Which of the following have mining companies been using to discover gold deposits?

In recent years Australian researchers have found that microscopic gold particles from underground ore deposits are present in leaves of eucalyptus trees. Eucalyptus tree roots can delve more than 40 metres (130 feet) deep underground in a search for water. Prior to this research, experts had disagreed over whether gold particles seen in eucalyptus leaves were merely wind-blown or truly represented ore traces transported by roots.

The average concentration of gold in the leaves is only about 46 parts per billion, less than 0.000005 percent of each leaf by weight. Although the gold content of the leaves is worthless in itself, for mining companies eucalyptus trees offer cheap and relatively reliable clues to the presence of gold deposits in a specific location, in comparison with wide-area drilling tests.