The sedimentation mechanism and preservation condition of the potassium evaporate series in the Majiagou Formation, east of Ordos Basin
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摘要:
鄂尔多斯盆地中央古隆起东侧奥陶系是海相固体钾盐矿的重要靶区。本文在岩心、测井和地震等资料的基础上，开展了该地区马家沟组中组合（马五5~马五10亚段）的沉积微相、蒸发岩分布、蒸发岩形成机制和保存条件研究。结果表明：马家沟组中组合总体为局限台地环境，可进一步划分出潮坪、潮缘滩、外潟湖、内潟湖等沉积微相，各微相沿古隆起呈环带状展布，内潟湖是含钾蒸发岩的主要形成环境；内潟湖沉积具有明显的旋回性，马五6、马五8、马五10时期为海退蒸发环境，以蒸发岩和白云岩为主，马五5、马五7、马五9时期为海侵环境，以灰岩为主；海退期，向心浓缩作用形成了大型米脂蒸发盐盆，进一步在“深水成盐”机制下沉积了大量厚层含钾蒸发岩系；岩溶地貌与含钾蒸发岩系分布的关系表明含钾蒸发岩系主体分布于岩溶盆地内幕。印支期岩溶作用的强度和深度分析进一步显示古岩溶作用在岩溶盆地范围内基本未破坏到马家沟组中组合的含钾蒸发岩系，具备了钾盐矿的沉积和保存条件。
Abstract:The Ordovician strata constitute an important target of marine potassium ore in the east of Central paleo-uplift, Ordos Basin. Based on core, logging and seismic data, the authors conducted researches on the middle combination of Majiagou Formation (from 5th to 10th intervals of the fifth member of Majiagou Formation), in the aspects of sedimentary microfacies, evaporate distribution, formation mechanism and preservation condition of evaporate. The results indicate that the middle combination of Majiagou Formation was deposited in a restricted platform environment which can be further divided into four sedimentary microfacies, i.e., tidal flat, tide marginal beach, outer lagoon and inner lagoon. These microfacies are distributed around the central paleo-uplift orderly. The inner lagoon is the main depositional environment of the potassium evaporate. The sediments of the inner lagoon present obvious cyclicity. The sedimentary environment of the sub-member 6, 8, 10 were under the regressive evaporation condition and produced evaporite and dolomite, while the sub-member 5, 7, 9 were of transgressive setting and formed limestone. In the regressive period, the centripetal enrichment effect formed a large salt sag in Mizhi area, Shaanxi Province. Furthermore, under the mechanism of"salt deposited in deep water", a lot of thick potassium evaporate series were deposited. These potassium evaporate series were mainly distributed under the post-karst basin coincidentally. The analysis of the intensity and depth of the Caledonian karst indicates that karstification did little damage to these potassium evaporate series.
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Keywords:
- potash mine /
- sedimentary environment /
- preservation condition /
- Majiagou Formation /
- Ordos Basin
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1. 研究目的(Objective)
通过实施2020年四川省自然资源厅省政府性投资地质勘查项目，对沐川地区上二叠统宣威组底部泥岩中赋存的稀有、稀土、稀散元素进行调查评价，了解“三稀”元素含量与找矿前景，希望在“三稀”找矿方面取得突破性进展，为乌蒙山贫困地区脱贫提供技术服务和支撑产业规划、扶贫开发提供资源保障。
2. 研究方法(Method)
主要采用了1∶10000地质填图、1∶500矿化带剖面测量、探槽工程、钻探工程、采样与测试分析等方法，对上二叠统宣威组底部的富镓泥岩进行了初步研究。
3. 研究结果(Results)
沐川地区位于扬子陆块西缘(图 1a)，峨眉山大火成岩省中带(图 1b)。研究区位于五指山背斜核部，其核部地层为上二叠统峨眉山玄武岩(P3e)，两翼向两侧依次出露上二叠统宣威组(P3x)、三叠系(T)、侏罗系(J)等(图 1c)。镓矿层产出于峨眉山玄武岩组(P3e)顶部、宣威组(P3x)底部的紫红色铁质泥岩、灰白色铝质泥岩、浅灰绿色泥岩、灰色泥岩、浅灰绿色泥岩及深灰色炭质泥岩中，呈层状分布，层位稳定(图 1d)；找矿标志：峨眉山玄武岩组与宣威组(P3x)的平行不整合界线之上，具“成矿界面”特征，颜色上有明显的紫红色、灰白色、灰色及深灰色等，特别是具有特征的紫红色，宏观上易识别；根据地球化学图解判别显示，其成矿物质来源有可能来源于峨眉山玄武岩及峨眉山地幔柱演化末期喷发的火山灰。
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图 1 沐川地区构造位置图(a)，峨眉山玄武岩分布简图(b)，区域地质简图(c)及镓矿层产出层位示意图(d)Figure 1. Structural location map of Muchuan area(a), Simplified geological map showing distribution of the Emeishan basalts(b), Regional geological map of Muchuan area(c) and Schematic diagram of gallium ore occurrence horizon(d)采集钻孔岩心1/2切分样品，送样至自然资源部成都矿产资源监督检测中心采用电感耦合等离子体质谱仪(ICP-MS)进行测试分析，结果显示ZK01钻孔镓(Ga)平均品位104 μg/g，矿层厚度6.27 m；ZK02钻孔镓(Ga)平均品位68.1 μg/g，矿层厚度8.67 m；ZK03钻孔镓(Ga)平均品位55.3 μg/g，矿层厚度13.87 m；三个工程的镓(Ga)平均品位为75.8 μg/g，平均厚度9.60 m。可以看出，沐川地区宣威组底部的泥岩中镓(Ga)品位较高，厚度较大，达到了中国现行的Ga矿资源工业指标要求(30 μg/g)。
4. 结论(Conclusion)
经初步估算沐川地区镓(Ga)资源量可达数万吨到数十万吨，有望找到超大型镓矿床(＞2000 t) 的潜力。沐川地区峨眉山玄武岩分布面积较广，为镓的富集成矿提供了丰富的物源。经地质填图及工程取样显示，宣威组底部富镓泥岩层厚度大，分布面积广泛，镓元素含量较高，具有巨大的找矿潜力。研究宣威组底部“三稀”元素成矿特征有助于中国在战略性关键矿产找矿方面取得重大突破，对地方国民经济发展具有重要意义。
5. 基金项目(Fund support)
本文为四川省自然资源厅2020年省政府性投资地质勘查项目(DZ20 2002)和四川省自然资源厅科技项目(kj-2022-6)资助成果。
致谢: 非常感谢审稿专家对本文所提出的宝贵建议；感谢本文责任编辑对文章所做出的细致修改！ -
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图 1 鄂尔多斯盆地东部马家沟组典型沉积相标志
Figure 1. The typical facies signs of Majiagou Formation in eastern Ordos Basin
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图 3 鄂尔多斯盆地古隆起东侧马家沟组中组合各亚段沉积微相分布图
Figure 3. The sedimentary microfacies map of sub-members of middle combination of Majiagou Formation in eastern Ordos Basin
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图 2 桃17井沉积微相与测井响应特征
Figure 2. Sedimentary microfacies and its log curve response characteristics of well Tao 17
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图 5 不同微环境下的蒸发盐赋存特征及沉积模式
Figure 5. Occurrence characteristics of the evaporate in different microenvironments and its sedimentary model
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