Mulatos - Estrella, El Salto, Mina Vieja, Escondida, El Victor, San Carlos, Cerro Pelon, La Yaqui Grande
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The Mulatos epithermal, high-sulphidation, disseminated gold deposits comprise the Estrella, El Salto, Mina Vieja, Escondida, El Victor, San Carlos, Cerro Pelon and La Yaqui Grande sub-deposits and satellites, and the sub-economic Gap and Puerto del Aire mineralised bodies. It is located in the Sierra Madre Occidental mountain range in the east central portion of the State of Sonora, Mexico, ~220 km east of the city of Hermosillo, and 300 km south of the US border (#Location: 28° 38' 25"N, 108° 44' 42"W).
The deposits are hosted within a sequence of mid Tertiary dacite, rhyodacite and associated volcaniclastic rocks in dacitic dome complexes and intrusive centres. Gold mineralisation is closely associated with silicic and advanced argillic alteration, enveloped by large areas of argillic alteration, part of a hydrothermal alteration system that covers >100 km2. High-grade gold mineralisation is also present locally, occurring as late stage native gold.
The Mulatos district was first discovered in 1635 by Jesuit priests and has been the centre of considerable activity with the bulk of historic production in the late 19th century. Gold production largely ceased during the Mexican Revolution in 1910. The area has been the target of prospecting and exploration activity by various groups throughout the 19th and 20th centuries, particularly after 1960, including many of the major mining companies. This work culminated in a 2004 Feasibility Study which identified a Measured + Indicated Mineral Resource of 62.2 Mt @ 1.51 g/t, Au, 0.6 g.t Ag based on exploration programs completed by Alamos Gold, Phelps Dodge, Kennecott and Minera Real de Angles. Construction of the Mulatos Mine Heap Leach began in the third quarter of 2004, for processing of mineralisation from the Estrella deposit. The first gold pour occurred in July 2005. In March 20, 2007 Alamos Gold reported a revised global resource for the Mulatos deposits of 91.2 Mt @ 1.26 g/t Au at a 0.5 g/t gold cut-off.
The Sierra Madre Occidental volcanic province comprises two distinct packages of volcanic rocks: i). a Lower Volcanic Series composed of early Oligocene 36 to 28 Ma fine-grained to porphyritic andesite lavas, tuffs and agglomerates, unconformably overlain by ii). the Upper Volcanic Series, dated as Miocene, between 24 and 18 Ma, comprising felsic to intermediate flows and tuffs and ignimbrites. Basement rocks are Palaeozoic to Cretaceous and early Tertiary sedimentary rocks. Several 65 to 60 Ma Laramide intrusives are also known in the area. The older Oligocene package typically dips at between 20 and 50°, whereas the Miocene rocks are typically flat lying, but may dip at up to 15°. The youngest rocks in the sequence are <10 Ma rift-related basalts.
North-northwest and northeast trending faults cut All of the rocks in the Mulatos district are cut by NNW an NE trending faults related to basin and range extension. The Mulatos deposits are exposed in the footwall uplift of the Mulatos extension fault, and the host sequence is tilted at ~25°NE in the mine area. The age of mineralisation at Mulatos has been bracketed at between 32 and 25 Ma. Only Oligocene rocks host gold mineralisation in this part of the Sierra Madre.
The 'layer cake' stratigraphic succession in the Mulatos district is as follows, from the oldest to youngest:
• Dacite Porphyry Flow, which is generally quartz free, representing the basal unit in the deposit area. It can contain gold and copper mineralisation, but it is not a major host unit. Copper is normally present in the upper levels of the unit.
• Rhyodacite Porphyry, which ranges from 50 to 200 m in thickness and is a coarse-grained rhyodacite porphyry flow with up to 1 cm quartz phenocrysts, as well as plagioclase, orthoclase, biotite and hornblende. It forms the main dome in the deposit area and also represents one of the main hosts for alteration and gold mineralisation, primarily in the southern portion of the Estrella Pit, but is present on all parts of the Mulatos deposit.
• Dacite Porphyry Flow/pyroclastic, which occurs in both the southern portion of the Mulatos deposit and locally within the northern portion of Mulatos and Escondida. In El Victor, it is rare to absent, and has been partially to completely removed by erosion prior to the deposition
of the overlying Aphanitic Tuff in the north part of the Estrella pit. It hosts gold mineralisation at Mulatos and Escondida.
• Aphanitic Tuff, a fine grained air-fall tuff or volcaniclastic sediment above the Dacite Porphyry Flow and/or inter-fingered with the Epiclastic Sediments detailed below. It is always present in the southern part of the Mulatos deposit with a thickness up to 100 m.
