Alkaline rocks and gemstones, Australia: A review ...

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Alkaline rocks and gemstones, Australia: A review and synthesis F. L. Sutherland a

a

Mineralogy and Petrology Section, Australian Museum, Sydney, NSW, 2000, Australia

Available online: 09 May 2007

To cite this article: F. L. Sutherland (1996): Alkaline rocks and gemstones, Australia: A review and synthesis, Australian Journal of Earth Sciences: An International Geoscience Journal of the Geological Society of Australia, 43:3, 323-343 To link to this article: http://dx.doi.org/10.1080/08120099608728259

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Australian Journal of Earth Sciences (1996) 43, 323-343

Alkaline rocks and gemstones, Australia: a review and synthesis F. L. SUTHERLAND

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Mineralogy and Petrology Section, Australian Museum, Sydney, NSW 2000, Australia

In Australia, valuable gemstones that occur in alkaline rocks include diamonds in lamproites and kimberlites and sapphires, zircons and rubies in alkali basalts. One gem zircon prospect is in carbonatite. This review combines geological and gemmological literature to discuss the tectonic settings and origins of Australian gem-bearing alkaline rocks. Marked contrasts exist between diamond and sapphire/zircon associations across the continent. More western cratonic areas exhibit episodic, sparse, deep alkaline activity from the diamond zone (2 Ga-20 Ma), while in eastern fold belt areas prolific Mesozoic/Cainozoic basalt volcanism carried up considerable sapphire and zircon (since 170 Ma). Some South Australian Mesozoic kimberlitic diamond events (180-170 Ma) represent ultradeep material rising through the mantle transition zone. Eastern Australian diamonds are unusual and at present their origin is contentious; Palaeozoic subductions that form young, shallow-origin diamonds for later basaltic transport underpin a novel model. Several different models compete in explaining eastern sapphire/zircon formation. They range from eruptive plucking of metamorphosed subducted materials to crystallisation from felsic melts, or carbonatitic reactions. Pb-U isotope zircon ages favour formation during Phanerozoic basaltic activity and not during earlier Palaeozoic subduction or granitic intrusion events. A problem for crystallising zircon from highly fractionated basaltic melts is negligible europium depletion in Rare Earth Element patterns. A model proposed here favours sapphire/zircon crystallisation from relatively small volume, little evolved, felsic melts generated from metasomatised mantle as the lithosphere overruns subdued hot spot systems, initiated at Tasman-Coral Sea rift/spreading margins. A unique ruby, sapphire, sapphirine, spinel assemblage from the Barrington basalt shield in New South Wales, marks a separate ruby/pastel-coloured sapphire genesis. Key words: alkali basalts, diamonds, hot spots, kimberlites, lamproites, sapphires, zircons.

INTRODUCTION Australia is a prolific producer of diamonds and sapphires, derived from alkaline igneous hosts and. their secondary alluvial deposits (Atkinson et al. 1990; Hughes 1990; Sutherland 1991a; Olliver & Townsend 1993; Jaques 1994). Around 42 million carats (over 81) of diamonds were produced in 1993, valued at A$565 million (Smith & Meyer 1994). The largest diamond found was 42.6 carats weight (a white diamond discovered in June 1994; Argyle Mines pers. comm. 1995). In comparison, sapphire production in eastern Australia in 1993-94 was some 65-75 million carats valued around A$20-25 million (Oakes et al. 1996; G. M. Oakes pers. comm. 1995). A variety of other minerals from alkaline igneous sources also contribute to local gem cutting needs, for example, ruby, zircon, olivine, feldspar, garnet (Sutherland 1991a). This paper reviews the distribution, nature and sources of the main gem minerals, examining the physical and chemical conditions of their origin and the tectonic settings involved. Specialist terms and acronyms used in the review are defined for the reader in the Appendix. The genesis of some diamond and sapphire sources remains under debate and receives some attention here. The main gem-producing alkaline rocks occupy three distinctive zones across Australia (Figure 1). They include: (i) a major diamond-bearing lamproite, kimberlite and ultramafic lamprophyre/melilitite/ monchiquite association in western and central northern Australia (Figure lb); (ii) a minor diamond-bearing

kimberlite and ultramafic lamprophyre/melilitite-nephelinite/monchiquite association in southern Australia (Figure lc); and (iii) a major sapphire, zircon, minor ruby and marginal diamond bearing ultramafic alkaline and alkali basalt association in eastern Australia (Figure Id).

