Subduction-Related Diamond Deposits? Constraints, Possibilities, and New Data from Eastern Australia
W.L. Griffin, Suzanne Y. O'Reilly, Rondi M. Davies, 1998. "Subduction-Related Diamond Deposits? Constraints, Possibilities, and New Data from Eastern Australia", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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Most significant alluvial deposits of diamonds can be traced back to their primary sources in kimberlite or lamproite intrusions in cratonic areas. However, some alluvial deposits occur in noncratonic regions, do not carry the usual mantle-derived indicator mineral suite, and cannot be related to known primary sources. The occurrence of such “headless placers”in young orogenic belts suggest that the diamonds are a result of Phanerozoic subduction processes, rather than being derived from old subcontinental lithosphere, and that they might be derived from metamorphic, rather than magmatic, primary sources. Microdiamonds have been reported in crustal rocks, metamorphosed at high pressure during continent-continent collisions, from Kazakhstan, eastern China, and western Norway. However, no macrodiamonds have been found in these deposits and their tectonic setting is distinct from those where significant headless placers occur. Macrodiamonds (altered to graphite) are abundant in pyroxenite layers within peridotite massifs in Spain and Morocco, but these massifs represent high-temperature diapirs, unrelated to subduction processes. Although microdiamonds reported from Tibetan ophiolites probably represent contamination, macrodiamonds have been found in several subduction-related peridotites, thereby indicating that diamonds are produced in at least some subducted plates. The thermal evolution of a subducting slab allows formation of diamonds at relatively shallow depths, but if the diamonds are to survive graphitization, the diamond-bearing rocks must either be obducted to the surface without heating or be sampled by ascending magmas within approximately 30 Ma of the end of subduction.
Several large headless diamond placers occur in Tertiary river deposits in the Paleozoic Tasman orogen of eastern Australia. New data on the diamonds from several fields show that two major populations are present. They consist of both peridotite and eclogite-calc-silicate parageneses and their proportions vary regionally. Surface morphology and its relation to internal structure suggest extensive resorption during transport by magmas. The diamonds may have formed during subduction of an oceanic plate in a variety of metabasaltic rocks (ranging from eclogite to metarodingite) and in the surrounding highly depleted serpentinites. The anomalously heavy carbon isotope signature (δ13C = –2 to 3‰) of one population can be explained by decarbonation-reduction reactions at 300° to 500°C within the diamond stability field. High degrees of nitrogen aggregation in the diamonds may be related to the intense deformation that is visible macroscopically and in cathodoluminescence images. If so, the N aggregation does not rule out a subduction-related origin. Unpublished 39Ar/40Ar dates (approximately 330 Ma) on pyroxene inclusions in the diamonds may suggest that the diamonds were carried to the surface by magmas related to the major Carboniferous subduction episode in the Tasman orogen. Surface abrasion and radiation damage indicate that many of the diamonds have spent considerable time in surface environments since erosion of their primary sources, and this may account for the scarcity of typical indicator minerals.
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The types of mainly metallic mineralization found in metamorphic terranes are reviewed and an attempt is made to define the genetic relations between the mineralization and the metamorphic events.The terms metamorphosed, metamorphic, and metamorphogenic as applied to ores are also considered.The development of thought and the history of investigations on ores in metamorphic terranes aretraced from the early work in the second half of the nineteenth century onward. Early conceptions ofmetamorphism as an ore-forming process (metamorphogenesis) were seemingly not followed up by theiroriginators, contemporaries, or immediate successors and were neglected until comparatively recentyears. The idea of metamorphism as a modifier of preexisting, mainly sulfidic, but also oxidic, mineralizationwon more immediate and general acceptance in the early decades of the present century. InNorth America, emphasis seems to have been mainly on the deformational aspects of the metamorphism,whereas elsewhere, especially in Europe, the textural and mineralogical results of the metamorphic recrystallizationalso received considerable attention and metamorphism as an ore-forming process hadwon a certain degree of acceptance. This difference in emphasis may perhaps be referred to the differentviews held regarding the initial genesis of the ores in the two regions.The late 1940s and the 1950s witnessed a considerable revision of ideas on ore genesis, especially regardingstrata-bound massive sulfide ores. A parallel revival of interest in the role of metamorphism,probably not unrelated to the foregoing, began in the early 1950s, to begin with concerning metamorphosedores. However, new thoughts concerning metamorphogenesis related to granitization or ultrametamorphismas an ore-forming process began to be published.The following decades witnessed an almost explosive increase in the number of publications dealingwith the effects of metamorphism on ore mineralization of practically all types, but with a definite emphasison sulfide ores of the strata-bound type. One of the most significant breakthroughs in this respectconcerned the world-famous Broken Hill deposit, New South Wales, although the metamorphosed natureof ores in the Scandinavian Caledonides, the North American Appalachians, the Lachlan fold beltof eastern Australia, the Sanbagawa terrane of Japan, the Urals, and Proterozoic fold belts in southernAfrica, have all been thoroughly documented.In recent years, however, the interpretation of many massive sulfidic ores in metamorphic terranes asmetamorphosed has been increasingly questioned, and syntectonic, metamorphogenic, origins havebeen advocated. There is obviously a great need to be able to distinguish more