Summary:
The Shahuindo mine is located on the eastern flank of the Andean Western Cordillera in northern Peru, within a regional fold and thrust belt of predominantly Mesozoic sedimentary rocks intruded by felsic stocks located along faults and cores of anticlinal structures. The principal zone of mineralization in the Shahuindo district occurs in a belt between two large-amplitude regional-scale folds, the Algamarca anticline and the San Jose anticline. Important structural elements such as fold flanks, axial fold surfaces, fold-related fractures, faults, extension-related fractures, dikes, and intrusive contacts as well as favourable stratigraphy were used in modeling. The mineralization is hosted within the siliciclastic sandstone-dominant Farrat formation and the underlying sedimentary Carhuaz formation. These sedimentary rocks have been intruded by at least three felsic stocks which tend to be located along faults and cores of anticlinal structures.
The oldest rocks on the Property are thinly bedded and laminated mudstones with occasional coal seams of the basal Lower Cretaceous Chicama Formation. This formation is only exposed in valleys and in the cores of anticlines.
Overlying the Chicama Formation is the Goyllarisquizga Group, which is comprised from oldest to youngest of the Chimu, Santa, Carhuaz, and Farrat Formations.
White orthoquartzite beds of the Chimu formation form the Algamarca anticline to the southwest of the Shahuindo deposit. The Santa Formation which is exposed on the flanks of the Algamarca anticline consists of mudstone with intercalations of limestone. Overlying the Santa Formation is the Carhuaz Formation, consisting of interbedded sandstone, siltstone, and mudstone, with many sandstones displaying cross bedding and amalgamated wedge-shaped sandstone beds. The Farrat Formation consists of cliff-forming siliciclastic strata dominated by sandstone. The Carhuaz Formation hosts gold and silver mineralization along the core of the Shahuindo deposit while the Farrat Formation hosts mineralization along the northwest, northeast, and southeast flanks.
Multiple intrusions have intruded the Cretaceous sedimentary sequence at Shahuindo. Intrusions include rocks described as andesite, dacite porphyry, and intrusion breccia. Compositions vary from diorite porphyry (known as “andesite” in the district) to dacite porphyry, fine-grained dacite porphyry, quartz diorite porphyry, and foliated quartz diorite porphyry. The three intrusive breccia phases are heterolithic breccia with biotite diorite matrix, heterolithic breccia with fine-grained dacite matrix, and heterolithic megabreccia with foliated quartz-biotite dacite matrix.
Diorite porphyry (andesite) observed in the southeast part of the Property corridor formed as sills in the Goyllarisquizga Group sediments. It is characterized by large (8 mm diameter) biotite phenocrysts, a lack of quartz, and no evidence of hydrothermal alteration. An isotopic age determination is reported to have been made (thought to be zircon U-Pb) on this intrusion that yielded an age of ~26 Ma (Bussey and Nelson, 2011).
The dacite porphyry is the most widespread intrusive in the district that forms a commonly argillically altered dike-like body with relatively steep discordant contacts in the north part of the main corridor that splays into a series of narrow dikes to the southeast. The dacite porphyry is thought to have been emplaced as sills concordant to Goyllarisquizga Group sediment bedding. An isotopic age determination (zircon U-Pb) of this intrusive yielded an age of ~16 Ma (Bussey and Nelson, 2011).
Three intrusive breccia phases are recognized on the Property and include; (1) heterolithic biotite diorite breccia, (2) heterolithic fine-grained dacite breccia, and (3) heterolithic megabreccia. Heterolithic finegrained dacite breccia occurs as narrow (< 3 m width) dike-like bodies with rounded to subangular clasts (up to 10 centimetres (cm) in diameter) of sandstone, siltstone, dacite porphyry, and rare shale in a matrix of fine-grained lithic clasts and clay with 1 - 3 mm quartz, biotite, and plagioclase crystals.
Mineralization
The principal zone of mineralization in the Shahuindo district occurs in a belt between two large-amplitude regional-scale folds, the Algamarca anticline and the San Jose Anticline. The Algamarca anticline is upright and symmetrical with amplitude of at least 400 m, whereas the San Jose fold is an asymmetric, overturned, northeast-vergent fold with a shallowly dipping axial surface and amplitude of at least 300 m. Important structural elements include fold limbs and fold axial surfaces, fold-related fractures, faults and related extension fractures, breccia dikes and irregular bodies, and igneous intrusive contacts.
Mineralization at Shahuindo has been identified over an area approximately 3.7 km southeast to northwest and 0.5 km southwest to northeast. Oxidation of mineralization extends to a depth of 150 m below surface. Sulfide mineralization has been identified by surface drilling to 700 m depth.
Mineralization at Shahuindo can best be described as an intermediate-sulfidation epithermal system, though high-sulfidation mineralization occurs at depth and in the core of hydrothermal breccias. The highsulfidation mineralization was pervasively overprinted by intermediate-sulfidation mineralization (pyrite, galena, sphalerite, Ag sulfosalts), which occurs at shallow levels and in “feeder structures”. Mineralization occurs on fracture surfaces, in breccia matrix, and as disseminations within the sediment packages.
The host rocks at Shahuindo are the Carhuaz and Farat sedimentary formations which are folded and locally fault offset, and cut by porphyritic dikes and stocks. Sandstone tends to be a better host to higher grades of gold and silver compared to siltstone. Brecciated structures with polylithic fragments consist of wall rock clasts, locally clasts of residual quartz (the vuggy texture indicating rock dissolution), as well as juvenile clasts of dike rock, the latter evidence of a syn-hydrothermal timing of dike emplacement.
In the oxide facies, which is interpreted to be the result of weathering processes, gold and silver are associated with the presence of jarosite and hematit. In the sulfide facies, gold is typically extremely fine-grained and the mineral species has not been identified. Fine-grained pyrite forms a close association with gold mineralization and occurs as disseminations, veinlets, and semi-massive replacement bodies.
Tetrahedrite, sphalerite, galena, arsenopyrite, stibnite, and covellite have also been reported as minute blebs adhering to zoned pyrite. Although native silver has been identified at San José and in the historic Shahuindo mine, silver is usually found in sulfosalts at Shahuindo.