Mineral. Deposita 23, 150-157 (1988)

MINERALIUM DEPOSITA Springer-Verlag 1988

Complex unmixed spinels in layered intrusions within an obducted ophiolite in the Natal-Namaqua mobile beltH. V. Eales 1, A. H. Wilson 2, and I. M. Reynolds 1

1 Department of Geology, Rhodes University, Grahamstown 6140, South Africa 2 Department of Geology, University of Natal, Pietermaritzburg 3200, South Africa Spinellids showing unmixed intergrowths of chromite or chromian spinel (sensu stricto) and magnetite or chromian magnetite are not known in mafic or ultramafic igneous rocks. They do occur within metamorphosed rocks that attained temperatures sufficiently high (upper amphibolite facies) for the formation of homogeneous A1-Cr-Fe3+-Ti spinel phases with compositions not matched in slowly cooled igneous rocks. In the Tugela Rand intrusion complex intergrowths of chromian spinel, chromian magnetite, ulv/Sspinel, ilmenite and a transparent aluminous spinel are observed and interpreted in terms of the thermal history of the rocks. Compositional differences between the separate areas of chromian spinel and chromian magnetite in complex intergrowths exhibited by the metamorphosed Tugela Rand and Mambulu Complexes confirm the extension of the magnetitehercynite solvus (Turnock and Eugster 1962) towards magnesium- and chromium-rich compositions. The Tugela Rand spinellids are compared with those from the Carr Boyd Complex (Purvis etal. 1972) and the ultramafic rocks of the Giant Nickel Mine (Muir and Naldrett 1973) and the Red Lodge district (Loferski and Lipin 1983). Significant differences between the spinels from the Red Lodge district compared to the other three occurrences may reflect the different metamorphic histories of theseAbstract.areas.

Spinels intermediate in composition between aluminian chromite and magnetite are found in mafic volcanic rocks and in the chilled margins of mafic intrusion (Eales and Snowden 1979; Eales and Reynolds 1983). Spinels of similar composition are, however, notably absent from the more slowly cooled portions of mafic and ultramafic intrusions. This feature suggests the existence of a solvus between chromite and magnetite analogous to that between magnetite and hercynite, investigated by Turnock and Eugster (1962). Experimental studies have also shown that extensive solid solution exists between magnesioferrite, spinel, picrochromite, magnetite, hercynite and chromite at high temperatures, but that varying degrees of immiscibility are present at lower temperatures (Ulmer 1969; Cremer 1969; Navrotsky 19.75; Muan 1975). Despite these solid solution relationships, there are no reports of Cr-spinels containing exsolved bodies of magnetite or chromian magnetite in mafic and ultramafic Offprint request to: A. H. Wilson

igneous rocks. Spinels showing these relationships have, however, been described by Purvis et al. (1972) and Muir and Naldrett (1973) and, more recently, by Loferski and Lipin (1983) in metamorphosed ultramafic rocks. This feature is, in part, due to the well-documented "spinel gap" that separates the crystallization of early-formed chromite from the later crystallizing Ti-magnetite in the more slowly cooled igneous rocks (inter alia, Hill and Roeder 1974; Eales et al. 1980). As a result, intermediate chromian titanomagnetite compositions that might be expected to dissociate by exsolution are apparently not found as primary phases in slowly cooled mafic and ultramafic intrusions. The present paper describes complex, exsolved Cr-bearing spinels and other spinels of the Tugela Rand and Mambulu intrusions of Natal and sketches the position of the solvus between A1-Cr-rich and Fe-Ti-rich spinels. The Tugela Rand and Mambulu complexes are located in the medium- to high-grade metamorphic terrane of the Natal-Namaqua metamorphic belt (Du Toit 1931) w h i c h is believed to form part of an obducted ophiolite suite and may thus represent a segment of transformed oceanic crust (Matthews 1972, 1981). The metamorphic nappe complex comprises a number of thrust sheets with the Tugela Rand intrusion located in the uppermost Tugela nappe sheet (Fig. 1) and the Mambulu intrusion in the structurally lower Mandleni sheet. The layered complexes were intruded prior to the final nappe tectonics and radiometric dates for the granitic rocks of the mobile belt indicate a time span for the tectonothermal events to be in the range 900 to 1,200 Ma (Cain 1975; Matthews 1981). The Tugela Rand intrusion comprises a complexly layered body of mafic and ultramafic rocks with an areal extent of about 70 km 2. The body has been partly deformed with layers dipping gently southward in the north, but becoming vertical and overturned in the south. There is no evidence to show complete overturning of the complex as postulated by Dix (1979). Recrystallization has taken place in some parts of the intrusion and locally in shear zones related to the folding, but for the most part primary igneous textures and mineral compositions are well preserved. Thermodynamic modelling (Wilson, in prep.) for the silicate minerals show equilibrium temperatures of 1,000 C to 1,200 *C at pressures of between 5 and 8 kb, these being indicative of the igneous conditions. The geochemistry indicates that the magma which gave rise to the intrusions had affinities to an alkali basalt with era-

