Mineral. Deposita 22, 1-10 (1987) MINERALIUM DEPOSITA

© Springer-Verlag 1987

Chromite deposits in the northern Oman ophiolite: Mineralogical constraints T. Aug~

Groupement d'Int~r~t Scientifique BRGM-CNRS, 1 A, rue de la Ftrollerie, 45071 Orleans, Cedex 02, France

Abstract. Chromite deposits in the northern Oman ophiolitic complex occur in three structural contexts, i.e., (1) at the base of the cumulate series, (2) in the top kilometer of the mantle sequence, and (3) in the deeper parts of the mantle. Types 1 and 2 are characterized by the diversity of interstitial silicates where in decreasing order of abundance olivine, clinopyroxene, orthopyroxene, plagioclase, and amphibole occur, as opposed to type 3 which contains only olivine. They differ however in ore texture. Similar silicates also occur as euhedral inclusions in chromite crystals, but their proportions are reversed. The composition of the interstitial silicates is comparable to that found in early cumulates. Type-1 and type-2 chromite deposits crystallized from a magma similar to that from which the basal cumulates formed (A1203, 15.1-16.1 wt%; FeO/MgO, 0.55-0.60). The type-3 chro- mites were derived from a magma of much lower A1203 content (12.5 wt%). It is considered that they belong to an older episode in the magmatic evolution of the complex.

Although chromite deposits in ophiolitic complexes represent a low percentage of the total known chromium reserves (Thayer and Lipin 1978) and are of relatively small tonnage, they are of strategic interest for the diversifi- cation of exploration targets. However, the mode of forma- tion of ophiolitic chromitite is not clearly established. The aim of this paper is to present a synthesis of mineral- ogical data on chromite occurrences from the northern part of the Oman ophiolite and to make a comparison with the mantle series and the base of the cumulate sequence to contribute to the understanding of chromite concentration processes. Special attention is paid to silicates associated with chromite ore. Their study has two main interests, i.e., to obtain information on the conditions of crystallization of the host ore and, as they are the first phases to crystal- lize from the upwelling magma, they can give information on the nature of the primitive parent magma. Finally, calculations are presented which give the composition of the magma from which chromite deposits have crystallized in different contexts.

Geological setting and field relations

The Cenomanian Oman ophiolite forms a 700-km arcuate mountain chain along the southern coast of the Gulf of

Oman. In its northern part, a complete, 14-km-thick se- quence of ophiolite stratigraphy is exposed. The lowermost 7 km consists of 80%-90% harzburgite, the remainder being composed of dunite, chromitite, and of dikes of various stages. This corresponds to the mantle sequence and is overlain by a crustal sequence comprising layered ultramafic and marie cumulate rocks, isotropic gabbro, diorite and trondjemite, a sheeted dike complex, and a pillow lava succession (Smewing 1980). The contact be- tween the mantle sequence and the crustal sequence is defined as the petrological Moho (Malpas 1978). The phase assemblage developed at the base of the layered sequence is variable. At some localities, a dunite unit up to 100m thick is present and may extend for as much as 5 kin, whereas at other localities (e.g., Rajmi) the first cumulate rock encountered is an orthopyroxene-bearing wehrlite. Elsewhere, gabbroic assemblages rest directly on harzburgite.

Most of the chromite occurrences are within the mantle sequence. This investigation has therefore concentrated on an area of northern Oman where this mantle sequence is well exposed (Fig. 1).

The characteristics of each deposit are summarized in Table 1. The following three environments of chromitite occurrence have been distinguished (Brown 1982):

1. The ultramafic cumulate layer (mainly dunitic) above the harzburgite unit. Deposits from this group (Farfar 4, 5, 6, 7; Aleya) are generally relatively thin and of limited extent.

2. The top 1,000 m of the mantle sequence, where dunite occurrences are more frequent. Deposits belonging to this group (Maharah 1, 2; Farfar 1, 2, 3; Jebba Gebba, Rajmi 1, 2, 3, 4) are the most abundant and the largest known.

3. The rest of the mantle sequence excluding the "transi- tion zone". Deposits located deeper in the mantle sequence (Ray), 1, 2; Al Ainah 1, 2) are generally'small.

Petrography of chromite ore

Most of the deposits in the mantle sequence (i.e., types 2 and 3) are tabular in form (Christiansen 1982) and appear concordant to subconcordant according to the classifica- tion of Cassard et al. (1981). In places, they are cut by later faults or dismembered into small rounded bodies. In type-2 chromite deposits the ore is submassive, containing

ics- 5 S & i

;[ , , - . . . iCS ">,

2 ' % . . .e ~ ,

I

; FIZH BLOCK "~- , - - ) - "

...... ... ,'" MS ~ . . ', / ~ ',.,

tl I

\ I

OOKM

oMA |, I

5KM

t N

Fig. 1. Location of the main chromite occurrences (e) studied in northern Oman (after Brown 1982); CS, cumulative sequence; MS, mantle sequence. Inset map, location of the Oman

more than 20% intersti t ial silicates. Chromite crystals consist o f subhedra l to anhedra l grains between 0.5 mm and 1 m m and show locally complex interfaces with the inter- stitial silicates (Aug6 and Roberts 1982; Christ iansen 1985). Most type-3 deposits are characterized by very massive ore of closely packed anhedral chromite grains many cut by secondary amphibole .

