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Abstract

Trace element geochemistry of detrital zircons from fluvial sediments can serve as a tracer for copper porphyry exploration and may be a useful novel method for identifying mineralization sources. However, this method has not been applied to a large data set in a geographically broad and geologically variable setting of a terrane-scale magmatic arc. I hypothesize that, even in this complex setting, detrital zircon samples from rivers downstream of ore bodies can be unmixed into multiple endmembers, one of whose geochemistry will be consistent with an ore-body source and whose mixing proportions will decrease with distance from the ore body. I test this hypothesis and exploration method by conducting geochemical analysis of zircons from modern river sands in southern Peru and northern Chile. I collected samples from three categories of catchments: (i) drainages hosting active copper porphyry mines, (ii) drainages with known but unmined copper porphyry centers, and (iii) adjacent drainages with no known mineralization. For each sample I calculated known copper porphyry indicator ratios, including Eu/Eu*, CeN/NdN, DyN/YbN, Ce/Ce*, Th/U, Ti and Hf. I used statistical methods, including Principal Component Analysis and clustering to identify the elements that account for the majority of the variability in the geochemical data set, and decomposed the multivariate dataset into endmembers using a Tucker-1 decomposition method implemented by DZgrainalyzer. Results suggested endmembers with geochemical distributions consistent with hydrous and oxidized (potentially Cu-permissive) and relatively anhydrous and unoxidized (likely Cu immobile) magmas. Of the analyzed ratios, Eu/Eu* and CeN/NdN provide the greatest discrimination between potential Cu-permissive and Cu-immobile igneous sources. Modal ages from the Cu-permissive endmembers are consistent with known mineralized intrusions and include younger populations from 0 Ma to 10 Ma that have Cu potential in the area. The proportion of Cu-permissive endmembers is directly proportional to sample’s proximity to the Cu-porphyry system. We conclude that the Tucker 1- decomposition successfully distinguishes Cu-permissive from Cu-immobile sources located up to 40 km from the Cu-body. These results suggest that detrital zircons can be used to identify Cu-porphyry systems in inaccessible terrain where sampling is only possible tens of kilometers downstream of the source.