Nickel sulphides occur in two deposit types: disseminated sulphides (trace to 5 modal percent) in olivine-rich cumulates (osMC, osAC, and osOC in the Harrier, Corella, Hannibals, and Harakka deposits) and sulphide-rich rocks (massive sulphide, sulphide breccia and osOC in the Wedgetail deposit) hosted by spinifextextured flows. The Harakka deposit is relatively small and will not be described.
The main deposits are described below in order of their stratigraphic preservation with the Harrier deposit being the best preserved and the Hannibals and Wedgetail deposits occurring in allochthonous fault blocks.
The Harrier deposit occurs in the south of the Honeymoon Well prospect along the eastern contact of the main oAC-bearing ultramafic unit (Fig. 2). The deposit contains 43.0 MT at 0.64% Ni (0.4% Ni cut off, <300 m depth). The nickel sulphides at Harrier are hosted by osMC with minor osOC. The deposit, which is overlain by between 55 - 120 m of overburden, extends along strike for at least 1700 m and varies in width from 30 to 140 m.
To the east, the ultramafic sequence is in fault contact with metamorphosed andesitic to dacitic lavas and volcaniclastic rocks, and thin, discontinuous olivine spinifex-textured komatiite units, in places containing disseminated and massive nickel sulphides. In plan view this faulted contact is sub-parallel to the stratigraphy (Fig. 1) although in cross section it cuts the stratigraphy at highly variable angles (Figs. 3 and 4).
The lower part of the ultramafic stratigraphy has been removed by the eastern boundary fault. The stratigraphy to the west of the fault, however, appears to be relatively intact, and indeed the Harrier deposit appears to be the least structurally disrupted of the Honeymoon We l l deposits. The nickel sulphide-bearing sequence grades up-sequence into barren oMC and then, in places, into oAC. North of 11000N the olivine cumulate pile bifurcates around the RMH (Fig. 2), here composed of spinifex-textured flows, andesitic and minor basaltic volcanic rocks and minor pyritic and cherty sedimentary rocks. This unit is up to 100 m thick and dips 60-75o to the east (Fig. 4), although younging directions derived from the flows are westerly indicating that the sequence is overturned.
Between 10500N and 11000N, where the oAC is relatively thick and is in direct contact with the mineralised sequence, the RMH is absent (Fig. 3). The RMH re-appears south of 10500N as a 50 m thick unit of oOC, minor recrystallised spinifex-textured rocks and bladed metamorphic olivine - pyroxene rocks, the latter similar to those in areas of amphibolite metamorphic facies in the south of the Agnew - Wiluna belt. These relationships suggest that the RMH has been thermally eroded within a 500 m wide channel-like feature by komatiite lava prior to the formation of the oAC. North of 11000N the oAC on the western side of the RMH thickens into the oAC body that forms the core of the Honeymoon Well ultramafic complex (Fig. 1) The nickel-sulphide mineralisation, north of 10700N, is constrained to one osMC unit with minor layers of osOC in the north.
This unit extends to the south where other sulphide-bearing units are also present. These units are separate igneous layers with different bulk compositions. The western-most unit is composed of osAC and is lens-shaped in plan view. It is located in the area where the RMH is absent (Fig. 2), at or near the base of a possible oAC-filled channel. Other mineralised units in this area appear to be igneous layers rather than structural duplicates of the extensive osMC horizon. The true width of the sulphide-bearing horizons is highly variable ranging from 10 m to 140 m. Just to the west of the southern-most part of the RMH, and effectively on the southern margin of the oAC-filled channel way, is a 20 - >100 m thick body composed of osOC (Fig. 2). The sulphides consist of pyrrhotite and minor pyrite and occur intergrown with magnetite in modified magmatic-textured lobate aggregates intergranular to former olivine. These rocks are strongly depleted in nickel such that despite sulphur contents of up to 1.7%, Ni contents are mostly below 1700 ppm (Table 2).
The mineralogy of the Ni sulphide host lithologies is dominated by lizardite-rich assemblages. A significant proportion of the host sequence has, however, been altered by carbonate-bearing fluids resulting in a zoned pattern with lizardite assemblages giving way to antigorite-carbonate assemblages which, in places, envelope talc-carbonate rocks. Most talc-carbonate rocks are strongly recrystallised, massive rocks or are foliated, although rare pseudomorphed igneous textures are present in low strain zones.