Deposits from the base o f the cumulates (type 1) commonly occur as al ternating layers of massive chromite, chromitiferous dunite, and dunite. Individual layers vary in thickness from 0.1 cm to 50 cm. The texture is distinct from that of the previous types, the ores being or thocumu- lates with euhedral chromite and olivine crystals enclosed by oikocrysts of plagioclase and pyroxenes. The textures include chromite net and occluded silicates (Brown 1980) and are very similar to those in chromiti te from stratiform intrusions (Jackson 1964). Similar stratiform chromite bodies have also recently been recognized by Ceuleneer and Nicolas (1985) in the southern par t of the Oman ophiolite.

In terstitia I silicates

Two kinds of silicate phase are associated with the ore, p r imary phases and al terat ion phases (tremolit ic amphi- bole, serpentine). ] 'he latter are not discussed here. The propor t ion of p r imary silicates varies from 99% (pure dunite) to 10% (submassive chromitite). In most ophioli t ic chromitite, e.g., Vourinos (Burgath and Weiser 1980) and New Caledonia (Moutte 1982), olivine is the only intersti- tial phase, but Oman type-1 and type-2 chromite deposits contain var ied silicate minera l assemblages. Olivine, cl inopyroxene, or thopyroxene, plagioclase, and pargasit ic amphibole were found either in association or separately in decreasing average order of abundance. However, olivine is the only silicate associated with chromite of

Table 1. Structural data of the studied chromite occurrences in northern Oman (after Christiansen 1982). A type of deposit. B distance to the Moho (kin); negative figures correspond to deposits in the cumulate series. C form of the deposit. D lateral extension (m). E average thickness (m). F layering in chromitite. G foliation in harzburgite

Deposit A B C D E F O

1 A1Ainah 1, 2 3 5.0 irregular 2x 1 1 170°/50 ° E 140°/45 ° NE 2Aleya 1 -0.1 layers 20× 5 0.1 500/25 ° SE 3 Farfar 1 2 0.3 pencil 210x 10 3 900/40 ° N ] 4Farfar2 2 0.5 tabular 15x 5 0.5 110°/40 ° N I 105°/60 ° N 5 Farfar 3 2 0.8 tabular 20 x 3 1 ? 6 Farfar4 1 -0.1 layers 40× 10 0.5 100°/30 ° N 7 Farfar 5 1 - 0.1 layers 15 x 10 0.3 - 8 Farfar 6 1 - 0.1 irregular 3 x 1 1 - 9Farfar7 1 -0.1 layers 20x 15 0.2 0°/40°E

10 Jebba Gebba 2 0.5? irregular 160x 40 1 ? ? 11 Maharah 1 2 0.3 tabular 250x 150 3 170°/30 ° E 350/55 ° SE 12 Maharah 2 2 0.6 ? ? ? ? 650/70 ° SE 13 Rajmi 1 2 0.1 tabular 12× 7 1 ghallow 140°/40 ° SW 14 Rajmi2 2 0.4 tabular 15x 3 1 150°/40 ° NE 200/75 ° E 15 Rajmi3 2 0.0 tabular 300× 10 ? 150°/45 ° NE 145°/70 ° NE 16 Rajmi 4 2 0.2 irregular ? 0.5 ? 140°/40 ° SW 17 Rayy 1, 2 3 6.0 tabular 10x 8 1 130°/40 ° NE 140°/45 ° NE

Fig. 2. a Euhedral chromite crys- tals containing silicate inclusions (mainly pargasite). Scale bar is 0.1 ram. b suggests a trapping dur- ing crystal growth. Farfar 7 chro- mite deposit

Table 2. Selected microprobe analyses and structural formulae of spinel and associated silicates from chromitite. 1, 2, and 3, chromite from type 1, 2, and 3 deposits respectively; 4, 5, olivine; 6, 7, orthopyroxene; 8, 9, clinopyroxene; 10, 11, plagioclase; 12, pargasite; 13, Na- phlogopite; 14, K-phlogopite. ~, silicate included in spinel

