MINERALOGICAL AND CHEMICAL VARIATIONS IN HYPOGENE AND SUPERGENE KAOLIN DEPOSITS IN A MOBILE FOLD BELT THE CENTRAL ANDES OF NORTHWESTERN PERU

In western Peru kaolin-alunite deposits occur in Lower Cretaceous and Tertiary elastic, volcaniclastic and volcanic, mostly rhyolitic, rocks . Alunites from hypogene kaolin deposits yield K/Ar ages of 11.5 +/- 0 .7 Ma and 13.3 +/- 0.4 Ma. In addition to kaolin and alunite, the foll owing minerals are present: white mica, smectite, barite, pyrophyllite , tridymite, cristobalite, alpha- and beta-quartz, chamosite, gibbsite , and aluminum-phosphate-sulphate minerals (APS). APS mineralizations with REE-bearing svanbergite and florencite originate from supergene a lteration. Woodhouseite, goyazite, crandallite and pure svanbergite de velop in hypogene and supergene kaolin deposits. The distinction betwe en hypogene and supergene kaolinization can be made using various elem ent ratios in kaolin (P vs. S, Zr vs. Ti, Cr + Nb vs. Ti + Fe, and Ce + Y + La vs. Ba + Sr). S, Ba, and Sr are considerably enriched in kaol in during hydrothermal alteration, whereas Cr, Nb, Ti and lanthanide e lements are concentrated mainly during weathering. Au and Ag become en riched during hypogene kaolinization (advanced argillitization). Kaoli nization is associated with the evolution of the Central Andes as foll ows: (1) during the Lower Cretaceous kaolinization characterizes phase s of relative tectonic quiescence during mountain building and took pl ace in a miogeosynclinal back-are basin. The kaolin-bearing sediments were laid down in flood plain to delta plain environments; (2) in the magmatic arc/back-arc basin (eugeosyncline) kaolinization was mainly a ssociated with uplift and peneplanation; (3) in the magmatic are prope r, late Miocene kaolinization of volcanic and volcaniclastic rocks has many features in common with the high sulphidation epithermal Au depo sits.