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San Nicolas |
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Zacatecas, Mexico |
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Cu Zn Au Ag
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Super Porphyry Cu and Au
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The San Nicolás volcanic hosted, massive sulphide (VHMS) copper-zinc-silver-gold deposit is located ~60 km ESE of Zacatecas City, in Zacatecas State, central Mexico, at an altitude of 2150 metres above sea level (#Location: 22° 36' 49"N, 101° 59' 0"W).
In 1996, Minera Teck S.A. de C.V., the Mexican subsidiary of Teck Corporation, formed a joint venture with Western Copper Holding Ltd., to explore the El Salvador district in Zacatecas, initially drilling an oxide copper showing exposed in a small pit. A pre-drilling inspection of the pit had interpreted the mineralisation as probably representing stratabound VHMS mineralisation of the type found elsewhere in the district. Drilling continued until hole SAL-5 intersected 2.1 m of massive sulphide @ 2.07% Cu, 1.53% Pb, 16.57% Zn, 3.68 g/t Au, 213 g/t Ag. Continued drilling outlined a small massive sulphide body hosted by siliceous sedimentary rocks at the contact between underlying felsic tuffs and breccias, and overlying porphyritic andesite, consistent with the VHMS interpretation (Thompson, 1999).
Subsequent to outlining a small body of VHMS mineralisation at El Salvador, geological mapping and sampling, and airborne magnetic, EM and radiometric geophysical surveys were conducted. These helped identify other prospective volcanic sequences and mineralised showings, but failed to define specific drill targets. During the summer of 1997, an orientation induced polarisation (IP) survey was undertaken over the known El Salvador mineralisation by Quantec Geofísica de México, S.A. de C.V. That mineralisation was successfully detected, and a Quantec gradient array was selected for further work, rated to be able to detect mineralisation similar to that at El Salvador within 200 m of surface. Twenty-two line km at 100 m line spacing was completed in the El Salvador area, defining a 500 x 500 m zone of high chargeability and low resistivity, centred ~2 km west of El Savador. Follow-up diamond drilling commenced in November 1997, intersecting 179.7 m of massive sulphide in drill hole SAL-25. By mid-1998, the San Nicolás deposit had been delineated with 58 diamond drill holes, and preliminary reserves were announced later that same year (Thompson, 1999).
Regional Setting
The San Nicolás deposit is located within the Guerrero Composite Terrane, which is interpreted to underlie much of the western half of Mexico. However, the Mesozoic metavolcano-sedimentary assemblages that define the Terrane, are only exposed over <5% of the surface area, occurring as scattered erosional windows through the extensive Tertiary and Quaternary volcanic and sedimentary strata of the Sierra Madre Occidental Province. The Guerrero Terrane is the composite product of complex subduction-related processes, major translation and periods of rifting during the Mesozoic along the western margin of Mexico (Campa and Coney, 1983; Centeno-García et al., 2008). VHMS deposits occur within two distinct belts in the Guerrero composite terrane: i). a coastal belt which includes the Cuale-Bramador, La Minita-Sapo Negro and Arroyo Seco deposits and prospects; and ii). another, inland, close to the eastern boundary of the Guerrero Composite Terrane, encompassing San Nicolás-El Salvador, the deposits in the Guanajuato Ranges, Tizapa-Santa Rosa, Tlanilpa-Azuláquez, Rey de Plata, Campo Morado-Suriana, etc.
The volcanic window which the San Nicolás-El Salvador deposits overlap is one of a series that exposed sections of a NW-SE trending belt of deformed Jurassic to Cretaceous marine sedimentary and volcanic rocks of the Guerrero Terrane that in the central part of Mexico have been defined as the Chilitos Group. This group unconformably overlies the Middle to Late Triassic, passive continental margin turbidite sequence of the Zacatecas Formation, which comprises a flysch sequence, containing pillow lavas and minor limestone. Discontinuous exposures of the Chilitos Group over a distance of >300 km, from Guanajuato to Zacatecas, suggest a gently folded, grossly conformable, stratigraphic succession, up to 1.5 km thick, that has been intruded by the Jurassic to early Tertiary El Saucito, Peñón Blanco and Comanja type plutons. As detailed above, the Mesozoic sequence is mostly covered by the Tertiary volcanic rocks, which in this region are the rhyolitic La Bufa Formation, other minor volcanics units and semi-consolidated alluvium.
