- Email: [email protected]
The formation of ferruginous pisoliths and the mobility of gold and pathfinder elements in the Yilgarn Craton, Western Australia
Goldschmidt Conference Abstracts 2006
Pedogenic evidence for climate change and aridification on Mars R. AMUNDSON1, S. EWING1, J. OWEN1, W. DIETRICH1, K. NISHIIZUMI1, O. CHADWICK2, B. SUTTER3, C. MCKAY3
The formation of ferruginous pisoliths and the mobility of gold and pathfinder elements in the Yilgarn Craton, Western Australia R.R. ANAND1, R.M. HOUGH1, C. PHANG1, M.D. NORMAN2
1
University of California, Berkeley, USA (earthy@nature. berkeley.edu; [email protected]; jowen@nature. berkeley.edu; [email protected]; [email protected]) 2 University of California, Santa Barbara, USA (oac@geog. ucsb.edu) 3 NASA Ames Research Center, USA ([email protected]; [email protected]) Data obtained by MERs reveal significant accumulations of sulfate in Mars soils. Here, we re-analyze the Mars soil data to show they have also lost significant quantities of most major rock-forming elements. We show that Earth soils, spanning its rainfall spectrum, provide evidence that Mars soil hydrological conditions have changed from those causing net elemental losses via leaching to conditions facilitating elemental gain. This is indicative of a profound aridification of the planet. Additionally, we show that shallow soil profile excavations by MERs suggest a late stage downward migration of salts, and small amounts of downward moving liquid water, even in more recent Martian time (Fig. 1).
Fig. 1. Fractional elemental gains (positive tau) and losses (negative tau) for average Spirit and Pathfinder soils, and Earth soils spanning semi-arid (310 mm rain) to hyperarid (1 mm) climates. All tau values are soil to parent material elemental comparisons, normalized to TiO2.
1
CSIRO Exploration and Mining, ARRC, Kensington, WA, Australia ([email protected]) 2 RSES, Australian National University, Canberra, ACT, Australia Ferruginous pisoliths are spherical bodies in regolith. They are abundant and widespread in the deeply weathered landscape of the Yilgarn Craton of Western Australia and they have been used successfully as a geochemical sampling medium for Au exploration. In places, ore-grade pisolith deposits have been mined. Pisoliths are developed in a variety of residual and transported materials including Tertiary sediments. There are a number of studies on the geochemical dispersion in pisoliths from the underlying primary mineralisation but very few on the origin and location of their contained metals. Without knowledge of the origin and mineral hosts of ore-derived trace metals in the regolith, understanding of anomalous concentrations, and selection of the best sample media is very empirical. This paper presents new information on (a) the nature and origin of pisoliths and (b) on the occurrence and concentration of specific trace elements and gold in a variety of minerals and mineralogical assemblages from the Moolart Well, Rose Dam and Mt Gibson gold deposits. Pisoliths are investigated by a variety of techniques, including bulk chemical composition, XRD, SEM, electron microprobe and laser ablation ICP-MS. There are four main types of pisoliths: (i) angular to subangular yellowish brown, (ii) massive black, (iii) red earthy and (iv) yellow concentrically structured. Angular nodules are unconformably overlain by black to dark reddish brown and red, earthy pisoliths. Concentric pisoliths commonly occur in the lower clayey part of Tertiary sediments. The most diagnostic morphology of residual, goethite–hematite-rich angular nodules is the preservation of monomictic distinctive bedrock fabrics. Black detrital pisoliths are massive and hematite–maghemite-rich; authigenic red pisoliths are hematite–goethite–kaolinite-rich; authigenic concentrically structured pisoliths have multiple cutans and are goethite-rich. Red, earthy and black grains both have thin (<1 mm) red or yellow cutans. Hematite has commonly altered to goethite by rehydration, resulting in goethite pisoliths. Microanalysis of pisoliths of different origins and mineralogies suggest multi-stage hydromorphic and biogenic dispersion of Au, Cu, As and Pb in goethite and hematite. The dispersion processes are linked with the relative ages of materials and groundwater regimes. doi:10.1016/j.gca.2006.06.044
