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The Shahuindo gold deposit is located in the district of Cachachi, province of Cajabamba, department and region of Cajamarca, northern Peru, 80 km SE of the town of Cajamarca, 15 km west of Cajabamba and 110 km ENE to NE of the coastal city of Trujillo (#Location: 7° 36' 42"S, 78° 12' 46"W).
The first mining in the Shahuindo deposit area, mainly as multiple short adits, was conducted by the Spanish after their conquest of the Inca Empire in the 1530s. Compañia Minera Algamarca S.A. undertook exploration activity in the area from 1945 to 1989 resulting in the discovery and operation of the Algamarca underground Cu-Ag-(Au) mine which produced ~1.5 Mt of ore. Small-scale mining of gold-silver mineralisation was also undertaken from the San José and Shahuindo mines located on the NE limb of the Algamarca anticline. The Shahuindo deposit is ~ 2 km ESE of the Algamarca mine. From 1990 to 1998, Alta Tecnología e Inversión Minera y Metalúrgica S.A. (Atimmsa), Asarco LLC, and Southern Peru Copper Corp. explored the Shahuindo Project area. Work by Asarco and Southern Peru identified four significant low-grade gold-silver zones at Shahuindo-San José, Porphyry, South Contact and East Zone, which are now part of the Shahuindo resource. Sulliden Gold Corporation acquired the property and undertook exploration activity between 2002 and 2012. Following their acquisition of Sulliden in 2014, Rio Alto Mining Limited continued infill drilling and exploration. In 2015, Rio Alto was acquired by Tahoe Resources Inc. Construction of an open pit heap leach operation was completed in early 2016.
For details of the regional setting, see the separate Peruvian Andes Cu-Au Province record.
The Shahuindo deposit lies in the eastern Cordillera Occidental of the Peruvian Andes. The regional geology is dominated by Cretaceous sedimentary rocks, which were deposited in the Western Trough and deeper sections of the Western Platform that represents the Cretaceous continental margin of South America (Benavides-Cáceres 1999; Scherrenberg et al., 2012). The deposit is located near a major bend in a fold-thrust belt whose structural elements formed at ~43 Ma, during the Incaic orogeny (Mégard 1984). In the deposit area, the regional structural regime is dominated by tight NW-SE trending and NE vergent folds and thrust faults. Individual folds are up to 80 km long, with widths of ~5 km. The deposit is also located along a localised belt of numerous intrusive bodies that are mostly distributed and elongated parallel to the dominant structural fabric in the fold-thrust belt. These intrusive rocks mostly occur as hypabyssal dacite and granular diorite stocks, dykes and sills, dated as Middle Miocene, at ~26 to 16 My in the immediate Shahuindo area (Hodder 2011). These intrusions are coeval with the Calipuy Group rocks of the main magmatic arc, an outlier of which is exposed around 15 to the SW, while the edge of the preserved main arc is ~20 km SW. The Calipuy Group volcanic sequence in the arc is mainly composed of subaerial tuffs, interbedded with andesitic lavas, with an agglomerate unit at the base. Intrusive bodies of andesitic to dacitic composition intrude the Calipuy Group volcanic suite, whilst andesite, dacite and quartz-feldspar porphyries also occur as isolated 26 to 16 Ma (Bussey and Nelson, 2011) stocks and dykes intruding the Mesozoic sedimentary sequence to the east of the preserved volcanic arc. The intrusions at Shahuindo are part of the latter group of stocks.
Overall, the sequence in the Shahuindo deposit area comprises a lower, shallow marine to deltaic siliciclastic sequence and an upper succession of finer grained siliciclastic units, with minor interbeds of carbonates in its lower sections, followed by thick beds of sandstone. All of these rocks are of Late Jurassic, but mainly Lower Cretaceous age.
The oldest country rocks, which are exposed in valley floors and anticlinal cores, are thinly bedded and laminated mudstone, minor siltstone and fine grained sandstone, with occasional coal seams of the basal Late Jurassic to Lower Cretaceous Chicama Formation. These are overlain by the Lower Cretaceous, dominantly clastic sedimentary sequence representing continental and shallow marine facies, and has been divided into the following units, from the base:
• Chimú Formation, which consists of white orthoquartzite with intercalations of carbonaceous shale.
• Santa Formation, which is dominantly black to grey shale containing 1 to 10% pyrite, including minor lenticular siltstones with a calcareous matrix at the base.
