中国三叠纪钾盐沉积——以四川为例

The Triassic potash deposits in China:an example from the Sichuan Basin

关键词:  三叠纪  蒸发岩  盐类沉积  钾盐矿产地质特征  钾卤演变  钾盐类型  找矿模型

Key words:  Triassic  evaporite  salt deposits  geological characteristics of potash deposits  K-bearing brine evolution  potash salt type  prospecting model

摘要:

中国三叠系蒸发岩分布于华南,面积超过100×10⁴km²,发现硫酸钾盐及富钾卤水,钾盐矿物有杂卤石、无水钾镁矾、硫锶钾石及多钙钾石膏等,富钾卤水矿化度255~382g/l,含钾量4~49g/l。蒸发盆地的发展受控于古构造及古地理,经历了开放台地一局限台地一堰塞湖一盐湖的发展阶段。钾盐沉积后,经历多阶段的复杂演变。富钾卤水中的钾主要来自硫酸钾盐的深滤,其次来自蒸发浓缩卤水及火山灰吸附钾的释放。杂卤石与富钾卤水的钾同位素测年值分别为210~150Ma及264~150Ma,说明固液相钾盐同源。从地质背景及成矿环境预测,三叠系的钾盐类型只能是硫酸钾盐,主要目的层为T₁j^5~2及T₂l^4~2,主要成矿带为构造分异及钾异常明显的四川盆地东、西部,富钾卤水的储集取决于蒸发岩地层的含钾性、碳酸盐夹层的裂隙发育程度及构造控矿条件。找矿模型应是油气钾卤兼探。

Abstract:

The Triassic evaporites occupy an area of more than 1 million km² in southern China. They have found to be involved in potash salts and K-rich brines,including polyhalite, langbeinite,kalistrontite and geogeyit.The mineralization degrees of the K-rich brines range between 255 and 382 g/1,and K contents vary from 4 to 49 g/l.The evaporite basins are generally delimited by palaeotectonics and palaeogeography,and show a depositional evolution from open platform,through restricted platform,to barrier lagoon and finally to salt lake.The potash deposits thus have undergone several stages of evolution.The K in the K-rich brines is derived mainly from the leaching of potash sulphate,and subordinately from the concentrated brines through evaporation and volcanic ash.The K isotopic age determinations range between 156 and 210 Ma,and 150 and 264 Ma,respectively,indicat-ing a same source of solid-and liquid-phase potash salts.It is inferred from geological background and mineral-forming environments that the Triassic potash salts only comprise one type,i,e,potash sulphate.The target horizons consist dominantly of T₁j^5-2 and T₂l^4-2, and mineralization zones are confined to the eastern part of the Sichuan Basin.The storage of K-rich brines depends upon the K-bearing potentiality of the evaporitic strata,fissure development in the carbonate rocks and structural controls on the potash deposits.The proposed model for the prospecting and exploration of the potash deposits suggests that further work should be conducted on both the potsh deposits and oil-gas deposits.