Open Collections

Mailey, Amy

University of British Columbia

Primary volcanic rocks mined for diamonds have historically been classified as Group I or Group II kimberlites; the latter now referred to as carbonate-rich olivine lamproites (CROL). Correctly identifying and classifying diamondiferous rocks has consequences for both fundamental and practical geology because most economic diamond mines throughout the world are hosted in kimberlites. This study investigates the impact of crustal xenolith assimilation on host rock classification using samples from the Late Devonian Pionerskaya pipe in the mined Lomonosov diamond deposit, part of the Zolotitsa cluster in NW Russia’s Arkhangelsk diamond province. The pipes in the Zolotitsa cluster have historically been classified as CROLs due to the presence of groundmass clinopyroxene as well as high modes of phlogopite with compositions that are atypical of kimberlitic phlogopite. The kimberlite and crustal xenoliths were studied using optical and electron microscopy, X-Ray diffraction, X-Ray fluorescence bulk rock compositional analysis, and electron microprobe analysis. The mineralogy of the crustal xenoliths was modelled with thermodynamic software, Perple_X. The pipe fill at Pionerskaya consists of two units of pyroclastic kimberlite (PK), hypabyssal kimberlite (HK), and transitional hypabyssal kimberlite (HKt). Pionerskaya contains granite, diorite, and metamorphic xenoliths (collectively called basement xenoliths) that were altered through reactions with the host kimberlite. Multiple lines of evidence support the kimberlitic affinity of the Pionerskaya pipe: 1) the metasomatic origin of groundmass clinopyroxene and phlogopite, both modelled as subsolidus, low-temperature phases (<500°C); 2) changes in phlogopite size, morphology, inclusion content, and compositional zoning trends near basement xenoliths; and 3) the presence of monticellite. The previous erroneous classification of Pionerskaya as a CROL relates to the presence of crustal xenoliths and their metasomatic reactions with the kimberlite. The results of this study indicate that Pionerskaya is a crustally contaminated Group I Kimberley-type pyroclastic kimberlite (KPK) with significant diamond potential.

Text

2024-11-15

Attribution 4.0 International

http://hdl.handle.net/2429/89671

Geological Sciences

University of British Columbia

UBCV

http://creativecommons.org/licenses/by/4.0/