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Paper No. 15-13
Presentation Time: 8:30 AM-6:30 PM
COMPARISON OF REE SIGNATURES OF PRE- AND SYN-MINERALIZATION PLUTONS ASSOCIATED WITH PORPHYRY COPPER DEPOSITS OF NW MEXICO
Rare earth elements (REE) may help to better understand the petrogenetic processes associated with the porphyry copper systems. This study examines the REE compositions of intrusive rocks collected along a NW-SW trending “Laramide” porphyry copper belt of northwestern Mexico. The analyses were done on samples collected from some of the most representative deposits and included pre- and syn-mineralization plutons. Chondrite-normalized REE patterns display similar shapes for corresponding pairs of equigranular (pre-) and productive (syn- mineralization) porphyritic intrusions. However, the latter show an interesting reduction in the size of the typically well-developed negative europium anomaly observed in the former plutons, which, in some cases, shifted into a positive anomaly. This behavior is particularly evident in samples of deposits located in the northern and central parts of the belt, where mineralized bodies were emplaced in a crust underlain by Proterozoic rocks of North American affinity. Here, the REE signatures are characterized by negative slopes defined by CeN/YbN ratios between ~8 and 28 and Eu anomalies (EuN/[SmN*GdN]1/2) between 0.8 and 1.6. In the southern part of the belt, where the porphyry copper deposits were emplaced in a more juvenile basement, characterized by Jurassic island-arc rocks of the Guerrero terrane, the REE patterns are clearly flatter (CeN/YbN ratios between 6.1 and 17.2), with minor negative or even positive Eu anomalies (EuN/[SmN*GdN]1/2 ~ 0.7-1.4). A similar behavior in the REE compositions was documented in several of the copper porphyry systems of Arizona, which was interpreted as a result of the magmatic stage that occurred in equilibrium with horblende. In this sense, assuming that changes in the chemical conditions of the magma favored Eu+2 over Eu+3, this phenomenon was more conspicuous in the north and central parts of the porphyry copper belt of northwestern Mexico.