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Los Filos - Bermejal, Guadalupe, Nukay
Guerrero, Mexico
Main commodities: Au Ag


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The Los Filos gold deposits are exploited from three open pits, Los Filos, Bermejal and Guadalupe, and two underground mines, Los Filos and Bermejal, located ~180 km south-southwest of Mexico City in the municipality of Eduardo Neri and district of Nukay, Guerrero State, Mexico (#Location: 17° 53' 8"N, 99° 40' 52"W).

The centre of the Bermejal pit is ~4 km SSE of the centre of the Los Filos pit. Nukay is <1 km to the NW of the margin of the Los Filos pit, whilst Guadalupe is <3 km SE of the Bermejal pit. It is also 15 km SSE of the Morelos Operation mines, which like Los Filos, are part of the wider Guerrero Gold Belt.

The Nukay underground mine, now part of the Los Filos ‌mine, was operated by Minera Guadalupe S.A. de C.V. from 1938 to 1940 and 1946 to 1961 to produce ~0.5 Mt @ 18 g/t Au. A new entity, Minera Nukay, commenced mining at an open pit mine at Nukay from 1984, and from 1987 to 2001 operated a plant near Mezcala to process ore from the Nukay, La Agüita, Subida and Independencia deposits in the same district. In 1993, a consortium, led by Teck Corporation conducted exploration in the district, culminatiing in the Los Filos discovery hole drilled in August 1995. In November 2003, Wheaton River Minerals gained 100% ownership of the Los Filos Mine Complex, before being taken over by Goldcorp in March 2005. Goldcorp also acquired the Nukay mine in 2008, which was subsequently integrated into the Los Filos operations as the Los Filos Underground mine. Exploration by Industrias Peñoles S.A. de C.V. in the Cerro Bermejal area in 1986 outlined gold values in the oxide zone and in jasperoids. Further exploration between 1988 and 1989 was followed by a pre-feasibility mineral resource estimate in 1994 for an open pit and heap leach operation. ‌In March 2005, Goldcorp's wholly owned Mexican subsidiary Luismin acquired the Bermejal gold deposit from Minera El Bermejal, S. de R.L. de C.V., a joint venture between Peñoles and Newmont. Feasibility studies of the Los Filos and Bermejal Open Pits and Los Filos Underground mine were completed between 2005 and 2007. Open pit mining commenced at the Los Filos Mine Complex in 2005. Underground production at Los Filos began in 2007 and the first gold was poured in the same year. Annual open pit ore production increased to >20 Mtpa by 2008. Underground production has varied from 280 tpd in 2009 to over 1100 tpd in 2015. Exploration drilling below the Bermejal Open Pit in 2013 encountered high-grade oxide mineralisation, now known as the Bermejal Underground deposit. In April 2017, the Los Filos Mine Complex operating entity Desarrollos Mineros San Luis S.A. de C.V. (DMSL) was purchased from Goldccorp by Leagold Mining Corporation. In March 2020, Leagold Mining was merged into Equinox Gold who then assumed control of the Los Filos operation.

Geology

The Los Filos deposits are located within the Guerrero Gold Belt, occurring near the centre of the circular, ~200 km diameter Morelos-Guerrero Sedimentary Basin which contains a thick sequence of Mesozoic platform carbonate rocks. This sequence is composed of the:
Morelos Formation - which comprises medium- to thick-bedded fossiliferous crystalline limestone and dolostone. This formation is the dominant suite in the Los Filos Mine Complex, although its lower contact is not exposed in the mine area;
Cuautla Formation - which transitionally overlies the Morelos Formation, and comprises a succession of thin- to medium-bedded silty limestone and sandstone with argillaceous partings and minor shale intercalations;
Mezcala Formations - which, in turn, transitionally overlies the Cuautla Formation and is composed of a platform to flysch-like succession of interbedded sandstones, siltstones and lesser shales, that have been extensively altered to hornfels near intrusive contacts.

These Cretaceous carbonates have been intruded by a number of early Tertiary granitoid bodies that are regionally distributed along NW-trending belts, thought to reflect emplacement controlled by pre-existing faults (de la Garza et. al., 1996). In the Los Filos Mine Complex area, the Tertiary intrusives are granodiorite that include:   i). the East and West Stocks of the Los Filos Intrusion, two circular bodies, that cover a total area of ~3.5 km2;   ii). the Bermejal Intrusion, an irregularly shaped oblong body that extends over a NW-SE length of 5 km with an area of at least 10 km2; and   iii). the Xochipala Intrusion which has an area of ~4 km2. Another unnamed granodiorite body with >4 km2 area is located in the northeastern portion of the complex.

Both the Cretaceous sedimentary rocks and Tertiary granitoid intrusions are partially overlain above an unconformable contact by a sequence of Tertiary intermediate to felsic volcanic rocks and alluvial sedimentary rocks that includes rhyolitic volcanic unit on the eastern edge of the complex, as well as red sandstones and conglomerates.

The sedimentary succession was folded into broad north-south trending paired anticlines and synclines, the result of an east-vergent, compressive, Laramide 80 to 45 Ma event. The Los Filos deposits lie within the transition between a domain of overthrust rocks to the west, and more broadly folded sequences to the east.

