RE: black kettleYes gobbly you are quite right about caliche.
'Caliche is a general term for any secondary calcium carbonate (CaCO3) that forms in sediments or in voids and crevices within bedrock just below the surface in semiarid regions, as a result of soil-forming processes (pedogenic caliche) or ground-water evaporation (ground-water caliche); it is material left behind by the evaporation of ground water or soil moisture that is no longer present at that level, although ground water may be present at much lower depths beneath the caliche.'
'In arid and semiarid regions, the CaCO3 comes from capillary rise and evaporation of CaCO3-charged ground water, from calcareous dust blown by wind and then driven into the soil by episodic rainfall, and from infiltration of soils, sediments, and rocks by runoff from areas containing sources of CaCO3 (primarily limestones).'
'Because water must be present in the soil or at the water table to evaporate and leave behind the CaCO3, formation of caliche requires a climate that is semiarid to subhumid'
https://www.tec.army.mil/research/products/desert_guide/terrain/desert/lsmsheet/lscal.htm
The main process by which caliche forms is the same one which I believe also brought the metals to surface. Upward movement of water by capillary action and subsequent evaporation of the water when it gets near to surface.
Owl