Zeolite occurrence and genesis in the Late-Cretaceous Cayo arc of Coastal Ecuador: Evidence for zeolite formation in cooling marine pyroclastic flow deposits

This paper describes the quantitative mineralogy, the mineral chemistry and the distribution of natural zeolites over the outcrop area of the Late Cretaceous Cayo Formation of Coastal Ecuador (> 1000 km2) and develops a model for zeolite alteration in the Cayo volcanic arc. Different zeolite types were identified: Ca-heulandite-type zeolites (clinoptilolite and heulandite), mordenite, laumontite, analcime, stilbite, epistilbite, chabazite, thomsonite and erionite. Zeolites occur over nearly the entire outcrop area and the entire stratigraphical thickness of the Cayo Formation, in percentages varying between less than 20 to nearly 100 wt.%. A substantial amount of the analyzed samples (1/8) has zeolite contents higher than 50 wt.% and could potentially be used in agriculture, aquaculture, for waste water treatment or as supplementary cementitious materials. A clear difference in zeolite type and content was observed when comparing the lower and upper unit of the Cayo Formation and the distribution of these units determines the zeolite distrubution over the outcrop area. In the upper unit, Ca-HEU-type zeolites are the main zeolite minerals and rarely laumontite and analcime occur. A smectite-rich smectite/chlorite (C/S) is the major associated alteration mineral, while quartz contents are relatively low. In the lower unit, the zeolite mineralogy is more variable and mordenite, Ca–HEU-type zeolites and laumontite are common. Stilbite, epistilbite and analcime occur rarely. Further quartz, albite, C/S and celadonite occur as associated alteration minerals. Little burial metamorphism or volcanically induced hydrothermal alteration have affected the deposits of the Cayo formation. Mineral alteration occurred mainly by interaction of hot pyroclastic glass with marine water, present as pressurized steam in cooling pyroclastic flow deposits on one hand or by low temperature diagenesis of already cooled pyroclastic or epiclastic deposits. A model similar to the “geoautoclave” model is proposed to explain the genesis of zeolites, in which an autoclave is formed by the hydrostatic pressure exerted by the marine water column overlying the pyroclastic deposits, preventing gas escape and promoting glass dissolution, zeolite formation and, conversion to higher-grade phases possible if heat can be maintained for a long enough period.status: publishe