DIAMOND ASSOCIATIONS Diamond-bearing alkaline rocks in Australia's cratonic and epicratonic regions, with their petrological affinities, ages, and known diamond characteristics, are summarised in Table 1. Diamond emplacement was sporadic from Precambrian into Tertiary time (ca 2000-20 Ma) with lamproites, particularly olivine lamproites, being the main diamond carriers. The Proterozoic Argyle pipe (1180 Ma) was the richest diamond carrier (Boxer & Jaques 1990) and since then its Tertiary exposure has distributed diamonds into local economic alluvial deposits (Smoke Creek, Limestone Creek and Bow Creek deposits: Boxer & Deakin 1990; Fazakerley 1990; Smith et al. 1990). Some Ellendale olivine lamproite pipes carry near-economic diamond grades (Ellendale 4, 7, 9 and 11; Hughes & Smith 1990), but others only contain minor or microdiamond suites. The source for the oldest Coanjula event (ca 2000 Ma) awaits discovery, but these micro-diamonds carry K-rich melt inclusions that are more aluminous (up to 21 wt% A1,O,; Lee et al. 1994) than the typical fresh kimberlite or lamproite compositions found in Australia (up to 11 wt% A1,O,; Jaques et al. 1986).

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(b) Indian Ocean

10*S-

16'S

Coral Sea

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20*S

30-S

Tasman Sea

•30-S 4O'S

133*E

145"E

150-E

155"E

Compiled from Olliver and Townsend (1993), Jaques (1994), Wyatt et al. 1994, Sutherland et al.

Diamond types

Argyle lamproitic diamonds show abundant and unusual surface etch features, attributed to transport in corrosive potassic melts, while Ellendale lamproitic diamonds gained a lustrous polish from their magmatic transportation (Jaques et al. 1986; Tombs & Sechos 1986). The predominant brown (> 80%) and yellow (< 20%) hued colours in Argyle gem diamonds generated a revolution in marketing as they were promoted as 'champagne' and 'cognac' diamonds and graded into 7 categories of colour (Brown & Chapman 1991). The brown, and rarer pink colours, in Argyle diamonds result from deformation gliding under plastic strain before diamond eruption. In the commercially valuable pink stones, an intensity of pink graining coincides with the internal linear strain

Figure 1 (a) Distribution of the main alkaline rock-gemstone associations in Australia showing location of insets (b-d). (b) Northwest Australian Kimberley diamond province showing Q , Kimberley Block and 0, Proterozoic mobile belts, (c) South Australian diamond province showing E3, the exposed Gawler Craton. (d) East Australian sapphire/zircon/diamond province showing B, main basaltic areas. • , lamproitic diamond hosts; O, kimberlitic diamond hosts; + , picritic/ monchiquitic/basaltic/ indeterminate diamond hosts; • , basaltic sapphire/zircon associations; O, basaltic ruby/sapphire associations. Lamproitic diamond hosts include: AM, Argyle Mine; E, Ellendale; BS, Bitter Springs; C, Calwynyardah; and N, Noonkanbah fields. Kimberlitic diamond hosts include: A, Aries; M, Maude Creek; P, Pteropus; Em, Emu; Cl, Cleve; and Eu, Eurelia fields. Picritic/monchiquitic and indeterminate diamond hosts include: Wa, Wandagee; Na, Nabberu; and Co, Coanjula fields. Main basaltic sapphire workings include: GI, Glen Innes-Inverell; RA, Rubyvale-Anakie; W, Willows; LP, Lava Plains; and B, Oberon-Blayney fields. Cratonic regions include: NAC, North Australian Craton; KB, Kimberley Block; KL, King Leopold Mobile Zone; HC, Halls Creek Mobile Zone; YB, Yilgarn Block; and GC, Gawler Craton. Other labelled features include: FT, Fitzroy Trough; CB, Canning Basin; KG, King George River; TAS, Tasmania; AFB, Adelaide Fold Belt; and TFB, Tasman Fold Belt. 1994a and author's unpubl. data.

birefringence (Hofer 1985) and the deepest pink cut stones reach individual values of over A$l-2 million (Sutherland 1991a; Weber & Sechos 1994). Yellows are prominent in Ellendale diamonds (Hughes & Smith 1990), while whites characterise Aires diamonds (Towie et al. 1994). Dodecahedral crystals dominate Argyle, Ellendale, Lissadell Road and Aires diamonds, but sharp planar octahedral and hemimorphic octahedral-rhombohedral forms appear, particularly in the small-sized diamond fractions (Jaques et al. 1986; Jaques et al. 1994). Rare tetrahedral forms at Argyle belong to the normal holosymmetric class rather than a hemihedral class and indicate pronounced deformation recrystallisation (X-ray topographic studies; Yacoot & Moore 1993). The dodecahedral diamonds contain either eclogitic, peridotitic or rare sulfidic suite inclusions (E-type,

ALKALINE ROCKS AND GEMSTONES Table 1

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Diamond-bearing alkaline suites (decreasing age), Australian cratonic-epicratonic associations.

