The Pilot Knob iron ore deposits in southeast Missouri, USA: A high-to-low temperature magmatic-hydrothermal continuum
The Pilot Knob iron ore deposits in southeast Missouri, USA: A high-to-low temperature magmatic-hydrothermal continuum
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DOI：
10.1016/j.oregeorev.2020.103973
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发表时间：
2021-04
影响因子：
3.3
通讯作者：
Bolorchimeg N. Tunnell;M. Locmelis;C. Seeger;R. Mathur;I. Dunkl;Brandon Sullivan;Lisa M. Lori
中科院分区：
文献类型：
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作者：
Bolorchimeg N. Tunnell;M. Locmelis;C. Seeger;R. Mathur;I. Dunkl;Brandon Sullivan;Lisa M. Lori
位于美国密苏里州东南部的中元古代圣弗朗索瓦山脉火成岩地体中，含有八个主要和几个次要的IOA/IOCG型矿床。本研究的重点是Knob试验性矿床，即块状试验性Knob磁铁矿(PKM)矿床和试验性Knob赤铁矿(PKH)矿床，该矿床位于PKM以上240m的地层中，由可变矿化的层状赤铁矿和赋存于角砾岩火山集块中的矿石组成。PKM矿床以前被证明是岩浆和岩浆-热液成因的，尽管它的形成还没有准确的年代。PKH矿床的成因(即沉积和热液)及其与PKM的成因关系仍存在争议。我们提供了PKM矿床中磷灰石与块状磁铁矿互生的新的U-Pb数据，并首次提供了PKM矿形成的1437.7 Ma±55.8Ma的准确年龄。对PKH矿石的岩石学观察、块体岩石成分和赤铁矿的矿物化学研究表明，PKH矿床中的赤铁矿是在200-250℃的酸性和高盐度热液中结晶的。来自PKH矿床的9个层状赤铁矿样品(δ56Fe=0.05-0.30‰，平均0.13‰)和3个角砾状赤铁矿样品(δ56Fe=−0.19-0.01‰，平均−0.06‰)的Fe同位素组成比已发表的PKM矿床磁铁矿δ56Fe结果(δ56Fe=0.06-0.27‰，)略轻。平均0.17‰)。然而，所有的同位素特征都在岩浆范围内，这表明这两个矿床中的铁最初来自岩浆。由于PKH矿床的热液成因，PKM和PKH矿石的铁同位素组成暗示着共享/相似的铁源，以及两者在空间上的接近，我们认为PKM和PKH矿床在成因上是相关的，是高低温岩浆-热液连续体的两个端元。在这种情况下，从岩浆中喷出的促进PKM矿床形成的矿液向上迁移，渗入地表附近的现有沉积构造，在保留原始层状和角砾结构的同时沉淀了热液赤铁矿。赋存PKM矿床的流纹岩/流纹岩的地球化学特征表明，这些岩石是A2型长英质岩石，侵位于碰撞后伸展环境。PKM矿床和围岩的整体硅酸盐地球标准化模式呈现从Cs到Lu的负斜率，Nb和Ta为负异常，表明流纹岩和流纹岩可能是俯冲改造的次大陆岩石圈地幔(SCLM)的水源区。这些地球化学特征支持圣弗朗索瓦山脉的构造背景，其中火成岩地体发育在不断增长的大陆边缘。幕式基性-中级岩浆作用，以及随后的热液喷发，可能在大约1500~1440 Ma之间在火成岩地体中形成了IOA/IOCG型矿床簇。在这样的背景下，PKM和PKH矿床可能代表了类似于形成IOA-IOCG连续体的过程的浅部、小规模快照：一种更深层次的岩浆事件，喷出形成上复矿体的热液。
The Mesoproterozoic St. Francois Mountains igneous terrane in southeast Missouri, USA, contains eight major and several minor IOA/IOCG-type deposits. This study focuses on the Pilot Knob deposits, i.e., the largely massive Pilot Knob Magnetite (PKM) deposit and the Pilot Knob Hematite (PKH) deposit, which is located 240 m stratigraphically above the PKM and consists of variably mineralized bedded hematite and ore hosted in brecciated volcanic agglomerates. The PKM deposit was previously shown to be of magmatic and magmatic-hydrothermal origin, although its formation has not been precisely dated. The origin of the PKH deposit (i.e., sedimentary vs. hydrothermal) and its genetic relationship to the PKM, remain controversial.We present new U-Pb data on apatite intergrown with massive magnetite in the PKM deposit and provide the first precise age for the formation of the PKM ore at 1437.7 ± 5.8 Ma. Petrographic observations of PKH ore, bulk rock compositions, and the mineral chemistry of hematite, which contains up to 2.7% Ti, suggest that the hematite in the PKH deposit crystallized from acidic and hypersaline hydrothermal fluids at a temperature between 200 and 250 °C. The Fe isotopic composition of 9 bedded (δ56Fe = 0.05–0.30‰, average 0.13‰) and 3 brecciated hematite samples (δ56Fe = −0.19 to 0.01‰, average −0.06‰) from the PKH deposit are slightly lighter than the published δ56Fe results of magnetite from the PKM deposit (δ56Fe = 0.06–0.27‰, average 0.17‰). However, all isotopic signatures fall within the magmatic range, indicating that iron in both deposits was originally sourced from a magma. Because of the hydrothermal origin of the PKH deposit, the iron isotopic compositions of the PKM and PKH ores that imply a shared/similar iron source, and the spatial proximity of both deposits, we argue that the PKM and PKH deposits are genetically related and represent two endmembers of a high-to-low temperature magmatic-hydrothermal continuum. In this scenario, ore fluids exsolved from the magma that facilitated the formation of the PKM deposit migrated upwards, infiltrated existing sedimentary structures near the surface, and precipitated hydrothermal hematite ore while preserving the original bedded and brecciated structures.Geochemical signatures of the rhyolites/rhyodacites that host the PKM deposit imply that these rocks are A2-type felsic rocks that were emplaced in a post-collisional extensional setting. Bulk silicate Earth normalized patterns of the PKM deposit and wall rocks display a negative slope from Cs to Lu with negative Nb and Ta anomalies, indicating a hydrous source for the rhyolites and rhyodacites, possibly a subduction-modified subcontinental lithospheric mantle (SCLM). These geochemical signatures support a proposed tectonic setting of the St. Francois Mountains, wherein the igneous terrane developed on a growing continental margin. Episodic mafic-to-intermediate magmatism, and subsequently exsolved hydrothermal fluids, may have formed the cluster of IOA/IOCG-type deposits in the igneous terrane between ~1500 and ~1440 Ma. Within such a context, the PKM and PKH deposits may represent a shallow, small-scale snapshot of processes similar to the ones that form the IOA-IOCG continuum: a deeper magmatic event that exsolved a hydrothermal fluid that forms an overlying ore body.