Explained by Marti L. Miller, Thomas K. Bundtzen, Dennis P. Cox, John E. Gray, and Jeffrey D. Phillips.
On the choice of deposit models
Pillow basalt or greenstone units that are associated with ophiolitic sequences may host Cyprus massive sulfide deposits (Singer, 1986). In most parts of southwest Alaska, we lack sufficient geologic information to separate the lava units from the rest of the ophiolite, therefore ophiolite assemblages in general are considered permissive.
On the delineation of permissive tracts
This tract, which consists of scattered ophiolite assemblages, is permissive for the occurrence of Cyprus massive sulfide deposits, although none are known in the region. Ophiolitic rocks are concentrated in two areas of southwestern Alaska. (1) In the north, ophiolite assemblages are in both Holy Cross and Iditarod quadrangles. In the Holy Cross quadrangle these rocks consist of altered basalt, diabase, and scattered ultramafic rocks (Csejtey, 1992; T.K. Bundtzen, written commun., 1992) that may correlate with ophiolitic rocks of the Angayucham or Tozitna terranes (Patton and others, 1992; 1994b). A mafic-ultramafic complex in the northern part of the Iditarod quadrangle (Miller and Bundtzen, 1994) was not included as permissive for Cyprus massive sulfide deposits, even though it too is ophiolitic. This complex was deleted from the permissive tract because it is mapped in enough detail that we are certain it does not contain basalt lavas (Miller, 1990). (2) In the southwestern part of southwest Alaska, ophiolitic rocks are in the Goodnews Bay and Hagemeister Island quadrangles. In this area, mafic-ultramafic complexes and associated pillow basalt and gabbro are interpreted to represent an ophiolite succession (Patton and others, 1994a). Stream-sediment samples from this area are locally anomalous in Co, Cu, Cr, and Ni (Kilburn and others, 1993).
Important examples of this type of deposit
No examples of Cyprus massive sulfide deposits are known in southwest Alaska.
On the numerical estimates made
Although permissive rock sequences are present, the lack of known occurrences of Cyprus massive sulfide type mineralization led to a low estimate of undiscovered deposits. The estimated minimum number of undiscovered Cyprus massive sulfide deposits, consistent with the tonnage and grade model of Singer and Mosier (1986), is:
Percentile 90 50 10 5 1
Estimated number of deposits 0 0 1 2 4
Csejtey, Bela, Jr., compiler, 1992, Reconnaissance geologic compilation map of the Holy Cross quadrangle, Alaska, plate 1 in, Csejtey, Bela, Jr., and Keith, W.J., eds., Preassessment Report on the Holy Cross quadrangle, Alaska: Menlo Park, California, U.S. Geological Survey unpublished administrative report, 52 p., 3 sheets, scale 1:250,000.
Kilburn, J.E., Goldfarb, R.J., Griscom, Andrew, and Box, S.E., 1993, Map showing metallic mineral resource potential in the Goodnews Bay, Hagemeister Island, and Nushagak Bay 1o x 3o quadrangles, southwest Alaska: U.S. Geological Survey Miscellaneous Field Studies Map MF-2228, 4 sheets, scale 1:250,000.
Miller, M.L., 1990, Mafic and ultramafic rocks of the Dishna River area, north-central Iditarod quadrangle, west-central Alaska, in Dover, J.H., and Galloway, J.P., eds., Geologic studies in Alaska by the U.S. Geological Survey, 1989: U.S. Geological Survey Bulletin 1946, p. 44-50.
Miller, M.L., and Bundtzen, T.K., 1994, Generalized geologic map of the Iditarod quadrangle, Alaska, showing potassium-argon, major-oxide, trace-element, fossil, paleocurrent, and archaeological sample localities: U.S. Geological Survey Miscellaneous Field Studies Map MF-2219-A, 48 p., scale 1:250,000.
Patton, W.W., Jr., Box, S.E., and Grybeck, D.J., 1994a, Ophiolites and other mafic-ultramafic complexes in Alaska, chap. 21, in Plafker, George, and Berg, H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-1, p. 671-686.
Patton, W.W., Jr., Box, S.E., Moll-Stalcup, E.J., and Miller, T.P., 1994b, Geology of west-central Alaska, chap. 7, in Plafker, George, and Berg, H.C., eds., The Geology of Alaska: Boulder, Colorado, Geological Society of America, The Geology of North America, v. G-1, p. 241-269.
Patton, W.W., Jr., Murphy, J.M., Burns, L.E., Nelson, S.W., and Box, S.E., 1992, Geologic map of ophiolitic and associated volcanic arc and metamorphic terranes of Alaska (west of the 141st meridian): U.S. Geological Survey Open-File Report OF 92-20A, scale 1:2,500,000.
Singer, D.A., 1986, Descriptive model of Cyprus massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 131.
Singer, D.A., and Mosier, D.L., 1986, Grade and tonnage model of Cyprus massive sulfide, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 131-135.