• Carhuaz Formation, that is dominantly composed of grey siltstones, with lesser impure white to cream-coloured sandstones that have a fine-grained to sugary texture, and minor grey shale. The formation is very lenticular, and contains beds that are massive and 0.1 to 1 m thick with little internal structure. This unit, with the overlying Farrat Formation, is the principal host to Au-Ag mineralisation in the central and southern parts of the deposit.
• Farrat Formation, composed of typically clean, yellow-white sandstones and quartzite, with minor interbeds of lenticular siltstones. This resistant and brittle unit forms cliffs and ridges, and is the main host to mineralisation in the northern part of the deposit area.
Lower Machay Group, regarded as part of the Goyllarisquizga Group in the source reports,
• Inca Formation, which consists of pyritic black shale with minor siltstones and sandstones, and occasionally has a calcareous matrix.
• Chulec Formation, a marine carbonate sequence, outcropping to the north of the Shahuindo deposit.
• Pariatambo Formation, which consists of thin layers of shales and bituminous limestone, with abundant ammonites.
This sedimentary succession is cut by a series of intrusions, the oldest of which were mostly emplaced as sills in folded units of the Goyllarisquizga Group. Later intrusive phases, including the quartz diorite porphyry and foliated quartz diorite porphyry, were emplaced as discordant stocks, plugs or dykes. From oldest to youngest, the intrusive rocks are (after Bussey and Nelson 2011):
• Diorite porphyry, locally termed 'andesite', is an early intrusive, only mapped to the SE and NE of the deposit. It is characterised by large (up to 8 mm) biotite phenocrysts, absence of quartz, and no evidence of hydrothermal alteration. In addition to biotite, it has abundant large plagioclase and hornblende phenocrysts. An isotopic age determination is reported to have been made on this intrusion and yielded a date of ~26 Ma (Bussey and Nelson, 2011).
• Dacite porphyry represents the most extensive intrusion in the district, occurring as a body in the northern part of the deposit area, which splays to the SE into a series of dykes that thin and lens out. It is argillically altered wherever observed, and is characterised by ~1 cm bipyramidal quartz as well as biotite and plagioclase phenocrysts, set in an aphanititc groundmass, and has been dated at ~16 Ma (U-Pb zircon; Hodder 2011). Much of the dacite porphyry occurs as a sills within the Goyllarisquizga Group, although the main intrusion of the Shahuindo corridor is a composite dyke like body with relatively steep discordant margins, similar to the dacite porphyry on the SW limb of the Algamarca Anticline.
• Quartz diorite porphyry, which has very similar grain-size and phenocryst types and content to the dacite porphyry, but is unaltered. It is mapped in the North Corridor area, 1.5 km to the NE of the main deposit. Clasts of altered dacite porphyry in the heterolithic breccia suggest that the quartz diorite porphyry is a younger intrusion.
• Foliated quartz diorite porphyry, which is characterised by phenocrysts of quartz (~50%), 5 mm biotite and plagioclase, set in a fine-grained groundmass. Where unaltered, disseminated accessory magnetite renders the rock magnetic. It has only been recognised in the NW part of the deposit area, where it occurs along a prominent NW-trending ridge. Foliation is defined by aligned biotite phenocrysts and, to a lesser extent, plagioclase phenocrysts, and in outcrop has a variable orientation but is often steep and parallel to the margin of the igneous matrix megabreccia, which it surrounds.
• Intrusive breccias - three such breccias are recognised in the deposit area: i). Heterolithic breccia with a biotite diorite matrix; ii). Heterolithic breccia with a fine-grained dacite matrix, which occurs as narrow dyke-like bodies, no more than 3 m wide, containing rounded to sub-rounded, up to 10 cm diameter clasts of sandstone, siltstone, dacite porphyry and rare shale, set in a matrix that includes fine grained lithic clasts and clay with 1 to 3 mm quartz, biotite and plagioclase. In some locations, the breccia contains fragments of mineralised sedimentary rock containing pyrite, sphalerite, quartz and white clay; iii). Heterolithic megabreccia with a foliated quartz-biotite matrix.
Whilst most structural elements of the regional fold-thrust belt were created during the Incaic II orogeny at ~43 Ma, geochronological data and field relationships suggest that mineralisation occurred during the Miocene, beginning at ~16 Ma. The Shahuindo deposit is associated with a localised belt of intrusive rocks that are mostly elongated along the dominant NW-SE structural grain of the fold-thrust belt, following structural elements, including fold limbs, axial surfaces, fold-related fractures, faults and related extensional fractures.
