Penasquito - Penasco, Chile Colorado,
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The Peñasquito gold, silver, lead and zinc deposit is located in the western half of the Concepción Del Oro district in the NE corner of Zacatecas State, Mexico, ~25 km south of Concepción Del Oro and ~200 km NE of the city of Zacatecas (#Location: 24° 38' 38"N, 101° 41' 15"W).
For a brief overview of the distribution and character of the deposits in the carbonate replacement and related vein Pb-Zn-Ag belt in Mexico and the western United States, and links to other deposits of that belt, see the Regional Setting section of the Fresnillo record.
The geology of the Peñasquito district is predominananty composed of Mesozoic sedimentary rocks that are intruded by Tertiary granodiorite and quartz monzonite stocks, and overlain by Tertiary terrestrial sedimentary rocks and Quaternary alluvium. The Mesozoic sedimentary sequence is a >2.5 km thick succession of Jurassic and Cretaceous marine strata, comprising ~2000 m of carbonaceous and calcareous turbiditic siltstones and interbedded sandstones, underlain by ~1500 to 2000 m of limestone. The sequence is as follows from the base:
• Zuloaga Formation - limestones and cherts with the low clastic content, consistent with deposition in a shallow epicontinental sea.
• La Caja Formation - a variably fossiliferous series of thinly-bedded phosphatic cherts and silty to sandy limestones, possibly representing a period of sea level fluctuations.
• Taraises Formation - limestones and argillaceous limestones, with increasing chert and disseminated pyrite near the top.
• Cupido Formation - massive limestone, which represents one of the favourable host rocks for much of the previously mined mineralisation of the district.
• La Peña Formation - cherty limestones.
• Cuesta del Cura Formation - thickly-bedded limestones.
• Indidura Formation - a series of shales, calcareous siltstones and argillaceous limestones, marking an abrupt change in sedimentation style, possibly indicating a shallowing of the marine depositional environment.
• Caracol Formation - primarily composed of interbedded siltstones and sandstones, representing a change to dominantly clastic sedimentation.
• Mazapil Conglomerate - deposited after a period of compressional deformation, uplift and subsequent erosion.
Large granodiorite stocks are interpreted to underlie much of the mineralised zones within the Concepción Del Oro District, including the Peñsquito area. A little younger suite of quartz-feldspar porphyries, quartz monzonite porphyries, and other feldspar-phyric intrusions occur as dykes, sills and stocks and cut the sedimentary units. These intrusions are interpreted to have been emplaced between the late Eocene to mid-Oligocene and have been dated at 33 to 45 Ma. Granodiorite and quartz-feldspar porphyry at and near Peñasquito yielded U-Pb age dates of 37 to 40 Ma and 36.2 to 37.1 Ma, respectively.
The Mesozoic sedimentary succession in the district surrounding the Peñasquito deposit, which lies within the Mazapil valley, was folded into arcuate east-west folds during the Laramide Orogeny. This was followed by Late Laramide extension that was accommodated by NW-, NE- and north-south striking faults, coeval with deposition of the Tertiary terrestrial sequence in fault-bounded basins. During the same period, granodiorite, quartz monzonite and quartz-feldspar porphyry were also intruded into this extensional regime. The present topography reflects the underlying geology, with anticlines cored by older Mesozoic rocks being exposed as ranges. Valleys are filled with alluvium and Tertiary sedimentary rocks overlying synclinal folds, the axes of which are occupied by younger Mesozoic units. Tertiary stocks and batholiths are best exposed in the ranges.
The Peñasco and Brecha Azul breccia pipes intrude Caracol Formation siltstones in the centre of the Mazapil valley. The Peñasco diatreme is the principal host to gold-silver-lead-zinc mineralisation at the Peñasquito deposit, whilst mineralised sedimentary rocks adjacent to and overlapping the Brecha Azul diatreme host the Chile Colorado deposit. These breccia pipes are interpreted to be related to quartz-feldspar porphyry stocks underlying the Peñasquito area. The present surface is estimated to be at least 50, and possibly several hundred metres below the original palaeo-surface when the diatremes were formed. The degree and character of the explosive brecciation suggest the diatremes breached to the surface, although any eruption craters and ejecta aprons have been assumed to have been eroded away.
The alluvium in the valleys averages 30 to 50 m in thickness at Peñasquito, and obscured the diatremes, apart from one small breccia outcrop near the centre of the Peñasco diatreme which protruded ~5 m above the valley floor. This single outcrop contained weak sulphide mineralisation along the south and west side of the outcrop, representing the uppermost expression of a much larger mineralised zone below.
