Stone line profiles: Importance in geochemical exploration

In vast tropical rain forest areas, weathering profiles are commonly characterized by a “stone line” overlain by a brown-yellow loose-clay horizon. Concordant with the topographic surface, such a stone line may be traced continuously over considerable distances. It is typically composed of coarse fragments of lithorelics, debris of laterite as Fe-oxides nodules, corroded quartz, gibbsitic aggregates, …, embedded in a clayey matrix. These materials cover the saprolitic weathering profile which is typically a few tens of metres thick.The origin of stone lines has given rise to much controversy and are still widely misunderstood. A broad range of processes, allochthonist or autochthonist, have been put forward in the literature. The findings in this paper conclude that these weathering profiles result from chemical leaching and differential movement between the matrix and the coarse fragments which accumulate by downward migration. Accumulation takes place at the lower limit of rain water impregnation and forms the stone line, whereas leaching and homogenization of fine material occur throughout the upper water-impregnated horizon. Although the materials of the loose-clay horizon and of the stone are extensively altered, the relics are chemically rather well recognizable.According to the above hypothesis, stone line weathering profiles should thus be mostly residual. The main aspects of geochemical dispersion processes of some stone line profiles in Gabon are presented as examples. These show that:

1. (a) The vertical redistribution of some major elements in the profiles, accumulation (Fe₂O₃, Al₂O₃, SiO₂) or leaching (K₂O, MgO, CaO, SiO₂,..) are different from the bedrock composition;

2. (b) In some situations, it is possible to characterize the bedrock by using groups of trace elements such as V, Ni,.. for basic rocks or Ba, Sr,.. for gneisses for instance; the contrasts obtained can be smoothed in comparison with results from deeper in the profile.

3. (c) The persistence of geochemical anomalies arising from mineralization, throughout the weathering profile, up to the main sampling media, the surface soil. A “mushroom” dispersion pattern can be recognized where the foot of the mushroom corresponds to the element dispersion pattern can be recognized where the foot of the mushroom corresponds to the element dispersion in the saprolite and the bedrock, with the top of the mushroom being partly in the stone line and partly in the loose clay horizon.

Such a dispersion pattern has two consequences on exploration: (1) the spreading out of the surficial signal favoring the identification of anomalies during follow-up on a relatively wide spaced grid; and (2) at the same time, a reduction of the extension of the signals by dilution and leaching according to the weathering process; therefore, relatively low anomaly contents must be taken into account in exploration.Thus, anomalies arising from stone line profiles tend to be well-dispersed, but of weak magnitude, and represent in situ transfer from the parent rock.