基于相化学研究老挝万象钾镁盐矿床形成的机制

A Study of the Formation Mechanism of the Vientiane Potash Deposit Based on Phase Chemistry

老挝万象钾镁盐矿床是一个典型的海相碎屑盐缺硫酸盐型钾盐矿床,该矿床形成于古近纪,是古海水蒸发浓缩沉积形成。老挝万象钾镁盐矿床中缺乏硫酸盐和碳酸盐沉积物,因此深入研究该矿床的形成机制很重要。本文研究探讨了该矿床形成时的古海水特点,根据相化学,分析成钾原始卤水的物理化学特性,从矿体形成的化学基础来研究老挝钾镁盐矿床形成的机制。结果表明:显生宙以来海水组分发生变化,经海相非骨骼灰岩和钾盐蒸发岩矿物学研究,发现这两种沉积岩长期以来连续变化,在"文石海"是MgSO4型蒸发盐,在"方解石海"是KCl型蒸发盐,从白垩纪晚期、第三纪早期的底部石盐溴含量及矿物学特征表明,此时处于"方解石海",古海水组分的特点是造成缺硫酸盐型钾盐矿床形成的物化基础;通过NaCl-KCl-MgCl2-H2O和NaCl-KCl-MgCl2-CaCl2-H2O两个体系相图的分析认为,当时所形成的成钾原始体系母液是高镁、低钾氯化物型的卤水,在母液蒸发过程中,由于原始海侵母液与残余高镁母液的掺杂作用,致使结晶路线直接从氯化钠区到E点母液或光卤石与氯化钠共饱线上,而没有通过氯化钠和氯化钾的共饱线,因而在矿体中氯化钾相很少或几乎不存在,由于外界CaCl2型水体的掺杂,使成钾母液进入光卤石相区,随着蒸发的进行,最终形成溢晶石矿物。

The Vientiane potash deposit in Laos is a typical marine clastic-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 evaporates 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.