Al- and Cr-rich chromitites from the Mayari-Baracoa ophiolitic belt (Eastern Cuba): Consequence of interaction between volatile-rich melts and peridotites in suprasubduction mantle
F. Gervilla et al., Al- and Cr-rich chromitites from the Mayari-Baracoa ophiolitic belt (Eastern Cuba): Consequence of interaction between volatile-rich melts and peridotites in suprasubduction mantle, ECON GEOL B, 94(4), 1999, pp. 547-566
Citations number
122
Categorie Soggetti
Earth Sciences
Journal title
ECONOMIC GEOLOGY AND THE BULLETIN OF THE SOCIETY OF ECONOMIC GEOLOGISTS
The Mayari-Baracoa belt occupies the easternmost part of the east-west-tren
ding Cuban ophiolitic belt. It comprises two large, chromite-rich massifs:
Mayari-Cristal and Moa-Baracoa. Neither of these massifs show a complete op
hiolite sequence, but they consist of a part of an ideal section made up of
(1) harzburgites grading upward into interlayered harzburgtes and dunites,
(2) interlayered harzburgites (with minor dunites) and gabbros, (3) gabbro
s, microgabbros, dolerites, and diabase dikes, and (4) pillowed basalt, che
rts, and radiolarites.
Chromite deposits can be grouped into three mining districts according to t
he chemistry of chromite ore: the Mayari district and the Sagua de Tanamo d
istrict, both in the Mayari-Cristal massif, and the Moa-Baracoa district in
the Moa-Baracoa massif. The latter is the most important as it contains mo
re than 5.5 million tons of ore. All chromitites mainly exhibit massive tex
ture, show a pseudotabular, lenticular shape, and are concordant with the f
oliation of the enclosing harzburgites. In Moa-Baracoa they tend to occur i
n the mantle-crust transition zone, commonly contain dunite and gabbro bodi
es oriented parallel to the elongation of the lenses, and are cut by late p
egmatitic gabbro dikes. By contrast, in Mayari, and to some extent in Sagua
de Tanamo, chromitites occur deeper in the mantle tectonites and are cut b
y websterite dikes. Intergranular minerals are olivine, serpentine, and chl
orite. Chromite has abundant, randomly distributed solid inclusions of oliv
ine and pargasite, and minor pyroxene, laurite, and millerite. Toward the c
ontact with the included gabbros, chromitite from Moa-Baracoa. shows increa
sing amounts of gabbro-related alteration products. Abundant clinopyroxene,
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 Mayari, where
the Cr no. ranges between 0.41 and 0.75, the Mg no. between 0.57 and 0.81,
and the TiO2 content between 0.09 and 0.52 wt percent. At Moa-Baracoa, the
Cr no, of chromite decreases and Toe content increases from harzburgite to
dunite and massive chromitite, positively correlated with the forsterite co
ntent of coexisting olivine. At Mayari, both the Cr no. and TiO2 content of
chromite, and the forsterite content of olivine increase from harzburgite
to dunite and chromitite. Bulk platinum-group element abundances in chromit
ite vary from 20 to 538 ppb and show a broad positive correlation with Cr2O
3 percent of chromite. The latter correlation is strongest in the Sagua de
Tanamo district.
Structural, textural, mineralogical, and chemical characteristics of the st
udied chromite deposits, as well as the lithophile trace element geochemist
ry of their host rocks, support a genetic model based on the crystallizatio
n of chromite from different types of melts (from back are basin basalts to
boninitic andesites) at around 1,200 degrees C, at variable fo(2). Chromit
e formed when calc-alkaline melts, formed by melt-rock reactions at increas
ing melt volume, 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 with
in the chromite liquidus field in the P-T-fo(2) space (Hill and Roeder, 197
4). Percolation of the hybrid melt through the dunitic channels promoted di
ssolution of preexisting silicate minerals and chromite crystallization. Th
e Al-rich chromitites formed at the mantle crust transition zone at high fo
(2) (approximate to log fo(2) = -7), whereas Cr-rich chromitites formed dee
per in mantle tectonites under more reducing conditions, at log fo(2) appro
ximate to -10, depending on Cr contents of the parental magma.