Altered chrome ores from the Coolac Serpentine Belt, New South Wales, include chloritic and nonchloritic assemblages. In the chloritic ores the primary chromite is usually an aluminum-rich variety which is partly replaced by secondary chromite, or by a chemically equivalent oxide mixture; and by chlorite. Chemical analyses of chromite concentrates and electron probe scans across chromite variants show the secondary chromites to contain more chromium, more total iron, more ferric but less ferrous iron, less aluminum and less magnesium than the primary chromites. Much of the chlorite developed from serpentine and pyroxene; when aluminum, released from chromite during oxidation, became available. Some chlorite which originated in other ways, however, occurs in the same ores.

In the nonchloritic assemblages, primary chromium-rich chromite contains magnetite within fissures and is associated with serpentine-containing accessory awaruite. Diffusion at some chromite-magnetite contacts however resulted in a mineral resembling the secondary chromite of the chloritic assemblages.

The non-chloritic assemblages developed during serpentinization in conditions of low oxygen activity, and tended to persist during subsequent conditions of increased oxygen activity which induced the oxidation of aluminum-rich chromite. In conditions of high oxygen activity chromium-rich chromite also oxidized and associated serpentine was chloritized. The involvement of rodingitic assemblages in these reactions ranged from negligible to intense.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.