New Mexico Geological Society Annual Spring Meeting — Abstracts

Recent work by Wallace (2016) and earlier work by Dunn (1982) recognized widespread, albeit relatively weak, phyllic alteration in the Copper Flat porphyry copper system of the Hillsboro district, Sierra County, New Mexico. We report shortwave infrared (SWIR) spectral absorption features of the phyllic alteration obtained using a probe mask in order to significantly reduce the area sampled by the spectrometer probe. This allows the detection of IR absorption anomalies on a much smaller spatial scale than previously possible. Analytical transects across individual samples of the early phyllic alteration that is widely distributed in the Copper Flat system generally show limited sample-specific wavelength variations for the main Al:OH bond absorption feature in the infrared spectrum. Detailed analyses utilizing the probe mask indicates that sericite in the altered mafic sites has slightly longer wavelength absorption by about 3-4 nanometers (nm) relative to sericite replacing adjacent plagioclase. This supports the conclusion of Wallace (2016) based on microprobe analysis that the sericite in this setting inherited a composition partly from its precursor silicate mineral, rather than being compositionally controlled by the fluid. On average, the early sericite has its main Al:OH absorption feature at around 2210nm, which is produced by mica of celadonitic composition. Analytical transects over samples showing multi-stage phyllic alteration have up to a 9nm wavelength variation in this absorption feature. Later stage phyllic alteration is characterized by well-developed sericite halos to thin quartz ±sulfide veinlets affecting all precursor minerals, but has limited representation in the system. The IR absorption features of this sericite have both longer (to 2214nm) and shorter wavelengths (2205nm) relative to the background sericite. The late sericite of longer-wavelength absorption tends to be a gray color and that of shorter wavelength is white. Based on the location of the Al:OH absorption feature, the late sericite ranges between muscovite and celadonite compositions (2205nm to 2214nm), and overlaps with the background sericite. No sericite of shorter wavelength has been found in the Copper Flat system. The Copper Flat hydrothermal system differs from other copper porphyry systems in a number of ways but the characteristics of the phyllic alteration are particularly noteworthy. As in other porphyry systems, the widespread background carbonate-bearing phyllic alteration likely evolved from earlier potassic alteration fluids, but these were much more restricted in volume, less oxidized and sulfur-poor at Copper Flat. Due to the fact that the host rock rock strongly buffered the phyllic fluid, the resulting alteration was weak and targeted specific minerals with the general preservation of the texture of the rock and the dominance of celadonitic sericite compositions. In other porphyry systems, greater stability of shorter-wavelength sericite (i.e., <2205nm), such as muscovite and paragonite, may result from various processes (absent at Copper Flat) that result in a reduction of the host rock’s capacity to buffer the hydrothermal fluid.