Abstract
To a first approximation, alteration minerals in greisen and related fluorine-rich porphyry deposits lie in the system K 2 O-Al 2 O 3 -SiO 2 -H 2 O-F 2 O (sub -1) . Equilibria in this model system can be depicted on quartz and fluid-saturated chemical potential diagrams whose axes are mu HF (acidity, where mu HF = mu HCl + mu (sub FCl (sub -1) ) ) and mu KF (salinity, where mu KF = mu NaCl + mu (sub KNa (sub -1) ) + mu (sub FCl (sub -1) ) ). Advantages of this approach are that it reflects the fact that the anion F, unlike Cl, is incorporated in greisen minerals (mainly topaz, fluorite, and micas) and that mineral stabilities can be correlated with possible ore-forming processes.The most important of these processes is probably vapor phase separation from an aqueous brine or magma (first or second boiling). Fluid inclusion studies have shown that boiling occurs in all porphyry and many (but not all) greisen deposits. Vapor phase separation from a magma (pneumatolysis) must similarly be responsible for the formation of greisenlike mineral assemblages in miarolitic topaz-rhyolite lava flows and is presumably also responsible for the formation of gem pockets in more deeply seated pegmatites and granites.The result of boiling is an increase in mu HF and a decrease in mu KF in the vapor phase, and the opposite in the residual brine or magma. In deep greisen veins, the muscovite stability field is encountered between the K-feldspar and topaz fields, whereas in F-rich, relatively shallow porphyry deposits, such as Henderson, Colorado, boiling causes aqueous fluids to pass directly from the K-feldspar ore zone to the overlying topaz (+ magnetite) zone. Muscovite (+ or - F-bearing Mn-Fe garnet) is only encountered farther out, as the fluids are condensed, neutralized, and diluted by cooler meteoric waters.When Ca, Fe, Mn, Be, W, O 2 , S 2 , and CO 2 . are added to the greisen model system, scheelite is shown to transform to wolframite with increasing mu HF , and beryl to phenakite with increasing mu HF or mu KF . Biotite transforms to magnetite, hematite, siderite, pyrite, chlorite, or fayalite beyond the K-feldspar field (its stability is enhanced by high mu HF ), and chlorite, cordierite, or Mn-Fe garnet transform to an Mn-Fe carbonate, oxide, or sulfide (or biotite) beyond the Al silicate field. The Ca content of plagioclase coexisting with fluorite decreases with an increase in either mu HF or mu KF , and thus provides an indicator of total fluorine in both acid and neutral species.
