Apatite chemistry in P-Si-S space

Apatites from felsic-intermediate rocks display considerable variability on a plot of P/lOO-Si-S (Fig. 4a). The ratio of Si to S is highly variable in the apatite, but the bulk of the analyses cluster crudely near a value of 1, consistent with the substitution Si + = 

Apatite associated with ore-systems (non- and ore-producing), have an excess of Si relative to S, which in all likelihood is due to additional substitutions in this site, for example REE + Si = Ca + P (Roeder et al. 1987, Ronsbo 1989, Sha and Chappell 1999) (see also discussion above) or SiO/ + COi = 2 P04 (Sommerauer and Katz-Lehnert 1985). This excess of Si relative to S in apatite from ore-related systems may seem curious given the high S contents (bulk) associated with many porphyry-type Cu and Mo deposits however, some studies (Core et al. 2001) suggest that some S in the melts is in a reduced form. 

Apatites associated with felsic-intermediate intrusive and extrusive rocks exhibit a relatively restricted range in composition F-Cl-OH space (Fig. 5a). Apatites from these rocks contain between 50-100% F, 0-50% OH, and 20% Cl, with few exceptions. The dominant exchange operating is F(OH).i, due in part to the similarity in radius for F and (OH) as compared to Cl. Apatites from felsic-intermediate pegmatites display an even more restricted range of compositions, but possibly with less Cl however, this may not be due to volatile exsolution (a topic discussed later). It is interesting to note that in some meteorites, for example the achondritic Nakhlite, the Z site is filled with F (1.6 wt %) and Cl (4.0-4.3 wt %) with no detectable OH (Bunch and Reid 1975) (not on figure). 

The use of halogen concentrations in apatite to make estimates of aqueous fluid compositions is well developed (Korzhinskiy 1981, Zhu and Sveijensky 1991,1992). Given the temperature of apatite crystallization (calculated as an AST), or some other independent estimate of temperature, and an estimate of the pressure of crystallization (although the calculation is fairly insensitive to pressure), the fugacity ratio 

Within a given granitic rock suite, the halogen concentrations in apatites commonly vary as a function of whole rock Si02. For example, in a study of the gold-mineralized Emerald Lake pluton. Tombstone Suite, Canada, Coulson et al. (2001) attempted to use fluid composition determined using apatite and biotite chemistry to infer the source of 

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