The structural and temporal constraints on regional propylitic alteration: A study of the Quellaveco Cu-Mo porphyry district, Peru

The propylitic halo is one of the least studied parts of the porphyry system, however advancements in geochemical vectoring and fertility studies have highlighted the importance of propylitic alteration for the future of porphyry exploration. This study furthers our understanding of the development of district-scale propylitic alteration by applying an innovative approach which considers the potential spatial, structural and temporal controls on district-scale propylitic alteration, generating a 4-dimensional model of the evolution of propylitic alteration in a specific region: the giant Quellaveco Cu-Mo porphyry in southern Peru. Over 20,000 structural data points and 107 samples were collected from two ~25 km transects in the Quellaveco region. SEM analysis of 82 samples yielded grain-scale petrographic and textural information of veining and alteration assemblages. This enabled the classification of 18 wall rock and vein alteration assemblages 10 of which represented propylitic-related assemblages. Specifically identified sites were selected from the propylitic minerals for geochronological (apatite, titanite, epidote and zircon) and geochemical trace element analysis (epidote and chlorite) using the LA-ICP-MS. Dating revealed an extensive period of propylitic alteration between 72-60 Ma with an alteration maximum concentrating at 65-69 Ma. This peak in propylitic alteration occurred ~8-12 My prior to the onset of mineralisation at the known porphyry regional centers: Quellaveco (58.4-54.3 Ma); Toquepala (57.0-54.0 Ma) and Cuajone (56.5-53.0 Ma). Fluctuations of propylitic alteration continued throughout this period, with geothermic and geochemical vectors indicating a period of high temperature fluids between 72-68 Ma, consistent with early Yarabamba Batholith emplacement. The principal NW-SE faults act as major fluid conduits during this period, with specifically the NNW-SSE Puca-Porquene splay-fault. Where the NW-SE fault intersect the major NE-SW faults there are apparent areas of fluid localization. In summary, this project’s multi-disciplinary approach elucidates the temporal and spatial complexity of the development of propylitic alteration in porphyry regions