Abstract

Faults divide the Big Burro Mountains into three separate and distinct blocks. The Tyrone stock occupies the northeastern part of the central Tyrone-Burro Peak block. This block is a horst, uplifted along the Austin-Amazon and Sprouse-Copeland faults. These bounding faults also form the approximate boundaries of the stock. The absence of roof pendants of metamorphic rocks into which the Burro Mountain batholith was intruded, and the deep erosion of the stock are additional evidence of the originally deeper position of the rocks of this block.Mineral deposits of the different blocks are characterized by different metal content. Most distinctive is the virtual absence in the Tyrone-Burro Peak block of all minerals except chalcopyrite, and the ubiquitous pyrite. In the adjacent blocks, lead, zinc, silver, gold, bismuth, and iron as specular hematite, are present in commercial amounts in addition to copper. This difference is believed to be a function both of vertical zoning and the relative uplift of the separate blocks. Copper minerals formed at greater depths, and presumably higher temperatures than other base metal and precious metal minerals, are thus exposed at the surface only in the uplifted central block. All base metal and precious metal deposits that may have existed above the chalcopyrite deposits in this area, have been removed as a consequence of the uplift.

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