The effect of the trisulfur radical ion on molybdenum transport by hydrothermal fluids
Authors: Kokh Maria A. , Laskar Clement , Pokrovski Gleb S. 
Published: March 2021

&lt;p&gt;Knowledge of molybdenum (Mo) speciation under hydrothermal conditions is a key for understanding the formation of porphyry deposits which are the primary source of Mo. Existing experimental and theoretical studies have revealed a complex speciation, solubility and partitioning behavior of Mo in fluid-vapor-melt systems, depending on conditions, with the (hydrogen)molybdate (HMoO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;-&lt;/sup&gt;, MoO&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;) ions and their ion pairs with alkalis in S and Cl-poor fluids [1-3], mixed oxy-chloride species in strongly acidic saline fluids [4, 5], and (hydrogen)sulfide complexes (especially, MoS&lt;sub&gt;4&lt;/sub&gt;&lt;sup&gt;2-&lt;/sup&gt;) in reduced H&lt;sub&gt;2&lt;/sub&gt;S-bearing fluids and vapors [6-8]. However, these available data yet remain discrepant and are unable to account for the observed massive transport of Mo in porphyry-related fluids revealed by fluid inclusion analyses demonstrating 100s ppm of Mo (e.g., [9]). A potential missing ligand for Mo may be the recently discovered trisulfur radical ion (S&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8226;-&lt;/sup&gt;), which is predicted to be abundant in sulfate-sulfide rich acidic-to-neutral porphyry-like fluids [10]. We performed exploratory experiments of MoS&lt;sub&gt;2&lt;/sub&gt; solubility in model sulfate-sulfide-S&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8226;-&lt;/sup&gt;-bearing aqueous solutions at 300&amp;#176;C and 450 bar. We demonstrate that Mo can be efficiently transported by S&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8226;-&lt;/sup&gt;-bearing fluids at concentrations ranging from several 10s ppm to 100s ppm, depending on the fluid pH and redox, whereas the available data on OH-Cl-S complexes cited above predict negligibly small (&lt;100 ppb) Mo concentrations at our conditions. Work is in progress to extend the experiments to wider T-P-composition range of porphyry fluids and to quantitatively assess the role of S&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;&amp;#8226;-&lt;/sup&gt; in Mo transport by geological fluids.&lt;/p&gt;&lt;ul&gt;&lt;li&gt;1. Kudrin A.V. (1989) &lt;em&gt;Geochem. Int. &lt;/em&gt;&lt;strong&gt;26&lt;/strong&gt;, 87&amp;#8211;99.&lt;/li&gt;
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