Mineral chemistry of magnetite from magnetite-apatite mineralization and their host rocks: examples from Kiruna, Sweden, and El Laco, Chile

Abstract

Magnetite-apatite deposits, sometimes referred to as Kiruna-type deposits, are major producers of iron ore that dominantly consist of the mineral magnetite (nominally [Fe 2+ Fe 3+ 2 ]O 4 ). It remains unclear whether magnetite-apatite deposits are of hydrothermal or magmatic origin, or a combination of those two processes, and this has been a subject of debate for over a century. Magnetite is sensitive to the physicochemical conditions in which it crystallizes (such as element availability, temperature, pH, fO 2 , and fS 2 ) and may contain distinct trace element concentrations depending on the growing environment.These properties make magnetite potentially a useful geochemical indicator for understanding the genesis of magnetite-apatite mineralization.The samples used in this study are from precisely known geographic locations and iv

Silician Magnetite from the Copiapó Nordeste Prospect of Northern Chile and Its Implication for Ore-Forming Conditions of Iron Oxide–Copper–Gold Deposits

“…As shown in Figure 7 [19]. The compositionally-zoned magnetite of the Chadormalu deposit spreads over distinct areas [20]; the silica-rich magnetite is enriched in Ca + Al + Mn contents in comparison with silica-poor magnetite, occupying mainly the Porphyry and IOA areas.…”

“…The average compositions of "impure" elements in the two samples (C-4, S-2) are plotted in the discriminant diagrams of Al + Mn vs. Ti + V, Ca + Al + Mn vs. Ti + V, and Ni/(Cr + Mn) vs. Ti + V (Figure 7), published by Dupuis and Beaudoin [1], in order to compare with data of silician magnetite from other types of Fe deposits [17][18][19][20]. The mean value of the fairly homogeneous C-4 sample plots within the IOCG field; however, that of the zoned S-2 sample plots in the skarn field.…”

“…This implies that the Copiapó Nordeste silician magnetite has a chemical characteristic between IOCG-and skarntypes. [4], El Laco (purple) [19], Chadormalu Si-rich magnetite (orange) [20], Chadormalu Si-poor magnetite (yellow) [20], and Chengchao (green) [17]. Modified from Dupuis and Beaudoin [1] and Nadoll et al [3].…”

“…Fe 2+ cations are observed to occupy half of the octahedral lattice sites, and alternatively Fe 3+ cations occupy the other octahedral lattice sites and all of the tetrahedral lattice sites [21]. In the case of silician magnetite, the replacement of Fe 3+ in the tetrahedral site by Si 4+ and the electrostatically-compensating substitution of Fe 3+ by Fe 2+ [6][7][8]21] can be assumed as follows: [4], El Laco (purple) [19], Chadormalu Si-rich magnetite (orange) [20], Chadormalu Si-poor magnetite (yellow) [20], and Chengchao (green) [17]. Modified from Dupuis and Beaudoin [1] and Nadoll et al [3].…”

Occurrence of the Iron–rich Melt in the Heijianshan Iron Deposit, Eastern Tianshan, NW China: Insights into the Origin of Volcanic Rock–hosted Iron Deposits

“…Long-standing controversy persists over the presence/absence of iron-rich melts in the genesis of this type of iron deposits. Some investigators have interpreted these deposits as direct crystallization from an iron-rich melt, with evidence including distinct magnetite volcanic flow structures, vesicles in the magnetite ore veins with chilled margins, degassing tubes similar to textures observed in basaltic flows, and the presence of 'ore breccia' (Nyström and Henriquez, 1994;Chen et al, 2010;Mao Jingwen et al, 2012;Tornos et al, 2016;Broughm et al, 2017). However, some others have invoked a hydrothermal model in which they propose that the above ore features can be explained by open-space filling of ascending hydrothermal fluids, as evidenced by the pervasive presence of hydrothermal alteration and the low-Ti feature of magnetite (Frotos and Oyarzun, 1975;Gu Lianxing and Ruan Huichu, 1988;Bookstrom, 1995;Sillitoe and Burrows 2002;Dare et al, 2015).…”