Regionally, the Chilitos Group has been divided into three conformable formations (Vassallo, 2012), as follows, the:
• Arroyo Chilitos Formation at the base (de Cserna, 1976), which dominantly comprises basaltic to andesitic sills and lavas;
• San Nicolás Formation, mainly composed of rhyolitic lavas, synvolcanic intrusions and volcaniclastic rocks. The crystallisation age of a rhyolite sampled from this unit gave an age range of 146.5 ±2.2 Ma (zircon 206Pb/238U; Mortensen et al., 2008):
• La Virgen Formation, the upper unit, which comprises lithic sandstone and greywacke with some basaltic lava, as well as syn-volcanic intrusions, and various volcaniclastic and sedimentary rocks. The basaltic lava yielded a hornblende age of 117.94 Ma (Iriondo et al., 2003);
The Chilitos Group is interpreted to have been tilted during Aptian faulting, and to probably represent the northeastern section of a large NW-SE trending overthrust structure, possibly formed during the mid-Cretaceous, when NE-vergent volcanic rocks were juxtaposed over shelf carbonate and silicate sedimentary rocks to the NE. The regional metamorphic grade increases from sub-greenschist to greenschist from east to west across Zacatecas state (Vassallo and Solorio-Munguia, 2005).
Of the major plutons in the area, those emplaced during the Tertiary, e.g., the El Saucito and Peñón Blanco plutons, postdated the end of Laramide deformation and constrains its termination to ~49.5 Ma (Aranda et al., 2007; Vassallo et al., 2008). The overlying Tertiary felsic volcanic flows and tuffs of the Sierra Madre Occidental have been locally dated at between 32 and 27 Ma at San Nicolás. A later phase of Miocene extensional faulting has been identified that cuts across the mineralised zone and and Eocene clastic units. All of the sequences are also cut by north, NW and west striking faults, the age of which are uncertain, although they are interpreted to range from syn-volcanic extensional to post-volcanic strike-slip (Johnson et al., 2000).
Geology
The San Nicolás deposit is one of several mound or lense-shaped VHMS deposits and occurrences that define the larger 500 km2 San Nicolás Mining District. Mineralisation at San Nicolás is hosted within the central part of the Upper Jurassic to Lower Cretaceous marine volcanic and sedimentary rocks of the Chilitos Group (modified by Vassallo et al., 2015 after Johnson et al. 2000).
The stratigraphic sequence of the rocks of the Chilitos Group within the San Nicolás deposit area is as follows, from the base:
LATE TRIASSIC
• Graphitic Mudstone that is >100 m thick, and represents the upper Zacatecas Formation. It is strongly folded, reflecting pre-Chilitos Group deformation, and is composed of black to medium grey graphitic mudstones with thin laminae and lenses up to 5 cm thick of grey fine-grained silstone and limestone;
LATE JURASSIC
• Mafic Volcanics - 120 m of green, massive, locally amygdaloidal, basaltic flows, containing phenocrysts of plagioclase, and commonly angular fragments of dark green glass that give the rock a mottled appearance. Intercalated cherty sedimentary rocks form units up to 15 m thick. This unit represents the regional Arroyo Chilitos Formation;
• Quartz Feldspar Rhyolite - a light yellowish green rhyolite dyke with disseminated pyrite that cuts both of the preceding units, and has been dated at 148.9 Ma;
• Rhyolite Flow Dome - an up to 300 m thick complex of massive rhyolite flows and breccias that are light grey to light green and contain ~5% phenocrysts of 1 to 2 mm long white feldspar, and disseminated pyrite. It has been dated at ~146.5 Ma. These rhyolites belong to the regional San Nicolás Formation;
• Massive Sulphide - that is laterally equivalent to the Rhyolite Flow Dome, up to 280 m thick, and composed of massive pyrite, sphalerite, chalcopyrite, barite and other minor ore minerals. The favourable host within which the massive sulphides are deposited, according to Vassallo et al. (2015), is an up to 300 m thick lithologically diverse unit that is characterised by coarse, 'quartz-eye' bearing volcaniclastic rocks, coarse porphyritic 'quartz-eye' intrusions and a few rhyolite breccias that is mineralised toform massive to semi-massive sulphide and massive to semi-massive barite bodies;
