Diamonds from the Buffalo Head Hills, Alberta: formation in a nonconventional setting

Abstract Kimberlite pipes K11, K91 and K252 in the Buffalo Head Hills, northern Alberta show an unusually large abundance (20%) of Type II (no detectable nitrogen) diamonds. Type I diamonds range in nitrogen content from 6 ppm to 3300 ppm and in aggregation states from low (IaA) to complete (IaB). The Type IaB diamonds extend to the lowest nitrogen concentrations yet observed at such high aggregation states, implying that mantle residence occurred at temperatures well above normal lithospheric conditions. Syngenetic mineral inclusions indicate lherzolitic, harzburgitic, wehrlitic and eclogitic sources. Pyropic garnet and forsteritic olivine characterize the peridotitic paragenesis from these pipes. One lherzolitic garnet inclusion has a moderately majoritic composition indicating a formation depth of ∼ 400 km. A wehrlitic paragenesis is documented by a Ca-rich, high-chromium garnet and very CaO-rich (0.11-0.14 wt.%) olivine. Omphacitic pyroxene and almandine-rich garnet are characteristic of the eclogitic paragenesis. A bimodal δ 13 C distribution with peaks at −5‰ and −17‰ is observed for diamonds from all three kimberlite pipes. A large proportion (∼ 40%) of isotopically light diamonds (δ 13 C b −10‰) indicates a predominantly eclogitic paragenesis. The Buffalo Head Terrane is of Lower Proterozoic metamorphic age (2.3-2.0 Ga) and hence an unconventional setting for diamond exploration. Buffalo Hills diamonds formed during multiple events in an atypical mantle setting. The presence of majorite and abundance of Type II and Type IaB diamonds suggests formation under sublithospheric conditions, possibly in a subducting slab and resulting megalith. Type IaA to IaAB diamonds indicate formation and storage under lower temperature in normal lithospheric conditions