非洲下刚果盆地钾盐矿床特征、沉积旋回与沉积模式

Mineral characteristics, salt rhythm and depositional model of potash deposit in Lower Congo Basin, Africa

下刚果盆地位于西非大陆边缘,形成于南美大陆与非洲大陆裂开初期,大西洋打开之时,在早白垩世形成了巨型钾盐矿床。文章在盆地内布谷马西矿区钻孔的岩芯编录的基础上,结合岩相学、地球化学和沉积韵律等研究,探究钾盐矿床沉积特征和盆地内蒸发岩沉积旋回,并总结了矿床沉积模式。下刚果盆地总体较平缓,西南低,北东高,自盆地边缘向中部,沉积厚度逐渐增大,钾盐矿床赋存于下白垩统Aptian阶蒸发岩系中。蒸发岩系主要由石盐岩与光卤石岩互层构成,局部夹(含水氯镁石)溢晶石岩、石膏岩和黑色碳质页岩夹层。矿床由多层水平_近水平层状矿体组成,单层矿体厚度几米至几十米,矿层比较稳定,延续范围广,主要矿层——光卤石层累计厚度超过100 m。主要矿石类型为光卤石型,仅局部为钾石盐型。矿石品位高,KCl平均品位为16.49%。成盐盆地在早白垩世只有一个成盐期,含盐系即为Ⅰ级韵律;根据沉积特征,又可划分为8个Ⅱ级盐韵律和若干个Ⅲ级盐韵律。整个成盐期总体沉积环境较稳定,未出现蒸发岩序列中断,但受地面的沉降变化和海水周期性灌入的影响,出现了石盐与光卤石的韵律互层沉积。下刚果盆地内蒸发岩沉积序列与正常的海相蒸发岩不同,底部缺乏碳酸盐和硫酸盐地层,指示其成钾物质来源于经过一定程度蒸发浓缩的古海水。在上述研究的基础上,结合成盐期盆地岩相古地理与构造条件,总结出下刚果盆地裂谷型钾盐矿床沉积模式。

Located on the continental margin of West Africa and formed at the beginning of the opening of the Atlantic Ocean in the Early Cretaceous period, the Lower Congo Basin deposited a huge amount of potash resources. Based on the results of the core catalogs in the Cuckoo marcy district of the basin, the authors tried to make out the sedimentary characteristics of the potash deposit and the sedimentary cycle of the evaporites, and to summa- rize the depositional model of the deposit. The basin is relatively flat; the thickness of evaporites is increasing from the edge to the center of the basin gradually. The deposit is hosted in the Early Cretaceous Aptian strata.The evaporites are mainly halite interbedded with carnallite, locally with banded tachydrite, bischofite and black shale. Orebodies are horizontal or nearly horizontal, and the thickness of a single-layer orebody is a few meters to tens of meters. In total, the thickness of carnallite layers are accumulated over 100 m. The potash deposit is mainly composed of brownish red carnallite, and the main ore type is carnallite type, with partially sylvite type. The ore grade is high, and the average KC1 grade is about 16.49 %. In the Early Cretaceous Aptian stage, there was only one salt-genetic period, and the salt-bearing strata constituted the first-order salt rhythm, which could be subdivided into eight second-order salt rhythms and several third-order salt rhythms. The sedimentary envi- ronment of the whole salt-genetic period was stable, and evaporite sequences had no interruption. Nevertheless, with the continual subsidence of the basin and the injection of seawater, there occurred rhythmic deposition of rock salt and potassium-magnesium salts. The salts are characterized by high percentage of potassium-magnesium and a scarcity of sulfates and carbonates, suggesting that it was already enriched before entering the basin. Based on the above studies, combined with Aptian lithofacies and paleogeographic distribution of the basin as well as the tectonic-paleoelimate condition, the authors summarized and exhibited the schematic diagram showing how the potash deposit was formed.