al- and cr-rich chromitites from the mayar[iacute]-baracoa ... etal_mayari... the mayarf district...

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  • Economic Geology Vol. 94, 1999, pp. 547•566

    A1- and Cr-Rich Chromitites from the Mayarf-Baracoa Ophiolitic Belt (Eastern Cuba): Consequence of Interaction between Volatile-Rich Melts and

    Peridotires in Suprasubduction Mantle

    JOAQUIN PROENZA, Departamento de Geologia, Instituto Superior Minero Metahirgico de Moa (Cuba), and Departament de Cristallografia,

    Mineralogia i DepOsits Minerals, Facultat de Geologia, Universitat de Barcelona, Martl i Franques s/n, 08028 Barcelona, Spain

    FERNANDO GERVILLA, {

    Instituto Andaluz de Ciencias de la Tierra, Universidad de Granada-CSIC, Facultad de Ciencias, Avda. Fuentenueva s/n, 18002 Granada, Spain

    JOAN CARLES MELGAREJO, Departament de Cristallografia, Mineralog•a i Dep6sits Minerals, Facultat de Geolog•a, Universitat de Barcelona,

    Marti i Franques s/n, 08028 Barcelona, Spain

    AND JEAN LOUIS BODINIER URM C,5569, Gdofluides-Bassins-Eau, ISTEEM, CNRS et Universitd de Montpellier II, Cc 57, Place Eugene Bataillon,

    34095 Montpellier Cedex 05, France

    Abstract

    The Mayar/-Baracoa belt occupies the easternmost part of the east-west-trending Cuban ophiolitic belt. It comprises two large, chromite-rich massifs: Mayarf-Cristal and Moa-Baracoa. Neither of these massifs show a complete ophiolite sequence, but they consist of a part of an ideal section made up of(1) harzburgites grading upward into interlayered harzburgites and dunires, (2) interlayered harzburgites (with minor dunites) and gab- bros, (3) gabbros, microgabbros, dolerites, and diabase dikes, and (4) pillowed basalt, cherts, and radiolarites.

    Chromite deposits can be grouped into three mining districts according to the chemistry of chromite ore: the Mayarf district and the Sagua de T•namo district, both in the Mayarf-Cristal massif, and the Moa-Baracoa district in the Moa-Baracoa massif. The latter is the most important as it contains more than 5.5 million tons of ore. All chromitites mainly exhibit massive texture, show a pseudotabular, lenticular shape, and are concor- dant with the foliation of the enclosing harzburgites. In Moa-Baracoa they tend to occur in the mantle-crust transition zone, commonly contain dunitc and gabbro bodies oriented parallel to the elongation of the lenses, and are cut by late pegmatitic gabbro dikes. By contrast, in Mayarf, and to some extent in Sagua de Tanarno, chromitites occur deeper in the mantle tectonites and are cut by websteritc dikes. Intergranular minerals are olivine, serpentine, and chlorite. Chromite has abundant, randomly distributed solid inclusions of olivine and pargasite, and minor pyroxene, laurite, and millerite. Toward the contact with the included gabbros, chromi- tite from Moa-Baracoa shows increasing amounts of gabbro-related alteration products. Abundant clinopyrox- ene, partly altered plagioclase, and rutile occur as inclusions in the chromite. The composition of the chromite ore varies from typical refractory grade (Al rich) at Moa-Baracoa to metallurgical grade (Cr rich) at Mayaif, where the Cr no. ranges between 0.41 and 0.75, the Mg no. between 0.57 and 0.81, and the TiO2 content be- tween 0.09 and 0.52 wt percent. At Moa-Baracoa, the Cr no. of chromite decreases and TiO2 content increases from harzburgite to dunitc and massive chromitite, positively correlated with the forsteritc content of coexist- ing olivine. At Mayari, both the Cr no. and TiO•. content of chromite, and the forsteritc content of olivine in- crease from harzburgite to dunitc and chromitite. Bulk platinum-group element abundances in chromitite vary from 20 to 538 ppb and show a broad positive correlation with Cr•O3 percent of chromite. The latter correla- tion is strongest in the Sagua de TSnamo district.

    Structural, textural, mineralogical, and chemical characteristics of the studied chromite deposits, as well as the lithophile trace element geochemistry of their host rocks, support a genetic model based on the crystallization of chromite from different types of melts (from back-are basin basalts to boninitic andesites) at around 1,200øC, at variablefo•. Chromite formed when talc-alkaline melts, formed by melt-rock reactions at increasing melt vol- ume, percolated through subhorizontal, porous dunitic channels and mixed with oxidized melts formed by low degrees of hydrous melting and low-temperature melt-rock reactions in suprasubduction zone mantle. Mixing of these two melts generated a hybrid melt whose bulk composition fell within the chromite liquidus field in the P-T-fo•. space (Hill and Roeder, 1974). Percolation of the hybrid melt through the dunitic channels promoted dissolution of preexisting silicate minerals and chromite crystallization. The Al-rich chromitites formed at the mantle crust transition zone at highfo• (-- logfo• = -7), whereas Cr-rich chromitites formed deeper in mantle tectonites under more reducing conditions, at logfo2 -- -10, depending on Cr contents of the parental magma.