• Dacite Porphyry Flow, which occurs above the Aphanitic Tuff in the northern part of Cerro La Estrella and possibly the El Salto area, but is absent in the southern part of Mulatos, Escondida and El Victor area. It may be part of the Epiclastic Sediments detailed below. It hosts gold mineralisation and may have a thickness up to 100 m.
• Epiclastic Sediments, interpreted to represent facies variations of fine to coarse grained pyroclastic and volcaniclastic sedimentary rocks derived from erosion and partial destruction of earlier dome complexes, including the Dacite Porphyry Flow/pyroclastics, Rhyodacite Porphyry and Dacite Porphyry Flow described above, which they unconformably overlie. They are overlain by an erosional contact with the dacitic and rhyodacitic flows discussed below. Relief on the underlying unconformity is up to 300 m, with maximum thicknesses near the central Estrella section line, where palaeo-erosion has completely removed the Rhyodacite Porphyry unit, and progressed into the basaal Dacite Porphyry Flow. Coarse grained conglomeratic facies generally contain fragments of all rock types comprising the original dome complex. The majority of the Mulatos deposit is hosted within the volcaniclastic rocks of this composite unit, which is composed of the following facies:
- a fine to medium grained, poorly sorted and stratified volcaniclastic sandstone containing abundant detrital quartz grains derived from erosion of the rhyodacite porphyry. It locally contains clasts of vuggy silica alteration, and also locally has a pyroclastic component, and is found high in the volcaniclastic sequence, reflecting relatively low-energy depositional environments.
- a coarse grained facies equivalent, composed of granule to cobble-sized clasts in a largely clast-supported conglomerate. Clasts are predominantly homolithic in the basal portions of the sequence, corresponding to the lithology of the nearby source terrain. The homolithic breccias are interpreted as homolithic debris resulting from mass wasting of the immediately adjacent host lithology. Heterolithic volcaniclastic conglomerates are present within the central portions of the volcaniclastic sequence. High-grade mineralisation at the historic workings at Mina Vieja is hosted within volcaniclastic rocks of this facies, with high-grade gold concentrations, generally >15 g/t, occurring as stratabound mineralisation at the contact between beds of the two facies. Similar volcaniclastic rocks are the primary host rock in the El Victor resource area.
Gradations between the two end member facies are common, with matrix supported conglomerates and apparent pyroclastic rocks common. In general, the sequence fines upward and outward from homolithic to heterolithic clast-supported pebble to boulder conglomerate, to heterolithic matrix-supported conglomerate, to volcaniclastic sandstone with occasional conglomeratic and pyroclastic material, to fine grained volcaniclastic sandstones and/or tuffs. Coarse-grained clast-supported conglomerates are pervasively silicified and mineralised and are the best host rocks, due to their high initial porosity and permeability. Matrix-supported conglomerates and finer-grained equivalents are predominantly altered to pyrophyllite and/or kaolinite, as they appear to have lacked significant porosity and permeability.
• Post Mineral Sequence, comprising nine distinct units and sub-units have been discriminated, although some indicate some mineralisation in the lower units. They form a relatively thick sequence to the NE of the Mulatos deposit, and extend from Puerto del Aire to the El Victor area, and have been consolidated in the resource model. The maximum thickness of the suite is ~200 m, but in general ranges from 0 to 150 m. The individual units are:
- Rhyolitic lithic crystal tuff, that is 5 to 70 m thick, with abundant biotite + quartz and flattened lithic fiamme;
- Lithic crystal tuff that is 30 to 40 m thick, with siliceous concretions or spherules at the base;
- Medium grained andesitic basalt porphyry that is 35 to 70 m thick;
- A lower lithic lapilli tuff with abundant waxy pumice lapilli, overlain by another lithic lapilli tuff containing abundant lithic fragments, that together are ~60 m thick;
- Another composite unit that is ~100 m thick, composed of a lower vitric tuff with abundant glass shards, overlain by lithic lapilli tuff with abundant pumice lapilli but no quartz, capped by crystal lithic tuff with quartz and sanadine;
- Aphanitic basaltic andesite with hornblende phenocrysts;
- A regionally extensive, moderately welded, rhyolite crystal tuff with abundant quartz and biotite;
• Quaternary Overburden, mainly composed of landslide material, located over all other units, just below the topographic surface. This overburden can contain gold mineralisation, e.g., on the western side of the Mulatos deposit near the Arroyo Mulatos. Overburden located above the Escondida deposit is barren.
