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Milpillas, La Parrena
Sonora, Mexico
Main commodities: Cu

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The La Parreña mine exploits the small, high grade, supergene enriched, Milpillas porphyry style copper deposit, located within the northern segment of the Cananea District in northeastern Sonora, Mexico.
(#Location: 31° 6' 32"N, 110° 25' 28"W).

  Exploration of the area surrounding Milpillas began in 1975, conducted by Minera Cuicuilco, later to be continued by Industrias Peñoles. Extensive exploration of the deposit area continued until 2001, sufficient for a feasibility study commenced in 1998 to estimate a resources of up to 35 Mt @ 1.95% Cu, and to justify construction of a mine. Exploration and delineation had involved a cumulative ~100 000 m of drilling by the two companies. Mining operations began in 2006 and continued until 2020 when they were indefinitely suspended, as high operating costs combined with a steep fall in copper price, made the operations economically unsustainable. However, in June 2022 mining, crushing, and metallurgical recovery activities were resumed to produce cathode copper and have continued to the present (2024). Mining is by underground methods, with ore stacked on leach pads, followed by electrowinning (SXEW).

Regional Setting

  The Cananea District lies on the southwestern margin of the North American craton, the basement of which locally comprises the 1.68 Ga late Palaeoproterozoic Pinal Schist, intruded by 1.41 to 1.48 Ga Mesoproterozoic anorogenic granites (e.g., Valencia et al., 2006). In Northeastern Sonora, overlying Palaeozoic sedimentary rocks represent the southern extension of the Cordilleran miogeocline and platform sequences, represented in the district by the Cambrian Bolsa and Abrigo, Devonian Martín, and Lower Carboniferous Escabrosa formations, capped by part of the Permian Naco Group (e.g., Valencia et al., 2006). Proterozoic and Palaeozoic rocks are overlain by Triassic to Jurassic volcanic rocks, which, in the Cananea District, are the Elenita and Henrietta Formations, that are intruded by Jurassic plutonic rocks. These rocks are part of a continental magmatic arc that extends from California in the USA to Durango in Mexico (e.g., Valencia et al., 2006). These were followed by the magmatic arc of the 75 to 50 Ma Laramide Orogen, with which many of the porphyry copper deposits in southwest North America are associated. After an ~20 m.y., quiescent period, during which time the magmatic arc migrated to the west, intensive magmatism recommenced, represented by extensive 30 to 25 Ma Oligocene volcanic sequences. Between 27 and 12 Ma, during the Miocene, mid-crustal extension and core-complex formation occurred in Sonora, causing disruption and rotation of the Cananea District (Carreón-Pallares, 2002). Continued extension then resulted in the vertical tectonics of the ~13 to 5.5 Ma Basin and Range Province. The Oligocene magmatism was related to the Sierra Madre Occidental large igneous province immediately to the SW. See also the separate Cananea record for a description of the geology of the Cananea District. Note, there are some differences in stratigraphic names and descriptions due to the different sources used to compile each record.