Chromite Olivine Orthopyroxene Clinopyroxene Ptagioclase Amphi- Phlogopite bole

1 2 3 4 5~, 6 7~ 8 9~ 10 11 12~ 13P, 14~

SiO2 0.00 0.06 0.00 41.80 42.26 57.23 56.10 53.82 54.34 48.06 44.19 44.32 42.41 39.21 T i Q 0.06 0.15 0.17 0.06 0.00 0.00 0.16 0.29 0.05 0.00 0.00 1.18 3.44 0.13 A1203 32.07 26.17 12.53 0.00 0.00 1.88 2.86 2.55 0.72 33.03 35.71 13.66 16.20 16.42 Cr203 31.96 42.61 56.27 0.00 0.17 0.61 1.62 0.63 1.82 0.00 0.00 1.85 1.58 2.49 Fe203 5.85 3.46 3.13 . . . . . . . . . . . FeO 17.78 10.22 13.00 4.90 3.19 5.07 3.80 2.87 1.29 0.06 0.00 3.05 1.05 1.41 MnO 0,14 0.08 0.09 0.03 0.04 0.10 0.06 0.11 0.02 0.00 0.00 0.00 0.00 0.00 MgO 12,64 16.89 13.49 53.55 54.02 34.00 35.44 17.54 17.33 0.02 0.00 17.80 24.33 27.75 CaO 0,10 0.00 0.00 0.00 0.03 1.04 0.24 21.64 24.59 16.07 19.54 11.96 0.03 0.09 Na20 0,00 0.00 0.00 0.00 0.00 0.00 0.06 0.42 0.30 2.08 0.31 3.00 6.43 1.21 K20 0.00 0.00 0.00 0.00 0.00 0.00 0.08 0.00 0.00 0.01 0.00 0.04 0.24 8.35 NiO 0.14 0.08 0.00 0.48 0.79 0.13 0.07 0.00 0.00 0.00 0.00 0.15 0.08 0.18 H20 . . . . . . . . . . . 2.11 4.47 4.35

Total 100.74 99.72 98.68 100.82 100.50 100.06 100.48 99.87 100.45 99.33 99.75 99.12 100.25 101.59

Si - 0.013 0.001 0.995 1.002 1.965 1.911 1.951 1.967 2.211 2.045 6.304 5.684 5.395 Ti 0.011 0.027 0.034 0.001 - 0.004 0.008 0.001 - - 0.127 0.347 0.013 A1 8.934 7.317 3.820 - 0.076 0.114 0.109 0.031 1.791 1.948 2.290 2.559 2.663 Cr 5.972 7.990 11.503 - 0.003 0.017 0.044 0.018 0.052 - 0.208 0.168 0.271 Fe 3+ 1.041 0.617 0.608 . . . . . . . . . . Fe 2+ 3.515 2.028 2.812 0.098 0.063 0.146 0.109 0.087 0.039 0.002 0.363 0.117 0.162 Mn 0.029 0.017 0.021 0.001 0.001 0.003 0.002 0.003 . . . . Mg 4.453 5.972 5.200 1.900 1.910 1.740 1.799 0.948 0.935 0.001 - 3.773 4.861 5.692 Ca 0.024 - - - 0.001 0.038 0.009 0.840 0.954 0.792 0.969 1.823 0.004 0.013 Na . . . . . 0.004 0.029 0.021 0.185 0.028 0.827 1.671 0.323 K . . . . . 0.003 . . . . 0.007 0.041 1.470 Ni 0.026 0.015 - 0.009 0.015 0.004 0.002 . . . . 0.017 0.009 0.020 OH . . . . . . . 1.000 2.000 2.000

Total 24.005 23.996 23.998 3.004 2.995 3.989 4.000 3.993 4.000 4.982 4.990 16.739 17.461 18.020

type-3 deposits. In type-1 and type-2 chromitite, distribu- tion of interstitial silicates varies from one deposit to another and even within a given deposit. For example, some deposits contain only olivine (Maharah 2, Aleya), and others (Maharah 1) have zones containing only olivine or only plagioclase or both pyroxenes. Ores where all the phases mentioned coexist within a thin section are rare. Flat contacts between parts containing only clinopyroxene and others containing only plagioclase or other silicates as interstitial phase have commonly been observed. Thus, heterogeneity of silicate distribution can be observed within a single thin section. The type of silicate present is not related to any other characteristic of a given chromite sample.

As well as interstitially, silicates also occur as inclusions in chromite. These are morphologically different from interfingering silicates in chromite grains since they fill negative chromite crystals or occur as euhedral crystals. Their size ranges 1-150 ~tm, and they occur either indi- vidually in the host chromite crystal or as a mass or forming a rectangle in section (see Fig. 2). The minerals are the same as those occurring interstitially except that mica can also be present. However, their proportions are reversed; pargasitic amphibole represents more than 50% of the inclusions followed in decreasing order by clinopy- roxene, orthopyroxene, olivine, mica, and plagioclase.

Although such inclusions have been observed in all chromitite samples, they are rare in type-3 chromite, and the best examples are found in stratiform deposits (type 1) where they occur in almost all chromite grains. They also exist in disseminated spinels in dunite of both mantle and cumulate sequences.

Mineral chemistry

All the minerals discussed in the following section have been analyzed under routine conditions using an automated Camebax microprobe (counting time of 6 sec., acceleration voltage of 15 kV, reference current of 10nA). Selected analyses of minerals from chromitite are given in Table 2.

Chromite

On the Cr-Al-FP + triangular diagram (Fig. 3), the three types of chromitite have been distinguished. The chromite ore is always relatively rich in A1. However, a clear dis- tinction is noticeable, i.e., chromite from type 3 plots at the Cr-rich end of the values for Oman but is still more A1- rich than Cr-rich ore from Vourinos, Greece, or the Tiebaghi massif of New Caledonia (Moutte 1982), for example. A distinction also appears between types 1 and 2, stratiform chromitite being richer in A1, with a slight overlap with type-2 chromitite. No significant variations in chromite chemistry within a single deposit have been ob- served, although variations in texture are common.

