The chemistry of five accessory rock-forming apatites

Chemical and physical data are given for five samples of rock-forming apatite from diverse geologic environments in Nevada and Colorado. Four of these apatites contain rare-earth assemblages in which the cerium group is well represented but the yttrium group predominates. The fifth apatite contains a highly fractionated assemblage of the lighter (cerium group) rare earths similar to the assemblage typical of alkulic rocks. Before the advent of modern techniques of mineral purification many minerals being considered for analysis were selected on the basis of their easy availablity, and thus many of the early analyses were performed on minerals from pegmatites, tactites, or drusy cavities, from which large crystals could be handpicked with minimal effort. Fewer analyses were performed on minerals from some of the more common igneous and metamorphic rocks, especially rather fine-grained rocks. During the past 20 years or so, many analyses of the more common rock-forming minerals have been published. However, for some of the accessory (as opposed to essential) mineralsincluding apatite there still is a dearth of chemical data on samples recovered from some of the more common rock types. This paper presents chemical data for four samples of accessory apatite and one sample of placer apatite. The apatites described here were purified by the same methods used on accessory apatites from the southern Snake Range, Nev. (Lee and others, 1973). Indices of refraction were determined by the immersion method, using a spindle stage (Wilcox, 1959) and focal masking technique (Wilcox, 1962). Cell parameters were obtained by least-squares refinement of powder diffractometer data, using an internal standard of CaF 2 and a self-indexing computer program developed by Evans, Appleman, and Handworker (1963). Semiquantilative spectrographic results are based on their identity with geometric brackets whose boundaries are 1.2, 0.83, 0.56, 0.38, 0.26, 0.18, 0.12, and so forth, and are reported arbitrarily as midpoints of these brackets, L, 0.7, 0.5, 0.3, 0.2, 0.15, and 0.1, respectively. The precision of a repotted value is approximately one bracket at 68-percent confidence, or two brackets at 95-percent confidence. FIELD SETTING OF APATITES Apatite 244-MW-60 was recovered from a muscovite-rich granitoid rock collected at lat 39°35'55" N.; long 114°7'55" W., in the Kern Mountains of White Pine County, Nev. This granitoid rock is almost identical with the (southern Snake Range) Pole Canyon Can Young Canyon intrusive type described in detail by Lee and Van Loenen (1971, p. 5, 38, 39), who ascribed its distinctive nature to assimilation of argillite. According to R. K. Hose and M. C. Blake, Jr. (oral commun., 1971), who prepared the geologic map of White Pine County, the distinctive nature of the granitoid rock from which apatite 244-MW-60 was recovered might also result from assimilation of argillite. Whatever the origin, the great similarity of these two granitoid rocks extends to the peculiar apatite-zircon relation described by Lee and Van Loenen (1971, p. 5, 39) and illustrated by Lee and others (1973, fig. 2). In each rock almost all the zircon is present as tiny, acicular inclusions in large, equant, rather poorly formed, and sparsely distributed apatite crystals. Apatite 300-DL-64 was recovered from a placer concentrate collected at the mouth of Hampton Creek at lat 39°14'45" N.; long 114°3'50"W., in White Pine County, Nev. Except for some Middle Cambrian limestones resting above a thrust surface, all the Hampton Creek drainage is underlain by Lower Cambrian metaquartzites and metashales, as shown on the geologic map of White Pine County, Nev. (Hose and Blake, 1970). The ultimate environment(s) in which the crystals of apatite 300-DL-64 may have formed is problematical. However, the sample analyzed appeared to be homogeneous, for it gave an X-ray diffraction pattern with sharp peaks which in turn gave a good refinement of the unit cell parameters (table 1). Moreover, the Lower Cambrian metasedimentary rocks were the source of most or perhaps almost all of this placer apatite. In connection with potassium-argon age studies in the area (Lee and others, 1970; and unpub. data), we have done mineral separation work on several samples of these metasedimentary rocks. Minor amounts of apatite were commonly recovered, and petrographic study shows the apatite to be present as well-formed crystals that appear to be part of the metamorphic assemblage.