Summary:
Deposit Types
Four distinctly different mineral deposit types have been identified within the property. The most extensively explored are the epithermal, low sulphidation, gold and silver-bearing, quartz-calcite veins of the San Luis vein system, while the others, hydrothermal breccia-hosted base metal, manto-hosted occurrences and copper-molybdenum porphyry type stockwork, appear related to a buried intrusion, possibly of calc-alkaline composition, central to the BP zone.
The vein-hosted gold-silver occurrences, and the hydrothermal breccia and manto-hosted base metal mineralization are relatively close, and appear to share common regional structural controls
Deposit Classification
Epithermal Low-Sulphidation Precious Metal Vein Mineralization
The gold and silver-bearing veins of the San Luis vein system have many characteristics typical of ‘epigenetic or epithermal, low sulphidation, precious metal vein deposits’. According to Simmons et al. (2005), “Epithermal deposits comprise epigenetic ores that are generally hosted by coeval or older volcanic rocks. Most commonly, mineralized bodies occur in veins with steep dips that were formed through dilation and extension. Some are hosted by major faults but more commonly they are hosted by minor faults with smaller displacements (< 10 m)” (Pincus and McCrea, 2006).
Copper-Molybdenum Porphyry and Related Base Metal Mineralization (BP Zone)
The BP zone has many features indicative of a central, buried intrusion responsible for locally intense brittle fracturing, hydrothermal breccia bodies, multiple alteration facies, peripheral ‘manto’ or skarn occurrences, and base and precious metal mineralogy with an intrusion-related, possibly calc-alkaline, porphyry-style signature. These deposits typically occur in association with the emplacement of highlevel stocks during extensional tectonism related to regional strike-slip faulting at convergent plate boundaries or back-arc spreading following continent margin accretion. Any type of country rock may be mineralized but commonly the high-level stocks and related dykes intrude their coeval and cogenetic volcanic piles (Panteleyev, 1995).
Mineralization
Four main types of potentially economic mineralization have been identified on the property to date, including:
• Gold and silver-bearing epithermal quartz veins comprising the “San Luis vein system”
• Zinc-rich sulphide replacement bodies, referred to as “mantos”
• Base metal-bearing sulphide mineralization hosted by brecciated and hydrothermally altered andesite and polylithic breccias of undetermined origin
• A copper-molybdenum porphyry system within the BP zone.
San Luis Vein System
Recent exploration has identified a number of mineralized quartz veins in different areas of the property concessions, but the most intense and advanced exploration has been undertaken on the San Luis vein system, especially the Ayelén and Inés vein structures. The San Luis vein system includes, from west to east, five principal veins known as: ‘Ayelén’, ‘Inés’, ‘Paula’ and ‘Paula Split’, ‘Regina’ and ‘Sheyla’. These en-echelon vein structures strike approximately west-northwest to northwest and are spaced at 400–600 m intervals within an east-west trend within the property. The combined strike length is almost 5 km, although less than one-quarter of this strike length has been explored to date.
The Ayelén and Inés vein structures were identified from surface trenching as carrying anomalous gold and silver values, and these became the focus of most of the 2006 and 2007 drilling programs since they were indicated to host the highest precious metal grades. Elsewhere, 21 other drillholes have tested some of the other quartz vein structures and stockwork-type mineralization identified to date.
The San Luis vein structures have continuous strike lengths over hundreds of metres and down-dip extents, based on drilling intercepts, exceeding 200 m. Vein thicknesses vary considerably due to local faulting, shearing and dyking; however, they range in true width from 50 cm to more than 10 m but commonly average 1.5 m to 3.5 m. The vein structures are controlled and hosted by normal faults that are sub-vertical to moderately to steeply inclined to the northeast.
At the Ayelén and Regina veins, rhyodacitic to rhyolitic dykes of 2–12 m thick have been emplaced along the controlling fault structures for the veining. Along the Ayelén vein structure, two or more felsic dykes have intruded both the fault and vein. It appears from drilling results that the dykes are dominantly postmineral relative to the main auriferous vein mineralization but locally they do host volumetrically minor gold and silver-bearing mineralization that may be either late-stage primary or of secondary origin.
Zinc-Copper-Lead-Silver (± Gold) Manto Mineralization of the BP Zone
The BP zone is situated approximately 6 km southeast of the San Luis vein system. It covers approximately 12 km2 where the Calipuy Formation is prominently stained by limonite, goethite and hematite derived from the pyritized andesitic country rocks, which is readily visible in the IKONOS satellite imagery. At the centre of this zone, within a glacial cirque on the northern flank of Cerro Huillcahuain, there are a number of small, abandoned trenches and short adits collectively known as the ‘Patococha’ mineral showings. These minor workings were reportedly active in the 1980s and possibly earlier where the local miners ‘high-graded’ sub-horizontal tabular ‘manto’ bodies of fairly coarse-grained pyrrhotite-sphalerite mineralization and transported the hand-cobbed mineralization away for milling.
The Patococha ‘manto’ or skarn occurrences are 50–250 cm thick. They have been traced along strike for several tens of metres, and dip -20° south to southwestwardly. According to Ferraris (2007), these mantos are commonly associated with quartz stockwork veining within the underlying andesitic volcaniclastics, and usually terminated by strike-slip faulting trending northwest and steeply dipping. They are hosted within a sequence of andesite lava flows and are interpreted by the project geologists to be metasomatic deposits where interbedded, fine-grained, sedimentary units have been partially replaced by hydrothermal intergrowths of pyrrhotite, sphalerite, galena, actinolite-tremolite and chlorite.
Besides pyrrhotite, sphalerite is the most common sulphide mineral found in the mantos with reported zinc contents ranging up to 12% (Burk, 2007). Galena may be nearly absent or occur in significant amounts. It is reported that some samples returned values of up to 6% lead (Burk, 2007). Chalcopyrite and possibly other sulphides, such tetrahedrite-tennantite, are locally present in trace to minor amounts.
Hydrothermal Breccia-Hosted Copper-Lead-Zinc-Silver Mineralization
Hydrothermally brecciated, intensely altered and mineralized andesite crops out a few hundred metres north of the Patoccocha manto deposits, central to the much larger gossan area defining the BP zone. Bedrock exposures occur around a small pond and bog locally known as Laguna Patococha, suggesting that the breccia body has an elliptical shape with surface dimensions in the order of 400 m by 200 m. Significant pyrite, chalcopyrite, sphalerite and minor galena mineralization occurs within a brecciated matrix.
Copper-Molybdenum Porphyry-Type Mineralization
At BP zone, in the central-east part of the San Luis Project, the dominated andesitic volcanic rocks are widely altered to pyrite- and pyrrhotite-bearing, biotite hornfels by a concealed intrusive centre. Within the southern part of the alteration zone, the andesitic rocks and a shallowly inclined rhyolite sill host a porphyry-type veinlet stockwork, with surface dimensions of at least 800 m by 600 m (Sillitoe, 2011). Additional stockwork veining is reportedly also present at surface some 1 km farther north.