Summary:
Deposit Types
Deposits at La Encantada are examples of polymetallic, high-temperature, intrusion-related carbonate-replacement and minor skarn-hosted deposits. The carbonate-replacement deposits are hosted by the Jurassic–Cretaceous Aurora Formation.
Mineralization
Mineralization consists of polymetallic, high-temperature, intrusion-related carbonate-replacement and minor skarn-hosted deposits. Mineralization occurs as tabular veins, mantos, massive lenses, breccia pipes, and irregular replacement zones.
The deposits were grouped into four geological zones: the Prieta complex, the San Javier–Milagros complex, the Vein systems, and Tailings Deposit No. 4.
Discordant, near-vertical deposits with irregular elongate shapes proximal to main intrusions are referred to as chimneys and breccia pipes. These include the San Javier, Milagros and Prieta breccia deposits.
Tabular sub-vertical replacement deposits are referred to as veins and can contain richer mineral shoots or small chimneys at the intersection of northwest-trending faults and fractures. Steeply dipping, tabular deposits of the Vein systems have a northeast orientation, and are commonly distal to main intrusions.
Massive lens replacement zones of the Prieta complex are proximal to a granodiorite source intrusion and formed adjacent to skarn alteration. Contact metamorphic features (recrystallization to marble, development of hornfels and skarnoid) normally occur peripheral to the skarn zone.
Mineralization consists of secondary oxide minerals including silver, iron, zinc, lead, copper oxides and native silver. Native silver and oxide minerals also occur with sulphides in skarn and carbonate replacement zones where sulphides are partially converted to oxide minerals. The sulphide minerals acanthite, pyrite, magnetite, marmatite (iron-rich sphalerite), galena, chalcopyrite, and covellite occur in the Prieta and the San Javier–Milagros complexes.
The Tailings Deposit No. 4 consists of cyanidation circuit paste tailings from previously processed ore that has been stacked on the surface close to cyanidation Plant No. 2.
The silver mineral deposits at La Encantada are high-temperature polymetallic replacement deposits hosted in sedimentary carbonate rocks related to felsic intrusions and controlled by local and regional structures. Carbonate replacement deposits are characterized by irregular shaped pods, lenses and massive lens, and roughly tabular masses of oxides. Some replacement deposits are associated with skarn alteration and mineralization also hosted by the sedimentary carbonate rocks.
Prieta Complex
The Prieta complex silver, lead, and zinc polymetallic deposits consist primarily of massive lens-type and breccia pipe carbonate replacement deposits that formed adjacent to the limits of skarn alteration in Aurora Formation limestones. The skarn alteration also hosts silver, lead, zinc, and gold mineralization in a dome shaped halo that surrounds a granitic intrusion.
The massive lens type deposits include Ojuelas, Cuerpo 660, and La Fe. Together, these deposits form a nearly continuous carbonate replacement zone encircling the skarn alteration, between 1,425–1,675 m elevation, that has lateral dimensions of approximately 550 by 350 m. The Ojuelas deposit is positioned on the east side of the skarn alteration and is fault offset from the Cuerpo 660 deposit to deeper structural levels by the Dike Escondida normal fault. The La Fe deposit is positioned on the south side of the skarn alteration between 1,550 and 1,650 m elevation with lateral dimensions of approximately 300 by 150 m. Skarn-hosted mineralization and irregular carbonate replacement deposits are found surrounding the massive lens deposits. Locally the carbonate replacement mineralization appears to conform to the bedding of the limestone, but in other areas the mineralization crosscuts bedding. Structurally above the Ojuelas deposit the Aurora Formation is intensively fractured, with hematite staining along fractures.
The Prieta breccia pipe is a high-grade, polymetallic silver, lead, zinc deposit that formed on the west side of the complex adjacent to and structurally above the skarn alteration. The breccia body is an irregular chimney-shaped deposit that extends from 1,600–1,800 m in elevation with lateral extents of 100 by 100 m. The deposit is intensely oxidized and comminuted, obscuring primary textures. It is believed to have had a massive to semi-massive sulfide matrix originally.
Fault zones associated with the development of the Prieta breccia pipe and the massive lens carbonate replacement deposits also host polymetallic silver mineralization. The Falla 35 fault strikes northwest, crosscuts the Prieta breccia pipe and hosts vein-style mineralization to the southeast of the breccia, extending to the La Fe deposit. The Falla Asuncion fault strikes northeast and crosscuts both Cuerpo 660 to the north and La Fe to the south. Falla Asuncion can show silver and higher levels of gold as this structure is hosted primarily within skarn alteration, which contains disseminated gold.
San Javier–Milagros Complex
The San Javier–Milagros complex consists of a quartz monzonite stock bounded by two silver breccia pipe deposits and associated chimney-shaped, silver-bearing, carbonate replacement deposits. These are the San Javier and the Milagros breccias with the adjacent Nucleo and Cuerpo 310 replacement deposits. The Milagros intrusion also hosts lesser silver mineralization near its margins. The San Javier-Milagros complex extends from the 1,400 m elevation to surface at the 2,000 m elevation and has lateral extents of approximately 400 by 175 m.
The San Javier breccia is a poorly consolidated and predominantly clast-supported, chimney-shaped breccia consisting of sub-rounded limestone fragments (monomictic breccia) ranging in size from tens of centimetres to several metres. Some of the clasts are recrystallized or replaced by iron and manganese oxides, and the matrix is usually fine-grained oxidized and comminuted rock. In contrast, the Milagros breccia is a matrix-supported, chimney-shaped breccia consisting of limestone and intrusive clasts (polymictic breccia) varying in size from centimetres to tens of centimetres. The matrix of the Milagros breccia is made up of fine-grained and oxidized and comminuted rock. Most rock fragments in this breccia are rounded to sub-rounded.
Vein Systems
The silver mineralization in the Vein systems consists of numerous, steeply-dipping, tabular-shaped deposits with open space-filling and carbonate replacement developed along northeast-trending faults and fractures. From north to south, are the Conejo, Bonanza, Dique San Francisco, 990, 990-2, Regalo, Azul y Oro and Buenos Aires vein systems.
Northwest-trending cross-faults intersect the northeast-trending veins, which occasionally favors the development of pipe-like chimneys or vein-hosted mineral shoots. Mineralogically, the veins consist of siderite, manganiferous calcite, calcite, hematite, goethite, pyrolusite, acanthite and native silver. Vein mineralization occurs commonly between 1,750– 1,950 metres elevation although greater vertical extents are encountered particularly at the intersections of the veins with the northwest-trending structures. The entire system has been recognized over an area that is approximately 2000 by 750 m. Veins typically pinch and swell and vein thickness varies between a few centimetres to several metres in the case of the Conejo and 990 vein systems. The Dique San Francisco is over 1,700 m in strike length and the vein structure contains an oxidized and argillized andesitic dike that has been mineralized along with the carbonate replacement adjacent to the dike.