Lithotectonic Controls on the Reduced Granite Belt of the Western Sierra Nevada and its Relation to the Mother Lode Gold: Trace Element, Mineral Chemistry & Isotopic Studi

内华达山脉西部还原花岗岩带的岩石构造控制及其与金矿母矿的关系：微量元素、矿物化学

• 批准号: 8915997
• 负责人: George Brimhall
• 金额: $ 8.52万
• 依托单位: University of California-Berkeley
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 1990
• 资助国家: 美国
• 起止时间: 1990-01-01 至 1991-12-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=8915997&HistoricalAwards=false
• 关键词: Lithotectonic; Controls; Reduced; Granite; Belt

We propose a two-year study focused on clarifying the field, age, spatial, and genetic relationships of strongly contaminated and reduced I-type "I-SCR" granite plutons, sensu stricto, with specific pre-batholith wall rocks occurring as roof pendants within the Western Sierra Nevada batholith along the south eastern projection of the Metamorphic Foothills belt near Mariposa, California (Fig. 1). We will also address the origin of gold and tungsten in the Mother Lode gold quartz vein belt which occupies the Melones Fault within the Metamorphic Foothills belt. The latter ends abruptly near Mariposa where invasion on the eastern and southern margins by the Sierra Nevada batholith in Jurassic and Cretaceous time position the northern end of the colinear reduced granite belt on strike with the Mother Lode and extends from there southwards towards the southern end of the Sierra Nevada batholith. This lithotectonic correlation with magma characteristics will be detailed through regional sampling of metamorphic wall rocks and plutons followed up with whole rock and mineral chemistry to better explain the distribution of reduced I-SCR granites which we have hypothesized to have form either by their derivation through partial melting of hydrous graphitic pelites or assimilation of these wallrocks by ordinary I-type magmas. To accomplish this we will perform graphite furnace atomic absorption (AAS) and ICP mass spectrometry trace element analysis of selected metals (Au, W, Sn, Mo, Ti, V, Mn, Fe, Co, Ni, Cu, Zn, Zr, Nb, C, S and Ag). it is impossible to resolve source rock problems without using isotopic tracers as trace element data alone are insufficient. Therefore two isotope studies will be conducted. First, in collaboration with Jason Saleeby of Cal. Tech, we propose to perform lead isotope analysis on University of California Geology Museum galena specimens from several Mother Lode deposit along the Melones Fault zone to the immediate north west of Mariposa and to compare these with whole-rock lead isotope ratios of nearby metamorphic wallrocks, including Shoo Fly, Calaveras, Mariposa, Don Pedro hypabyssal rocks and each of the pluton types we have identified near Mariposa including I- SCR, as well as I-WC, MC, and SC. This study will be directed at inferring the nature of wall rocks traversed by hydrothermal fluids coursing through the vein flow path. Secondly, in collaboration with Don DePaolo at Berkeley, we propose to use Rb- Sr and Nd-Sm isotopes to confirm conclusions made from Pb systematics on magmatic source rocks and to establish the extent and reductive efficacy of metamorphic wall rock assimilation on magma properties. Using the empirical igneous amphibole aluminum geobarometer of Hammarstrom and Zen (1986) to expand on our earlier conclusions about the regional depth of crystallization within the batholiths, we will place magma-wallrock interaction effects and hydrothermal gold quartz veins in a more quantitative geobarometric framework focussing on potential source rocks and selective release mechanisms of ore metals, particularly Au, Ag, Pb, W, Sn. However, since I-SCR plutons rarely contain amphiboles, only pluton types other than I-SCR which do contain amphibole, are of similar age to the veins, and have appropriate bulk compositions for application of the geobarometer are suitable for this purpose.

我们提出了一项为期两年的研究，重点是澄清该领域， 年龄，空间和遗传关系的强污染 严格意义上的I型“I-SCR”花岗岩岩体， 作为屋顶悬垂物出现的特定前岩基围岩 在西塞拉内华达州岩基内，沿着南部 变质山麓带的东部投影， 马里波萨，加州（图1）。 我们还将讨论 母矿脉金石英脉带中的金、钨 它占据了梅洛内斯断层， 腰带 后者在马里波萨附近突然结束， 东部和南部边缘的山脉内华达州岩基 在侏罗纪和白垩纪时期， 与主矿脉走向共线的还原花岗岩带， 从那里向南延伸到南端， 内华达州山脉岩基。 这种岩石构造与岩浆特征的相关性 将通过变质壁的区域取样进行详细说明 岩石和岩体，随后是整个岩石和矿物 更好地解释还原I-SCR的分布 我们假设的花岗岩是由 含水石墨泥的部分熔融衍生 或被普通I型岩浆同化。 到 实现这一目标，我们将进行石墨炉原子 原子吸收光谱法和电感耦合等离子体质谱法痕量元素分析 选择的金属（Au、W、Sn、Mo、Ti、V、Mn、Fe、Co、Ni、Cu、Zn、 Zr、Nb、C、S和Ag）。 不可能解决烃源岩， 不使用同位素示踪剂作为痕量元素数据的问题 单靠这些是不够的。 因此，将进行两项同位素研究。 进行。 首先，与加州大学的杰森·萨利比合作。 技术员，我们建议对大学进行铅同位素分析 加州地质博物馆方铅矿标本来自几个母亲 沿着梅洛内斯断层带向北的矿脉存款 马里波萨西部的岩石中， 附近变质围岩，包括Shoo Fly、Calaveras、马里波萨、Don Pedro浅成岩石以及 我们在马里波萨附近发现的岩体类型包括I- SCR，以及I-WC，MC和SC。本研究将针对 推断热液穿过的围岩性质 流体流过静脉流动路径。其次，在 与伯克利的Don DePaolo合作，我们建议使用Rb- Sr和Nd-Sm同位素证实了Pb 岩浆源岩的系统分类， 变质围岩同化作用对 岩浆性质 使用经验火成角闪石铝质地质压力计 Hammarstrom和Zen（1986）的研究，以扩展我们早期的 关于区域结晶深度的结论 岩基，我们将把岩浆-围岩相互作用的影响 和热液石英脉中的金含量 以潜在烃源岩为重点的地质压力框架， 矿石金属，特别是Au，Ag， 铅、钨、锡。 然而，由于I-SCR岩体很少含有 角闪石，只有I-SCR以外的岩体类型， 角闪石，年龄与矿脉相似， 用于应用地质压力计的本体组合物 适合这个目的。