Feldspar hydrolysis is a common pervasive type of hydrothermal alteration. The thermodynamics of hydrothermal alkali feldspar-mica-aluminosilicate equilibria were evaluated by Sverjensky et al. [1991], who derived an internally consistent set of thermodynamic data for these minerals and relevant aqueous species that will be useful in modeling fluid-rock interactions. Revised values for the dissociation constant of HCl, an important source of acidity, were also derived.
The spatial scale of hydrothermal circulation and alteration in crustal
rocks is important because of its implications for the volume of rock that
can be leached of metals to form concentrated ores. Cathles [1993]
used O isotopic data on altered rocks to conclude that a major, long-lived
hydrothermal convection cell, centered around a pluton in the Noranda
district, Quebec, had penetrated to depths of greater than 8 km. The
most intense and coherent zones of 
O depletion coincided with the
highest tonnage massive sulfide deposits.
Hydrothermal alteration of oceanic crust is important to understanding
the formation of seafloor massive sulfide deposits, the geochemical cycle
of sulfur in the oceans, and ultimately the origin of magmatic sulfur
erupted from volcanic arcs over subduction zones. Sulfur mass balance
and isotopic systematics accompanying hydrothermal alteration of oceanic
crust by seawater were developed by Alt [1994], based on studies
of ophiolite complexes. Sulfur is redistributed from the lower dike and
gabbro sections to the upper dike section, and additional sulfur is added
to the upper dike section by reduction of convecting seawater sulfate.
However, this latter gain is balanced by oxidative loss of sulfur from the
volcanic section. These processes result in exchange of crustal sulfur for
seawater sulfur, resulting in enrichment of 
S in altered crust.