LA-ICP-MS trace element mapping: Element mobility of hydrothermal magnetite from the giant Beiya Fe-Au skarn deposit, SW China

摘要

Graphical abstractDisplay OmittedHighlights?The visible element mobility in skarn magnetite is poorly understood.?This is a suitable resolution for 2D/3D quantitative element mapping approach.?Which could be able to reveal the element mobility of magnetite.?These maps provide “see though” technique for the multiple heating events.AbstractSkarn alteration occurs over a wide range of temperatures and geological settings, and hydrothermal magnetite (a common mineral in skarn) represents a potentially useful temperature and tectonic indicator. Here we describe a new 2D/3D quantitative element mapping approach to evaluate the element mobility of magnetite during skarn alteration/mineralization. Magnetite grains from the giant Beiya Fe-Au skarn deposit (Yunnan, SW China) are either coarse-grained euhedral (hexagonal) with core-rim zoning (type I) or subhedral unzoned (type II) coexisting with (some enclosing) garnet. Concentrations of Na, Mg, Al, Si, Ca, Sr, Cr and Mn in the type I magnetite core and rims are drastically different, which are high in the core and outer rim and low in the inner rim. This suggests fluid geochemical fluctuations, probably led by multiple phases of ore fluid incursions that are common in many world-class skarn deposits. Concentrations of Mg, Al, Si, Ca, Mn and Sr in the garnet are higher than those in type II magnetite. This is likely because the earlier garnet crystallization (during the prograde skarn alteration) had depleted these elements in the later magnetite ore fluids. Both the garnet and the magnetite core are characterized by high Ti concentration. The high Ti concentration in the garnet is governed by the crystal structure, while that of Ti in hydrothermal magnetite is governed by Ti mobility. The 3D-mapping and simulation modeling have shown clear element variation patterns in some magnetite grains that were previously determined to be compositionally homogeneous by spot analysis. Our new 2D/3D-mapping have revealed the presence of mineral inclusions such as garnet and calcite as constrained by the trace element mobility during the replacement of garnet by magnetite, as shown in the euhedral zoned magnetite in garnet pseudomorph.]]>