Summary:
The Siembra Minera deposit lies within a portion of the lower Caballape Formation volcanic and volcanic-related sedimentary rocks. The units present are (1) andesitic to rhyolitic tuffaceous volcanic beds, (2) related sedimentary beds, and (3) a tonalitic intrusive body. All rocks have been tilted and subjected to lower greenschist facies metamorphism. In the main mineralized trend, moderate to strong foliation is oriented N10°E and dipping 30° to 55° northwest. This foliation appears to be parallel to the original bedding and tends to be strongest in the finer-grained rocks. A much weaker foliation orientation appears in outcrop exposures, striking north-northwest and dipping to the southwest.
The Siembra Minera Project consists of three rock types. Hard rock ore comprises approximately 87% of the material that will be processed. The remaining 13% of the mineralized material is saprolite with a split composed of approximately 43% oxide saprolite and 57% sulphide saprolite.
BRISAS
A possible deposit analogy is of a copper porphyry forming over a magmatic source (yet to be discovered) that was very rich in boron. A peraluminous granite might fit the boron requirements and a sufficient volume of basaltic/andesitic rock could provide the copper. Thin lenses of high Cu and Mo extending away along bedding/foliation planes could be the result of periodic high confining pressures within the Blue Whale that forced mineralizing fluids outward along these planes. The fluids replaced crystals and lithic fragments, evidence of which can be viewed in drill core.
There are four distinct types of gold and copper mineralization present at Brisas, defined by geometry, associated minerals, and the gold-copper ratio. These zones are the Blue Whale body, disseminated gold + pyrite ± copper, disseminated high copper, and shear-hosted gold. Only the first three types are encountered within the proposed pit geometry.
The Blue Whale mineralized body is a discrete, sharply bounded, flattened, cigar-shaped feature that trends more or less parallel to the local schistosity and plunges approximately 35° southwest.
Mineralogically, the Blue Whale is a sericite-tourmaline-pyrite-chalcopyrite-quartz schist, with a smaller volume of quartz-tourmaline-sulphide breccia.
The bulk of mineralization occurs in disseminated, coalescing, lensoid bodies, and high in gold and in most cases low in copper. These bodies lie almost exclusively in the lapilli-rich, rapidly alternating sequence of tuffaceous units and are clearly aligned along foliation. Together, these lenses form a generally well defined mineralized band, which mimics the dip of the foliation/bedding and remains open at depth. It remains at a similar thickness from the northern concession boundary for a distance of 1.4 km south, after which, it tapers rapidly. Alteration minerals characteristic of these lenses are epidote, chlorite, secondary biotite, and sericite.
Stratiform lenses of high copper with or without high gold underlie the gold+pyrite lenses described above. These lenses outcrop in the northern part of the deposit, and plunge to the south in a manner similar to the Blue Whale and high gold/low copper lenses but with variable dips.
Shear-hosted gold occurrences exist in the southern part of the concession, running parallel to the foliation as with mineralization further north. Stratigraphically, they occur above the large disseminated lenses previously described. The gold and copper grades are erratic and discontinuous.
CRISTINAS
In terms of classification, Cristinas has been assigned to shear zone-hosted systems by some geologists, and to a porphyry association by others; however, several key elements of the Cristinas deposit must be satisfied in any attempt to classify the deposit.
The main two styles of mineralization present at Cristinas are:
1. Stratiform mineralization at Conductora, Morrocoy, and Cordova.
2. Hydrothermal breccia-hosted mineralization at Mesones-Sofia.
The Conductora (including Cuatro Muertos and Potaso), Morrocoy, and Cordova areas contain over 95% of the gold resource at Cristinas.
Mineralization occurs in a greater than three-kilometre long, north-trending zone that dips moderately (30° to 40°) to the west, sub-parallel to the volcanic stratigraphy and to the pervasive (S1) cleavage. Gold mineralization is associated with a sulphide assemblage that consists essentially of pyrite and chalcopyrite.
Alteration mineral assemblages in Conductora are secondary biotite, minor potassium feldspar, calcite, chlorite and minor epidote and sericite. Calcite is ubiquitous, occurring mainly as disseminations, in addition; in carbonate-sulphide veinlets, carbonate-only veinlets, and quartz-carbonate veinlets. Silicification is minimal in Conductora-type mineralization. Minor tourmaline disseminations occur in some parts of Conductora, but in much lower concentrations than in the Mesones-Sofia area. The most consistent gold mineralization occurs in zones in which secondary biotite is most intensely developed. Many sulphide clots within these biotite-dominated alteration zones are rimmed by a green chlorite alteration that has overprinted the secondary biotite.
Pyrite and chalcopyrite constitute the only sulphide species of significance in primary ore. The average pyrite/chalcopyrite ratio is greater than five. Sulphides occur principally as disseminations, but also in narrow veinlets 1 mm to 2 mm wide. These veinlets are variable in composition ranging from sulphide-only to sulphide-calcite and sulphide-calcite-quartz. These veins have selvages of secondary biotite, chlorite, or chlorite-epidote.
Quartz-sulphide veins are rare, but where they do occur, they are in zones of intense secondary biotite development against which they have indistinct margins and are associated with multi-ounce gold values. Higher than average gold grades (>2 g/t) are associated with areas in which pyrite occurs as coarse clots up to 2 cm in diameter in zones of intense secondary biotite alteration. Generally, however, the sulphides are fine-grained, and much more so than in Mesones-Sofia.
Molybdenite is locally quite abundant, occurring in quartz-calcite-sulphide veinlets, and disseminated with pyrite and chalcopyrite. The Potaso area contains disseminated molybdenite that appears to have no spatial relationship with pyrite and chalcopyrite on a handspecimen scale.
Mineralization in Mesones-Sofia is concentrated in the quartz-tourmaline-sulphide-calcite vein breccias and extends laterally into the adjacent country rocks. The breccias are sufficiently closely spaced that the country rock between them also constitutes ore in the central part of Mesones-Sofia. Grades in the country rock on the periphery of the system decrease as the distance between the breccias increase.
Discrete auriferous quartz veins are located adjacent to the Cristinas deposit. Such veins include the Los Rojas and Albino veins that lie approximately one kilometre to the east of the Conductora area, and the Hofman vein, which lies about one kilometre to the west of the Cordova area. These veins consist of quartz with gold mineralization associated with pyrite (there is no appreciable chalcopyrite). The veins have chlorite selvages about 50 cm wide. Although gold mineralization in these veins does not constitute part of the Cristinas resource, they are considered to be genetically related, and peripheral, to the Cristinas deposit.