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Gneiss omphibolite MADIDIMA NAPPE

[]==~ Gneissic omphiboliteN KON10 N A P P E D Granite gneiss, gneissic amphibolite TUGELA NAPPE Amphi bol ires, metagabbro, p l a g i o g r o n i t e , gneissic amphi bol ires, kyanite gneiss

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the ultramafic rocks shows variable degrees of serpentinization with the dunite usually completely serpentinized. Interstitial phases in the ultramafic rocks comprise augite, bronzite and plagioclase with local alteration and replacement of these minerals by hornblende, talc and tremolite. Rocks marginal to the intrusion have primary postcumulus hornblende probably resulting from hydrous interaction of the original magma with the country rocks. In some samples plagioclase is extensively replaced by calcite and hydrogrossular. The spinels are present in the ultramafic rocks as isolated or clustered octahedra or as roundish crystals located interstitially to the cumulus olivines and pyroxenes. More rarely, spinels are enclosed within cumulus olivine or pyroxene. Grain size ranges between 10 and 100 Ftm with the occasional crystal reaching 0.4 mm in diameter. Chromite grains in the seams are very much coarser with the average size ranging from 1 to 4mm. In some marginal rocks spinels show hopper-type textures and characteristically enclose small crystals of pyroxene or olivine. The spinels within a single sample commonly show a range in colours with those rich in aluminium appearing translucent yellow and becoming greenish-brown and opaque with increasing iron and chromium content. Chromite grains from the seams are usually opaque or translucent red in thin sections. The spinels are considered to have undergone modification during the amphibolite grade metamorphism with possibly some reaction involving local silicate phases. This is in contrast to the silicate minerals which have resisted metamorphism because of the exceptionally dry nature of these rocks and higher blocking temperatures (Wilson, in prep.) and is also indicative of a metamorphic history of relatively short duration. Two types of spinel are present in the Tugela Rand Complex: 1. homogeneous aluminium-rich spinels and; 2. complex, exsolved intergrowths of several spinel species.

placement occurring in thickened oceanic crust analogous to an oceanic intraplate-type setting. The Mambulu Intrusion (Fig. 1) comprises similar lithologies and textural characteristics to the Tugela Rand intrusion, but appears to represent overall a more evolved analogue (Wilson, in prep.) but arising from differentiation of a magma of similar composition. The complex covers an area of about 30 km 2 and is saucer-shaped with layers dipping in towards the centre at between 10 and 40 . The striking feature of the complex is the cyclic nature of the lithologies of alternating pyroxenite and gabbro/anorthosite layers. Titaniferous magnetite layers are also developed within the layered sequence. This Mambulu complex has suffered significant deformation and fairly extensive recrystallization although relict igneous textures are still preserved. The greater deformation of the Mambulu Complex compared to Tugela Rand probably arises because of the location of this complex in one of the structurally lower nappe sheets.

Homogeneous spinelsThese grains are not in contact with olivine and exhibit relatively low Fe203 contents and are usually high in aluminium. Their compositions are comparable with those of Cr-spinels for ultramafic rocks that have undergone amphibolite grade metamorphism as described by Evans and Frost (1975). These grains show no evidence of intergrowths with other spinellids or zoning under reflected light.

