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Cozamin |
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Zacatecas, Mexico |
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Cu Ag Zn Pb
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Super Porphyry Cu and Au
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The Cozamin copper-silver-zinc-lead mine is located in the Municipality of Morelos, ~3.5 km to the north-northwest of the centre of Zacatecas City, in Zacatecas State, Mexico (#Location: 22° 48' 7"N, 102° 35' 10"W).
In pre-Hispanic times, the indigenous Huichol people mined native silver from the oxidised zone of argentiferous veins in the Zacatecas Mining District. In 1546, a Spaniard Juan de Tolosa, guided by a local Huichol, examined argentiferous occurrences in the district, and in 1548, production commenced at three locations: the Albarrada mine on the Veta Grande system, and the San Bernabe and Los Tajos del Panuco mines on the Mala Noche Vein (MNV) system. Initially, only oxides were worked for silver and some gold, although later, the sulphide zones were exploited for base and precious metals. During the chaos of the Mexican Revolution, between 1910 and 1917, mining was essentially halted by numerous floods and cave-ins. Foreign companies worked other mines in the district for base metals from 1936 to 1948, although the lack of electric power, labour problems and low metal prices resulted in closure of what became unprofitable mines. From 1972, the Consejo de Recursos Minerales worked mines that included parts of the vein system mined at Cozamin. Historical production from the immediate Zacatecas district is estimated by the Consejo de Recursos Minerales (Cardenas et al, 1992) as 23 330 tonnes of silver from 20 Mt of ore averaging >900 g/t Ag and ~2.5 g/t Au. Whilst lead, zinc and copper have also been recovered, neither production totals nor ore grades are available.
Minera Cozamin was established in 1982, consolidating title over much of the Mala Noche Vein (MNV) system, and operating the 250 tpd San Roberto mine and plant until October 1996, over which time, an estimated 1.2 Mt of ore was processed. In October 1996, the Cozamin Mine changed ownership, and in 1997, the mill was upgraded to a 750 tpd flotation plant, which processed ore from 1997 to the end of 1999, mainly from shallow, oxide zone workings, and from the San Roberto mine. Near the end of 1998, the Cozamin operation was closed due to low metal prices, undercapitalisation of the asset, and inadequate mine management. In December 2005, the Canadian company Capstone Gold (later Capstone Copper) acquired a 90% interest in the Cozamin Mine and commenced commercial production in August 31, 2006, which it has operated continuously to the present (i.e., 2024).
Regional Setting
The Cozamin mine is located within the Zacatecas Mining District, a belt of epithermal and mesothermal vein deposits containing silver, gold, copper, lead and zinc within the larger Mexican Silver Belt, as outlined in the Regional Setting section of the Fresnillo record. It lies between the Real de Angeles mine, 75 km to the SE and the Fresnillo mine, 75 km to the NW. These deposits fall within the Southern Sierra Madre Occidental Province, near the boundary with the Mesa Central Province of north-central Mexico. Mineralised structures are all of Tertiary age.
Geology
The bulk of the mineralisation of the Cozamin Mine is hosted by the Mala Noche Vein (MNV) system within and below the San Roberto Mine. The stratigraphic succession that hosts this vein system is as follows, from the base:
• Zacatecas Formation - an Upper Triassic marine unit, composed of pelitic sedimentary and carbonate rocks that hase been metamorphosed to sericite schists, phyllites, slates, quartzites, metasandstone, flint, metaconglomerate and recrystallised limestone. It hosts the El Bote and Pimienta vein systems to the west of the city of Zacatecas and the Cozamin Mine.
• Chilitos Formation - an Upper Jurassic to Lower Cretaceous marine sequence of andesitic to basaltic volcanic rocks with pillow structures and some limestone lenses, locally referred to as 'greenstones' or the 'Zacatecas microdiorite'.
• Zacatecas Red Conglomerate - which contains fragments of Chilitos and Zacatecas Formation rocks and is interpreted to be of Early Tertiary, Paleocene to Eocene age.
