Abstract: Albitite, locally aegirine- and riebeckite-bearing, formed as a result of sodium metasomatism of felsic dykes and argillites along the Melones Fault Zone near Jamestown, California. Pyrite, magnetite, hematite and titanite are common in small amounts in altered dykes. The dykes were originally plagioclase-hornblende porphyritic, and had major and trace element abundances typical of calc-alkaline rocks, whereas they now have Na₂O contents as high as 11.40%. Associated fracture-filling veins are dominated by albite, but locally include aegirine, analcime, paragonite, calcite and sodic scapolite. Quartz is present in most albitic rocks, but is absent in riebeckite- and aegirine-bearing samples. Albitization predated CO₂ metasomatism and formation of sericite-pyrite assemblages that are typical of gold deposits of the Mother Lode Belt.

Alkaline fluids responsible for Na-metasomatism had elevated Na⁺/K⁺ and Na⁺/H⁺ relatively high f_O2, and low a_H4SiO4. The presence of titanite indicates fluid. The presence of titanite indicates fluid X_CO2 was low, in contrast to fluids that formed later carbonate-bearing assemblages. Sodic scapolite suggests that, at least locally, the fluids attained very high salinities.

Mass balance calculations indicate that alteration involved addition of large amounts of sodium, and the removal of SiO₂ and K₂O. Textural preservation, combined with volume factors calculated from specific gravity and whole rock analytical data, indicate that Na-metasomatism was essentially isovolumetric.

Sodium-rich zones along the Melones Fault Zone are closely associated with fault-bounded bodies of ultramafic rock, typically altered to talc-carbonate or quartz-magnesite-Cr muscovite assemblages. Carbonatization and talc-forming reactions in the ultramafic rocks may lead to SiO₂-undersaturated fluids. Expansion of the muscovite stability field in terms of Na⁺/K⁺-Na⁺/H⁺, as a result of incorporation of Cr (up to 7.7% Cr₂O₃) in muscovite, would result in H⁺- and K⁺-depletion as the fluid interacts with ultramafic rocks. This could lead to fluids with elevated Na⁺/K⁺ and high pH, as documented in this occurrence.