Abstract: In the Pohang area of Korea, heulandite occurs as cements in conglomerate and sandstone of a Miocene marine fan-delta system resting on Eocene dacitic volcanics. Three types of heulandite cements are distinguishable in the fan-delta sediments on the basis of texture, chemical composition and authigenic mineral association. The earliest type I heulandite (Si/(Al+Fe): 3.5–3.8) occurs as microcrystalline (10–30 µm) in situ crystallites that replace volcanic matrix and are intermixed with early-formed smectite. Type II heulandite (Si/(Al+Fe): 3.2–3.6) occurs as medium-grained (30–60 µm) crystal aggregates rimming intergranular cavities. Type III heulandite (Si/(Al+Fe): 3.6–4.1) is the last to form and is a composite phase of heulandite-clinoptiloite, which occurs as unusually coarse (50–200 µm) single-crystal cement associating with late-formed smectite and hematite.
These characteristic heulandite cements were formed by alteration of volcaniclastic sediment during shallow burial (burial temperature: 40–60 °C and uplift in marine pore fluid diluted by meteoric water. Sr isotope data for heulandite II (87Sr/86Sr: 0.706565–0.706598) and heulandite III (87Sr/86Sr: 0.707347–0.707432) indicate that the pore fluid was progressively mixed with meteoric water during burial and uplift but the Ca in the pore-filling heulandites has been derived mainly from dissolution of carbonate cements. Heulandite III, heulandite-clinoptilolite, was formed with an unusual coarsening and chemical zoning at somewhat diluted and disequilibrium conditions caused by the migration of oxygen-rich meteoric water during or after uplifting.