• Tertiary Volcanic and Volcaniclastic Rocks - deposited south of the Zacatecas Caldera, described by the Consejo de Recursos Minerales (Cardenas et al., 1992) as rhyolitic tuffs with intercalations of rhyolite flow rocks that were extruded during the Oligocene to Eocene. The rhyolitic rocks have a moderate to high silica and high potassium content.
• Tertiary Rhyolitic Subvolcanic rocks - occurring as dykes, lava-flow domes, tuffs, ignimbrites, pipes and autoclastic breccias. The dykes and other subvolcanic intrusives bodies are structurally controlled by radial or concentric faults and fractures of the caldera structure, and are concentrated in the central part of the Zacatecas district, forming a northwest-southeast trending zone, where they are spatially associated with mineralisation within the San Roberto mine and Mala Noche Vein (MNV) system. A group of three rhyolite flow domes together surround the western third of the MNV which to date represents the zone within which, the only economically significant copper mineralisation has been found. These domes form three hills, the Cerro La Sierpe, Cerro San Gil and Cerro El Grillo which are 500 m NNW, 1.5 km WNW and 750 m SSW of the San Roberto shaft respectively.
The host rocks to the MNV are intercalated carbonaceous meta-sedimentary and andesitic volcanic rocks of the Zacatecas and Chilitos Formations and Tertiary rhyolite intrusive rocks and flows. The host lithology does not appear to have influenced the grade of the copper-silver mineralisation, although the rheological contrasts between rock units may have some control on vein emplacement, e.g., the Mala Noche Footwall Zone (MNFWZ) is intimately associated with rhyolitic dykes where mineralised veins often fall within, or follow dyke contacts with the country rock. The spatial association between the copper mineralisation and rhyolite flow domes and that of the MNFWZ veining has been taken to infer this mineralisation is developed relatively close to a volcanic to sub-volcanic magmatic centre.
Mineralisation within the MNV appears to have been episodic, the first, and most important phase of which is interpreted to have seen the introduction of copper-silver, which was subsequently enveloped, overprinted or brecciated by a moderate to strong zinc-lead-silver pulse.
Structure
The principal vein system, the Mala Noche Vein (MNV) has the characteristics of open spaces infill, controlled by brittle faulting along the Mala Noche Fault System (MN Fault). The MNV is curvilinear overall, with a strike that ranges from WNW in the San Roberto mine in the west, to NE on the eastern margin of the Cozamin mine, before rotating to east-west and eventually in the far east to NW-SE again, and then bifurcating. It's dip varies from 38°N to vertical. There is a clear association between higher copper grades and steeper dips of the Mala Noche fault.
Other faults include the:
• El Abra fault, which is sub-parallel to and forms an anastomosing association with the Mala Noche Fault, in both strike and dip directions. Ore grades in the San Roberto Mine are strongest where the two fault sets coalesce. The dominant alteration associated with this fault is silica-calcite-pyrite.
• Rosita fault, which is also sub-parallel to the Mala Noche Fault, but is mostly to the north of, and in it's hanging wall. The principal alteration association is coarsely crystalline calcite.
• San Ernesto fault, which also strikes WNW and dips at ~60°NNE. Mineralisation within the fault encountered to date has been zinc and lead dominant.
• MNFWZ Fault, which with the MNV is the principal structure hosting Cozamin Mineral Resources. It is a fault splay from the footwall of the Mala Noche Fault System, striking ~30° oblique to the MNV at ~145°, with an average dip of 54°. Mineralised veins and rhyolite dykes both exploit and closely follow the structure.
These four fault sets are mineralised and largely sub-parallel to the Mala Noche Fault. The remainder are cross structures that are only poorly altered and may offset the MNV. They are distributed irregularly over the the length of the MNV in the Cozamin property, with a range of attitudes as indicated below.
• Margarita Fault, which strikes NNE, dips at 70°WSW, and has undergone minor argillic alteration.
• Josefina Fault, striking SE, dipping at ~55°NE, which intersects the Margarita Fault and has also been subjected to minor argillic alteration.
