Honeymoon Well Geology


The ultramafic sequence occurs below a 50-120 m cover of cemented sands and grits (5-25 m), transported clays and minor basal gravels (0-40 m) and a residual regolith (30-60 m). The residual regolith varies markedly over the ultramafic rocks compared to adjacent lithologies. Over the ultramafic rocks the regolith profile is highly variable and has been variably stripped since its formation (Mitchell, 1997). Where complete it consists of an upper brown, strongly leached, highly porous and partially silicified, goethite-rich upper saprolite, a green-grey clay-rich lower saprolite, and then saprock overlying the bedrock. Within the saprolite over oAC and oMC are hard, highly porous, laterally discontinuous silica-minor goethite layers that range up to 30 m in thickness.

Bedrock Lithologies

Ultramafic Rocks

Where igneous textures are preserved oAC display polygonal-textured olivine pseudomorphs and triple-point junctions with adjacent pseudomorphs and have negligible igneous porosity (see Donaldson and Bromley, 1981). Olivine grain sizes range up to 2.5 cm but are mostly 4-10 mm. Stichtite after chromite forms lobate, porous aggregates at some triple-point junctions. Within oMC olivine pseudomorphs are mostly 3-8 mm across while stichtite content (0.5- 5%) and aggregate size is highly variable. Olivine orthocumulates have a wide variation in igneous porosity and texture. Low porosity orthocumulates generally have coarser, more evengrained olivine pseudomorphs, whereas high porosity rocks show a wide range in olivine grain size, sometimes within the same rock, and with grain shapes ranging from euhedral to hopper to harrisitic. These latter rocks are mostly associated with spinifex-textured rock sequences. At a few localities, delicate igneous textures are very well preserved within 20-40 m thick sequences of thin spinifex-textured flows. Mostly, however, deformation and metamorphic recrystallisation has destroyed spinifex textures, with former thin flows now represented by interlayered tremolite-chlorite and serpentine-tremolitechlorite schists or talc-carbonate altered equivalents.

Some spinifex-textured rocks and associated oOC show evidence of partial melting and high-temperature recrystallisation. In such rocks textures and mineral compositions differ markedly from those in typical komatiites (Gole et al., 1990). Spinifex olivine plates are bent and interstitial spaces are composed of polygonal aggregates of partially altered clinopyroxene, orthopyroxene, minor olivine, plagioclase and spinel. Two pyroxene thermometry yields temperatures of 1055 to 1140ºC, just below the low pressure komatiite solidus ( Thy, 1995). These features reflect metamorphism, partial melting and subsequent slow cooling within a 40-60 m thick sequence of spinifex-textured flows that are located between two thick stratigraphic sequences of olivine-rich cumulates. Thermal modelling shows that post-magmatic conductive cooling of these two cumulate piles, if they were formed within a period of about ten years, would re-heat the intervening flows to the observed metamorphic temperatures (Gole et al., 1990). These very distinctive rocks, which require a very restrictive set of circumstances to form, are part of an horizon, termed the Recrystallised Marker Horizon (RMH; Fig. 1), that is critical in the stratigraphic reconstruction of the Honeymoon Well ultramafic complex.

No traceable stratigraphic layering has been recognised within the main mass of oAC but has been recognised in various states of preservation within the marginal ultramafic sequences (Fig. 1). Strain within the ultramafic rocks has been markedly heterogeneous with widespread preservation of igneous textures in massive-textured rocks with foliated and shear fabrics restricted to relatively discrete zones. However most rocks have a well developed network of anastomosing veins, many of which appear to be minor faults. It is thus likely that the bulk strain of rock units, even those with well preserved igneous textures, will be high.

Many of the talc-carbonate rocks are foliated reflecting penecontemporaneous alteration and deformation within fault/shear zones permeable to CO2-bearing fluids. Lizardite + graphite schists reflect alteration and deformation within H2O-rich fluid regimes.

Rock Compositions

Typical compositions of the main ultramafic rock types are given in Table 1. Variations in composition between oOC, oMC and oAC largely reflect differences in igneous porosity (ie. olivine packing density). The MgO/(MgO+FeO) ratio of oMC and oAC varies widely from ~0.7 to 0.95, but the extent to which this reflects differences in original olivine composition or effects of metasomatism during serpentinisation is difficult to determine on current data. The low CaO value in the oMC is due to loss during serpentinisation as is common in such rocks (Donaldson, 1983).

