Miguel Gonzales, BSc
M. Gonzales1, J. Cerrón2,3P. Aguilar, 3C. Moscoso, 4M. Robles
1Department of Geology, National University of San Antonio the Abbot of Cusco, Cusco, Perú
2Department of Geology, National University of San Marcos, Lima, Lima, Perú
3Bonanza Mining Group S.A., Guayaquil, Ecuador
4Pontifical Catholic University of Perú, Lima, Perú
The Pijilí Cu–Au porphyry project is located in south-central Ecuador, on the western flank of the Western Cordillera of the Andes. The regional geology comprises volcanic rocks of the Pallatanga Unit (Middle Cretaceous). Overlying, volcanic and pyroclastic rocks of the Saraguro Group (Eocene–Oligocene) are recognized. These units are intruded by tertiary dioritic bodies of the Chaucha Batholith, with ages between 14 and 15 Ma. Pijilí project lies in a prospective Miocene metallogenic belt, between the Gaby–Papagrande porphyry (20.6 Ma; Re–Os dating) and the Chaucha porphyry (9.5–9.92 Ma; Re–Os dating). However, mineralization at the project has yielded a Re–Os age of 31.15 Ma, which introduces some controversy regarding the temporal evolution of the system and its implications for exploration. This study focuses on the trace-element geochemical analysis of rock samples using multi-element ICP–MS data (four-acid digestion). The analyzed samples were collected from continuous channel sampling and drill cores. The main objective is to model in three dimensions and recognize key porphyry formation petrogenetic patterns associated with porphyry formation and mineralization processes. Lithogeochemical analysis was conducted using ratios of immobile and incompatible trace elements, such as Sc, V, P, Ti, Y, Yb, U, La, and Sr. This approach enabled the geochemical classification of lithologies, plus the identification of magmatic differentiation trends and inference of petrogenetic mineral fractionation, supported by petrographic observations. Equally, several geochemical ratios commonly applied to infer magmatic fertility and petrogenetic evolution were evaluated in relation to Au and Cu grades, to establish correlations. Finally, 3-D analysis of these geochemical ratios using Leapfrog Geo–Edge, Montecarlo simulation in Python, and ioGAS allowed visualization of their spatial distribution and correlation with Au and Cu grades. High values of V/Sc, Sr/Y and La/Yb, interpreted as indicative of hydrated and oxidized magmas, are spatially associated with near-surface Au-Cu mineralization, observed down to 30 m depth and dated at 31.15 Ma. This mineralization is interpreted as a remnant of a partially eroded porphyry. In contrast, Au-Cu mineralization encountered at depth between 200 and 300 m, is characterized by low ratio values. suggestive of less hydrated and oxidized magmas, representing only an expression of a deeper, younger, yet-unexplored mineralizing center. The results support the existence of at least two mineralizing magmatic events: an older, exposed and partially eroded event, and a deeper event with an inferred Miocene age.