Zincochromite from the Guaniamo river diamondiferous placers, Venezuela: evidence of its metasomatic origin
Résumé
Zincochromite associated with chromite and magnesiochromite was found in heavy-mineral concentrates from the Quebrada Grande placer deposit in the Guaniamo River diamondiferous district, Venezuela. It occurs in subhedral to anhedral grains from 0.1 to 0.7 mm in size. Reflectance measurements show an increase of reflectance values with FeFe2O4 content in spinel solid solution. The Vickers microhardness (load 100 g) is 1367(84) kg/mm(2). Zincochromite containing 60 mol.% ZnCr2O4 has a unit cell with a value of a equal to 8.271(3) angstrom. As revealed by EPMA, zincochromite grains are markedly zoned, with the rim and microfissures enriched in Zn. Furthermore, the chromite crystals partly replaced by zincochromite indicate high-temperature metasomatic processes involving a Zn- and Mn-bearing fluid phase. All chromian spinel compositions observed underwent, more or less intensely, this replacement. The chemical formula of the zincochromite richest in Zn from Guaniamo can be written: (Zn6.499Fe2+ Mn-0.613(2+) (0.511MgNi0.022)-Ni-0.434)(Sigma 8.079)(Cr(9.639A)l(4.742)Fe(3+) 1.262Ti0.213V (3+)(0.040))Sigma O-15.921(32). A large extent of Mn-for-Fe2+ substitution has been observed. In some grains, the number of Mn atoms per formula unit exceeds Fe2+. Statistically, taking into account the Fe3+ concentration, zincochromite was formed at higher oxygen fugacities than its chromite and magnesiochromite precursors. EPMA traverses of zoned zincochromite and Zn- bearing magnesiochromite crystals indicate the substitution mechanisms 3Zn -> 2(Fe2+ + Mn) + Mg and 4Zn -> (Fe2+ + Mn) + 3Mg, besides the Zn -> (Fe2+ + Mn) and Zn -> Mg replacements. Chromite and magnesiochromite contain low, but consistent Zn concentrations, which demonstrate an early Zn enrichment of the magmatic system. Their chemical compositions are compatible with chromian spinels occurring in diamondiferous kimberlites and lamproites. Considering the presence of Zn- bearing chromites in several kimberlite-bearing areas on a worldwide scale, the occurrence of zincochromite at Guaniamo points to its use as a mineral indicator of kimberlites.