Deposit Geology

  The Milpillas deposit is located within the extensional, north-south trending, Basin and Range Cuitaca Graben, where it is hosted within a 7 km wide down-dropped block. The eastern section of the graben is at a shallower level than the western portion, and is dominated by Tertiary gravels, Quaternary alluvium, and erratic outcrops of Laramide age volcanic units. The deeper western portion of the graben is dominated by Quaternary alluvium. Close to the eastern boundary of the Cuitaca Graben, a small horst exposes scarce altered and oxidised rocks representing the upper part of the Milpillas ore deposit.
  The Mesoproterozoic 1440 ±15 Ma (U-Pb in zircon; Anderson and Silver, 1977) Cananea Granite is the oldest rock in the Cananea District. It is overlain by a Lower Palaeozoic platform sequence, mostly composed of quartzite and carbonates, succeeded by the conformably overlying Upper Palaeozoic carbonates of the Naco Group (Meinert, 1982). Whilst all of these units are extensively exposed elsewhere in the District, neither has been recognised in the Milpillas area, but may be present at depth (Valencia et al., 2006). These are overlain by the Late Triassic to Early Jurassic Elenita Formation, which consists of a sequence of volcaniclastic and sedimentary rocks that include rhyolitic flows and tuffs, interbedded with andesites, sandstones, quartzites and conglomerates that are exposed in the Cananea District. This unit has been correlated with the 220 and 192 Ma Mount Wrightson and Fresnal Canyon Formations in southern Arizona.
  The Jurassic Henrietta Formation is the oldest exposed volcanic unit at Milpilas, where it is represented by a volcaniclastic sequence of calc-alkaline dacitic and rhyolitic flows and tuffs, interbedded with agglomerates, latites and andesites (Wodzicki, 2001). Elsewhere in the Cananea region, this unit overlies the Elenita Formation.
  The main volcaniclastic host in the Milpillas area is the Laramide Mesa Formation, which unconformably overlies the Henrietta Formation. It is an ~1500 m thick, ~69 Ma, calc-alkaline volcaniclastic unit that outcrops extensively throughout the Cananea District (Wodzicki, 1995). The volcanic rocks of this unit have a medium to high potassium content and comprise trachybasaltic to andesitic agglomerates, flows and tuffs, including dacite and trachydacite, with dominant andesitic compositions throughout the sequence. In the broader Cananea District, this unit usually also includes significant interbedded volcanic sandstones and agglomerates, as well as a basaltic flow unit, synvolcanic dolerite sills and domes, that together are referred to as the Mariquita Formation.

The Henrietta and Mesa Formations are intruded by a suite of small porphyry stocks. These porphyries, which vary from quartz monzonite to monzonite in composition, are composed of 2 to 5 mm quartz, feldspar and biotite phenocrysts set in a matrix of aphanitic to fine quartz and orthoclase. They are typically overprinted by strong sericitic alteration and are the main host to the Cu mineralisation, which extends outward into the immediate intruded volcaniclastic country rocks. These porphyry stocks are spatially related to the late stages of a Laramide batholitic pluton complex, known locally as the Cuitaca-Tinaja Batholith, and are assumed to be a phase of that intrusion. This batholith is extensively exposed throughout the broader Cananea Mining District. It is a granodiorite that contains biotite and hornblende as the main accessories, with minor magnetite and sphene, and has been dated at 64 ±3.0 Ma (U-Pb, zircon; Anderson and Silver, 1977). No porphyries crop out in the Milpillas area, although there are some isolated outcrops of altered and leached volcanic host rocks.

  Three main lineaments have been described in the Milpillas area: i). a pre-mineral north-south trend; ii). a syn-mineral NE linear trend; and iii). a post-mineral NW trend (de la Garza et al., 2003). In addition, a detailed structural analysis of quartz veins and mineralised structures by Carreón-Pallares (2002) revealed a flat radial and concentric structural pattern with preferential dips to the NE, east and SE. These dip trends are also the same primary orientations reported for Laramide stocks throughout Arizona (Rehrig and Heidrick, 1972; Heidrick and Titley, 1982), whereas the concentric pattern has been recognised in Sierrita, Arizona (Titley, 1982).

Mineralisation and Alteration

  The hypogene mineralisation in the broader Cananea District variously occurs as breccias, stockwork and/or disseminated sulphide minerals accompanied by skarn altered and mineralised bodies where pre-Laramide carbonate host rocks are present. High-grade mineralisation occurs in breccias pipes, such as in the Cananea Mine. High-grade, but low tonnage, replacement Cu-Zn-Pb mineralisation has also been locally developed in Palaeozoic carbonate rocks interbedded with quartzites. Most of the porphyry/skarn style copper deposits of the Cananea District have been dated at ~60 ±4 Ma to ~54 ±2 Ma (K-Ar; Damon and Mauger, 1966; Damon et al., 1983; Wodzicki, 1995).

