129-13 Utilizing XRF and Crush-Leach Analysis as Exploration Tools for Gem-Bearing Pegmatite Pockets in the Pikes Peak Granite, Colorado

Abstract:

The Pikes Peak Batholith (PPB) is a prolific ~1 Ga granitic body that hosts pegmatites containing smoky quartz, amazonite, topaz, fluorite & others that attract prospectors worldwide. Exploration for pegmatite gemstone pockets traditionally relies upon surface clues such as “following float”, the presence of iron-rich clay, following zoning patterns & increasing euhedral crystal habit. Even with these clues, the discovery of pockets is still hit-or-miss, requiring time & labor. The pegmatites are classified as Niobium-Yttrium-Fluorine (NYF)-type pegmatites.

During the last stages of pegmatite formation, minerals crystallize from volatile-rich fluids in miarolitic cavities. Afterwards, the residual fluid can migrate into nearby fractures & crystallize as quartz veinlets containing indicator elements of LCT pegmatites such as Zr, Mo, Y, U & Rare Earth Elements (REEs). The veinlets erode to become part of the overlaying soil.

This project utilized soil geochemistry to attempt to identify unique geochemical signatures that can lead to gem-bearing pockets. Clay fraction sediment samples were collected from both known crystal-producing & non-producing pockets & analyzed using portable X-Ray Fluorescence (XRF), while quartz separates underwent crush-leach analysis for fluid inclusion composition. Results focused on calculating enrichment factors (EF) of the above elements compared against average crustal abundances.

The XRF results show significant enrichment in Mo & Zr (max EF values of 42.5 & 10.3, respectively) & moderate enrichment of Th, U, Y & Pb (max EF values of 13.1, 11.1, 7.6 & 7.9, respectively). Crush-leach data revealed minimal REE enrichment. However, other pegmatites within different districts of the PPB are known for enriched REEs (i.e. the South Platte District). The elevated Pb contents could indicate amazonite potential as it’s a contributing chromophore.

Differences between samples & grain sizes indicate that these elements are not uniformly distributed & may be concentrated in surface coatings, fractures, or fluid features rather than the crystal lattice. Combining XRF & crush-leach analyses provides a more complete geochemical picture, revealing both the composition & localized enrichment processes responsible for trace element variability & thus has good potential for exploration upon analyzing more sites.

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