Geochemical Analysis and Classification of the Gates-Adah Kimberlite Dike

Abstract:

The Gates-Adah Kimberlite dike is a NW-SE striking vertical ultramafic igneous intrusion located in Adah, southwestern Pennsylvania. A previous compositional study of the kimberlite considered classifying the kimberlite, either Group I or Group II, to be problematic. Major and trace element (including REE) analysis using X-ray Fluoresence and ICP-MS was performed on a sample collected from the dike by the Washington State University GeoAnalytical lab in order to better classify the kimberlite as Group I or II. Comparison of major elements to South African kimberlite suggests that the Gates-Adah kimberlite most closely resembles a Group I kimberlite. Comparable major element concentrations between South African Group I kimberlite and Gates-Adah kimberlite include TiO₂, Al₂O₃, FeO, MgO, and CaO. Assessment of calculated clay mineral and tectosilicate content relative to unaltered phlogopite and olivine was performed using a contamination equation in order to understand the extent of emplacement conditions and weathering of the kimberlite using weight percent of normalized major elements (contamination index C.I.). Uncontaminated Group I kimberlite has a C.I. near 1.0 and some apparently fresh and contamination-free micaceous Group II kimberlite has a C.I. up to 1.5 . The Gates-Adah kimberlite has a C.I. level of 1.14 suggesting greater similarity to a Group I kimberlite. Similarly an Ilmenite index (Ilm.I.) was calculated using the weight percents of normalized major elements to further classify the Gates-Adah kimberlite. Group I kimberlite and Group II kimberlite should not exceed 0.52 and 0.47, respectively and the Gates-Adah kimberlite has an Ilm.I. of 0.42. Two thin sections were produced from the Gates-Adah kimberlite dike. Minerals present in Group I kimberlite include: Olivine, phlogopite, serpentine, ilmenite, and diopside. Phenocrysts of anhedral serpentinized olivine were found along with ilmenite, diopside, enstatie, and phlogopite in thin section. Large phenocrysts of olivine were more heavily weathered then smaller grains within the groundmass. Hand samples show macrocrysts and phenocrysts of olivine and phlogopites. Thin sections, hand samples, and analytical techniques show characteristics of Group I kimberlites.