Fluid control on low‐temperature mineral formation in volcanic rocks of Kahrizak,Iran

The Kahrizak volcanic field, south Tehran, in Iran, is composed dominantly of basalt and basaltic andesite that experienced variable degrees of alteration because of the low‐grade metamorphism (stage I) and hydrothermal activity (stage II). Stage I alteration, which occurred in response to the burial of volcanic rocks and their interaction with heated groundwater, is characterized by the formation of low‐temperature zeolite facies minerals in vesicles consisting mainly of fine‐grained mafic phyllosilicate (smectite, chlorite/smectite mixed layer) and zeolites (thomsonite, chabazite, gonnardite, natrolite, analcime, heulandite, and mordenite). Stage II mineralization occurred because of the activity of hydrothermal fluids that formed large crystals of heulandite, stilbite, mesolite/scolecite, natrolite, and analcime along with quartz and calcite in cavities and fractures. The elements necessary for the formation of alteration minerals (i.e. zeolites and mafic phyllosilicates) in Kahrizak were derived from the hydrolysis of olivine and volcanic glass as well as the alteration of plagioclase. Various mineral assemblages formed during stages I and II reflect changes in temperature, pressure, and fluid composition. The change from mafic phyllosilicates to zeolites species is caused by the decrease in Mg and Fe relative to Ca fluid activities. Zeolite assemblages of stage I, known to be formed at lower temperatures, show the general sequential order from older to younger: chabazite, thomsonite, gonnardite, and natrolite. This sequence is consistent with a hypothetical fluid evolution path with increasing Na⁺ relative to Ca²⁺ activity. The change to stage II, which consists of zeolites species (stilbite, scolecite, natrolite, mesolite, analcime, and heulandite) that formed at higher temperatures, can be attributed to a temperature increase and fluid influx caused by hydrothermal activity related to a later magmatic event in the region.