George V. Albino 1995 <p><span>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 Na</span>_{<span class="sub">2</span>}<span>O 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 CO</span>_{<span class="sub">2</span>}<span>&nbsp;metasomatism and formation of sericite-pyrite assemblages that are typical of gold deposits of the Mother Lode Belt.</span></p><p><span>Alkaline fluids responsible for Na-metasomatism had elevated Na^{<span class="sup">+</span>}/K^{<span class="sup">+</span>}&nbsp;and Na^{<span class="sup">+</span>}/H^{<span class="sup">+</span>}&nbsp;relatively high&nbsp;<i><span class="italic">f</span></i>_{<span class="sub">O2</span>}, and low <i>a</i>_{^H<span class="sub">4</span>^SiO<span class="sub">4</span>}. The presence of titanite indicates fluid. The presence of titanite indicates fluid&nbsp;<i><span class="italic">X</span></i></span>_{^{<span class="sub">C</span><span class="sub">O</span>}2}&nbsp;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.</p><p><span>Mass balance calculations indicate that alteration involved addition of large amounts of sodium, and the removal of SiO</span>_{<span class="sub">2</span>}<span>&nbsp;and K</span>_{<span class="sub">2</span>}<span>O. Textural preservation, combined with volume factors calculated from specific gravity and whole rock analytical data, indicate that Na-metasomatism was essentially isovolumetric.</span></p><p><span>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 SiO_{<span class="sub">2</span>}-undersaturated fluids. Expansion of the muscovite stability field in terms of Na^{<span class="sup">+</span>}/K^{<span class="sup">+</span>}-Na^{<span class="sup">+</span>}/H^{<span class="sup">+</span>}, as a result of incorporation of Cr (up to 7.7% Cr_{<span class="sub">2</span>}O_{<span class="sub">3</span>}) in muscovite, would result in H^{<span class="sup">+</span>}- and K^{<span class="sup">+</span>}-depletion as the fluid interacts with ultramafic rocks. This could lead to fluids with elevated Na^{<span class="sup">+</span>}/K^{<span class="sup">+</span>}&nbsp;and high pH, as documented in this occurrence.</span></p> application/pdf 10.1180/minmag.1995.059.396.02 en Cambridge University Press Sodium metasomatism along the Melones Fault Zone, Sierra Nevada Foothills, California, USA article