Forrestania Group - Cosmic Boy, Digger Rocks, New Morning, Spotted Quoll, Flying Fox, Lounge Lizard
Western Australia, WA, Australia
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The Forrestania group of Ni-Cu deposits are located approximately 400 km east of Perth, 210 km south-west of Kalgoorlie and 150 km SSE of Southern Cross in the Forrestania Greenstone Belt which is some 150 to 200 km west of the main Norseman Wiluna Greenstone Belt that hosts many of the other significant Ni deposits of the Yilgarn Craton (#Location: 32° 25' 16"S, 119° 41' 28"E).
The 2.9 Ga, 300 x 40 km Forrestania Greenstone Belt is a SSE trending continuation of the Southern Cross Greenstone Belt which lies to the north. It is bounded by Archaean granitoid and gneisses, intruded by less deformed granite and pegmatite and cut buy east-west trending Proterozoic dolerite dykes.
The greenstone pile comprises a lowermost sequence of of tholeiitic basalt with up to six ultramafic members after komatiites, and numerous thin banded iron formations and chert units. This sequence is overlain by psammitic to pelitic schists that are found in the core of a regionally north-plunging syncline. Dips are moderate to steep and locally overturned, with only the western ultramafic belt dipping east. The sequence has been subjected to upper amphibolite facies metamorphism and multiple (at least three) deformation events. Thin komatiites have been recrystallised to assemblages of tremolite, chlorite, serpentine, anthophyllite, enstatite and metamorphic olivine. The thicker adcumulate to mesocumulate komatiites have preserved cores of original olivine cumulate that comprise bladed to granular metamorphic olivine, serpentine, talc, anthophyllite and enstatite. Basalt is recrystallised to amphibolite, while the banded iron formations are well preserved. The psammitic and pelitic sequence has been metamorphosed to quartz-muscovite-sillimanite schists.
There are four main areas of mineralisation along the trend of the greenstone belt, namely: Cosmic Boy, Digger Rocks, New Morning and Flying Fox - Lounge Lizard, and numerous areas of disseminated mineralisation. Other occurrences in close proximity to and along strike from the main deposits/zones include Spotted Quoll, Liquid Acrobat, New Morning, Mt Hope, Beautiful Sunday, Carr Boyd, Seagull and Rat Rat. All of the deposits occur within the Eastern and Western ultramafic belts, mostly in the lowermost of the two main ultramafic units. Cosmic Boy and Digger Rocks occur in the Eastern ultramafic belts, while Flying Fox, Spotted Quoll, New Morning and Beautiful Sunday are in the Western ultramafic belt.
Flying Fox Zone
The Flying Fox deposits are found around 30 km NW of Cosmic Boy, within the centre of the Western ultramafic belt, where it dips at around 40°E and comprises a well layered, strongly foliated succession of intercalated ultramafic flow units, sulphidic cherts, basalts and psammitic to pelitic meta-sediments, all overlying a thick sequence of psammitic schists. The lowermost ultramafic unit, which is up to 50 m thick, is the thickest and most olivine rich of the entire sequence. It is capped by pyroxenite and thin komatiite flows. The succession changes upwards from relatively thin, olivine-rich flows to multiple thin-flow sequences of less olivine-rich flows. The succession is cut by flat lying granitoid and pegmatite dykes. Faults located on the margins of the dykes show a consistent displacement of the lower block to the east.
The Flying Fox mineralisation occurs as a series of high grade massive sulphide lenses of around 0.2 Mt of ore with 5 to 9% Ni. These lenses are within a dipping zone that is around 400 m in strike length, plunging at 25 to 80°E to depths in excess of 1300 m below the surface. Individual lenses are 400 m long and extend around 200 m down dip and are 0.1 to 10 m in thickness. The uppermost lens contained around 0.2 Mt @ 3.1% Ni. The next down-dip is T1, with 0.342 Mt @ 5.1% Ni, followed by T4 with around 0.315 Mt @ 4% Ni, then T5 with approximately 0.55 Mt @ ~1% Ni. Disseminated sulphides with up to 1.5% Ni are found in the basal ultramafic host unit above the massive sulphide and as a low grade halo to the north, south and up-dip.
The upper-most massive sulphides are within the oxidation zone and have been replaced by a supergene assemblage of pyrite-violarite and large pyrite porphyroblasts. The lower massive sulphide lenses are below the supergene zone. The primary sulphide assemblage comprises pyrrhotite-pentlandite±pyrite and chalcopyrite.
Digger Rocks Zone
The Digger Rocks and Diggers South nickel sulphide occurrence are continuous and may be regarded as the same zone. Both are located in the basal portion of a large, complexly zoned ultramafic which is around 5 km long and up to 425 m thick, which comprises a central, 100 to 150 m thick section with some preserved cumulate textures that grades out into bladed-granular forsterite-talc±anthophyllite±enstatite rock. Below the cumulate zone, there is a zonation from forsterite-talc to forsterite-talc-anthophyllite and forsterite-talc-enstatite near the mineralisation. Another 10 to 75 m thick, separate ultramafic known as the 'footwall ultrabasic unit' occurs below the mineralisation, composed of alternating forsterite-tremolite-anthophyllite-enstatite rocks.
