REE SIGNATURES OF MAGMATIC SYSTEMS ASSOCIATED WITH PORPHYRY CU-MO DEPOSITS IN NORTHWESTERN MEXICO: A POSSIBLE PETROGENETIC TRACER

A belt of porphyry Cu-Mo deposits was emplaced in northwestern Mexico during the Late Cretaceous and Paleogene (90-40 Ma). These deposits are closely associated with the emplacement of shallow (~1-3 km depth) porphyritic stocks, linked with the cooling of a deeper and much larger equigranular pluton of calcalkaline composition. Normalized REE patterns help to better constrain the petrogenetic process associated with the evolution of the magmatic system that controlled the progressive concentration of metals in the liquid phase, the hydrothermal fluid circulation and the final emplacement of the mineralization. This work evaluates the REE data of both, pre-ore and ore-related plutons of the most representative deposits within this region. The normalized REE patterns display rather similar shapes for both plutonic phases; however, there is a nearly systematic reduction of the size of the typically negative europium anomalies, which, in several cases, even reaches a complete reversal of the polarity of the anomaly. This is particularly noticeable in the porphyry Cu-Mo deposits of the northern and central parts of the belt, which is underlain by Proterozoic rocks of North American affinity. At the moment, the aim of this work is limited to report this finding. However, the research work to try to understand the petrological reasons is in progress.