Model for evolution of Honeymoon Well Complex

An interpreted stratigraphic reconstruction of the Honeymoon Well ultramafic complex is shown in Fig.15. The lowermost komatiite unit contains all the known Honeymoon Well Ni sulphide deposits. This correlation is made on the basis of; (a) physical linking of similar stratigraphic packages between Corella and Harrier; ( b) the western sulphide deposits are all hosted in fault blocks with some contacts being mylonites; (c) the presence of significant Ni sulphide mineralisation and the similarity in mineralisation style and composition between the western and eastern disseminated deposits; and (d) the presence of recrystallised spinifex-textured thin flows along the length of the RMH as well as within Hannibals. These latter two points are particularly powerful arguments for the correlation.

It would be expected that magmatic sulphides from temporally different igneous sequences or even perhaps from widely separated localities within the same sequence would have undergone different magmatic histories and thus have dissimilar compositions (eg. Naldrett and Barnes, 1986; Barnes et al., 1997). The very similar compositions, particularly the similar Ni/Pd, Pt/(Pt + Pd) and Pd/Ir ratios (Table 2), of the disseminated deposits thus strongly argues for these sharing the same magmatic history. The recrystallised spinifex-textured rocks require such restricted and unusual circumstances to form (Gole et al., 1990) that these are unlikely to be duplicated within two different sequences in the same general locality. Honeymoon Well is the only known locality for these rocks in the world.

Lateral variations in stratigraphic profiles along this unit reflect crystallisation in different volcanic facies (see Hill et al., 1995, 1996). These range from crystallisation within 5-8 km wide, long-lived turbulent flow channels sustained by very high lava supply rates that form thick, layered olivine-cumulate dominated sequences to periodically emplaced, thin, rapidly cooled lava flows that results in sequences of spinifex-textured flows.

Although Wedgetail is within a fault slice and thus not the complete stratigraphic sequence, and also that the massive sulphide is interpreted to be physically remobilised along a fault, the presence of spinifex- textured flows and of matrix and massive sulphide suggests that in volcanic setting it is generally similar to that of a Kambalda style deposit (Type 1 of Barnes et al., 1994) and is likely to have formed near the flow front of an advancing komatiite flow field or marginal to any regional-scale lava channel way (Hill et al., 1996). Reconstruction of the stratigraphy of the disseminated deposits shows significant differences between the deposits with spinifex-textured flows occurring within the Hannibals mineralised sequence, and relatively thick, laterally persistent oOC and osOC units occurring within the Corella sequence, whereas at Harrier olivine-rich cumulates dominate the mineralised sequence.

These differences may reflect more marginal (at Hannibals) to more central positions (at Corella) relative to a major flow channel. The variable but relatively high modal abundance of sulphide within the Honeymoon Well disseminated deposits suggests that much or all of the magmatic sulphide was physically introduced into the deposits within turbulently flowing silicate lava rather than being the result of insitu olivine-sulphide cotectic crystallisation as proposed at the slightly lower Ni grade deposits at Mt Keith and Yakabindie (Hill et al., 1996). The similarity of the sulphide compositions between Honeymoon Well disseminated deposits suggests that the magma from which the deposits were formed came from a very large homogeneous source.

The mineralised unit is capped by spinifex-textured rocks which are in turn overlain by basalt and andesitic volcanic and minor sedimentary rocks. The base of the unit is not preserved but originally the unit was probably, in places, >250 m in thickness.

The upper komatiite unit formed within a few tens of years after the lower unit (Gole et al., 1990). Its character varies along strike and in the Honeymoon Well area contains a coarse-grained oAC body, originally 500 - 1000 m thick by >6 km long in apparent cross section, formed in a massive komatiite lava channel (Hill et al., 1990, 1995). To the south the oAC body thins into an oOC-dominated sequence whereas to the north proximal lateral equivalents are not preserved. The unit is capped by spinifex-textured flow rocks and in places fractionated sequences of pyroxenite and gabbro. Thermal erosion occurred along the basal contact of this very broad lava flow channel, removing in places the underlying intermediate and mafic lavas and volcanoclastic rocks and part of the top section of the mineralised komatiite sequence. During igneous cooling remnants of this top section were partially melted and metamorphosed to high temperatures (Gole et al., 1990). Some of the scattered disseminated Ni sulphides with Ni grades of 0.35-0.6% Ni within the central oAC body appear to have formed by local magmatic S-saturation and cotectic crystallisation of olivine and sulphide. The pyrrhotite-rich sulphides present in osOC and osMC east of and up-sequence of the RMH at Harrier occur in relatively high modal proportions and must have been physically introduced to their present position as highly fractionated sulphide liquid within strongly chalcophile-element depleted silicate lava.

The ultramafic stratigraphy outlined above is generally similar to that proposed by Barnes et al. (1988) for the Perseverance Ni deposit in the southern part of the Agnew-Wiluna belt. Early deformation of the Honeymoon Well sequence occurred within a D1 thrust duplex with an apparent sense of movement of north block to the south. The geometry of this duplex was probably greatly influenced by the size and competency of the unmetamorphosed oAC body. The oAC unit itself was stacked and thin thrust slices of the lower mineralised ultramafic horizon were emplaced along the top (now western) margin of the duplex.

Folding of the greenstone belt sequence and strike-slip faulting during D2 was accompanied by lower greenschist facies metamorphism. Strike-slip faulting resulted in further duplication of the stratigraphy in the southern part of the Honeymoon Well prospect, again with an apparent sense of movement of north block to the south, as well as removing the northern proximal equivalents of the Honeymoon Well ultramafic horizons.

At Wedgetail the bulk composition of massive sulphide dominantly reflects magmatic processes, and apart from relatively minor redistribution of Cu and the addition of As to parts of the deposit there appears to have been relatively little adjustment of the sulphide bulk composition during metamorphism. The Ni-rich bulk sulphide compositions of the disseminated sulphide deposits are, however, a function of interaction with metamorphic fluids. The sulphide deposits are located adjacent to major faults that have acted as pathways for migration of larg e volumes of fluid. Highly reduced fluids that may form during progressive alteration of ultramafic rocks (Frost, 1985) have caused complete leaching of Cu and partial loss of Fe, S and Zn from parts of the sulphide deposits and of Fe from large volumes of barren ultramafic rocks. In the latter, magnetite is absent from lizardite-brucite-iowaite dominated alteration assemblages resulting in low magnetic susceptibility for such serpentinites present, for example, south east of Wedgetail and east of Hannibals-Harakka (see Fig. 5, Bourne, 1996). In the disseminated sulphide deposits the metasomatism has formed a zoned pattern in Cu content, whole rock Ni/S ratio and primary sulphide assemblages.

The sulphide zones are defined by pentlandite-minor pyrrhotite-chalcopyrite, pentlandite-only, pentlandite-heazlewoodite and heazlewoodite-only assemblages. Zones containing pyrrhotite and chalcopyrite represent the least altered sulphide assemblages whereas heazlewoodite-only assemblages occur in the most strongly leached rocks.