A Study of the Formation Mechanism of the Vientiane Potash Deposit Based on Phase Chemistry
基于相化学研究老挝万象钾镁盐矿床形成的机制

The Vientiane potash deposit in Laos is a typical marine elastic-type MgSO4-absent potash deposit.This potassium-magnesium salt deposit belongs to chemical sedimentary evaporation deposit,whereas the ancient sea water evaporation and salt-bearing strata belong to Palaeogene Tagong Formation.The study of the formation mechanism of this ore deposit is quite important because of its absence of sulfate and carbonate.In this paper,the characteristics of ancient sea water during the formation Of the potash deposit was studied,the physical-chemical peculiarities of potassium-forming brines were analyzed by using the phase diagram,the formation mechanism of this mineral deposit was investigated based on the chemical foundation of its formation conditions.The composition of the seawater has changed rapidly since Phanerozoic,and the prolonged mineralogical changes of marine non-skeletal limestone and potash evaOorates occurred in a phase on a 100-200 Ma.time scale;therefore the period of "aragonite seas"was synchronous with MgSO4 evaporates,and the period of "calcite seas" was synchronous with KCl evaporates.Based on the increasing Br content of basal halite since the Early Cretaceous,the authors deduced an extensive change in the composition of seawater: the period of "calcite seas" existed during the Late Cretaceous and the Early Tertiary. The above change in composition constituted the physical-chemical condition for the absence of sulfate and carbonate in the Vientiane potassium-magnesium salt deposit. The mother liquid of potassium-forming brines seemed to be high-magnesium and low-potassium brines, as shown by the NaCl-KCl-MgCl2-H2O and NaCl-KCl-MgCl2-CaCl2-H2O two system phase diagram. In the evaporation process of the mother liquor, the route of the crystallization directly moved from the sodium chloride phase zone to the E point or the saturated line of sodium chloride and carnallite, but did not pass the saturated line of sodium chloride and potassium chloride, because of the mixing of the original transgression mother liquor with the residual high-magnesium mother liquor. As a result, the potassium chloride is rarely existent or virtually nonexistent in the potash deposit. The mother liquid of potassium-forming brines entered the carnallite phase zone due to the participation of the external CaCl2-type liquid, and the tachydrite was formed during the evaporation process.