Complex spinel intergrowthsThese consist largely of a host phase that is dark grey in reflected fight and which contains more Fe203 than the homogeneous grains with comparable levels of Cr203. This phase acts as host to numerous lighter-coloured, Ferich globular or blebby segregations that occur individually or as stringers within the grain or along the margins of the host (Figs. 2-4). These Fe-rich phases are predominantly chromian magnetite with similar levels of Cr to their hosts, and are easily recognizable by virtue of their higher reflectivity. A small number of ilmenite granules is also present amongst the inclusions. The exsolution de-

Petrography and mineralogyLithologies of the Tugela Rand intrusion comprise dunites, wehrlites, harzburgites, lherzolites, pyroxenites and gabbroic rocks. The spinels studied are present as accessory phases in the ultramafic rocks or as discontinuous and podiform-type chromitite layers. Olivine in

152

Fig. 2. A spinal grain showing complex intergrowths. The larger white granules are an Fe-rich Crspinel enclosed within the dark grey matrix of a Cr- and N-rich spinel. Granules of a similar Ferich composition are also located at the margins of the grain. Intergrown within the matrix is a set of very fine vermicular bodies of another Fe-rich spinel imparting the generally blotchy appearance to the grain. Incident light, oil immersion Fig. 3. An area of the grain in Fig. 2 under higher magnification and showing the very fine (< 1 micron) vermicular intergrowths of Fe-rich spinel (white) in the areas between the large granules. Note the absence of the fine Fe-rich spinel in the matrix (Cr- and AIrich spinel) immediately surrounding the larger bodies. Incident light oil immersion Fig. 4. A complex spinel grain showing the development of coarse Fe-rich spinel external granules (white) and a fine micrometre-sized intergrowth of Fe-rich spinel parallel to (100) with the darker Cr- and N-rich spinel. Incident light, oil immersion

Fig. 5. The central upper granule in Fig. 4. viewed under higher magnification. This Fe-rich spinel body hosts several large ilmenitelamellae parallel to (111) (light grey) that are rimmed with bodies of N-rich spinel (darker grey). A further set of finer Al-rich spinel intergrowths (darker grey) is also present in a grid-like pattern within the areas between the ilmenite lamellae

scribed is quite distinct from ferritchromite which occurs as an alteration product of chromite grains in ultramafic rocks. Ferritchromite usually occurs on the margins or along fractures of chromite grains (Beeson and Jackson 1969; Bliss and Maclean 1975). A second set of very much finer, micrometre-sized magnetite bodies that are close to the limits of optical resolution appears in the areas between the larger chromian magnetite blebs (Figs. 3 and 4). These form tiny lamellae oriented parallel to (100) of their hosts and are notably absent from narrow zones surrounding the larger chromian magnetite bodies (Fig. 3). Also, they decrease in size towards the margins of their hosts. The larger chromian magnetite segregations (bright areas of Figs. 2 and 4) are themselves highly complex, and commonly show the development of sparse ilmenite lamellae parallel to (111) (Fig. 5), with micrometre-wide rims of transparent spinel along the ilmenite-magnetite contacts. A further set of fine, micrometre-sized transparent spinel lamellae is developed parallel to (100) in the areas between these ilmenite plates. Yet another pinkish phase (ulv6spinel) is occasionally discernible as fine micrometre-sized lamellae parallel to (100) of the host chromian magnetite. This latter exsolved phase is not always present for its place may be taken by sets of micrometre-sized ilmenite "trellis lamellae" developed parallel to (111) of the host chromian magnetite.

The spinels of the Mambulu intrusion display a more complex relationship of lamellar and granular unmixing and oxidation/exsolution than that normally encountered in rocks of this nature and reflect the reactivation of exsolution processes during regional metamorphism. These also contrast with the spinels from the Tugela Rand Complex and probably result from the overall bulk compositional and mineralogical differences between the two intrusions and the more intense regional and cataclastic metamorphism experienced by the Mambulu Complex. Ti-magnetite ores from this body are highly aluminous as indicated by the abundance of green pleonaste that occurs as discrete grains and as exsolution bodies within the Ti-magnetite. The pleonaste comprises 2 to 10 percent of the ore by volume and the grain size ranges from 0.2 mm to 1.5 mm. Small grains of h6gbomite are commonly present around the margins of the larger crystals and along fracture planes (Reynolds 1981). The pleonaste grains sometimes show alteration to diaspore or may be replaced by fine aggregates of corundum and magnetite. The Mambulu ores are characterized by abundant (15 to 30 volume percent) medium- to fine-grained ilmenite interstitial to the larger Ti-magnetite crystals. Small crystals of pleonaste often surround the ilmenite grains, particularly where these are in contact with Ti-magnetite. Spinel lamellae are also developed parallel to (0001) in certain ilmenite crystals (Reynolds 1986). Spinel lamellae

