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Chapter 4.4 Paleoarchean Mineral Deposits of the Pilbara Craton: Genesis, Tectonic Environment and Comparisons with Younger Deposits

Chapter 4.4 Paleoarchean Mineral Deposits of the Pilbara Craton: Genesis, Tectonic Environment and Comparisons with Younger Deposits

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412



Chapter 4.4: Paleoarchean Mineral Deposits of the Pilbara Craton



4.4-2. Paleoarchean Geological Evolution of the Pilbara Craton



413



Fig. 4.4-1. Generalised geological map of the Pilbara Craton showing the location of mineral deposits formed before 3200 Ma. MB = Mallina Basin. MCB = Mosquito Creek Basin. SSZ = Sholl

Shear Zone.



4.4-2. PALEOARCHEAN GEOLOGICAL EVOLUTION OF THE PILBARA CRATON

The Pilbara Craton (Fig. 4.4-1) has a geological history that spans over 800 million

years, from ca. 3655 Ma, the age of a tonalitic gneiss in the Warrawagine Granitic Complex,

to ca. 2830 Ma, the youngest age of granites of the Split Rock Supersuite (Van Kranendonk et al., 2006a, 2007a, this volume). Supracrustal rocks of the craton were deposited in

two supergroups, from at least 3515 through to 3165 Ma (Pilbara Supergroup), and from

3020 Ma to 2930 Ma (De Grey Supergroup). The Pilbara Craton is unconformably overlain by the volcano-sedimentary Mount Bruce Supergroup, with an age of 2775–2450 Ma

(Nelson et al., 1999), giving a total geologic history in the Pilbara region of over one billion

years. For a detailed geologic history of the Pilbara Craton, see Van Kranendonk et al. (this

volume).



4.4-3. PALEOARCHEAN MINERAL DEPOSITS OF THE PILBARA CRATON

Of all Paleoarchean terrains, the Pilbara Craton is perhaps the best mineralized. In addition to barite deposits, Paleoarchean rocks in the Pilbara Craton contain lode-gold, VHMS,

porphryry Cu-Mo and epithermal deposits (Fig. 4.4-1). Of these, lode-gold deposits have

been mined historically and, at this writing, feasibility studies are being undertaken on

VHMS and porphyry Cu-Mo deposits. Although not economically important, BIFs are also

present and provide important constraints on the composition of Paleoarchean seawater.

4.4-3.1. Volcanic-Hosted Massive Sulphide Deposits and Stratiform Barite Deposits

The Warrawoona Group is host to the oldest VHMS deposits. Although this group is dominated by basaltic rocks, significant andesitic to rhyolitic intervals are present, including

the ca. 3471–3463 Ma Duffer Formation and ca. 3459–3428 Ma Panorama Formation

(Thorpe et al., 1992a; Nelson et al., 1999; Smithies et al., this volume). The ca. 3490 Ma

Dresser Formation, which contains minor felsic volcaniclastic rocks, hosts the VHMSrelated North Pole barite deposits. The Duffer Formation contains the world’s oldest significant base metal accumulations at the Big Stubby deposit and the Lennons Find district

(Fig. 4.4-1).

4.4-3.1.1. The North Pole barite deposits

The North Pole Dome contains massive barite lenses hosted by the Dresser Formation.

This unit comprises pillowed basalt interbedded with up to five stromatolitic, carbonate-



414



Chapter 4.4: Paleoarchean Mineral Deposits of the Pilbara Craton



Fig. 4.4-2. Geology of the North Pole Dome, showing the locations of the Dresser barite deposits, the

Normay lode-gold deposit, and the Miralga Creek epithermal Au-Ag-base metal prospect. Modified

from Van Kranendonk et al. (2001).



and barite-bearing chert units (Van Kranendonk, 2006). The basal North Pole Chert hosts

barite lenses over an 8 km strike length (Fig. 4.4-2), with the largest barite accumulation

in the southern part of the dome, at the Dresser mine, which was active between 1970

and 1990, with total production of nearly 0.130 Mt of barite (Abeysinghe and Fetherston,

1997). Lead isotope analyses of galena indicate a model age of around 3490 Ma (Thorpe

et al., 1992b).

