AMMONIUM ABUNDANCES IN THE EOCENE BULLRUN CREEK AND GROUSE CREEK STOCKS AND ASSOCIATED WALLROCKS, NE OREGON

The 36 Ma Bullrun Creek (BR) and the 34 Ma Grouse Creek (GC) stocks in the Blue Mountains of northeastern Oregon were emplaced into shale and sandstone of the Izee terrane; the BR was also emplaced into serpentinized peridotite. The tonalitic GC is host to porphyry Cu mineralization, surrounded by propylitic alteration, and has low delta 18O values (+2.2 to +4.9 per mil). Most GC samples contain secondary epidote, chlorite, calcite, hematite, and/or pyrite. The BR comprises equigranular tonalite, lacks an alteration halo, and has delta 18O more typical of magmatic values (+7.3 to +8.6 per mil). Now-altered dikes of GC intruded the BR. Major and trace element compositions of the GC and BR samples are nearly identical. Gold-bearing hydrothermal actinolite + apatite + magnetite veins within the BR occur normal to the contact between the BR and the serpentinite. Ammonium (NH4+) was extracted from whole rock samples by a modified Kjeldahl method and analyzed colorimetrically. Unaltered BR rocks generally contain low NH4+ values (1.8-6.3 ppm), which are considered to be magmatic; a high-K2O granitic dike has 7.2 ppm NH4+. In contrast, GC rocks contain slightly higher NH4+ (7.8-14.7 ppm). A weak correlation exists between NH4+ and K2O abundances for these samples. Carbonaceous shale from outside the mineralized zone of the GC contains very high NH4+ (104.2 ppm), but shale and skarn samples adjacent to the stock have variable NH4+ contents (4.2-15.3 ppm). GC and BR samples exhibit a weak negative correlation between delta 18O and NH4+ values, which suggests that samples with higher NH4+ (> about 7 ppm) and lower delta 18O (<+7 per mil) have been altered by heated meteoric water (propylitic alteration). Unaltered BR samples exhibit a “magmatic” trend of increasing K+/NH4+ with increasing NH4+ from tonalite to granite. This either suggests that they are related by fractional crystallization or that the granites are partial melts of metasedimentary wallrocks. In contrast, altered GC and BR samples follow an “alteration” trend of decreasing K+/NH4+ with increasing NH4+. These data suggest that NH4+ analyses provide an additional constraint for identifying magmatic versus alteration trends in plutonic suites.