153 in ilmenite have also been reported from the Tellnes deposit in Norway (Gierth and Krause 1973). Titaniferous magnetite is the dominant ore phase of this intrusion and occurs as polygonal grains 0.5 to 3 mm in diameter. Both ilmenite and pleonaste exsolution lamellae are abundant in the magnetite. Linear, arcuate or circular trains of rounded pleonaste bodies are also numerous within the magnetite. Compositions of the spinels from this body are not dealt with in detail here, but analytical data of nearly pure spinel and magnetite that are intimately intergrown have been included to extend the range of data points in Fig. 6.AI 3+

Compositional relationshipsCompositions of the various phases have been established by microprobe analysis (for operational details, see Eales et al. 1980). Where homogeneous, the grains examined present no analytical problems and compositions of coarse intergrowths are also readily obtained. However, complex grains with fine-scale intergrowths require repeated analysis along short traverses to achieve consistent averages. The data used in the plots are based on analyses of 210 separate areas, and Fe is assigned to Fe 2 and F P + on the assumption of stoichiometry, with all Ti being expressed as the ulv6spinel (F%TiQ) molecule. Three fields are clearly revealed by the analytical data i (Fig. 6) representing (a) grains which appear compositionally homogeneous on the basis of microprobe traverses and polished sections; (b) Fe-enriched host phases to the segregations; and (c) the strongly Fe- and Ti-enriched, highly reflecting chromian magnetite segregations. The homogeneous grains are consistently displaced towards the A1-Cr join on the ternary plot of Fig. 6, relative to these grains which occur as hosts to the chromian magnetite segregations. Several data points plotting close to the (Fe3++2 Ti)-Cr join are from rare grains (encountered in only three samples) which do not show segregations and represent a further group of spinels of homogeneous composition. The distribution of these data on the ternary diagram indicates the limb of a solvus close to the A1-Cr join with the other limb close to the (Fe3++2Ti)-Cr join. The homogeneous grains with low (Fe~++2 Ti) or low AI would lie outside the solvus on this projection. Where analysis of the host and segregated phases is permitted by the sufficiently coarse grain size the data points are linked by solid lines shown in Fig. 6. The compositions representing unmixing on the solvus plot on a narrow arc. The range of compositions encountered for the Mambulu Complex is restricted essentially to co-existing aluminous spinel and titaniferous magnetite with very low chromium content ( 0.30 for all the occurrences. The marked depletion of Ti in the spinels from Red Lodge compared to those from the other bodies is puzzling and, although this may reflect primary magmatic compositions, a different metamorphic history may have resulted in granule exsolution of ilmenite in the former, although this is not mentioned by Loferski and Lipin (1983). The wide range in Ti contents at low magnesium numbers for the other occurrences probably results from variable oxidation in exsolution ofilmenite.

Amongst the minor elements, Mn shows partitioning in favour of the Fe2+-rich phase, but the behaviour of Ni is unpredictable.Discussion and conclusions

Development of homogeneous solid solutions during metamorphismThe unmixing of chromian magnetites in subordinate amounts from Al-rich host phases with different levels of Cr (Fig. 6) points to the previous existence in these rocks of a series of homogeneous solid solutions at higher temperatures, with compositions somewhat more enriched in Fe than the present host phases of the intergrowths. Putative compositions such as these have not been reported for layered mafic complexes, although they are exhibited in various rocks that experienced rapid quenching (Eales and Reynolds 1983). It is of interest to note that the unusual Al-enrichment trend exhibited by Cr-spinels of the Rhum intrusion (Henderson and Suddaby 1971; Henderson 1975) extends their compositional range into the arcuate field shown in Fig. 7, but (except for one point) not into the area circumscribed by the 'solvus' (Fig. 9) for the Tugela Rand, Giant Nickel Mine and the Carr Boyd Rocks Complex. This reaction, which Henderson (op. cir., p. 1041) identifies as plagioclase+olivine+highCr spinel --, high-A1 spinel+liquid, does not therefore offer a plausible mechanism for generating the required homogeneous parent phase. Chromite is a stable phase within mafic igneous rocks (apart from minor exsolution of titanian phases) but sub-solidus equilibration between chrome spinel and olivine does lead to increase in (XMg/XFe)olivine/(XMg/XFe)spinel,where Xi represents mole fractions. This reaction has been formulated on thermodynamic grounds by Irvine (1965) and also confirmed by studies of natural assemblages (e.g. Medaris 1975; Wilson 1982) and experimental products (Roeder et al. 1979), but again in slowly cooled igneous rocks does not lead to compositions appropriate to the putative homogeneous parent phase. Several possibilities for generating the required compositions exist under conditions of prograde metamorphism. Bliss and MacLean (1975) have, for example,