As the Warrawoona Group in the North Pole Dome is one of the best preserved Paleoarchean supracrustal belts in the world, and as the Dresser Formation within it contains

some of the earliest known stromatolites, its sedimentological and structural history is well

described (e.g., Barley et al., 1979; Walter et al., 1980; Buick and Dunlop, 1990; Nijman

et al., 1998; Ueno et al., 2001; Van Kranendonk et al., 2003; Van Kranendonk, 2006). The

North Pole Chert and barite lenses formed in shallow water to emergent conditions (i.e.,

<200 m: Lambert et al., 1978; Van Kranendonk, 2006). Nijman et al. (1998) indicated that



4.4-3. Paleoarchean Mineral Deposits of the Pilbara Craton



415



deposition of the North Pole Chert was controlled by syn-volcanic, listric growth faults,

with the barite mounds forming in slightly deeper water adjacent to these faults.

Individual barite lenses are commonly more than 20 m thick, with strike lengths in excess of 50 m. The barite mounds contain 10–20 cm thick layers of coarsely crystalline

barite. Barite also forms thin layers within the chert, locally with habits suggestive of replacement of gypsum, and in chert veins in the underlying basalts (Lambert et al., 1978;

Buick and Dunlop, 1990; Abeysinghe and Fetherston, 1997; Nijman et al., 1998). Minor

base metal sulfides (to 3.7% Zn) have been intersected in some barite lenses (Fergusson,

1999). These deposits differ from other VHMS deposits by the lack of significant basemetal mineralization and the extreme abundance of barite, characteristics that may result

from low temperatures of hydrothermal deposition and are consistent with formation at

shallow water depths (cf. Van Kranendonk and Pirajno, 2004; Van Kranendonk, 2006).

Van Kranendonk and Pirajno (2004) indicate that the barite lenses are associated with a

zone of intensely altered rocks that extend up to 1.5 km stratigraphically below the baritic

horizon. This contrasts with rocks above the baritic horizon, which are not altered. Within

this alteration zone, massive kaolinite at the area of most massive barite mineralization

passes downwards into a white-mica-rutile assemblage. This alteration zone is associated

with a swarm of silica ± barite veins that are interpreted to have fed the syngenetic barite

lenses (Nijman et al., 1998; Ueno et al., 2001; Van Kranendonk, 2006). Quartz veins with

epithermal-like textures, consistent with the emplacement of the veins and the barite lenses

under a low confining pressure, are also present (Van Kranendonk and Pirajno, 2004).

4.4-3.1.2. The Big Stubby deposit

The first direct indication of the antiquity of mineral deposits in the East Pilbara Terrane

came from the small (0.1–0.2 Mt), but high grade (13.8% Zn, 4.5% Pb, 350 g/t Ag and

20% Ba; Reynolds et al., 1975) Big Stubby VHMS deposit, which is hosted by the Duffer

Formation. Sangster and Brook (1977) reported a Pb-Pb model age of ca. 3500 Ma that

was later recalculated at ca. 3472 Ma (Thorpe et al., 1992b), an age consistent with the

independently determined age of the host rocks using the U-Pb on zircon method. Descriptions of the Big Stubby deposit have been presented by Reynolds et al. (1975) and, more

recently, by Samieyani (1993), upon which the following summary is based.

The deposit consists of six stacked lenses hosted by felsic to intermediate volcanic

rocks. The basal Big Stubby tuff-breccia contains felsic clasts in an intermediate matrix.

This unit is intruded by rhyolite domes, which are overlain by the massive sulfide lenses,

jaspilite, and intermediate volcaniclastic rocks. This sequence is intruded by dikes that

disrupt both the host sequence and the massive sulfide lenses (Samieyani, 1993).

The ores are simple, consisting of low-Fe sphalerite (0.68–1.7 mole% FeS), pyrite, and

galena, with minor chalcopyrite and trace tetrahedrite and acanthite. Gangue minerals include tourmaline, barite, quartz, sericite, chlorite and calcite. The associated alteration

zone is characterized by a quartz-sericite-pyrite-carbonate assemblage (Reynolds et al.,

1975; Marston, 1979; Samieyani, 1993). Samieyani (1993) reported fluid inclusion homogenization temperatures of 230–390 ◦ C, with most data between 250–350 ◦ C.



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Chapter 4.4: Paleoarchean Mineral Deposits of the Pilbara Craton



4.4-3.1.3. The Lennon’s Find deposits

Of the VHMS districts of the northern Pilbara Craton, the Lennon’s Find district is one

of the more poorly described, despite containing the earliest (ca. 3470 Ma; Thorpe et al.,

1992b), significant VHMS deposit in the world. Production in the district was 50.8 tonnes

and a non-JORC-compliant 1.2 Mt resource grading 0.43% Cu, 7.76% Zn, 1.94% Pb, 100

g/t Ag and 0.3 g/t Au has been outlined for the Hammerhead zone (Ferguson, 1999), one

of five mineralized zones that occur over a strike length of 5 km (Fig. 4.4-3).

