New Mexico Mineral Symposium — Abstracts

High-resolution X-ray Computed Tomography (HRXCT) provides imagery of the interior of rock specimens. This new technique, advanced by researchers at the University of Texas at Austin, has been used to create three-dimensional imagery of ore samples in the Cripple Creek mining district, Colorado for a variety of studies. HRXCT, unlike conventional medical CAT-scanning, can resolve extremely small details—down to a few tens of microns in size.

The Cripple Creek mining district is centered on a circular-shaped, Oligocene-age, alkaline volcanic complex that covers just over 7 mi². The complex is surrounded by Precambrian rocks (biotite gneiss, granodiorite, quartz monzonite, granite). Regional tectonic extension at the intersection of these four Precambrian rock units localized the volcanic activity. Laccoliths, cryptodomes, dikes, and sills composed of syenites, phonolites, phonotephrites, and lamprophyres were formed at shallow depths.

The mineralization at the Cripple Creek mining district is characterized by high-grade gold-telluride veins and low-grade disseminated gold and gold tellurides. High-grade gold mineralization is associated with major structural zones and also occurs as sheeted vein zones.

Underground mining operated in the district from 1891 until the 1960s. A small amount of underground mining is currently taking place in the district. Small-scale surface mining, using the heap-leach method, started in 1971 and was followed by large-scale, low-grade, surface mining at Cripple Creek & Victor Gold Mining Company's (CC&V) Cresson project in 1994. The project is expected to continue operating into 2012. CC&V's gold production for 2006 was 283,000 ounces. CC&V reached an important milestone in August 2007 when it passed 3 million ounces of produced gold.

HRXCT technology was applied to samples collected from high-grade gold zones. HRXCT scans sense differences in X-ray absorption due to differences in density within the sample and emphasize these density variations. The digital information is processed from each ore sample, producing a series of two-dimensional images or "slices" of the sample. These series of scans are then stacked in order to generate a three-dimensional data set that will render the rock nearly transparent so that the interior can be seen. These images can be rotated, sliced, and viewed from any angle—allowing a detailed analysis and measurement of internal features. These three-dimensional HRXCT investigations of ore samples examine grain-scale relationships between gold and associated ore minerals, reveal spatial distribution of mineral grains in rocks, and provide insight regarding the nature of structural features and their relationship with gold distribution (preferred mineralization sites). These studies contribute to the understanding of hydrothermal ore-forming systems.

A three-dimensional image was made of an exploration drill core approximately 3.25 inches in diameter and 9 inches long in order to evaluate the economic potential of the exploration target. The image of the opaque rock was nearly transparent with gold-tellurides appearing as red and yellow specks clustered in a single plane. The variation in color of the gold grains is a function of their size and their composition (generally calaverite, sylvanite, or native gold). As a result, the gold content of this piece of drill core is estimated to range between 8 and 23 ounces per ton.

HRXCT technology provides a new way to image rock specimens and has the potential to help understand the emplacement mechanisms that result in high-grade veins and may assist in targeting new resources in ore districts. The evaluation of HRXCT' applications in mining, ore studies, and precious metal recovery is continuing.
 

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