TiO2 content ranges between 0.03 wt% and 0.45 wt% (Fig. 4), but such a range may be observed within any deposit of any type, Ruffle exsolution lamellae have been seen in chromite crystals, but this does not explain the range of variation since they are more common in the samples richest in titanium. They indicate simply that the solubility limit of titanium has been reached. However, in volcanic rocks spinel containing larger amounts of TiO2 without ruffle exsolution is well known (Clocchiatti et al.

Fe 3+ ~'~' ~-

• A ~ B B

v

3 , , . . . . . ~, v

Cr

A

B \ c \

AI

Fig. 3. Composition of chromite in part of the A1-Cr-FP + triangu- lar diagram. A disseminated chromite in basal cumulate rocks (zx gabbro, • dunite [] wehrlite); B chromitite 1, 2, 3, chromite de- posits type 1, 2, and 3, respectively. C spinel from mantle rocks • dunite, [] harzburgite

A,cumulate

0.1 0.2 Q3 0~4 05 O'b 1'.0 1.1

I i i m ! i

• . [ - ]

C, dunite

P L D,harzburgite

L & & o.3 o;4 o;s &

TiO 2 wt % SPINEl_

Fig. 4. Histogram of the TiO2 contents of spinel in different rock types

1979). The mechanism of rutile exsolution in chromitite is not clearly understood. A process involving initial non- stoichiometry due to the presence of TP + in the structure is favored, leading to the exsolution of ruffle during cooling, a mechanism that could be inhibited in volcanic rocks by rapid cooling.

The Mg/Fe ratio of the chromite ore tends to be rela- tively high (Fig. 5); it decreases with increasing proportion

of silicates but does not vary with the type of deposit. This correlation has been attributed here to subsolidus diffusion between silicate and chromite, mainly of Fe and Mg, during cooling. Lehmann (1983) has shown that Mg and Ni diffuse from chromite to olivine and Fe and Mn from olivine to chromite. Hence, it is considered that in the most massive samples, where silicates are absent, the Mg/Fe ratio of the chromite has not been affected by this process and reflects primary composition.

Disseminated chromite from other environments

Chromite is an ubiquitous accessory phase in mafic and ultramafic, residual or cumulate ophiolitic rocks. In the Oman complex, it occurs up to the level of cumulate gabbro (Aug6 1983). Spinels from cumulate rocks are characterized by large variations in Cr/A1 ratio (in the same range as that defined by the ore, Fig. 3) and relative Fe ~+ and Ti enrichment (Figs. 3 and 4). Spinels from mantle dunite and harzburgite show Cr/A1 variations in the same range (Fig. 3), but harzburgite spinels are charac- terized by lower Fe~O~ and T i Q values. Fe~O~ in dunitic spinel lies between ore and cumulate values, and it is clearly enriched in titanium relative to spinel from harzburgite (Fig. 4). The iron enrichment of the rock- forming minerals in the cumulate series also occurs in its spinel; the highest Fe~+/Mg values occur in gabbro (Fig. 5).

Pyroxenes Interstitial clinopyroxene in chromite ore is chrome diop- side, more Mg-rich than in dunite, harzburgite, and basal cumulate (Fig. 6). It is slightly sodic (Na20 = 0.0-0.3 wt%), titaniferous (TiO2 = 0.0-0.3 wt%), and contains an average of 2.4 wt% A1203. Inclusions of clinopyroxene in chromite crystals differ from interstitial clinopyroxene in that their Al~O3 is lower (average of 1.4 wt%), and they contain an average of 0.4 wt% Na20 and 0.2 wt% TiO~. Clinopyroxene in basal cumulates is the same as that in mantle dunite but it is marked by rapid iron enrichment farther up in the sequence (Fig. 6).

The enstatite content of the interstitial orthopyroxene in chromite ore varies 91%-93%, while the most Mg-rich orthopyroxene in the cumulate sequence is En88. Ortho- pyroxene from residual rocks plots in the gap between the two compositions (Fig. 6). The Cr203 in interstitial ortho- pyroxene is constant (around 0.50 wt%); its A1203 content varying between 1.3 wt% and 2.0 wt% (average of 1.8 wt%). In cumulates it is characterized by a rapid upward decrease in Cr203 (0.5-0.1 wt%). Inclusions of ortho- pyroxene in chromitite vary in composition between ]~ng~ and Engs and are marked by distinctly low Wo contents (Fig. 6).

Olivine

Olivine composition in chromitiferous rocks is related to the modal silicate/chromite ratio. The Fo content varies from 93.5% in disseminated ore to 94.8% in more massive chromitite and to 96.0% in olivine included in chromite crystals. The NiO content also increases from 0.4 wt% in interstitial olivine to 0.6 wt% in included crystals. Olivine in dunite exhibits slight Fo variation (Fo9o s-Fo93.0), more restricted (Fogo.2-Fo92.o) in harzburgite. In the cumulate

0.7

0.6

0.5

0.4

J i i

X Mg spinel

t~

Z~

' 8 " 5 ' '

[]

[]

• 2 I ~ 4 A •

I DI u I~ vi i •

°°% ,P. •

J

Fo % olivine . . . . '

Fig. 5. Correlation of the Mg/(Mg+Fe 2+) ratio of chromite and olivine (forsterite content) in massive ore (A), disseminated ore ( • ) , harzburgite (D), dunite (m), wehrlite (~), and gabbro (zx). e, olivine included in spinel from chromite ore

W o . . . . . .