The depth to the oxide/supergene sulphide boundary ranges from 70 to 100 m being deepest in the north. The width of the supergene zone is variable being almost non-existent on some sections but up to 65 m thick on 11600N. The transition zone is generally 20 - 30 m thick and again is thicker in the north. In the primary zone the sulphide content of the mineralised ultramafic rocks typically varies between 1 to 5 modal percent, being highest in osOC which may contain >3% Ni. In lizardite-rich osMC sulphides typically form lobate aggregates between former olivine grains whereas in osOC sulphides in the intercumulus space are blebby. In antigorite-carbonate and particularly talc-carbonate rocks strong recrystallisation of gangue minerals has destroyed much of the magmatic texture of sulphide aggregates, and sulphides are commonly intergrown with antigorite blades, talc aggregates or relatively coarse-grained carbonate grains. Arsenic - bearing minerals are patchily associated with these latter rocks.
Within the primary sulphide zone pentlandite + trace chalcopyrite, pentlandite - heazlewoodite and minor heazlewoodite-only are the dominant sulphide assemblages in lizardite and most antigorite-rich rocks. In lizardite-rich assemblages chalcopyrite mostly occurs in veins with pyroaurite, brucite, magnetite and pyrite, and is only rarely present in relict magmatic sulphide aggregates. In talc and some antigorite-rich rocks the sulphides consist of pentlandite, millerite, trace chalcopyrite, pyrrhotite and patchy gersdorffite, niccolite and maucherite.
As in the other disseminated sulphide deposits the mineralisation can be divided into two types based on Ni/S ratio and Cu contents (Table 2): 1.High-Cu mineralisation, containing 40 - 2000 ppm Cu, having relatively low Ni/S ratios (range 0.5 - 1.3) and with the sulphide mineralogy dominated by pentlandite with trace tochilinite, chalcopyrite and in places pyrrhotite. Alteration to violarite and minor pyrite is relatively abundant even deep within the deposit. This mineralisation type is generally similar to that in the Mt Keith and Yakabindie disseminated Ni sulphide deposits further south in the Agnew - Wiluna belt (Burt and Sheppy, 1975; Hill et al., 1990). 2.Low-Cu mineralisation containing 0 - 40 ppm Cu, having high Ni/S ratios (range 1.2 - 3.0) and with the sulphides being pentlandite and heazlewoodite with minor alteration to millerite.
In rocks with high Ni/S ratio magnetite is absent from the mineral assemblage. Most samples of this type contain no detectable Cu (detection limit of 5 ppm) despite Ni grades of up to 2% and elevated Co and PGE. This ore type forms about 40% of the Harrier resource. These mineralised types form large coherent blocks within the deposit (Fig. 5). There are slight differences in the gangue mineralogy between these two ore types. The high-Cu ore is commonly hosted in dark green lizardite- rich assemblages and some antigorite - carbonate assemblages with veins containing a relatively high proportion of carbonate. The low-Cu ore is mostly hosted by dark to very light green lizardite-rich rocks containing a low proportion of antigorite-assemblages and the vein mineralogy is dominated by brucite and pyroaurite/iowaite. Based on Cu, the contact between ore types is sharp (over 2 - 6 m) whereas other geochemical differences (S, Fe) are gradational (over 5 - 50 m). These geochemical gradients are reflected in a zoned pattern in primary sulphide + magnetite assemblages of Ni-poor pentlandite - chalcopyrite + pyrrhotite-magnetite (Ni/S ratio <0.8), pentlandite + chalcopyrite + magnetite, (0.8 - 1.3), Ni-rich pentlandite - heazlewoodite + trace magnetite (1.3 - 2.0) and heazlewoodite + rare magnetite (2.0 - ~3.0).
The Corella deposit is located along the north east contact of the Honeymoon Well ultramafic complex. The deposit contains 53.5 MT at 0.62% Ni (0.4% Ni cutoff, <300 m depth) and extends for 1400 m with widths of 20 to 100 m (Fig. 6). The Corella sequence is composed of a wide variety of lithologies including oAC, osAC, oMC, osMC, high porosity osOC and oOC and minor spinifextextured flows. These flows have been recrystallised under high temperature metamorphic conditions (Gole et al., 1990) and together with minor andesitic and basaltic volcanic rocks form the RMH. Spinifex-textured flows within this horizon show both westerly and easterly younging directions. The latter are thought to be due to local folds associated with shear zones.