Locality and State

Rock type and variants

Age Ma (method)

Diamond characters: colours and crystals

Diamond grade

Reference*

microdiamonds, 2 types 71% coloured fibrous cubic 24% colourless octahedra undescribed mostly brown, lesser yellow, mostly resorbed 1 dodecahedra (1580 Ma); sharp octahedral

often greater than kimberlites and lamproites subeconomic 5% gem, max 43 CM 40% subgem, av. 5 CM/t

1,3,4,5

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WESTERN AND NORTHERN ASSOCIATIONS

Coanjula, Murphy Inlier NT

undetermined potassic silicate melt

2050-1950 (host beds)

Marymia Dome WA Argyle pipe, E. Kimberley WA

Kimberlite, lamprophyre ol-lamproite, tuffs, breccias, dykes

1700-1900 1177 + 47 (Rb-Sr)

Lissadell Road, E. Kimberley WA

ol-lamproite (altered) dyke train

undated related to Argyle?

Aires pipe, E. Kimberley WA Pteropus Ck, N. Kimberley WA

kimberlite breccias, dykes kimberlite breccia pipe

820 (Rb-Sr, mica) 802 ± 14 (Pb-U, zircon)

Maude Ck, E. Kimberley WA

kimberlite dyke

E. Mu I and II McArthur Basin NT

30 1-*, 6-16

high 6.4 CM from 42 m3

1,2,4

very high gem % 5 CM/1001 negligible

1,2,17,18

undated, mostly microdiamonds related to Pteropus?

low ca 1 CM/1001

1,2,3,4

kimberlitic

360

small and microdiamonds

uneconomic ca 0.12 CM/1001

1,3,4

Merlin pipes Batten Trough NT

kimberlite clustered pipes

undated

microdiamonds (0.1-0.4 mm)

5 diamonds/kg

29

Nabberu, Nabberu Basin WA

ultramafic lamprophyres (alnoite etc.)

305 ± 7 (Pb-U, zircon)

microdiamond

negligible

1,3,4,19

Wandagee, Carnarvon Basin WA

ol-rich monchiquite tuffs, breccias, sills, dykes

160 ±10 (Pb-U, zircon)

small ( < 0.5mm)

4 diamonds

1-4,20

Ellendale, W. Kimberley WA

ol-lamproites and lamproites pipes, plugs, lava

23-21 (K-Ar and Rb-Sr)

small and microdiamonds

subeconomic up to 14 CM/100 t

21,23

E. Lennard, Lennard Shelf WA

lamproites and ol-lamproites pipes, plugs, lava

22-20 (K-Ar and Rb-Sr)

small and microdiamonds

very low

1-2,9,11

Calwynyardah, Fitzroy Trough WA

lamproites pipes, plugs, lava

20-22 (K-Ar and Rb-Sr)

small and microdiamonds

very low

1-2,9,11

Noonkanbah, Fitzroy Trough WA

lamproites, rare ol-lamproite pipes, plugs, lava

18-20 (K-Ar and Rb-Sr)

small and microdiamonds

very low

1,9,11

kimberlite, dykes, tuffs

180 ± 3 (Pb-U, perovskite)

small and microdiamonds

very low

1,3,4,24

Eurelia, kimberlites Adelaide Fold Belt SA dykes

170 ± 2 (Pb-U, zircon)

colourless, brown, rare grey octahedral, dodehedral, irregular

low

24,25,26

Kayrunnera, kimberlite-melilitite Wonaminta Block NSW breccia pipe

260 Ma (fission track, zircons)

microdiamond

negligible

27,28

mostly white, rare yellow; mostly dodecahedral microfragment

1,2,3,4,11

1—4,12,16,

SOUTHERN ASSOCIATIONS

Cleve, Gawler Block SA

* 1, Atkinson et al 1990; 2, Jaques et al. 1986; 3, Smith et al. 1990; 4, Jaques 1994; 5, Lee et al. 1994; 6, Harris & Collins 1985; 7, Richardson 1986; 8, Boxer et al. 1989; 9, Jaques et al. 1989a; 10, Jaques et al. 1989b; 11, Pigeon et al. 1989; 12, Jaques et al. 1989c; 13, Sobolev et al. 1989; 14, Boxer & Jaques 1990; 15, Jaques et al. 1990; 16, Taylor et al. 1990; 17, Edwards et al. 1992; 18, Towie et al. 1994; 19, Hamilton & Rock 1990; 20, Jaques et al. 1989d; 21, Hughes & Smith 1990; 22, Stachel et al. 1994; 23, Jaques et al. 1994; 24, Wyatt et al. 1994; 25, Scott-Smith et al. 1984; 26, Black et al. 1993; 27, Danchin et al. 1985; 28, L. M. Barren et al. 1996; 29, Lee et al. 1995; 30, Shee et al. 1996.

