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

我们提议进行一项为期两年的研究，重点阐明强污染和还原的I型“I-SCR”花岗岩的场地、年龄、空间和成因关系，严格意义上讲，特定的前岩基围岩作为屋顶吊坠出现在内华达山脉西部岩基内，沿着加利福尼亚州马里波萨附近变质山麓带的东南投影(图1)。我们还将讨论位于变质山麓带内的Melones断层内的母脉金石英脉带中金和钨的来源。后者在马里波萨附近突然结束，在那里侏罗纪和白垩纪内华达山脉岩基侵入东部和南部边缘，使共线还原花岗岩带的北端与母亲矿脉走向，并从那里向南延伸至内华达山脉岩基的南端。这种岩石构造与岩浆特征的相关性将通过变质围岩和深成岩体的区域采样以及全岩和矿物化学来详细说明，以更好地解释还原的I-SCR花岗岩的分布，我们假设这些花岗岩是通过含水石墨质泥岩的部分熔融或这些围岩被普通I型岩浆同化而形成的。为此，我们将对选定的金属(金、钨、锡、钼、钛、钒、锰、铁、钴、镍、铜、锌、锆、铌、碳、S和银)进行石墨炉原子吸收(AAS)和电感耦合等离子体质谱(ICPMS)痕量元素分析。不使用同位素示踪剂是不可能解决生油岩问题的，因为仅有微量元素数据是不够的。因此，将进行两项同位素研究。首先，与加州大学的杰森·萨利比合作。技术方面，我们建议对沿着Melones断裂带到Mariposa紧邻西北的几个母脉矿床的加州大学地质博物馆方铅矿样品进行铅同位素分析，并将其与附近变质围岩的全岩铅同位素比值进行比较，这些围岩包括ShoFly、Calveras、Mariposa、Don Pedro浅成岩和我们在马里波萨附近识别的每一种深成岩类型，包括I-SCR，以及I-WC，MC和SC。这项研究将旨在推断热液流体通过脉状流动路径穿过围岩的性质。其次，我们与伯克利的Don DePaolo合作，提出用Rb-Sr和Nd-Sm同位素来证实铅系统学对岩浆源岩的结论，并确定变质围岩同化作用对岩浆性质的程度和还原效果。利用Hammarstrom和Zen(1986)的经验火成岩角闪石铝地压计，对我们早先关于岩层内区域结晶深度的结论进行了扩展，我们将把岩浆-围岩相互作用效应和热液金石英脉置于一个更定量的地压框架内，重点关注潜在的源岩和矿石金属的选择性释放机制，特别是Au，Ag，Pb，W，序列号。然而，由于I-SCR深成岩体很少含有角闪石，因此只有含有角闪石、与矿脉年龄相近、具有适合应用地压计的块体成分的I-SCR深成岩类型才适合用于这一目的。

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.