Caracterización geoquímica y mineralógica de la sección 5303350 del depósito epitermal Loma de la Plata, Proyecto Navidad, Chubut, Argentina

Abstract

The intermediate sulfidation epithermal deposit Loma de la Plata is located in the Chubut Province along with other seven deposits (Loma Navidad, Loma Galena, Loma Baritina, Loma Calcita, Loma Calcita NW y Valle Esperanza). It is part of the Navidad district, which is placed in the western Macizo Norpatagónico among the Cañadón Asfalto Basin. Loma de la Plata has resources of 4500 t Ag with mean grades of 169 g/t and shows a distal mineralization style with an 187,8 Ag/Pb ratio. Mineralization is hosted in Volcanic Unit C of the Cañadón Asfalto Formation (Lower Jurassic). The beginning of the hydrothermal activity, in the analyzed section, has been recorded with Episode I, which precipitates calcite, barite, scarce magnetite and trace pyrite. Episode II consist of three states, the first and most important inputs Cu-Zn-Pb with Sb and abundant Ag, which precipitates chalcopyrite>sphalerite>tetrahedrite>galena as result of fluid mixing. The last state is given by Ag exsolution from previous sulfides and sulfosalts to form stromeyerite and acanthite Hydrothermal alteration minerals products (potassic feldspar, chlorite, titanite and laumontite) suggest the rock interacted with neutral to alkaline oxidant fluids with temperatures range between 350 to 220°C and fluctuating dissolved CO2 content. The presence of late illite-smectite mixed layer and smectite indicates the circulation of neutral CO2-rich steam heated waters or cooling of the hydrothermal system. Zoning of these minerals suggest structural control of fluids. Loma de la Plata shares some mineralogical and textural characteristics with other distal deposits of the district (Valle Esperanza), although it differs in the presence of laumontite related to mineralization, geochemical zoning (base metal and Ag-rich central zones and outer Zn-As), and zoning of hydrothermal alteration minerals (proximal illite-smectite and distal smectite). Both distal mineralization style deposits, Loma de la Plata and Valle Esperanza, show marked differences with Loma Galena given by a higher Ag/Pb ratio, Ag contained in Cu-Fe and Cu-Ag sulfides and sulfosalts rather than framboidal pyrite-marcasite, and the absence of abundant kaolinite alteration product.