GRANOPHYRES OF THE STILLWATER COMPLEX: EVIDENCE FOR A COMMON ORIGIN

Granophyres are present at several locations within the Stillwater Complex. The granophyres consist of quartz-plagioclase intergrowths, along with chlorite and microcline. Previous work on Stillwater granophyres was performed at Picket Pin Mountain (PP) in the Gabbronorite III zone (vein and leucogranite samples)¹ and near Contact Mountain in the Gabbronorite I zone (medial section of a mafic pegmatite)² .The present study utilizes granophyre samples collected at outcrops and adjacent veins from the Olivine-bearing I zone in the Stillwater Mine and compares them to samples from the previous studies.

Numerous similarities are present between samples from the three locations. All have sharp contacts with no chilled margins against the undeformed host rock. Plagioclase in all three is An-poor. All are leucocratic with few mafics, and are compositionally similar with SiO₂ exceeding 74 wt. %, and low K₂O (< 1.5 wt. %). Samples from the three areas have matching REE patterns with LREE enrichment, variable negative Eu anomalies, and HREE enrichments. The negative Eu anomaly has been attributed sphene crystallization affecting REE available for feldspar incorporation¹. Though no Eu anomaly has been found in the parent melt³, the granophyres have REE patterns similar to the parent melt, albeit with greater LREE enrichment.

Mechanisms previously proposed for the Stillwater granophyres include (1) Rapid precipitation from an aqueous chloride solution in which the granophyres are not the result of melt evolution¹ or (2) Channelization of residual silicate liquid via cumulate compaction and in which the granophyres do result from melt evolution². Due to the marked similarities such as textures and REE patterns noted above in PP, CM, and mine samples, the granophyres from all three locations may have formed from the same process. Given the compositional similarities between the granophyres and the parent liquid, the second process is probably the more likely scenario.

1. Czamanske et al., Am. Min. 76 (1991) 1646-1661.

2. Hanley et al., J. Pet. 49 (2008) 1133-1160.

3. Helz, MBMG Spec. Pub. 92 (1985) 97-177.