Nickel in olivine as an exploration indicator for magmatic Ni-Cu sulfide deposits: A data review and re-evaluation

Abstract

Nickel contents of olivine have been widely used as petrogenetic indicators and as fertility indicatorsfor magmatic sulfide potential of mafic-ultramafic intrusions, on the assumption that olivines crystallized from magmas that had equilibrated with sulfide liquid should be relatively depleted in Ni compared with a sulfide-free baseline. This has given rise to a large accumulation of data that is brought together here, along with data on volcanic olivines, to critically evaluate the effectiveness of the approach. We identify multiple sources of variance in Ni content of olivine at given Fo content, including: variability in mantle melt composition due to depth, water content (and possibly source); subsequent fractional crystallization with and without sulfide; recharge and magma mixing; batch equilibration between olivine and sulfide at variable silicate-sulfide ratio (R) and olivine/liquid ratio; and subsequent equilibration during trapped liquid crystallization in orthocumulates. Baselines for Ni in olivine in relation to Fo content are somewhat lower in orogenic belt settings relative to intrusions in continental Large Igneous Provinces (LIPs). This is probably related to differences in initial parent magma compositions, with plume magmas generally forming deeper and at higher temperatures. No clear, universal discrimination is evident in Ni in olivine between ore-bearing, weakly mineralized and barren intrusions even when tectonic setting is taken into account. However, sulfide-related signals can be identified at intrusion scale in many cases. Low-R factor, low-tenor sulfides are associated with low-Ni olivines in a number of examples and these cases stand out clearly.

Predicting olivine formation environments using machine learning and implications for magmatic sulfide prospecting

“…However, the use of Ni-olivine as a fertility tool requires significant a priori knowledge of the olivine formation processes and the exploration regions; thus, this indicator is of low effectiveness. In addition, a recent empirical data-driven approach has shown that no universal, clear discrimination can be made between sulfide-bearing and sulfide-barren intrusions using the olivine Ni content at a given Fo content (Barnes et al, 2022). The same results were observed when trace elements were considered (Mao et al, 2022).…”

“…Olivine is an important and ubiquitous mineral in global volcanic and plutonic ultramafic rocks and occurs in a wide range of ages and geological environments (Fig. 1; Lehnert et al, 2000;Sobolev et al, 2007;Li et al, 2012;Barnes et al, 2022). It is crucial for understanding the Earth's upper mantle and the genesis and evolution of mantle melts (Foley et al, 2013;Herzberg et al, 2016).…”

“…The compatible character of Ni leads to a strong tendency to partition into olivine , while its chalcophile character results in much stronger partitioning into sulfide liquid (Rajamani and Naldrett, 1978;Li and Ripley, 2010;Kiseeva and Wood, 2015;Yao et al, 2018). Hence, the olivine Ni content is often considered an indicator of magmatic sulfide deposits in potential host rocks (Li and Naldrett, 1999;Barnes et al, 2004Barnes et al, , 2022Le Vaillant et al, 2016). However, the use of Ni-olivine as a fertility tool requires significant a priori knowledge of the olivine formation processes and the exploration regions; thus, this indicator is of low effectiveness.…”

Lithogeochemistry in exploration for intrusion-hosted magmatic Ni-Cu-Co deposits

“…It is probably true (although untested) that within any give province the most Mg-rich olivines are most likely to be associated with ore, but this is not an a priori predictor between provinces. A detailed analysis of olivine compositions in relation to mineralisation is provided by Barnes et al (2022b).…”

“…We take a broadly similar approach to that of Halley (2020) of utilising the typical large multi-element ICP-MS datasets that are now routinely collected during regional exploration programs, asking what kinds of information can be extracted from them, at what scale, and with what degree of confidence. A complementary mineral-chemistry based approach, using the Ni content of olivine, has been assessed in a companion paper (Barnes et al 2022b).…”