干旱区生态环境知识资源中心

Using mass balance techniques we test the prevalent view that laterite genesis is dominated by in situ residual enrichment during chemical weathering of bedrock. Through calculation of net mass fluxes through the laterite soils in Mali, West Africa, we show that residual enrichment by removal of mobile elements with a corresponding increase in bulk porosity and decrease in bulk density, contributes only a very minor fraction of the enrichment of Al, Fe, Si and Au. Instead, we demonstrate that the abundance of these elements is due to the influx and accumulation by selective retention of chemically mature detritus of local and foreign origin clearly evident in micromorphological infilling features. At the same sample depths that accumulation reaches extreme values, we show that volumetric expansion in excess of 200% has occurred locally. We infer that these spatially coincidental zones of mass influx of rock-forming metals Fe, Al, Si, and also Au with dilational hyperstrains result from a mutually reinforcing, mechanical interaction between material influx and the effects of subsurface deformational processes such as shrink-swell cycles and root growth and decay. We propose that with progressive infilling of available connected voids by illuvial microsedimentary deposits of insoluble resistate and neoformed minerals, the capacity for the combined skeleton and plasma to remain isovulumetric is exceeded. We speculate that the resultant space problem is relieved by upwards expansion towards the overlying free surface. Continued translocation and void infilling occur and are limited to the depth where the size of translocational particles is smaller than that of the connected voids. Consequently, progressive hyperstrains accumulate above this critical depth by the long-term influence of a proposed translocational wedge of chemically-resistant minerals against which numerous generations of plant roots have exerted stresses.

Eclectic surficial contaminants involved are continuously derived from above leaving no indication of a relict source region within the present soil profile from which they might have been extracted. Instead, the source region is largely the existing regolith column itself which releases local material and in addition, is supplemented by deposition of colluvial detritus shed nearby by escarpment retreat and from contributions by foreign eolian sources. The regolith undergoes progressive reduction by wind deflation, erosion. local dissolution and reprecipitation. This open-system in situ fractionation process may be characteristic of tectonically-stable continental weathering in general where the relative stability of the landscape contributes to the accession, superficial recycling and retention of resistant minerals by interrelated mechanical, chemical and biological mechanisms. Residual enrichment in lateritic weathering profiles which has an increasingly-eclectic grain character near its top is then best viewed as only the first step of in situ modification of parent material. As such. residual enrichment dominates only the basal saprolitic portions of laterites but higher up is subordinate too the accumulated effects of translocation and accumulation of mineral detritus within voids. Lithostructure is progressively destroyed by bioturbation, mixing and expansion. The migration of surficial detritus into the ocean basins is thereby delayed and widespread cumulative lateritic soils become the long-term repositories of an aged continental residuum.

Gold is one mineral which is highly concentrated in the zone of lateritic weathering above mineralized Precambrian volcano-sedimentary greenstone protoliths and this surficial enrichment is of considerable economic importance. It appears that because of its small particle size and unusually high grain density, gold particles contained in the parent material are effectively enriched mechanically during regolith reduction as a captive constituent in the laterite column and are selectively separated from gangue constituents which are physically or chemically carried away by eolian and fluviatile regolith reduction and in solution in groundwater discharge.