Weathering, Landscape Evolution, Supergene Enrichment, and Ore Genesis: Implications to Mineral Exploration Paulo Vasconcelos
1 Lateritic/Oxide Au, Fe, Mn, Al, Ni, Cu, Zn,
2 Lateritic/Oxide Au, Fe, Mn, Al, Ni, Cu, Zn, 2
Processes: weathering and erosion Scale of approach: continental Focus: Australia and Brazil Temporal Scale: Mesozoic to Present
65 Ma Why Brazil and Australia? Present
Weathering is Function of:! Bedrock! Climate - Biological activity! Relief! Time! Carajás, Amazon, Brazil Tom Price Western Australia
Bedrock! Channel! Erosion! Landsliding! And Rockfall! Diffusive! Hillslope! Transport! Bedrock! Weathering! Fluvial Erosion,! Transport, and! Deposition! Tectonic! Uplift! (Tucker and Slingerland, 1993)
Grotzinger et al., 2007!
Significance of Landscape Evolution Models to Supergene Ore Genesis?
Landscape Evolution Cycles: Implications to Mineral Exploration
How do we quantify supergene processes?! Climate Weathering Erosion Tectonics
Weathering geochronology H Shuster et al., GCA (2005) dh/dt=weathering rate Vasconcelos et al., GCA (1994) Vasconcelos (1999a,b) Carmo and Vasconcelos (2006)
Main stages of the 40 Ar/ 39 Ar method The 40 Ar/ 39 Ar dating method provides high analytical precision and accuracy, and high spatial resolution, as well as yield statistically confident data sets for the study of many geological processes.
Weathering geochronology by 40 Ar/ 39 Ar geochronology H Erosion Shuster et al., GCA (2005)
(U-Th/He) Goethite Dating Ken Farley Caltech P. Vasconcelos J. Heim Shuster et al. 2005. Geoch. Cosmoch. Acta Heim et al. 2006. Geology
Resolution: mm-size grains 600 ka to age of the earth Goethite Pisolith
Study Sites in Australia
Dugald River, Mount Isa n samples = 35 n grains = 108 Vasconcelos and Conroy, GCA 2003
Vasconcelos and Conroy, 2003 Dugald River Interpretation Leads to the Same Hypothesis
Cross-section of Yandi Channel Iron Deposit 505m Alluvium 495m Clay 480m 475m Upper CID 10m 35m BarimunyaMember 10 samples of iron locations oresample collected along a vertical traverse Sample locations Lower CID Munjina Member Weeli Wolli Fm. hmt hmt goe goe 1 cm hmt goe wood goe
(U-Th)/He results of goethite (Apparent ages not corrected for helium diffusive loss) 11.9 ± 0.4 Ma 10.0 ± 0.3 Ma Yan-02-08-4 Yan02-03 4.6 ± 0.1 Ma 5.8 ± 0.2 Ma 8.5 ± 0.3 Ma 7.1 ± 0.2 Ma 1 cm Yan02-04-A Analytical error ~ 3% (2σ), which greatly underestimates the error associated with helium diffusive loss
505m 10m 495m 35m (C) Schematic cross-section of Yandi Channel Iron Deposit (CID) Alluvium Clay 480m 475m 470m (D) Stratigraphic Column Alluvial deposits Oakover Fm- calcrete,silcrete Iowa Eastern mbr- clay Barimunya member Lower CID { Munjina mbr- clay, conglomerate Weeli Wolli Fm- BIF, shale, dolomite Present water table Sample locations Barimunya mbr- Upper/Lower CID Upper CID Munjina member Weeli Wolli Fm Marillana Ck. 10km 8.6, 10.3 Ma 10, 11.9 Ma 8.5, 7.2 Ma 4.5-6 Ma ~9-14 Ma (A) Location Map (B) Yandi-Marillana Catchment Mine site 22 o 45 S 119 o 00 E Yandicoogina Ck.
Goethite age versus elevation graphs Yandi Channel Conclusions Ferruginization of channel sediments moves downwards Ferruginization occurs at the ground-water atmosphere interface 500 490 480 y=2.72x R 2 + 454 =0.62 y=1.17x + 463 R 2 =0.93 (D) Partial dissolution and re-precipitation of goethite in upper parts of the channel 470 20% helium loss 0 5 10 15 20 25 30 35 40 Age (Ma) Heim et al., Geology, 2006
Study Sites
Carajas
Carajas 40Ar/39Ar Geochronology Deep (>100m) weathering profiles on plateaus Shallower (<30m) profiles on dissected plains Old to young weathering profiles on plateaus Young weathering profiles on dissected plains
70 Ma dh/dt = 6.89 m.ma -1 300 m 12 Ma
SE Brazil
Carmo and Vasconcelos, ESPL, 2003 Carmo and Vasconcelos, ERPSL, 2005
Carmo & Vasconcelos, in prep.
Weathered granite 0 m 0 y = -110.72 + 8.823x R= 0.9665 42 and 45 m 60 m Mn Oxides Depth (m) -20-40 -60-80 0 2 4 6 8 10 12 Plateau Age (Ma) Carmo and Vasconcelos, EPSL, 2006
Borborema
Borborema Study Maria da Guia Lima, PhD João Marinho Morais Neto, PhD 29 Sites sampled for 40Ar/39 Ar ~ 20 sites sample for goethite (U-Th)/he ~ 70 sites sample for 10Be ~ 15 sites sampled for feldspar 40Ar/39 Ar thermochronology > 50 sites sampled for AFFT and AHe Fonte: SRTM/USGS/EROS DATA CENTER
Weathering Profile Regional Distribution Vasconcelos (1999)
Weathering Geochronology Results, Borborema Province
Weathering Geochronology Results, Borborema Province old and young weathering profiles on Borborema Surface young weathering profiles only at Sertaneja Surface intermediate-age profiles at the Coastal Plateaus 800 Idade X Age x Elevation Elevação (m) 700 600 500 400 300 200 100 Planalto da Borborema Depressão Sertaneja Planicies e Tabuleiros Costeiros 0 0 10 20 30 40 Idade (Ma)
65 Ma Hamersley Australia Present Quadrilatero Ferrifero Brazil
Landscapes Have Ages! Hamersley Quadrilatero Ferrifero
How does the relative tempo of weathering and erosion controls supergene metal accumulation? Take home messages:! The weathering record is episodic;! Episodes at the Early Cenozoic correlate between Brazil and Australia;! Australian and Brazilian record strong weathering in the Miocene;! Weathering geochronology permits dating propagation of weathering! fronts through time;! Calculated weathering front propagation rates between Australia and! Brazil differ by a factor of two;! This difference is potentially related to the difference in climatic! histories between the two continents.
How to Quantify Erosion Rates? Weathering Climate Erosion Tectonics
Take Home Message! Old landscapes (Cretaceous) best but rare! Miocene Landscapes: significant weathering! and supergene enrichment.!
Take Home Message! Understand Miocene Landscapes!