155AI 3+

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Fig. 9. Al-enrichment trend exhibited by Rhum complex spinels(data of Henderson and Suddaby, 1971 and Henderson, 1975). Compositions approach but, except for a single sample, do not impinge upon area circumscribed by arcuate field of unmixed spinels

mineral group at magmatic temperatures, but their lowertemperature phase relationships are known with far less certainty (Muan 1975). The consolute temperature of the magnetite-hercynite solvus is at 860C (Turnock and Eugster 1962) and that of magnetite-ulv6spinel at approximately 600 C (Vincent et al. 1957; Basta 1960; Price 1979); that of chromite-magnetite is not accurately known but has been estimated at approximately 500 C by Evans and Frost (op. cit.) who also estimated that of chromite-A1 spinel at approximately 700C. These data refer to endmember compositions and the positions of the solvi might be considerably influenced by more complex solid solution involving Cr-AI-Fe 3+ such as is indicated by the spinellids from the four occurrences. The thermal history will be an important factor in determining the nature of the exsolution. A single metamorphic event of limited duration is indicated for the Tugela Rand Complex. On the other hand, the Red Lodge ultramafic rocks have undergone two metamorphic events with the earlier locally up to granulite facies (Loferski and Lipin 1983) as compared to the lower temperature event of Tugela Rand. A more protracted cooling history of the Red Lodge rocks would be consistent with unmixing processes persisting to lower temperatures resulting in a wider solvus being defined for the coarser exsolution. The finer exsolution (Type C of Loferski and Lipin, op. cit.) indicates the lower temperature limits at which unmixingOccurs.

modelled the formation of ferritchromite (A1- and Mgpoor chromian magnetite) rims to chromite as a two-stage process. Magnetite released from olivine during serpentinization is deposited around original chromite grains. During prograde metamorphism to the epidote amphibolite facies, cation exchange by diffusion between the spinellid species yields intermediate ranges of composition. Such intermediate products are characterized by depletion in A1, Cr and Mg, and enrichment in Fe 2+ and Fe 3+ relative to original magmatic chromite. The study by Evans and Frost (1975) of metamorphism of ultramafic rocks within the upper parts of the amphibolite facies showed that spinels follow a trend towards N-enrichment as aluminous chlorite (MgsA12SisOlo(OH)8) is consumed. Thus, at temperatures above the chromian spinel - chromian magnetite solvus, conditions would be appropriate to the formation of a range of compositions intermediate between these species. Only rather crude estimates can be made of the temperatures attained during the formation of the solid solutions. The data of Bliss and MacLean (1975) suggest that zoned grains with chromite cores and ferritchromite rims are produced in the epidote amphibolite facies metamorphism of serpentinites of central Manitoba. Higher temperatures than this are indicated by the absence of zoning in the Tugela Rand spinellids. Diagnostic assemblages are somewhat obscured in our samples by the effects of retrograde metamorphism and possible metasomatic alteration leading to the development of hydrogrossularite (de Waal 1969) but the relict assemblage olivine + enstatite + tremolite is indicative of upper amphibolite facies metamorphism (Evans and Frost 1975). Cain (1975) assigns an average grade of almandine amphibolite facies to the region. Rather crudely, the temperatures may be assessed as 550-650C (Winkler 1976). Extensive solid solution is possible within the spinel

