An integrated accessory mineral approach to understanding postsubduction magmas and mineralisation

Accessory minerals, such as zircon, apatite, and sulfides, can be used to decipher distinct events that can occur over protracted magmatic histories. Although studies of accessory minerals are now well established, there are caveats to their use, such as the loss of their petrogenetic context. This study examines the textural characteristics of zircon, crust and mantle contributions to granitic melts, the chemistry of sulfides, and the petrogenetic context of apatite in the Colorado Mineral Belt to provide a transcrustal story told by accessory minerals. The Colorado Mineral Belt is chosen due to the presence of syn- and post-subduction granitoids that are abundant in accessory minerals, and in close spatial and temporal proximity.

Granite isotope chemistries from the Colorado Mineral Belt are sourced primarily from crustal assimilation. εHf values range from -5 to -12 (55 to 80% crustal contribution) in syn-subduction magmas, and from εHf -6 to -10 (55 to 70% crustal contribution) in post-subduction magmas. Post-subduction magmas lack a significant inherited zircon component compared to syn-subduction, despite their similar isotopic and whole-rock chemistries. Adiabatic ascent and zircon dissolution, more open pathways, and more voluminous melting likely resulted in the differences between syn- and post-subduction magmas.

Sulfide inclusions in mid-crustal clinopyroxene and magnetite-rich xenoliths have chalcophile element chemistries that indicate chalcophile element (Au-Cu-Te) transfer from immiscible sulfide liquids into silicate melts. The observed chalcophile behaviour primed magmas beneath the San Juan Volcanic Field for the eventual formation of mineral deposits in the upper crust.

Textural-chemical relationships in apatite determine in situ and ex situ crystallisation of granitoids. A new approach, using Zeiss’ Mineralogic software, provides repeatable, rapid, and quantitative investigations of apatite host mineralogy. Apatites become more depleted in trace elements during crystallisation, but inclusion into host mineralogy occurs at or near the eutectic point, resulting in the decoupling of these two records.