Two dominant fault trends are recognised in the Mulatos district, namely a NW to NNW, and a NNE set. The NW striking San Francisco Fault trend represents the regional alignment related to basin and range extension. It is a north side-down normal fault that strikes at ~300° with a near vertical dip that appears to
flatten at depth, and intersects and is offset by the Escondida fault to the NW. The latter is a west side-down normal structure that runs NE from the Mulatos fault. It has a strike of 30° and a near vertical dip. Prior to the doming event, multiple NNE striking faults are interpreted to have been present in the area. The near NNE striking Mulatos fault trend probably reflects a deep-rooted structural trend that existed prior to the emplacement of the Rhyodacite Porphyry dome. When doming commenced, a NNE striking fault set, parallel to the present Mulatos fault, acted as the main structural control for emplacement of the dome complex and controlled the original mineralising fluid flow. The Mulatos Fault is a major bounding normal fault on the west side of the Mulatos deposit with up to 400 m of down-drop displacement of the western block. It dips at ~70°W and juxtaposed unaltered, post-mineral volcanic unit adjacent to highly altered dome complex rocks. NW striking Basin and Range style faults were active during the late stages of mineralisation and partially controlled superficial silicic alteration and acid leaching. Four major sub-vertical faults have been defined in the Mulatos/Escondida area where post-mineral displacement is significant. These are, from oldest to youngest: i). North Fault, which is concealed beneath post-mineral volcanic rocks. It is a south-side down normal fault that strikes at 75° and dips at 80°SE. It forms the northern boundary of the Escondida zone and offsets mineralisation by ~100 m, with the Escondida zone being preserved in the down-thrown block. ii). San Francisco Fault; iii). Escondida Fault; and iv). Mulatos Fault. The last three have been described above. In the El Victor area, two main faults are recognised. These are the i). The Northwest Fault, a syn- to post-mineral normal fault separating the NE and SW portions of the El Victor zone. It strikes at 320° and dips 40°SW, with the NE block up-thrown by ~80 to 100 m relative to the SW block; ii). El Victor Fault, an inferred post-mineral structure forming the southern boundary to mineralisation in the El Victor area. It strikes at 45° and has a near vertical dip. Displacement appears to be 80 to
100 m, with the SE side up with respect to the NW side
The volcanic units immediately above the Rhyodacite Porphyry dome have been structurally deformed into an anticlinal shape. Growth faults along the dome
margins result in normal displacement of these units. Thus, lithologic units on the east and west limbs of the dome have been down-dropped to their present positions, with displacements of the order of generally <30 m.
The Mulatos deposits have been subjected to an alteration zonation typical of high sulphidation deposits, ranging from low-temperature distal propyllitic → illite → kaolinite → proximal pyrophyllite-dickite → silicic. Gold has a close spatial relationship with alteration, predominantly hosted within the silicic zone, with the highest grades in vuggy silica. Significant gold is also found in pyrophyllite-dominant argillic alteration within and immediately adjacent to the silicic alteration. The distribution of alteration is controlled by lithology, contacts between units, and structure. Two stages of alteration, and possibly mineralisation, have been interpreted within the Mulatos deposit. The first is apparently confined to volcanic rocks of the original
volcanic dome, while the second event occurred after partial erosion and destruction of the dome complex, and is hosted within the volcaniclastic rocks of
the depositional basin. The coarse-grained facies of the Epiclastic Sediments unit contains clasts of variably altered silicified/mineralised rocks that have been overprinted by a second alteration/mineralisation event. Vuggy silica clasts occur in strongly argillised fine grained facies, or in matrix-supported coarse-grained material. At least two stages of vuggy silica alteration are recognised.
Silicic alteration occurs as pervasive cryptocrystalline silica, as pervasive fine grained crystalline, and vuggy, interpreted to reflect distance from source. The vuggy silica has the highest gold grades, while the crypto-crystalline silica is often low grade to barren, except where cut by late fractures. Silicic alteration generally occurs as stratabound bodies localised along the contact between the rhyodacite porphyry and the overlying dacite porphyry, and is pervasive within the coarse-grained, clast-supported volcaniclastic rocks, whereas fine-grained equivalents are largely argillised. The stratabound silicic alteration within volcaniclastic rocks is often texturally destructive. Vuggy silica alteration formed from intense acid leaching of host rocks and appears
to be strongly lithology dependent, best developed in porphyritic rocks, particularly the rhyodacite porphyry. However, an extensive area of vuggy silica alteration occurs within pervasively silicified coarse-grained volcaniclastic rocks in the northern part of the Mulatos deposit. The overall trend of silicic alteration within the Estrella portion of the Mulatos deposit is NNW, while the prominent trend to silicic alteration in the Puerto del Aire, Escondida, Gap, and Victor areas is to the NE. Prominent NE-trending structurally-controlled zones of silica+pyrophyllite alteration were mapped in the northern and southern part of the Mulatos deposit, where alteration/mineralisation is less pervasive. NE structure appears to have been a dominant control on alteration.
