The provenance of Borneo's enigmatic alluvial diamonds: A case study from Cempaka, SE Kalimantan

Gem-quality diamonds have been found in several alluvial deposits across central and southern Borneo. Borneo has been a known source of diamonds for centuries, but the location of their primary igneous source remains enigmatic. Many geological models have been proposed to explain their distribution, including: the diamonds were derived froma local diatreme; theywere brought to the surface through ophiolite obduction or exhumation of UHP metamorphic rocks; they were transported long distances southward via major Asian river systems; or, they were transported from the Australian continent before Borneo was rifted from its northwestern margin in the Late Jurassic. To assess these models, we conducted a study of the provenance of heavy minerals from Kalimantan's Cempaka alluvial diamond deposit. This involved collecting U-Pb isotopic data, fission track and trace element geochemistry of zircon as well as major element geochemical data of spinels and morphological descriptions of zircon and diamond. The results indicate that the Cempaka diamonds were likely derived from at least two sources, one which was relatively local and/or involved little reworking, and the other more distal which records several periods of reworking. The distal diamond source is interpreted to be diamond-bearing pipes that intruded the basement of a block that: (1) rifted from northwest Australia (East Java or SWBorneo) and the diamonds were recycled into its sedimentary cover, or: (2) were emplaced elsewhere (e. g. NW Australia) and transported to a block (e. g. East Java or SWBorneo). Both of these scenarios require the diamonds to be transported with the block when it rifted from NWAustralia in the Late Jurassic. The local source could be diamondiferous diatremes associated with eroded Miocene high-K alkaline intrusions north of the Barito Basin, whichwould indicate that the lithosphere beneath SWBorneo is thick (similar to 150 kmor greater). The ` local' diamonds could also be associated with ophiolitic rocks that are exposed in the nearby Meratus Mountains. (C) 2016 International Association for Gondwana Research. Published by Elsevier B. V. All rights reserved.