INVESTIGATING THE CRITICAL MINERAL POTENTIAL AND PETROLOGY OF A AU-CU EXPLORATION TARGET, THE MINERAL HILL ALKALINE INTRUSIVE COMPLEX, NORTHEASTERN WYOMING

Mineral Hill is a Tertiary, ring-shaped, multi-phase alkaline igneous complex that intruded Precambrian schists and lower Paleozoic units on the northwest flank of the Black Hills uplift along the Wyoming-South Dakota border. The complex is located 30 km southeast of the carbonatite-hosted REE deposit in the Bear Lodge Alkaline Complex, with rocks of similarly silica-undersaturated and alkali-rich composition. Alkaline igneous systems are important sources of Au, Cu and critical elements (e.g., REE, HFSE, Te). At Mineral Hill, areas of epithermal-style Au-Ag mineralization and porphyry-style Au-Cu mineralization have been identified. The critical mineral potential of the Tertiary intrusive rocks has received comparatively little attention.

The purpose of this study was to assess the critical mineral potential of the alkaline intrusive rocks and improve existing rock classifications based on petrography and geochemistry. Major and trace element concentrations were determined for 103 samples from two core holes by WDXRF, ICP-OES and ICP-MS. The concentrations of CO₂, F, S, Fe²⁺, H₂O and PGE were also determined. Mineralogy was characterized using polarized light microscopy, powder X-ray diffraction and cathodoluminescence.

Major lithologies identified in drill hole 1 include: (1) heterolithic breccia (0-245 m), (2) foid syenite (245-515 m), (3) melteigite-feldspathic ijolite (515-600 m), and (4) porphyritic quartz trachyte-latite porphyry (600-686 m). Drill hole 2 is dominated by coarse-grained clinopyroxene cumulate (jacupirangite) with variable feldspathoid content and up to 15 volume percent apatite. Melanite-bearing melteigite-ijolite samples have the highest total REE concentrations (910-1237 ppm), whereas quartz trachyte samples have the lowest total REE concentrations (249-309 ppm). The range of total REE concentrations for all samples is 220-1562 ppm, which is lower in comparison to silicate rocks of the Bear Lodge Alkaline Complex. All Mineral Hill samples have chondrite-normalized REE patterns steeply dipping from La to Lu, with no Ce or Eu anomalies. Based on the geochemistry of rocks examined during this study, the overall critical mineral potential of the complex is low, although this does not preclude the possibility of critical mineral-enrichment at depth.