Summary:
Property Geology
Localized intrusive centers (e.g., La Cantera, Middle Zone, El Limon, and La Garrucha) comprise a series of intermediate composition porphyries and related intrusive (emplacement) breccias.
Deposit Types
The La Mina Property hosts copper-gold mineralization associated with sub-volcanic porphyry stocks intruding a late Miocene-age volcanic-sedimentary sequence of the Combia Formation. These rocks are related to an extensive magmatic arc that developed along the northern South American plate margin (the Chocó block margin).
Past and current exploration in and around the La Mina district has been aimed at Au-Cu porphyry, and/or epithermal Au styles of mineralization. In the specific cases of La Cantera Middle Zone, and La Garrucha the principal style of mineralization can be classified as Au-Cu porphyry.
Mineralization can occur in various styles and many combinations of disseminations, veins, stockwork, fractures, and breccias. As in the case of La Mina, multi-phase intrusions and inter-mineral phases are important factors in assessing porphyries, along with their wall-rock conditions, host rocks, structural conduits, and various chemical parameters (pH, water content, etc.).
Mineralization
La Cantera and Middle Zone constitute two of the four drill-tested mineralized porphyry intrusive and breccia bodies on the La Mina property. In both deposits, the intrusive centers are characterized by a series of porphyry stocks and related breccias that together make up porphyry copper-gold deposits. In the case of La Cantera, the core of the deposit is cut out by a late, barren porphyritic stock resulting in a “doughnut” pattern (plan view) whereby the copper- and gold-bearing rocks form a concentric pattern around the late, barren porphyritic stock. In the case of Middle Zone, the barren core is an amorphous feature that appears to have intruded preferentially along pre-existing planes of weakness.
The La Cantera deposit is slightly elliptical in plan-view (long axis NW-SE), measuring approximately 200 m by 190 m in plan-view on surface with a depth extent of 350 m - 600 m based on the results from 26 drill holes. Average grades are close to 0.9 g/t Au with 0.3% Cu and 1.7 g/t Ag.
The Middle Zone deposit lies approximately 400 m north of La Cantera and consists of a more pronounced elliptical body in plan-view (long axis NE-SW), which remains open at depths of over 600 m, based on the results of 54 drill holes. Faults appear to have offset the western and eastern lobes of mineralization. Faults also appear to delimit the western edge. Mineralization here is of two types. The first is characterized by a high copper-gold ratio, similar to what is observed at La Cantera. The second is characterized by high gold with relatively low copper. Overall, the grades are lower than La Cantera, close to 0.5 g/t Au with 0.1% - 0.2% Cu, over true widths of up to 100 m.
Mineralization in the La Garrucha porphyry intrusive complex is similar to that described for La Cantera and Middle Zone prospects comprising a calcic-potassic core, grading out to sodic-calcic, and an outer argillic zone. Magnetite alteration is ubiquitous throughout all of the porphyry phases.
La Cantera Prospect Mineralization
The principal minerals associated with the Au-Cu porphyry mineralization at La Mina are chalcopyrite and lesser bornite, both with associated gold mineralization. Secondary copper minerals (chalcocite, azurite, malachite and chrysocolla) do occur locally in the upper portions of the La Cantera prospect. Overall gold mineralization greater than 0.3 g/t Au is sulfide-poor and typically contains less than 1% total sulfides. In this type of mineralization chalcopyrite ± bornite are more abundant than pyrite.
Minor silver, lead, and zinc mineralization is associated with calcite ± quartz-tetrahedrite-sphalerite veins that cut earlier potassic alteration. These veins may be related to argillic alteration, which is commonly present where these veins are found.
The most sulfide-rich with alteration and mineralization at La Cantera are the sericitic and argillic assemblages that commonly contain more than 3% total sulfides. However, this mineralization typically contains less than 0.3 g/t Au and is not economically important.
Middle Zone Propsect Mineralization
The principal ore minerals associated with the Au-Cu porphyry mineralization at Middle Zone consist of chalcopyrite, pyrite, and, in very rare cases, bornite. Secondary copper minerals (chalcocite, cuprite, malachite and chrysocolla) do occur locally in the shallow portions at Middle Zone prospect; they represent supergene alteration of primary hypogene copper mineralization. Generally, gold mineralization greater than 0.3 g/t Au occurs with sulfides, but total sulfide content is normally less than 3% (with pyrite > chalcopyrite).
Unlike La Cantera, Middle Zone mineralization falls into two distinct classes. The first is Au-rich, relatively Cu poor mineralization occurring in the X3 and X3 Breccia. It occurs at relatively shallow levels, primarily where the X3 unit drapes over the X1 Porphyry. In Figure 7-10, examples of this mineralization type are marked in ellipses labeled ‘A’. The second mineralization type is Cu-rich with variable Au, and predominates in the X1 Porphyry and X1 Breccia units. In Figure 7-10, examples of this type are shown in the ellipse labeled ‘B’. The deepest drilling in Middle Zone terminates in this second mineralization type.
Minor silver, lead, and zinc mineralization is associated with cross-cutting calcite ± quartz- sphaleritegalena veins (late in the paragenetic sequence, as listed in the previous section). These veins are more common in the pervasive argillic alteration zone peripheral to the deposit. They also occur in contact margins between early and late porphyries. In the latter case, sub- epithermal veins occur predominantly in fault zones.
La Garrucha Prospect Mineralization
The principal ore minerals associated with the Au-Cu porphyry mineralization at La Garrucha consist of chalcopyrite and lesser amounts of bornite and covellite. Secondary copper minerals (chalcocite, cuprite, malachite and chrysocolla) do occur locally in the shallow portions at La Garrucha but are rare and do not account for significant Au-Cu values volumetrically. Pyrite mineralization for the most part is low at La Garrucha except where secondary QSP alteration has overprinted magnetite. Typically, the total sulphide content of the gold-copper zone at La Garrucha is less than 2% whereas the magnetite content averages approximately 3% - 5%.
Chalcopyrite is much more common than bornite. Bornite typically occurs in trace amounts and usually indicates higher Au values. Chalcopyrite occurs as disseminations and in various veins types as disseminations, patches and ribbons. In a typical moderately-to-well mineralized zone at La Garrucha the chalcopyrite will rarely exceed 1 vol% and typically averages 0.3 to 0.4 vol%. Chalcopyrite in veins however can make up to 20% by volume but these veins are typically less than 1 mm - 2 mm wide.
El Limon Prospect Mineralization
Gold-copper mineralization at El Limon is sporadic and associated with the L2 porphyry event and the strong potassic alteration (potassium feldspar-magnetite and biotite-magnetite) event cutting the L1 porphyry at depth in drill holes LMDDH-021 and 030. Mineralization in the potassic zones of LMDDH-021 and 030 is comprised of chalcopyrite disseminations in weakly developed quartz and quartz-magnetite veins. Mineralization in the L2 porphyry and associated L2 breccia of small amounts of chalcopyrite within quartz veins, quartz-magnetite veins, magnetite veins and fine-grained disseminations in the porphyry.