P-type, S-type diamonds), which form mixed suites in rare cases. The proportion of eclogitic diamonds is much greater at Argyle than at Ellendale (85% cf. 43%: Jaques et al. 1989c; probably as much as 93%: Sobolev et al. 1989). Argyle eclogitic diamonds formed around 1580

Ma, 400 million years before their discharge by eruption (Richardson 1986). Some probably formed over a significant time period, during solid-state growth under fluid infiltrations into an eclogitic protolith (Griffin et al. 1988). Nitrogen in Argyle and Ellendale eclogitic dia-

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Table 2 Alkaline rocks accociated with reported diamonds; eastern Australian orogens. Locality and State

Rock type and variants

Age Ma (method)

Diamond characters

Comments

Oaky Creek, Copeton NSW

altered alkali dolerite, dyke

post-285 Ma (field data)

white, irregular dodecahedra

11? from 220 tonnes 1,2 max 2.5 mm, 0.3 CM

Ovens, Sydney Basin NSW

basaltic diatreme

post-263Ma (fission track, zircon)

Ruby Hill, Bingara NSW

basanite-ne hawaiite, breccia pipe, dykes

167 +Ma (K-Ar, feldspar)

Umbiella Creek, Sydney Basin NSW

nephelinite plug breccia

180 Ma (plug) 60 Ma (breccia)?

Watsons Creek, Gloucester NSW

ultramafic lamprophyre, breccia pipe

Mesozoic (?)

microdiamond

Prince Charles Ck Gloucester NSW

monchiquitic lamprophyre, breccia pipe

Mesozoic (?)

microdiamond

Jugiong NSW

ultramafic/monchiquitic breccia pipes

Mesozoic (?)

Browns Ck, near Kyogle NSW

ultramafic breccia

41±5Ma (fission track, zircon)

Brickclay Creek Walcha NSW

ne-mugearite, dyke, breccia

Brigooda, near Proston Qld

alkali basaltic? maar-breccia

unusual pits on crystals

Reference*

reported diamond

3

10 crystals', over 4 CM total

1,4

reported diamond

1,3 1,3,5

0.01 CM from 24 m3

1,3

reported diamond

1,3,6

microdiamond

0.0005 CM

1,3

36 ± 1 Ma (K-Ar, feldspar)

white, yellow dodecahedroids

17, up to 1 CM, low to high

2,7

0.4 Ma ± 0.04 Ma (K-Ar, feldspar)

pale yellow dodecahedroids

curved faces, 0.24 CM

8

* 1, MacNevin 1977; 2, Sutherland et al. 1994a; 3, L. M. Barron, S. R. Lishmund, G. M. Oakes, B. J. Barron, F. L. Sutherland unpubl. data 1996; 4, A. L.Jaques & L. R. Raynor unpubl. data 1996; 5, Rock 1991; 6, Danchin et al. 1985; 7, Meyer et al. 1995; 8, Robertson & Robertson 1994.

monds shows a striking contrast between the two suites (Taylor et al. 1990). Argyle eclogitic diamonds show Baggregation defects and other effects that suggest resetting on high temperature isotherms (1220-1305°C), compatible with high temperatures estimated from the inclusion suites (1085-1580°C; Jaques et al. 1989c). Some Ellendale eclogitic diamonds exhibit only A-defects suggesting storage associated with a lower temperature isotherm (1000-1100°C), a feature taken to indicate a younger Phanerozoic formation compared to Proterozoic Argyle eclogitic diamond formation (Taylor et al. 1990). However, other Ellendale peridotitic diamonds yield higher inclusion temperatures (1115-1220°C; Jaques et al. 1989c), indicating higher formation temperatures. The octahedral Argyle and Ellendale diamonds (P-type) contain peridotitic inclusions, which fits the presence of octahedral diamonds in garnet peridotite xenoliths at Argyle (Jaques et al. 1990). They formed in the Precambrian at temperatures between 1055 and 1235°C (inclusion T estimates; Jaques et al. 1989c). The nitrogen characteristics in Ellendale peridotitic diamonds differ from those in the lower temperature Ellendale eclogitic types and suggest storage under moderate to hot isotherms (1145-1240°C; Taylor et al. 1990). Argyle, Ellendale and Coanjula diamonds all have isotopically light 8I3C values. Argyle E- and P-type diamonds overlap in their ranges, with E-types extending to more negative values (-5 to -19 cf. -3 to -9; Jaques et al. 1989c; Sobolev et al. 1989). However, different 813C ranges exist within Argyle E-type diamonds, when partic-

