Zonation in skarns and the controlling factors

摘要: The major element composition of intrusive rocks has been demonstrated by Meinert (1995) to be associated with the metal content of a skarn. Trace elements, especially the volatile elements, can also affect the formation of skarns and the zoning patterns, especially F. Fluorine can dramatically increase the solubility of Al in hydrothermal fluids by forming strong Al-F complexes (Tagirov et al., 2002; Tagirov et al., 2002), which makes it possible to form abundant garnet (grossularitic and subcalcic) even under reduced conditions at distal locations, such as in many Sn and W skarns (Newberry, 1998).Fluorite can also promote the formation of endoskarns by facilitating the dissolution of magmatic silicate minerals, which provides the space and the supply of Al for skarn formation. For example, at the Empire Cu-Zn skarn, Idaho, USA, there is more endoskarn than exoskarn (Chang & Meinert, 2004, 2008). The endoskarn occurs mostly as veins in the causative granite porphyry, one of the intrusive phases in the Mackay Stock. The main-stage endoskarn veins are typically composed of a central zone of dominantly garnet with minor pyroxene and locally vesuvianite, an envelope of wollastonite and/or green pyroxene±Carich plagioclase, and a halo containing disseminated, fine-grained alteration minerals of the same assemblage as the envelope. Where many veins intersect, endoskarn is massive. Endoskarn veins are present up to~ 250m from the contact into the intrusion, whereas exoskarn is less than 50m wide. The high fluorine content in the magma and the hydrothermal fluids are evidenced by the high F in magmatic hornblende and biotite, the …