GEOCHEMICAL AND PETROLOGIC EXAMINATION OF CORUNDUM KNOB LITHOLOGIES

The purpose of this study is to examine Corundum Knob lithologies, located within the Buck Creek ultramafic complex in Western North Carolina, using standard geochemical and petrologic tools by the WCU Fall 2023 Earth Materials course participants. Corundum Knob primarily contains metatroctolite, metadunite, and amphibolite. Five samples were collected near the summit that were processed into petrographic thin-sections as well as powders for X-Ray Diffraction (XRD) and Fluorescence (XRF) at WCU and Inductively Coupled Plasma Atomic Emission (ICP-AES) and ICP-Mass Spectrometry (ICP-MS) at ALS Global Geochemistry.

Preliminary XRF analyses show these samples are SiO₂-poor (30.7 to 36.1 wt.%) and MgO-rich (12.3 to 19.8 wt.%), similar to the marginal troctolites reported by Tenthorey et al. (1996). Our samples also indicate an elevated Al₂O₃ content (26.1 to 33.6 wt.%), which is reflected in the thin-sections that are dominated by plagioclase crystals. XRF data also show that our samples have a relatively low abundance of Ni (285 to 447 ppm), consistent with low abundance of olivine in the thin-sections, that contrasts with the Ni abundances reported by Tenthorey et al. (1996) for two troctolite groups (1090-1240 ppm).

Preliminary XRD analyses indicate the presence of various hydrous (e.g., tremolite, edenite) and anhydrous (e.g., kaersutite) amphiboles in all samples. All but one sample also potentially contain a form of Obertiite, a relatively uncommon anhydrous amphibole. Petrologic examinations are on-going to confirm the presence/absence of these minerals within the thin-sections, particularly with respect to the presence/absence of coronas and symplectites. Three samples contain abundant coronas with varying degrees of development, but the remaining two appear to lack them. The corona-free samples have higher Cr (909-1096 ppm) and K₂O (0.05 wt.%) as well as marginally lower SiO₂ (30.7-34.0 wt.%) in contrast to those with coronas (Cr: 193-515 ppm; K₂O: <detection; SiO₂: 35.6-36.1 wt.%), but otherwise the groups have similar XRF and XRD results.

Ongoing work will focus on thorough examinations of the thin-sections including investigations into the presence of coronas and symplectites and more detailed geochemical results (ICP-AES and -MS) to better understand the samples’ history.