冰川底部富铁溶液氧化沉淀:塔里木北缘新元古界库鲁克赛铁矿床的成因

Oxidation and deposition of iron-rich saline water below glacier cap:Genesis of Neoproterozoic Quruqsay iron deposit on northern margin of Tarim craton

新元古代沉积变质铁矿床是继大氧化事件(GOE)后,沉积间断10亿年(~1800 Ma至~750 Ma)之后,再次大规模出现的一种沉积铁建造类型。这类铁建造与新元古代冰碛岩密切伴生,是新元古代雪球地球事件的重要证据。文章选择与新元古代雪球地球事件有关的沉积变质赤铁矿床—库鲁克赛铁矿进行研究,通过锆石U-Pb定年和区域地层对比工作限定其形成时代为新元古代青白口纪末期—南华纪早期。锆石年龄谱值对比和岩相学研究表明,铁矿床中的碎屑物质主要来自于青白口系下部独断山组石英砂岩地层。主、微量元素研究表明,库鲁克赛铁矿形成于相对富氧或者从贫氧向富氧变化的环境,其成矿元素应主要与陆源物质风化有关,可能有少量成矿元素来自于低温海底热液或海水。笔者认为,库鲁克赛铁矿的形成与成冰纪冰水沉积作用有关,来自冰下水体、从冰下通道中流出的富铁缺氧水溶液与富氧的表层海水混合时,成矿元素快速氧化沉淀,胶结冰水中的近源砾石,进而形成了此种富铁砾岩型铁矿。

Banded Iron Formations （BIFs） reappeared during the Neoproterozoic, after about 1 billion years of absence in sedimentary records from the last deposition of Superior-type BIFs at 1.8 Ga. This type of iron formation is commonly associated with deposition of Neoproterozoic glacial facies which proceeded with the Cambrian radiation of metazoans, reflecting dramatic environmental change. A detailed understanding of mechanisms of Fe oxidation and accumulation during the Cryogenian period provides insights into evolution of the Earth as a system and biological innovations during this critical geological interval. In this paper the authors studied the hematite-rich Quruqsay iron deposit on the north margin of Tarim craton, which is associated with Neoproterozoic glaciation but was rarely reported in previous studies. Based on zircon U-Pb dating and regional stratigraphic correlation, the authors constrained the deposition age of the Quruqsay iron deposit between the early Qingbaikou period and the late Nanhua period in Neoproterozoic. Zircon age spectrum and petrographic features show that the clastic materials associated with hematite deposits were mainly from the Duduanshan Group, which was the upper part of the formation of Qingbaikou system. Major and trace element compositions indicate that hematite from the Quruqsay iron deposit was formed in an oxygen rich environment, and the ore-forming materials were mainly derived from continental crust, with only a small proportion of contribution from hydrothermal fluids or seawater. Geological and geochemical evidence suggests the close affinity between iron and continental glacier in the Quruqsay iron deposit, which is radically different from the Cryogenian Raptian-type BIFs, for which hydrothermal origin of Fe is commonly proposed. The authors argue that the iron source for the Quruqsay deposit originated from saline water below the glacier cap, which liberated Fe2＋ from subglacial rocks during the progression of glaciers, and the subglacial Fe2＋ fluids were oxidized at the edge of the glacier when it migrated into the ocean. Such mechanism of Fe2＋ mobilization and oxidation has been observed from a modern glacier system at the Taylor Glacier in Antarctica, known as the famous ＂Blood Falls＂. The authors hold that the Quruqsay iron deposit is an example of the ＂Blood Falls＂-type iron formation, which has a unique iron mineralization model sourced from surficial continental rocks, which is distinct from commonly proposed Archean BIF models and the Raptian BIF models.