Volume on geology of the Ellsworth MountainsâProgress in 1984 ...
Volume on geology of the Ellsworth Mountains—Progress in 1984-1985 G.F. WEBERS Macalester College St. Paul, Minnesota 55105
J.F. SPLETTSTOESSER Minnesota Geological Survey University of Minnesota St. Paul, Minnesota 55114
Much of the past year was engaged in continued coordination of the production of a volume on the geology and paleontology of the Ellsworth Mountains. Production of the volume has undergone a transition from data reduction and processing of rock and fossil material by senior investigators since the 1979 - 1980 field season, to preparation and submittal of final manuscripts, most of which had been reviewed by the end of May 1985. Final versions of all 25 chapters planned for the volume are expected to be completed by the authors by mid-1985, and submitted by the editors (C. Craddock, J. F. Splettstoesser, and G. F. Webers) to the Geological Society of America (GSA) shortly afterward. Printing is expected in 1986. The volume is proposed for the GSA Memoir series. A highlight of the volume will be the colored geologic map (scale 1:250,000) of the entire range, which will be printed in one sheet by Williams and Heintz Map Corporation.
Study of sulfide mineral distribution in the Dufek intrusion J.L. DRINKWATER, A.B. FORD, and G.K. CZAMANSKE U.S. Geological Survey Menlo Park, California 94025
We have investigated the occurrence and distribution of sulfide minerals in the layered gabbroic Dufek intrusion (82°30'S 50°W) in preparation for mineralogical studies by electron-microprobe analysis. Studies of cumulus pyroxenes and oxides (Himmelberg and Ford 1976, 1977) and plagioclase (Abel, Himmelberg, and Ford 1979) show compositional (fractionation) trends generally comparable to those of other differentiated layered mafic intrusions (Wager and Brown 1968). Presently little-studied sulfide minerals include pyrite, pyrrhotite, and chalcopyrite; bornite(?) is reported by Walker (1961). Our study was made by reflected-light examination of approximately 2,300 standard thin sections representing all lithologies from throughout the exposed layered sequence (samples were 50
Some of the rock and fossil material will continue to be studied by many of the investigators who participated in the 1979 1980 field season under the direction of G.E Webers, and also by other specialists. Several unresolved geologic problems remain enigmatic, and further field work is contemplated for a later time, especially to supplement existing faunal collections and to examine in detail structural relations of selected areas in the Heritage Range. A review of geologic studies in the Ellsworth Mountains was presented as a poster paper at the 97th Annual Meeting of the Geological Society of America, 5 - 8 November 1984, Reno, Nevada (Splettstoesser, Webers, and Craddock 1984). The annotated bibliography of the Ellsworth Mountains (Webers and Splettstoesser 1982), which is being compiled on a continuing basis, included about 150 citations as of May 1985. The bibliography is stored on a word processor disc at the Minnesota Geological Survey. A copy is available from the authors on request. This research was supported by National Science Foundation grants to Macalester College (G.F. Webers, principal investigator).
References Splettstoesser, J.F., G.F. Webers, and C. Craddock. 1984. Geologic studies in the Ellsworth Mountains, Antarctica—A 25-year journey from the unknown. Geological Society of America Abstracts With Programs,
16(6), 665. Webers, G.E, and J.E Splettstoesser. 1982. Geology, paleontology, and bibliography of the Ellsworth Mountains. Antarctic Journal of the U.S., 17(5), 36 - 38.