• Mafic Flows and Sediments, comprising ~100 m of black mudstone that is intercalated with light green mafic flows and overlies the flow dome and massive sulphides. The mudstone is massive to laminated, commonly internally brecciated, siliceous and mildly carbonaceous, containing 1 to 5 mm pyrite laminae. The flows are pale green to olive green, aphyric and amygdaloidal;
EARLY CRETACEOUS
• Mafic Flows and Breccias, which are 50 to 70 m thick, and are light green to olive, distinguished by the presence of 1 to 10 % euhedral hornblende phenocrysts that are 2 to 5 mm across and dark brown to dark green when fresh. The latter are typically replaced by clay minerals or epidote. The flows contain up to 15 % plagioclase and abundant amygdales filled with green chlorite and white calcite ±quartz;
• Mafic Flows, Volcaniclastic and Sediments, that are ~50 m thick and composed of mafic flows, fine to coarse grained volcaniclastic rocks,
and minor siliceous sediments and greywackes. The Early Cretaceous units, and the underlying 'Mafic Flows and Sediments' are attributed to the regional La Virgen Formation;
TERTIARY
• Conglomerates and Volcaniclastic Breccias, of the La Bufa Formation that unconformably overlie the Cretaceous sequence and are >150 m thick. They are medium to dark orange-brown, massive and polymictic, containing a wide variety of felsic to mafic volcanic and minor sedimentary clasts of older rocks, supported in an iron stained clay matrix. They are younger than 32.32 Ma.
The cumulative sequence is ~890 m thick.
Based on the distribution of mineralisation and alteration (described below), the bulk of the mineralisation is interpreted to have formed between emplacement of the rhyolitic lava dome of the San Nicolás Formation and the deposition of the mafic extrusives rocks of the La Virgen Formation above.
Two separate generations of faulting have been identified at San Nicolás (Johnson et al., 2000; Vassallo 2003):
i). a NW-SE trending set which cut the footwall rhyolitic rocks and are interpreted to be of Jurassic to Cretaceous age, and to have been active during deposition of the Chilitos Group and possibly reactivated during Tertiary extension accompanying the Sierra Madre Occidental magmatism. Immediate footwall to equivalent rhyolite rocks have been locally displaced by this set into an apparent hanging wall position within the deposit, while faults of this generation are intruded by Tertiary sub-volcanic rocks (Vassallo et al., 2015);
ii). NE-striking group of normal faults belonging to the Miocene Basin and Range Province, that largely post-dated deposition of the Sierra Madre Occidental volcanic cover, and cut the massive sulphide deposits and Eocene clastic units (Vassallo, 2003; Vassallo et al., 2015).
At San Nicolás, the Chilitos Formation sequence has been uplifted and exposed in the core of a NW striking local horst over which the Tertiary volcanics have been eroded. However, volcanic rocks of Tertiary age have also intruded along the bounding faults of the horst, and along north-striking faults to it's west. Within the broader horst, the same extension produced localised Jurassic to Cretaceous horsts and grabens, also bounded by NW-SE trending faults. The deposit itself, which is bounded to the SW by the NE flank of a rhyolite dome, and by a SW dipping fault to it's NE, falls within one of these grabens. The fault that bounds it to the NE, the La Panza Fault, marks the margin of this graben (Vassallo et al., 2015).
Mineralisation
The deposit is divided into an upper Main zone of massive to semi-massive polymetallic sulphide, and a copper-rich Lower zone. The top of the Main zone massive sulphide is 150 to 220 m below the surface, covered by up to 100 m of unmineralised mafic volcanic flows, fragmental rocks, and volcaniclastic and argillaceous sedimentary rocks, interpreted to be part of the same sequence as the underlying volcanic hosts and massive sulphides. These rocks are, in turn, covered by 50 to 150 m of Tertiary volcaniclastic breccias, which locally contain saline ground waters.
The Main zone of massive sulphide has a generally conformable upper surface, underlain by a 'keeled boat-shape' lens that is 400 to 200 m wide, tapering downward, to be up to 280 m thick, and is 900 m long. A thick pile of felsic volcanic rocks, that marks the SW margin of the massive sulphide body, is interpreted to be a rhyolite dome (Johnson et al., 1999).
