Sulfur isotope compositions in the weathering profile of magmatic Ni-Cu deposits in SW Australia
Authors: Sullivan Kaj , Drummond Justin , Polito Paul , Stoltze Amanda , Leybourne Matthew I. 
Published: March 2022

&lt;p&gt;The Nova-Bollinger Ni-Cu sulfide ore deposit is hosted in layered mafic and ultramafic intrusive rocks of the Mesoproterozoic Albany-Fraser zone, located about 160 km east-northeast of Norseman, Western Australia. Nova and Bollinger are two adjacent but spatially distinct orebodies with a combined pre-mining resource estimate of 13.1 million tonnes (Mt) with about 2 % Ni, 0.8 % Cu, and 0.1 % Co (IGO Ltd., unpublished data, 2018) &lt;sup&gt;1&lt;/sup&gt;. Significant challenges are posed in exploring for magmatic Ni-Cu deposits that are buried under post-mineral cover. For example, electromagnetic and gravity surveys identify numerous targets but are unable to distinguish economic mineralization. Previously, it was suggested that the regular pattern of S isotope compositions (&amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt;) of surficial sulfate in lakes and groundwaters in southern Australia provides an ideal baseline against which to search for anomalous &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; values associated with base-metal or gold mineralization &lt;sup&gt;2&lt;/sup&gt;. In the absence of lakes and readily accessible groundwaters in prospective areas, soils and rocks make a convenient sampling medium. Here, we investigated the exploration potential of &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; of the trace sulfur content of unconsolidated surface sediments, saprolite, and bedrock samples above Nova and two nearby sub-economic prospects, Griffin and Chimera. The &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; values likely reflect a two end-member system, with values ranging from -5.8 at depth to 21.4 &amp;#8240; near the surface, showing little dependence on lithology. Values in samples closer to the surface are similar to modern seawater sulfate that has a globally homogenous &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; value of 21.0 &amp;#177; 0.2 &amp;#8240; &lt;sup&gt;3&lt;/sup&gt;, whereas at depth, values approach typical mantle S isotopic compositions of 0 &amp;#177; 2 &amp;#8240; &lt;sup&gt;4&lt;/sup&gt;. In support of this, rocks at Nova have a &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; of around 0 &amp;#8240; and regional metagabbro are between -2 and 4 &amp;#8240; &lt;sup&gt;5&lt;/sup&gt;. On a regional scale, in both Western Australia (Yilgarn Block) and South Australia, the &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; values of surficial gypsum have a regular pattern over distances of up to 1000 km, with the highest values (~ 21 &amp;#8240;) near coastlines decreasing to &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; values of ~ 14 &amp;#8240; further inland &lt;sup&gt;2&lt;/sup&gt;. This is suggested to be predominantly the result of the delivery of salts to the Australian landscape as aerosols, with volatile biogenic S compounds of mostly marine origin (&amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; of ~ 1 &amp;#8240;) that proportionately increase in importance further inland resulting in decreasing &amp;#160;&amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; values &lt;sup&gt;2&lt;/sup&gt;. Located approximately 200 km inland, &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; results in samples within 10 metres of the surface at Nova, Griffin, and Chimera are in agreement with this and range from 12.6 to 20.4 &amp;#8240;. Given that near-surface &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; values above Nova, Griffin, and Chimera appear to be mostly related to seawater-derived sulfate with minimal magmatic influence, &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; shows little potential as a field sampling technique to vector for deposits buried under post-mineral cover. However, at depth, &amp;#948;&lt;sup&gt;34&lt;/sup&gt;S&lt;sub&gt;CDT&lt;/sub&gt; shows a clear relationship between the mixing of seawater sulfate and magmatic S weathering into the environment, indicating that analysis of S isotopes of otherwise apparently barren cores has utility in mineral exploration.&lt;/p&gt;