National mineral assessment tract AK-BR08 (Mississippi Valley Type)

Tract AK-BR08
Geographic region Brooks Range, AK
Tract area 49,825sq km
Deposit type Mississippi Valley Type
Deposit age Mississippian - Pennsylvanian

Rationale

Explained by Jeanine M. Schmidt, Karen D. Kelley, Richard I. Grauch, Byron R. Berger, John W. Cady, Gregory T. Spanski, James E. Kilburn
On the choice of deposit models
Two Mississippi Valley type (MVT) deposits are described in Cox and Singer (1986). The differences between Southeast Missouri Pb-Zn (model 32a; Briskey 1986b) and Appalachian Zn (model 32b; Briskey, 1986a) are subtle enough that the grade and tonnage models for the two deposit types are plotted together with no differentiation between types (Mosier and Briskey, 1986). In Alaska, the lack of detailed geologic information prevents distinguishing between the two deposit types, and a generic Mississippi Valley type model has been used.
The geologic setting and host rocks, together with several known fluorite occurrences, suggest that this tract is permissive for Mississippi Valley type deposits,
Two Mississippi Valley type (MVT) deposits are described in Cox and Singer (1986). The differences between Southeast Missouri Pb-Zn (model 32a; Briskey 1986b) and Appalachian Zn (model 32b; Briskey, 1986a) are subtle enough that the grade and tonnage models for the two deposit types are plotted together with no differentiation between types (Mosier and Briskey, 1986). In Alaska, the lack of detailed geologic information prevents distinguishing between the two deposit types, and a generic Mississippi Valley type model has been used.
On the delineation of permissive tracts
This tract is defined by the carbonate rocks of the Mississippian to Pennsylvanian Lisburne Group. These are mainly platform carbonates, with some slope and deeper-water lithologies.
This tract comprises the combined areas of all the tracts throughout the state that are permissive for Mississippi Valley type deposits, and is defined predominately by the distribution of carbonate rocks. These are mainly platform carbonates, but also include slope and deeper-water lithologies.
Important examples of this type of deposit
Several occurrences of fluorite within the Lisburne Group carbonates in the eastern Brooks Range (Porcupine River etc.) are unexplained but could indicate fluid movement associated with Mississippi Valley type mineralization.
The Reef Ridge deposit in the Medfra quadrangle of west-central Alaska is an example of a Mississippi Valley type deposit (Andrews and Rishel, 1982). The Powdermilk deposit in the Brooks Range (Schmidt and Folger, 1986) and the Step deposit in East Central (Schmidt, 1997) may also be Mississippi Valley type deposits.
On the numerical estimates made
Although grade and tonnage curves are available for Mississippi Valley type deposits, geologic knowledge about this tract inadequate to attempt an estimate of the number of undiscovered deposits. No basinal sources of fluids have been identified because of the difficulty of reconstructing the pre-Jurassic folding- and thrusting paleogeography, and the timing of any possible Mississippi Valley type mineralizing episodes is totally unconstrained.
No basinal sources of fluids have been identified because of the difficulty of reconstructing the pre-Jurassic folding and thrusting paleogeography, and the timing of any possible Mississippi Valley type mineralizing episodes is totally unconstrained. In addition, the geology of most of the individual tracts is poorly known. Therefore, we have attempted to look at the state as a whole to project what little knowledge is available over a larger area. The estimated minimum number of undiscovered Mississippi Valley Type deposits in Alaska, based on the grade and tonnage model of Mosier and Briskey (1986) is:
Percentile 90 50 10 5 1
Estimated number of deposits 1 2 4 6 6
References
Briskey, J.A., 1986a, Descriptive model of Appalachian Zn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 222-223.
Briskey, J.A., 1986b, Descriptive model of southeast Missouri Pb-Zn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 220-221.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 130.
Mosier, D.L., and Briskey, 1986, Grade and tonnage model of southeast Missouri Pb-Zn and Appalachian Zn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 224-226.
AK02 Mississippi Valley Type Pb-Zn Alaska
by: Thomas D. Light, Jeanine M. Schmidt, Byron R. Berger, Larry J. Drew, Bruce M. Gamble, Donald Grybeck, Marti L. Miller, and Frederick H. Wilson
Andrews, T., and Rishel, John, 1982, 1981 Annual Report-Reef Ridge I-XIII project areas, Kuskokwim Block, Volume 1 of 3: unpublished report, Patino Inc., Anchorage, Alaska, 49 p.
Briskey, J.A., 1986a, Descriptive model of Appalachian Zn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 222-223.
Briskey, J.A., 1986b, Descriptive model of southeast Missouri Pb-Zn, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 220-221.
Cox, D.P., and Singer, D.A., eds., 1986, Mineral deposit models: U.S. Geological Survey Bulletin 1693, 379 p.
Mosier, D.L., and Briskey, 1986, Grade and tonnage model of southeast Missouri Pb-Zn and Appalachian Zn deposits, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 224-226.
Schmidt, J.M., 1997, Strata-bound carbonate-hosted Zn-Pb and Cu deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 90-119.
Schmidt, J.M., and Folger, P.F., 1986, Pb-Zn-Ag mineralization in Paleozoic dolostones, Powdermilk prospect, Baird Mountains B-4 quadrangle, in Bartsch-Winkler, Susan, and Reed, K.M., eds., Geologic studies in Alaska by the U.S. Geological Survey during 1985: U.S. Geological Survey Circular 978, p. 19-21.

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