• I Q []

~ A ~ A • A A Z X • ~ ZX

En 95 90 85

Fig. 6. Pyroxene composition. Same symbols as in Fig. 5; (e) pyr- oxene inclusions in chromite crystals, (A) pyroxene interstitial il~ chromite ore

section F0914 has been recorded in a dunite interlayered with gabbro. However, even in the basal dunite, olivine is iron rich (Fig. 5) relative to mantle rocks.

Plagioclase

Plagioclase is common in chromitiferous rocks in Oman and has been described in aluminous chromitite from other ophiolitic complexes (Thayer 1970; Leblanc and Violette 1983). As inclusions, however, it has only been found in one sample. In contrast to the other phases in chromitite, plagioclase composition varies within the same deposit (Ansi-An97). Variations of 4% An have also been measured in the same thin section (Fig. 7), but no zoned crystals have been observed. The composition of the in- clusion is An92. When comparing these compositions with cumulate plagioclase, we observe the same range of values with small-scale variations and heterogeneities within a single sample (Fig. 7). Albitization is unlikely to explain variations in composition, and it is probable that they are primary.

Amphibole

Interstitial magmatic amphibole is uncommon in chro- mitite, whereas it constitutes more than 50% of the

95

90

85

80

I

X Mg cpx ÷

An % plag. I I I I

80 85 90 95

Fig. 7. Correlation Mg/(Mg+ Fe 2+) in clinopyroxene and An% in coexisting plagioclase (average composition with indication of standard deviation). (A) gabbro, ([]) wehrlite, (e) chromitite

2D

I.O

AI iv I I

1.9 1.6 2.9 1 •

1A zo~5o .~mo'~A • • • u~ ~O 0.9 •

• 0.5 0,5 0.5

1.3 • 0.4

~ • 0 . 4 - '03

0.4

[.7

0•~2

0 O 0.1 0.2

O 0.2

o.o o Na +K 0.1

I I

0 0.5 LO

Fig. 8. Amphibole composition expressed as (Na+K) and AP atoms per formula units with indication of the TiO2 content in wt%. • included in spinel from chromitite, • from dunite, © alter- ation amphibole in chromitite

inclusions. In both contexts, it is chromitiferous and titaniferous pargasitic hornblende (Fig. 8). Magnesium-rich with a XMg ratio of about 0.95, its TiO2 content varies between 0.4 wt% and 2.9 wt%; its Cr203 between 2.1 wt% and 3.1 wt%, and Na20 between 1.6wt% and 3.2wt%, while it contains only traces of K20 and an average of 0.1 wt% NiO. Figure 8 shows that there is solid solution between the pargasitic and the hornblendic end members (Oba 1980).

Primary amphibole is easily distinguished from that due to alteration, which is tremolite never containing more than 0.2 wt% TiO2 (Fig. 8). From its position in the host crystal and its composition, the pargasitic hornblende is

considered to be magmatic. No primary amphibole has been found in other rocks, not even in the basal cumulate series. However, inclusions of pargasitic amphibole exist in disseminated spinels from mantle and cumulate dunites.

Mica

Mica is relatively rare and occurs only as inclusions in chromite crystals. Two compositional poles have been found, a K-phlogopite and a Na-phlogopite, with solid solution between the two with Na / (Na+K) ranging 1-0.56. TiO2 content ranges 0.1-4.1 wt%, and Cr203 1.6-2.7 wt%. It also contains 0.1-0.4 wt% NiO. Like all the mineral phases included in chromitite, it is Mg-rich with a Mg/(Mg + Fe) ratio of about 0.96. Natural occurrences of Na-phlogopite are rare. In the same context, Irvine (1975) mentioned, as inclusion in chromite from the Muskox intrusion, "a chromian titanian phlogopite and its sodium analogue" while Johan et al. (1983) in different ophiolitic complexes discovered several sodium-rich phases as in- clusions in submassive chromitite, including chromium- rich pargasite, pure sodic nepheline, and the sodium analogue of phlogopite. Schreyer et al. (1980) described and analyzed sodium phlogopite in a metamorphosed evaporite sequence. The reason for this rarity is its low- temperature hydration into hydrates I and II (Carman 1974) that precludes occurrence of Na-phlogopite. Hy- drate I contains 8.6 wt% H20 while the ideal end member contains 4.5 wt%. The composition given by Schreyer et al. (1980) contains about 8% H20, which seems to correspond to a composition close to that of hydrate I, while com- position given in Table 2 is close to the theoretical H~O content of Na-phlogopite. The position of the mica trapped in chromite crystals prevents any exchange with the environment and explains its preservation.

