The Paleoproterozoic Ironwood Iron-Formation, a Superior-type banded iron formation located in the western Gogebic Iron Range in Wisconsin, is one of the largest undeveloped iron ore resources in the United States. Interest in the development of this resource is complicated by potential environmental and health effects related to the presence of amphibole minerals in the Ironwood, a consequence of Mesoproterozoic contact metamorphism. The presence of these amphiboles and their contact metamorphic origin have long been recognized; however, recent interest in this resource has highlighted the lack of detailed knowledge on their distribution, mineral chemistry, and morphology. Optical microscopy, X-ray diffraction, scanning electron microscopy, and electron microprobe analysis were utilized to investigate the origin, distribution, morphology, and chemistry of amphiboles in the Ironwood. Amphibole is present in the western portion of the study area due to regional-scale contact metamorphism associated with the intrusion of the 1.1 Ga Mellen Intrusive Complex. Locally amphibole is also present, adjacent to diabase and/or gabbro dikes and sills in the lower-grade Ironwood in the eastern portion of the study area. In both localities, amphiboles in the Ironwood most commonly developed in massive and prismatic habits, and locally assumed a fibrous habit. Fibrous amphiboles were recognized locally in the two potential ore zones of the Ironwood but were not observed in the portion likely to be waste rock. Massive and prismatic amphiboles show a wide range of Mg# [molar Mg/(Mg+Fe2+)] values (0.06 to 0.87), whereas Mg# values of fibrous amphiboles are restricted from 0.14 to 0.35. Factors that influenced the compositional variability of amphiboles in the Ironwood may have included temperature of formation, morphology, bulk chemistry of the iron formation, and variations in prograde and retrograde metamorphism. The presence of amphiboles in the Ironwood is a known issue that will need to be factored into any future mine plans. This study provides an objective assessment of the distribution and character of amphiboles in the Ironwood to aid all decision-makers in any future resource development scenarios.

中文翻译：

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威斯康星州 Gogebic 铁矿区 Ironwood Iron-Formation 中的变质角闪石：对潜在资源开发的影响

位于威斯康星州西部 Gogebic Iron Range 的古元古代 Ironwood Iron-Formation 是一种高级型带状铁地层，是美国最大的未开发铁矿资源之一。由于铁木中角闪石矿物的存在（中元古代接触变质作用的结果）相关的潜在环境和健康影响，因此对该资源的开发的兴趣变得复杂。人们早就认识到这些角闪石的存在及其接触变质起源。然而，最近对该资源的兴趣突出了其分布、矿物化学和形态方面的详细知识的缺乏。利用光学显微镜、X 射线衍射、扫描电子显微镜和电子探针分析来研究其起源、分布、形态、Ironwood 中角闪石的化学和化学。由于与 1.1 Ga Mellen 侵入杂岩体侵入相关的区域尺度接触变质作用，角闪石存在于研究区的西部。局部角闪石也存在，毗邻研究区东部低品位铁木的辉绿岩和/或辉长岩堤坝和窗台。在这两个地方，铁木林中的角闪石最常发育成块状和棱柱状习性，并在当地呈现出纤维状习性。在 Ironwood 的两个潜在矿区局部发现了纤维状角闪石，但在可能是废石的部分未观察到。块状和棱柱形闪石的 Mg# [摩尔 Mg/(Mg+Fe2+)] 值范围很广（0.06 到 0.87），而纤维状闪石的 Mg# 值限制在 0.14 到 0.35 之间。影响 Ironwood 中角闪石成分变异性的因素可能包括形成温度、形态、铁形成的整体化学以及顺行和逆行变质作用的变化。Ironwood 中角闪石的存在是一个已知问题，需要将其纳入任何未来的采矿计划。本研究对 Ironwood 中角闪石的分布和特征进行了客观评估，以帮助所有决策者在未来的任何资源开发方案中。Ironwood 中角闪石的存在是一个已知问题，需要将其纳入任何未来的采矿计划。本研究对 Ironwood 中角闪石的分布和特征进行了客观评估，以帮助所有决策者在未来的任何资源开发方案中。Ironwood 中角闪石的存在是一个已知问题，需要将其纳入任何未来的采矿计划。本研究对 Ironwood 中角闪石的分布和特征进行了客观评估，以帮助所有决策者在未来的任何资源开发方案中。