Green leaf tree, nature

At Barns Gold Prospect (25 km north of Wudinna, South Australia), previous studies have shown that vegetation samples, soil and calcrete have anomalously high Au contents.

Like Freddo, Barns is an undisturbed site well-suited to Au mobilization studies. The Barns Au deposit lies within Archean rocks of the Gawler Craton that have been weathered to 50 m depth. The Barns regolith consists of saprolite overlain by up to 8 m high aeolian sand dunes that have developed over the last 20,000 years. Gold is patchily distributed in the saprolite and there is a sub-economic Au supergene deposit (500 m by 200 m) 35 m below the dune.

Freddo

Samples from the large (>10 m tall) Eucalyptus trees at Freddo were found to be highly anomalous in Au exclusively and directly over the buried deposi The maximum Au concentrations in dried leaves, twigs, bark, litter and soil were 80, 44, 4, 12 and 41 p.p.b., respectively, with background concentrations of 0.1 p.p.b. for plant material, 1 p.p.b. for litter and 6 p.p.b. for soil. Thus some tree parts (leaves and twigs) were richer in Au than others (bark). Tree trunk (heartwood and sapwood) samples contained between 0.1 and 0.7 p.p.b. Au. Owing to its large mass, the tree trunk represents a major storage organ.

Barns exudates

Exudate samples collected from vegetation over the Barns deposit had 20 times more Au (maximum of 0.004 μg) than the background samples 800 This indicates that Eucalyptus trees, in addition to compartmentalizing Au in Ca oxalate crystals (as found for the Freddo samples), expel soluble Au through their leaves. As exudates evaporate from the leaf surface Au salts are precipitated which, under normal circumstances, would be dislodged by wind or dissolved by rain and transferred to soil.

Greenhouse study

In a separate, related laboratory experiment, Eucalyptus and Acacia seedlings were grown experimentally under greenhouse conditions in sand pots and dosed with 1,000 p.p.m. Au. Scanning electron microscopy (SEM) (equipped with an energy-dispersive spectrometer (EDS)) analyses of the leaves showed that they contained Au particles with some partly coating Ca oxalate crystals in a similar manner to our observations in the natural.

Methods

Soil samples (about 1 kg) were collected with a plastic trowel within 1 m of the base of the tree trunks and put into plastic bags. Sampling depth was from 0–10 cm. Bark samples (about 300 g) were collected from base of the tree trunk and placed into calico bags. Litter samples (about 300 g) were collected into calico bags from above where the soil sample was taken. Foliage (collected as branches, 2 kg) was removed from trees using Teflon-coated secateurs and a truck mounted cherry picker (or basket crane) at Freddo or from ground level at Barns. The leaves and twigs were separated from the main branches with Teflon-coated secateurs and placed into calico bags. All samples were collected during one campaign.

Soil profile

A soil profile was excavated using a back hoe excavator. The face of the soil profile was cleaned down with a brush and samples taken using a plastic trowel approximately every 0.1 m.

Leaf exudates

Dissolved leaf exudates were sampled and analysed from the foliage of Eucalyptus trees growing on the dunes using transpiration collectors. Large plastic (transpiration) bags (1.2 m by 0.5 m) were placed around branches containing foliage (leaves and twigs). The opening of each bag was tied around the branches, supporting the foliage, using a plastic cable tie. A corner of the bag was tied to the trunk in such a way as to allow exudates to drain freely from the foliage into the bag. Bags were left tied around branches overnight for approximately 15 h after which time the plastic bag corner was cut and the exudates drained into a screw-capped plastic sample container. These were directly analysed by inductively coupled mass spectrometry (ICP-MS).

Sample preparation

For the foliage, leaves were separated from the branches in the laboratory to make the twig and leaf samples. The leaves, twigs, bark and litter were milled with a cross beater mill to a fine powder. Soil samples were sieved through a 250 μm plastic sieve and the sample milled to a nominal <75 μm particle size in a ring mill.

General sample analyses

About 10 g of organic-rich sample (that is, bark, litter, twigs and leaves) was submitted for analysis. Standards and duplicates analysed at the same time were in good agreement (errors <10%). About 4 g of material was digested with 10 ml of nitric acid overnight and then a further 10 ml of nitric and hydrochloric acids were added and digested for 2 h at 90 °C. The digest was diluted to a set volume and analysed by ICP-MS.

Greenhouse study

Seeds of Eucalyptus salmonophloia and Acacia aneura were grown in 150 × 40 × 40 mm pots containing acid-washed beach sand and pelleted slow-release nutrient. The seedlings underwent four treatments (25 seedlings for each treatment) of Au solution: 0, 10, 100 and 1,000 p.p.b. Au. Gold was added daily (10 ml) in solution as a cyanide complex (KAuCN) dissolved in deionised water using a pipette. The experiment continued for 3 months. At the completion of this treatment one each of the Eucalyptus and Acacia plants of the grown plants from the 1,000 p.p.b. Au treatment had 10 ml of 1,000 p.p.m. Au added to them. Samples of these organic materials were mounted in epoxy resin and polished exposing the plant tissue ready for SEM analyses.

Synchrotron analyses

Leaf discs were punched out from the Freddo leaves using a 6 mm diameter tungsten steel hole punch. Three discs were punched from each of 20 leaves at random to make 60 samples. These were mounted directly on a Perspex holder using Mylar tape that was fixed, (in turn) in front of the beam source using locating magnets. The X-ray fluorescence analyses were performed with a Maia 384 detector array at the Australian Synchrotron facility and X-rays focussed into a beam spot of ~1.5 μm, dwell of ~1–2 ms per pixel, energy of 16.5 KeV using the Kirkpatrick–Baez-mirror-based lens system on the X-ray Fluorescence Microprobe (XFM) beamline Platinum, Mn, Ni and Fe foils were used to calibrate the beam. This system enables rapid analysis (8 h) of a large area (~2 cm2) at about 1–2 μm spatial resolution; this is at least an order of magnitude more efficient compared with conventional X-ray detectors to identify Au in soil samples.

Robert Rudd

Robert Rudd

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