Development of intergrowthsduringcoolingAny model for the development of the microstructures observed in the Tugela Rand spinels should account for: 1. the persistence of homogeneous, highly aluminous (chromian) spinels free of segregations, adjacent to others with conspicuous mottling of the aluminous spinel hosts by chromian magnetite, 2. the occurrence of both ulv6spinel and ilmenite within the chromian magnetite blebs, with protrusion of ilmenite lamellae beyond the margins of the chromian magnetite blebs, 3. the occurrence of minute, transparent, aluminous spinel granules among the planar contacts of ilmenite blades with their chromian magnetite hosts, and 4. micrometre-scale intergrowth of aluminian spinel and chromian magnetite in the areas between the larger chromian magnetite blebs, and the existence of clear haloes devoid of segregations around such blebs. The following sequence is indicated by the microstructures: 1. Initial segregation of the parent solid solutions into weakly aluminous chromian titanomagnetite blebs within chromian spinel (dominant phase) as the solvus is intersected on cooling. Grains with compositions plotting to the convex side of the arcuate field indicated in Fig. 6 were sufficiently displaced from the solvus (by virtue of higher Cr levels) to cause their cooling trajectories to intersect the solvus only at lower temperatures such that diffusion was too sluggish to permit segregation. Such grains have remained homogeneous to the present time, at least on the microscopic scale.

156 2. The formation of the chromian titanomagnetite blebs would almost certainly have been initiated by heterogeneous nucleation (Champness and Lorimer 1976) largely at grain-boundary imperfections, leading to the formation of external granules. Relicts of imperfectly homogenized magnetite rims would have acted as favourable sites. The larger internally exsolved blebs would also belong to this stage. These processes require relatively long-range migration of ions and would thus have been confined to higher temperatures prevailing during the earlier stages of cooling. The decline in ionic diffusion rates with falling temperatures and the gradual widening of depleted zones around the blebs would progressively retard the growth of the blebs until it effectively ceased. 3. It is known that Ti is only sparingly accommodated within Cr-A1 spinels (Eales et al. 1980, Fig. 6) so that even minor amounts of Ti are readily exsolved from typical magmatic chromites. The small smount of Ti originally present in the homogeneous solid solution was therefore strongly partitioned into the relatively small volume of chromian titanomagnetite. Magnitite (Mt)-ulvtspinel (Usp) solutions are very sensitive to fO2, increase of Which leads to oxidation of the ulvtspinel component to ilmenite. The latter is only sparingly soluble in magnetite solid solution and is exsolved at high temperatures as external granules and lamellae parallel to (111) of their host phase (Buddington and Lindsley 1964). This process may have been initiated early in Stage 2, as the larger ilmenite lamellae protrude from the chromian magnetite blebs into the aluminous spinel host phase. 4. The growth of ilmenite lamellae would have created a halo of N-enrichment in the chromian magnetite immediately surrounding such lamellae. Furthermore, dislocations along the planar ilmenite-magnetite grain boundaries would have provided particularly favourable sites for heterogeneous nucleation and subsequent growth of transparent, N-rich spinel as the solvus between Mt-Usp solid solution and chromian magnetite was intersected with falling temperatures. The lowered temperatures at this stage would have favoured only short-range diffusion and the growth of these micrometre-sized granules would have ceased as soon as the immediate surroundings had been depleted of appropriate components. 5. The growth of all larger exsolved blebs and plates would have slowed and effectively ceased with falling temperatures in response to a decrease in the effectiveness of long-range diffusion. Concentration gradients would thus be established, with supersaturation of the exsolving phases increasing with distance from the larger blebs or plates. Further decline in temperature would initiate either homogeneous or heterogeneous nucleation, but subsequent growth would be restrained, yielding micrometresized segregations at the greatest distances from earlierformed coarse blebs. Three sets of lamellae characterized this stage in the Tugela Rand samples: a) fine chromian magnetite lamellae within the dominant host phase, b) fine ulvtspinel lamellae, and c) aluminous spinel lamellae, within chromian magnetite. These lamellae all developed within the (100) planes of their hosts, imparting a fine "cloth texture" at the limits of optical resolution. In some samples, the occurrence of ilmenite parallel to (111) planes of their host chromian magnetite is indicative of somewhat higher fO2.

