The Casposo District is located in the Cordillera Frontal, in the center-west of San Juan Province, Argentina. It is characterized by the presence of a low sulfidation epithermal system hosted in volcanic rocks of the Choiyoi Group, where the Mercado-Kamila and Julieta are the more important sectors, concentrating the largest resources of the district. Julieta consists of a quartz-calcite (Au-Ag) vein system, with colloform-banded and brecciated textures, as well as lattice-bladed replacements. In this study, a detailed analysis of the mineralogy and textures of the Julieta Vein system identifies 9 pulses grouped into 4 main events, 3 hydrothermal and 1 tectonic-hydrothermal. Pulses P1, P2 and P3 are barren and carbonated in nature. Pulses P4 and P5A are of chalcedony composition with a colloform-banded texture, where the last one shows cream-colored adularia bands and black ginguro bands, to which high Au (5,5 g/t) and Ag (52 g/t) values are associated. Pulse P5B is of silicic nature as well, although with a brecciated texture, whereas pulse P6 is siliceous carbonate. The composition of these pulses migrates to carbonates represented by P7 and P8, both of brecciated texture. Finally, the system closes with the tectonic-hydrothermal pulse P9. Julieta Vein has a dominant NW-SE strike, with minor subveins of E-W and N-S orientation, that can be divided into 3 segments: North, Central and South. The greater thicknesses, as well as the greater inclinations and high precious metals values are associated with inflections between the 3 segments. Concerning the ore shoots geometry, the structure has a main lithologically controlled subhorizontal ore shoot, while the secondary subvertical ore shoots are structurally controlled. Differences in the rheology of the host rock would control the vein morphology, forming a vertical jog. Factors such as the evolution of the mineralizing events, the vein morphology, and the precious metals distribution, allowed us to identify the importance of the lithological and structural controls in the development of mineralization in the Julieta Vein.
espanolEl yacimiento de oro de la mina Guanaco ubicado en la Segunda Region de Chile es un deposito epitermal de alta sulfuracion, caracterizado por la presencia de un sistema de “ledges” sub-paralelos que se disponen en trenes estructurales ENE-OSO. La mineralizacion esta contenida en rocas volcanicas y volcaniclasticas de edad Paleocena. El objetivo del trabajo consistio en generar una reconstruccion de la litologia y de las mineralizaciones en un modelo tridimensional del sector Dumbo, donde se incluye la geologia previa a la explotacion del yacimiento mediante un openpit de aproximadamente 500 m de diametro. El modelado se construyo sobre la base de la informacion contenida en los logueos de la empresa y se controlo con revision de los testigos de las perforaciones a partir de secciones transversales a las estructuras separadas 50 m. El modelado geologico permitio obtener solidos de cada una de las litologias, las alteraciones y las estructuras mineralizadas presentes en el sector Dumbo. Asimismo se elaboraron superficies de isoconcentraciones de oro del momento previo a la explotacion. El modelado tridimensional puede constituirse como una poderosa herramienta para evaluar la geologia y mineralizaciones no solo en proyectos de exploracion avanzados sino en minas que estan productivas o que ya han dejado de serlo. EnglishThe Guanaco mine is a gold ore located in the Chilenian second region. Guanaco is a high sulfidation epithermal deposit, with sub-parallel ledges arranged in an ENE-OSO structural trend. The mineralization is hosted in volcanic and volcaniclastic Paleocene rocks. The aim of this contribution was to reconstruct the Dumbo sector lithology and mineralization with a three dimensional model. This model includes the geology of the deposit, previous to the mining operations. The Dumbo sector was mined by a 500 m diameter open-pit. The model was constructed using the log records provided by the actual owner company, and it was controlled by re-logging of cutting in transversal cross-sections to the mineralized structures with 50 m spacing. Geological modeling allowed obtaining solids for each lithology, alteration and ledges in the Dumbo sector. Additionally, iso-surfaces of gold were interpolated, reconstructing the Au concentrations before mining operations took place. Geological modelling proves to be a very powerful tool to evaluate the geology and mineralization of an ore deposit, not only in advanced exploration projects but also in productive or in inactive mines
The Pingüino deposit, located in the low sulfidation epithermal metallogenic province of the Deseado Massif, Patagonia, Argentina, represents a distinct deposit type for this region.It evolved through two different mineralization events: an early In-bearing polymetallic event that introduced In, Zn, Pb, Ag, Cd, Au, As, Cu, Sn, W and Bi, represented by a complex sulfide assemblage, and late Ag-Au quartz-rich veining that crosscut and overprinted the early polymetallic mineralization.Three In-bearing mineralization stages were identified within the polymetallic event.Iron-rich sphalerite of the second stage is the major In carrier in the deposit.The spatial distribution of In within the veins shows its maximum mean values in the Marta Centro and Ivonne Norte veins, in a pattern similar to the Zn distribution, showing a close spatial relation with the unexposed Kasia intrusive complex.At the vein scale, and in the hypogene zone, the In distribution is associated with variations in the temperature of mineralizing fluids.The vertical distribution of In within the veins exhibits consistent values both below and above the oxidation level, resembling the behavior of Pb and Sn, and therefore suggesting that In behaves as an immobile element during vein weathering.
