ALUMINUM PHOSPHATE MINERALIZATION FROM THE HYPOGENE LA VANGUARDIA KAOLIN DEPOSIT (CHILE)

In the region of Illapel, Chile, dioritic and andesitic rocks of Creta ceous to Paleocene age from the Unidad San Lorenzo Formation have suff ered strong alteration resulting in the formation of a kaolin-rich zon e, which is mined in the La Vanguardia open pit near Combarbala. Detai led mineralogical and chemical investigations of channel samples from this mine, involving a combination of optical microscopy, XRD, SEM-EDX , XRF and TEM, permitted recognition of a zone of silicification and a zone of advanced argillic alteration, both of which contain alunite s .s. and aluminium-phosphate-sulphate (APS) minerals of the woodhouseit e group with subordinate amounts of gorceixite, florencite and goyazit e. These minerals are found to have originated from infiltration of me teoric waters. The passage from silicification into advanced argillic alteration was associated with an increase in sulphate activity as man ifested by the increase of SO42- at the expense of PO43- in the alunit e s.s. and by an increase of the Au content from 0.01 to 0.03 mg/kg Au . This high sulphidation type of wallrock alteration may be roughly co mpared with the Nansatu type of White (1991).