Both breccia pipes have upward flaring funnel-shapes, and are defined by brecciated sedimentary and intrusive rocks, cut by dykes. The Peñasco diatreme has a diameter of 800 to 900 m immediately below the alluvial cover, with breccia persisting to at least 1000 m below surface. The smaller Brecha Azul diatreme, lies ~700 m to the SE of Peñasco and is ~500 m in diameter immediately below the alluvium. Brecciation in this structure also extends to at least 1000 m below surface. Porphyritic intrusive rocks intersected in drilling within and beneath the breccias are assumed to connect the two pipes at depth. Chile Colorado is a mineralised stockwork located to the SW of and overlapping the Brecha Azul breccia pipe, hosted by sedimentary rocks of the Caracol Formation. It has plan dimensions of ~600 x 400 m immediately below the alluvial cover, and extends to at least 500 m below the surface.
Polymetallic mineralisation is hosted by the diatreme breccias, intrusive dykes and in the surrounding siltstone and sandstone units of the Caracol Formation. The breccias are divided into three units, from top to bottom within the breccia column, determined by clast composition, namely: i). Sediment-clast breccia; ii). Mixed-clast breccia containing both sedimentary and igneous clasts; and iii). Intrusive-clast breccia. The sedimentary rock clasts comprise Caracol Formation siltstone and sandstone, whilst the intrusion clasts are dominantly quartz-feldspar porphyry. A variety of dykes with a range of textures from porphyry breccia, to quartz-feldspar and quartz-eye porphyries, to porphyritic, to aphanitic micro breccias cut the breccia pipes and the immediately adjacent clastic wall-rocks. Within the deposit area, three intrusive lithology groupings are distinguished; i). brecciated intrusive rocks, ii). felsites and felsic breccias; and iii). quartz-feldspar porphyry.
The two breccia pipes are considered to be the result of Tertiary intrusion-related hydrothermal activity. Alteration, mineral zoning, porphyry intrusion breccia clasts, and dykes are all interpreted to indicate the diatreme-hosted deposits represent mineralisation distal to and above an underlying quartz-feldspar porphyry system. A complex structural setting related to thrusts and associated tear folds and fissures are interpreted to have generated the structural conditions for the ascent and placement of magma, which, when entering into contact with phreatic water, provoked violent explosions and brecciation, giving rise to the phreatomagmatic breccias. A number of mineralised fault zones have also been identified.
Alteration and Mineralisation
Both breccia pipes are the focus of a hydrothermal alteration zonation of nested shells ranging from a proximal phyllic sericite-pyrite-quartz (QSP) suite → distal sericite-pyrite-quartz-calcite (QSPC) assemblage → a peripheral pyrite-calcite (PC) alteration halo. An inverse relationship has been recognised between the degree of alteration and the organic carbon content of the Caracol Formation sedimentary rocks, implying organic carbon was mobilised or destroyed during alteration.
As outlined above, the Peñasquito deposit is centred on the Peñasco diatreme breccia pipe, whilst the Chile Colorado deposit comprises mineralised sedimentary rocks adjacent to and overlapping the Brecha Azul breccia pipe. Both the diatreme and sedimentary rocks host and are surrounded by disseminated, veinlet and vein-hosted sulphides and sulphosalts containing base metals, silver and gold. Mineralisation comprises disseminations, veinlets and veins of various combinations of medium to coarse-grained pyrite, sphalerite, galena and argentite (Ag2S). Abundant sulphosalts of various compositions are found locally, including bournonite (PbCuSbS3), jamesonite (PbSb2S4 ), tetrahedrite, polybasite [(Ag,Cu)16(Sb,As)2S11], and pyrargyrite (Ag3SbS3). Stibnite (Sb2S3), rare hessite (Ag2Te), chalcopyrite and molybdenite have also been recognised. Telluride minerals are the main gold-bearing phase, with electrum and native gold also observed.
Gangue minerals includes calcite, sericite and quartz, with rhodochrosite, fluorite, magnetite, hematite, garnet (grossularite-andradite) and chlorite-epidote. Carbonate is the more abundant gangue mineral compared to quartz in veins and veinlets, particularly in the 'crackle breccia' that is commonly found at the diatreme margins. Sulphides occurring as disseminations within the matrix dominate the breccia-hosted mineralisation, with lesser disseminations and veinlets within clasts. All three breccia types host mineralisation, although much of the mineralisation within both pipes lies within the intrusion-clast breccia. All of the dyke lithologies may also be mineralised, and they are almost always strongly altered. Mineralisation within these dykes occurs as breccia matrix fillings, disseminations and minor veinlet stockworks at intrusion margins, and in veinlets or veins cutting the more massive dykes. Mineralised dykes are more important in the Peñnasco diatreme but are less abundant in Brecha Azul.
Where the Caracol Formation clastic sedimentary rocks are cut by the diatremes, mineralisation dominantly occurs as sulphide replacement of calcite matrix in sandstone beds and lenses and as disseminated sulphides and sulphide clusters in sandstone and siltstones. Crosscutting vein and veinlet mineralisation within these sedimentary rocks comprises sulphide and sulphide-calcite fillings.