The upper layer of the Main massive sulphide body is composed of 2 to 35 m of finely banded, polymetallic sulphide, dominated by fine-grained pyrite, with high concentrations of sphalerite and chalcopyrite, and is rich in Zn, Au and Ag, with minor barite. It is overlain by black mudstones. The bulk of the underlying Main zone massive sulphide is Cu-rich, and is composed of fine-grained pyrite, which is locally brecciated and contains minor chalcopyrite and sphalerite, passing down to the basal contact that is typically uneven, and grades into stringer mineralisation. Gold and silver concentrations increase towards the upper part of the zone.
The Lower zone is a tabular chalcopyrite-rich SW dipping body, which appears to join the southeastern part of the Main zone at a depth of ~400 to 450 m below surface. The Lower zone largely comprises stringer and replacement mineralisation, composed of sulphides that are granular and fragmented (Johnson et al., 1999).
All of the sulphide assemblages are predominantly composed of sphalerite, chalcopyrite and pyrite. Barite occurs in veinlets, mainly forming stockworks beneath the polymetallic lenses. The stringer mineralisation commonly follows flow laminations in host rhyolites, whilst textures within the massive sulphides, such as curved sulphide 'laminations' that mimic volcanic flow lamination, and hyaloclastite breccias with jigsaw textures replaced by sulphides, suggest they were partly formed by replacement of such rhyolites (Johnson et al., 2000). Sulphide-bearing stringers occur either above, below, or laterally to the massive core of the Lower sulphide zone.
Footwall rocks exhibit propylitic alteration assemblages, mainly chlorite, calcite and siderite-rich associations, but with localised intense chlorite plus quartz-sericite alteration of the felsic and mafic host rocks. Stringers of calcite and iron carbonates, as well as the barite, are common throughout the footwall of the deposit, whilst barite veinlets locally cut the sulphide masses.
Alteration
The alteration at the San Nicolás deposit is both texturally and mineralogically diverse and variable (Vassallo et al., 2015). A conformable alteration zone underlies the massive sulphide lenses and zone of mineralisation over an area of several tens of square kilometres, and persists over a stratigraphic thickness of at least 200 m below the several ore lenses that constitute the deposit. According to Vassallo et al. (2015), this alteration varies, with increasing intensity, from:
i). weak chlorite-sericite → weak chlorite-pyrite-sericite, which are particularly common in the hanging wall volcanic rocks; →
ii). moderate to intense chlorite-pyrite-sericite, which is confined to the footwall volcanics, below the massive sulphides; →
iii). primary feldspar-destructive muscovite-chlorite-biotite-rich with minor pyrite → mottled, strong chlorite-pyrite-sericite with secondary K feldspar - which occupies the bulk of the alteration blanket; →
iv). quartz-K feldspar with intense quartz-pyrite-sericite, which locally occurs on the fringes of the footwall alteration zone; →
v). disseminated tremolite and patchy carbonate-tremolite-chlorite with albite-calcite-epidote, which occurs sporadically in the footwall
alteration zone, generally in the top of the rhyolite flow dome, and in association with the tectonic breccias of the SW Massive sulphide lens; →
vi). intense, quartz-pyrite and silicification with stringers of chalcopyrite-pyrite that are developed as conformable bands around the massive
sulphides and as discordant zones within the footwall.
Minor zones of intense alteration cut across the footwall alteration zone and extend up to the massive sulphide bodies.
Carbonate alteration (dolomite and/or ankerite) is regarded as representing the initial phase of hydrothermal activity. This was followed by diffuse acidic hydrothermal fluids causing dissolution of carbonates and destruction of primary feldspars (plagioclase), precipitation of pyrite, and formation of sericite, chlorite, and clay minerals. Continued and intensified influx, and evolution, of these fluids led to mottled biotite, secondary K feldspar-chlorite-pyrite-sericite, more intense quartz-pyrite alteration, and deposition of mineralisation (Vassallo et al., 2015).
The altered mafic rocks show large variations in the concentrations of Na, Ca, Mg and Fe. Poorly altered to fresh mafic rocks contain between 3 and 8 wt.% Na2O+CaO. Increasing alteration corresponds to a gradual decrease in Na2O and CaO, culminating in intensely altered basalts having Na2O+CaO values of <1 wt.%. This trend is taken to indicate plagioclase destruction and alteration to muscovite/sericite. Intense pyritic alteration is characterised by high MgO+FeO values, commonly between 3 and >20 wt.%, due to the abundance of pyrite and chlorite. Calcareous alteration above
the ore body shows extreme CaO and MgO enrichment, related to a combination of carbonate (dolomite) and chlorite. Samples of altered quartz-eye porphyry have
MgO+Fe2O3 values mainly between 6 and 10 wt.% and their Na2O+CaO content ranges between <1 and 4 wt.%, overlapping the least altered, weakly altered, and moderately altered footwall rhyolite.