• Lorena Fault, which strikes NE and dips at ~70°SE. The Lorena, Josefina and Margarita Faults each intersects the MN Fault over a 250 m interval, in the western third of the MNV, whilst the Lorena and Josefina faults intersect to its south. The Lorena Fault is also characterised by weak argillic alteration.
• Anabel Fault, which strikes NNE and dips at ~60°. It offsets the MNV by ~10 m near it's centre, ~ 1 km to the east of the Josephina fault. Mineralisation west of this fault is strongly diminished. Alteration within the fault is silicification.
• Lupita Fault, ~800 m east of the Anabel Fault, striking NE and dipping at ~65°SE, with minor silicification.
• Karla Fault, ~900 m east of the Lupita Fault, towards the eastern extremity of the MN Fault, and has only been mapped on one level.
Mineralisation
The dominant mineralised vein systems at Cozamin are the MNV (Mala Noche Vein) and the MNFWZ (Mala Noche Footwall Zone). The MNV has been mapped over a strike length of ~5.5 km at surface, striking ~east-west, with an average dip of 60°N. The largest historical mined area within the MNV System is the San Roberto Mine with a strike length of 1.4 km, whilst the second largest is the San Rafael Mine exploited over a strike length of 500 m, both prior to Capstones acquisition of the mine. The MNV system averages 5 m in thickness, and has been drill tested to a depth of 1500 m, where it remains open down dip. The MNFWZ is not exposed at surface, although drilling shows it persists over a strike length of >2.2 km to a depth of at least 1450 m. It strikes at 145° with an average dip of 54°NE, and where tested, mineralisation varies from 0.5 to 15.0 m true thickness. It reamins open at down-dip, up-dip and to the east and west.
The MNV system occupies a complex of anastomosing faults, best developed where the individual faults coalesce into a single fault zone. Some of the strongest mineralisation within the MNV system of the San Roberto Mine is where it plunges to the west within the veins at around minus 50° from the horizontal. Post mineralisation offsets of the MNV are minimal and occur where cut by high angle, normal faults that generally strike NE.
The MNFWZ is composed of multiple veins that occur in close spatial association with rhyolite dykes which they locally cross-cut. The strongest mineralisation within the MNFWZ veins plunges to the NW at around -10° from the horizontal. The relative age of copper mineralisation within these veins ranges from contemporaneous with, to possibly a little later than the rhyolite magmatism. Similar to the MNV, post mineralisation offsets of the MNFWZ are minimal and only occur where cut by high angle normal faults.
As mentioned above, mineralisation within both the MNV and MNFWZ of the Cozamin Mine has been interpreted to be episodic. Early intermediate sulphidation pyrite-pyrrhotite-chalcopyrite dominant mineralisation is surrounded, overprinted or brecciated by younger sphalerite dominant intermediate sulphidation epithermal alteration and mineralisation in what is regarded to be a telescoped, intrusive related hydrothermal system. A third set, best described as low sulphidation, well-banded quartz, or quartz-carbonate veins are also present, but are generally volumetrically insignificant in the Cozamin mine area. These veins display open space filling textures with quartz druse vug linings. In the San Roberto workings, the sulphides of the MNV occur as disseminations, bands and masses, and includes both the copper dominant and zinc dominant epithermal events. In contrast, in the San Rafael area in the east, only the zinc dominant epithermal event is evident.
The dominant vein sulphide in the MNV of the San Roberto mine and within the MNFWZ veins is pyrite, making up 15% of the former and 20% of the latter. It is found as fine disseminations and veinlets, coarse crystalline replacements and pseudomorphs of epithermal textured carbonate minerals, and possible barite. Locally, minor arsenopyrite occurs as microscopic inclusions in pyrite.
The second most common sulphide is pyrrhotite, although it is only found at intermediate and deeper levels in the MNV of the San Roberto Mine, and in the up-dip sections of the MNFWZ. It ranges from monoclinic to hexagonal, or a combination of the two, and is found as replacement masses, pseudomorphs of platy masses, and acicular replacements, probably after amphibole. Pyrrhotite commonly occurs as an envelope surrounding, or is intermixed with, strong chalcopyrite mineralisation.
