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 clastic, 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 following minerals are present: white mica, smectite, barite, pyrophyllite, tridymite, cristobalite, α- and β-quartz, chamosite, gibbsite, and aluminum-phosphate-sulphate minerals (APS). APS mineralizations with REE-bearing svanbergite and florencite originate from supergene alteration. Woodhouseite, goyazite, crandallite and pure svanbergite develop in hypogene and supergene kaolin deposits. The distinction between hypogene and supergene kaolinization can be made using various element 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 kaolin during hydrothermal alteration, whereas Cr, Nb, Ti and lanthanide elements are concentrated mainly during weathering. Au and Ag become enriched during hypogene kaolinization (advanced argillitization). Kaolinization is associated with the evolution of the Central Andes as follows: (1) during the Lower Cretaceous kaolinization characterizes phases of relative tectonic quiescence during mountain building and took place in a miogeosynclinal back-arc 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 associated with uplift and peneplanation; (3) in the magmatic arc proper, late Miocene kaolinization of volcanic and volcaniclastic rocks has many features in common with the high sulphidation epithermal Au deposits. Received: 11 August 1995 / Accepted: 8 May 1996