Electron microprobe analyses of chromite ores from Baramiya, Seifein, Siwigat, Ashayer, Um Salatit and Ras Shait, Egypt, reveal two compositional groups. The unaltered chromites from Baramiya and Seifein have high Al and low Cr contents [Cr/(Cr + Al) = 0.56] whereas the remainder have low Al and high Cr contents [Cr/(Cr + Al) = 0.73]. Such bimodality characterises Alpine-type peridotites. The second group probably crystallised at higher T and lower P than the first group and has a composition characteristic of a type III alpine-type peridotite, i.e. of arc or possibly ocean plateau origin but not of mid-ocean ridge origin. The crystallisation setting of the first group is less certain but not inconsistent with the same environment as the second group so that considering the likely higher pressure of crystallisation of the first group overall favours plutonic crystallisation in the roots of an arc for both groups.

Rim, patchy and fracture alteration of the chromite occurred possibly partly of late magmatic (deuteric) origin but mainly connected with fluid movements, serpentinisation and tectonism. The final composition of the resultant ferritchromite is variable and depends largely on the original chromite composition; the composition of the ferritchromite developed in highly cataclased chromites deviates markedly from that of the original chromite presumably due to unmixing and migration.

Ferritchromite is rarely reported in forearc mantle peridotites. This contribution describes ferritchromite alteration and zoned Cr-spinel in serpentinites from the Rio San Juan Complex in the Dominican Republic. These rocks originated from the forearc mantle and protruded along lithosphere-scale faults in the mid Eocene. The cores of the Cr-spinel grains have Cr# ratios [i.e.atomic Cr/(Cr + Al)] between 0.48 and 0.66; such values are relatively high and are considered to represent primary compositions. Relatively high Zn contents in the grain cores (0.46 c 0.95 wt.% ZnO) are also thought to be primary; they reflect exceptionally cool conditions in the northern Caribbean forearc mantle. A progressive change in the zoning of Cr-spinel is recorded in the samples. Weakly zoned grains of Cr-spinel have rims with lower Mg# ratios [i.e.atomic Mg/(Mg + Fe2+)] and slightly higher Cr# ratios than the cores. More strongly zoned grains of Cr-spinel, in addition to low Mg# and high Cr# in their rims, have a marked increase in Fe3+# [i.e.Fe3+/(Fe3+ + Al + Cr)] of up to 0.35 in their rims and are partially coated by Mg-rich chlorite. All grains show core-to-rim decreases in their Zn content and increases in Ti, Mn and V. The association with Mg-rich chlorite and the compositional zoning are reminiscent of those reported for ferritchromite. Ferritchromite (with Fe3+# >0.5) is common in ultramafic rocks in amphibolite-grade terranes; however, the serpentinite samples described herein show little evidence of high-grade metamorphism. The lowtemperature serpentine-group mineral lizardite is dominant and high-temperature antigorite is either very rare or absent; other high-temperature minerals, such as talc, tremolite and cummingtonite, are trace constituents. The observed zoning in the Cr-spinel is thought to represent 'immature' ferritchromite, probably formed in response to a short-lived thermal event. This event appears to have been on too short a timescale to produce either proper ferritchromite or significant quantities of high-temperature minerals. It may be related to the emplacement of the nearby Rio Boba Intrusion, or the upward protrusion of the serpentinites along the lithosphere-scale Septentrional fault zone from the base of the mantle wedge through its hotter interior. We suggest that such alteration is rare in forearc serpentinites because they are not commonly heated during exhumation along the plane of subduction. This work demonstrates that Cr-spinel compositions can be modified by relatively low-grade metamorphism.