REMOTE DETECTION OF ARGILLIC ALTERATION IN QUARTZITES AND QUARTZ ARENITES ABOVE AND DISTAL TO PORPHYRY CU AND MO DEPOSITS: IMPLICATIONS FOR ASSESSMENTS OF CONCEALED DEPOSITS

Hydrothermal alteration in volcanic rocks above porphyry deposits is easy to detect and has been extensively studied and mapped. However, where erosion has removed the volcanic cover, traditional field methods typically do not detect subtle alteration in sedimentary and metasedimentary rocks. Ongoing deposit studies in western Utah that incorporate the analysis of satellite (ASTER) and airborne (AVIRIS) remote sensing data for mineral mapping have identified argillic and local advanced argillic alteration (alunite, pyrophyllite, dickite, or kaolinite) in quartzites and quartz arenites situated stratigraphically above (1 km) and distal (6 km) to known but concealed porphyry Mo and porphyry Cu deposits.

In the Bingham district, an argillic alteration halo was mapped in Permian calcareous sandstone and orthoquartzite (kaolinite with local dickite and supergene alunite along fractures). The halo envelops the Melco distal disseminated Au-Ag deposit and extends 6-8 km farther north than the potassic and phyllic halos that surround the associated porphyry Cu deposit.

In the Park City district, pyrophyllite + sericite ± goethite were mapped on exposed alpine ridges along faults in quartz arenites of the Triassic Woodside Formation. These exposures are ~1 km above and 6-7 km to the side of a concealed porphyry Cu resource.

In the Tintic district, breccia pipes and zoned polymetallic replacement, porphyry Cu, and vein deposits are associated with a cluster of Oligocene intrusions. Pyrophyllite or dickite ± pyrite ± hematite were mapped along fracture zones in Cambrian Tintic quartzite adjacent to the intrusions, as well as in an area 6 km NW of Eureka that possibly indicates an unrecognized cupola in that area.

Near Marysvale, dickite was identified in Jurassic Navajo Sandstone ~1 km above the Deer Trail Mo- and F-bearing polymetallic manto deposits and just below magmatic steam alunite veins in volcanic cover above the Navajo.

In the Pine Grove district, advanced argillic alteration along fractures in faulted sericitic quartzites forms a halo that extends more than 6 km from the buried (~1 km) Pine Grove porphyry Mo deposit.

These field-verified results suggest that remote detection of the argillic halos can be used to efficiently target concealed porphyry Cu, Mo, and related mineral resources in sedimentary terranes.