El objetivo general de la presente tesis es avanzar en el entendimiento de la geología y las mineralizaciones presentes en el yacimiento argentífero Mina Martha. Dentro de este esquema se intentará avanzar sobre los siguientes aspectos: · Reconocer y estudiar en detalle las unidades geológicas presentes en el área, determinar su ambiente y procesos de formación. · Obtener datos petrográficos, geoquímicos e isotópicos de las unidades geológicas, que permitan reconstruir los procesos volcánicos actuantes en el área. · Caracterizar las mineralizaciones: alteraciones hidrotermales, pulsos actuantes, mineralogía, tipos de fluidos mineralizantes, edad de las mineralizaciones y origen de los metales. · Obtener datos petrográficos, calcográficos, mineralógicos, geoquímicos, isotópicos y de inclusiones fluidas de las mineralizaciones y sus rocas asociadas, que permitan caracterizarlas en detalle. · Determinación de la relación espacial, temporal y genética entre las mineralizaciones y las unidades geológicas presentes en el área. · Estudiar la estructura del área, caracterizando y determinando la relación de la misma con las unidades geológicas y la mineralización. · Definir la génesis del yacimiento argentífero Mina Martha y comparar los resultados obtenidos con otros sectores del Macizo del Deseado.
The complexity of high-grade ore-shoot formation is addressed through a detailed study of the Martha mine epithermal deposit (Deseado Massif, Patagonia, Argentina). The mine was active from 2002 to 2012, producing over 21 million oz of silver from an intermediate sulfidation vein system hosted in a pyroclastic sequence erupted during Upper Jurassic times (40Ar–39Ar age: 157.6 ± 1 Ma). The Martha deposit is composed of more than 15 different veins, 8 of which show high grade ore-shoots (locally up to 45,000 g/t Ag). Veins are hosted in a sinistral transtensive horst structure delimited by two NW–SE trending first-order veins (up to 600 m long and 5 m thick). Inside this horst, a series of E–W trending veins (up to 450 m long and 2.5 m thick) developed as second-order structures in response to growth and linkage of the two overlapping first-order vein structures. Ore is characterized by a complex paragenetic sequence of Ag–As–Sb sulfosalts and Cu–Pb–Zn sulfides, with an adularia-quartz-illite gangue. Hydrothermal alteration is zoned outward and rarely extends for more than 15 m away from the vein walls; five alteration zones were defined: an adularia zone close to the veins, an intermediate halo characterized by illite and illite–smectite zones, and a distal smectite zone, additionally a late and supergene kaolinite zone can be found near the surface in close association with the mineralized structures. High grade "bonanza" ore-shoots are the result of the interplay of a series of structural, hydrothermal (mostly physicochemical) and supergene processes. The mineralization evolved through three main phases: a main hydrothermal tectonically controlled phase, a late tectonic–hydrothermal phase and a post-mineral supergene phase. The main hydrothermal phase was the result of mineral deposition from low salinity (0.5 to 3.5 wt.% NaCl eq.) near-neutral to weakly alkaline chloride aqueous hydrothermal solutions with estimated temperatures ranging from 215.5 °C to 316.5 °C, and isotopic composition suggesting high proportions of meteoric waters (δ18OH2O from − 8.1‰ to − 2.7‰). Relatively constant fluid inclusion liquid-to-vapor ratios and a trend of decreasing Th and δ18OH2O with increasing time suggest that cooling and dilution were the main mechanisms controlling ore deposition. These physicochemical conditions, combined with an efficient network of structural conduits (vein intersections, jogs and step-over zones) were the first step in the construction of the high grade ore shoots that characterize the vein system. 40Ar–39Ar dating in adularia crystals constrains the hydrothermal activity to the Upper Jurassic (156.5 ± 0.9 Ma). Afterwards, a late tectonic–hydrothermal phase occurred through the development of irregularly-shaped bodies of massive fault breccias and foliated cataclasites related to tectonic reactivations of the vein system in synchronism with the waning stages of the hydrothermal system. The interplay between deformation and fluid circulation resulted in mechanical and chemical remobilization of the previously deposited Ag-rich sulfosalts by lower temperature (138 °C to 168 °C) and higher salinity fluids (7.7 and 9.9 wt.%. NaCl eq.), possibly reflecting a late magmatic input into the waning system. This late tectonic–hydrothermal phase strongly contributes to increase the grades in some veins, and therefore is considered an essential process in order to build the highest-grade portions of the ore-shoots found at the mine. Finally, a post-mineral supergene phase defined a 20 m thick oxidized zone close to the surface, followed by a 20–30 m thick secondary enrichment zone below it, developed in response to the circulation of descending cold meteoric waters. Despite its small size and vertical extension, the secondary enrichment level at Martha mine constitutes an important economic factor for the mining operations as it helped to increase the silver grades in the shallower portion of the vein system. The Martha mine vein system constitutes an excellent example of how the interplay of hydrothermal processes controlling mineral deposition, structural factors building up an efficient plumbing system, tectonic ore-remobilization, and late supergene secondary enrichment processes, are combined in order to build "bonanza"-grade ore shoots in the epithermal environment.