ular inclusion suites are considered (-5.5 to -16.8 for coesite-garnet suite cf. -10.9 to -17.5 for kyanite-highCa garnet suite: Sobolev et al. 1989). Coanjula microdiamonds extend to even higher negative 813C values (cubes -10 to -27; octahedra -3 to -22). Similar cubic microdiamonds with such values (-15 to -28) from north Kimberley alluvial deposits were compared with metamorphic diamonds (Sobolev et al. 1989). However, growth stages and inclusions in the Coanjula diamonds suggest a magmatic origin after initial growth in garnet lherzolite mantle (Lee et al. 199A). Diamonds from southern and eastern alkaline Australian associations exhibit some distinctive characters compared to the western and northern associations. Within the southern kimberlitic association, diamonds from Eurelia contain enstatite and magnesiowiistite inclusions, which suggest a lower mantle paragenesis (deeper than 650 km; Scott-Smith et al. 1984; Jaques 1994). Within the eastern ultramafic lamprophyre, melilitite-nephelinite and alkali basalt association (Table 2), diamonds in the Copeton alkali dolerite resemble the strongly resorbed diamonds found in the adjacent Copeton-Bingara alluvial deposits. These diamonds carry an unusual coesite-diopside/omphacite-high-Ca garnet inclusion suite and are isotopically heavier (8I3C -2 to' + 3) than other Australian diamonds (Sobolev 1984, pers. comm. 1989; Taylor et al. 1990; L. M. Barron et al. 1994, 1996; Sutherland et al. 1994a). Further work on these unusual Copeton-Bingara alluvial diamonds (Meyer et al. 1995) revealed three main types: (i) yellow,

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ALKALINE ROCKS AND GEMSTONES high-N type IaB diamonds with regular morphology and no internal zonation; (ii) white, low N diamonds with irregular morphology and variable and sometimes complex zonation; and (iii) yellow/white diamonds of intermediate-N content and some internal structure. Some Copeton diamonds (mostly intermediate-N type) fit an isotherm of 1145°C based on an assumed lithospheric residence of 1600 Ma (Taylor et al. 1990), but most fell off this assumed isotherm. New studies suggest temperatures of 1140°C for the high-N diamonds and 1155°C for low-N diamonds, using an assumed 300 Ma isotherm and a 3Ga isotherm only increases T by about 50°C (Meyer et al. 1995). In contrast, diamonds from the Walcha alkaline dyke include high-N diamonds, of Type IaA with the N almost entirely in A defects, indicating either low-temperature, long-term storage or a young formation age (Sutherland et al. 1994a).

Lithospheric/asthenospheric relationships for Australian diamond events The richest diamond-bearing rocks are lamproites within Proterozoic mobile belts that frame the Kimberley Basin and show basement ages of 1800-1900 Ma for final stabilisation (Jaques 1994). Diamond-bearing kimberlites intrude greater areas of the North Australian craton (1600-1800 Ma basement ages) and include some ultramafic and transitional alkaline hosts for example Maude Creek (Rock 1991). Argyle-like diamonds in the north Kimberley Basin come from unidentified sources and hosts (Sobolev et al. 1989). Elsewhere in Western Australia, diamond carriers intrude 1000-2000 Ma basement and the ultramafic lamprophyric/picritic monchiquitic hosts contain only traces of diamonds (Nabberu, Wadagee). The diamondiferous lamproites in east and west Kimberley (Table 1) show striking contrasts in age (1180 Ma compared to 20 Ma), number of intrusions (several compared to over a hundred), structural controls (major north-northeast-south-southwest shear zone, [Deakin & White 1994] compared to deep northwesttrending basement shears and rifts [Hughes & Smith 1990]), diamond grades (av. 5 CM/t, cf. 1 CM/100 t), proportions of diamond types (over 90% eclogitic cf. subequal eclogitic and peridotitic types) and thermal histories of their diamonds (common high temperature IaB type cf. common low temperature IaA types). This indicates significant differences in underlying lithospheric sources at times of diamond formation and eruption. Olivine lamproites dominate as diamond hosts but paucity of diamonds in leucite lamproites could mark fractionation and resorption effects rather than differences in depth or character of melt sources (Jaques 1994). An experimental study of a west Kimberley primary olivine lamproite suggested two possibilities for melt generation (Foley 1993). One is partial melting of potassium mica-bearing harzburgite at 4.5-5.5 GPa; the other more likely possibility is a complex melting process involving a veined source rock rather than homogeneous mica harzburgite. The latter process is favoured by rare earth element (REE) measurements on Argyle olivine lamproite (Tainton & McKenzie 1994). Heavy REE (Tm,

