Mineral Reactions in Altered Sediments From the California State 2–14 Well: Variations in the Modal Mineralogy, Mineral Chemistry and Bulk Composition of the Salton Sea Scientific Drilling Project Core

Abstract

X ray diffraction‐reference intensity method (XRD‐RIM) modes, whole rock chemistry, and mineral chemistry were determined on 36 samples from the Salton Sea Scientific Drilling Project (SSSDP) core. These samples display a wide variation in grain size (shale to sandstone), bulk composition (SiO2 = 36.2–81.2 wt %; Al2O3 = 5.1–17.1 wt %; CaO = 1.2–21.1 wt % and (total Fe as Fe2O3) Fe2O3/(Fe2O3 + MgO) wt ratio = 0.48–0.98) and mineral assemblage. Seven mineral assemblages have been identified in this sample suite: (1) chlorite + calcite + quartz + plagioclase ± K‐feldspar ± illite (476–2559 m), (2) chlorite + quartz +plagioclase + K‐feldspar (918.7–1984.3 m), (3) chlorite + epidote + quartz ± plagioclase ± calcite ± Kfeldspar ± illite ± magnetite (905.6–2954.8 m), (4) chlorite + biotite ± calcite + quartz + plagioclase + K‐feldspar (2700.0–2819.2 m), (5) epidote + quartz ± plagioclase (1424.2–2954.8 m), (6) epidote + actinolite + quartz ± biotite ± chlorite ± plagioclase ± K‐feldspar ± anhydrite (2880.2–3021.2 m), and (7) epidote + clinopyroxene + actinolite + quartz (2484.7 m). These assemblages define three metamorphic zones in the SSSDP well: calcite‐chlorite zone, biotite zone, and actinolite zone. Accompanying these transitions in mineral assemblages are changes in the Si‐Al ordering in K‐feldspar and an increase in the anorthite component in plagioclase. Fe/(Fe+Al) in epidote shows very little systematic variation downhole (in the sediments) but can be highly zoned and shows considerable systematic variation between vein systems and sedimentary pore space. Quantitative modal mineralogy and whole rock chemistry can be utilized in determining mineral reactions and their importance and in evaluating the evolution of mineral assemblages. For example, epidote‐forming reactions precede actinolite‐epidote‐ and biotite‐epidote‐forming reactions in numerous bulk compositions and result in the formation of high modal abundances of epidote. The production of fluids with high XCO2 from epidote‐, actinolite‐, biotite‐, and diopside‐forming reactions in rocks in equilibrium with fluids with CO2 < 0.1 indicates extensive interaction between fluid and the shales. This is also indicated by the leaching of Cu, Zn, and Mn and the enrichment of Sr. Numerous elements, however, do not show evidence of exchange between sediments and fluid.