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

Laura A Ward,David A Holwell,Tiffany L Barry,Daryl E Blanks,Shaun D Graham

doi:10.1016/j.gexplo.2018.01.018

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 >300 ppm). 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.

Highlights

Different generations of magnetite are texturally, geochemically classified, with petrogenesis discussed, and we propose a new discrimination diagram that can become a powerful tool in surface geochemical exploration for Ni-Cu-platinum group element (PGE) sulfide deposits. 117 The Munali Ni sulfide deposit The mafic-ultramafic Munali Intrusive Complex (MIC) is located in the Zambezi Supracrustal Sequence (ZSS) within the medium-high metamorphic grade Zambezi Belt, southern Zambia (Evans 2011; Holwell et al 2017a; Fig. 1A)
There are a number of generations of magnetite that are abundant in the MIC, which have discernible geochemical characteristics; most importantly, between magnetite associated with sulfides, and those in barren rocks
The following discussion firstly briefly addresses the processes that determine the variability in magnetite chemistry at Munali, and focusses on the application of this to exploration. 431 Origin of different styles and chemical signatures of magnetite There are three separate origins of the magmatic magnetite in our study: (1) that formed from the fractional crystallisation of a silicate liquid; (2) that formed from the crystallisation of a sulfide melt; and (3) that formed during silicate-silicate or –sulfide interaction

Summary

Introduction

Indicator mineral chemistry has been a successful early stage exploration technique for many years.The key features of successful indicator minerals are that they have a much higher abundance than the main commodity; they survive in weathering processes; and have distinctive geochemical signatures (Layton-Matthews et al 2014).This approach is well established in diamond exploration, where indicator minerals (e.g. ilmenite, olivine, Cr51 diopside, garnet) are present in much higher quantities than diamonds in overburden; and their abundance, co-occurrence and chemistry have been used to identify the presence and fertility of kimberlites (Gurney 1984; Fipke et al 1995).Early stage geochemical exploration for base and precious metals, has traditionally relied primarily on bulk elevations of certain elements in overburden and transported (e.g. stream) samples (Cameron and Hattori 2005), alongside geophysical anomalies (Balch 2005).Recently, there has been a drive to develop indicator mineral geochemistry for base metal deposits in fresh rock (e.g. Mao et al 2016); most significantly for porphyry Cu-Au deposits in arc-related magmatic rocks, where plagioclase, apatite, and magnetite have all been shown to be useful in identifying key processes that determine fertility (Williamson et al 2016; Bouzari et al 2016; Pisiak et al 2017). 63 Magmatic Ni-Cu-platinum group element (PGE) deposits are the world’s most important source of Ni and PGEs, accounting for ~56% of the world’s Ni production and over 96% of Pt, Pd, and the other PGE production (Mudd and Jowitt 2014).Such deposits are hosted in ultramafic/mafic intrusions that have undergone sulfide saturation and the separation of an immiscible sulfide liquid from the silicate magma, which scavenged chalcophile elements like Ni, Cu, and the PGE (Barnes et al 2017).Exploration for these deposits has traditionally 69 relied on geophysics; primarily gravity and magnetic surveys to identify the host ultramafic/mafic complexes; and overburden geochemistry to identify elevated levels of 71 elements like Cu, Ni, Co, As, and Cr (Rose et al 1979; Cameron and Hattori 2005). The key features of successful indicator minerals are that they have a much higher abundance than the main commodity; they survive in weathering processes; and have distinctive geochemical signatures (Layton-Matthews et al 2014) This approach is well 50 established in diamond exploration, where indicator minerals (e.g. ilmenite, olivine, Cr51 diopside, garnet) are present in much higher quantities than diamonds in overburden; and their abundance, co-occurrence and chemistry have been used to identify the presence and fertility of kimberlites (Gurney 1984; Fipke et al 1995).Early stage geochemical exploration for base and precious metals, has traditionally relied primarily on bulk elevations of certain elements in overburden and transported (e.g. stream) samples (Cameron and Hattori 2005), alongside geophysical anomalies (Balch 2005).

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