Discussion

On the implications of silicate inclusions in chromite

In comparing the proportion of included and interstitial silicates a fundamental difference appears, i.e., amphibole dominates the inclusions whereas it is uncommon as an interstitial phase. Thus, inclusions cannot be the result of a simple trapping of the silicates during chromite crystal- lization; otherwise, we would expect to find the same pro- portion of interstitial and included species. The abundance of chromite crystals containing inclusions precludes the possibility that the inclusions originated in another environment.

The interstitial amphibole forms a thin rim around euhedral chromite crystals, which suggests the following mechanism: after formation the amphibole rims, at a certain stage, destabilize or react with the magma; at the same time the growth of chromite crystals recommences trapping relict amphibole and its destabilization products (see Fig. 2B). This would explain the position of the inclusions in the host crystal and their nature. Boyd (1959), studying the stability field ofpargasite in a synthetic system, found that above 1,050°C the following reaction is observed: pargasite ~ diopside+ forsterite+nepehline + spinel+ anorthite + vapor. Pure pargasite is not stable above 1,050 °C. However, the pargasite studied here con- tains up to 2.2wt% TiO2. The effect of titanium is to

increase the thermal stability field of the amphibole, the limits of which are not known. Nepheline, not observed here, is not stable and has reacted with the magma. Two- phase inclusions (pargasite-clinopyroxene and pargasite- olivine) can be formed by this mechanism. The difference in composition between included and interstitial silicates is also explained by their differing origins.

Phlogopite does not seem to be directly associated with this system and could have crystallized directly from the magma. The stability field of this mica has been studied by Carman (1974). In synthetic systems, it is not stable above 1,000 °C. However, the natural phase analyzed here con- tains up to 4.1 wt% T i Q . By analogy with K-phlogopite, it is likely that the effect of titanium is to increase the stability field of the Na-phlogopite to limits which are as yet undetermined (Forbes and Flowers 1974).

Apart from the fact that olivine is the only inclusion found in type-3 chromitite, the composition of silicate in- clusions is not specific to a particular type of deposit.

On the formation of chromite bodies

Since chromium is only a minor constituent of magmatic liquids, a concentration process must be invoked to form large chromite deposits. Two possible mechanisms may be considered, i.e., a mechanical concentration of crystals after their formation from a given liquid and a chemical preconcentration of chromium through a fluid phase, a mechanism recently proposed by Johan etal. (1983). Whatever the process invoked, the volume of magma involved to form chromitite must be at least 500 times greater (average value of the Cr crystal/liquid partition coefficient, Maurel and Maurel 1982 b) than that of the ore. A structural approach to this problem enabled Lago et al. (1982) to propose a model for the formation of such deposits based on the accumulation of chromite crystals in a vertical conduit, but the reason for chromite crystalliza- tion is not clearly explained. Neither the relationship be- tween the dunite envelope and the chromite body, espe- cially the fact that interstitial silicates in chromitite, other than olivine, have not been found in the surrounding dunite. The second model (Johan etal. 1983) is not affected by the geometry of the system since the fluid phase can evolve independently of the magma and, at a certain stage, be injected into the harzburgite series, but practical evidence of this phenomenon is lacking. Ex- perimental studies, however, have shown that chromium can be extracted from a magma by a fluid phase and be carried by it (Johan et al. 1983). Reduction of this phase provokes massive crystallization of chromite.

Stratiform deposits in Oman, occurring at the base of the cumulate sequence, show textures clearly formed by accumulative processes similar to those occurring in stratiform intrusions (Jackson 1964). The similarity in mineral composition and chemistry between stratiform deposits (type 1) and mantle deposits (type 2) in Oman argues in our opinion for formation through similar process, i.e., crystallization/accumulation, occurring at the base of the main magma chamber for type 1 and in magma pockets within the mantle for type 2, from a liquid of similar composition. The differences in ore texture can be attributed to contrasts in dynamics of crystallization (Aug6 and Roberts 1982). Moreover, the association of plagioclase, pyroxene, and olivine with chromite ore,

a paragenesis similar to that of the overlying gabbros (with very similar composition, as mentioned above), also argues for the crystallization of chromite from a magma having a composition not far from that of the liquid from which the basal cumulates crystallized.

Dunlop and Fouillac (in press), based on oxygen isotope data on primary minerals and Sr-Nd isotopic com- position, opted for a magmatic, as opposed to hydrother- real origin for the three types of Oman chromitite. The role of a fluid phase is questionable here. The systematic association of hydroxyl-sodic phases (amphibole and mica) with chromite implies that they crystallized from a magma enriched in H20 and Na20, in quantities that cannot be precisely defined because of the lack of experi- mental studies in this system. Cawthorn (1976) from his experimental work concluded that basaltlike liquids con- taining less than 3% Na20 cannot crystallize amphibole. In any case, the presence of these phases as inclusions in spinel from barren dunite in both the mantle and cumulate sequences implies that the existence of hydroxyl phases and the formation of chromite bodies themselves are not necessarily related. In basaltic liquids, the presence of alkalis and H20 can drastically modify liquidus phases and their composition. Its effect on chromite crystalliza- tion has been mentioned by Onuma and Tohara (1981), leading to the conclusion that high H2OP is favorable for the incorporation of Cr into the spinel structure. However, very little is known about the effects of H20 and alkalis on the stability field of chromium-rich spinel. On the other hand, the role of temperature and oxygen fugacity has been clearly established. Maurel and Maurel (1982b) showed that a decrease in chromium solubility in basaltic liquid, provoking chromite crystallization, can be due either to an increase in fO2 or to a decrease in tempera- ture, which is the mechanism favored here.

