Several case studies are presented to demonstrate that significant migration of elements in regolith over mineral deposits can reach the earth surface through thick layers of superimposed regolith. Data obtained from drill-holes in Mo-W and Mo-Ag mineral deposits in Eastern Inner Mongolia, China using portable X-ray fluorescence (pXRF, Niton XL3t 950) and data from other areas reported in the literature have been modeled using power-law decay functions that describe the regolith decay trends with increasing distance from the underlying altered rocks or saprocks. The results demonstrate that the element concentration in the surface media can be very low due to decay and mask effects, even if a thin layer of overburden exists. In order to characterize the decay behaviour of geochemical concentration of an element in a vertical regolith profile caused by complex mechanisms, a new non-linear differential equation was proposed which assumes the decay rate of concentration is negatively proportional to the concentration itself, with a functional coefficient dependent on vertical distance from the underlying surface of mineralised rocks. Applying Taylor series expansion to the coefficient function, the differential equation can be approximated by four simple dynamic systems, each with explicit solutions including Gaussian functions, exponential functions, power-law functions and exponential functions with inverse distance. These functions can be utilized as either separate or combined models to fit observed data by means of simple linear regression or multivariate regression. The combined mode is useful for evaluating the comprehensive effect of element concentration distribution due to various mechanisms.