Detrital microdiamonds & heavy mineral study of the 2.8 Ga Slave basement cover sequence

Proponents: Aleksandar Miskovic (NTGO), , Luke Ootes (NTGO), Larry Heaman (U. of Alberta)

Introduction
The oldest diamond in the terrestrial geologic record is reported to occur as microdiamond in detrital zircons from the Archean Jack Hills conglomerate in Western Australia. Zircons with diamond and graphite inclusions have an age range of 4252 to 3058 Ma (Menneken et al., 2007; Fig. 1), and show a unique, as of yet unexplained, range in δ13C, with a δ13C as low as –58 ‰ being some of the lightest C ever measured (Nemchin et al., 2008). On the other hand, the oldest known host rocks of macrodiamonds on the Earth's surface are paleoplacers of the 2.89 to 2.82 Ga Central Rand sequence of the Witwatersrand Basin in South Africa (Williams, 1932; Raal, 1969; Fig. 1).

Figure 1. Age ranges of diamond deposits, diamond-bearing rocks, and diamond inclusion minerals. Sedimentary host rocks: JHC = Jack Hills conglomerate, with age range of zircons containing microdiamond inclusions; WR = Central Rand sequence of Witwatersrand Supergroup containing oldest macrodiamonds, Wa = Wawa, Bi = Birimian containing oldest paleoplacer deposits (open diamonds); increasing thickness of solid line represents increasing frequency of paleoplacers (Gurney et al., 2010). Yellow star = the Slave basement cover sequence.

Objectives

• An approximately coeval sedimentary unit of the Slave Craton has never been systematically examined for the microdiamond content.
• The 2.8 Ga Late Mesoarchean Slave cover sequence consists of BIFs, basal conglomerates and massive to well-bedded fuschitic quartzites that unconformably cover the Paleoarchean Slave basement (Fig. 2).

Figure 2. The Slave basement cover sequence within the Archean stratigraphic column (Bleeker et al., 2000).

• This research effort is aimed at a comprehensive geochemical, geochronological and isotopic survey of detrital heavy minerals such as zircons, micro-diamonds and various KIMs (garnet, Cr-rich clinopyroxene, Fe-Ti and Cr-spinels and titanite).
• If kimberlite indicator minerals are found together with diamonds in the heavy mineral concentrates, they would provide indirect evidence for ancient kimberlitic magmatism and associated mantle processes within the Slave Craton as concluded with Witwatersrand diamonds (Wilson, 1982).
• A corollary of this project is a possibility to isolate the lower Paleoarchean and Hadean mineral detritus from the Slave cratonic basement. The evidence for the existence of an ultra-old, 4.2 Ga mineral detritus was provided in 2006 by Iizuka et al. despite the fact that their sampling conducted in the Acasta River region of the western Slave craton was relatively random and the age came from a float boulder.
• Unlike the previous efforts (Bennett et al., 2008) which were concentrated on the Paleoproterozoic Snare Group in the southern Wopmay Orogen, the geographically dispersed samples of the Paleoarchean cover sequence (Fig. 3), especially the petrologically mature quartzites are better candidates for a repository of a significant Paleoarchean and Hadean-age component.

• Ultra-old grains (> 3.9 Ga; Fig. 4) are undergoing additional ICP-MS (REE, Hf), SIMS (O), and TIMS dating which will enable us to couple very precise geochronometry with the tracer isotope systematics and trace-element thermometers (Ti-in zircon). This will shed light on a range of tectono-magmatic conditions pertinent to diamond genesis (e.g. confirmation of anomalously low heath flow regime in Hadean that might have been conducive for ultrahigh-pressure diamond formation; Menneken et al., 2007).

Figure 3. Locations of the 2.8 Ga Slave basement cover sequence where sampling was conducted (after Bleeker et al., 2000).

Figure 4. Probability density distribution diagram of U-Pb ages from the Slave Craton quartzites.