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.