Nickel sulphide mineralisation appears to be structurally controlled, with footwall rocks varying from banded iron formation in the lower northern part of Digger rocks, to the footwall ultrabasic unit below the main Digger Rocks, while an 8 to 10 m thick tectonic sliver of quartz-biotite-garnet schist is found in the southern part of Digger Rocks and Diggers South. Hanging wall rocks comprise tholeiitic amphibolites.
Mineralisation occurs over strike length of 1200 m and up to a width of 125 m, comprising both massive and disseminated matrix sulphides, which is best developed towards the northern end of the zone, the Main Digger Rocks zone. The Main Digger Rocks zone is around 240 m in strike length and up to 125 m thick, dipping at 50 to 65°W and pitching at 50°N. A typical cross section of this mineralisation comprises a lower massive nickel sulphide accumulation up to 10 m thick containing 6 to 7% Ni, overlain by 10 to 40 m of heavily disseminated to matrix mineralisation with 0.8 to 2.5% Ni, which is in turn overlain by 30 to 80 m of disseminated mineralisation containing 0.4 to 1.0% Ni. The southern two thirds of the overall mineralised interval, Diggers South, comprises disseminated pyrrhotite-pentlandite-pyrite-chalcopyrite that forms a homogeneous mass ranging from 9 to 25 m in thickness, dipping steeply west. This latter interval is similar to Cosmic Boy, but is richer in pyrrhotite.
The bulk of the higher grade portions of the Digger Rocks lens lies between 30 and 80 m below the surface, directly below a siliceous weathering cap and is strongly influenced by supergene alteration characterised by an Fe-rich assemblage of pyrite-violarite-pentlandite-pyrrhotite-magnetite with a variable Ni:Cu ratio. Most of the massive sulphide mineralisation occurs at the contact with the 'footwall ultrabasic unit', although an appreciable portion occurs as cross-cutting stringers of pyrrhotite-pentlandite-pyrite that cut matrix mineralisation and extend for up to 20 m into the footwall banded iron formation. The disseminated sulphides in the Main Digger Rocks zone consists of pyrrhotite-pentlandite-pyrite with variable supergene alteration to pyrite-violarite. An additional 200 m long by up to 20 m thick Ni-sulphide zone is found in the hanging wall, within the central cumulate ultramafic, comprising disseminated pentlandite-millerite with up to 1% Ni, which is intergranular to the cumulus olivine grains.
Cosmic Boy Zone
This deposit occurs within the Eastern ultramafic belt in an interval where the generally west-facing ultramafic belt is characterised by relatively thin (<60 m thick), laterally extensive ultramafic units with intercalated banded iron formation and tholeiitic basalts which dip at ~50°W. The orebody is found on the basal, eastern contact of the lowermost ultramafic unit. The lowermost ultramafic unit is composed of several, up to 900 m long, discrete lenses of olivine mesocumulate which are linked by differentiated flow units that are up to 50 m thick. Other smaller, relatively minor olivine mesocumulate lenses occur in the lowermost ultramafic unit and within most other ultramafic units. The ultramafic sequence grades upwards to the west to more evolved, less olivine rich, thin spinifex-textured komatiite flows.
The deposit occurs as two parallel ore zones. The basal orebody is ~800 m in strike length and extends down dip for more than 500 m at the base of a 40 to 60 m thick olivine mesocumulate, overlying a prominent BIF unit. The up- and down-dip peripheries are the result of thinning of the sulphide zone, while the lateral margins to the north and south are faulted. The sharp footwall is defined by a fault with the underlying BIF, while the upper margin is marked by a reduction in Ni grade to the background 0.3% Ni of the 'barren' ultramafic.
The primary sulphide assemblage comprises a strong dissemination of pyrrhotite-pentlandite±pyrite and chalcopyrite to between 20 and 40% sulphides by volume, and Ni grades of 0.7 to 5.0%. Violarite is a significant component in the supergene zone. Sulphides form either triangular domains within bladed metamorphic olivine-talc grains and as partially connected networks around silicate minerals. No massive sulphides are found in the basal orebody.
The upper orebody occurs as a complex series of thin, discontinuous lenses of disseminated sulphides hosted by a 10 to 20 m thick olivine mesocumulate unit, close to the 'upper' contact of the second ultramafic unit, which may be an isoclinal repetition of the lower ultramafic unit.
The total reserves/resources at Forrestania in 1981 were 10.8 Mt @ 2.46% Ni, or 14 Mt @ 1.3% Ni.
Reserve and resource figures in mid 2008, as quoted by Western Areas NL, 2008, were:
Flying Fox + Diggers - 3.307 Mt @ 2.8% Ni
Flying Fox Zone:
Flying Fox - 2.307 Mt @ 4.7% Ni,
New Morning/Daybreak - 2.144 Mt @ 1.4% Ni,
Spotted Quoll - 0.545 Mt @ 6.3% Ni,
Beautiful Sunday - 0.480 Mt 1.4% Ni
Total Flying Fox Zone - 5.477 Mt @ 3.3% Ni
Cosmic Boy Zone - 0.376 Mt @ 2.4% Ni
Diggers South Core - 3.000 Mt @ 1.48% Ni,
Diggers South Halo - 4.8 Mt @ 0.74% Ni,
Digger Rocks Core (indicated) - 0.055 Mt @ 1.1% Ni,
Digger Rocks Core (inferred) - 0.172 Mt @ 1.1% Ni,
Digger Rocks Halo - 1.441 Mt @ 0.7% Ni,
Total Diggers Zone - 10.028 Mt @ 1.0% Ni.
