Firstly is it a truism or a fallacy that potassic (microcline) feldspar is only formed (replacing Ca and Na and lower T K-orthoclase in orebody wall-rocks where fluids are >400deg.)?
Secondly, I think mass-balance affecting replacement silicification predates the K-alteration of ore-stage veins, the vein selvages and in surrounding host-rocks. Thus silica-addition is partly responsible for the brittle K-spar-dominant veining of ore formation, and for in-situ fracture network permeability facilitating mineralization of the adjacent host rocks at a lower level.
However quartz is often included in the Potassic alteration assemblage by many authors, though they do not make it clear what quartz and where is referred to, or if it is associated with the inner K-spar or the outer biotite sub-zone.
If the former is the quartz that is considered part of the potassic zone simply the quartz that makes up part of the vein gangue assemblage in open space rather than silica addition to the host-rocks?
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