The origin of dunite in ophiolitic complexes is beyond the scope of this discussion. We have noticed however that all the types of dunite, including that associated with orebodies, have the same mineral composition. The presence of silicate inclusions in spinel from all types of dunite, similar to those found in massive ore, and similar TiO2 contents in spinel from dunite and from chromitite argue for a similar origin, i.e., crystallization of olivine and spinel from a magma with removal of the residual liquid without a process leading to chromite concentration.

Composition of the magma

Maurel and Maurel (1982a) have shown that the Cr content of spinel is not directly related to that of the melt, while its A1 content can be related to that of the liquid by the following formula:

(A120~)Sp=0.035 (A1203)12iq 2 (A1203 inwt%).

The A1203 contents of the liquids in equilibrium with the three types of Oman deposit have been calculated. The results are given in Table 3. The slight differences between type 1 (16.1 00.4) and type 2 (15.1 a0.5) can be explained by minor differences in the fractionation of the liquid. Type-3 (12.5 00.7) deposits must have been derived from a melt of very different composition; this is also shown by the different nature of its interstitial silicates (olivine only). Their higher degree of deformation (Christiansen 1982) could indicate an earlier event. The average calculated

composition of the hquid in equilibrium with dunite and harzburgite spinel is similar. However, in both cases large variations are recorded implying heterogeneities in com- position of the liquid in equilibrium with these mantle rocks. For cumulate rocks, the presence of plagioclase modifies the equilibrium conditions. On plagioclase-free samples, the values obtained (15.4 00.9) are in agreement with those obtained for chromitite.

Another important point to be established is the FeO/MgO ratio of the melt. Large variations of this ratio observed in olivine and spinel in different rock types have been partly explained by subsolidus reequilibration. In order to calculate this ratio we require the composition of minerals supposed not to have undergone reequilibration, which is the case for olivine in dunite and chromite in massive ore. The composition of olivine gives the FeO/MgO ratio of the liquid in equilibrium (Roeder and Emslie 1970). Maurel (1984) has shown that the same ratio can be obtained for spinel using the following formula:

FeO FeO L n - - - 0.47 - 1.07 Y~ + 0.64 Y~+ + Ln -

MgO Sp MgO liq

(FeO and MgO in wt%).

This calculation has to be treated with reservation since the most massive ore still contains 5%-10% silicates with which reequilibration could have taken place. Results are given in Table 3.

If our hypothesis is correct, the FeO/MgO ratio of the melt from which massive chromitite has crystallized is similar to that in equilibrium with the dunite and basal cumulate, in the range 0.55-0.60. Note the good agree- ment of the values obtained for dunite and cumulate (calculation based on olivine) and chromitite (calculation based on spinel). Values outside this range (either from dunite or chromite) show the effect of reequilibration processes. This value is in good agreement with the com- position obtained by Pallister (1984) by mass balance of the basal columnar section of cumulate in the southern part of the Oman ophiolite. The value obtained here for the A12Ox content of the liquid (14.9-16.1) in equilibrium with chromitite is also in agreement with his figure of 16.9 wt%.

Pallister (1984) discussed the meaning of the average composition obtained for a possible parent magma for the cumulate sequence. Plagioclase crystallizing from this composition should be An83. However, he found An92-An95 in a chromite-olivine adcumulate. Such An-rich plagio- clases are described here. Pallister proposed two hypotheses to explain the presence of highly calcic plagioclase, i.e., (1) his "average" composition does not reflect local heterogeneity, (2) the possibility of "second-stage mantle melts" which should crystallize more calcic plagioclase. The hypothesis favored here is crystallization under PH2o. Yoder (1969) has shown that plagioclase crystallizing under high water pressure was more calcic than that

Table 3. Calculation of A1203 content and FeO/MgO ratio of the liquid in equilibrium with different rock types

A120~ liquid FeO/MgO liquid

Average Range Maurel (1984) Roeder and Emslie (1970)

Chromitite type 1 (CS) 16.1 00.4 15.5-16.4 type 2 (MS) 15.1 o 0.5 14.8-15.9 type 3 (MS) 12.5 o 0.7 11.4-13.5

Dunite 13.9 o 1.4 11.3-16.2

Harzburgite 14.6 a 1.1 11.9-16.3

Cumulate gabbroic 14.4 o 0.5 13.7-14.9 plag-free 15.4 or0.9 14.3-16.7

Massive 0.62 00.02 0.29 00.03 ore

1.39 00.26 0.55 o0.07

1.31 00.22 0.56 o0.04

1.21 00.27 0.57 00.09

800 I000 P

900 I100

Harzburgite

800 1000

Dunite

70O

Chromitite

Iooo

I000 1200

Lehmann

(1983)

& 900 1200

Wood & Banno

. . ~ (1973)

900 I100

Cumulate

Fig. 9. Histogram of temperatures obtained using Lehmann (1983) and Wood and Banno (1973) equations in harzburgite, dunite, chromitite, and basal gabbro

crystallizing under anhydrous conditions. The presence of amphibole and mica associated with chromite is further evidence for the presence of water in the system. Variations in the ~n content from one sample to another could be due to local variations in PH2o in the liquid.

