are leached, leaving either casts or pseudomorphs of goethite-hematite. Abundant goethitic and hematitic staining is evident in much of the vein near surface, and some secondary silver minerals are probably developed, although definitive mineralogy has not been done. Banding of the fissure veins is common, with the bands generally sub-parallel to the walls of the veins, and open vugs commonly between bands in the upper part of the deposit. Breccia clasts of host rock are common in the veins, and the clasts are generally strongly altered either by quartz replacement and/or by argillization of feldspars and choritization of mafic phases and some feldspars.
The walls of the vein structure sometimes have sharp boundaries, but it is also quite common for the “vein” to consist of anastomosing veinlets and stockwork veinlets within a fragmented and fractured zone. In the latter case, the boundary of the vein system is defined by logging a gradual decrease in the intensity of sheeted veinlets. These walls of individual veins commonly have a 1 to 30 centimetres halo of weak silicification, and propylitic alteration (chlorite-epidote-calcite-pyrite) is commonly developed 2 to 25 metres from the vein.
These propylitic halos have variable 1 to 5% pyrite. There are two major veins that have been recognized in the Guadalupe y Calvo Project. The Rosario vein is by far the larger, consisting of an 8 to 60 metres wide zone of hydrothermal breccias, banded veining, and extensive stockworks and sheeted veins in the walls of the vein.
The vein strikes from N15°W to N35°W and dips 45° to 70° to the southwest. The greatest thicknesses of veining occur near surface, and especially near the intersection with the Nankin vein.
The second major vein is the Nankin vein, which is mostly an east-west striking vein, dipping shallowly to the south at an average of 30°. This structure has an obvious deflection where it intersects the Rosario vein, creating a complex pattern of stockwork and sheeted veinlets that provide a possibility for bulk-mining either in open pit or large underground stopes.
The deflection in the vein’s strike is obvious on level maps, where it can be seen to have suffered either fault drag or is simply a major split off the Rosario, forming a 60° bend in the strike. The Nankin vein is another target for exploration especially where it intersects the Rosario vein.
To the southeast, the projection of the Rosario and Nankin veins is masked under the unconformity of the post-mineral tuffs. A parallel structure, the “Vein of the Sun”, is parallel to the Nankin, and is interpreted as a footwall limit to what is sometimes a stockwork-sheeted vein system below the Nankin vein.
Schematic Surface Map of the Guadalupe y Calvo Project showing the Main Veins
The Concordia vein is a footwall split from the Rosario vein but has a weak surface expression and has been discontinuous in drill results to date. Numerous other small vein splits have been cut in drilling, but none has been followed over substantial lengths to date. Previous exploration drilling has been focused on testing the Rosario structure, and drill intersections on the Nankin vein were made only when the vein fell between the drill collar and the Rosario target. The area tested by infill drilling was approximately 600 metres in strike and 550 metres in vertical dimension (approximately 600 metres in the plane of the Rosario vein).