Gregory, Daniel1, Lui, Timothy1, Wu, Selina1, Large, Ross2
1University of Toronto, Toronto, Canada, 2ARC Centre of Excellence in Ore Deposits (CODES), School of Physical Sciences, University of Tasmania, Hobart, Australia.
Over the past 10 years it has become increasingly common for people to view host rocks, often sedimentary rocks, to be a source of metals for orogenic gold systems. For sediment hosted deposits it has been proposed that syngenetic and/or diagenetic pyrite may be an important source of the gold. This model states that the gold is released during metamorphism, after which it migrates with metamorphic fluids and is deposited in trap sites. During the same time period it was shown that pyrite trace element content, including gold, varies significantly through geologic time. Thus, it stands to reason that an initial step to finding new gold districts may be to identify basins / periods of time when gold is elevated in sedimentary pyrite. Databases of trace element content of sedimentary pyrite show that gold was elevated at approximately: 3 Ga, 2.5-2.7 Ga, 1.9 Ga, 0.9 Ga, 550 Ma, 450 Ma, and 300 Ma (Precambrian ages +/- 100 Ma; Phanerozoic +/-50 Ma to encompass the length of time that sedimentary pyrite is generally elevated in gold). These are also the ages of the host rocks for many important gold districts (for example, the Superior and Yilgarn craton orogenic gold deposits, the Witwatersrand deposits, the Sukoi Log deposit, the Bendigo district, and the Carlin district). In this study we went to an area where gold deposits occur but is not one of these more famous districts where gold is well established to be hosted by sediments the same age as those with elevated gold in pyrite: Queensland Australia. Approximately 8658 known gold deposits or occurrences are present in Queensland. Of these 3023 (35%) are host by the stratigraphy of the ages given above, a further 898 (10%) are within 1 km and 1579 (18%) are within 5 km. Furthermore, the geologic units of prospective age encompass less than 5% of the land area of Queensland. This suggests that indeed gold deposits are more likely to form in areas that are likely to have elevated gold in sedimentary pyrite and these stratigraphic packages should be prioritized when searching for new gold districts.
Daniel Gregory is an Assistant Professor at the University of Toronto. He completed his PhD focussing on pyrite chemistry at CODES, University of Tasmania in 2014 before spending 3 years as a post doc at the NASA Astrobiological Institute at the University of California Riverside.