Jingfeng Guo ? Suzanne Y.

O’Reilly
William L. Griffin

Corundum from basaltic terrains:

Received: 15 March 1995 y Accepted: 30 June 1995

Abstract This paper investigates the origin of corun- ic model for corundum megacrysts is experimentally
dum megacrysts that occur in many basaltic terrains, and testable.
which are considered to be eroded from basaltic rocks.
Geochemical data for over 80 primary mineral inclu-
sions within corundum megacrysts are used to gain a new Introduction
insight into the petrogenetic history of the corundum
megacrysts. A wide spectrum of minerals is present as Most of the world’s current sapphire production is taken
inclusions in the corundum; the most common are from recent or palaeo-alluvial deposits in basaltic ter-
Nb2Ta oxides (such as titaniferous columbite and uran- rains sparsely distributed in eastern Australia, Thailand
pyrochlore), alkali feldspar, low-Ca plagioclase (albite- and eastern China. The lack of understanding of corun-
oligoclase) and zircon. Rare inclusions include Fe,Cu- dum genesis and the geology of corundum-bearing par-
sulphide (low in Ni), cobalt-rich spinel, Th,Ce-rich ent rocks in basaltic terrains has greatly limited the
phosphate and uraninite. The similar chemistry of some search for new economic deposits.
inclusion minerals from corundum occurring in widely There is an empirical relationship between the occur-
separated areas suggests that the corundum megacrysts rence of alluvial sapphire deposits and the presence of
in basalts have a similar petrogenesis. Geochemical basaltic rocks in certain volcanic provinces worldwide,
characteristics of the inclusions indicate a bimodal and basaltic rocks have been considered the source of the
grouping, which is best explained by a mixing-hybridisa- corundum. Examples include eastern Australia (e.g.
tion process. This study indicates that the corundum MacNevin 1972; Stephenson 1976; Coldham 1985;
megacrysts are not cogenetic with their basaltic hosts Krosch and Cooper 1990), eastern China (Keller and
but are crustal fragments accidentally incorporated into Wang 1986; Guo et al. 1992), the Indochina region of
the erupting magma. It is suggested that interactions southeastern Asia (e.g. Vichit et al. 1978; Barr and Mac-
between a silicic component and an intruding carbon- Donald 1978), Nigeria (Kiefert and Schmetzer 1987),
atitic or similar Si-poor magma is responsible for Al- and Scotland (Upton et al. 1983; Aspen et al. 1990).
oversaturation, resulting in locally distributed lenses Weathering and erosion of basaltic lavas, pyroclastics,
of corundum-bearing rock of mixed paragenesis (“ hy- plugs and diatremes, and subsequent alluvial processes
brid rock hypothesis ”). Feldspar exsolution textures resulted in economic deposits of the gem-quality variety
provide strong evidence that this hybridisation occurred sapphire in eastern Australia, eastern China, the In-
at mid-crustal levels. Subsequent volcanic eruptions dochina region of southeastern Asia (Thailand, Vietnam
brought the corundum-bearing rocks (later disintegrated and Cambodia) and northern Africa (Kenya and Nige-
in the magma) up to the Earth’s surface. This petrogenet- ria). Notwithstanding these observations, petrological
evidence suggests that corundum could not crystallise
from basaltic magmas (e.g. Green et al. 1978).
J-F. Guo (✉) ? S.Y. O’Reilly
Key Centre for Geochemical Evolution and Metallogeny However, apart from the empirical observation and
of Continents (GEMOC), experimental evidence above, few studies have been car-
School of Earth Sciences, Macquarie University, ried out to elucidate the origin of the alluvial corundum
Sydney, NSW 2109, Australia as well as the corundum megacrysts found in basaltic
W.L. Griffin rocks. Many aspects of the “ sapphire enigma ” are still
CSIRO Division of Exploration and Mining, untouched although corundum clearly forms part of the
PO Box 136, North Ryde, NSW 2113, Australia well-documented megacryst suite in alkalic basaltic
Editorial responsibility: J. Touret rocks. A knowledge of corundum genesis may improve
our understanding of the whole megacryst suite. This recovered from the surrounding alluvial materials clear-
