Determining the Magma Genesis of Mo Porphyry Deposits

Abstract:

The high flux of magma associated with super eruptions is hypothesized to rebuild the deep crust, altering the source(s) of subsequent magmatism. Climax-type Mo deposits are commonly generated immediately after eruption of large ignimbrites within a volcanic field, and provide an opportunity to understand the evolution of magma sources following high flux events. The Questa caldera of the Latir volcanic field, NM exposes a 10 Ma long record of pre-, syn- and post-ignimbrite intrusive and extrusive rocks, and hosts the Questa Climax-type Mo deposit. New detailed geochronology and geochemistry from Questa (including extensive sampling of subsurface rocks in the mine) permit detailed reconstruction of the temporal evolution of magma sources through the waxing and waning stages of super eruption magmatism.

Comparison of chemical and isotopic data waxing, ignimbrite, Mo-mineralizing and waning stage magmas reveals several patterns. Waxing and waning magmas (waxing: 29-25.7 Ma; waning: 24.5-19 Ma) have intermediate trace elements and radiogenic isotopes relative to other magmatism (⁸⁷Sr/⁸⁶Sr_i=0.7050 to 0.7070, ɛ_Nd=-5.2 to -7.2). Ignimbrite magmatism (25.5 Ma) is depleted in incompatible elements, enriched in MREE and HREE’s and has more evolved radiogenic isotopes (⁸⁷Sr/⁸⁶Sr_i=0.7095, ɛ_Nd=-8.0). Molybdenum mineralizing magmas (24.9-24.5 Ma), are enriched in incompatible elements, depleted in MREE and HREE’s and have distinct radiogenic isotopes (⁸⁷Sr/⁸⁶Sr_i=0.7055 to 0.7075, ɛ_Nd=-4.2 to -5.7). We suggest the lower crustal source of magmas changed during ignimbrite generation, and as a result, subsequent mineralizing magmas incorporated more juvenile, mafic components. This mantle influence is the metallogenesis for Climax-type deposits and indicates that deep crustal hybridization, rather than upper crustal differentiation, is pivotal in their generation. These results indicate that a lower crustal source of magmatism for a volcanic field is altered due to super eruptions.