Resources and Reserves
As at 31 December, 2022, Teck Resources estimated the San Nicolás deposit to contain (Teck Annual Information Form, February 21, 2023):
Ore Reserves
Proved - 47.00 Mt @ 1.26% Cu, 1.6% Zn, 0.41 g/t Au, 23.9 g/t Ag,
Proved - 57.50 Mt @ 1.01% Cu, 1.4% Zn, 0.39 g/t Au, 20.9 g/t Ag,
Proved + Probable - 105.2 Mt @ 1.12% Cu, 1.48% Zn, 0.40 g/t Au, 22 g/t Ag.
Mineral Resources
Measured - 0.50 Mt @ 1.35% Cu, 0.4% Zn, 0.08 g/t Au, 6.4 g/t Ag,
Indicated - 6.10 Mt @ 1.17% Cu, 0.7% Zn, 0.20 g/t Au, 11.9 g/t Ag,
Inferred - 4.90 Mt @ 0.94% Cu, 0.6% Zn, 0.13 g/t Au, 9.3 g/t Ag.
Total Mineral Resources - 11.50 Mt @ 1.08% Cu, 0.64% Zn, 0.16 g/t Au, 10.55 g/t Ag.
NOTE: Mineral Resources are separate from Ore Reserves.
Agnico Eagle acquired a 50% interest in the project in April 2023 from Teck Resources Limited and the two companies formed a long-term 50/50 joint venture partnership to advance permitting and development of the deposit. They aim to submit an Environmental Impact Assessment and Permit application for the property in the first half of 2023 and are targeting completion of a feasibility study in early 2024.
The information in this summary has been drawn from:
Press release "Teck and Agnico Eagle Announce Agreement on the San Nicolás Copper-Zinc Project located in Zacatecas, Mexico, September 16, 2022."
Thompson, J.H.F., 1999 - Discovery of New Deposits at Depth Examples of a Flexible Approach to Exploration - Keynote Address, SMEDG and AIG., Exploration Undercover Symposium, Ken and Joan Smith Auditorium, Shore, Blue Street, North Sydney, 24th September, 1999.
Teck Annual Information Form, February 21, 2023.
UBC Coarse Notes - Applying Geophysical Inversion at the San Nicolás Deposit, adapted from Phillips, N., 2002 - Geophysical Inversion in an Integrated Exploration Program: Examples from the San Nicolas Deposit, unpublished M.Sc. thesis, by N. Phillips. and
other sources, as cited below.
The most recent source geological information used to prepare this decription was dated: 2017.
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.
San Nicolas
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Selected References:
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Camprubi, A., Gonzalez-Partida, E., Torro, L., Alfonso, P., Canete, C., Miranda-Gasca, M.A., Martini, M. and Gonzalez-Sanchez, F., 2017 - Mesozoic volcanogenic massive sulfide (VMS) deposits in Mexico: in Ore Geology Reviews v.81, pp. 1066-1083. doi.org/10.1016/j.oregeorev.2015.07.027.
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Johnson, B.J., Montante-Martinez, J.A., Canela-Barboza, M. and Danielson, T.J., 2000 - Geology of the San Nicolas Deposit, Zacatecas, Mexico: in Sherlock, R.L. and Logan, M.A.V. (eds.), 2000 VMS Deposits of Latin America, Geological Association of Canada, Special Publication, 2, pp. 71-85.
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Phillips, N., Oldenburg, D., Chen, J., Li, Y. and Routh, P., 2001 - Cost effectiveness of geophysical inversions in mineral exploration: Applications at San Nicolas: in The Leading Edge, December, 2001, pp. 1351-1360.
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Vassallo, L.F., Aranda-Gomez, J.J. Solorio-Munguia, J.G., 2015 - Hydrothermal alteration of volcanic rocks hosting the Late Jurassic-Early Cretaceous San Nicolas VMS deposit, southern Zacatecas, Mexico: in Revista Mexicana de Ciencias Geologicas, v.13 pp. 254-272.
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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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