Geothermometry Two mineral pairs have been used for geothermometric calculation, olivine-spinel (Lehmann 1983) and ortho- pyroxene-clinopyroxene (Wood and Banno 1973). Results are given in Fig. 9. The same average temperature is obtained by both methods for the gabbroic rocks (1,050°C). The Lehmann geothermometer gives an average of 930 °C for dunite and harzburgite, while that of Wood and Banno gives 1,055 °C for harzburgite. The most interesting point here is the difference obtained for the chromitite, 800°C with the Lehmann equation, i.e., the lowest temperature recorded, and 1,100°C with the Wood and Banno equa- tion, i.e., the highest temperature calculated. The result obtained in the first case obviously corresponds to low- temperature reequilibration. The temperature given by the second method is still too low for magmatic conditions and reflects minor requilibration in pyroxenes.

Conclusions

In the Oman ophiolitic complex, the following three types of chromite deposit have been distinguished according to their structural position:

I. stratiform deposits at the base of the cumulate sequence, 2. deposits located at the top of the mantle sequence, and 3. deposits located deeper in the mantle.

The analogy in mineral composition and chemistry between stratiform deposit and deposits at the top of the mantle sequence lead to the conclusion that both types are formed by the same process, i.e., crystallization-accumula- tion from a basaltic liquid occurring at the base of the main magma chamber for type 1 and in pockets within the mantle for type 2. This liquid has a composition similar to that from which the cumulate sequence has crystallized (15.1-16.1 wt% A1203 with FeO/MgO 0.55-0.60). A similar result has been obtained by Pallister (1984) by mass balance calculations for the base of the series. Our results indicate that a certain amount of water has to be envisaged in the parent liquid. Local variations Of PH~o could explain the stabilization of amphibole and highly calcic plagioclase associated with chromitite.

Silicate inclusions (mainly a pargasitic amphibole and its destabilization products) trapped in chromite crystals confirm that the magma from which chromite has crys- tallized was not anhydrous, bu t processes of chromite con- centration do not seem to be related to the presence of water in the system.

Oversaturation in chromium of this liquid, provoking chromite crystallization, will occur for different reasons (drop in temperature, increase in fO2) mainly before the liquid enters the main magma chamber to form type-2 chromite deposits in "minichambers" (Neary and Brown 1979; Lago et al. 1982) where the magma coalesces before being injected in the main chamber and, to a minor extent, early in the main chamber forming stratiform deposits.

Type-3 chromite deposits belong to another magmatic episode. Based on their higher degree of deformation (Christiansen 1982) this could be older.

Acknowledgements. This paper is part of an EEC program on chromite mineralization (contract number MSM-038F and MPP-122-F). The EEC is thanked for financial support and permission to publish the results, and the Department of Petro- leum and Minerals, Sultanate of Oman, for assistance. Field work in Oman was organized by the Open University, U.K.Z. Johan, S. Roberts, F.G. Christiansen, and C. Maurel are acknowledged for numerous discussions, John Kemp for improving the English of the manuscript, and two anonymous reviewers for their critical comments.

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10

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Received: July 30, 1985 Accepted: June 6, 1986

A nnouncements

VI. International Congress of the International Committee for the Study of Bauxite, Alumina and Aluminium (ICSOBA) from May 11-20, 1988, with pre-session excursion Sao Paulo/ Brazil.

Scientific Programme

Topics

1. Bauxite and other Aluminium raw materials Geology, mineralogy, ore deposits - exploration and exploi- tation, alternative ores.

2. Alumina Alternative ores and relevant technologies, developments in Bayer process and technology, new processes, economy, and energy problems.

3. Aluminium Developments in aluminium and alloy, production, processes, and technology.

The technical sessions will be held on the period of May 1 lth to 16th, 1988.

One pre-session excursion and two post-session excursions are be- ing planned, having each one about 4-5 days duration.

I. Amazon Region - May 6th-10th - bauxite deposits formed from clastic sediments of Pliocene age.

II. Pocos de Caldas (MG), Passa Quatro (SP) - May 16th-20th - bauxite deposits associated with alkaline rocks.

III. Ouro Preto, Cataguases (MG) - May 17th-20th - bauxite de- posits originated from different lithological types of Pre-Cam- brian age. Visit to aluminium plant.

For further information contact: Prof. A. J. Melfi VI. International Congress of ICSOBA Istituto Astronomico e Geof[sico Caixa Postal 30.627 01051-S~o Paulo, SP-Brazil Telex no. 011 36221 IAGM BR