Browsing by Subject "whole-rock geochemistry"

This study consists of a comprehensive characterization of the geology, geochemistry, alteration, and mineralization at the Ronaldo prospect as well as an evaluation of its ore potential. Previous mapping campaigns of the prospect, which lies in the Central Andes in Peru at an elevation of 4300 metres, have identified intrusions overlain by a volcanic package. The intrusions are crosscut by silicified ridges that host epithermal mineralization. Satellite imagery reveals that the topographically elevated areas exhibit strongly altered rocks identified as an advanced argillic-altered lithocap. The methods used to define and better understand the geology and the evolution of the hydrothermal system included reconnaissance field mapping, whole-rock geochemistry, short-wave infrared spectroscopy, petrography, and geochronology. Previous studies have shown that only high-sulfidation epithermal mineralization can be spatially and temporally linked to porphyry Cu mineralization, and therefore this study investigates – among other aspects – what type of epithermal mineralization is present at Ronaldo in order to evaluate the potential for concealed at-depth porphyry Cu mineralization. Two separate lower Miocene intrusive units were identified, a porphyritic diorite and a porphyritic granodiorite, whose average age difference is 1.79 Ma. The intrusive units display intermediate argillic alteration. The overlying extrusive units are Sacsaquero Formation basaltic andesites and ignimbrites that are either unaltered or display propylitic alteration. The basaltic andesite roof pendants observed at Ronaldo indicate that the tops of the intrusions are preserved. At high elevations, advanced argillic alteration composed of pyrophyllite, kaolinite, and dumortierite was observed. This area is the remnant, deeper zone of a larger lithocap. The steeply dipping silicified ridges that display sericitic alteration were inferred to be the root zone of this lithocap. Elevated values of trace elements such as Te, Bi, As, and Sb suggest that the Ronaldo prospect is mostly situated in a sericitic alteration zone related to a porphyry-like magmatic-hydrothermal source located at greater depth. Isolated magnetite aggregates were observed in magmatic-hydrothermal breccia, which indicates that the sericitic alteration may have overprinted potassic alteration. A few intermediate-sulfidation epithermal veins and porphyry-related veins, including a banded molybdenite quartz vein, were observed in the creek near the major fault. At Ronaldo, high silica content and sericitic alteration correlate well with elevated concentrations of Ag, Au, and Mo, whereas Cu concentration does not correlate well with any alteration type or with silica content. Quartz veinlets in the silicified ridges that host abundant Ag and Au mineralization were interpreted to have formed at a slightly later stage and to be unrelated to the magmatic-hydrothermal system. This mineralization was interpreted to be low-sulfidation epithermal in origin due to features such as abundant adularia, lattice-textured bladed calcite replaced by quartz, crustiform banding, banded quartz-chalcedony veins, druse-lined cavities, and high Ag/Au ratios. In conclusion, the Ronaldo prospect comprises a hydrothermal system in which the deep, root zone of an advanced argillic lithocap is exposed. The exploration potential for low-sulfidation epithermal mineralization in the silicified ridges is rather significant, whereas the potential for porphyry Cu mineralization is minor due to the lack of appreciable Cu and Mo mineralization, typically shallow-depth porphyry-related hydrothermal alteration, and the lack of high-sulfidation epithermal mineralization.