The link between volcanism and plutonism in epizonal magma systems; high-precision U–Pb zircon geochronology from the Organ Mountains caldera and batholith, New Mexico

The Organ Mountains caldera and batholith expose the volcanic and epizonal plutonic record of an Eocene caldera complex. The caldera and batholith are well exposed, and extensive previous mapping and geochemical analyses have suggested a clear link between the volcanic and plutonic sections, making this an ideal location to study magmatic processes associated with caldera volcanism. Here we present high-precision thermal ionization mass spectrometry U–Pb zircon dates from throughout the caldera and batholith, and use these dates to test and improve existing petrogenetic models. The new dates indicate that Eocene volcanic and plutonic rocks in the Organ Mountains formed from ~44 to 34 Ma. The three largest caldera-related tuff units yielded weighted mean [superscript 206]Pb/[superscript 238]U dates of 36.441 ± 0.020 Ma (Cueva Tuff), 36.259 ± 0.016 Ma (Achenback Park tuff), and 36.215 ± 0.016 Ma (Squaw Mountain tuff). An alkali feldspar granite, which is chemically similar to the erupted tuffs, yielded a synchronous weighted mean [superscript 206]Pb/[superscript 238]U date of 36.259 ± 0.021 Ma. Weighted mean [superscript 206]Pb/[superscript 238]U dates from the larger volume syenitic phase of the underlying Organ Needle pluton range from 36.130 ± 0.031 to 36.071 ± 0.012 Ma, and the youngest sample is 144 ± 20 to 188 ± 20 ka younger than the Squaw Mountain and Achenback Park tuffs, respectively. Younger plutonism in the batholith continued through at least 34.051 ± 0.029 Ma. We propose that the Achenback Park tuff, Squaw Mountain tuff, alkali feldspar granite and Organ Needle pluton formed from a single, long-lived magma chamber/mush zone. Early silicic magmas generated by partial melting of the lower crust rose to form an epizonal magma chamber. Underplating of the resulting mush zone led to partial melting and generation of a high-silica alkali feldspar granite cap, which erupted to form the tuffs. The deeper parts of the chamber underwent continued recharge and crystallization for 144 ± 20 ka after the final eruption. Calculated magmatic fluxes for the Organ Needle pluton range from 0.0006 to 0.0030 km3/year, in agreement with estimates from other well-studied plutons. The petrogenetic evolution proposed here may be common to many small-volume silicic volcanic systems

Triggers for the formation of porphyry ore deposits in magmatic arcs

Porphyry ore deposits are the source of much of the copper, molybdenum, gold and silver used by humans. Porphyry ore typically forms in magmatic arcs above subduction zones. However, generation of the largest deposits is often restricted to specific arc segments and limited periods of time. Here, I outline a hierarchy of four key triggers that may be involved in the formation of large porphyry deposits. The first process is characterized by a cyclical enrichment of magmas with metals and water in the deep crust. Second, saturation of the magma with sulphide facilitates the concentration of metals into smaller volumes of material from which they can later be released. The third process is an efficient transfer of metals into hydrothermal fluids that are exsolved from the magmas. Finally, localized processes trigger the precipitation of ore minerals in the crust. Although some or all of these processes must act in concert to generate large ore deposits, I argue that sulphide saturation of the magma is the most important step and that this can explain the temporal and spatial distribution of ores. Consequently, the fingerprint of sulphide saturation in igneous rocks could be used to identify those parts of magmatic arcs that are particularly predisposed to ore formation.© 2013 Macmillan Publishers Limited. All rights reserved. The attached document is the author’s final accepted version of the journal article. You are advised to consult the publisher’s version if you wish to cite from it