New Mexico Geological Society Annual Spring Meeting — Abstracts

The Capitan Pluton is a hypabssal, peraluminous alkali granite intrusion which is located in Lincoln County, south-central New Mexico. Comprising the bulk of the Capitan Mountains, the intrusive body is the largest Tertiary pluton in the state and has been dated at 26.5 Ma by the K-Ar method (Allen, 1988).

Previous work (Allen, 1988; Allen and McLemore, 1991) has shown that the Capitan Pluton displays textural and mineralogical zonation which is thought to represent chemical fractionation in the source magma. Faulting and/or tilting of the pluton after emplacement has exposed three textural zones: an outer rind of granophyre at the western end of the pluton, a middle zone of aplite, and a porphyritic core at the eastern end.

Work by Phillips (1990) on primary/pseudosecondary fluid inclusions in Th-U-REE quartz/fluorite vein deposits in the Capitan Pluton has suggested that these veins resulted from the concentration of high-temperature, high-salinity magmatic-hydrothermal fluids in fractured and brecciated zones within the cooled outer carapace of the pluton. If this model is correct, these same fluids might also be expected to have been trapped as secondary fluid inclusions in the igneous quartz of the pluton. In addition, the plutons cross-sectional exposure allows the distribution of the original fluids within the pluton to be studied.

Microscopic examination of igneous quartz in 44 thin sections of the Capitan Granite indicates a greater abundance of secondary fluid inclusions with daughter minerals in the outer textural zones (granophyre and aplite) of the pluton. Fluid inclusions with daughter minerals are usually absent in the porphyritic core. In contrast, inclusions containing liquid + vapor or entirely vapor are equally abundant in all three textural zones.

Optical and microthermometric observations of 50 secondary fluid inclusions in igneous quartz from a sample of granophyre correlate fairly well with the data from the Th-U-REE quartz/fluorite vein deposits, suggesting that the fluid inclusions in igneous quartz were formed by the same or similar fluids as the veins. Vapor homogenization temperatures of high salinity inclusions range from 204-509°C with an average of 384°C. Halite dissolution temperatures of high salinity inclusions range from 442-583°C with an average of 525°C. Total homogenization temperatures are not as well documented due to many of the high salinity inclusions decrepitating before total homogenization. Samples of miarolitic quartz also contain high salinity inclusions, but microthermometric data is too sparse at present to sufficiently constrain the nature of the trapped fluids.

These observations suggest that the magmatic-hydrothermal fluids responsible for vein mineralization were not only concentrated in the vein systems, but were widely disp'ersed within the outer textural zones of the pluton. Furthermore, the greater abundance of fluid inclusions in the outer portions of the pluton suggests that the interior of the pluton had not yet fully crystallized when these fluids ex solved from the magma.