Lithochemical Surveying

metallometric surveying, the most important geochemical method of prospecting for minerals; it is based on a systematic study of the chemical composition of rocks and their weathering products.

Lithochemical surveying makes it possible to discover unseen indications of minerals by means of highly sensitive (for example, spectral) quick analyses of samples taken on a specific grid. As a result lithochemical anomalies are identified—high (or low) concentrations of valuable elements or the elements accompanying a mineral. Among the anomalies important for geological prospecting are secondary (hypergenic) dispersion trains and halos which form as a result of the migration of ore elements in the weathered layer, and primary, in particular endogenic, halos of the deposits which originate during the process of ore formation. Dispersion halos and trains greatly exceed the dimensions of ore bodies and are more accessible to discovery from the surface than deposits that occur at depth, which, under favorable conditions, makes for high efficiency of lithochemical surveying. If, after a detailed study, the geological evaluation is favorable in the lithochemical anomaly zone, mine shafts are sunk or boreholes drilled to expose the mineral and evaluate it further commercially.

Lithochemical surveying is carried out in all stages and phases of geological exploration, the methodology being determined by the stage.

In the stage of geological surveying, in ore regions with rugged relief, lithochemical surveying samples the alluvial-proluvial deposits of rivers, streams, and dry channels to find mineral deposits by dispersion trains. This kind of lithochemical surveying is usually done on a scale of 1:200,000 with an average sampling density of 2.0 X 0.5 km.

In the stage of medium-scale geological surveying (1:100,-000–1:25,000) and in not very rugged regions, loose eluvial-deluvial formations of ore-bearing rocks are sampled during exploratory lithochemical surveying. The rocks are usually taken from a depth of 15–20 cm on a rectangular grid (for example, 500 X 50 m, scale of 1:50,000). In sections where secondary dispersion halos of the main ore body are found, selective, detailed lithochemical surveying is done on a scale of 1:10,000 (grid of 100 X 20 m) or on a larger scale.

In areas where ore-bearing rocks are covered by a mantle of young deposits carried in from elsewhere and where the formation of “superimposed” dispersion halos is not guaranteed, conventional lithochemical surveying from the surface is not effective. In such cases, in particularly promising ore regions where the mantle is up to 100 m thick, deep lithochemical surveying is carried out by boring special holes (for example, on a grid of 1,000 X 100–200 m) and sampling the upper horizons of the ancient weathering mantle to find deposits by their secondary “buried” dispersion halos.

In the stage of exploring and exploiting commercial deposits and also when evaluating ores that have been discovered, the primary ore-bearing rocks are sampled from the surface, by core samples from exploratory boreholes, and in underground workings in order to find and predict concealed mineralization. This very detailed lithochemical surveying (a testing interval is 2–5–20 m), which is based on discovering the primary halos of ore bodies and analyzing their zonation, is used primarily in prospecting for blind ore bodies on the flanks and deep levels of ore fields of endogenic deposits.

In all cases lithochemical surveying is combined with geological and geophysical surveys on the same or approximately the same scale.

As a result of lithochemical surveying, maps, sections, and diagrams of the concentration of elements that are indicators of minerals are drawn up and lithochemical anomalies are identified, taking into consideration the statistical parameters of the local geochemical background (C_b) and the direction of the ore-controlling structures.

The results of detailed lithochemical surveying are usually represented by concentration isobars. The total number of anomalies discovered is significantly greater than the number of commercial deposits, which makes a correct interpretation of the anomalies very important. On the basis of the interpretation and with due regard for other data, decisions are made on the advisability of further study or exploration by drilling.

Many deposits of nonferrous and rare metals have been discovered in the USSR and abroad by lithochemical surveying. For example, the Muryntau gold ore deposit (Uzbek SSR) was discovered by lithochemical surveying of the secondary dispersion halos of arsenic.