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

我们建议进行一项为期两年的研究，重点阐明强污染和还原i型“I-SCR”花岗岩岩体的场域、年龄、空间和成因关系，严格意义上，在马里波萨附近变质山脚带东南凸起的内华达山脉西部基岩中，特定的前基围岩作为顶板垂体出现。加州（图1）。本文还讨论了位于变质山麓带内梅洛内斯断裂带的母矿脉金石英脉带中金钨的来源。后者在马里波萨附近突然结束，侏罗纪和白垩纪内华达山脉基基在马里波萨东部和南部边缘侵入，位于与主矿脉走向的共线减少花岗岩带的北端，并从那里向南延伸至内华达山脉基基的南端。这种与岩浆特征的岩石构造对比将通过对变质围岩和岩体的区域取样以及随后的整个岩石和矿物化学来详细说明，以更好地解释还原型I-SCR花岗岩的分布，我们假设这些花岗岩是由含水石墨泥质部分熔融或同化而形成的普通i型岩浆围岩。为此，我们将对选定的金属（Au、W、Sn、Mo、Ti、V、Mn、Fe、Co、Ni、Cu、Zn、Zr、Nb、C、S和Ag）进行石墨炉原子吸收（AAS）和ICP质谱分析。不使用同位素示踪剂是不可能解决烃源岩问题的，因为仅靠微量元素数据是不够的。因此，将进行两项同位素研究。首先，我们与加州理工学院的Jason Saleeby合作，提议对加利福尼亚大学地质博物馆的方铅矿标本进行铅同位素分析，这些方铅矿标本来自沿着Melones断裂带的几个母矿岩矿床，位于马里波萨的西北部，并将其与附近变质围岩的全岩铅同位素比例进行比较，包括Shoo Fly， Calaveras, Mariposa，Don Pedro浅成岩和我们在Mariposa附近发现的每一种岩体类型，包括I- SCR、I- wc、MC和SC。这项研究将旨在推断热液流经脉流路径的围岩的性质。其次，与加州大学伯克利分校的Don DePaolo合作，提出利用Rb- Sr和Nd-Sm同位素验证岩浆岩源岩Pb分系统的结论，确定变质围岩同化作用对岩浆性质的影响程度和还原作用。利用Hammarstrom和Zen（1986）的经验火成岩角闪孔铝地球气压计来扩展我们先前关于岩基内结晶区域深度的结论，我们将把岩浆-围岩相互作用效应和热液金石英脉放在一个更定量的地球压力框架中，重点研究潜在的烃源岩和矿石金属（特别是Au、Ag、Pb、W、Sn）的选择性释放机制。然而，由于I-SCR型岩体很少含有角闪孔，因此只有除I-SCR型岩体以外的含有角闪孔的岩体类型，其年龄与矿脉相似，并且具有适合地球气压计应用的体积成分，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.