VHMS deposits in the Lennon’s Find district are located just below the contact between

mafic schist of the Apex Basalt and underlying, mainly felsic, schist of the Duffer Forma-



Fig. 4.4-3. Geology of the Lennons Find volcanic-hosted massive sulphide district.



4.4-3. Paleoarchean Mineral Deposits of the Pilbara Craton



417



tion (Fig. 4.4-3: Ingram, 1977). Thorpe et al. (1992a) presented a conventional zircon U-Pb

date for the Duffer Formation along strike from the Lennon’s Find district of 3465 ± 3 Ma,

which is consistent with a galena Pb-Pb model age of ca. 3473 Ma (Thorpe et al., 1992b;

see also Huston et al., 2002).

The Duffer Formation comprises three laterally persistent units in the Lennon’s Find

area (Fig. 4.4-3). The basal quartzo-feldspathic schist contains mainly fine-grained quartzmuscovite-biotite ± feldspar schist and lesser, knotty quartz-feldspar-muscovite-biotite

schist with quartz microphenocrysts. This unit, interpreted as metamorphosed felsic volcanic rocks, has a true thickness of 250–300 m. These schists are overlain by a 80–150 m

thick metasedimentary unit containing fine-grained quartz-muscovite-chlorite schist with

up to 20%, 5–20 mm andalusite porphyroblasts (metapelite), and fine-grained quartzmuscovite-chlorite schist (metapsammite). Bedding is defined in pelites by andalusite-rich

layers (Fig. 4.4-4(a)). The uppermost unit of the Duffer Formation consists of 0–75 m thick

quartz-muscovite schist that is locally augen-textured near the top. The unit contains all significant gossans and massive sulfide lenses and thins in unmineralized zones (Fig. 4.4-3).

It is interpreted as hydrothermally altered felsic volcanic rock. The Apex Basalt consists of

strongly foliated mafic schist, with lesser talc ± carbonate schist and chert units near the

base.

Foliated leucogranite intrudes quartzo-feldspathic rocks at the base of the Duffer Formation. Minor, apparently late, pegmatite dykes intrude the foliated granite. The granite

and the volcano-sedimentary sequence are intruded by foliated 0.5–20 m dolerite sills and

dykes. The last intrusive phase is narrow (0.5–1 m) felsite sills and dykes most likely associated with the nearby ca. 3324 Ma Wilina Pluton (Collins et al., 1998).

The rocks hosting the Lennon’s Find district form a broadly arcuate belt that trends 075◦

in the west and 045◦ in the east (Fig. 4.4-3). These rocks dip moderately (40–70◦ ) to the

south. Although no map-scale folds are present in the area other than this arching, folds

with amplitudes up to 2 m are present in outcrop.

Five mineralized zones have been defined at Lennon’s Find based on the location of

old workings and on surficial geochemistry (Fig. 4.4-3). These zones are localized at two

stratigraphic horizons: an upper horizon within the quartz-muscovite schist, and a lower

horizon near the upper contact of the underlying psammo-pelitic rocks. The upper horizon

is more intensely mineralized and contains the Hammerhead and three other mineralized

zones, two of which (Tiger and Bronze Whaler) have significant (>1% Zn) drill-hole

intersections. The lower horizon contains the Grey Nurse zone, which also has significant drill-hole intersection. A third horizon, defined by geochemical anomalies and by

weak pyritic gossan, occurs below the Grey Nurse position, near the contact between the

psammo-pelitic unit and the underlying quartzo-feldspathic schist (Fergusson, 1999; not

shown in Fig. 4.4-3).

The Hammerhead zone consists of a thin (<5 m true thickness) lens of baritic semimassive to massive sulfide (gossan at surface: Fig. 4.4-4(b)) that dips moderately to the

SE. This lens has a moderate to steep rake to the east, which may be controlled by regional structural elements. Mineralogical studies by Marston (1979) indicated that this

mineralized zone consists of sphalerite (1.46–3.56 mole% FeS), chalcopyrite and galena



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Chapter 4.4: Paleoarchean Mineral Deposits of the Pilbara Craton



Fig. 4.4-4. Photographs showing rock relationships at the Lennons Find volcanic-hosted massive

sulphide district: (a) Bedding in pelitic rocks defined by andalusite-rich layers. (b) Surface gossan

expression of Hammerhead zone massive sulphide-barite lens. (c) Iron-oxide (after pyrite) stringers

in chlorite-sericite schist in the stratigraphic footwall to the Hammerhead zone.



