Supergene alteration of the Mt Windarra nickel sulphide ore deposit, Western Australia

Abstract

Three main zones of progressive oxidation, termed the transition, violaritepyrite and oxide zones, can be delineated in the supergene profile o£ the Mt Windarra massive/matrix ore deposit. In the broad transition zone from pure primary ore, pentlandite is progressively oxidised to an iron rich violarite of composition Co0.0eFel.asNil.60 $4, releasing Fe 2+ and Ni 2+ ions into solution. Up to 43 % of this Ni 2+ moves to nearby pyrrhotite margins which are replaced firstly by nickeliferous smythite and then by a second lamellartextured violarite with an even higher iron content but lacking in cobalt (approximately Fel.6Nil.4S4). On completion of violaritisation of the pentlandite, violaritisation of the pyrrhotite also ceases and the remainder of the pyrrhotite is rapidly replaced by secondary pyrite/marcasite, siderite and void space, this reaction defining the top of transition zone. Both sulphur and nickel are extracted from solution and further Fe 2+ ions are released into solution. The violarite-pyrite zone is characterised by the absence of pentlandite and pyrrhotite and continued stability of violarite and secondary iron disulphides. Most, if not all, of the iron generated by these oxidation reactions precipitates as magnesian siderite at the expense of magnesite, giving rise to solutions containing mainly Mg 2+ and Ni 2+ ions. At and just above the water table atmospheric oxygen is reduced while the sulphides are oxidised to sulphate and hydroxides. Much of the iron remains in situ as characteristic goethite relicts while nickel and copper are leached, producing the enrichment below the water table. The overall genetic model proposed is electrochemical and is analogous to the corrosion of a piece of metallic iron partially immersed in differentially aerated water.

Chemical Constraints on the Weathering of Serpentinites Containing Nickel-Iron Sulphides

“…Eh measurements have shown that the oxidation potential in a nickel sulphide deposit undergoing weathering increases upward (Thornber, 1975), and mineralogical observations have confirmed that the original sulphide assemblage changes to a more highly oxidized one toward the water table (Nickel et al, , 1977aWatmuff, 1974;Thornber, 1975).…”

“…Changes in the sulphide mineralogy (below the water table) have been well documented (Nickel et al, , 1977aWatmuff, 1974;Thornber, 1975).…”

“…Thornber (1972) first suggested that the oxidation of Ni-Fe sulphide orebodies can be interpreted on an electrochemical model, and this suggestion has been borne out by subsequent mineralogical studies Watmuff, 1974;Thornber, 1975). Measurements made on material from Kambalda indicate pH values between 8 and 9, and Eh values as low as -0.3 volt, at depths of 100 m or more below the water table.…”

Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23°N, MAR)

“…In contrast, chalcopyrite is often retained when pentlandite is altered to violarite (Table 4). This may be related to weathering-related aerated oxidizing conditions, in which violarite is typically formed (Watmuff, 1974).…”

Gossans

“…Thornber, 1972;Nickel et al, 1974;Watmuff, 1974;Thornber, 1975a). Such sulphide deposits exhibit the following features that promote deep oxidation: (a) at least one of the primary sulphides is characterized by a high metal: sulphur ratio, which enables oxidation reactions to take place at oxidation potentials below that at which sulphur oxidizes; (b) the massive sulphide assemblage provides a continuous conductor for the electrons generated by the anodic reactions; and (c) the top end of the orebody intersects the water table where there is abundant oxygen for the cathodic reactions to take place.…”