327

Yb, Lu) concentrations indicate an original garnet and chromian spinel-bearing mantle source, depleted by some 20% extraction of melt before subsequent addition of 4-10% of metasomatising melts rich in light REE (LREE). These last melts also introduced incompatible elements such as K, Rb, Sr, P, Th, U, Nb and Zr and their ratios and isotopic characteristics mark old enrichments up to 2 Ga in age (Jaques 1994). The enrichments are attributed to some 0.5% partial melting of underlying mid ocean ridge basalt (MORB) mantle sources (Tainton & McKenzie 1994). The K, Sr and U introduction was accompanied by high Ba enrichment, and the high Ba/La and Pb/La and low K/Ba and Sr/Nd ratios with negative Eu anomalies in the olivine lamproites suggest Palaeoproterozoic crust recycled into the underlying mantle source regions (Nelson & McCulloch 1989). The younger west Kimberley lamproites are very enriched in Ba, Rb, Th and LREE (Jaques 1994), which may indicate greater recycling under the King Leopold Mobile Zone than the Halls Creek Mobile Zone. The diamonds in these source regions were extracted by lamproite melts, estimated to form by 0.3-0.4% degree partial melting (Tainton & McKenzie 1994). The diamondiferous kimberlitic suites within the Kimberley block include micaceous kimberlites resembling Group II kimberlites and micaceous kimberlites from Africa (Jaques 1994). Such kimberlites possess mineralogical, geochemical and isotopic characteristics that indicate deep lithospheric sources rather than Asthenospheric sources for other kimberlites (Mitchell 1994). The Aires kimberlite is an unusual variant, resembling South African Group II kimberlites in mineralogy, the West African micaceous kimberlites in geochemistry and Group I kimberlites in Nd isotopes (Edwards et al. 1992; Towie et al. 1994). This complexity of characters suggests that anomalous mantle underlies the central-east Kimberley region. In contrast to the northern Australian lamproite/ kimberlite associations, the southern Australian kimberlites resemble Group I kimberlites (Scott-Smith et al. 1984; Wyatt et al. 1994) and suggest deeper asthenospheric sources. The kimberlites intrude 1000-2700 Ma basement and these sparsely diamondiferous suites (Cleve, Eurelia) pass eastwards into largely barren melilitite/nephelinites derived from shallower levels (Kayrunnera, New South Wales). The Terowie kimberlites lack obvious diamonds, but their proximity to a microdiamond find at Pine Creek (South Australian Department of Mines and Energy 1990) makes them a potential diamond source. A lamproite dyke reported at Terowie is now assigned to the nephelinitic suite (Smith et al. 1990), but leucite has been identified by X-ray diffraction in some dyke samples (D. J. Colchester pers. comm. 1995). Experiments on a synthetic Group I kimberlite composition suggest formation by small degrees of partial melting of mantle over 300-650 km deep (Ringwood et al. 1992). Based on experiments to determine oxide partitioning between such mineral phases (Kesson & Fitzgerald 1992) such depths may apply to the Eurelia kimberlites that contain magnesiowiistite-enstatite bearing diamonds. Kimberlites originating from lower mantle depths are termed

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superkimberlites and may be initiated by mantle plumes rising from the core-mantle boundary (Haggerty 1994). Eastern Australia's reported diamond hosts encompass ultramafic, melilititic/nephelinitic and alkali basaltic rocks of Late Palaeozoic to Late Cainozoic age (Sutherland et al. 1994a and Table 2). The apparent presence of atypical diamond hosts of relatively shallow lithospheric/asthenospheric origin (< 150 km depth) prompted proposals for unusual diamond sources at such depths (L. M. Barren et al. 1994, 1996). The envisioned process involved thick, cool subducting oceanic slabs, which developed diamonds under relatively low pressures before the termination of subduction. The abnormal, heavy, 8I3C isotope diamonds from Copeton-Bingara, New England, were correlated with diamond formation during a heavy carbon pulse in the global carbon pool, probably around 310 Ma (Late Palaeozoic). The unusual calcic garnet-calcic pyroxene-coesite inclusion suite in these diamonds is attributed to calc-silicate source rocks and probably calcsilicate metasediments for the N-rich diamonds (Taylor et al. 1990). Some crystal impressions in the Copeton diamonds contain euhedral faces which resemble garnet morphology, suggesting diamondiferous eclogites and/or garnetites in the source rocks (L. M. Barron et al. 1996). The New England diamonds are found within a complex Palaeozoic orogen, which shows no obvious Precambrian cratonic elements and passes into underlying (seismic) mantle at 30-35 km (Finlayson & Collins 1993; Korsch et al. 1993). This orogen includes Palaeozoic ophiolites, arc subduction sequences and granitic plutons, and recent dating and reconstructions suggest that it was obducted westwards by thrusting prior to Early Mesozoic time (Aitchison & Ireland 1995; Woodward 1995). The orogen is thrust 75-85 km west over Early Palaeozoic Lachlan Fold Belt rocks, which may extend under this decollement to the New South Wales coast (Woodward 1995; figure 6). Thus, remnant subduction sequences within the orogen may be disconnected from their original subducted slab positions. Some older diamondiferous alkaline intrusions may be detached, but later intrusives (e.g. Gloucester, Browns Creek and Walcha) would remain connected to their mantle sources. Decoupled older lithospheric diamond sources under the Palaeozoic orogens were proposed by Taylor et al. (1990). The recent models of New England obduction improve these possibilities, but evidence for such remnants remains elusive. Probable Precambrian crust underlies some areas in south central Victoria (Gray & Willman 1991) and north and central Queensland (Wellman 1992; Sutherland et al. 1994a) where surficial diamonds are found, but precise sources for these diamonds are uncertain. Diamond formation and eruption Australian diamonds show considerable variations in inclusion suites, C isotope ranges, thermal histories based on N states and transporting alkaline host rocks (Figure 1; Tables 1, 2). Eruptions of distinct diamond suites in over 5 separate magmatic episodes across the continent over the last 2000 million years suggest protracted, even if not prolific, diamond formation and discharge during Australia's subcontinental evolution.

