TANTALITE ASSOCIATED WITH MICA REPLACEMENT: A MAGMATIC-METASOMATIC ORIGIN?

A common style of mineralization in LCT pegmatites is Ta oxides associated with mica replacement (micas typically range from muscovite to lepidolite in composition). Mica rather than K-feldspar can be stable in highly fluxed silicate melts at low temperature thus the metasomatism may be caused by melts rather than aqueous fluids. In order to test this possibility a series of experiments were conducted at 200 MPa, 700 to 600°C and at fluid oversaturated and undersaturated conditions using a synthetic pegmatite glass, pure H₂O and K-feldspar. The glass is peraluminous with 5.5, 2.8, 1.5 and 2.8 wt% F, P₂O₅, Li₂O and B₂O₃, respectively, and 200 and 1400 ppm MnO and Ta₂O₅, respectively. No replacement textures were observed in any of the fluid oversaturated runs. For H₂O undersaturated melts no mica replacement was present at 700°C, but at 675°, 650°, 625° and 600°C the mica content increased. The mica occurs as crystals in the quenched glass (a liquidus phase), but it is also observed to have replaced the K-feldspar crystal, particularly in the 600°C H₂O undersaturated experiment. Preliminary analyses indicate that the micas contain roughly 1500 ppm Ta₂O₅, for an apparent mineral-melt partition coefficient of about 1. The experiments demonstrate that magmatic metasomatism is a viable process. In addition, natural micas rarely contain more than a few hundred ppm Ta₂O₅, thus these values may also reflect the maximum Ta contents of evolved melts.