Geochemistry and high-precision geochronology of the mass transfer between a skarn and sapphire metapelite within the Whitehorn Stock magmatic-hydrothermal system, Colorado: a model for hydrothermal corundum

Abstract

This study examines the metapelite-hosted hydrothermal corundum occurrence at the sheared contact of a syngenetic skarn and related magmatic-hydrothermal evolution of the Laramide calc-alkaline I-type Whitehorn Stock in central Colorado. Prior researchers have suggested that leaching away quartz within quartz-bearing protoliths will enhance residual Al2O3 so that corundum can be stabilized. This study demonstrates, using field relations, microscopy, whole-rock analysis, electron microprobe analysis (EPMA), and laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS), that Al hydrothermal mass transfer is possible and quartz-bearing protoliths can be modified to form hydrothermal corundum. Our model shows that Si–Al-alkali–alkaline-halide-rich magmatic-hydrothermal fluids derived from the Whitehorn Stock metasomatically interacted with a pure limestone protolith juxtaposed with the metapelite-hosted corundum occurrence. Skarn mineralization of the limestone resulted in an Al-alkali-rich and Si-undersaturated magmatic-devolatilization-derived modified fluid before interacting with the quartz-bearing pelitic schist. Disequilibrium reactions between the quartz and modified fluid led to significant changes in mineralogy and whole-rock composition of the quartz-bearing pelitic schist by the formation of primarily feldspar and corundum as Si and alkalis were consumed at ~681°C and 2 kbar. This study also demonstrates that metapelite-hosted corundum and skarn-hosted garnet trace element geochemistry reflects the involvement of igneous intrusions and skarn formation. Corundum and garnet show similar geochemical signatures enabling interpretations of metasomatic vs. pelitic signatures across a pelitic- and metasomatic-derived compositional continuum from which the corundum is derived. A set of petrogenetic indicator chemical criteria are established using molar-element ratio analysis of trace elements in corundum to distinguish among the protolith, magmatic-hydrothermal fluids, and skarn-related fluids to establish corundum gemstone (sapphire) origins. Chemical abrasion-isotope dilution-thermal ionization mass spectrometry U–Pb zircon high-precision dating was used to bracket the emplacement-to-crystallization and magmatic-to-hydrothermal ages of the Whitehorn Stock. The emplacement-to-crystallization duration was determined by determining the ages of a paragenetically early-crystallized diorite sill and related paragenetically later-crystallized granodiorite. The magmatic-to-hydrothermal age was bracketed by dating a felsic anorthosite hosting both magmatic and hydrothermal zircons. The magmatic-to-hydrothermal transition was further bracketed by dating hydrothermal zircon occurring within the metapelite-hosted hydrothermal corundum. The results support models where multiple injections accrete igneous intrusions, the metasomatic phase was derived and sustained by incremental magmatic pulses for approximately 300 kyr, and that hydrothermal mineralization was concluded before complete Whitehorn Stock solidification.