Acknowledgements. Grateful acknowledgement (IMR and AHW)is made to the CSIR, Pretoria, for financial assistance during this investigation. Colleague Roger Jacob is thanked for criticisms of the manuscript.References

Basta, E.Z.: Natural and synthetic titanomagnetites. (The system Fe30-Fe2TiO-FeTiO3.) Neues Jahrb. Min. Abh. 94:1017-1048 (1960) Beeson, M.H., Jackson, E.D.: Chemical composition of altered chromites from Stillwater Complex, Montana. Amer. Mineral., 54:1084-1100 (1969) Bliss, N.W., Maclean, W.H.: The paragenesis of zoned chromite from central Manitoba. Geochim. Cosmochim. Acta 39: 973-990 (1975) Buddington, A.F., Lindsley, D.H.: Iron-Titanium oxide minerals and synthetic equivalents. J. Petrology 5: 310-357 (1964) Cain, A.C.: A preliminary review of the stratigraphic relationships and distribution of metamorphism in the northern part of the Natal-Namaquarides, South Africa. Geol. Rundschau 64: 192-216 (1975) Champness, P.E., Lorimer, G.W.: Exsolution in silicates. In: Electron Microscopy in Mineralogy (H.R. Wend, Ed.) pp. 174-204. Berlin-Heidelberg-New York: Springer 1976 Cremer, V.: Die Mischkristallbildung im System Chromit-Magnetit-Hercynit zwischen 1,000C und 500C. Neues Jahrb. Min. Abh. 111:184-205 (t969) De Waal, S.A.: On the origin of hydrogrossularite and other calcium silicates in serpentinites. Trans. Geol. Soc. South Africa 72:23-27 (1969) Dix, O.R.: High-grade metamorphism and possible overturning of the Tugela Rand layered intrusion in southeast Africa. Geology 9:155-160 (1979) Du Toit, A.L.: The Geology of the Country Surrounding Nkandhla, Natal. South African Geol. Surv. Expln. Sheet No. 109:111 p. (1931) Eales, H.V., Reynolds, I.M.: Factors influencing the composition of chromite and magnetite in some southern African Rocks. In: Proc. 1st Int. Conf. Applied Mineralogy (ICAM) Randburg, 1981 (1983) Eales, H.V., Reynolds, I.M., Gouws, D.A.: Spinel-group minerals of the Central Karoo tholeiitic province. Trans. Geol. Soc. South Africa 83: 243-253 (1980) Eales, H.V., Snowden, D.V.: Chromiferous spinels of the Elephant's Head dike. Mineral. Deposita 14:227-242 (1979) Evans, B.W., Frost, B.R.: Chrome spinel in progressive metamorphism. Geochim. Cosmochim. Acta 39:959-972 (1975) Gierth, E., K_rause, H.: Die ilmenite-lagerstatte Tellnes (Sud Norwegen). Norsk Geol. Tidsskr. 53: 359-402 (1973) Henderson, P.: Reactive trends shown by chrome spinels of the Rhum layered intrusion. Geochim. Cosmochim. Acta 39: 1035-1044 (1975) Henderson, P., Suddaby, P.: The nature and origin of the chromespinel of the Rhum layered intrusion. Contr. Mineral. Petrology 33:21-31 (1971) Hill, R., Roeder, P.: The crystallization of spinel from basaltic liquid as a function of oxygen fugacity. J. Geol. 82:709-729 (1974) Irvine, T.N.: Chromian spinel as a petrogenetic indicator Part 1. Theory. Can. J. Earth Sci. 2:648-672 (1965) Loferski, P.J., Lipin, B.R.: Exsolution in metamorphosed chromite from the Red Lodge district, Montana. Amer. Mineral. 68: 777-789 (1983) Matthews, P.E.: Possible Precambrian obduction and plate tectonics in southeastern Africa. Nature Phys Sci. 240:37-39 (1972)

