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Location: 8 km W from José María Patoni, Durango, Mexico
Carr. San Juan Coneto de Comonfort Km. 12San Juan del RíoDurango, Mexico34480
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Deposit TypeEl Castillo is interpreted to be a porphyry-style gold system related to Eocene granodiorite– diorite porphyries that intrude Cretaceous clastic and carbonate sediments in an extensional tectonic setting. Gold mineralization occurs throughout the magmatic-hydrothermal system in space and time and is related to sulphide mineralization spatially associated with early potassic development and genetically related to an overprint of phyllic alteration. Supergene alteration, formed as a product of acid leaching, has resulted in argillic-quartz alteration assemblages within the oxide zone of the deposit. The main gold event is believed to be associated with magmatic hydrothermal fluids corresponding to phyllic alteration.The El Castillo gold system is similar to that found at Andacollo, Chile (Reyes, 1991 and Oyarzun, et al., 1996). Andacollo is a Cretaceous diorite to granodiorite porphyry copper-gold system with central porphyry copper-gold mineralization related to a classic porphyry alteration assemblage and a distal sediment-hosted (manto) low sulphidation and epithermal-level gold satellite system. Fluid inclusion work by Oyarzun et al. (1996) indicates that the manto epithermal gold deposits may in fact be related to another intrusive that has not been recognized at the surface due to high temperature fluid inclusions (about 365ºC) that are found about 5 km from the porphyry centre.Supergene oxidation of shallow (<200 m) pyrite-gold mineralization is the most important factor affecting mine viability at El Castillo. Oxidized or partially oxidized transition and sulphide material located relatively near surface is a main component of the Mineral Resources and mine plan. The depth of oxidation has been strongly influenced by the density and depth of natural rock fracturing, total pyrite content of hypogene mineralization and the thickness of post-mineral cover. The deeper sulphide portion of the mineral system remains open at depth.MineralizationThe moderately dipping granodiorite–diorite sills, siltites and argillites are the most favourable host lithologies. Argillic–quartz alteration is often closely associated with the intrusive contacts and can be indicative of higher-grade zones of mineralization.The dominant controls on the gold mineralization include structural channeling along contacts between intrusive sills and metasedimentary units and a broad zone of northeast-striking, steeply-dipping faults and fractures that acted as conduits to help spread mineralization. Gold precipitation may be somewhat dependent on a chemically-favourable environment within the sediments but does not appear to be strongly influenced by rock composition. The hydrothermal fluids and their contained metals are believed to have been derived from a magmatic source and are a primary volatile component of the porphyry intrusion that is host to much of the mineralization.Gold is spatially and genetically associated with pyrite occurring as disseminations, fracture fillings and stockworks often occurring within areas of hydrothermal brecciation. Within the mine the host environment for gold mineralization is dominated by an alternating pattern of sediments and parallel intrusive sills that strike to the northwest and dip moderately to the northeast. The sedimentary units generally vary from 20 m to 40 m thick, as do the intrusive sills. Many of the sills appear to have intruded along bedding planes by splitting the tabular sedimentary blocks into their present positions. This geologic event resulted in the unique alternating pattern of sediments and intrusive sills that are observed throughout the mine area. Extensive fracturing and brecciation of the sedimentary blocks, especially along contacts, created favourable secondary permeability for the deposition of gold mineralization predominately associated with pyrite. As a result, the northwest-striking sediments are often better mineralized than the surrounding intrusive rocks.There is typically a transition zone of partially oxidized mineralization that lies between the fully oxidized material and lower non-oxidized, sulphide material. The transition zone varies from 5 m to 50 m thick and is generally influenced by degree of fracturing and level of erosion.The sulphide zone is generally identified by the presence of pyrite mineralization. The occurrence of sulphides, either fracture-related or disseminated, is usually a good indicator of gold mineralization. The sulphide veinlets are most commonly 0.5 cm to 4.0 cm wide.There are two preferred trends to mineralization. The most obvious of these reflects the generally stronger mineralization within the sedimentary units. The favourable permeability related to increased fracturing within the sediments enhanced the distribution and broader geometry of mineralization. The second trend of mineralization is to the northeast and reflects the dominant structural controls to mineralization. These structures are considered to be important conduits that helped channel the mineralizing system. The combination of these geologic controls resulted in a northeast-elongated gold zone that measures approximately 1,800 m by 1,500 m.
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