Magmatic-hydrothermal fluids transport more than dissolved solutes

• Main Authors: Simon, Adam, Knipping, Jaayke L., Reich, Martin, Barra, Fernando, Deditius, Artur P., Anonymous
• Format: Conference Object
• Language: English
• Published: V.M. Goldschmidt Conference 2018
• Subjects: Chile; crystallization; Economic geology; geology of ore deposits; fluid dynamics; hydrothermal conditions; iron ores; iron oxides; Los Colorados Mine; magmas; magmatism; melts; metal ores; microlite; mineral deposits; genesis; ore-forming fluids; oxides; silicate melts; silicates; South America; tantalates; Kiruna
• Online Access: https://researchportal.murdoch.edu.au/esploro/outputs/abstract/Magmatic-hydrothermal-fluids-transport-more-than-dissolved/991005560461807891; https://researchportal.murdoch.edu.au/view/delivery/61MUN_INST/12139975310007891/13142067780007891

The ability for magmatic-hydrothermal fluids to scavenge and transport elements from silicate melt to form ore deposits is well accepted. Less well understood is the capacity for magmatic-hydrothermal fluids to sweep up and transport microlites that serve as nucleation sites for exsolving bubbles in silicate magma. Magnetite from the Los Colorados Kiruna-type iron oxide-apatite (IOA) deposit in the Chilean Iron Belt preserves evidence for the transport of igneous magnetite microlites by magmatic-hydrothermal fluid. IOA deposits are spatially and temporally associated with magmatic activity in arc environments. However, existing genetic models cannot successfully explain the geochemical signature of Kiruna-type IOA deposits, or their spatial association with magmatic activity. Here, we use trace element concentrations, and Fe, O and H stable isotope abundances in magnetite from Los Colorados to develop a new genetic model that explains IOA deposits as a combination of igneous and magmatic-hydrothermal processes. The novel genetic model invokes 1) near-liquidus crystallization of magnetite microlites from an intermediate silicate melt; 2) nucleation of gas bubbles on crystal faces of magnetite microlites; 3) coalescence of the volatile phase and encapsulation of magnetite microlites to form a magnetite-fluid suspension; 4) scavenging of Fe and other metals from the melt; 5) buoyant ascent of the suspension along structurally enhanced dilatant zones during regional extension; 6) growth of originally igneous magnetite microlites that source Fe from the decompressing magmatic-hydrothermal fluid; and 7) deposition of magnetite. The model explains the origin of Kiruna-type IOA deposits, and the globally observed temporal and spatial relationship between magmatism and IOA deposits, and provides a valuable conceptual framework to define exploration strategies.