Hydrothermal zircons and zircon overgrowths, Sierra Blanca Peaks, Texas^*

Zircons from the Sierra Blanca Peaks, a group of mildly peraluminous, rare-element-enriched rhyolite laccoliths in Trans-Pecos Texas, display textures that indicate formation and/or alteration in a hydrothermal environment. These zircons can be categorized into three types according to host rock, texture, and mineral chemistry: (1) magmatic zircons hosted by intrusive igneous rocks; (2) hydrothermal (or late magmatic) zircon overgrowths on magmatic zircons; and (3) hydrothermal zircons hosted by replacement bodies of fluorite in limestone. Rims of magmatic zircons from the Round Top intrusion generally are enriched in Y, Hf, Th, and U relative to cores. Magmatic zircons from the Sierra Blanca intrusion show a negative correlation between Y and U contents. Overgrowths from the Round Top intrusion have generally higher Hf contents (5.1 to 7.6 wt% HfO₂) than do their magmatic substrates (3.9 to 6.3 wt%). Hydrothermal zircons in replacement bodies contain much less Hf (1.1 to 2.2 wt% HfO₂). Compositions of magmatic zircons vary between intrusions.

Magmatic zircons in most of the intrusions display amoeboid texture and, in samples from the more Th-rich intrusions, host numerous thorite inclusions. Some magmatic zircon grains from Round Top have conspicuously inclusion-free zircon overgrowths. Round Top also has zircon veinlets and tiny “stringers” that connect to the overgrowths. Zircon overgrowths also occur in the Triple Hill intrusion, where the magmatic zircons tend to be less corroded. Hydrothermal fluorspar replacement bodies in limestone that are associated with intrusion of the Sierra Blanca laccoliths contain subhedral to anhedral, commonly corroded, inclusion-free zircon grains.