forms the subject of the present investigation. ly is derived from the breakdown of the corundum-bear-
ing basalts (Guo et al. 1992). Recently, a unique, large
corundum aggregate (5 cm32 cm) was found in situ in
basaltic scoriaceous lava matrix in the Chudleigh Vol-
Occurrence
canic Province, northern Queensland (M. Zhang and O.
Gaul, personal communication 1994); the host basalt
Kopecky et al. (1967) were among the fist to report contains numerous peridotite xenoliths.
corundum occurring as discrete crystals in diatremes of In several basaltic terrains, corundum crystals are
kimberlitic and carbonatitic affinity; they were consid- found in the weathered residuals or soil profiles lying
ered to be xenocrysts. Reports of in situ occurrence of directly above the basaltic plugs, lavas or pyroclastic
corundum in basaltic rocks (and volcanic rocks in gener- rocks; these include localities of the Inverell-Glen Innes
al) have been rare. MacNevin (1972) documented rare in region, NSW, Australia (MacNevin 1972), Lava Plains,
situ occurrences of corundum megacrysts in basalt from northern Queensland, Australia (Krosch and Cooper
the Inverell-Glen Innes region, northern New South 1990), Bo Ploi, Thailand (Bunopas and Bunjitradulya
Wales, Australia. These corundum megacrysts show 1975), Wenchang, Hainan, China (Wang 1988; Guo
many similarities to the corundum recovered from the 1993) and Luhe, Jiangsu, China (Guo 1993). Similar
nearby alluvial deposits. Stephenson (1976) reported the corundum occurrences were reported at the Pailin rubyy
occurrence of corundum-anorthoclase composite sapphire deposit, Cambodia (Jobbins and Berrangé
megacrysts and a number of euhedral corundum crystals 1978).
in basalts of the Hoy Volcanic Province in central Alluvial corundum is equivalent to the corundum
Queensland, Australia. megacrysts found in situ in basalts in many aspects (Guo
At two localities in Scotland, Loch Roag in the west 1993). This confirms the empirical observation of a
and Ruddon’s Point of Fife in the east, corundum-bear- corundum-basalt association. The similarities are:
ing xenoliths are found in alkali olivine basalt host rocks 1. They are all coloured stones ranging from very dark
(Upton et al. 1983; Aspen et al. 1990). The Scottish blue (nearly black), through blue, green to less com-
corundum forms part of a cogenetic assemblage domi- mon yellow.
nated by alkali feldspar; other phases include zircon, 2. They have similar trace element fingerprints, which
magnetite, ilmenorutile, yttro-niobate, and probably bi- are distinctly different from those of corundum from
otite and apatite. Many large euhedral corundum crystals non-basaltic terrains (Guo et al. in preparation).
have been extracted directly from basaltic lava flows at 3. Their crystal habits and surface features are directly
Changle, China (Guo et al. 1992). At this locality, corun- comparable. The corundum megacrysts are typically
dum is concentrated in massive alkali basalt flows that of barrel-shaped habit (Fig. 1 a). The alluvial corun-
locally contain abundant ultramafic xenoliths (forming dum commonly occurs as broken fragments, but some
up to 30–40% by volume), and there appears to be a of these preserve original crystal faces and shapes
positive correlation between the abundance of xenoliths identical to those of the megacrysts. Both the
and of corundum (Fig. 1 a). The gem-quality corundum megacrysts and the alluvial corundum exhibit surface
features typical of high temperature corrosion
(Fig. 1 b).
Fig. 1 a A deep blue corundum megacryst in the matrix of alkali Indirect evidence also supports the megacryst origin of
basalt from Changle, Shandong Province, China; the yellow frag- the alluvial sapphires, e.g.
ments adjacent to the megacryst are altered peridotite xenoliths.
b SEM image of the surface of a corundum from Lava Plains, i. Some alluvial corundum exhibits only minor me-
Queensland, Australia, showing irregular corrosion patterns re- chanical abrasion along the crystal edges and the
sulting from the corundum-basalt interaction original surface features are well preserved, which