Alteración argilica avanzada de rocas piroclasticas en función de su estructura en la zona de rodalquilar (Almeria)

Abstract

Advanced-argillic alteration process is studied in different types of pyroclastic volcanic rocks: dacitic tuffs, riodacitic ignimbrites and andesitic agglomerates located in the Rodalquilar area (Almería, SE Spain). Tuffaceous material show massive alteration with vertical zonal patterns. In the lower zones silicification and propylitization processes (quartz, chlorite, illite ± pyrite) predominate and evolve into advanced-argillic paragenesses (illite, kaolinite ± alunite) at intermediate zones and to a quartz-alunite rock in the upper rones. Ferrie aureolaes and limonitic crusts are developed between the intermediate and upper zones. Agglomeratic and ignimbritic rocks show similar a1teration arranged in a more complex fissural patterns probably related to the intense fracturation displayed in that materials. Fissural zones are characterized by the incidence of supergenic processes with an increase in the size, idiomorphism and crystallinity of quartz crystals. Zones of massive argillie a1teration with alunite show important contents in Cl (0.4%), while limonitic crusts and ferrie aureolaes concentrate Sb (30 ppm.) and As (500 ppm.). The Arsenic is also concentrated in quartz levels (80 ppm.). A genetic model related to sea water hydrothermal flow in a «hot spring» environment is discussed: The influence of weathering and supergenic processes in the redistribution of some analyzed minor elements (Cl, As, and Sb) and Fe is discussed toghether with the evolution in morphology and crystallinity of siliceous minerals.