The compositions of rocks containing fine-grained random olivine spinifex and flow-top breccia sampled from all the Honeymoon Well deposits are generally similar, with anhydrous MgO contents of 27-29% (Table 1). Spinifex-textured rocks and spatially associated, high porosity oOC at Honeymoon Well commonly have very significant S values due to metasomatic addition.

Other Lithologies

Rocks in contact with, and in places included within the ultramafic complex consist of a wide range of volcanic rocks that include tholeiitic basalt and related gabbro, andesite, dacite and minor rhyolite as well as intermediate and minor felsic volcaniclastic rocks (Harrison, 1995; Russell, 1995). Minor pyritic black shale and chert are also present. Within the intermediate to felsic rock sequence immediately east of Harrier are discontinuous, thin layers of Ni sulphide-bearing peridotitic komatiite. The stratigraphic significance of these is unknown. The basalts and gabbros are composed of actinolite, albite, chlorite, quartz, and leucoxene (Donaldson and Bromley, 1981). Some gabbros contain metamorphic magnetite formed by alteration of igneous ilmenite and are highly magnetic. The andesitic and dacitic rocks contain feldspar, quartz, chlorite, mica, actinolite and carbonate in widely varying proportions. Metasomatised rocks contain abundant carbonate, mica and minor arsenopyrite and tourmaline.

Lithological Distribution

Coarse-grained oAC forms the core and makes up the bulk of the komatiite complex, with oMC, oOC, spinifex-textured rocks and minor, medium-grained oAC occurring around the margins and as narrow horizons within the complex (Fig. 1). Dips of lithological units and structures are highly variable ranging from 30º to vertical.

Most contacts between major rock units, both within and bounding the ultramafic complex, are faults. There are two very broad types: a) discrete faults defined by 5 - 20 cm wide mylonite zones with well preserved igneous-textured rocks on either side and with little associated carbonate alteration. These are interpreted to be relicts of D1 thrust faults: b) foliated and brecciated zones, highly variable in width and definition, that appear to be contemporaneous with metamorphic alteration. Some of these zones, particularly those along the margins of the ultramafic complex, are associated with intense carbonate alteration. Some appear to be strike slip faults and some may be reactivated D1 thrust faults. Within ultramafic rocks these faults may change along strike or down dip from relatively well-defined shear zones into a wide, poorly-defined anastomosing network of veins and small shears.

The Harrier and Corella sulphide deposits are interpreted to be hosted by equivalent stratigraphic sequences. This correlation is based on several features. In terms of host lithology, mineralisation style and sulphide compositions (Table 2) the deposits are very similar. Secondary they are linked by patchy zones of bedrock disseminated Ni sulphides. Further, to the west and stratigraphically overlying these deposits, is the distinctive RMH, a discontinuous horizon not only containing recrystallised spinifex-textured flows but also intermediate to mafic volcanic rocks and minor sedimentary units (Fig. 1).

The two main ultramafic horizons in the south of the prospect are thought to be stratigraphic equivalent units that have been duplicated by a strongly foliated, talc-carbonate altered D2 strike slip fault located along the east side of the Harrier deposit and extending northwards along the eastern contact of the komatiite complex (Fig. 1).

The host sequences of the Hannibals - Harakka and Wedgetail deposits and other weakly mineralised spinifex-textured sequences occur as thin, fault-bounded units along the western ultramafic contact (Fig. 1). These are interpreted to be D1 thrust slices. Based on the presence of Ni sulphide mineralisation, the similarity in sulphide composition between the Hannibals and Harrier-Corella deposits (Table 2) and particularly the presence within the Hannibals deposit of recrystallised spinifex rocks (see below), it is thought that these fault slices were derived from lateral equivalents to the Harrier-Corella mineralised horizon.

Augite and augite-plagioclase cumulates, tremolitechlorite rocks (after spinifex-textured rocks) and high porosity, fine-grained olivine orthocumulates located along the western contact of the south east ultramafic sequence appear to define the stratigraphic top of the main Honeymoon Well ultramafic unit. Similar ultramafic and mafic rocks together with felsic rocks that form a structurally disrupted block within the northern part of the ultramafic complex are also probably remnants of this upper stratigraphic contact. This top unit is, however, not preserved elsewhere within the ultramafic complex.

The thickness of the oAC in the central and northern part of the prospect is thought to be a result of repetition by D1 thrust faults of an originally extensive and thick oAC body. The abrupt northern termination of the ultramafic complex, where the width changes from 3 km to 0 m over 1.5 km of strike, occurs at the intersection of these thrust faults with a D2 strike slip fault that defines much of the eastern ultramafic contact (Fig. 1). No ultramafic rocks are present along strike for at least 5 km to the north of the Honeymoon Well complex.