Mineralisation

Regional mineralisation in this part of Mexico occurs as skarn-hosted; epithermal precious metal oxide; and volcanic hosted massive sulphide deposits. In Guerrero State, there are two adjacent arcuate belts, namely the Guerrero Gold Belt to the east of the volcanic hosted massive sulphide belt to the west, both of which trend NW-SE, and are ~30 km wide and over 100 km in length.

The Los Filos Mine Complex deposits lie within the Guerrero Gold Belt. They occur as oxide skarn developments at or near the contacts between Tertiary intrusions and the carbonate-rich Morelos Formation sedimentary rocks. Garnet-rich skarn predominates at the base the deposits, with traces of silica, grading upward to chlorite and epidote, plus abundant silica toward the top. Sericite is abundant, but is restricted to the apexes of the intrusive stocks. The formation of oxide skarn occurred in three stages, with late hydrothermal veining overprinting the sequence as a fourth stage:
Stage 1, Prograde Skarn - comprising garnet-pyroxene endoskarn with lesser exoskarn, including massive magnetite, forming an envelope around the stock.
Stage 2, Retrograde Skarn - extensive chlorite-epidote, tremolite-actinolite and phlogopite-serpentine assemblages with lesser talc, muscovite and sericite which predominate in the upper 400 m of the stock. This peripheral halo can be up to 170 m wide within the intrusion.
Stage 3, Late Skarn - which consists of garnet veins that cut through both early prograde and later retrograde skarn.
Stage 4, Late Veining - which comprises two successive gold-bearing stages of silica as well as phlogopite and amphibole veins. Earlier quartz-pyrite-hematite veins were followed by quartz-pyrite, opal chalcedonic quartz veins, and silica flooding along structures as well as within the intrusive matrix. Pervasive jasperoids are typically found in association with the late veining stage, replacing skarn and intrusive rocks, and forming a silica cap.

Massive magnetite, hematite, goethite and jasperoidal silica, with minor associated pyrite, pyrrhotite, chalcopyrite and native gold are typically found within these veins and metasomatic replacement bodies at the contacts between the Morelos Formation platform carbonates and the intrusive rocks. Extensive, deep, syn-mineral oxidation of the deposits altered the mineralisation into material amenable to cyanidation recovery techniques without the need of pre-treatment by roasting or other equivalent techniques.

Mineralisation is distributed as follows in the various of the deposits. Within the Los Filos Open Pit, it is associated with a shallowly east-dipping sill and with the upper portion of the east stock. The Los Filos Underground is divided into the Los Filos Norte and Sur Sectors along the north and south side of the circular West Stock. The principal mining areas in the Norte Sector are Nukay, Conchita, Peninsular, Chimenea, Independencia-Subida, and in the Sur Sector include Sur, Zona 70 and the Creston Rojo deposits. Mineralisation in the Bermejal area is along the contact of the Bermejal Stock and the carbonate rocks of the Morelos Formation. The Bermejal Open Pit mineralisation is typically at the top, or on the flanks of the upper portion, of the Bermejal Intrusive. Mineralisation extends below the Bermejal Open Pit into the Bermejal Underground deposit, down the steeply dipping to vertical flanks of the Bermejal Intrusive, and at the northern end of the same intrusive.

The total circumference of the Los Filos area intrusive stocks is ~8 km, whilst the Bermejal Intrusive has a circumference of ~16 km. Mineralisation extends from surface to >700 m depth, with variable grades and widths along the contact between the intrusives and Morelos Formation carbonate rocks.

Reserves and Resources

Mineral Resources, as at 31 December 2012 (GoldCorp Inc, NI 43-101 Technical Report, 2012):
  Measured + Indicated Resources - 69.622 Mt @ 0.83 g/t Au, 6.7 g/t Ag for 58 t of contained gold;
  Inferred Resources - 236.97 Mt @ 0.79 g/t Au, 6.0 g/t Ag, for 187 t of contained gold.

Ore Reserves and Mineral Resources, as at 25 April, 2022 (Equinox Gold, Reserves and Resources Satatement, 2022):
  Proved + Probable Reserves - 104.22 Mt @ 1.31 g/t Au, for 137 t of contained gold;
  Leach pad inventory - 3.54 tonnes of contained gold;
  Measured + Indicated Resources - 326.31 Mt @ 0.93 g/t Au, for 304 t of contained gold;
  Inferred Resources - 98.204 Mt @ 0.83 g/t Au, for 82 t of contained gold.
NOTE: Resources are inclusive of reserves.

The bulk of the information in this summary is drawn from "Arseneau,G., Lincoln, N., Olin, E., Rykaart, M., Olson, T., Nicholas, D. and WInkelmann, 2019 - Independent Technical Report for the Los Filos Mine Complex, Mexico; an NI 43-101 Technical Report, prepared by SRK Consulting (Canada) Inc., for Leagold Mining Corporation, 513p."

The most recent source geological information used to prepare this decription was dated: 2019.    
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.


Los Filos

  References & Additional Information

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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