FORMATION AND OCCURRENCES OF LAUMONTITE AND RELATED MINERALS IN THE CAROLINAS AND VIRGINIA, MIDDLE MESOZOIC ZEOLITE FACIES METAMORPHISM SOUTHERN APPALACHIAN PIEDMONT

A 175 -195 Ma hydrothermal zeolite grade metamorphic event altered most pre-Jurassic rocks in the southern Appalachian Piedmont from central Georgia, South and North Carolina to southern Virginia.

Laumontite fills fractures and replaces plagioclase in exposed and in cored subsurface rocks. Veinlets of pink asicular laumontite fill single and multiple fractures and laumontite replaces the plagioclase forming partial to complete mosaic pseudomorphs. Intensely altered rock in several areas contains up to 60% replacement and fracture filing laumontite. Cuttings from a well in north central Rowan County show that laumontization extend to 790 feet veinlets of pink fracture filling laumontite cut sheared diorite.

Considerable disparity exists between theoretical and experimental temperature, pressure limit determinations, detailed paragenesis studies and independently determined substantially lower temperatures of natural laumontite formation. Experimentally determined temperatures are both higher and lower than values obtained under natural conditions. The lowest recorded temperature of formation is 43º C.

The parameters thought to control the temperature and pressures of zeolites crystallization are: fluid pressure gradients of CO2 and H2O, activities of CO2, H20, ratio of water and CO2, the permeability, nucleation and reaction kinetics, oxygen fugacity and presence of additional ions. Swarms of 200 Ma northwest/north striking diabase dikes were injected and basalt flowed into into rift valleys during initial Atlantic rifting, followed by filling of half grabens with immature clastic sediments. Fractures (joints and faults) served as conduits for calcium-saturated warmed water that crystallized laumontite, prehnite, calcite and other zeolites in granites, metadiorites and quartz monzonites.

This period of extensive hot spring activity, zeolite grade thermal alteration, low pressure metamorphism is widespread.