New Mexico Geological Society Annual Spring Meeting — Abstracts

The prospect is located in the Capitan Mountains Several miles north-east of the town of Capitan. The veins are hosted by a igneous stock (lacolith?) of akaskite composition (50% feldspars, 40% quartz, 3% hornblende, 7% accessory minerals) and contains: allanite, titanite (sphere), quartz, plagioclase, chlorite and clay minerals. Allanite exhibits a similar crystal habit as epidote and contains the rare-earth Cerium (Ce). Some quartz at the prospect exhibits Japanese twining.

The paragenetic sequence of minerals in the vein is obscured by poor exposure and the presence of clays. The general area is characterized by few outcrops and thick cover.

Through microscopic textural relationships, a tentative paragenesis has been constructed (fig. 1). Fluid inclusions have been found in the sphere and quartz at the deposit. These inclusions have several characteristics that make them very interesting.

1) The salinities are very high, averaging 65 eq. wt. % NaCl.

2) Homogenization temperature ranging from 480580 C.

3) Homogenization by halite disappearance (e.i., vapor homogenizes before halite). This may be explained by the fluid being saturated with halite at the time of deposition. Further evidence for this are solid inclusions of halite found in quartz.

4) The inclusions contain an average of 8 daughter minerals.

Positive identification of the daughter minerals in complicated by their small size, but tentative identification of halite, sylvite, hematite and anhydrite has been made based on optical properties and behavior upon heating. The rest are unidentified.

Allanite is commonly associated with felsic igneous rocks and pegmatites. The high salinities and homogenization temperatures along with the close proximity to a igneous body point to a magmatic source for the mineralizing fluid.