思茅盆地石盐矿物的原位元素含量特征对成矿模式的启示

Characteristics of in situ elemental composition of rock salt from the Simao Basin:New metallogenic insights

思茅盆地江城含盐带勐野井地区在“二层楼”钾盐成矿理论的指导下,逐步在侏罗系钾盐资源调查中取得重要成果,但盆地内其他含盐带研究程度薄弱。本文以整董含盐带的磨黑L2井勐野井组(K_(1)m)盐岩样品为研究对象,通过显微镜观察、扫描电镜能谱分析、石盐矿物元素含量电子探针微区原位测试方法的建立,精细研究了样品岩相学、石盐矿物学及元素含量特征。同时,测试了江城含盐带勐野井地区MK-1井花开左组(J_(2)h)石盐样品。对比研究两个井样品中石盐矿物微量元素K和Br含量及10^(3)Br/Cl值(质量分数比)特征,结合研究区地质演化,取得以下认识。L2井K_(1)m(含)泥砾盐岩中石盐矿物发育两类产状:一类是胶结碎屑颗粒的主体石盐,具塑性流变特征;另一类是析出在碎屑中被盐类或黏土矿物碎屑包裹的石盐,两类包裹体特征指示均属次生成因。电子探针测得L2井K_(1)m的主体石盐、碎屑包裹和勐野井MK-1井J_(2)h含钾盐层石盐的K含量分别为≤0.09%、≤0.18%和≤0.13%,Br含量分别为≤60×10^(-6)、70×10^(-6)~410×10^(-6)和70×10^(-6)~500×10^(-6),10^(3)Br/Cl值分别为≤0.10、0.12~0.71和0.12~0.85,主体石盐显著低于后二者,处于陆源或海陆混合源石盐阶段或重结晶石盐阶段,而后二者数值接近,大多处在海源石盐阶段,小部分处在海源母液结晶钾石盐阶段和光卤石阶段。推断磨黑L2井碎屑包裹的石盐属深部古盐体刺穿贯入的证据,可能的成矿模式为深部中侏罗统海相古盐体受盐底辟作用迁移到浅层下白垩统勐野井组后,部分被盆地内侧向迁移来的中侏罗世残留海水、盆地周缘汇入的陆源水以及深部热液的共同溶蚀淋滤和混染改造形成新的卤水,部分以固体古石盐砾保留下来,在早白垩世晚期新母液卤水蒸发成盐过程中被形成的盐类和陆源碎屑矿物包裹,后期在母液结晶的主体石盐胶结下沉积成岩和成矿。以上认识完善了“二层楼”成矿理论在整董含盐带的勘探实践。

The“two-storey”potash deposit model has led to significant progress in the exploration of Jurassic potassium resources in the Mengyejing area,Jiangcheng salt belt,Simao Basin.However,studies of other salt belts in the basin are lacking.In this paper,salt rock samples from the Lower Cretaceous Mengyejing Formation(K_(1)m)in well L2 in the Mohei area,Zhengdong salt belt were analyzed through microscopic observation,scanning electron microscope and energy dispersive X-ray spectroscopic analysis(SEM-EDS),as well as in situ elemental analysis of rock salt by electron probe microanalysis(EPMA)to study their petrography,mineralogy and elemental content in detail.Meanwhile,the rock salt samples from the Middle Jurassic Huakaizuo Formation(J_(2)h)in well MK-1 in the Mengyejing area,Jiangcheng salt belt were also studied.The K and Br trace elemental contents and 103Br/Cl(mass fraction)ratios in these samples were compared between the two wells in the context of geological evolution of the study area.In the K_(1)m formation in well L2,there are two types of rock salt in the mud-gravel salt rocks.The main type is cement-like detrital halite particles with plastic rheological characteristics,the other is clastic halite precipitate wrapped in detritus of salt or clay minerals.The characteristics of both rock salt types indicate secondary origins.The EPMA results show that the maximum K contents were 0.09%in the main and 0.18%in the inclusion types of rock salt from the K_(1)m formation in well L2 and 0.13%in the potassium-bearing rock salt in the J 2 h formation in well MK-1;while the Br contents were 60×10^(-6) at most,(70410)×10^(-6) and(70500)×10^(-6),respectively,and the 103Br/Cl(mass fraction)ratios were 0.10 at most,0.120.71 and 0.120.85,respectively,in the three types of rock salt.The main type,with significantly lower mineral contents compared to the other two types,corresponds to terrestrial source,sea-continent mixed source or recrystallized halite.The other two types,similar in mineral contents,correspond mostly to marine halite,with minor potassium and carnallite crystallized from seawater.It is inferred that the detritus-wrapped halite is an evidence of salt diapir associated with ancient saline sediment.A metallogenic model is proposed on the basis of the above analyses as follows:After Middle Jurassic deep marine sediment migrated to the K_(1)m formation via salt diapirism,one part—modified via leaching,melting and mixing by a combination of Middle Jurassic residual seawater migrated from the inner basin,continental source water flew from the basin margin and deep hydrothermal fluid—formed new brine,while the other part remained as ancient salt gravel.In the late Early Cretaceous during evaporation of the new brine,ancient salt gravel became enwrapped by newly crystallized salt and terrigenous clastic minerals.In the later period,cement-like halite deposit was formed via brine evaporation.This metallogenic model epitomizes the application of the“two-storey”metallogenic model in the Zhengdong salt belt.