Abstract

The Lone Star porphyry copper deposit in the Safford District of southeastern Arizona was discovered in the late 1800's but never mined on a large scale. In addition to typical copper oxide species such as chrysocolla, the upper part of the deposit has zones of mineralization in which the chemical assays of core samples have higher amounts of copper than can be visually assigned to the observed copper-bearing minerals. The goal of this study is to identify the Cu-bearing minerals, which is crucial because the efficiency of the extraction processes is strongly dependent upon the mineralogy. Samples from seven cores with a range of copper contents and observed copper minerals were collected for analysis. Elemental compositions have been determined by X-ray fluorescence spectrometry and show a large variability in copper content (ranging from 360 ppm to 4.7 wt. %). Mineral assemblages were determined by optical microscopy, energy dispersive spectroscopy on the scanning electron microscope, and X-ray diffraction analysis and show varying concentrations of possible Cu-hosting minerals such as chlorite, biotite, iron oxides/hydroxides, and clay. Copper element maps were created for selected samples using an electron microprobe and areas of elevated copper concentration were more closely mapped with quantitative analyses taken of many points. This identified some common copper minerals that were simply too small to be seen in hand sample. However, several other minerals also contain high concentrations of Cu including chlorite and biotite (up to 19.3 wt. % CuO), iron oxides/hydroxides (up to 5.2 wt. % CuO), and clay (up to 7.3 wt. % CuO). While it has been determined that there is copper substitution into the structure of these minerals, transmission electron microscope analysis shows some of the copper in the chlorite and biotite is in native Cu blebs between the phyllosilicate sheets. The iron oxides and hydroxides are nanometer-sized particles with large surface areas for adsorption of copper. This presents a multi-phase system of copper minerals that cannot be seen during normal core logging. The presence of ore-grade concentrations of copper in the oxidized zone is likely due to insufficient acid production, as Lone Star is a sulfur-poor deposit, and the neutralizing effect of the andesite that hosts the deposit.

Degree

College and Department

Physical and Mathematical Sciences; Geological Sciences

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