    *Corresponding author, email: [email protected] es

    0361-0128/99/2070/547-20 $6.00 547

  • 548 PROENZA ET AL.

    Introduction

    POD•ORM chromitites from ophiolitic complexes are the only source of refractory-grade chromite (chromite ore with Al203 > 20 wt %, low Fe and (Cr203 + A12Os) > 60 wt %) and com- monly occur within the same complex with chromitites of metallurgical-grade chromite (chromite ore with Cr•Os > 40 wt % and Cr/Fe = 2.2-4.0). Although both types of chromi- tires can be locally interspersed, they tend to occur at differ- ent depths in the mantle tectonites, with the Al-rich chromi- tites being the most shallow (Leblanc and Violette, 1983). The origin of this bimodal compositional distribution has been related to the crystallization of chromite from parental melts generated by (1) different degrees of partial melting (Arai, 1992), (2) progressive fractionation (Graham et al., 1996), and (3) partial melting or melt-rock reactions related to the percolation of melts through variably depleted peri- dotites (Zhou et al., 1994; Melcher et al., 1997; Zhou and Robinson, 1997). Most of the relevant research is focused on understanding both the nature and origin of the parental melts of chromitites. However, Arai and Yurimoto (1994), Zhou and Robinson (1997), and Zhou et al. (1994, 1996) have presented a model based on the mixing of a differentiated basaltic melt and a primitive melt in open fractures at shallow levels of the lithospheric mantle, and Melcher et al. (1997) suggest that the crystallization of chromite takes place at rel- atively low temperatures (

  • CHROMITITES, MAYAR[-BARACOA OPHIOLITIC BELT (EASTERN CUBA) 549

    25 ø

    20 ø

    a) I I

    85 ø 70 ø

    SSIF

    MAYARi-BARACOA OPHIOLITIC BELT

    80 ø 75 ø

    Dunitc and • Gabbros and harzburgite diabase dikes Mainly harzburgite i!• Ophiolitic melange

    Cretaceous Gabbros • volcanic rocks

    Chromite deposits

    •;:• "La Corea" melange • Post-orogenic

    sedimentary rocks

    • Fanlt

    b) MAYAPd DISTRICT •C':-• Q• MOA-BARACOA DISTRICT • \

    0 5 •m !:5::: ::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: '•

    FIG. 1. a. Distribution of ophiolite-related rocks in Cuba and location of the Mayaif- Cristal and the Moa-Baracoa mas- sifs, which constitute the Mayaff-Baracoa ophiolitic belt. b. Sketch map of the Mayaff-Cristal massif, showing the distribu- tion of chromite deposits in the Mayaft and Sagua de TSnamo districts (modified from Kravchenko and VSzquez, 1985; and Nekrasov et al., 1989). c. Sketch map of the Moa-Baracoa massif, showing the distribution of chromite deposits (modified from Nagy et al., 1976).

    structure), chert, and limestone occur at the top of the crustal sequence (Iturralde-Vinent, 1996). Commonly, this sequence is tectonically inverted, where the upper volcanosedimentary unit is thrusted over by the gabbro complex and/or the man- tle tectonites (Rios and Cobiella, 1984).

    Chromite Deposits The chromite deposits of the Mayarf-Baracoa ophiolitic

    belt can be grouped into three mining districts: (1) the Mayarf district, (2) the Sagua de Tanarno district, and (3) the Moa- Baracoa district. The Mayaft district is located in the western part of the Mayari-Cristal massif and includes two large de- posits with more than 200,000 t of metallurgical-grade ore (Caledonia and Casimba), five deposits with slightly more than 10,000 t of ore, and 32 minor deposits (Lavaut et al., 1994). The Sagua de Tanarno district lies in the easternmost part of the Ma•varf-Cristal massif, in an area with a complex structure characterized by the imbrication of different tec- tonic sheets of ophiolite-related, mainly serpentinized ultra- mafic rocks (Fig. 1). This district contains 35 small deposits of both refractory (25 deposits) and metallurgic (10 deposits) chromite ore (Murashko and Lavandero, 1989). The Moa- Baracoa district is the most important from an economic point of view. It comprises the whole Moa-Baracoa massif and includes more than 100 chromite deposits of refractory-grade ore (Ostrooumov, 1986). Most of these deposits are of small

    size; however, four (Amores, Loro, Yarey, and Piloto) contain more than 100,000 t of ore and the calculated reserves of the Mercedira mine exceed 5 Mr. In addition, the Cayo Guam and Potosf deposits were extensively exploited in the past (see Thayer, 1942; Guild, 1947) and they produced more than 800,000 t of ore.

    Chromitite bodies have irregular, tabular to lenticular shapes (they are called lenses by the Cuban miners; Fig. 2) and are extremely variable in size. The central b