COMPLEX MAGMA IMMISCIBILITY IN THE FORMATION OF A MAGNETITE-APATITE SYSTEM, EL LACO, CHILE

The nature of the processes involved in the formation of magnetite-apatite (MtAp) deposits has been debated for many decades. Here, we evaluate the magmatic processes at the Pleistocene El Laco Volcanic Complex (ELVC), Chile, that hosts one of the youngest and best preserved MtAp deposits in the world. The present study focuses on ~10-100µm melt inclusions trapped in plagioclase phenocrysts in the El Laco host andesite which preserve a “snapshot” in the evolution of the plumbing system underlying ELVC. The results of our high-resolution transmission electron microscopy, field emission gun-electron probe microanalysis, image processing, and microthermometry reveals the microtextures, chemical compositions, phase proportions, and melting temperatures of these melt inclusions up to the sub-micron scale. The individual melt inclusions comprise clinopyroxene (augite-pigeonite)-magnetite (cpx-mt) globules enclosed in high-Si, Al-K-Na rhyolitic composition glass and are homogenized at 1145°C. Commonly, the globules host subspherical to spherical Cu_xS globules of digenite to covellite composition. The cpx-mt globules also host fluorapatite that, in turn, encloses spherical, <100 nm in diameter glassy melt inclusions of C-Si-Cl-Al-rich composition with daughter ilmenite with Th-Nb-Y-U-Zr. These melt inclusions demonstrate the presence of interconnected immiscible melts; Si-rich and Fe-rich melts, the latter hosting sulfide-rich, phosphate-rich, and residual melts. The composition of these melts replicates the mineral assemblage of the MtAp ore bodies at El Laco with early crystallization and deposition of massive magnetite-rich ore bodies with later crystallization of pegmatite-like bodies with Ca-Mg silicates, REE-rich fluorapatite, magnetite, and also variable amounts of anhydrite. These data suggest that the individual melt inclusions track the small-scale evolution of magma immiscibility, that at large-scale can successfully produce MtAp deposits. The similar C-rich composition of the residual melts at El Laco and other MtAp systems (e.g., Buena Vista, Nevada, Iron Springs, Utah, USA) link the crustally contaminated silicate magmas and the Fe-rich melts that can produce MtAp deposits worldwide while the Cu_xS globules suggest the link to iron oxide-copper-gold and porphyry systems.