in association with barite and pyrite. Three textural styles are present in the Hammerhead

zone: (a) massive banded, which contains 25–80% sulfide with quartz, barite, chlorite,

carbonate, tourmaline, muscovite and biotite gangue; (b) thinly-banded and disseminated,



4.4-3. Paleoarchean Mineral Deposits of the Pilbara Craton



419



Fig. 4.4-4. (Continued.)



which contains 5–20% sulfide, mainly pyrite, in 1–20 mm bands and as disseminated

grains; and (c) disseminated, which contains 1–2% disseminated sulfide, again mainly

pyrite (Sjerp, 1983). The massive-banded sulfide is located at the top of the mineralized

sequence, overlying thinly-banded and disseminated sulfide. The thinly-banded zone described by Sjerp (1983) is probably a stringer zone (Fig. 4.4-4(c)). The Hammerhead zone

has a Pb-Ag-rich top and a Zn-Cu-rich base.



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Chapter 4.4: Paleoarchean Mineral Deposits of the Pilbara Craton



Alteration facies in the Lennon’s Find area are restricted to disseminated pyrite in the

upper part of the Duffer Formation and quartz-sericite schist at the top of the Duffer Formation (Fig. 4.4-3). No hydrothermal alteration assemblages were noted in the Apex Basalt.

4.4-3.1.4. Deposits hosted by the Sulphur Springs Group

The Panorama (or Strelley) Zn-Cu-Pb deposits are located 120 km southeast of Port Hedland in the East Pilbara Terrane (Figs. 4.4-1 and 4.4-5). The history of mineral exploration

at Panorama is discussed in Morant (1998) and Buick and Doepel (1999).

Base metal mineralization is hosted by the Sulphur Springs Group in the Soanesville,

Pincunah and East Strelley greenstone belts of the East Pilbara Terrane (Fig. 4.4-5(a); Van

Kranendonk, 1998; Van Kranendonk and Morant, 1998). Volcanic rocks of the Sulphur

Springs Group and the consanguineous Strelley Monzogranite have been dated at 3235–

3238 Ma (Buick et al., 2002). Model Pb-Pb ages for galena from VHMS and vein prospects

are mostly 3220–3260 Ma (data from Vearncombe (1995), Brauhart (1999), and Sipa Resources Ltd, using the Pb evolution model of Thorpe et al. (1992b)). Metamorphic grade

and strain are mostly low, such that primary textures and structures are well preserved in

the deposits (Vearncombe et al., 1995), and in volcanic and sedimentary rocks (Brauhart et

al., 1998).

The Sulphur Springs Group includes three volcano-sedimentary formations and the

Strelley Monzogranite (Fig. 4.4-5(a): Van Kranendonk et al., 2006a). The lowermost

Leilira Formation comprises a ∼3900 m succession of turbidites and chert, which are

locally intruded by subvolcanic rhyolitic. The Kunagunarrina Formation comprises up to

3000 m of mafic and komatiitic volcanics, with thin intercalated volcaniclastic deposits and

chert. The uppermost Kangaroo Caves Formation comprises up to 1700 m of tholeiitic volcanics and subvolcanic sills ranging from basalt to rhyolite (Morant, 1998; Brauhart, 1999;

Vearncombe and Kerrich, 1999). A regionally extensive marker chert at the top of this

formation comprises silicified sedimentary rock with locally abundant black kerogenous

chert veining. The marker chert, which defines a transition from volcaniclastic to epiclastic

sedimentation, is about 5 m thick regionally, but is up to 100 m thick at Sulphur Springs. It

includes unusual local rocks in the hangingwall of the Sulphur Springs (olistostrome and

tholeiitic rhyodacite sills) and Kangaroo Caves (calc-alkaline rhyodacite) deposits.

The Strelley Monzogranite is a subvolcanic intrusion, which geochemical and geochronological studies have shown to be consanguineous and coeval with the tholeiitic volcanics

of the Kangaroo Caves Formation (Brauhart, 1999; Vearncombe and Kerrich, 1999; Buick

et al., 2002). It is a magnetite series hornblende-biotite bearing granite that is interpreted

to have driven hydrothermal circulation in the Kangaroo Caves Formation that resulted in

the formation of the VHMS deposits (Brauhart et al., 1998).

Three major (Sulphur Springs, Kangaroo Caves and Bernts) and seven small VHMS

deposits and prospects are hosted by the Sulphur Springs Group in the Panorama district.

