Deposit Footprints and Mineral System Science
As mineral exploration moves into regions dominated by transported cover, conventional techniques (e.g., lag gravel) may not be applicable and thus, increasingly, there is a need for new, innovative approaches. To develop these approaches, potential mechanisms that transfer metals from buried mineral deposits through cover to the surface need to be identified. This paper presents an overview of some of the experimental and field trials conducted in Australia as part of an industry-supported three-year CSIRO/AMIRA project. The objective was to define vadose zone processes that might form elemental anomalies at surface over buried deposits in semiarid and arid terrains, and to compare methods that detect these anomalies. Studies were conducted at seven sites representing orogenic Au, volcanogenic massive sulfide (VMS; Cu-Zn-Ag), and magmatic Ni mineralization with transported cover ranging in thickness from 2 to 30 m. Three vertical metal migration mechanisms are important in vadose environments: (1) biological, (2) gaseous, and (3) capillary. An integrated approach, combining different mechanisms with the nature and evolution of transported regolith and climatic settings, was considered to obtain the best prediction of metal transfer. Upward element transfer by vegetation (Acacia aneura and Eucalyptus spp.) occurs in areas of transported cover up to 30 m thick, but not in environments which lack supergene enrichment and have hypersaline acid groundwater. Microbial populations are different in soil over mineral deposits than in those from background sites. Metals, detected by gas collectors, are transferred to surface as gases. Soil pit experiments show that strong geochemical anomalies can form rapidly (over 7 months) through 2 m of transported cover, and assist in understanding the genesis of natural geochemical anomalies. Seasonal variations suggest that migration of elements from source to surface may vary in time and intensity. Anomaly formation in the pit experiments is an episodic process largely driven by capillarity, in which batches of metals in water-soluble form are translocated. Soil-forming processes may form false anomalies and the data need to be interpreted with care.