The main zone of mineralisation is located between two large-amplitude, NE vergent regional-scale folds, the Algamarca and San José anticlines, to the SW and NE respectively.
The Algamarca anticline has an amplitude of at least 400 m and is an upright symmetrical, box shaped fold with a generally vertical to steeply SW dipping axial plane, and a sub-horizontal axis that trends to the WNW. The anticline exposes a core of Chimú Formation, and has historic mine workings on a number of strike-parallel and transverse veins. The Algamarca anticline is only found to the NW of the NE striking La Cruz fault that cuts through the centre of the field, perpendicular to the fold axes. This fault only terminates the Algamarca anticline, but not the San José fold. Vertical displacement on this fault, at the cliff-face exposing the anticline, is at least 600 m. This same fault also closely corresponds to a marked narrowing of the mineralised zone from ~ 1 km in the SE, to 300 to 600 m to the NW. The La Cruz and a series of other parallel faults are interpreted to be steeply dipping to vertical tear faults (Hodder, 2010; Hodder et al., 2010). The Algamarca anticline has been interpreted to be either an allochthonous fault-bend fold in the hanging wall of a duplex roof thrust, or an anticlinal stack of folded strata and thrust faults above a postulated sub-horizontal roof thrust below the base of the Algamarca anticline. However, Defilippi et al (2016) conclude the geometry of the fold is more consistent with a detachment, than a fault-bend fold.
The San José anticline, which is spatially associated with the Shahuindo resource, has a lower amplitude, and occurs on both sides of the La Cruz transverse fault. The anticline is cut by the NW-trending San José fault zone, which is considered to be a major control on Shahuindo
mineralisation, comprising a series of parallel and anastomosing structures which dip steeply to the SW. The main San José anticline has an amplitude of at least 300 m, and is an asymmetric, overturned NE vergent fold, with a low angle dipping, 15 to 20° axial plane. It also includes a series of subsidiary short wavelength (<150 m) folds, with varying axial plane orientations, involving the Carhuaz Formation. Many of these anticlines and some synclines have breccia dykes along their axial surfaces. The San José anticline appears to be interrupted to the NE by a SW dipping fault, and thrust over the asymmetric, overturned, NE vergent Minas Azules Anticline with an amplitude of at least 300 m and low angle axial plane dipping at 15 to 20°SW. This structure passes to the east into an overturned syncline with a core of Pariatambo Formation bituminous limestone.
Mineralisation and Alteration
Mineralisation at Shahuindo, defined by the 0.1 g/t Au cut-off, has been identified over a NW-SE oriented area of ~3.7 x 0.5 to 1 km. Oxidation extends to ~150 m below the surface, and sulphide mineralisation has been traced to depths of at least 700 m.
Mineralisation has been described as an intermediate sulphidation assemblage. However, high sulphidation phases which have been identified at depth and in the core of hydrothermal breccias, but has been overprinted by the intermediate sulphidation pyrite, galena, sphalerite, Ag-sulphosalts assemblage that occurs at shallower levels, and in 'feeder structures'. Mineralisation occurs on fracture surfaces, in breccia matrix and as disseminations within the sedimentary rocks.
Mineralisation in the main Shahuindo corridor is characterised by millimetric quartz veinlets that locally grade into narrow breccia zones that can have euhedral quartz druse lining cavities. There is a close correlation between euhedral druse quartz and gold mineralisation, although the quartz extends beyond the resource area. These quartz veinlets cross-cut pure pyrite veinlets, indicting they formed during a later stage of mineralisation. Quartzite-like silicification textures are only evident in sandstone, and this are thought to be related to the original porosity and permeability of this lithology.
The host rocks of the Carhuaz and Farrat formations are folded and locally fault offset, and cut by stocks and dykes of porphyry, with better Au-Ag grades within sandstones, as compared to shales. Mineralised structural breccias contain polylithic fragments of wallrocks, and locally contain clasts of vuggy residual quartz, as well as juvenile dyke lithologies, providing evidence of the syn-hydrothermal timing of dyke emplacement.