  At Milpillas, only scarce, low-grade, 0.15 to 0.20% Cu hypogene mineralisation has been recognised, and the ore mined is principally from high-grade supergene enriched, chalcocite blankets that are entirely masked by commonly up to 50 to 250 m of Cenozoic alluvial sediments (de la Garza et al., 2003).
  The supergene blankets dip gently to the west and vary from <20 up to near 100 m in thickness, cutting, from east to west, the Cretaceous Mesa Formation; Tertiary quartz monzonite porphyry and Jurassic Henrietta Formation over a length of >1 km (Valencia et al., 2006).
  Supergene enrichment has produced a vertical zonation from an upper leached cap and oxide zone that overlies a secondary enrichment zone which commonly contains grades that range from >1, to >10% Cu. These high grades are found in at least six sub-horizontal blankets that occur at depths that range from 150 to 750 m below the surface. Evidence for at least three zones of secondary enrichment are recognised within these blankets (Anderson, 1982; Titley and Marozas, 1995; Gilmour, 1995).
• The upper three are reworked earlier blankets and contain oxide mineralisation that have a complex mineralogical assemblage, comprising: i). green Cu-oxides and carbonates, including antlerite, brochantite, malachite, azurite and chrysocolla; ii). red Cu-oxides, including cuprite, native copper, delafossite and minor 'pitch' limonite; and iii). black Cu-oxides, including neotocite, melaconite, tenorite and minor 'copper wad' (de la Garza et al., 2003).
• Below these three blankets, there is an intermediate horizon that contains a mixture of oxides and secondary sulphides.
• The two deepest blankets are dominantly composed of supergene sulphide minerals, mainly chalcocite and minor covellite (de la Garza et al., 2003).

The quartz monzonite porphyry unit that host the mineralisation at the Milpillas deposit, yielded a crystallisation age of 63.9 ±1.3 Ma (U-Pb zircon, ICP-MS). Dating of hypogene molybdenite mineralisation from a quartz-sericite-molybdenite and a quartz-sericite-molybdenite-chalcopyrite-pyrite vein in two drill core samples from >500 m depth yielded an identical age of 63.1 ±0.4 Ma (Re-Os Molybdenite ages; Valencia et al., 2006). As such, the Milpillas mineralisation age corresponds to one of the three discrete pulses recognised within the Cananea District at ~63, ~61 and ~59 Ma (Valencia et al., 2006).

Reserves and Resources

  Prior to the commencement of operations, the supergene mineral resources at Milpillas were estimated at (Valencia et al., 2006 after Servicios Industriales Peñoles)
    30 Mt @ 2.5% Cu

  Ore Reserves at 31 December, 2008 (Industrias Peñoles Annual Report, 2008)
    37.498 Mt @ 1.67% Cu
  Total processed during the year - 1.404 Mt of ore.

  Remaining Ore Reserves at 31 December, 2014 (Industrias Peñoles Annual Report, 2014)
    19.001 Mt @ 1.26% Cu
  Total processed during the year - 2.400 Mt of ore.

  Remaining Ore Reserves at 31 December, 2019 (Industrias Peñoles Annual Report, 2019)
    14.701 Mt @ 0.97% Cu
  Total processed during the year - 2.506 Mt of ore.

  Remaining Ore Reserves at 31 December, 2022 (Industrias Peñoles Annual Report, 2022)
    4.007 Mt @ 0.81% Cu
  Total processed during the year - 0.895 Mt of ore (mine closed for 6 months of year).

The most recent source geological information used to prepare this decription was dated: 2006.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.

Mina Millpilas

  References & Additional Information
   Selected References:
Valencia, V.A., Noguez-Alcantara, B., Barra, F., Ruiz, J., Gehrels, G., Quintanar, F. and Valencia-Moreno, M.,  2006 - Re-Os molybdenite and LA-ICPMS-MC U-Pb zircon geochronology for the Milpillas porphyry copper deposit: insights for the timing of mineralization in the Cananea District, Sonora, Mexico: in    Revista Mexicana de Ciencias Geologicas   v.23, pp. 39-53

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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