Peridotitic octahedral diamonds from Argyle and Ellendale probably represent the oldest diamond formation, within residual peridotites left by major tholeiitic basaltic extraction in Archaean or early Palaeoproterozoic time (Jaques et al. 1990). In this respect they resemble the 3.3 Ga peridotitic diamonds from the African lithosphere (Kirkley et al. 1991). Archaean asthenosphere/lithosphere conditions were analysed by Green (1991) and his model depicts an upper mantle under reducing conditions, where oxygen fugacity lay above levels for iron-wiistite and where fluid phases contained CH4 and H r The temperature-depth estimates for diamond-bearing peridotite in Argyle xenoliths (1140-1290°C and 150-180 km; Jaques et al. 1990) lie below an experimental fertile peridotite + CH 4 >H 2 O solidus, but intersect the peridotite +H 2 O solidus (Green 1991). Lamproitic melts generated in more metasomatised post-Archaean lithosphere under hydrous, but still reduced, conditions (HjO > CH4; Foley 1993) then carried up both early peridotitic diamonds and later formed eclogitic diamonds. The Coanjula microdiamonds (formed about 2 Ga) may also link into early peridotitic diamond formation. Rare, octahedra appear and single crystalline cores to the fibrous cubic diamonds contain peridotitic inclusions (Lee et al. 1994). The outer cubic coatings enclose eclogitic and other inclusions, suggesting subsequent metasomatic and alkaline silicate melt reactions. Although preceding Argyle eclogitic diamond formation by over 400 million years,-the exact ages of core and coating events remain unclear. Significant diamonds formed in the basal east Kimberley lithosphere after 1.6 Ga in an eclogitic setting (Richardson 1986). Variable formation temperatures (up to 400°C in difference) and disequilibrium within inclusion suites suggest diamonds grew progressively in the solid state within an open system where carbon-bearing fluids modified an evolving lithosphere (Griffin et al. 1988; Jaques et al. 1989c). N aggregation states suggest high-temperature storage (around 1260°C) for up to 400 million years for the brown diamonds (Taylor et al. 1990). Further diamond formation probably extended below the central-east Kimberley region prior to the 820 Ma emplacement of the Aires kimberlite pipe. These colourless diamonds lack the deformation features found in the Argyle brown diamonds, but inclusions are rare and give little guide to the source lithology. Nevertheless, the presence of Type II kimberlites within the 800-820 Ma central-east Kimberley province suggest some potassic mica metasomatism within the lithospheric sources. Younger diamonds developed in the west Kimberley lithosphere, as Ellendale eclogitic diamonds exhibit distinctly lower temperature N characteristics (Taylor et al. 1990). Two Ellendale peridotitic diamonds show greater N aggregation extending into the Argyle domain and may relate to the older diamond events. These disparate diamonds under Ellendale came up in young lamproitic melts (20 Ma) generated in long evolved, highly enriched, lithosphere (Jaques 1994). Migration in lamproitic ages southwards suggests passage of deep metasomatised diamond-bearing lithosphere over an asthenospheric thermal anomaly. Diamond formation periods under the southern cratons are only constrained by host eruptive ages (170-260 Ma,

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ALKALINE ROCKS AND GEMSTONES Table 1). South Australian diamonds, associated with Group I kimberlite, suggest an asthenospheric origin for the melts. While most diamonds are inherited xenocrysts (Meyer 1985), diamond was recently crystallised experimentally from a silicate melt of Group I African kimberlite composition between 1800 and 2000°C and 7.0-7.7 GPa (Arima et al. 1993). Hence Mesozoic crystallisation is a possibility for the origin of Eurelia diamonds that contain ultra-deep, highly reduced inclusion suites. The relative redox states of asthenosphere and lithosphere are under debate and Ballhaus and Frost (1994) suggest that asthenospheric sources (represented by South Australian Group I kimberlites) would be more reduced than lithospheric sources (represented by Western Australian Group II kimberlites), due to oxidation as melts rise to the surface. Limited data also hinders understanding of diamond formation below the eastern Palaeozoic fold belts. One scenario derives diamonds from delaminated Proterozoic lithosphere during Mesozoic eruptions (Taylor et al. 1990), another develops younger diamonds in Palaeozoic slabs terminated during subduction and delivers them by Mesozoic-Cainozoic eruptions (L. M. Barron et al. 1994, 1996). Diamond formation ages are critical to separate these models, as a cited 300 Ma Copeton diamond date (L. M. Barron et al. 1994) and immature N aggregation in a Walcha diamond (Sutherland et al. 1994a) favour post-Proterozoic scenarios. The 1140-1155°C formation temperature derived using a 300 Ma Copeton diamond age (Meyer et al. 1995) can provide minimum depth estimates when extrapolated to a potential Late Palaeozoic geothermal gradient (EMAC geotherm; Pearson et al. 1991). This suggests depths around 75 km and compares with depths around 50 km for a hotter Tertiary New England geothermal gradient (Sutherland et al. 1994b). These depths lie above those for melilitite/nephelinite/ basanite generation from the Phanerozoic mantle (Green 1991).

