Collaborative Research: Time-Dependent Hydrothermal Convection within the Great Basin Nevada

合作研究：内华达大盆地内随时间变化的热液对流

• 批准号: 0809644
• 负责人: Mark Person
• 金额: $ 24.38万
• 依托单位: New Mexico Institute of Mining and Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-10-01 至 2011-09-30
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0809644&HistoricalAwards=false
• 关键词: Collaborative; Research; Time; Dependent; Hydrothermal

Groundwater flow systems within the Great Basin, Nevada are remarkable in many respects. Despite the arid conditions and relatively low permeability volcanic rocks and basin fill, this region hosts active geothermal systems and world class Eocene age gold deposits. Temperature profiles, fluid inclusion studies, and isotopic evidence suggest that modern and fossil hydrothermal systems share many common features including the absence of a clear magmatic fluid source, discharge areas restricted to fault zones, and remarkably high temperatures ( 200 °C) at shallow depths (200-1500 m). Many of the Great Basin geothermal systems exhibit some of the highest shallow crustal heat flow levels ever recorded ( 2000 mW/m2 at Beowawe) within the continental crust. Geochemical and isotopic data collected at the Beowawe geothermal system suggests that fluid circulation is deep ( 5 km) and comprised of relatively unexchanged Pleistocene meteoric water with small ä18O ( 2.5 ?) shifts from the meteoric water line (MWL). Fossil ore-forming hydrothermal systems associated with Carlin-type gold mineralization have similar temperature patterns but exhibit fluid-rock interactions with larger ä18O shifts of 5 to 20 ? from the MWL.The goal of this proposal is to understand the three-dimensional plumbing, fluid flow impelling mechanisms, and temporal evolution of modern and fossil hydrothermal flow systems within the Great Basin. We wish to evaluate two end member questions regarding the nature of hydrothermal circulation within the Great Basin: A) Is flow restricted to high permeability fault planes driven by free-convection? or B) is flow driven by water-table topographic gradients with some combination of matrix and fault controlled fluid circulation? Because of the lack of broad low heat flow anomalies adjacent to Great Basin geothermal fields, we suspect that these flow systems must be episodic in nature due to permeability reduction associated with silica mineralization. By including the systematics of silica precipitation (and associated porosity/permeability reduction), we hope to constrain the duration of these geothermal systems. We will develop a suite of 3D, single-phase, hydrothermal models using a new parallel finite element code (PGEOFE) for two field sites within the Great Basin. We will develop geologically/geophysically constrained, three-dimensional hydrothermal models of the modern Beowawe and fossil Carlin geothermal systems; two sites with rich , isotopic, geochemical and geothermal data sets. Using LaGrit mesh generation software, these hydrogeologic models will honor known fault geometries, widths, and stratigraphy. A unique feature of the proposed work is that we will use multiple constraints including temperature profiles, shallow heat flow maps, fluid/rock ä 18O composition, and the age of hot springs deposits to test our models. We will also develop more sophisticated reactive-transport geochemical models using PFLOTRAN incorporating porosity-enhancing carbonate dissolution reactions to constrain how long the Carlin flow systems remained active before becoming clogged by gangue mineralization. By dating organic matter (pollen) within the hot springs deposits at Beowawe using 14C dating methods, we hope to determine whether or not these hot springs deposits formed during a single event or in several episodes.Our study may help document the existence of time-dependent natural convection systems within the Great Basin. Understanding the mechanisms and patterns of fluid circulation within this region is of great societal relevance because this region may soon host our nation?s high level nuclear wastes. The project will support two graduate students at New Mexico Tech and University of Missouri at Columbia.

内华达州大盆地内的地下水流系统在许多方面都是引人注目的。尽管干旱的条件和相对低渗透性的火山岩和盆地填充，该地区拥有活跃的地热系统和世界级的始新世金矿。温度剖面、流体包裹体研究和同位素证据表明，现代热液系统和化石热液系统具有许多共同特征，包括缺乏明确的岩浆流体来源，排放区域仅限于断层带，以及在浅深度（200-1500米）处的显着高温（200 °C）。许多大盆地地热系统显示出大陆地壳内有史以来最高的浅层地壳热流水平（Beowawe为2000 mW/m2）。在Beowawe地热系统收集的地球化学和同位素数据表明，流体循环是深（5公里），由相对未交换的更新世大气降水与小？从大气水位线（MWL）偏移。与卡林型金矿化有关的化石成矿热液系统具有相似的温度模式，但表现出流体-岩石相互作用，具有较大的Δ 18 O位移5至20？该提案的目标是了解大盆地内现代和化石热液流系统的三维管道、流体流动推动机制和时间演化。我们希望评估两个端员问题的性质，热液循环在大盆地：A）是流动限制高渗透率断层面驱动的自由对流？或B）流动是由地下水位地形梯度驱动的，并具有基质和断层控制的流体循环的某种组合？由于缺乏广泛的低热流异常毗邻大盆地地热田，我们怀疑，这些流动系统必须是情节性的，由于渗透率降低与硅矿化。通过包括二氧化硅沉淀（和相关的孔隙度/渗透率降低）的系统学，我们希望限制这些地热系统的持续时间。我们将开发一套3D，单相，热液模型使用一个新的并行有限元代码（PGEOFE）的大盆地内的两个现场。我们将开发现代Beowawe和化石卡林地热系统的地质/地质约束的三维热液模型;两个站点具有丰富的同位素，地球化学和地热数据集。使用LaGrit网格生成软件，这些水文地质模型将荣誉已知的断层几何形状，宽度和地层。拟议工作的一个独特之处是，我们将使用多种约束条件，包括温度剖面，浅层热流图，流体/岩石的18 O组成，以及温泉沉积物的年龄来测试我们的模型。我们还将开发更复杂的反应性运输地球化学模型，使用PFLOTRAN结合孔隙度增强碳酸盐溶解反应，以限制卡林流动系统在被脉石矿化堵塞之前保持活跃的时间。通过14 C测年法测定贝奥瓦韦温泉沉积物中的有机质（花粉），我们希望确定这些温泉沉积物是在单一事件中形成的，还是在几个事件中形成的，我们的研究可能有助于记录大盆地内依赖时间的自然对流系统的存在。了解该地区的流体循环机制和模式具有很大的社会意义，因为该地区可能很快就会成为我们国家的东道主。高水平的核废料。该项目将支持新墨西哥州理工学院和哥伦比亚的密苏里州大学的两名研究生。