Robert G. Eppinger

Porphyry Cu indicator minerals in till as an exploration tool: example from the giant Pebble porphyry Cu-Au-Mo deposit, Alaska, USA

ABSTRACT Porphyry Cu indicator minerals are mineral species in clastic sediments that indicate the presence of mineralization and hydrothermal alteration associated with porphyry Cu and associated skarn deposits. Porphyry Cu indicator minerals recovered from shallow till samples near the giant Pebble Cu-Au-Mo porphyry deposit in SW Alaska, USA, include apatite, andradite garnet, Mn-epidote, visible gold, jarosite, pyrite, and cinnabar. Sulphide minerals other than pyrite are absent from till, most likely due to the oxidation of the till. The distribution of till samples with abundant apatite and cinnabar suggest sources other than the Pebble deposit. With three exceptions, all till samples up-ice of the Pebble deposit contain <10 grains/10kg of garnet (0.25–0.5 mm). Samples in the immediate vicinity of the Pebble deposit contain 10–20 grains, whereas samples with the most grains (>40grains/10kg) are in close proximity to smaller porphyry and skarn occurrences in the region. The distribution of Mn-epidote closely mimics the distribution of garnet in the till samples and further supports the interpretation that these minerals most likely reflect skarns associated with the porphyry deposits. All but two till samples, including those up-ice from the deposit, contain some gold grains. However, tills immediately west and down-ice of Pebble contain more abundant gold grains, and the overall number of grains decreases in the down-ice direction. Furthermore, all samples in the immediate vicinity of Pebble contain more than 65 % pristine and modified grains compared to mostly re-shaped grains in distal samples. The pristine gold in till reflects short transport distances and/or liberation of gold during in-situ weathering of transported chalcopyrite grains. Jarosite is also abundant (1–2 500 grains/10kg) in samples adjacent to and up to 7 km down-ice from the deposit. Most jarosite grains are rounded and preliminary Ar/Ar dates suggest the jarosite formed prior to glaciation and it implies that a supergene cap existed over Pebble West. Assuming this interpretation is accurate, it suggests a shallow level of erosion of the Pebble deposit by glacial processes. Overall the results of this study indicate that porphyry Cu indicator minerals in till samples may be useful in the exploration for porphyry deposits in SW Alaska.

Laser-excited fluorescence of rare earth elements in fluorite: Initial observations with a laser Raman microprobe

Fluorescence emission spectra of three samples of fluorite containing 226–867 ppm total rare earth elements (REE) were excited by visible and ultraviolet wavelength lines of an argon ion laser and recorded with a Raman microprobe spectrometer system. Narrow emission lines (< 1 nm) due to 4f-4f electron transitions in individual trivalent REE (Pr, Nd, Sm, Eu?, Tb, Dy, Ho, Er, Tm) were observed in the wavelength range of 400–900 nm. Emission from individual REE occur in bands of overlapping lines in the wavelength intervals of 470–495 nm, 535–560 nm, 565–580 nm, 585–620 nm, 640–643 nm, 671.4 nm, and 758.2 nm. A broad band at 419 nm excited by the 363.8-nm ultraviolet line of the laser is due to a 4f-5 d transition in Eu2+. Two bands of enigmatic origin are a narrow line at 682.8 nm present at all excitation wavelengths in only one sample and a broad band at 720 nm. We have tentatively assigned individual REE to specific lines in each emission band based on selection rules for strongly and weakly allowed 4f-4f transitions and the position of absorption and emission bands documented in the literature for REE in CaF2 and LaF3 host crystals. Working curves of integrated peak intensity of emission from Er3+ and Eu2+ vs. ppm measured by ICP-MS give linear log-log fits with R2 > 0.9 for Eu2+ and 0.99 for Er3+. Detection limits for three micrometer spots are about 0.01 ppm Eu2+ and 0.07 ppm Er3+. These limits are less than chondrite abundance for Eu and Er, demonstrating the potential microprobe analytical applications of laser-excited fluorescence of REE in fluorite. However, application of this technique to common rock-forming minerals may be hampered by competition between fluorescence emission and radiationless energy transfer processes involving lattice phonons.

Biogeochemical Characterization of an Undisturbed Highly Acidic, Metal-Rich Bryophyte Habitat, East-Central Alaska, U.S.A

ABSTRACT We report on the geochemistry of soil and bryophyte-laden sediment and on the biogeochemistry of willows growing in an undisturbed volcanogenic massive sulfide deposit in the Alaska Range ecoregion of east-central Alaska. We also describe an unusual bryophyte assemblage found growing in the acidic metal-rich waters that drain the area. Ferricrete-cemented silty alluvial sediments within seeps and streams are covered with the liverwort Gymnocolea inflata whereas the mosses Polytrichum commune and P. juniperinum inhabit the area adjacent to the water and within the splash zone. Both the liverwort-encrusted sediment and Polytrichum thalli have high concentrations of major and trace metal cations (e.g., Al, As, Cu, Fe, Hg, La, Mn, Pb, and Zn). Soils in the area do not reflect the geochemical signature of the mineral deposit and we postulate they are influenced by the chemistry of eolian sediments derived from outside the deposit area. The willow, Salix pulchra, growing mostly within and adjacent to the larger streams, has much higher concentrations of Al, As, Cd, Cr, Fe, La, Pb, and Zn when compared to the same species collected in non-mineralized areas of Alaska. The Cd levels are especially high and are shown to exceed, by an order of magnitude, levels demonstrated to be toxic to ptarmigan in Colorado. Willow, growing in this naturally occurring metal-rich Red Mountain alteration zone, may adversely affect the health of browsing animals.

An exploration hydrogeochemical study at the giant Pebble porphyry Cu-Au-Mo deposit, Alaska, USA, using high-resolution ICP-MS

A hydrogeochemical study using high resolution ICP-MS was undertaken at the giant Pebble porphyry Cu-Au-Mo deposit and surrounding mineral occurrences. Surface water and groundwater samples from regional background and the deposit area were collected at 168 sites. Rigorous quality control reveals impressive results at low nanogram per litre (ng/l) levels. Sites with pH values below 5.1 are from ponds in the Pebble West area, where sulphide-bearing rubble crop is thinly covered. Relative to other study area waters, anomalous concentrations of Cu, Cd, K, Ni, Re, the REE, Tl, SO42− and F− are present in water samples from Pebble West. Samples from circum-neutral waters at Pebble East and parts of Pebble West, where cover is much thicker, have anomalous concentrations of Ag, As, In, Mn, Mo, Sb, Th, U, V, and W. Low-level anomalous concentrations for most of these elements were also found in waters surrounding nearby porphyry and skarn mineral occurrences. Many of these elements are present in low ng/l concentration ranges and would not have been detected using traditional quadrupole ICP-MS. Hydrogeochemical exploration paired with high resolution ICP-MS is a powerful new tool in the search for concealed deposits.