Ultramafic Rock Mineralogy

Olivine adcumulate and oMC do not retain igneous minerals except for minor to trace amounts of partly altered chromite that may be present within stichtite aggregates (see also Donaldson and Bromley, 1981). The dominant metamorphic assemblages are lizardite - pyroaurite - stichtite - magnetite + brucite + chromite and lizardite - brucite - pyroaurite - stichtite (i.e. magnetiteabsent). This latter assemblage, which occurs in zones 20 m wide up to areas 0.5 x 1.5 km in extent, is highly unusual in serpentinised olivine-rich cumulates. Pyroaurite includes other sjogrenite group minerals, particularly iowaite (Donaldson and Bromley, 1981). Antigorite - carbonate - magnetite + stichtite and particularly talc - carbonate + magnetite assemblages, although they may dominate in large volumes of rock, are mostly associated with faults and shear zones particularly where these occur along ultramafic/country rock contacts. Olivine orthocumulate and most spinifex-textured rocks consist of variable proportions of serpentine (mostly antigorite), tremolite, chlorite, carbonate, magnetite and relict igneous chromite. Chromite generally occurs as equant grains, rimmed and veined by magnetite although partly skeletal chromites are present within osOC at Wedgetail and Corella. In places oOC contain minor igneous kaersuitite and rarely clinopyroxene.

Recrystallised spinifex-textured rocks and associated oOC generally retain coarse-grained pyroxenes and, except in some rocks from drill hole 74HWD003 (Gole et al., 1990), only rarely retain other minerals from their high temperature metamorphic assemblage, these minerals being altered during regional metamorphism. Augite and augite-plagioclase cumulates may retain igneous clinopyroxene but mostly consist of tremolitechlorite and actinolite-albite-epidote-chlorite-leucoxene assemblages respectively.

In the ultramafic rocks the presence of pseudomorphic lizardite, antigorite and talc-carbonate assemblages indicates that these formed directly from the igneous mineralogy (Wicks and Whittaker, 1977). Carbonate-rich fluids appear to have been spatially related to relatively discrete permeable zones. Non-pseudomorphic (i.e. igneous texture destroying) antigorite-carbonate is commonly superimposed on pseudomorphic lizardite assemblages and occurs in selvedges to carbonatebearing veins, shears and faults. Talc-carbonate rocks are mostly non-pseudomorphic suggesting that many of these assemblages also developed in already altered rocks. This assemblage is, however, commonly spatially associated with faults and shears, movement of which would cause recrystallisation and thus also result in formation of non-pseudomorphic textures. T h e s e relationships and the spatial distribution of assemblages suggest that, over time, carbonate fluids expanded away from initially relatively restricted CO2 fluid pathways. In restricted areas, for example at Wedgetail and the central parts of Harrier, late arsenic - carbonate + gold-bearing fluids have further altered the rocks.

The lizardite-dominant rocks in particular have a high vein density. At Honeymoon Well vein densities, measured as veins (>1mm wide) per metre, are on average 10 - 12 whereas at Mt Keith and Yakabindie casual observations suggest values of 1 - 4. Vein minerals consist of pyroaurite, iowaite, brucite, carbonate, magnetite, serpentine, trace amounts of gypsum and, within or near the sulphide deposits, minor Fe, Cu and Zn sulphides. The vein assemblages are markedly Mg - Fe-rich and Si-poor relative to their host rocks and suggest that, in places, extensive metasomatism has accompanied serpentinisation. Pyroaurite - iowaite - brucite-rich and carbonate-rich veins tend to be mutually exclusive and occur in separate zones although individual veins are commonly laminated with different assemblages. In antigorite and particularly talc - carbonate rocks the vein mineralogy is less complex being dominated by carbonate and magnetite.

Sulphide Textures

Disseminated Sulphides

In olivine sulphide mesocumulate (osMC) and olivine sulphide adcumulate (osAC) sulphide aggregates were moulded between closely-packed olivine pseudomorphs and hence have a general lobate shape (see Donaldson and Bromley, 1981, for detailed descriptions). Sulphide aggregates in olivine sulphide orthocumulate (osOC) tend to be more blebby and may be relatively coarse-grained (up to 5 mm) due to the greater space between former olivine grains. Metamorphic reconstitution of the rocks has, in many cases, greatly modified the shape and the mineralogy of these aggregates. In rocks that have been strongly recrystallised, and particularly where nonpseudomorphic antigorite - carbonate or talc - carbonate has replaced a large proportion of lizardite, all evidence of former igneous sulphide aggregates has been destroyed and sulphides have been redistributed into irregular, scattered and smaller aggregates. In rocks with relatively low Ni/S ratios sulphide aggregates are commonly veined and partly surrounded by magnetite, whereas in rocks with higher Ni/S ratios magnetite is a minor component or is entirely absent. In these latter rocks heazlewoodite - pentlandite and heazlewoodite-only assemblages occur and sulphide aggregates are commonly not located along former olivine grain boundaries, have ragged to irregular outlines and do not retain igneous lobate shapes. In these rocks igneous textures are rarely preserved due to extensive recrystallisation of serpentine. Sulphide aggregates are also commonly intergrown with antigorite and carbonate. Even in lizarditedominated lithologies (i.e. most osMC and osAC) antigorite needles and carbonate occur along former olivine grain boundaries and in interstitial areas and hence are commonly intergrown with sulphides. In places, antigorite needles and, particularly in osOC, chlorite and talc are strongly intergrown with sulphides such that the sulphides fill angular, wedge-shaped areas between silicate grains.

Massive sulphides

In both massive sulphide and massive sulphide breccia, which are present in the Wedgetail deposit, interlocking anhedral to blocky grains of pyrrhotite, pentlandite, pyrite and minor to trace gersdorffite generally form a massive fabric, and there is only minor development of weakly foliated massive sulphide. Minor chalcopyrite forms small i n t e rg r a n u l a r, irregular shaped grains. Gersdorffite is related to arsenic - gold - carbonate alteration that probably significantly postdates the main regional metamorphism (Hagemann et al., 1995) suggesting that the massive sulphide ores acquired their microtextures very late in the structural/metamorphic evolution of the area.

Supergene Alteration

Supergene alteration of primary sulphide assemblages, similar to that seen in other Ni deposits in the Yilgarn Craton (e.g. Butt and Nickel, 1981) occurs in the upper, very weathered parts of the deposits (Mitchell, 1997) and in patches deep within the deposits. Violarite, pyrite and magnetite are the common alteration products in rocks with a Ni/S ratio below about 1.3. In rocks with Ni/S ratio >1.3 millerite may occur as alteration along grain boundaries and fractures in pentlandite and heazlewoodite or be finely intergrown with violarite.