MINERAL CONTROLS OF ELEMENT MOBILITY DURING SODIC-CALCIC ALTERATION

Sodic +/- calcic metasomatism occurs in diverse environments including regional and seafloor metamorphism, and is associated with IOCG and porphyry ore deposits. Element mobility and reaction mechanisms that occur during this style of alteration are of great interest. Alteration zones cut a granodiorite sill complex related to the Tertiary-aged Whiterock Pluton that intruded the Maroon Formation in the Elk Mountains of Colorado. Sodic +/- calcic alteration there consists of symmetrical selvages around fractures and bedding planes. Alteration envelopes are typically zoned from unaltered wall rock to a diopside-rich outer selvage and a white, plagioclase-rich central zone.

Guided by cathodoluminescence, EPMA and LA-ICP-MS mineral analyses help link whole rock chemical changes to specific mineral reactions. The green, outer selvage is characterized by igneous plagioclase, replacement oligoclase/albite after alkali feldspar, and diopsidic pyroxene or actinolite + titanite ± epidote after biotite, amphibole and Fe-Ti oxides. The white zone contains oligoclase/albite + titanite and relatively Na-rich relict portions of igneous plagioclase. Replacement feldspar compositions range from albite to oligoclase and span the peristerite gap. The presence of sharp reaction fronts, microporosity, and pseudomorphism indicate the feldspar replacement mechanism interface-coupled dissolution-reprecipitation.

Whole rock powder and in situ handheld XRF analyses across the alteration zones constrain elemental gain/loss. Sodium gain was accompanied by Fe, Mn, Mg, Ba, and K loss. Diopside-zone samples have gained Ca, Cr, and Sc, but there was loss of these elements in the diopside-free, most-altered zones. Rare earth elements show differential mobility. The most-altered zone preferentially gained HREE; elsewhere REE behaved similarly to typically immobile elements. LA-ICP-MS analyses show that titanite is the main reservoir of REE in the most altered-zone and controlled mobility behavior of these elements. Linking elemental gain/loss to specific minerals provides greater understanding of element mobility and is a powerful tool for deciphering reaction mechanisms.