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Compositional zoning in Zn-chromites from the Cordillera Frontal Range, Argentina

Published online by Cambridge University Press:  05 July 2018

Ernesto A. Bjerg ,
Milka K. De Brodtkorb  and
Eugen F. Stumpfl
Ernesto A. Bjerg
Affiliation:
Institute of Geological Sciences, Mining University, 8700 Leoben, Austria Universidad Nacional del Sur, Dpto. de Geologia-CONICET, 8000 Bahia Blanca, Argentina
Milka K. De Brodtkorb
Affiliation:
CONICET, Paso 258-9 A, 1640 Martinez, Argentina
Eugen F. Stumpfl
Affiliation:
Institute of Geological Sciences, Mining University, 8700 Leoben, Austria
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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In intensely altered chromites which are spatially related to sulphide mineralisation, the primary chromite cores have been replaced by Zn-rich ferritchromit (7 wt.% ZnO) and they are rimmed by Cr-magnetite.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 Alpinetype 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

chromitezoningferritchromitCordillera Frontal RangeArgentina

Information

Type
Mineralogy
Information
Mineralogical Magazine , Volume 57 , Issue 386 , March 1993 , pp. 131 - 139
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1993

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