The only visible copper sulphide recognised at Cozamin is chalcopyrite, which like pyrrhotite, is most common at the intermediate and deeper levels of the mine. It is found as disseminations, veinlets and replacive masses. The latter appear to be fractured and brecciated at intermediate levels in the mine. Chalcopyrite is the dominant base metal sulphide in the MNFWZ mineralisation, whilst in contrast, the main MNV is polymetallic in nature.
Sphalerite is the principal economic sulphide in the zinc zones of MNFWZ, and in the MNV of the San Roberto-Zinc zone and San Rafael Mine. It is mostly marmatic, occurring as disseminations and coarse crystalline masses, and is commonly marginal to the chalcopyrite-dominant portion of the veins. Minor sphalerite is found locally within the MNFWZ copper veins, but dominates in the zinc veins. Some zinc within the MNV also occurs as the oxide franklinite, although, where present, results in lower zinc metallurgical recoveries.
Galena is generally closely associated with sphalerite, but is less abundant. It usually occurs as coarse crystalline replacement masses, although, whether coarse or fine, is usually argentiferous. The most common silver mineral is argentite, which has beeen identified under the microscope, occurring as inclusions in chalcopyrite and pyrite.
The principal gangue minerals are quartz and calcite, with locally developed rhodochrosite, gypsum, barite or ilvaite. The quartz is found as coarse-grained druse crystalline masses, and as cross-cutting quartz veinlets. Moderate propylitic wall rock alteration occurs as up to 3 m wide halos in both the hangingwall and footwall of the MNV and MNFWZ vein systems.
Reserves and Resources
Remaining NI 43-101 compliant Mineral Resources are as follows (Capstone Copper Mineral Resource Statement as of January 1, 2023):
MNV Copper-Silver, Copper-Zinc and Zinc Zones
Measured + Indicated Resource - 3.506 Mt @ 0.86% Cu, 45.5 g/t Ag, 2.10% Zn, 0.42% Pb;
Inferred Resource - 5.730 Mt @ 0.49% Cu, 37.5 g/t Ag, 2.54% Zn, 0.24% Pb.
MNFWZ Copper-Silver, Copper-Zinc and Zinc Zones
Measured + Indicated Resource - 16.159 Mt @ 1.74% Cu, 47.2 g/t Ag, 0.86% Zn, 0.41% Pb;
Inferred Resource - 6.553 Mt @ 0.91% Cu, 39.0 g/t Ag, 1.48% Zn, 1.34% Pb.
TOTAL Mineral Resource (MNV + MNFWZ)
Measured Resource - 0.400 Mt @ 1.25% Cu, 53.8 g/t Ag, 1.23% Zn, 0.40% Pb;
Indicated Resource - 19.264 Mt @ 1.59% Cu, 46.8 g/t Ag, 1.08% Zn, 0.41% Pb;
Measured + Indicated Resource - 19.644 Mt @ 1.58% Cu, 46.9 g/t Ag, 1.08% Zn, 0.41% Pb;
Inferred Resource - 12.283 Mt @ 0.72% Cu, 38.3 g/t Ag, 1.97% Zn, 0.83% Pb.
Remaining NI 43-101compliant Ore Reserves are as follows (Capstone Copper Mineral Reserve Statement as at January 1, 2023):
TOTAL Ore Reserve (MNV + MNFWZ)
Proved + Probable Reserve - 10.210 Mt @ 1.65% Cu, 43.44 g/t Ag, 0.54% Zn, 0.29% Pb;
NOTE: None of the reserves quoted above have been estimated as Proved, all are only Probable. The Reserves are included in the Resources.
See the second paragraph of this record outlining the history of the deposit for past production estimates.
The information in the description above has been largely drawn from: Amelunxen, P., Craig, C., Hardy, J., Jalbout, A., McLennan, V. and Moncrieff, J., 2023 - Technical Report on the Cozamin Mine, Zacatecas, Mexico, an NI 43-101 Technical Report Prepared for Capstone Copper Corp. 283p.
The most recent source geological information used to prepare this decription was dated: 2023.
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.
Cozamin
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