Unnamed (south of Monte Cristo Creek) (ARDF #NB036) Mo

Unnamed (south of Monte Cristo Creek)

Occurrence, Inactive

Commodities and mineralogy

Main commodities	Mo
Ore minerals	molybdenite; pyrite
Gangue minerals	gypsum; quartz

Geographic location

Quadrangle map, 1:250,000-scale	NB
Quadrangle map, 1:63,360-scale	A-5
Latitude	62.1988
Longitude	-143.0179
Nearby scientific data	Find additional scientific data near this location
Location and accuracy	This occurrence is at an elevation of about 4,500 feet on the south valley wall of Monte Cristo Creek, about 1.1 miles southeast of elevation 6210, and 1.4 miles east-northeast of elevation 5535. The site is in the NW1/4 of section 21, T. 5 N., R. 13 E. of the Copper River Meridian. This is National Park Service locality WRST-21 (unpublished data) and the coordinates for this occurrence were provided by the National Park Service.

Geologic setting

Geologic description An unpublished National Park Service map and accompanying database identify this locality as a porphyry molybdenum prospect.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.

Production and reserves

Indication of production	None
Additional comments The prospect is in the Wrangell-St. Elias National Preserve.

References

References Richter, D.H., 1976, Geologic map of the Nabesna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Series Map I-932, 1 sheet, scale 1:250,000. Richter, D.H., Singer, D.A., and Cox, D.P., 1975, Mineral resources map of the Nabesna quadrangle, Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-655-K, 1 sheet, scale 1:250,000.
Reporters	Travis L. Hudson (Applied Geology)
Last report date	11/24/2002