新疆西天山莫托萨拉热水沉积型铁锰矿床矿物学与地球化学特征

莫托萨拉铁锰矿床位于西天山阿吾拉勒成矿带东端,研究程度相对薄弱,在矿床成因方面存在热水沉积、沉积-热液改造、胶体化学沉积等争论。本文详细研究了莫托萨拉最上层锰矿及其围岩的矿物组成、结构构造和地球化学特征,并综合前人资料对整个铁锰矿床的成因做了进一步探讨。本研究首次在矿区发现了热液长石岩,其主要由钠长石、钾长石以及少量重晶石、霓石、锌铁黄长石等矿物组成,类似于"白烟型"热水沉积岩。莫托萨拉最上层锰矿主要由锰橄榄石、褐锰矿、红硅锰矿、磁锰铁矿以及少量重晶石、方铁锰矿等矿物组成,发育有典型的热水内碎屑结构,指示其沉积于海底热液喷流口附近。该层锰矿的Al/(Al+Fe+Mn)值很低(0～0.02)、Si/Al值较高(7.9～10.9)、Fe/Ti值很高(428～1353),通过UCC标准化后发现明显富集Zn、Ba、Pb等元素,而Co、Ni、Cu等元素未见富集,以上地球化学特征与现代海底热液成因铁锰沉积物一致。在Fe/Ti-Al/(Al+Fe+Mn)、Si O2-Al2O3、10×(Co+Ni+Cu)-Fe-Mn、100×(Zr+Ce+Y)-15×(Cu+Ni)-(Fe+Mn)/4等判别图中,莫托萨拉的锰矿层和铁矿层样品均落在海底热液沉积区。锰矿层和铁矿层的稀土元素经PAAS标准化后具有明显的Ce负异常、Eu正异常和Y正异常,与现代海底热液成因铁锰沉积物的稀土配分模式非常相似。综合分析本次研究的矿物学、岩石学、地球化学特征以及前人资料,本文认为莫托萨拉铁锰矿床为海相热水沉积成因,成矿与同期海底火山的间歇性活动密切相关,海底热液的化学组分、温度高低和活动强弱都具有明显的脉动性。莫托萨拉矿区铁锰共存但各自独立成矿,且铁锰分离程度较高,这在显生宙沉积型锰矿中独具特色。鉴于前人曾报道莫托萨拉铁矿石中存在菌藻类微生物化石,我们推测,该矿床的铁锰分离过程除了受控于沉积环境的氧化还原条件变化外,微生物的选择性氧化沉淀可能也发挥了重要作用,值得开展深入研究。

Mineralogical and geochemical characteristics of Motuosala exhalative sedimentary Fe-Mn deposit in the West Tianshan, Xinjiang, NW China

The Motuosala Fe-Mn deposit is located in the eastern end of the Awulale metallogenic belt in the West Tianshan, and the study about it is relatively weak. So far, the origins of this deposit include exhalative sedimentary, sedimentary-hydrothermal alteration, and colloidal chemical deposition. In this paper, the mineralogical composition, texture-structure and geochemical characteristics of the uppermost manganese ore bed and its wall rocks are studied in detail, and the origin of this Fe-Mn deposit is further discussed based on previous research results. For the first time, we found hydrothermal feldspar rock in Motuosala area. It is mainly composed of albite, K-feldspar and minor barite, aegirine, justite etc., which is similar to "white smoker"-type exhalative rock. The uppermost manganese ore bed is mainly composed of tephroite, braunite, parsettensite, vredenburgite and minor barite, bixbyite etc., which shows typical intraclastic texture indicating that it is deposited near the seafloor hydrothermal vent. Manganese ore bed have low Al/(Al+Fe+Mn) values (0~0.02), relative high Si/Al values (7.9~10.9) and high Fe/Ti values (428~1353). After UCC normalization, manganese ore bed are obviously enriched in Zn, Ba and Pb; while Co, Ni and Cu are not obviously enriched. The above geochemical characteristics are consistent with those of modern submarine hydrothermal iron-manganese sediments. In the discrimination diagrams of Fe/Ti-Al/(Al+Fe+Mn), SiO₂-Al₂O₃, 10×(Co+Ni+Cu)-Fe-Mn, 100×(Zr+Ce+Y)-15×(Cu+Ni)-(Fe+Mn)/4 and so on, the Mn and Fe ore beds samples of Motuosala completely fall into the submarine hydrothermal sedimentary area. PAAS-normalized REE patterns of Mn and Fe ore beds of Motuosala have significant negative Ce anomalies, positive Eu anomalies and positive Y anomalies, which is typical to modern submarine hydrothermal Fe-Mn sediments. Based on a comprehensive analysis of the above mineralogical, petrological and geochemical characteristics and previous research results, we suggest that the Motuosala Fe-Mn deposit is exhalative sedimentary in origin, and the composition, temperature and intensity of seafloor hydrothermal solution changed alternately, which may be closely related to the intermittent activities of contemporaneous submarine volcanoes. The Fe and Mn ore beds coexist in Motuosala but the separation degree between Fe and Mn is relatively high, which is unique in the Phanerozoic sedimentary manganese deposits. In view of the previous reports on the existence of bacteria and algae-like microbial fossils in motuosala iron ores, we speculate that the Fe and Mn separation process in Motuosala is not only controlled by the redox changes of the sedimentary environment, the selective oxidative precipitation by microorganisms may also play an important role, which is worthy of further study.