Heavy Metal Enrichments in the Kimberley Bedrocks: Evidence of AN Ore Deposit at the Source?

Three years ago, the Curiosity rover reached a sedimentary formation called Kimberley that is compositionally very different from previous and subsequent analyses. These sandstones contain elevated amounts of K2O with an average of 2.1 wt. % according to ChemCam instrument [1], explained by the occurrence of potassic minerals: a sandstone named Windjana has been analyzed by CheMin instrument, showing large amounts of sanidine [2]. Mafic minerals (augite, pigeonite, magnetite) and minor phases including F-apatites and sulfides have also been identified [2]. ChemCam analyzed several points where micas may have been sampled [1,3-4]. All these minerals are thought to be detrital having originated from igneous sources like potassic and mafic rocks in the northern rim [1-4]. High Zn and Cu contents (up to 2000ppm and 1010ppm, respectively) have been measured in these K-bedrocks [5-7]. The ChemCam instrument allows the analysis of materials with depth: a LIBS point is ablated by 30-150 laser-shots, measuring the composition of the first micrometers [8]. These depth profiles show evidences of the occurrence of a Cu-phase within K-spars. In a potassic sandstone containing up to 1010 ppm of Cu and 250 ppm of Ge according to ChemCam and APXS analysis respectively, a Cu-phase is potentially hosted in clays or micas. These high values would be related to local hydrothermalism at the igneous source region of the Kimberley detrital minerals [5,9]. These observations and the occurrence of 800 ppm of Cu in a porphyric alkali feldspar within a trachyandesite [10], suggest that these Cu enrichments may be due to a porphyry copper deposit at the source region of the potassic minerals. Another hypothesis is the presence of an ore deposit related to an impact-induced hydrothermalism. Hence, circulation of high temperature fluids would have happened at the magmatic source region of the Kimberley minerals, favoring the formation of a metallic deposit. [1] Le Deit et al, JGR 121, 784-804 (2016); [2] Treiman et al, JGR 121, 75 (2016); [3] Forni et al, JGR 42, 1020-1028 (2014); [4] Payré et al, JGR 122, 650-679 (2017); [5] Berger et al, JGR (2017); [6] Goetz et al, LPSC #2942 (2016); [7] Payré et al, LPSC #2097 (2017); [8] Wiens et al, Space Sci. Rev. 170, 167-227 (2012); [9] Thompson et al, JGR 121, 1981-2003 (2016); [10] Cousin et al, Icarus 288, 265-283 (2017)