Geologic descriptionAn unpublished National Park Service map and accompanying database identify this locality as a porphyry molybdenum prospect. Bedrock in this part of Monte Cristo Creek is marine and subaerial volcanic and volcaniclastic rocks of the Lower Cretaceous, Chisana Formation (Richter, 1976). Nearly horizontal volcanic rocks of the upper Tertiary and Quaternary, Wrangell Lava unconformably overly the Chisana Formation. The Chisana Formation is extensively hydrothermally altered and characteristically replaced by gypsum-quartz-sulfide assemblages (Richter, 1976). Tertiary hornblende-plagioclase porphyry crops out downstream in Monte Cristo Creek in the Nabesna A-4 quadrangle. A K/Ar age for this porphyry is 17 Ma (Richter, 1973). A porphyry molybdenite prospect (NB037) located downstream just inside the Nabesna A-4 quadrangle is at the contact of a granodiorite pluton inferred to be mid-Cretaceous in age (Richter, 1976). |
Geologic map unit |
(-143.020024845583, 62.1984084255318) |
Mineral deposit model |
Porphyry Mo, low-F (Cox and Singer, 1986; model 21b). |
Mineral deposit model number |
21b |
Age of mineralization |
Cretaceous or Tertiary? Hydrothermal alteration may be related to emplacement of hornblende-plagioclase porphyry downstream in the Nabesna A-4 quadrangle that has a 17 Ma K/Ar age (Richter, 1973). A porphyry molybdenite prospect (NB037) downstream just inside the Nabesna A-4 quadrangle is at the contact of a granodiorite pluton inferred to be mid-Cretaceous (Richter and others, 1975). |
Alteration of deposit |
Gypsum-quartz-sulfide (pyrite?) replacement of Chisana Formation volcanic rocks. Oxidation is evidenced by abundant iron-oxide staining. |