GEOLOGY AND GENESIS OF THE WATERLOO AND LANGTRY SILVER-BARITE DEPOSITS, CALIFORNIA (ORE, PRECIOUS METALS).

摘要

The Waterloo and Langtry deposits are within the Calico mining district which is part of a northwest-trending belt of precious-metal districts associated with Tertiary volcanic centers in the Western Mojave Block of southern California. The two deposits represent disseminated hydrothermal silver-barite mineralization within Miocene Barstow Formation lacustrian siltstone and sandstone; the Langtry deposit is a faulted portion of the Waterloo hydrothermal deposit which has been offset 1.9-3.2 km to the north across the right-lateral strike-slip Calico Fault.;Mass-balance calculations of whole-rock components indicate the following metasomatic changes in wallrock resulting from hydrothermal alteration: net removal of calcium, magnesium, carbon, sodium, and water, and net addition of sulfur, barium, silicon, and potassium. Aluminum remained largely unchanged, and variations of iron and manganese are ambiguous.;Studies of fluid inclusions in quartz and barite from veins and altered wallrock of the early barite stage and the silver-silicification stage indicate modal temperatures of formation around 185-195(DEGREES)C and fluid salinities of 4-5 wt percent equivalent sodium chloride. No daughter minerals were observed, and carbon dioxide is negligible. Depths of formation as shallow as 26-30 m are calculated from populations of inclusions entrapped during boiling.;Geological relationships and two potassium-argon radiometric age dates of altered wallrocks constrain the age of the hydrothermal event to approximately 17 Ma, which requires that it was contemporaneous with deposition of the host Barstow Formation sediments. A sublacustrian syngenetic-syndepositional model involving mixing of hydrothermal fluids with lake and connate water is deduced.;Three stages of alteration and mineralization are identified: (1) an early barite stage, (2) an intermediate silver-silicification stage, and (3) a late calcite stage. The early barite stage produced widespread removal of wallrock calcite and pervasively deposited barite plus minor quartz and silver. The silver-silicification stage produced K-feldspar alteration of wallrock plus pervasive silicification, and was responsible for the bulk of silver mineralization. The late calcite stage is characterized by minor calcite (+OR-) quartz veins and probably represents a late retrograde stage. The primary silver minerals are acanthite and native silver.