Diamonds from nepheline mugearite? A discussion of ‘Garnet websterites and associated ultramafic inclusions from a nepheline mugearite in the Walcha area, New South Wales, Australia’

Diamonds of multiple origins from New South Wales: further data and discussion

“…The varied, evolved nature of the Brickclay Creek olivine nephelinite and nepheline mugearite intrusions [MgO 4–6 wt%, Ni 131–187 ppm, Cr 178–238 ppm, Zr >400 ppm (Stolz 1984; Sutherland et al . 1985) ()] indicates this melt had fractionated within a small magma chamber in the lithosphere.…”

“…The fractionation from a more primitive nephelinite or basanite probably involved crystallisation of olivine, subcalcic clinopyroxene, and spinel represented as megacrysts (Stolz 1984) (). Other observed megacrysts (kaersutitic amphibole, titaniferous biotite, magnetite, apatite and possibly anorthoclase, zircon) may represent crystallisation from more salic melts (Stolz 1984; Sutherland et al . 1985) ().…”

“…Seventeen diamonds were recovered in the 1970s during alluvial gold mining operations at Brickclay Creek, 15 km west‐southwest of Walcha township (Nundle 1:100 000 Topographic Sheet 9135I: AMG 352600 6563850). The diamonds came from a narrow zone that exposed a steep, northeast‐dipping dyke and associated breccia that extended southwards from a small 36 Ma alkaline intrusion (Sutherland et al . 1985).…”

“…The intrusions at Walcha (Figure 2) range from fractionated nepheline mugearite to predominant olivine nephelinite in mineralogy and chemistry (Stolz 1984; Sutherland et al . 1985) ().…”