New contributions to the understanding of Kiruna-type iron oxide-apatite deposits revealed by magnetite ore and gangue mineral geochemistry at the El Romeral deposit, Chile

Abstract

2018)New contributions to the understanding of Kiruna-type iron oxide-apatite deposits revealed by magnetite ore and gangue mineral geochemistry at the El Romeral deposit, Chile. Ore Geology Reviews New contributions to the understanding of Kiruna-type iron oxide-apatite deposits revealed by magnetite ore and gangue mineral geochemistry at the El Romeral deposit, Chile, Ore Geology Reviews (2018), doi: https://doi. ABSTRACTIron oxide-apatite (IOA) or Kiruna-type deposits are an important source of iron and other elements including REE, U, Ag, and Co. The genesis of these deposits remains controversial, with models that range from a purely magmatic origin to others that involve variable degrees of hydrothermal fluid involvement. To elucidate the formation processes of this deposit type, we focused on the Chilean Iron Belt of Cretaceous age and performed geochemical analyses on samples from El Romeral, one of the largest IOA deposits in northern Chile. We present a comprehensive field emission electron microprobe analysis (FE-EMPA) dataset of magnetite, apatite, actinolite, pyroxene, biotite, pyrite, and chalcopyrite, obtained from representative drill core samples. Two different types of magnetite grains constitute the massive magnetite bodies: an early inclusion-rich magnetite (Type I); and a pristine, inclusion-poor magnetite (Type II) that usually appears as an overgrowth around Type I magnetite. High V (~2500-2800 ppm) and Ti concentrations (~80-3000 ppm), and the presence of high-temperature silicate mineral inclusions (e.g., pargasite, ~800-1020°C) determined by micro-Raman analysis indicate a magmatic origin for Type I magnetite. On the other hand, high V (2300-2700 ppm) and lower Ti (50-400 ppm) concentrations of pristine, inclusion-poor Type-II magnetite indicate a shift from magmatic to hydrothermal conditions for this mineralization event. Furthermore, the composition of primary actinolite (Ca-and Mg-rich cores) within Type-II magnetite, the presence of F-rich apatite and high Co:Ni ratios (>1-10) of late stage pyrite mineralization are consistent with a high temperature (up to 840°C) genesis for the deposit. At shallow depths of the deposit, the presence of pyrite with low Co:Ni ratios (<0.5) and OH-rich apatite which contains higher Cl concentrations relative to F record a dominance of lower temperature hydrothermal conditions (<600ºC) and a lesser magmatic contribution. This vertical zonation, which correlates with the sub-vertical shape of the massive iron ore bodies, is concordant with a transition from magmatic to hydrothermal domains described in several IOA deposits along the Chilean Iron Belt, and supports a magmatic-hydrothermal model for the formation of the El Romeral. The close spatial and temporal association of the deposit with the Romeral Fault System suggests that a pressure drop related to changes in the tectonic stress had a significant impact on Fe solubility, triggering ore precipitation. radiometric 40 Ar/ 39 Ar ages of ca. 128 Ma (Rojas, 2017).Brecciated rocks with magnetite clast...

Accumulation of magnetite by flotation on bubbles during decompression of silicate magma

“…Supersaturation caused by fast decompression rates lead to rapid, diffusion-limited crystal growth, such as hopper crystals 28 , which entrap melt inclusions within eventual large (up to ~100 μm), euhedral crystals 25 . Such melt inclusions are consistent with polycrystalline silicate inclusions observed in magnetite “cores” from IOA deposits and in chromite from podiform chromite deposits that are interpreted as igneous artifacts 10,19,21,29,30 . Our experimental results provide clear evidence that polycrystalline silicate inclusions in oxides can be primary igneous features resulting from rapid oxide crystallization from silicate melt.…”

“…1c). This model explains the apparently contrasting geochemistry within and among magnetite grains at Los Colorados 10 and other Chilean IOA deposits 19–21 . However, it was unclear if the attachment force between degassing bubbles and magnetite would be strong enough to segregate magnetite from silicate melt, and how much degassing is necessary for efficient separation of magnetite.…”

“…Importantly, depending on the Cl concentration of the exsolved volatile phase, the magnetite-fluid suspension will contain a significant fraction of Fe dissolved as FeCl 2 in the fluid 18 . The solubility of FeCl 2 in fluid decreases during cooling (600–400 °C), resulting in precipitation of secondary magmatic-hydrothermal magnetite, a process that is even more effective at large decompression rates 19 . Thus, changing tectonic stress in the late Lower Cretaceous within the Atacama Fault System - host to the Chilean Iron Belt – allowed the ascent of the magnetite-fluid suspension into shallow crustal hydraulic fractures with concomitant precipitation of magmatic-hydrothermal magnetite surrounding primary igneous magnetite (Fig.…”

WITHDRAWN: Thermal Evolution of Andean Iron Oxide-Apatite (IOA) Deposits as Revealed by Magnetite Thermometry

“…Magnetite is the main constituent of iron oxide-apatite (IOA) ore deposits, commonly referred to as Kiruna-type deposits, which can host hundreds of millions to several billion tonnes of magnetite. The microtextures and trace element and isotopic compositions of magnetite samples from Andean IOA deposits have been widely studied 10,11,15,[17][18][19][20][21][22][23][24][25][26][27][28] . These studies scaffold a framework to better understand the ore-forming processes, revealing that the Fe ore forms by mechanisms that involve growth of magnetite under a wide range of conditions, spanning from high-temperature, purely igneous settings, to lower-temperature, uid-dominated hydrothermal environments.…”

“…This raises uncertainties on genetic models proposed to explain the genesis of Andean IOA deposits, which over the years have invoked: (1) immiscible Fe-, Si-, and carbonate-sulfate-rich melts [29][30][31][32][33] ; (2) metasomatic replacement 17,34,35 ; and (3) magmatic-hydrothermal processes 10, 11, 21, 24, 25, 36, 37 . Temperature estimations for Andean IOA deposits have been determined using oxygen isotope thermometry between magnetite-actinolite and magnetitepyroxene pairs 19,23,24,27,29 , thermometry of magnetite-ilmenite pairs 38 , actinolite thermometry based on Fe contents 19,22,27 , and uid and melt inclusion thermometry in apatite, pyroxene, quartz, anhydrite and calcite 11,33,[39][40][41][42] . However, temperature data obtained directly from magnetite in these deposits are either scarce or unavailable.…”