Anne Hammond HBSc thesis abstract

Thesis Title: 
Accessory Mineralogy of Swartruggens Orangeite

Orangeites occur within a sub-parallel en-echelon dike system on several farms near Swartruggens, Transvaal.  Emplaced during the upper Jurassic in an approximately 1 km thick sequence of lavas of the Pretoria series, they are underlain by approximately 1.5 km of highly fractured shale/slate which overlies Transvaal dolomite.  The incompetent and fractured nature of the shale is believed to be responsible for the meandering, splitting, and intertwining nature of the dikes which may vary considerably in thickness over short distances or pinch out rapidly.  Composite dikes are common, with different levels and segments of individual dikes displaying petrographically different character and degrees of alteration.  Evidence exists for multiple intrusions with relatively slow magma-emplacement rates, as demonstrated by the preservation of delicate flow banding as dikes curve around irregular host rock protrusions.

This study of the Swartruggens orangeite was limited to the investigation by transmitted light and electron microscopy of samples from mining levels 3, 4, 6, and 7 of th Helam Mine.  Flow banding is evident in samples from levels 6 east and west and 7 east.  Levels 6 and 7 also contain the greatest diversity and abundance of accessory minerals which commonly include apatite, barite, and calcite.  Less common but important phases are perovskite, wadeite, and an unidentified Ca-Ti-Fe silicate.  Minor phases include strontianite, an unidentified Ca-REE phosphate, zircon, an unidentified Ca-Zr silicate, and diverse oxides and sulphides.  Strontium is a common component in apatite, barite, calcite, and perovskite, with barium present to a lesser extent in some of the apatite and calcite.  Both SrO and BaO contents within these minerals may vary considerably within samples from different levels.  REE-oxides are a minor component of some of the apatite and are present in perovskite in moderate amounts.

Compositionally distinct from archetypal kimberlites, Swartruggens orangeite bears closer similarities to lamproites, but differs sufficiently to preclude classification as such.  The accessory minerals apatite, barite, perovskite, and wadeite are common to both Swartruggens orangeite and lamproites.  Differences include the presence of calcite, a relatively rare mineral in lamproites, in Swartruggens orangeite.  An unidentified Ca-Ti-Fe silicate is unique to Swartruggens.  Common lamproitic phases such as leucite, sanidine, shcherbakovite, and K-Ti richterite are absent from Swartruggens.

Lamproite and orangeite magmas are believed to have similar origins in being the products of partial melting of metasomatized lithospheric mantle.  Unique differences resulting from long term evolutionary patterns of diverse lithospheric sources are believed to be responsible for the production of orangeite magmas within the Kaapvaal craton and lamproite magmas on other cratons, and could produce other types of potassic rocks elsewhere.  A proposal has been suggested to include all potassic magmas within a broad group termed the Metasomatized Mantle Magma (MMM) group reflecting similar petrogenetic processes in their origins.