Formation of the El Laco magmatic magnetite deposits by Fe-Si melt
immiscibility and bubbly suspension flow along volcano tectonic faults
Abstract
The origin of Kiruna-type magnetite-apatite deposits, which are thought
to form by magmatic and/or hydrothermal processes, has recently come
under renewed scrutiny. Geological and geochemical studies of
volcanic-hosted magnetite deposits that include magnetite lava flows and
ash layers at El Laco, a volcano in the Central Volcanic Zone, northern
Chile, suggest a formation by eruptive emplacement of an iron oxide-rich
melt. The generation of such exotic high density, low viscosity melts by
dissociation from an andesitic host magma contaminated by shallow
crustal sediments has only recently been shown experimentally. The
dynamics of volcanic emplacement have remained enigmatic because the
high density of iron-rich melts seems to negate their eruption
potential. Yet, observations of ubiquitous vesiculation, degassing
structures, and steam-heated alteration provide important clues that
volatiles had a pivotal role in the volcanic emplacement. Here, we posit
a scenario in which an iron-rich immiscible liquid gravitationally
separates from its andesitic parent magma in a shallow magma reservoir
and subsequently rises as a bubbly suspension along volcano-tectonic
faults extending to the flanks of the edifice. We test this hypothesis
through numerical models that capture both the deformation of the
volcanic edifice as well as the melt transport within. Preliminary
results indicate that separation of a low-viscosity, iron- and
volatile-rich melt from a silicic magma within a reasonable time is
possible only if an interconnected melt drainage networks forms at the
granular scale. Results further suggest that magma reservoir deflation
and/or minor local extension combined with the topographic load of the
edifice may explain normal faults connecting the magma reservoir with
magnetite flow locations on the volcano flanks. Finally, our models show
that hydrostatically driven flow of iron-rich melts into these faults at
depth may trigger volatile exsolution and bubble expansion to provide
sufficient driving force for an eruptive emplacement. Although the case
for such magmatic ore formation is perhaps strongest at El Laco,
evidence from other localities suggests that similar processes have been
at work. The new insights derived from our models may, therefore, apply
more generally to Kiruna-type deposits elsewhere.