The San Agustin mine is 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:
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. A component of the pyrite is thought to be premineral and associated with early phyllic alteration. The mineral system has very little silica and is more related to sulphide fracture filling. Epithermal boiling textures were observed locally such as bladed textures, coliform silica, or drusy quartz. These epithermal textures are not common. Some structures with cryptocrystalline jasperoid have also been found in deeper drill intercepts within sulphide zones. Two late phases of mineralization were identified with one carrying sphalerite and pyrite, and the other, galena and sphalerite.
The Main Fault, an important northwest striking and westerly dipping post-mineral fault, bisects the mineralized area showing differences in mineralization on either side. On the hanging-wall (west side) it is common to find structures rich in manganese and barite that are not observed in the footwall. The hanging-wall block also has higher silver and lead grades than the footwall block.
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 predominant alteration type is phyllic alteration characterized as an assemblage of sericitequartz-pyrite mineralization. In some areas it appears that the host rock was pervasively altered, destroying the original texture and converting biotite and feldspars to sericite. The matrix also shows the presence of sericite, silicification, and disseminated pyrite. In some areas veinlets of jarosite and alunite are observed and thought to be products associated with acid leaching of pyrite as opposed to hydrothermal alteration.
A phase of early potassic alteration was observed but is less common. These zones are characterized by the presence of moderate to pervasive secondary biotite associated with veinlets of quartz-magnetite and disseminated magnetite. Phyllic alteration is superimposed on this early potassic alteration with the latter being closely associated with mineralization.
In the areas more distal to mineralization, the intrusion is typically phaneritic with a coarse porphyritic texture with only propylitic alteration shown by moderate chlorite replacement of ferromagnesian minerals.
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.