The ultramafic sequence is strongly deformed and sheared with many lithological units, including mineralised lithologies, occurring as fault-bounded boudins on various scales. Lithologies interdigitate as a result of both original stratigraphic relationships and structural dislocation. Because of the shearing the density and thickness of veins are significantly higher at Corella than at the other deposits.
Between the shears and faults within the ultramafic rocks, igneous textures are generally well preserved. The metamorphic mineralogy of the osOC and oOC consists dominantly of pseudomorphic antigorite - carbonate with minor chromite, tremolite, chlorite and rarely kaersuititic amphibole. The originally more olivine-rich rocks are mostly altered to lizardite - brucite - stichtite-bearing assemblages. In places these assemblages are altered to non-pseudomorphic antigorite - carbonate and talc - carbonate assemblages. Talc - carbonate alteration occurs mostly along the eastern ultramafic fault contact and is generally outside the limits of the Ni sulphide resource.
The ultramafic sequence is sub-vertical over the N-S extent of the deposit although in the south, the deeper parts of the sequence dip to the west (Figs. 7 and 9). To the east the ultramafic rocks are in fault contact with basalts and minor gabbros in the northern part, and with andesitic rocks to the south (Fig. 1 and 6). In plan this fault is sub-parallel to the mineralised sequence. In cross section however the fault has variable attitudes. In the north it dips to the west and truncates the mineralised sequence at depth (Fig. 8), and in the south it dips to the east away from the mineralisation (Fig. 7). Due to this fault the lowermost part of the ultramafic stratigraphy is missing. Within the preserved part of the ultramafic sequence the interpreted stratigraphy is, from east to west, barren oAC, the mineralised sequence of osAC, then osMC with overlying osOC and oOC.
This sequence was originally capped by spinifex-textured flows and mafic to intermediate volcanic rocks that form the RMH. Up-sequence and to the west of the RMH is minor oMC, then medium-grained oAC grading into the very coarse-grained oAC that forms the central core of the Honeymoon Well ultramafic complex. T h i s stratigraphy is very similar to that at the Harrier deposit. The patchy distribution of the RMH along the eastern side of the Honeymoon Well complex (Figs. 1 and 6) is a function of probable extensive thermal erosion by komatiite magma from which the overlying oAC unit formed and also later structural dislocation.
The mineralised sequence varies along strike being dominated by osOC and osMC in the north to osAC and minor osOC in the south (Figs. 6, 8 and 9). To the north the sequence thins and grades into barren oAC whereas to the south the mineralised sequence is truncated by a NW trending fault. The depth to the oxide/supergene boundary is 55 to 75 m, and is the shallowest of the deposits because the transported clay layer is absent over most of the deposit. The thickness of the supergene zone is mostly 10 m or less. In the central and northern parts of the deposit, transition zone sulphide assemblages occur in patches and zones throughout the deposit, even at the deepest levels. In the south the transition zone occurs only as a narrow horizontal layer below the supergene zone and at depth the sulphides are almost exclusively primary sulphides.
The thicker mineralised parts of the deposit in the area of 16100-16200N and 16700-17000N appear to result from stratigraphic repetition by faulting, presumably related to early thrusting. Although the thicker parts of the mineralisation appear to form coherent blocks, in detail a combination of original stratigraphy and deformation has resulted in numerous small, discontinuous lenses of mineralisation occurring in otherwise barren rocks. This is particularly the case on the sections south of about 16400N. Nickel grades vary from 0.35 to >2.5% being highest in the osOC. As at Harrier both high-Cu and low-Cu mineralisation types are present although the proportion of the low-Cu type is relatively low.
The Hannibals deposit is located along the western contact of the Honeymoon Well ultramafic complex within a fault-bounded, west-younging sequence of oMC and minor oAC, oOC and spinifex-textured rocks that can be traced for about 2.4 km along strike (Fig. 1). This same sequence hosts the Harakka deposit. The deposit contains 36.1 MT at 0.70% Ni (0.4% Ni cut off, <300 m depth). It is divided into western and eastern overlapping fault blocks (Fig. 10), which are stratigraphic equivalents repeated along a central fault zone (probably a reactivated D1 thrust fault). The eastern boundary of the eastern fault block and much of the western block are against the central oAC along a major, discrete, planar fault with mylonite fabrics preserved in places. This fault transgresses the mineralised sequence at a low angle to the stratigraphy and is interpreted as a relatively well preserved D1 thrust fault.