Advanced argillic assemblages have also been recognised, comprising high-temperature pyrophyllite and/or dickite. They occurs within or marginal to silicic alteration, and hosts significant gold, with concentrations comparable to silicic alteration. Alunite is very rare, and regarded as of probable supergene origin. Lower temperature illite to kaolinite forms a distal alteration envelope around silicic and advanced argillic alteration, and is pervasive in the deposit area. Illite to kaolinite assemblages are generally only geochemically anomalous in gold. Remnant propylitic alteration is included in the low-temperature assemblage.
Three main mineralised assemblages have been recognised (Staude, 2001), from oldest to youngest: i). quartz + pyrite + pyrophyllite + gold; ii). quartz + pyrite + kaolinite + gold + enargite; iii). kaolinite + barite + gold.
Mineralisation occurs as hypogene sulphides, mixed oxide-sulphide and supergene mineralisation that has been tilted, such that the mixed zone-sulphide zone interface occurs closer to the surface in the southern rhyodacite dome area, and is much deeper in the northern Estrella breccia area. These zones may be summarised as follows:
• Sulphide Mineralisation, where the mineral assemblage includes pyrite, enargite, chalcopyrite, molybdenite, gold, chalcocite, covellite, bornite, tetrahedrite/tennantite, marcasite, and specularite. Pyrite is the dominant sulphide mineral at Mulatos. Within the sulphide and mixed zones, metallurgical test work indicates that gold recovery is inversely proportional to sulphide content.
• Mixed Oxide and Sulphide Mineralisation, contains both oxide and sulphide minerals in any proportion. The zone generally mimics the geometry of the overlying oxide zone, occurring as an intermediate 'blanket' between the completely oxidised rocks and the deeper hypogene sulphide zone. It frequently has irregular deeper tongues of leaching in cross sections, generally corresponding to fault zones. The mineral assemblage includes those described for the oxide zone below, as well as the sulphide minerals pyrite, enargite, chalcopyrite, molybdenite, chalcocite, covellite, bornite, tetrahedrite/tennantite, marcasite and specularite. Free gold can sometimes be found in hematite-filled fractures in this zone. Two mixed zone styles are recognised, namely: i). characterised by fractured controlled oxidation and i). those characterised by a weak to moderate pervasive oxidation.
• Oxide Mineralisation, which makes up a small proportion of the total volume of the resource, primarily occurring near the surface, the result of surface weathering and leaching. It generally mimics the surface topography, with some deeper zones of oxidised material that are generally found in zones of more intense fracturing that has enhanced permeability. Important oxide minerals include: hematite, limonite, jarosite, goethite and copper oxides. Kaolinite is more common in the shallower, oxidised portions of the deposit, whereas pyrophyllite is more common in deeper sections.
NI 43-101 compliant Mineral Resources and Ore Reserve estimated as at 31 December, 2011 at a 0.5 gt Au cutoff (Keane et al., 2012) were:
Mineral Resources - exclusive of Ore Reserves
Measured + Indicated Resource - 84.991 Mt @ 1.01 g/t Au, for 86 tonnes of contained gold;
Inferred Resource - 17.432 Mt @ 0.9 g/t Au, for 15.7 tonnes of contained gold;
Proved + Probable Reserve - 63.447 Mt @ 1.15 g/t Au, for 72.9 tonnes of contained gold.
Total contained gold ~175 tonnes.
Remaining NI 43-101 compliant Mineral Resources and Ore Reserve estimated as at 31 December, 2021 at a 0.5 gt Au cutoff (Alamos Gold Inc. website viewed April, 2022) were:
Mineral Resources - exclusive of Ore Reserves
Measured + Indicated Resource - 8.204 Mt @ 1.34 g/t Au, for 11 tonnes of contained gold;
Inferred Resource - 1.724 Mt @ 1.06 g/t Au, for 1.8 tonnes of contained gold;
Proved + Probable Reserve - 29.369 Mt @ 1.64 g/t Au, for 48 tonnes of contained gold;
The information in this summary is drawn from: "Keane, J.M., Jutras, M., Balleweg, K.J., Welhener, H., Odell, M., Btowne, R., Ames, S. and Garcia, D., 2012 - Minas de Oro Nacional, S.A. de C.V. - Mulatos Project, Technical Report Update; prepared for Alamos Gold Inc., 305p."
The most recent source geological information used to prepare this summary was dated: 2012.
This description is a summary from published sources, the chief of which are listed below.
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