The use of magnetite as a geochemical indicator in the exploration for magmatic Ni-Cu-PGE sulfide deposits: A case study from Munali, Zambia

Abstract

Magmatic sulfide deposits hosted by mafic-ultramafic intrusions are the most important source of Ni and PGE on Earth. Exploration strategies rely on geophysics to identify the host intrusions, and surface geochemistry to identify anomalous concentrations of Cu, Ni, Co, Cr, As and other associated elements. The use of geochemical indicator minerals in overburden is used widely in diamond exploration and mineral chemistry in fresh rock is increasingly used to identify proxies for mineralisation in magmatic-hydrothermal systems. However, no indicator mineral techniques are routinely applied to magmatic sulfides. Magnetite represents an ideal indicator mineral for this mineralisation style due to its ubiquity in such deposits, its resistance to weathering, its recoverability from soil samples, and its chemical variability under differing conditions of formation. We use the Munali Ni sulfide deposit to test the use of magnetite as an indicator mineral. Magnetite from mafic, ultramafic, and magmatic sulfide lithologies in fresh rock at Munali show discernible differences in the most compatible elements (V, Ni, Cr). We propose a new Cr/V versus Ni discrimination diagram for magnetite that can be used to indicate fractionation of the parent magma (Cr/V increases from ultramafic to mafic), and the presence of co-existing sulfides (Ni contents >300ppm). The signatures of these three elements at Munali are comparable to sulfide-related magnetites from other deposits, supporting the broad applicability of the discrimination diagram. Samples taken from overburden directly on top of the Munali deposit replicate signatures in the fresh bedrock, strongly advocating the use of magnetite as an exploration indicator mineral. Samples from areas without any geophysical or geochemical anomalies show weak mineralisation signatures, whereas magnetite samples taken from prospects with such anomalies display mineralisation signatures. Magnetite is a thus a viable geochemical indicator mineral for magmatic sulfide mineralisation in early stage exploration.

Composition of iron oxides in Archean and Paleoproterozoic mafic-ultramafic hosted Ni-Cu-PGE deposits in northern Fennoscandia: application to mineral exploration

“…Dare et al (2014) presented a Ni/Cr vs. Ti diagram, which can discriminate between magmatic and hydrothermal magnetite, and Dupuis and Beaudoin (2011) introduced a Cr + Ni vs. Si + Mg diagram to discriminate magnetite between Ni-Cu-PGE deposits and hydrothermal and other types of magmatic ore deposits. Ward et al (2018) showed that the Cr/V ratio vs. Ni diagram can be used to distinguish magnetite generations that were formed in mineralized and barren systems. Liu et al (2015) introduced a Ga + Co vs. Ge diagram, which can discriminate magnetite from Ni-Cu deposits, massif-type anorthosites, and evolved parts of mafic-layered intrusions.…”

“…This is most likely because of the alteration of the samples. In the Cr/V vs. Ni diagram (Ward et al 2018), all the magnetite compositions from the Lomalampi, Kevitsa, Vaara, Pechenga, and Tainiovaara deposits are inside the ore-related field (Fig. 9b).…”

“…Till geochemistry and indicator-mineral signatures have also been utilized in exploration for magmatic Ni-Cu-PGE deposits, especially in Canada (e.g., McClenaghan et al 2011 and references therein). Ward et al (2018) successfully used magnetite as a geochemical indicator for a Ni-Cu-PGE deposit in the Munali area, Zambia by sampling and analyzing soil overburden samples. A methodology of extracting magnetite grains from till samples, analyzing their composition with EPMA and LA-ICP-MS, applying selected discrimination diagrams to the resulting data, and studying mineral inclusions within the grains could be developed into a sound exploration strategy for discovering new magmatic Ni-Cu deposits.…”

“…Magnetite in Ni-Cu-PGE sulfide deposits is characterized by relatively high Ni and Cr contents. Ward et al (2018) introduced a discrimination diagram to distinguish magnetites from mineralized and barren mafic-ultramafic systems. Nadoll et al (2014) studied the composition of magnetite from various types of hydrothermal ore deposits and proposed several discrimination diagrams and elemental concentration boundaries to discriminate between hydrothermal magnetite and igneous magnetite.…”

Magnetite Geochemistry of the Jinchuan Ni-Cu-PGE Deposit, NW China: Implication for Its Ore-Forming Processes

“…It commonly contains many trace elements, such as Si, Al, Ti, Ca, Mn, Mg, V, Cr, Co, Ni, Zn, Ga, Ge, Y, Zr, Hf, Nb, Mo, and Ta [2,3,5], that can substitute Fe 2+ and Fe 3+ in magnetite under many parameters of the similarity of the ionic radii and the valence of the cations, magma/fluid compositions, temperature, and oxygen fugacity [1,2,[7][8][9][10]. Such trace element geochemistry of magnetite can be used as clues in deposits to provide information on origin, features, and processes for ore-forming fluids or systems [11][12][13][14][15][16][17][18][19][20][21], ore deposit provenance [2,3,9,11,15,16,[20][21][22], and mineral exploration [12,19,23].…”