Gold and platinum group elements in drainage between the River Erme and Plymouth Sound, south Devon

Significant amounts of gold have been found in drainage sediment at several localities in the area between the estuary and lower part of the River Erme and Plymouth Sound in south Devon. Gold is particularly abundant at a site about 2 km to the southwest of Holbeton where many of the grains are dendritic or otherwise extremely irregular in shape. The dendritic gold shows internal zonation of Pd abundance very similar to that seen in material within carbonate veins in limestone at Hope's Nose, Torquay. Other grains show incomplete rims rich in Pd (up to 13%) or are close to pure gold in composition. Grains with silver-rich rims are much less common than in drainage and overburden samples from east of the River Erme. Associated with the dendritic gold from southwest of Holbeton are dendritic, elongate knobbly and subrounded grains rich in either Pt or Pd or both, forming around 5% of the total. Though some grains are either pure potarite (PdHg) or Au-bearing potarite without significant internal compositional variation, the majority, including all those containing a significant amount of Pt, show very complex compositional zonation. Six PGE-bearing compositional types have provisionally been recognised. The cores of grains are generally Au-rich either Au with minor contents of other metals or Au with significant Pd and Hg. Rims are generally Pt-rich, often with a significant Au or Cu content which may be recognisable as a distinct thin zone. Other compositions include Pt with a significant Hg content and a combination of roughly equal amounts of Pd, Hg, Au and Pt. Some grains show clear concentric zonation and there may be very major compositional differences between zones only a few microns thick. Cinnabar occurs in small amounts in several drainage samples from the area but is particularly abundant (up to 2000 ppm Hg) in samples derived from an area underlain by Middle Devonian chiefly mafic volcanic rocks in the northeast of the area. The distribution of Mg,Ca,Ti,Mn,Fe,S,Ni,Cu,Zn,As,Rb,Zr,Sn,Sb,Ba, W,Pb and Bi in drainage samples is also discussed. Au concentrations show greatest positive correlation with Mn and Fe but a correlation with Sn is also significant.