Biogeochemical exploration for gold in tropical rain forest regions of Papua New Guinea

Biogeochemical methods have been widely used for mineral exploration, particularly in boreal forests and semi-arid regions, but there have been fewer applications in tropical areas. This paper describes a biogeochemical method of exploration for Au in equatorial regions. After investigation of several plant species, Astronidium palauense, a small- to moderate-size tree, was found to have many suitable attributes. (1) It is widely distributed in the southwest Pacific, although its occurrence may be limited at elevations greater than 1000 m. (2) The tree is easy to identify and is sufficiently common (e.g., one tree per 100 m(2) on Simberi and Lihir Islands, Papua New Guinea) for detailed sampling. (3) The outer bark is easy to obtain and the ashed bark reliably indicates Au concentrations in the substrate. (4) The root system reaches at least 4 m depth, allowing greater penetration than surface soil samples, which is important in volcanic terrains where geochemical targets may be buried by post-mineralization volcanic eruption or debris flows. (5) The areal distribution of the root system samples a large volume of soil (ca. 100 m(3)), which reduces the nugget effect for Au. (6) The ease of sampling and low weight of bark reduces the time and cost over soil surveys, for example 6 minutes per site compared with 15 minutes per 1 m deep soil. Bark can be ashed in the field, 200-500 samples in 2 to 3 days, then shipped for multi-element (Au, As + 32 elements) instrumental neutron activation analysis (INAA). Field tests on Simberi and Lihir Islands, PNG, show that biogeochemical surveys have a high level of reliability for identification of prospects.