FYI, a passage from...

FYI, a passage from https://www.researchgate.net/publication/305619289_An_Overview_of_Nickel_Mineralisation_in_Africa_with_Emphasis_on_the_Mesoproterozoic_East_African_Nickel_Belt_EANB

In contrast, exploration for high-grade Ni sulphide deposits in the EANB has proven to be rather difficult and expensive. After the initial discovery of massive sulphides at the Kabanga Main intrusion in 1978, the next major discovery was in 1993 at Kabanga North, and Tembo was only found in 2006, after years of near-continuous exploration effort. There are a number of difficulties in exploring for high-grade Ni sulphides, some of which are fairly frequentlyencountered worldwide, whereas others are specific to the EANB. These can be listed as followś The economic massive sulphide mineralisation has a small “footprint” area, due to the upright folding style which has resulted in most bodies being steeply plunging.ó High grade mineralisation does not necessarily have a widespread “halo” of low-grade mineralisation or alteration, as it often occurs in small, discrete chonolith bodies.ó There are a large number of strong magnetic anomalies in the region due to the predominance of ferromagnetic minerals in both ultramafic rocks (magnetite from olivine alteration) and host metasedimentary rocks (monoclinic pyrrhotite laminae).ó The sedimentary pyrrhotite laminae and graphitic horizons can be strongly conductive in some structural circumstances, leading to large numbers of false anomalies in airborne EM surveys.ó The strong tropical weathering has mobilized the elements of interest in the lateritic profile and resulted in strongly leached true gossans and a multitude of false “gossans” derived from ultramafic and Fe-rich metasedimentary rocks.ó Subsequent drainage incision due to recent tectonics has led to strong contrasts of geochemical background levels between different regolith regimes, making the identification of real anomalies more difficult.The optimum direct detection method for massive sulphide mineralisation is based on its very high conductivity. On the regional scale, however, airborne EM surveys, which necessarily operate at medium to high frequencies (>20 Hz), have been ineffective in distinguishing “superconductors” such as massive Ni sulphide bodies, from structurally-controlled barren metasedimentary conductors (Wolfgram and Golden, 2001). The most effective airborne geophysical method for regional-scale exploration has proven to be relatively close-spaced magnetic surveys, combined with high resolution radiometric measurement, which can reliably detect olivine and sulphide-bearing ultramafic rocks at surface and to depths of 500m. The airborne anomalies have to be followed-up on the ground and for this, low-frequency time-domain EM techniques, particularly those methods adapted for measuring response from “superconductors”, such as the UTEM system and the step-response measurement pulse EM systems (Ravenhurst, 2001), are still the best on a prospect-scale.Apart from the initial regional exploration carried out by governmental agencies in the 1970s, geochemical methods have tended to be used in a secondary role to airborne and ground geophysical methods, for verification or ranking of magnetic or EM targets. Geochemical identification of mineralised intrusions has generally relied on standard aqua regia digestions of soil and stream sediment media and multi-element analysis by inductively coupled plasma (ICP)-atomic emission spectrometry, with anomalies being signalled by coincident Ni-Cu-Co highs. Modern partial digest techniques and low-level analysis of the PGE and Au by ICP-mass spectrometry, do show promise in distinguishing higher tenor magmatic sulphide mineralisation from lower tenor sulphide and ultramafic-dominated geochemical signatures but have not been tested in the EANB.In summary, exploration for Ni sulphide deposits in the EANB has been most successful using a combination of airborne magnetic surveys to locate the olivine-rich bodies that invariably accompany sulphides, and ground EM methods supplemented by geochemistry to locate the actual massive sulphides. As always, a thorough knowledge of regional and local geology is needed to interpret the anomalies generated by these methods.