To the west the mineralised sequence is in fault contact with a sequence of tholeiitic basalt flows, some of which have doleritic to gabbroic centres, and minor interflow carbonaceous and pyritic shale. The ultramafic rocks along this contact are mostly schists derived from spinifex-textured flows. In places spinifex textures are preserved and indicate both easterly and westerly younging directions. Flows further from the contact are consistently west-younging and the easterly directions are due to folding along the faulted contact.
In the eastern block the stratigraphy of the sulphidebearing sequence is relatively intact and igneous layering is sub-vertical (Fig. 11). A high Ni-grade core within the mineralisation plunges in a general SE direction. Thin oOC and spinifex-textured rock units occur lateral to the high grade core suggesting that the sulphide shoot fills a possible 30-80 m deep, 150 m wide channel. In the western fault block the lithological distribution appears to be more complicated due to possible layerparallel faulting. The block extends southward for several hundred metres as a narrow (20-60 m wide) sequence between the central fault and metabasalts to the west. A high Ni-grade core appears to be sub-horizontal within the mineralised sequence which thins dramatically to the north and south. The mineralised sequence bottoms at depth against the folded, western metabasalt/ultramafic fault contact and is truncated on the east by a fault contact with oAC (Fig. 11 ) . Recrystallised spinifex-textured rocks and associated fine-grained oOC occur in fault slices (Fig. 10) that are interpreted to be derived from the RMH.
Mineralisation consists of disseminated sulphides containing trace to ~3 modal percent sulphide with Ni grades ranging from 0.35 % to 2.4%. The sulphide-bearing units are variously preserved igneous horizons with gradational to sharp contacts with barren or weakly mineralised oMC. Many of the sharp mineralisation, contacts are on minor faults. Down dip and along strike from the central portions of the mineralisation the sulphide layers thin and generally become lower grade and interdigitate with barren rocks. The depth to the oxide/supergene boundary ranges from 60 to 104 m and the supergene zone averages 15 m thick.
Most sulphides typically occur as scattered aggregates enclosed by oxide, carbonate and silicate gangue minerals. Sulphide aggregates typically have modified lobate shapes which reflects the predominance of mesocumulate-textured host rocks and general lack of strong recrystallisation. Chalcopyrite mostly occurs in veins with pyroaurite, brucite, magnetite and pyrite and is only rarely present in relict magmatic sulphide aggregates.
The mineralisation can be divided into low and high Cu types with the low-Cu ore type forming about 60% of the volume of the deposit. The eastern and western fault blocks have different mixes of these two mineralisation types with the eastern block having a higher proportion of the low-Cu type, hence having a lower overall Cu content and significantly higher Ni/S ratio together with higher lizardite, pyroaurite/brucite and lower carbonate contents compared to the western block.
The Wedgetail deposit is located along the north western margin of the oAC complex. The deposit has a strike length of 1700 m and varies in width from 10 to 80 m (Fig. 12). The deposit contains an estimated 22.9 MT at 1.08% Ni (0.5% Ni cut off, <300 m depth). Of this 2.5 MT at 3.36% Ni represents a high grade zone which includes massive sulphide and massive sulphide breccia mineralisation.
The Wedgetail deposit comprises disseminated and massive sulphides hosted by a north-striking, westyounging, moderate to steep easterly dipping sequence of oOC and spinifex-textured rocks. In the south the mineralised sequence is in fault contact with the central oAC (Fig. 13). At depth and to the north the sequence is separated from the oAC by a fault wedge of felsic volcanic rocks up to 80m thick (Fig. 14). To the west the mineralised sequence is in fault contact with variably deformed felsic to mafic metavolcanic and metasedimentary rocks that typically have a strong subvertical cleavage. North of the inflection in the oAC contact around section 18800N, the mineralised sequence pinches out at depth where the faults that mark the eastern and western contacts join. On some sections a thin layer of breccia massive sulphide extends down dip along the combined fault plane (Fig. 14). South of 18800N the sequence is open below the depth of drilling (300-500 m vertical depth).
