Phosphorus as a typological and mineralization potential indicator: the Cape Granite Suite of the Saldania belt as a case study

Scheepers R. ; Ozendaal A. (1995)


Granitoids of the Cape Granite Suite can be classified as S-, I- and A-types according to their P2O5 contents. The S-type granitoids dominate in the western Tygerberg terrane, the I-type granitoids in the central Swartland as well as the eastern Boland terranes and the A-types are intrusive in both the Swartland and Tygerberg terranes. The subdivision of the three major groups into six granitic associations, namely two Stype (Sa and Sb), two I-type (Ia and Ib) and two A-type associations (Aa and Ab) was performed by a combination of mineralogical and petrochemical parameters utilizing P2O5, Th, Nb, Zr, Na2O, K2O and the rare earth elements. The S-type granitoids are enriched in P2O5 above values for theoretical apatite saturation curves. A distinction is made between the Sa1 association with P enrichment regardless of the degree of differentiation and the Sa2 association with considerable enrichment during advanced stages of differentiation. Granitoids of the Ia association follow high temperature apatite saturation curves. Fractionated Ib association granitoids adhere to these curves up to advanced stages of differentiation, where P2O5 is slightly enriched. A-type granitoids of both associations are depleted in P2O5. The mineralization potential of granitoids of the Suite is specific [WMo(Cu) or WSn(Au, As) or MoCu(Au, As)] and related to granitoid association as well as geochemical parameters in each association. High P2O5 granitoids (S-types) have the potential for WSn(Au, As) mineralization when late differentiates (Sa2 association) are particularly enriched in P2O5 following a trend perpendicular to theoretical apatite saturation curves. Intermediate P2O5 granitoids (1-types) are potentially mineralized [WMo(Cu)] when they follow high temperature apatite saturation curves and have high Ca/P values, as well as a wide range in Ca/P ratios. A-type granitoids with MoCu(Au) mineralization are low in P2O5 and high in Nb, Th, Zr and CO2. Barren A-type granitoids are low in P2O5 and CO2 and high in Y and Th. High P2O5 peraluminous Stype granitoids are late orogenic to post-orogenic. The intermediate P2O5 metaluminous I-type granites are anorogenic and intruded during a long time span (560-517 Ma) in a marginal position with respect to a Proterozoic subduction zone. P2O5 depleted granitoids of the Aa and Ab associations have the typical characteristics of anorogenic alkali granites. © 1995.

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