上扬子区(四川盆地)寒武系的含盐性与地质背景

SALTNESS AND GEOLOGIC BACKGROUND OF THE CAMBRIAN STRATA IN THE SICHUAN BASIN IN THE UPPER YANGTZE AREA

关键词:  蒸发岩  含盐标志  地质背景  成因解释  寒武系

Key words:  evaporite  salt-bearing indicator  geologic background  genetic interpretation  Cambrian strata

摘要:

据记载,分布于上扬子区寒武系的石膏岩仅有零星资料。如今,大量蒸发岩如溶蚀角砾岩、硬石膏、石盐岩及富钾卤水等已被发现。沉积相经历了从盆地相、台地相到蒸发岩相的演化过程。在中、下寒武统中找到海退沉积序列。古构造-古地理背景是蒸发岩沉积的重要控矿条件之一。周边板块运动及塑性基底的褶皱,导致台缘隆起及台内坳陷的形成。岩相古地理图展示从碳酸盐、硫酸盐到氯化物盐类的"泪滴式"沉积相带。蒸发岩沉积于台缘隆起及礁生长进入堰塞潟湖至盐湖阶段。在稳定坳陷区沉积盆地中蒸发岩系发育;石盐岩较厚,其溴氯比值为0.2-0.4;古气候炎热、干燥;在蒸发岩体周围有沉积(封层)水及溶滤水,其含钾(K⁺)量为0.10-4.76g/L;水文地球化学比值(如钾氯比值、钾盐比值及钾溴比值等)指示含钾异常;盐湖浓缩卤水达到氯化物盐类沉积阶段及相应的构造封闭程度;含盐地质标志广泛分布等等。
鉴于上述,估计该区可能具备钾盐成矿条件。然而目的层埋藏太深,因此笔者建议,应在浅埋部位进行有效的成矿预测,为普查指出远景区。

Abstract:

As a matter of record,little research was made on the Cambrian gypsoliths in the Upper Yangtze area.A wide range of evaporites such as corroded breccia,anhydrite,halite and K-rich brines have now been noticed in this area.Sedimentary facies have experienced the evolutionary stages from the basin facies through platform facies to evaporite facies.The regressive sedimentary sequences have been found in the Middle and Lower Cambrian strata.The palaeotectonic and palaeogeographic settings are believed to be the important ore-controlling conditions.The movement of the peripheral plates and folding of the plastec basement resulted in the formation of the periplat-formal upliftS and intraplatformal depressions.The sedimentary facies and palaeogeographic maps display a pattern of sedimentary facies belts as the "tear-drop pattern" ranging from carbonates through sulfates to chlorides.The evaporites rest on the periplatformal uplifts,and the reef growth has graded into the barrier lagoon to saline lake stages.
The evaporite series are developed in the sedimentary basins in steadily subsiding area,with thicker halite rocks whose Br/Cl ratios range between 0.2 and 0.4.The palaeoclimates are dry and hot.The sedimentary(confined)water and leach water occur around the evaporite rock bodies, with K(K⁺)contents varying between 0.1 and 4.76g/1.The hydrologic-geochemical values(such as K/Cl ratios,sylvite ratios and K/Br ratios)indicate a normal K content.The concentrated brines in the saline lakes reach the depositional stages of the chlorides.There exist the corresponding tec-tonically enclosed conditions.The salt-bearing geological indicators occur on a wide range of scales. It is concluded that there may be the mineralization conditions for potassic salts in the study area. However the target strata are buried too deeply,and therefore the effective metallogenic prognosis should be made in the shallower depths.