SAPPHIRE-ZIRCON ASSOCIATIONS Sapphires and zircons are common alluvial associates shed from eastern Australian basaltic fields (Figure 1) and several fields support mining and prospecting operations (Coldham 1985; Mumme 1988; Olliver & Townsend 1993). Major fields include New England, New South Wales (MacNevin & Holmes 1980; Coenraads 1990; Pecover 1993; Oakes et al. 1995) and central Queensland (Stephenson 1990; Krosch & Cooper 1991; Robertson & Sutherland 1992). Sapphires and zircons appear in basalt (sometimes together in single samples e.g. Tumbarumba and Duncans Creek, New South Wales; Australian Museum collections), but most have been shed from volcaniclastic deposits formed by pyroclastic and epiclastic flow processes (Robertson & Sutherland 1992; Pecover 1994; Oakes et al. 1996). The term zircospilic was used to characterise the common heavy mineral association (zircon, corundum, .spinel, i/menite; Hollis 1985), but rare corundumanorthoclase and zircon-anorthoclase composites are known (O'Reilly & Griffin 1987; Stephenson 1990;

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Robertson & Sutherland 1992; J. D. Hollis, pers. comm. 1995): a more general term zircospilian to include anorthoclase is used here. Corundum-magnetite and zircon-magnetite composites (F. L. Sutherland & R. R. Coenraads unpubl. data 1996) indicate a role for ferrian spinels, as well as magnesian spinels, in the association. An apparent paradox in prolific sapphire/zircon producing basaltic fields is the scarcity of felsic rocks (Stephenson et al. 1989; Coenraads 1994), which are usually required for corundum/zircon crystallisation (Irving & Frey 1984; Irving 1986). Some feldspathoidal mugearites, phonolites and felsic imprinted volcaniclastic deposits may occur, e.g. McBride Province (Stephenson 1989) and New England gemfields (Wilkinson & Hensel 1991; Barron 1992; Sutherland et al. 1993a), but clearly the zircospilian association largely represents subvolcanic felsic crystallisations disintegrated by, and carried up in, basaltic eruptions. Sapphire types Sapphire crystals are typically bipyramidal, stepped and tapering or barrel shaped and surfaces are commonly corroded and etched due to transport within a melt environment (Coenraads 1992a). In basalt, crystals have pleonaste spinel reaction rims (Stephenson 1990). Morphology and growth relationships of natural sapphire and ruby in comparison to synthetic stones are described by Keifert and Schmetzer (1991). Colours range from white into yellow, orange, green, blue, violet and pink hues, which exhibit dichroism, and common colour banding parallels prism and pyramidal faces and basal planes (Coldham 1985; Coenraads 1992b). Unusual multi-layer colouring (blue, orangy pink, yellow) is described from Queensland (Koivula & Kammerling 1989) and rare zoned sapphires show cores of ruby (Brown et al. 1990). Black sapphires may reach large sizes (e.g. a 1156 carat, Queensland cabochon) and contain exsolved hercynite, hematite or ferrian ilmenite which obscures the underlying colour (Moon & Phillips 1986; Hughes 1990). Typical blue New England sapphire contains about 17 000 ppm Fe, 2000 ppm Ti, 1000 ppm Ga and 800 ppm Ni, with other trace element contents not exceeding 200 ppm (Mumme 1988). Fe is high (up to 1.1 wt%) in Australian and Thai sapphires, when compared to sapphires from volcanic fields in Kenya and China (Pearson 1982; Tombs 1991; Guo et al. 1992b). Optical adsorption spectra for royal blue and green Australian sapphires show that the blue Fe2+ /Ti4+ charge transfer band reduces the Fe3+ crystal field transmission in the green, compared to the Fe3+ dominance in green stones (Moon & Phillips 1986). Rare colour change Queensland sapphires (olivine green in sunlight, reddish brown in incandescent light) show Fe (1.1 wt%) as the only significant trace element (Koivula et al. 1993). Changes due to systematic heat treatment of Queensland sapphires are documented in Themelis (1995). Iron and titanium oxides (rutile, ferrian ilmenite, hercynite, magnetite, hematite) dominate epigenetic mineral inclusions in Australian sapphires and produce silk and asterism (Moon & Phillips 1985, 1986). Rare multiple chatoyancy, showing bicoloured stars in a 12-ray

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Table 3 Summary of REE analyses of selected zircons from eastern Australian gemfields.

Hf(wt%) Y (ppm) La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

BH4*

HG5

HG6

HG7

BK8

BTcol

BTclr

1.08 663 0.03 1.95 0.04