Total resource - 15.880 Mt @ 1.8% Ni.
Reserve and resource figures in at 31 March, 2013, as quoted by Western Areas NL, website in 2013, were:
Flying Fox Area - 1.7288 Mt @ 4.0% Ni - Probable
Spotted Quoll - 2.9420 Mt @ 4.2% Ni - Probable
Digger South - 2.0160 Mt @ 1.4% Ni - Probable
Digger Rocks - 0.0930 Mt @ 2.0% Ni - Probable
TOTAL reserves - 6.7798 Mt @ 3.3% Ni - Probable
Flying Fox and Lounge Lizard:
High grade massive ore - 1.7321 Mt @ 5.7% Ni - indicated + inferred
Disseminated ore - 4.9830 Mt @ 0.8% Ni - indicated + inferred
TOTAL Flying Fox and Lounge Lizard - 6.7151 Mt @ 2.1% Ni - indicated + inferred
Massive ore - 0.3218 Mt @ 3.7% Ni - indicated
Massive ore - 0.0931 Mt @ 3.5% Ni - inferred
Disseminated ore - 1.0698 Mt @ 0.9% Ni - indicated
Disseminated ore - 0.6592 Mt @ 0.9% Ni - inferred
TOTAL New Morning/Daybreak - 2.1439 Mt @ 1.4% Ni - indicated + inferred
Massive ore - 2.3795 Mt @ 6.0% Ni - indicated
Massive ore - 0.5397 Mt @ 5.1% Ni - inferred
TOTAL Spotted Quoll - 2.9192 Mt @ 5.1% Ni - indicated + inferred
TOTAL Beautiful Sunday - 0.4800 Mt @ 1.4% Ni - indicated
TOTAL Western Belt - 12.2582 Mt @ 2.8% Ni - indicated
Cosmic Boy Area
Cosmic Boy - 0.1809 Mt @ 2.8% Ni - indicated
Seagull - 0.1950 Mt @ 2.0% Ni - indicated
TOTAL Cosmic Boy Area - 0.3759 Mt @ 2.4% Ni - indicated
Diggers South Core - 3.0000 Mt @ 1.48% Ni - indicated
Diggers South Halo - 4.8000 Mt @ 0.74% Ni - indicated
Digger Rocks Core - 0.0549 Mt @ 1.1% Ni - indicated
Digger Rocks Core - 0.1723 Mt @ 1.1% Ni - inferred
Digger Rocks Halo - 1.4410 Mt @ 0.7% Ni - inferred
Purple Haze - 0.560 Mt @ 0.9% Ni - inferred
TOTAL Diggers Area - 10.0282 Mt @ 1.0% Ni - indicated + inferred
TOTAL Western Areas resource - 22.6623 Mt @ 2.0% Ni.
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 24/6/2013
This description is a summary from published sources, the chief of which are listed below.
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References to this deposit in the PGC Literature Collection:
Collins J E, Barnes S J, Hagemann S G, McCuaig T C and Frost K M, 2012 - Postmagmatic Variability in Ore Composition and Mineralogy in the T4 and T5 Ore Shoots at the High-Grade Flying Fox Ni-Cu-PGE Deposit, Yilgarn Craton, Western Australia : in Econ. Geol. v.107 pp. 859-879|
Collins J E, Hagemann S G, McCuaig T C and Frost K M, 2012 - Structural Controls on Sulfide Mobilization at the High-Grade Flying Fox Ni-Cu-PGE Sulfide Deposit, Forrestania Greenstone Belt, Western Australia : in Econ. Geol. v.107 pp. 1433-1455|
Frost K M, Woodhouse M, Pitkajarvi J T 1998 - Forrestania nickel deposits: in Berkman D A, Mackenzie D H (Ed.s), 1998 Geology of Australian & Papua New Guinean Mineral Deposits The AusIMM, Melbourne Mono 22 pp 365-370|
Marston R J, Groves D I, Hudson D R, Ross J R 1981 - Nickel sulfide deposits in Western Australia: a review: in Econ. Geol. v76 pp 1330-1363|
Porter D J, McKay K G 1981 - The nickel sulfide mineralization and metamorphic setting of the Forrestania area, Western Australia: in Econ. Geol. v76 pp 1524-1549|
Prichard H M, Fisher P C, McDonald I, Knight R D, Sharp D R and Williams J P, 2013 - The Distribution of PGE and the Role of Arsenic as a Collector of PGE in the Spotted Quoll Nickel Ore Deposit in the Forrestania Greenstone Belt, Western Australia : in Econ. Geol. v.108 pp. 1903-1921|
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