Genetic Types of Australian Diamonds and Their Origin Features

Based on a series of English literature about Australian diamonds, the possible origin features of Australian diamonds from different fields are discussed, such as quality, colour, shape, surface characteristics, growth structure and trace elements, inclusions, carbon isotopes of the diamonds.Research indicates that Australian diamonds have three genetic types, namely, lithospheric mantle, super deep mantle and subduction environment related origin.First, Arygle diamonds and Ellendale diamonds from North Australian Craton present similar trace element characteristics of Mn, Ni, Cr loss and rich in Na, K, Ti, Zn, Cu, Ga, Rb, Sr, which indicates a lithosphere mantle origin.The diamonds from the two localities can be distinguished from the following aspects.Firstly, only a small percentage of gem quality diamonds are discovered from the Argyle mine, most of which are brown and less are rare pink.However, Ellendale diamonds (60%-90%) are of gem quality, and high quality yellow diamonds are famous.Secondly, primary inclusions of Argyle diamonds are overwhelmingly of eclogitic paragenesis (75%), and these eclogitic diamonds, unlike the small percentage of Argyle peridotitic diamonds, show marked corrosion and deformation features, which display a certain relationship between inclusions and crystal morphology.However, the diamonds from Ellendale pipes NO.4 and NO.9 contain approximately equal proportions of peridotitic and eclogitic inclusions, and no relation between inclusions and crystal morphology is indicated.Thirdly, by carbon and nitrogen isotope features and nitrogen contents, diamonds from Argyle and Ellendale can be distinguished.On a plot of ¹³C and content of N, the diamonds from Argyle and Ellendale are clustered into two separate groups.Ellendale diamonds show a negative correlation of δ¹³C and Ntot with a large range of N contents, whereas Argyle diamonds show no correlation of δ¹³C-N tot and low N andδ¹³C contents.Furthermore, on a plot ofδ¹⁵N-δ¹³C, the Ellendale NO.9 group is the most diffuse and overlaps the Argyle and Ellendale NO.4group, which are mutually distinct.Second, apart from North Australian Craton, a small percentage of diamondiferous kimberlites are discovered in the Adelaide Fold Belt of Eurelia area, about 20km north of Orroroo.Diamond placer deposits within South Australia are located in the Echunga area and the Springfield Basin.Since only five diamonds from Echunga field are still known to exist, they are not included in this research.Diamonds from Springfield Basin, occur exclusively in the basal conglomerate of Permian.The mineral inclusions ferropericlase-bearing of the diamonds from Springfield Basin are similar to the inclusions of diamonds from kimberlites at Eurelia, which suggests the part of diamonds from the two localities is of unusually deep, mantle origin.It would be hard to make a distinction between diamonds from Springfield Basin and Echunga, since they have similar crystal shapes, surface characteristics, colours and distribution of nitrogen.However, they can be discriminated from diamonds of other fields around the world according to carbon isotope features.δ¹³C value of the former locates at-6‰, whileδ¹³C values from other places in the world range mostly are from -5.0‰ to -4.0‰.Third, diamonds occur in several alluvial diamond deposits such as Wellington, Bingara, Copeton and Airly Mountain in New South Wales, which have two distinct groups, A group and B group, according to different characteristics.Most diamonds from Wellington and Airly Mountain belong to A group, and they are similar to those found in kimberlites and lamproites globally and are thought to have formed in traditional lithospheric mantle.However, 95% diamonds from Bingara and Copeton belong to B group, all of which have undergone strong deformation, being of elongated, flattened or irregular dodecahedra with corrosive and glossy surfaces.Their carbon isotope compositions (δ¹³C) are higher than normal values with a peak between 1.0‰ and 2.0‰.Their mineral inclusion assemblage is also unique, with rich Ca and poor Na garnet and pyroxene, termed as "calc-silicate" paragenesis and regarded as a subgroup of the eclogitic paragenesis.The origin of B group diamonds is still unknown, but it has been suggested that the diamonds formed in Phanerozoic subduction systems since they occured in areas of convergent-margin tectonics, without the involvement of conventional kimberlitic or lamproitic volcanism.Besides, their unusual mineral inclusions and ¹³C-enrichment are also consistent with the subduction origin.In general, certain distinctions exist among diamonds from different fields, although it's not easy to identify Australian diamonds as a whole.Only in combination with diamond genetic types, can we obtain deeper understanding of diamond feature and their significance, which help to better comprehend the particularity of different diamond sources.