HIGH-PRECISION U-PB CA-TIMS GEOCHRONOLOGY AND ISOTOPE GEOCHEMISTRY OF POLY-METALLIC PORPHYRY INTRUSIONS OF THE LATIR MAGMATIC CENTER, NEW MEXICO: LARGE SYSTEM WITH MULTIPLE METALS, OR MULTIPLE SYSTEMS WITH DIFFERENT METALS?

Porphyry poly-metallic mineralization within long-lived magmatic centers is commonly interpreted as the result of fractionation of a large magma body, with porphyry magmatism emanating from a single, evolving chamber. Alternatively, individual metal mineralization events may occur via intrusion of geochemically distinct magmas. We use CA-TIMS U/Pb zircon single crystal ages and Nd whole rock isotopes, to evaluate these models for poly-metallic mineralization in the Latir magmatic center, northern New Mexico.

The magmatic history of the Latir includes pre-caldera, caldera (eruption of the Amalia Tuff at 25.5 Ma; Tappa et al., 2011) and post-caldera intrusions. Mineralization climaxed shortly after eruption of the tuff (Questa Mo, 24.9-24.5 Ma; Rosera et al., 2013) and was followed by the emplacement of Au, Ag, Cu and additional Mo mineralizing magmas. Isotope composition of intrusions emplaced following the eruption of Amalia Tuff mark a shift away from pre-caldera values (ɛNd = -6.5), with a protracted, ~1.5 Ma rebound which includes pre-Mo- (ɛNd = -4.3 to -4.9), syn-Mo- (ɛNd = -4.9 to -5.9) and post-Mo- (ɛNd = -5.9 to -7.5) intrusions. Porphyry Au (23.838 +/- 0.070 Ma; ɛNd = -5.49) and Ag (23.628 +/- 0.067 Ma; ɛNd = -6.59) mineralization occurred adjacent to the Mo deposit, but postdate it. Silver mineralizing magmatism was coeval and isotopically similar to a barren sill intercepted by deep drilling (23.662 +/- 0.073 Ma; eNd= -6.69). Subeconomic Mo deposits to the south postdate Questa mineralization (20.518 +/- 0.057 Ma). This magmatic period ended with the emplacement of the barren Lucero Peak pluton (19.875 +/- 0.040 Ma; eNd= -6.24 to -6.72).

Metal deposits within the Latir magmatic center were associated with temporally and geochemically distinct porphyry intrusions. Therefore, we interpret the poly-metallic mineralization to be the result of intrusion of multiple magmas, individually saturated in different precious metals rather than a bulk exsolution from a large crystallizing body. Poly-metallic mineralization in porphyry systems may therefore be the result of overlapping mineralization haloes associated with multiple discrete intrusions.