Using cocrystallization coefficients of isomorphous admixtures for determination of element concentrations in ore-forming solutions (by the example of Mn/Fe ratio in magnetite)

Abstract

The cocrystallization coefficient of Mn and Fe (DMn/Fe) in magnetite crystals is determined in hydrothermal-growth experiments with internal sampling at 450 and 500 °C and 100 MPa (1 kbar). It is weakly dependent on temperature in the studied PT-region and is constant over a wide range of Mn/Fe values. This permits using the magnetite composition as an indicator of Mn/Fe in the fluid under equilibrium: (Mn/Fe)aq ≈ 100 (Mn/Fe)mt. Since Mn is often a macrocomponent of the fluid and a microcomponent of magnetite, local analysis of fluid inclusions for Mn might help to determine Fe even in iron minerals. This will permit evaluation of the contents of other ore metals if the DMe/Fe values are known. For fine crystals (< 0.1–0.2 mm) with low contents of Mn (< 0.01–0.02%), it is necessary to take into account the fractionation of Mn into the surficial nonautonomous phase, in which its content can reach several percent. Comparison of these data with earlier data on the distribution of Mn in the system magnetite–pyrite–pyrrhotite–greenockite–hydrothermal solution shows that DMn/Fe remains constant in the presence of sulfur and sulfides. Precipitation of magnetite, in which Mn is a compatible admixture, cannot affect radically Mn/Fe in the solution because of the low DMn/Fe value. This effect is still more unlikely for pyrrhotite and pyrite, in which Mn is an incompatible admixture. The most probable mechanism of Mn fractionation into the solid phase is crystallization of FeOOH at lower temperatures. This is indirectly supported by the strong fractionation of Mn into the nonautonomous oxyhydroxide phase on the surface of magnetite crystals. The necessity of a more rigorous validation of “the new Fe/Mn geothermometer for hydrothermal systems” is substantiated.

Trace element fractionation in magnetite as a function of Fe depletion from ore fluids at the Baijian Fe-(Co) skarn deposit, eastern China: Implications for Co mineralization in Fe skarns

“…Fractionation of V, Cr, Mn, Co, Ni, and Zn in magnetite. The following elements, V, Cr, Mn, Co, Ni and Zn, were selected for modeling because: (1) They are among the main discrimination elements for magnetite from various settings (Nadoll et.al., 2014) and their concentrations in magnetite are generally above the detection limits of LA-ICP-MS; (2) Their concentrations are low in the host dolomitic limestone and limestone as well as in the gangue minerals such as diopsidic pyroxene, tremolite, phlogopite, and serpentine (Wen, 2017), such that fluid-rock reactions and the precipitation of gangue minerals has a limited influence on the budget of these elements in the hydrothermal fluid; and (3) The distribution coefficients for these elements between magnetite and chloride solutions are available (Ilton and Eugster, 1989;Tauson et al, 2015Tauson et al, , 2017Smagunov et al, 2021).…”

“…Corresponding distribution coefficients are as follows: 𝐾 𝑑 (Mn) = 0.009 ± 0.002; = 3 ± 1.5; and 𝐾 𝑑 (Cr) = 1.2 ± 1. We used the mean distribution coefficients for V, Cr, Mn, Co, Ni, and Zn between magnetite and fluid from Tauson et al (2015Tauson et al ( , 2017 and Smagunov et al (2021) for the Rayleigh and equilibrium fractionation modeling. The P, T, and fO 2 conditions for these distribution coefficients are comparable to those during magnetite mineralization at Baijian (Wen 2017) and other skarn Fe deposits worldwide (e.g., Meinert, 1984;Rose et al, 1985;Pons et al, 2009;Li et al, 2019).…”

“…In this study, we analyze the trace element composition of magnetite from the Baijian Fe-(Co) skarn deposit in the North China Craton (NCC). Building upon recent experimental findings on magnetite-fluid trace element partitioning (Tauson et al, 2015(Tauson et al, , 2017Smagunov et al, 2021), we perform Rayleigh and equilibrium fractionation modeling in order to illustrate how progressive magnetite precipitation influences both the fluid and magnetite chemistry during formation of an Fe skarn skarn deposits that combined have total Fe reserves over 900 million tons (Mt). Those deposits are hosted in the diorite and monzodiorite intrusions, along the contact zones between the intrusions and surrounding carbonate sequences, or within fracture zones of the carbonate proximal to the intrusions.…”

“…Zn are incompatible relative to Fe in magnetite in equilibrium with chloride solutions at 600 to 800 ℃ and 200 MPa. Recent experimental partitioning studies byTauson et al (2015Tauson et al ( , 2017 andSmagunov et al (2021) confirmed the incompatibility of Mn and Zn along with Co in magnetite equilibrated with chloride solutions at 450 ℃ and 100…”