Invisible gold in the high-sulphur Tertiary coals of Northeast India

During an investigation of some high-sulphur coals from
Meghalaya, Northeast India, with respect to their trace
metal content, we recorded the occurrence of gold in
iron sulphides (pyrites/marcasite) associated with such
coals. A maximum of 6 mg/l gold has been recorded in the
pyrite concentrates. Gold is lattice-bound and occurs
in ‘elemental form’, forming solid solutions within iron
sulphides. It is inferred that the iron sulphides are
mostly syngenetic with coal and gold is concentrated
in iron pyrites by biological activity during their precipitation and by inorganic adsorption process during
recrystallization in a reducing environment.