Nickel in olivine as an exploration indicator for magmatic NiCu 1 sulfide deposits : a data review and re-evaluation 2 REVISION 2 3

13 Nickel contents of olivine have been widely used as petrogenetic indicators and as fertility indicators 14 for magmatic sulfide potential of mafic-ultramafic intrusions, on the assumption that olivines 15 crystallized from magmas that had equilibrated with sulfide liquid should be relatively depleted in Ni 16 compared with a sulfide-free baseline. This has given rise to a large accumulation of data that is 17 brought together here, along with data on volcanic olivines, to critically evaluate the effectiveness of 18 the approach. We identify multiple sources of variance in Ni content of olivine at given Fo content, 19 including: variability in mantle melt composition due to depth, water content (and possibly source); 20 subsequent fractional crystallization with and without sulfide; recharge and magma mixing; batch 21 equilibration between olivine and sulfide at variable silicate-sulfide ratio (R) and olivine/liquid ratio; 22 and subsequent equilibration during trapped liquid crystallization in orthocumulates. Baselines for Ni 23 in olivine in relation to Fo content are somewhat lower in orogenic belt settings relative to intrusions 24 in continental Large Igneous Provinces (LIPs). This is probably related to differences in initial parent 25 magma compositions, with plume magmas generally forming deeper and at higher temperatures. No 26 clear, universal discrimination is evident in Ni in olivine between ore-bearing, weakly mineralized 27 and barren intrusions even when tectonic setting is taken into account. However, sulfide-related 28 signals can be identified at intrusion scale in many cases. Low-R factor, low-tenor sulfides are 29 associated with low-Ni olivines in a number of examples and these cases stand out clearly. 30 Anomalously high-Ni olivines are a feature of some mineralized intrusions, in part due to trapped 31 liquid reaction effects. However, in some cases, this mechanism cannot account for the magnitude 32 of enrichment. In these cases, enrichment may be due to re-entrainment of “primitive” Ni-rich 33 sulfide by a more evolved Fe-rich magma, driving the olivine to become Ni-enriched due to Fe-Ni 34 exchange reaction between sulfide and olivine during transport. An extreme case of this process 35 may account for ultra-Ni enriched olivine at Kevitsa (Finland), but more subtle signals elsewhere 36 could be positive indicators. A lack of clear mineralized/barren distinction in specific groups of 37 related intrusions, e.g. the deposits of NW China or the Kotalahti Belt in Finland, may well be due to 38 “false negatives” where undiscovered mineralization exists in specific intrusions or in their feeder 39 systems, or may also be due to a multiplicity of confounding factors. Wide variability of both Fo and 40 Ni between related intrusions at regional scale may be a useful regional prospectivity indicator, 41 more than an intrusion-scale discriminant, and is certainly informative as a petrogenetic indicator. In 42 general, the use of Ni-olivine as a fertility tool is more likely to generate false negatives than false 43 positives, but both are possible, and the technique should be used as part of a broader weight-of44 evidence approach. 45