The San Agustin and the El Castillo mines are owned and operated by Minera Real del Oro, S.A. de C.V., which is a subsidiary of Argonaut Gold Inc.
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Summary:
The El Castillo Complex includes two operating mines, El Castillo and San Agustin.
El Castillo
Deposit Type
El 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.
Mineralization
The 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.
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.
San Agustin
Deposit Type
The San Agustin Project does not fit entirely into an epithermal classification. The San Agustin deposit appears genetically and spatially related to a quartz monzonite stock with intense phyllic alteration and local tourmaline breccias. These factors may point towards a telescoped system associated with a deeper porphyry centre. This is supported by broad zones of potassic alteration that are overlapped by pervasive phyllic alteration; however, locally on the surface and in some drill holes, boiling textures, suggestive of an epithermal system do occur. Mineralization is mainly associated with pyrite that fills fractures, is disseminated, and occurs in the matrix of hydrothermal breccias. These form an extensive system of sulphide stockworks and disseminated mineralization dominated by pyrite.
San Agustin is interpreted to be a porphyry-style gold system related to Eocene aged intrusions emplaced into Cretaceous clastic and carbonate sedimentary rocks in an extensional tectonic setting. Gold mineralization occurs throughout the magmatic-hydrothermal system in space and time and is spatially related to early potassic development and an overprint of phyllic alteration. Supergene alteration, formed as a product of acid leaching, resulted in argillic-quartz alteration assemblages within the oxide zone of the deposit. The main gold event is associated with magmatic hydrothermal fluids corresponding to phyllic alteration. The gold system was overprinted by a younger structurally controlled epithermal system dominated by silver and zinc. The difference in style of mineralization from the nearby El Castillo deposit is possibly due to San Agustin having undergone less erosion than El Castillo thus preserving a larger volume of this late epithermal overprint. In support of this, late-stage high-level tourmaline breccias are prevalent at San Agustin but are not exposed at El Castillo.
Mineralization
The host rocks for mineralization at San Agustin are quartz monzonite-dacite bodies and the sedimentary sequence they intrude. Mineralization is emplaced through a strong and widespread system of sulphide rich veins, veinlets, and fissure fillings that make the system similar to a disseminated deposit. Fracture systems follow two main project-scale trends that run northeast and northwest. Locally mineralization can be observed following lithological controls in the sedimentary rocks, especially where they run parallel to sediment-intrusive rock contacts. Mineralization is also observed in the flow facies of the intrusion and is usually characterized by disseminated pyrite and in parallel veinlets.
The sulphide boundary is located within a range of 30 m to 170 m below the surface with an average depth of about 65 m. The boundary is reached when the rock colour turns grey and disseminated pyrite becomes visible. The transition zone is commonly less than 1 m wide. The boundary’s surface is undulating and erratic across the deposit, due to the many faults and fractures controlling ground water in the area.
The San Agustin deposit is roughly 1,500 m long by 800 m wide. The average depth of oxide material is 65-100 m below surface. Gold mineralization is found along faults and fractures within the host igneous and sedimentary rocks and as disseminations in halos across the deposit. Sulphide mineralization extends, where drilled, down to an average depth of about 200 m with the deepest tested areas extending to 400 m below surface.