南华系大塘坡组菱锰矿矿床地质特征及成因新见解

黔湘渝鄂毗邻地区新元古界大塘坡组菱锰矿矿床属于典型的海洋化学-生物化学沉积型菱锰矿床。大塘坡式菱锰矿属于一种海洋自生碳酸盐岩,在横向上常与含锰白云岩、白云岩、灰岩呈过渡关系,其结构构造又类似于灰岩、白云岩。作者认为,要形成区域上那么多的大中型菱锰矿床,必须要在成锰期之前漫长的地壳演化中,由岩浆-火山作用、各种海底喷流、古大陆风化剥蚀和搬运、古天然气渗漏等多种地质作用或事件,源源不断地为海洋盆地水介质提供大量的锰质,通过长期的各种地质作用和海解作用,使海洋盆地水介质中积累储存了丰富的Mn2+离子。在成锰期,其沉积盆地位于一个较宽阔的和多障壁地形的温暖、阳光充足、清澈的局限至半局限海岸-浅海陆架区,海流频繁,其中,浅海陆架盆地相带是菱锰矿沉积聚积的最佳相带。菱锰矿形成时的p H值在7~8,Eh值在-0.3EV左右。在此缺氧还原、碱性的底层水介质条件下,海水中的Mn2+离子易与HCO3-离子结合生成MnCO3析出沉淀。适宜的聚锰环境长期稳定、反复存在也是形成该区域内一些菱锰矿床中的多层富厚矿体的时间因素之一。

The Neoproterozoic Datangpo formation siderite deposit in the adjacent area of Guizhou,Hunan,Chongqing and Hubei is a typical marine chemical biochemical sedimentary siderite deposit.Datangpo type rhodochrosite belongs to marine authigenic carbonate rock,which is in transition with manganese bearing dolomite,dolomite and limestone in transverse direction,and its structure is similar to that of limestone and dolomite.The author thinks that in order to form so many large and medium-sized siderite manganese deposits in the region,it is necessary to continuously provide a large amount of manganese for the water medium of the ocean basin during the long crustal evolution before the manganese forming period,by various geological processes or events such as magmatic volcanism,various submarine exhalations,weathering,denudation and transportation of the ancient continent,and leakage of ancient natural gas The accumulation and storage of abundant Mn^(2+)ions in the water medium of the ocean basin are caused by the action of water and sea solution.During the manganese forming period,the sedimentary basin is located in a wide and multi barrier terrain,which is warm,sunny and clear,and is limited to semi limited coastal shallow shelf area with frequent ocean currents.Among them,the shallow shelf basin facies belt is the best facies belt for siderite deposition and accumulation.The pH value of rhodochrosite is 7-8,and the Eh value is about-0.3EV.Under the condition of anoxic reduction and alkaline bottom water medium,Mn^(2+)ion in seawater is easy to combine with HCO_(3)-ion to form MnCO_(3) precipitation.The long-term stability and repeated existence of suitable manganese accumulation environment is also one of the time factors for the formation of multi-layer rich and thick orebodies in some rhombic manganese deposits in this area.