157 Matthews, P.E.: Eastern or Natal sector of the Namaqua-Natal mobile belt in southern Africa. In: Precambrian of the Southern Hemisphere (D.R. Hunter, ed.) pp. 705-715. Elsevier, Amsterdam 1981 Medaris, L.G.: Coexisting spinel and silicates in alpine peridotites of the granulite facies. Geochim. Cosmochim. Acta 39: 947-958 (1975) Muan, A.: Phase relations in chromium oxide-containing systems at elevated temperatures. Geochim. Cosmochim Acta 39: 791-802 (1975) Muir, J.E., Naldrett, A.J.: A natural occurrence of two-phase chromium-bearing spinels. Can. Mineral. 11:930-939 (1973) Navrotsky, A.: Thermochemistry of chromium compounds. Geochim. Cosmochim. Acta 39:819-832 (1975) Price, G.D.: Microstructures in titanomagnetites as guides to cooling rates of a Swedish intrusion. Geol. Mag. 116: 313-318 (1979) Purvis, A.C., Nesbitt, R.W., Hallberg, J.A.: The geology of part of the Carr Boyd Rocks complex. Econ. Geol. 67:1093-1113 (1972) Reynolds, I.M.: The mineralogy and petrography of some vanadium-bearing titaniferous iron ores of the Mambula Complex, Zululand. In: Mineral Deposits of Southern Africa. l l. (C.R. Anhaeusser and S. Maske, Eds.) Geol. Soc. S. Afr. 1695-1708 (1986) Reynolds, I.M.: An occurrence of iron-rich h6gbomite in metamorphosed titaniferous iron ores of the Mambula Complex, Zululand. S. Afr. J. Geol. 90:64-71 (1987) Roeder, P.L., Campbell, I.H., Jamieson, H.: A re-evaluation of the olivine-spinel geothermometer. Contr. Mineral. Petrology 68: 325-334 (1979) Stevens, R.E.: Composition of some chromites of the Western Hemisphere. Amer. Mineral. 29:1-4 (1944) Turnock, A.C., Eugster, H.P.: Fe-AI oxides: phase relationships below 1,000 C. J. Petrology 3: 533-565 (1962) Ulmer, G.C.: Experimental investigations of chromite spinels. In: Magmatic Ore Deposits, Monograph4 (H.D.B. Wilson, ed.) pp. 114-131. Econ. Geol. Publ. Co. (1969) Vincent, E.A., Wright, J.B., Chevallier, R., Mathiew, S.: Heating experiments on some natural titaniferous magnetites. Mineral. Mag. 31:624-655 (1957) Wilson, A.H.: The geology of the Great "Dyke", Zimbabwe. J. Petrology 23: 240-292 (1982) Winkler, H.J.F.: Petrogenesis of Metamorphic Rocks. 4th ed. Berlin-Heidelberg-New York: Springer 1976

Received: July 22, 1986 Accepted: October 29, 1987

A nnouncementsNorthern Hydrocarbon Development in the Nineties: A Global PerspectiveSeptember 30-October 4, 1988 Yellowknife, Northwest Territories and Calgary, Alberta, Canada. For more information contact: Mr. F. Frankling, Geotechnical Science Laboratories, Carleton University, Ottawa, Ontario, Canada K1S 5B6. Scope: The Symposium Gold 89' - In Europe will highlight the recent results and advances made in Europe in the characterization of gold deposits, their exploration and beneficiation. Despite the emphasis put on Europe, contributions from other continents are warmly welcome. The four major types of gold deposits will receive equal attention: massive sulfide and sedimentary exhalative deposits, epithermal veins, shear zones and placers. The different sessions will be introduced by keynote speakers. Time and place: May 23 through 25, 1989 at the Universit6 Paul Sabatier, Toulouse, France. Field trips: Four field trips of 4 or 5 days each are planned, before or/and after the Symposium: 1 - Gold deposits in the French Montagne Noire (active Salsigne mine: strata-bound and vein mineralization; exploration, exploitation and beneficiation) and in Cataluna (France and Spain). 2 - The gold districts of the Limousin (NW French Massif Central): Exploration of abandoned mines (Le Chatelet, Vige, Villerange); active mines of Cros Gallet-Le Bourneix and of Lauri~ras. 3 - The gold district of NW Spain. 4 - Epithermal gold veins of Rodalquilarin a MioPliocene caldera (NE Spain). Information: Francis Tollon = Laboratoire de Minrralogie 39 allres Jules Guesde - F-31400 Toulouse Cedex - France Phone: 61 53 02 35 1LP. Foster - Department of Geology - The University - Southampton, S09 5NH - United Kingdom - Phone: (0703) 55 91 22 Telex: 47 661

The SGA is pleased to announce the following meetingOctober 23-30, 1988 In honour of Prof. Pietro Zuffardi, former president of the SGA, a meeting will be held in Cagliari, followed by a visit to the Sardinian mineral deposits. For further information please contact: Secretariat Zuffar'days 88, Isfituto di Giacimenti Minerari, Piazza d'Armi, 09123 Cagliafi (Italy). Telephone: (070) 28 08 37

GOLD 89' - In EuropeOrganized by the Universities of Toulouse (France) and Southampton (UK) and the Bureau de Recherches Grologiques et Minirres (France) in cooperation with the International Liaison Group on Gold Mineralization. Co-sponsored by the Society for Geology Applied to Mineral Deposits