Of these, eight are hosted by volcanic rocks of the Kangaroo Caves Formation, including

the Bernts deposit, which is in a structurally detached block (Fig. 4.4-5(a)). Other styles of

mineralization hosted by the Sulphur Springs Group include:



4.4-3. Paleoarchean Mineral Deposits of the Pilbara Craton

421



Fig. 4.4-5. Geological maps showing distribution of (a) lithological units and (b) alteration facies in the Panorama volcanic-hosted massive

sulphide district (modified after Brauhart et al., 1999).



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Chapter 4.4: Paleoarchean Mineral Deposits of the Pilbara Craton



(1) magmatic Cu-Zn-Pb-Sn veins near the top of the Strelley Monzogranite (Wheal of

Fortune prospect: Brauhart et al., 1998);

(2) molybdenite, fluorite and topaz associated with greisen zones in the Strelley Monzogranite (Drieberg, 2004);

(3) Cu-Au mineralization in quartz stockworks hosted by spherulitic dacite of the Kangaroo Caves Formation (Electra-Riviera prospects);

(4) Ni mineralization near the base of a peridotitic intrusion in the Kunagunarrina Formation (Daltons prosect); and

(5) Au mineralization in the Leilira Formation (Obelix prospect) and in granite-hosted

quartz veins.

The Panorama VHMS deposits and prospects are mainly of the Zn-Cu type (Large,

1992) and sulfide mineralization is typically zoned downwards from Zn (±Pb)-rich to Curich. Bernts and Mad Hatters are of the Zn-Pb-Cu type, and are comparatively rich in barite,

but poor in Cu. The deposits are low in gold (generally 0.2 g/t) and silver (generally

<40 ppm) relative to other Australian VHMS deposits (Large, 1992).

At Sulphur Springs (10 Mt @ 1.4% Cu, 3.5% Zn and 17 g/t Ag: Fig. 4.4-6(a)) and

Kangaroo Caves (1.7 Mt @ 0.6% Cu, 9.8% Zn, 0.6% Pb, 18 g/t Ag and 0.1 g/t Au:

Fig. 4.4-6(b)), massive sulfide mineralization is hosted at the top of a laterally extensive,

200 m thick tholeiitic dacite sill intruded near the top of a sequence of pillowed and hyaloclastic andesite-basalt (McPhie and Goto, 1996; Morant, 1998). The Bernts deposit (0.6

Mt @ 0.3% Cu, 7.8% Zn and 1.7% Pb) and the Breakers, Man O’War, Anomaly 45 and

Jamesons prospects, are hosted by a comagmatic suite of tholeiitic rhyolite and rhyolitic

volcaniclastic rocks. The Cardinal prospect is associated with felsic volcaniclastic rocks

towards the base of the formation.

Zinc-Cu zones at Sulphur Springs are immediately beneath the marker chert, within the

marker chert, and, rarely, in the hangingwall rhyodacite. Mineralization in the marker chert

is generally Zn-rich, and the larger accumulation beneath the marker chert has a massive

Zn-rich cap, a massive Cu-rich middle, and a stringer-style, Cu-bearing base. The Zn and

Cu zones may be separated by up to 10 m of sub-economic pyrite mineralization (Morant,

1998).

Much of the high-grade Zn mineralization at Kangaroo Caves is hosted by the marker

chert in a shallowly northeast-plunging shoot that has been intersected by drilling up to

1.5 km down plunge from the gossan outcrop (Fig. 4.4-6(b)). Copper-rich mineralization,

although not well developed, is present in strongly altered volcanic rock beneath the marker

chert.

The ore assemblage at the three main deposits includes pyrite, low-Fe sphalerite,

chalcopyrite and galena. Secondary Cu sulfides, arsenopyrite, tennantite-tetrahedrite,

pyrrhotite, rutile, bournonite, famatinite, bismuthinite and Se-bearing weibullite are present

at Sulphur Springs (Outokumpu Research Oy, unpublished data, 1994; Vearncombe, 1995).

Quartz, chlorite, sericite, ankerite and barite are the main gangue minerals at both deposits.

The very low strain and metamorphic grade have preserved sulfide textures (Fig. 4.47), from which textural zoning has been recognized at the Sulphur Springs and Kangaroo Caves deposits (Fig. 4.4-7: Vearncombe, 1995; Vearncombe et al., 1995). Dendritic



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Chapter 4.4 Paleoarchean Mineral Deposits of the Pilbara Craton: Genesis, Tectonic Environment and Comparisons with Younger Deposits

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