doi:10.1016/j.gca.2006.06.043
A15
Pedogenic evidence for climate change and aridification on Mars R. AMUNDSON1, S. EWING1, J. OWEN1, W. DIETRICH1, K. NISHIIZUMI1, O. CHADWICK2, B. SUTTER3, C. MCKAY3
The formation of ferruginous pisoliths and the mobility of gold and pathfinder elements in the Yilgarn Craton, Western Australia R.R. ANAND1, R.M. HOUGH1, C. PHANG1, M.D. NORMAN2
1
University of California, Berkeley, USA (earthy@nature. berkeley.edu; [email protected]; jowen@nature. berkeley.edu; [email protected]; [email protected]) 2 University of California, Santa Barbara, USA (oac@geog. ucsb.edu) 3 NASA Ames Research Center, USA ([email protected]; [email protected]) Data obtained by MERs reveal significant accumulations of sulfate in Mars soils. Here, we re-analyze the Mars soil data to show they have also lost significant quantities of most major rock-forming elements. We show that Earth soils, spanning its rainfall spectrum, provide evidence that Mars soil hydrological conditions have changed from those causing net elemental losses via leaching to conditions facilitating elemental gain. This is indicative of a profound aridification of the planet. Additionally, we show that shallow soil profile excavations by MERs suggest a late stage downward migration of salts, and small amounts of downward moving liquid water, even in more recent Martian time (Fig. 1).
Fig. 1. Fractional elemental gains (positive tau) and losses (negative tau) for average Spirit and Pathfinder soils, and Earth soils spanning semi-arid (310 mm rain) to hyperarid (1 mm) climates. All tau values are soil to parent material elemental comparisons, normalized to TiO2.
1
CSIRO Exploration and Mining, ARRC, Kensington, WA, Australia ([email protected]) 2 RSES, Australian National University, Canberra, ACT, Australia Ferruginous pisoliths are spherical bodies in regolith. They are abundant and widespread in the deeply weathered landscape of the Yilgarn Craton of Western Australia and they have been used successfully as a geochemical sampling medium for Au exploration. In places, ore-grade pisolith deposits have been mined. Pisoliths are developed in a variety of residual and transported materials including Tertiary sediments. There are a number of studies on the geochemical dispersion in pisoliths from the underlying primary mineralisation but very few on the origin and location of their contained metals. Without knowledge of the origin and mineral hosts of ore-derived trace metals in the regolith, understanding of anomalous concentrations, and selection of the best sample media is very empirical. This paper presents new information on (a) the nature and origin of pisoliths and (b) on the occurrence and concentration of specific trace elements and gold in a variety of minerals and mineralogical assemblages from the Moolart Well, Rose Dam and Mt Gibson gold deposits. Pisoliths are investigated by a variety of techniques, including bulk chemical composition, XRD, SEM, electron microprobe and laser ablation ICP-MS. There are four main types of pisoliths: (i) angular to subangular yellowish brown, (ii) massive black, (iii) red earthy and (iv) yellow concentrically structured. Angular nodules are unconformably overlain by black to dark reddish brown and red, earthy pisoliths. Concentric pisoliths commonly occur in the lower clayey part of Tertiary sediments. The most diagnostic morphology of residual, goethite–hematite-rich angular nodules is the preservation of monomictic distinctive bedrock fabrics. Black detrital pisoliths are massive and hematite–maghemite-rich; authigenic red pisoliths are hematite–goethite–kaolinite-rich; authigenic concentrically structured pisoliths have multiple cutans and are goethite-rich. Red, earthy and black grains both have thin (<1 mm) red or yellow cutans. Hematite has commonly altered to goethite by rehydration, resulting in goethite pisoliths. Microanalysis of pisoliths of different origins and mineralogies suggest multi-stage hydromorphic and biogenic dispersion of Au, Cu, As and Pb in goethite and hematite. The dispersion processes are linked with the relative ages of materials and groundwater regimes. doi:10.1016/j.gca.2006.06.044
doi:10.1016/j.gca.2006.06.043
A15