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

内华达州大盆地内的地下水流动系统在许多方面都是非凡的。尽管该地区气候干旱，火山岩和盆地充填物渗透率相对较低，但地热系统活跃，始新世期金矿床世界级。温度剖面、流体包裹体研究和同位素证据表明，现代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，排放区域仅限于断裂带，浅层深度（200-1500米）温度极高（200°C）。许多大盆地地热系统在大陆地壳中表现出一些有记录以来最高的浅层地壳热流水平（Beowawe的2000 mW/m2）。在Beowawe地热系统收集的地球化学和同位素数据表明，流体循环较深（5 km），由相对不交换的更新世大气水组成，与大气水线（MWL）有较小的ä18O（2.5 ?）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，ä18O位移较大，为5 ~ 20 ？来自MWL。本研究的目的是了解大盆地内现代和古热液流动系统的三维管道、流体驱动机制和时间演化。我们希望评价关于大盆地内热液环流性质的两个末端成员问题：A)流动是否局限于由自由对流驱动的高渗透断层面？还是B)流动是由地下水位地形梯度驱动的，并结合了基质和断层控制的流体循环？由于大盆地地热田附近缺乏广泛的低热流异常，我们怀疑这些流动系统在本质上一定是偶发性的，因为与硅矿化相关的渗透率降低。通过包括二氧化硅沉淀（以及相关的孔隙度/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。我们将使用新的并行有限元代码（PGEOFE）为大盆地内的两个现场开发一套3D单相热液模型。我们将开发地质/地球物理约束的现代Beowawe和化石Carlin地热系统的三维热液模型；这两个地点拥有丰富的同位素、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遵循已知的断层几何形状、宽度和地层学。这项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ä 18O成分和温泉沉积物的年龄，来测试我们的模型。我们还将使用PFLOTRAN开发更复杂的反应-输运地球化学模型，其中包含孔隙度增强的碳酸盐溶解反应，以限制卡林流系统在被脉石矿化阻塞之前保持活跃的时间。通过使用14C测年方法对Beowawe温泉沉积物中的有机质（花粉）进行测年，我们希望确定这些温泉沉积物是在一次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随时间变化的自然对流系统的存在。了解该地区流体循环的机制和模式具有重大的社会意义，因为该地区可能很快就会成为我们国家的所在地。美国高水平核废料。该项目将支持新墨西哥理工大学和密苏里大学哥伦比亚分校的两名研究生。

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.