- © The Mineralogical Society Of America
In contrast to those deposits formed by the alteration of volcanic ash in saline, alkaline lakes discussed in the chapter by Hay and Sheppard (this volume), large volumes of tuffaceous sediments around the world have been transformed to zeolites and other authigenic silicate minerals by the action of percolating water in open hydrologic systems. These include systems that have experienced significant chemical and hydrologic exchanges with the surrounding environment. Tephra sequences exposed to open-system alteration commonly show a vertical zonation of zeolites and other authigenic minerals that reflects the chemical changes in meteoric water moving through the system. Flow in an open hydrologic system can either be downward or have a downward component where meteoric water enters the system, resulting in a vertical or nearly vertical zonation of water composition and authigenic minerals. The original pyroclastic materials of tuffaceous sediments may have been laid down in the sea close to the volcanic sources, air-laid onto the land surface, or reworked into fluviatile and freshwater lacustrine environments. Open-system zeolite deposits are most common in nonmarine rocks, although many are known in sediments that were deposited in shallow marine environments. Zeolite deposits of the open-system type are commonly several hundred meters thick and can be traced laterally for several tens of kilometers. Many examples of zeolite deposits that formed from land-laid tephra have been recognized in the western United States, but most of the large marine deposits are in Japan and in southern and southeastern Europe. Because of their relatively large and commonly discontinuous areal extent, open-system deposits have not been studied in as much detail as some other types of zeolite deposits in sedimentary rocks; however, studies of several key areas have provided a sound basis for the current understanding of this type of zeolite body.
Zeolitic alteration can take place …