Geochronology and geology of the Pebble Cu-Au-Mo porphyry and the sill Au-Ag epithermal deposits, southwest Alaska
Schrader, Christian Michael
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The Late Cretaceous Pebble Cu-Au-Mo deposit is a large Cu and Au resource within a belt of Cu and Au deposits in southwest Alaska. This study presents precise 40Ar/39Ar igneous and alteration ages and a Re-Os mineralization age from the Pebble deposit as well as an alteration age from the Tertiary Sill epithermal Au-Ag deposit, which less than 5 km from the Pebble deposit. These data have utility in regional exploration modeling and provide an opportunity to relate the timing of intrusion to alteration and mineralization. The geologic and geochronologic data document Cretaceous igneous and hydrothermal events from ~96 Ma to 83.5 Ma. These include 1) an early suite of biotitemagnetite- pyroxenites and diorite dated at 95.9 ± 0.3 to 95.3 ± 0.3 Ma, 2) a molybdenum mineralization event in the porphyry stocks dated at 89.5 ± 0.3 Ma, 3) potassic alteration related to copper mineralization dated at 88.5 ± 0.3 Ma (biotite) and 86.0 ±.0.3 Ma (Kfeldspar), and 4) a hypabyssal syenite intrusion dated at 84.1 ± 0.3 Ma. The range of mineralization and alteration ages is due in part to the lower closure temperatures of the alteration silicate minerals relative to molybdenite, and in part to protracted hydrothermal circulation from multiple porphyry stocks. Over 35 m.y. later, in the Tertiary, the Sill latite host rock crystallized and the Sill epithermal mineralizing event occurred.