Whilst the Chile Colorado deposit, SW of the Brecha Azul diatreme, is the largest accumulation of sediment-hosted mineralisation, others are also found adjacent to Peñasco (e.g. El Sotol), and between the diatremes (e.g. La Palma).
The richest sulphide and sulphosalt mineralisation is associated with the strongest phyllic sericite-pyrite-quartz (QSP) alteration, whilst a halo of generally lower-grade disseminated Zn-Pb-Au-Ag mineralisation lies within the distal sericite-pyrite-quartz-calcite assemblage surrounding the two breccia pipes.
Manto-style sulphide replacement of carbonate strata occurs within and beneath the Caracol Formation, adjacent to the breccia pipes and beneath the clastic-hosted disseminated sulphide zones. This mineralisation comprises semi-massive to massive sulphide replacements of sub-horizontal limestone beds, as well as structurally-controlled cross-cutting chimney-style, steeply dipping, fracture and breccia zones filled with high concentrations of sulphides. The sulphides are usually predominantly sphalerite and galena, although significant pyrite is also present. Gangue minerals, which are commonly carbonates, are subordinate in the mantos and chimneys, which are characterized by their very high Zn, Pb and Ag contents, with variable copper and gold grades.
Garnet skarn hosted Cu-Au-Ag-Zn-Pb mineralisation within dissolution breccias has been identified at depth between the two main breccia pipes, occurring within the Indidura, Cuesta del Cura, Taraises and La Caja Formations. The principal trend of this mineralisation is NW-SE. The skarn alteration envelope has plan dimensions of ~1000 x 1200 m, and is open at depth as of 2018. This polymetallic mineralisation, which is hosted within garnet skarn and associated breccias, mainly occurs as chalcopyrite and sphalerite with some gold and silver. Gangue minerals include pyrite, calcite, garnet and magnetite. The skarns are often surrounded by halos of hornfels, particularly in siliciclastic units, and/or marble and recrystallised limestone in carbonate units.
Deep drilling below the deposit has encountered quartz feldspar porphyry with strong sericite-pyrite-quartz-calcite and potassic alteration, embracing occasional quartz-molybdenite veinlets and veins with secondary biotite and magnetite disseminated in the wall rocks, suggestive of porphyry-style mineralisation.
Reserves and Resources
NI 43-101 compliant Mineral Resources and Ore Reserves as at 30 June 2017 (Vdovin et al., 2018 for Goldcorp Inc.) were:
Proved + Probable Reserve - 515.03 Mt @ 0.53 g/t Au, 32.5 g/t Ag, 0.31% Pb, 0.68% Zn;
Measured + Indicated Resource - 250.4 Mt @ 0.27 g/t Au, 26.9 g/t Ag, 0.23% Pb, 0.52% Zn;
Inferred Resource - 23.53 Mt @ 0.29 g/t Au, 18.8 g/t Ag, 0.16% Pb, 0.59% Zn;
Proved + Probable Reserve - 9.72 Mt @ 0.36 g/t Au, 21.5 g/t Ag.
Measured + Indicated Resource - 24.92 Mt @ 0.21 g/t Au, 25.9 g/t Ag;
Inferred Resource - 0.14 Mt @ 0.21 g/t Au, 8.9 g/t Ag.
NOTE: Mineral Resources are exclusive of Ore Reserves.
These reserves and resources total 358 t of contained gold and 27 770 t of contained silver.
Mineral Resources and Ore Reserves as at 31 December 2020 (Newmont Reserve and Resource Report, 2021) were:
Proved + Probable Reserve - 348.2 Mt @ 0.60 g/t Au, 35.10 g/t Ag, 0.34% Pb, 0.83% Zn.
Measured + Indicated Resource - 276.8 Mt @ 0.27 g/t Au, 26.75 g/t Ag, 0.28% Pb, 0.60% Zn;
Inferred Resource - 150.9 Mt @ 0.40 g/t Au, 26.36 g/t Ag, 0.28% Pb, 0.53% Zn;
Proved + Probable Reserve - 39.6 Mt @ 0.33 g/t Au, 25.81 g/t Ag, 0.33% Pb, 0.49% Zn.
NOTE: Mineral Resources are exclusive of Ore Reserves
These reserves and resources total 355 t of contained gold and 24 625 t of contained silver.
This summary is principally drawn from "Vdovin, V., Pareja, G. and Lind, P., 2018 - Peñasquito Polymetallic Operations, Zacatecas State, Mexico; an NI 43-101 Technical Report prepared for Goldcorp Inc., Canada, 200p."
The most recent source geological information used to prepare this summary was dated: 2018.
This description is a summary from published sources, the chief of which are listed below.
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