ISOTOPIC AND GEOCHEMICAL STUDIES OF HOST ROCK PYROXENITES ASSOCIATED WITH CU-NI-PGE MINERALIZATION IN THE DUKE ISLAND COMPLEX, ALASKA

Recent exploration of the Duke Island Complex, southeastern Alaska, has outlined extensive zones of massive to disseminated sulfide segregations in the olivine clinopyroxenite unit of the Complex. Assays show a maximum of 2.08% Cu, 0.25% Ni, and 1 gram/ton combined Pt + Pd. Ni in olivine ranges from less than 100 ppm to ~1000 ppm. Olivine in sulfide-poor rock types shows a negative correlation between Ni and Fe, which is a characteristic of fractional crystallization. Marked depletion of Ni in olivine from sulfide rich domains is indicative of Fe-Ni exchange between olivine and sulfide minerals.

¹⁸⁷Re/ ¹⁸⁸Os ratios in clinopyroxenites show a wide range between 7 and 460. ¹⁸⁷Os/¹⁸⁸Os ratios are also elevated (2.7 to 4.8) with γ_Os values computed for 110Ma (U-Pb zircon age) of 2000 to 5000. The supra-chondritic Os isotopic values are suggestive of assimilation of crustal materials, which is also corroborated by the spread in the δ ³⁴S values of the sulfides between -7 and +4‰. δ ¹⁸O values of clinopyroxene in the clinopyroxenite and olivine clinopyroxenite samples range between 5 and 6‰, whereas those for olivine range between 4.4 and 5.7‰. The δ ¹⁸O values of clinopyroxene and olivine are not indicative of contamination by high-δ ¹⁸O crustal rocks and strongly suggest that assimilation was highly selective.

The scatter found in the Re-Os data suggests that hydrothermal activity may have perturbed the system. Interaction with hydrothermal fluids is further evidenced by the serpentinization of olivine and the olivine δ ¹⁸O values that are below 5‰. The δ ¹⁸O and δD values of the hydrothermal fluid calculated from the δ ¹⁸O and δD values of serpentine, using appropriate fractionation factors and a temperature of 300°C are near 3 and -73‰, respectively. The fluid isotopic composition can be explained in the following ways: (1) mixing of magmatic, meteoric and sea water, (2) the initial fluid causing the serpentinization was predominantly sea water, but the serpentine underwent subsequent hydrogen isotopic exchange with meteoric water to result in its present composition, (3) the serpentinization was caused by meteoric water with a low time integrated water/rock ratio. The hydrothermal system at Duke Island may have caused the redistribution of base metals and PGE in the intrusion.