In the sulphide mineralisation below the base of oxidation, gold is typically extremely fine grained, and the mineral species has not been determined, although it is closely associated with fine grained pyrite, which occurs as disseminations, veinlets and semi-massive replacement bodies. Minute blebs of tetrahedrite, sphalerite, galena, arsenopyrite, stibnite and covellite are found adhering to zoned pyrite. Although native silver has been reported in historic workings, Ag is usually confined to sulphosalts at Shahuindo.
The surface expression of the ore zone comprises the occurrence of voids and molds after pyrite and other sulphides, iron oxide in boxworks, limonite or gossanous coatings, fine grained euhedral quartz druse or veinlets and vugs in brecciated zones, crystalline white clay or white mica, and jarosite or scorodite in veins or veinlets. In the sub-surface oxide zone, which is interpreted to be the result of weathering, Au and Ag accompanies jarosite and hematite after pyrite. Estimates of the original pyrite content suggests an increase towards the ESE within the resource area. Jarosite (KFe3(OH)6(SO4)2), which usually forms in low pH environments due to oxidation of pyrite-rich rocks in the near-surface environment, is found in veins and as breccia matrix, and has a good correlation with the degree of known mineralisation. Scorodite (FeAsO4•2H2O), which often forms through the weathering of rocks containing arsenic-bearing sulphides, is an important mineral in the deposit, and has been found in the eastern and east-southeastern part of the resource area.
There is a deep occurrence of dickite, pyrophyllite and alunite, particularly along fractures and dykes that may mark feeder zones. In the southeastern part of the deposit, this assemblage is accompanied by a broad zone of sericite/illite at shallower depths, that may be associated with a white-mica stable mineralising fluid. The latter are interpreted to have formed by hydrothermal alteration of original micas from a Fe-Mg bearing phengite → to Na-rich paragonite → to K-bearing illite-muscovite.
Trending from SE to NW through the deposit, there is an alteration trend of the following assemblages in the sediment hosts, from silica-pyrophyllite → silica-paragonite → illite-muscovite-paragonite, interpreted to indicate a general trend of decreasing temperature and increasing pH from SE to NW. Similarly there is a decrease in temperature and increase in pH outward from the core of the mineralisation in the southeast part of the deposit.
The intrusions are altered to and assemblage of illite-kaolinite.
Reserves and Resources
Published ore reserves and mineral resources at 1 November, 2015 (pre-mining) were (Defilippi et al, 2016):
Ore reserves at an in situ dry tonnes cut-off of 0.18 g/t Au, including a 5% mining dilution and 98% mining recovery,
Proved reserves - 82.7 Mt @ 0.54 g/t Au, 6.92 g/t Ag;
Probable reserves - 29.2 Mt @ 0.51 g/t Au, 6.54 g/t Ag;
TOTAL reserves - 111.9 Mt @ 0.53 g/t Au, 6.82 g/t Ag, for 59 t Au and 763 t Ag.
Mineral resources - Oxide ore at an in situ at a cut-off of 0.14 g/t Au equiv.,
Measured resource - 96.5 Mt @ 0.50 g/t Au, 6.73 g/t Ag;
Indicated resource - 46.6 Mt @ 0.49 g/t Au, 6.53 g/t Ag;
Measured + Indicated resources - 143.1 Mt @ 0.50 g/t Au, 6.67 g/t Ag, for 71 t Au and 954 t Ag.
Inferred resource - 2.6 Mt @ 0.42 g/t Au, 7.4 g/t Ag, for 1.12 t Au and 19.5 t Ag.
Mineral resources - Sulphide ore at an in situ at a cut-off of 0.5 g/t Au equiv.,
Inferred resource - 87.7 Mt @ 0.71 g/t Au, 21.8 g/t Ag, for 62 t Au and 1849 t Ag.
This summary is drawn from: "Defilippi, C., Dyer, T.L. and Tietz, P., 2012 - Technical Report on the Shahuindo Heap Leach Project, Cajabamba, Peru; an NI 43-101 Technical Report prepared by Kappes, Cassiday and Associates for Sulliden Gold Corporation Ltd., 439p." and "Defilippi, C., Muerhoff, C.V. and Williams, T., 2016 - Technical Report on the Shahuindo Mine, Cajabamba, Peru; an NI 43-101 Technical Report prepared by Kappes, Cassiday and Associates and Tahoe Resources Inc. staff for Tahoe Resources Inc., 307p."
The most recent source geological information used to prepare this summary was dated: 2016.
This description is a summary from published sources, the chief of which are listed below.
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