Spinifex-textured rocks within the mineralised sequence usually occur as small patches and zones generally close to the western contact. Complete flow profiles are mostly not preserved, although rarely there is sufficient textural preservation in A-zones to allow a younging direction to be determined. However at depth in the southern part of the deposit a 30 m thick sequence of very well preserved spinifex-textured flows is present that indicate a younging direction to the west, i.e. the sequence is overturned (Fig. 13). Mineralised rocks occur above and below the flows which grade laterally into osOC, probably reflecting a change from crystallisation within a channel way (osOC) to a temporary overbank environment (rapidly cooled thin flows). Similar lateral changes occur within disseminated mineralisation at the Perseverance nickel deposit (Barnes et al., 1988). It must be born in mind, however, that Wedgetail occurs in a fault block, and as such is only a slice of the original stratigraphic sequence. Olivine orthocumulates and osOC mostly contain nonpseudomorphic antigorite assemblages with only minor preservation of lizardite assemblages. Talc-carbonate assemblages are generally confined to contacts although much of the southernmost part of the mineralised sequence is altered to assemblages containing talc, carbonate, and chlorite.
Nickel grades within the mineralised sequence generally increase from east to west, with massive sulphide present in places along or close to the western faulted contact. Sulphide breccia, which comprises 30-90% sulphide and contains 1-20 cm sized, angular clasts of ultramafic and foliated country rock, also occurs along this contact. The massive sulphide/sulphide breccia varies in thickness from a few centimetres to several metres (maximum 12 m). No massive sulphide is recognised as being in its original stratigraphic position, all being thought to have been remobilised along faults and shears. The base of the oxide zone ranges from 70 to 90 m with the underlying supergene zone extending to 117 to 204 m depth. The transition zone ranges from 178 to 265 m depth with some deeper patches along fractures and faults.
Primary zone sulphides consist of pyrrhotite, pentlandite, pyrite, minor chalcopyrite and trace gersdorffite. Massive sulphide consists of interlocking polygonal shaped, relative coarse grains (up to 1.5 mm) of pyrrhotite and pentlandite, with pyrite in some samples. Chalcopyrite forms small intergranular, irregular shaped grains. A preferred alignment of grains or compositional layering is only rarely observed in the massive sulphide. In osOC the sulphides are strongly intergrown with metamorphic gangue minerals and form irregular aggregates that mostly do not show typical magmatic shapes.
Very minor low-Cu ore, comprised of pentlandite and heazlewoodite, is restricted to low grade zones along the eastern contact of the mineralised sequence. Nickel/sulphur ratios within Wedgetail mineralisation are significantly lower than those of the other Honeymoon Well deposits, reflecting the Fe-S-rich nature of the sulphide assemblages (Table 2). Massive sulphide exhibits Ni/S ratios of 0.17-0.4, whereas ratios for osOC range from 0.2-0.9, with only very minor areas of higher values.
Minor to trace sulphides occur as layers and patches 1- 20 m thick in places within the main mass of oAC. Some of these disseminated sulphides form lobate aggregates between former olivine grains and appear to be modified igneous sulphides. Others, however, occur as ragged grains within olivine pseudomorphs and, although they may be associated with elevated whole rock Ni values (mostly 0.3-0.45%, rarely higher), do not have elevated Cu, Pt or Pd. Such sulphides appear to be of metasomatic origin, forming during serpentinisation from Ni and Co released from altered olivine and from S introduced with the metamorphic fluids. In places rocks containing such sulphides have Ni grades above that of barren rocks suggesting some loss of Mg and other major element components during serpentinisation to allow residual enrichment of barren Ni values to these slightly elevated values. Within and around the margins of the Ni sulphide deposits are veins containing trace to 90% sulphide. Vein thickness ranges from 0.5 mm to rarely >1 m. The veins mostly occur in lizardite-rich rocks and apart from sulphide contain pyroaurite, brucite, magnetite, lizardite, chrysotile and rarely antigorite and carbonate. The vein mineralogy suggests that they are part of the greenschist facies regional metamorphic and metasomatic alteration of the ultramafic rocks.
In lizardite-dominant mineralisation a high proportion of chalcopyrite occurs within such veins. Around the margins of deposits the primary zone vein sulphides are pyrrhotite, pyrite, tochilinite, chalcopyrite, trace pentlandite and sphalerite. The distribution of the sulphide-bearing veins around Ni sulphide deposits appears to be highly erratic.