The Metates property is composed of twelve contiguous concessions totaling 4,261 hectares in area. These concessions are held in the name of American Gold Metates, S. de R.L. de C.V., an 99.9%-owned subsidiary of Chesapeake.
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Summary:
The local geology shows that the Metates site is situated within a window of Mesozoic basement rocks, exposed by erosion of the extensive flat-lying Tertiary volcanic cover.
The Mesozoic sedimentary sequence generally strikes northwest and dips between 30° and 40° to the northeast. Folding generally parallels the strike of the sedimentary rocks and tends to be of a concentric to isoclinal nature. Faults have often developed parallel to isoclinal fold axes. Soft sediment deformational features are common in the sedimentary units. The Metates Intrusive is oriented in a northwest-southeast direction, dipping 40° to the northeast, and is broadly conformable to the trend of the surrounding sediments. The upper contact of the Metates Intrusive is a sedimentary or erosional type breccia or conglomerate (owing to the rounded clasts), while the lower contact is quite regular and could be a fault.
Major high angle faults in the project area trend northwest to northeast. Faults of a northeast orientation may possibly offset the Metates Intrusive, and have caused deformations and drag folds where they cut the sediments. Northwest trending faults are common, and trend parallel to the strike of the sediments. Within the sedimentary rocks, fracture directions and sulphide veinlets most commonly trend north-northeast. Stockwork mineralization follows fractures in the intrusive body, with the most common orientation being northwestsoutheast. The Upper and Lower Volcanic Sequences are not deformed and tilt in a homoclinal nature gently to the east.
Three types of alterations have been recognized at Metates. These alteration effects are most recognizable within the more reactive Metates Intrusive, where phyllic alteration has been very extensive, with feldspar and biotite phenocrysts being replaced by sericite and pyrite, respectively, and the groundmass being almost entirely replaced by fine-grained sericite and quartz. Argillic alteration within the Metates Intrusive body is poorly developed and is of only local extent. Variable but weak amounts of silicification have been noted, but in general there is no pervasivesilicification within either the intrusive body or the surrounding sedimentary rocks. Because of the fine-grained and generally non-reactive nature of the sedimentary rocks, visible alteration effects are not well developed, although a weak propylitic alteration is generally quite common and widespread. No distinct contact metamorphic effects have been noted surrounding or adjacent to the contact between the intrusive body and the enclosing sedimentary rocks, suggesting it was emplaced at a relatively low temperature and possibly high level.
Sulphide mineralization within the project area is thought to be both syngenetic and epigenetic in origin. Syngenetic mineralization is fairly widespread within the sedimentary rocks and is typical of rocks formed in a black-shale or euxinic environment. Very little, if any, precious metal mineralization is thought to be associated with this phase of predominantly pyritic mineralization. Epigenetic mineralization may have occurred as two separate mineralizing events in both the sedimentary rocks and in the intrusive rocks, but it is possible that the mineralization in the sediments represents an earlier and more distal event that is related to an emerging intrusive dome, which subsequently intruded part of the sedimentary hosted mineralization.
Mineralization is most typically expressed as sulphide stockwork veinlets or disseminations. Within both the sedimentary and intrusive rocks, veinlets are typically composed almost completely of pyrite, sphalerite, arsenopyrite, and galena, with very little gangue mineralization, such as quartz, calcite, or barite. Veinlets are typically 1 to 5 mm in thickness, sometimes exceeding 1 cm, and are generally banded with layers of pyrite, sphalerite, and/or galena. Within the intrusive, feldspar and biotite phenocrysts are commonly replaced by pyrite and sphalerite, with the individual pyrite crystals generally several millimeters in size. Sphalerite and galena inclusions are common within disseminated and veinlet pyrite.
Extensive mineralogical investigations indicate that some amount of native gold and electrum occurs as both rare free mineral grains, as micron-sized grains that are generally enclosed within the pyrite grains, or as solid solution within the crystal structure of the pyrite in both sedimentary and intrusive host rocks. The majority of the gold is associated with pyrite either as solid solution or as inclusions although there is some amount of coarser, visible gold (>20 micron). Extensive metallurgical investigations have demonstrated that the gold is largely refractory or not amenable to routine cyanidation, even when the material is finely ground. Most silver mineralization is associated with the mineral pyrargyrite or as a solid solution within the copper mineral tetrahedrite. Commonly both of these minerals are found as inclusions within galena (AMTEL, 2020).
Gold and silver mineralization is associated with the sulphides replacing feldspar and biotite phenocrysts, with sulphide veinlets, and with sulphide stockworks. Sulphide sulphur content of mineralized sedimentary and intrusive rocks is typically in the range of 3% to more than 10% by weight, a reflection of the high percentage of pyrite in these rocks. There is generally a positive correlation between pyrite content and gold and silver grades. The sedimentary rocks also contain up to about 1% of organic carbon, which results in the mineralization in these rocks having both refractory and mildly “preg-borrowing” characteristics. (Preg-borrowing occurs when the gold and silver extracted by cyanide is then adsorbed on the organic carbon rendering it unavailable for routine recovery.) Multiple mineralizing episodes are suggested, based on the cross-cutting and mineralized breccia clast/host relationships. Oxidation of the Metates mineralized system has been very limited, with the depth of oxidation generally not exceeding 5 to 10 m, and it is not uncommon to see fresh sulphides at surface.