Assessing the Role of Crustal S Contamination in the Generation of PGE Mineralization in the Stillwater Complex, Montana: A multiple S and Re-Os Isotopic Study

评估蒙大拿州斯蒂尔沃特杂岩地壳硫污染在 PGE 矿化生成中的作用：多重硫和铼锇同位素研究

• 批准号: 1249702
• 负责人: Edward Ripley
• 金额: $ 30万
• 依托单位: Indiana University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2013
• 资助国家: 美国
• 起止时间: 2013-04-01 至 2017-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=1249702&HistoricalAwards=false
• 关键词: Assessing; Role; Crustal; Contamination; Generation

The bulk of the world?s supply of platinum is provided by what are known as ?reef-type? deposits. These deposits are thin layers that are found in igneous rocks and are characterized by the presence of 1 to 5 volume percent of disseminated sulfide minerals, principally those rich in iron, copper and nickel. PGE (platinum?group element) sulfide minerals occur in much smaller concentrations but provide the economically important metals such as platinum, and also palladium. Our need for platinum-group elements is projected to increase in coming years, driven particularly by advances in the automotive industry. The largest reserves of platinum are found in South Africa and Zimbabwe, with deposits found in the Bushveld Complex and the Great Dyke. Large reserves are also found in Russia (Siberia) associated with a somewhat different assemblage of igneous rocks that are very rich in sulfide minerals (sometimes nearly 100 % sulfides). The igneous rocks of the Stillwater Complex in Montana host the only two PGE mines in the United States in what is called the J-M Reef. The palladium production from the mines accounts for a few percent of the world total. Because of our utilization of PGEs the United States needs to increase our reserves and reduce our dependence on foreign sources. Research of the type described in this proposal is needed to improve models for both exploration and development of reef-type PGE deposits, similar to those found in the Stillwater Complex.The importance of sulfur derived from sedimentary rocks of the earth?s crust has not been previously emphasized in models dealing with the generation of reef-type deposits, largely because sulfur isotopic values are near those typically assigned to sulfur derived from the earth?s mantle. Because country rocks to the Stillwater Complex are in excess of 2.6 billion years (referred to as Archean age), the utilization of isotopic ratios that involve both 33S and 34S values provides another method to test if country rock derived sulfur has been essential for the production of PGE-enriched magmas. Sulfur from Archean sedimentary rocks may show a distinct departure from a slope of ~ 0.515 in ratio plots involving 33S and 34S; the relation defines what is known as the Terrestrial Fractionation Line (TFL). The origin of the anomalous departure from the TFL is still a matter of debate, but the presence of anomalous sulfur isotopic values in igneous rocks confirms the assimilation, or incorporation, of Archean-aged sulfur of country rock origin. In order to test the hypothesis that crustal sulfur derived from Archean country rocks was involved in the formation of the J-M Reef and other sulfide-rich zones in the Stillwater Complex we propose to collect multiple sulfur isotope data from samples collected in traverses through the Stillwater Complex in two locations that include the current workings at the Stillwater and East Boulder mines. Recent mapping in the Stillwater Mine has clearly defined a number of sulfide-rich lenses that constitute the PGE mineralization in the J-M Reef. Our sampling will include the individual lenses, with detailed petrographic (microscopic) analyses done to interpret interfaces between mineralized and non-mineralized domains. Sulfur concentrations from all samples will also be collected; the combined data set will permit rigorous mass balance calculations that will be used to quantify the importance of crustal sulfur involvement in the Stillwater Complex. The proposed work is required to evaluate if large PGE deposits owe their origins to the interaction of magmas with sedimentary country rocks that supplied sulfur far in excess of that which could have been sequestered from the mantle source of the magmas. Our results will constrain models for further PGE exploration and shed light on the processes of magma-country rock interaction that may be of significance in the generation of several types of metallic ore deposits.

世界上大部分的铂金供应是由所谓的“礁石型”提供的。存款。 这些矿床是在火成岩中发现的薄层，其特征是存在 1 至 5 体积％的浸染状硫化物矿物，主要是富含铁、铜和镍的矿物。 PGE（铂族元素）硫化物矿物的浓度要低得多，但提供了经济上重要的金属，例如铂和钯。 预计未来几年我们对铂族元素的需求将会增加，特别是在汽车行业进步的推动下。南非和津巴布韦发现了最大的铂储量，在布什维尔德杂岩和大堤坝中发现了矿床。俄罗斯（西伯利亚）也发现了大量储量，与某种不同的火成岩组合有关，这些火成岩富含硫化物矿物（有时几乎 100% 硫化物）。蒙大拿州斯蒂尔沃特综合体的火成岩中蕴藏着美国仅有的两座铂族金属矿，即 J-M 礁。这些矿山的钯产量占世界总量的百分之几。由于我们使用铂族金属，美国需要增加我们的储备并减少对外国来源的依赖。需要对本提案中描述的类型进行研究，以改进礁石型 PGE 矿床的勘探和开发模型，类似于斯蒂尔沃特杂岩中发现的矿床。来自地壳沉​​积岩的硫的重要性尚未得到重视。先前在处理礁石型沉积物生成的模型中强调了这一点，主要是因为硫同位素值接近通常分配给来自地幔的硫的值。由于斯蒂尔沃特杂岩的围岩年龄超过 26 亿年（称为太古宙年龄），因此利用涉及 33S 和 34S 值的同位素比提供了另一种方法来测试围岩衍生的硫是否对生产至关重要富含 PGE 的岩浆。在涉及 33S 和 34S 的比率图中，太古宙沉积岩中的硫可能显示出明显偏离 ~ 0.515 的斜率；该关系定义了所谓的陆地分数线 (TFL)。 TFL 异常偏离的起源仍然是一个有争议的问题，但火成岩中异常硫同位素值的存在证实了太古宙年龄的围岩来源的硫的同化或掺入。为了验证源自太古代围岩的地壳硫参与了斯蒂尔沃特杂岩体中 J-M 礁和其他富含硫化物区域的形成的假设，我们建议从穿越斯蒂尔沃特杂岩体时采集的样品中收集多种硫同位素数据在两个地点，包括斯蒂尔沃特和东博尔德矿的当前工作区。 最近对斯蒂尔沃特矿进行的测绘清楚地确定了一些富含硫化物的透镜体，这些透镜体构成了 J-M 礁中的 PGE 矿化体。我们的采样将包括各个透镜体，并进行详细的岩相（显微镜）分析，以解释矿化区域和非矿化区域之间的界面。还将收集所有样品中的硫浓度；合并后的数据集将允许进行严格的质量平衡计算，用于量化斯蒂尔沃特复合体中地壳硫参与的重要性。拟议的工作需要评估大型铂族元素矿床的起源是否归因于岩浆与沉积围岩的相互作用，这些围岩提供的硫远远超过了岩浆地幔源中可能封存的硫。我们的结果将限制进一步 PGE 勘探的模型，并揭示岩浆-岩石相互作用的过程，这对于几种类型的金属矿床的生成可能具有重要意义。