Compositional Zoning in Zn-Chromites from the Cordillera Frontal Range, Argentina

Ernesto A. Bjerg1, 2, Milka K. de Brodtkorb3 and Eugen F. Stumpfl1
1 Institute of Geological Sciences, Mining University, 8700 Leoben, Austria
2 Universidad Nacional del Sur, Dpto. de Geología-CONICET, 8000 Bahía Blanca, Argentina
3 CONICET, Paso 258-9 A, 1640 Martinez, Argentina

Abstract: Serpentinised ultramafic bodies containing zoned chromite grains occur in the Cordillera Frontal Range, western Argentina. Chromites show Zn-rich Al-chromite cores (4.04 wt.% ZnO) surrounded by ferritchromite rims (1.3 wt.% ZnO) and outer Cr-magnetite rims. In intensely altered chromites which are spatially related to sulphide mineralisation, the primary chromite cores have been replaced by Zn-rich ferritchromit (7 wt.% ZnO) and they are rimmed by Cr-magnetite. In the Al-chromites the mean [Cr/(Cr + Al)] ratio is 0.53 and the [Mg/(Mg + Fe)] ratio is 0.53; they plot in the field of Alpine-type intrusions. Ferritchromit has a mean [Cr/(Cr + Al)] ratio of 0.93 and [Mg/(Mg + Fe)] ratio of 0.4. In Cr-magnetites the mean [Cr/(Cr + Al) is 0.98 and the [Mg/(Mg + Fe)] ratio is 0. Ferritchromit is always surrounded by a Cr-magnetite rim and it was formed as a reaction product owing to the irreversible dissolution of primary chromite cores. The dissolution of these cores provided the essential components for ferritchromit growth. Zn was introduced into the chromite cores and ferritchromit rims during the formation of the latter. Step-scan profiles have shown that the Zn content in the cores increases from the centre to their outer border, where they show the highest Zn values. It is suggested that Zn was introduced by the fluid phase involved in the alteration process that affected the cores and lead to the formation of the zoned chromite grains. This alteration process was also responsible for the changes in [Cr/(Cr + Al)] and [Mg/(Mg + Fe)] ratios.

Keywords: chromite • zoning • ferritchromit • Cordillera Frontal Range • Argentina

Mineralogical Magazine; March 1993 v. 57; no. 386; p. 131-139; DOI: 10.1180/minmag.1993.057.386.13
© 1993, The Mineralogical Society
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