Two geochemical surveys were undertaken in the Solomon Islands between 1976 and 1983 as part of a combined geological mapping and mineral exploration project.
The survey of Choiseul and the Shortland Islands was carried out between 1976 and 1979 by the Institute of Geological Sciences (now the British Geological Survey) with support from staff of the Geology Division of the Ministry of Land, Energy and Natural Resources, Solomon Islands. The project produced 12 geological maps at 1:50,000 scale as well as a series of unpublished reports which are listed in Section 6 of this note. The geochemical results are summarised in Ridgway (1984).
The survey of the New Georgia Group of islands was undertaken between 1979 and 1983. The project produced 7 geological maps at a scale of 1:100,000 and a regional map of the entire island group at a scale of 1:250,000. The geology of the islands is described in a report by Dunkley (1986), that includes a summary of the results of the geochemical survey. A series of multielement geochemical anomaly maps were produced at a scale of 1:100,000 to accompany each of the published geological maps. Master copies of these are held at the Geological Survey in Honiara. Anomalies were standardised for background lithology by partitioning the data prior to interpretation using the rock codes and informal lithostratigraphic codes included in the dataset and summarised in the tables below. Full descriptions of the methods used are described in the margins of the anomaly maps.
A total of 8848 stream sediment samples were collected from Choiseul and 7441 from the New Georgia Group, resulting in an average sampling density for the two areas of 2.68 samples per km² and 1.47 samples per km² respectively. Sampling in the Shortland Islands was confined to the larger islands, 187 were collected from the Fauro Island group, 148 from Alu and 69 from Mono.
Field data was recorded on field cards, which were described in Ridgway (1980). A sample preparation and analytical laboratory was set up in Honiara. The samples were dry sieved and the fraction passing -80 mesh B.S. (177 microns) was analysed. A hot concentrated nitric acid digestion was used prior to analysis by atomic absorbtion spectrophotometry (AAS) for Co, Cu, Pb, Zn, Ni, Ag, and Mn. Samples from the vicinity of the Siruka Ultramafic Complex were determined for Cr by AAS after digestion by a bisulphate fusion technique. In the New Georgia survey Mo was also determined routinely by the zinc dithiol colorimetric method. Molybdenum analyses were not included in the digital database as almost all were below the detection limit (approximately 2 ppm) for the analytical method, except at the Kele River Prospect of Vangunu.
Samples from the Shortland Islands group were analysed colorimetricly for Mo, As, W and Sn, as well as for Co, Cu, Pb, Zn, Ni, Ag and Mn by AAS, and an interpretation of the results was published in Ridgway (1984). BGS does not hold a copy of this small dataset.
Two standard samples were prepared from locally collected stream sediment, and were used to monitor analytical precision. These samples were subjected to confirmatory analysis by the BGS analytical laboratories in London. The standard samples were analysed routinely in each analytical batch, and time series control charts were plotted to monitor between-batch variation and precision. Analyses of control samples outside the range mean ± 2 standard deviations indicated departure from the control and the batches in question were reanalysed. One sample in twelve was reanalysed, as were all apparently anomalous samples. Analytical precision was estimated from the duplicate analyses using the method of Thompson & Howarth (1978) (J. Geochemical Exploration. 9, 23-30). Precision was arbitrarily set at 20%, which was deemed sufficient for exploration, although for most elements (except Ag) better precision was actually obtained.