Low-K granophyres of the Stillwater Complex, Montana

Small bodies of granophyre occur as a volumetrically insignificant but ubiquitous component of the Banded series of the Stillwater Complex. White to pink granophyre typically occurs as veins, 1-12 cm thick and as much as 100 m long. A geochemically similar body of coalescing alaskite dikes, associated with an occurrence of Pt-group elements in the Banded series of the complex, crops out approximately 2 km south-southeast of Picket Pin Mountain over an area 130 by 210 m. Analyzed samples contain 77-79 wt% SiO, and, with one exception, less than 0.12 wt% K20. Albite or oligoclase and quartz, commonly in granophyric, plumose, or graphic intergrowths, commonly make up more than 97 vol% of the veins. Rare-earth-element contents of whole-rock samples and feldspar separates are as much as 100 times those of chondrites; typically, there is a pronounced negative Eu anomaly. Considering host rocks and chemistry, these rocks are comparable to the most siliceous examples of oceanic plagiogranite. The Stillwater granophyres, however, are enriched in Si, Th, U, and LREEs, and depleted in K, Fe, and Eu, relative to oceanic granophyres. The relatively voluminous granophyres of classic diabase-granophyre associations contain 1.9-5.0 wt% K,O and 57-74 wt% SiO,; they are readily interpreted as crystallization differentiates. In contrast, we interpret granophyres of the type found in the Stillwater Complex, characterized by very low K,O content, near-constant high Si02 content, and varying, commonly anomalously high CaO content, to have evolved during the latest stages of consolidation, in equilibrium with a high-temperature aqueous chloride solution. We suggest that these granophyres may typically represent material transported by and precipitated from these solutions.