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

Abstract

Granitoids of the Cape Granite Suite can be classified as S-, I- and A-types according to their P 2O 5 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 P 2O 5, Th, Nb, Zr, Na 2O, K 2O and the rare earth elements. The S-type granitoids are enriched in P 2O 5 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 P 2O 5 is slightly enriched. A-type granitoids of both associations are depleted in P 2O 5. The mineralization potential of granitoids of the Suite is specific [WMo(Cu) or WSn(Au, As) or MoCu(Au, As)] and related to granitoid association as well as geochemical parameters in each association. High P 2O 5 granitoids (S-types) have the potential for WSn(Au, As) mineralization when late differentiates ( Sa2 association) are particularly enriched in P 2O 5 following a trend perpendicular to theoretical apatite saturation curves. Intermediate P 2O 5 granitoids (1-types) are potentially mineralized [WMo(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 MoCu(Au) mineralization are low in P 2O 5 and high in Nb, Th, Zr and CO 2. Barren A-type granitoids are low in P 2O 5 and CO 2 and high in Y and Th. High P 2O 5 peraluminous Stype granitoids are late orogenic to post-orogenic. The intermediate P 2O 5 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. P 2O 5 depleted granitoids of the Aa and Ab associations have the typical characteristics of anorogenic alkali granites.