gathered during the 1965 - 1966, 1976 - 1977, and 1978 - 1979 austral summers). Sulfide amounts were estimated and ranked as "trace" (less than 0.1 percent), "minor" (0.1 to 0.5 percent), and "major" (greater than 0.5 percent). Samples containing largest amounts were point counted for greater accuracy. Of the intrusion's estimated 8 to 9 kilometers thickness (Ford 1976), about 1.8 kilometers of lower units are exposed in Dufek Massif (Ford, Schmidt, and Boyd 1978), and about 1.7 kilometers of uppermost units make up the Forrestal Range (Ford et al. 1978). Hidden parts inferred are a 1.8- to- 3.5-kilometer-thick basal section and a 2- to- 3-kilometer thick intermediate interval covered by an icefield between the two ranges. Exposed rocks are chiefly well-layered gabbroic cumulates that contain interlayers of pyroxenitic and anorthositic cumulates. Iron-titanium oxides (chiefly of the magnetite series) are scarce in Dufek Massif, but common (2 to 10 percent) and locally dominant (layers of magnetitite) in the Forrestal Range. Granophyre of a capping layer and felsic dikes are the magma's latest differentiates. General occurrence. Sulfides chiefly occur disseminated between cumulus silicates and oxides, and concentrations vary significantly along and across layering. They are much more common in upper than lower parts of the intrusion, in which occurrences are generally of a different type. Conspicuous ANTARCTIC JOURNAL
bright-green weathering efflorescences of atacamite typically make occurrences appear more significant than warranted by the few primary sulfide grains generally present. Dufek Massif (lower) section. Many rocks of Dufek Massif appear barren of sulfides, but trace or minor amounts occur locally. Of 1,200 samples examined, only about 12 percent were found to contain sulfides, commonly a few small grains and generally less than about 0.5 percent. Chief occurrences are within small autoliths (inclusions) of noncumulus fine-grained gabbro or metasedimentary xenoliths of hornfels and calc-silicate rock, or in cumulates adjacent to the inclusions. The inclusions are probably fragments carried in from a concealed contact zone and deposited by magma currents (Himmelberg and Ford 1983). A maximum of about 4 modal percent sulfides was found in one caic-silicate inclusion. An origin by contamination with sulfur of country rocks and minor redistribution of sulfides into cumulates seems to be indicated. Forrestal Range (upper) section. About 22 percent of the 1,100 samples examined from the Forrestal Range were found to contain sulfides, of which about 10 percent contain 0.1 to 3.0 modal percent sulfides (mostly in the lower half of the range). A maximum sulfide amount of 12 modal percent occurs in one sample from a layer of leucogabbroic cumulate. Sulfides generally occur in magnetite-rich gabbro or in and near thin layers of magnetitite. They seem to be mostly concentrated in an interval about 200 to 500 meters above the base of the section and in another about 100 to 300 meters below the capping layer of granophyre. Minor sulfide amounts occur in a layer of noncumulus mafic-rich diorite and quartz diorite, up to 50 meters thick, underlying the granophyre. Traces occur locally in the granophyre and in some aplite dikes. In the cumulates, sulfides commonly occur within interstitial space fillings; as inclusions within oxide grains; as fracture fillings; and, more locally, as (replacement?) stringers, lenses, and patches in altered postcumulus amphibole and biotite. Discussion. During fractional crystallization of Dufek Massif cumulates, the magma evidently was undersaturated with respect to sulfur, and an immiscible magmatic sulfide phase was not present. Sulfide occurrences and concentrations in Forrestal Range cumulates are generally similar to those in the ironenriched upper zone of South Africa's Bushveld Complex (von Gruenewaldt 1976). A magma may reach sulfur saturation due to lowering of its iron content, for example by crystallization of magnetite or other iron-rich minerals (von Gruenewaldt 1976, 1979). As in the Bushveld magma's later crystallization stage (upper zone), the Dufek magma eventually crystallized large amounts of magnetite, and at times small quantities of immiscible sulfide droplets separated and settled to the floor. The largest Bushveld sulfide deposits (critical zone), however, are at a much lower stratigraphic position that is probably comparable in the Dufek intrusion to a height in the middle or upper part of its hidden basal section. Sulfides are collectors of platinumgroup elements (PGE) from magmas (Campbell, Naldrett, and
1985 REVIEW
Barnes 1983), and PGE may become concentrated in a magma and fractionate from late iron-enriched melts (Parry 1984). This process may account for the markedly greater, though low, PGE contents in Forrestal Range as compared to Dufek Massif cumulates (Ford et al. 1983). Our further study will focus on the mineralogy and compositional variations of the sulfide minerals and on relations between sulfide concentrations and magnetiterich layers of the intrusion. This work was supported in part by National Science Foundation grant DPP 80-20753. References Abel, K.D., G.R. Himmelberg, and A.B. Ford. 1979. Petrologic studies of the Dufek intrusion: Plagioclase variation. Antarctic Journal of the U.S., 14(5), 6 - 8. Campbell, I.H., A.J. Naldrett, and S.J. Barnes. 1983. A model for the origin of the platinum-rich sulfide horizons in the Bushveld and Stillwater Complexes. Journal of Petrology, 25(2), 133 - 165. Ford, A.B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion, Antarctica. (U.S. Geological Survey Bulletin 1405-D.) Washington, D.C.:
U.S. Government Printing Office. Ford, A.B., R. E. Mays, J. Haffty, and B. P. Fabbi. 1983. Reconnaissance of minor metal abundances and possible resources of the Dufek intrusion, Pensacola Mountains. In Oliver, R.L., J.R. James, and J.B. Jago (Eds.), Antarctic Earth science. Canberra: Australian Academy of Science. Ford, A.B., D.L. Schmidt, and W.W. Boyd, Jr. 1978. Geologic map of the Davis Valley quadrangle and part of the Cordiner Peaks quadrangle, Pensacola Mountains, Antarctica (scale 1:250,000). (U.S. Geological Survey
Map A -. 10.) Washington, D.C.: U.S. Government Printing Office. Ford, A.B., D.L. Schmidt, W.W. Boyd, Jr., and W.H. Nelson. 1978. Geologic Map of the Saratoga Table quadrangle, Pensacola Mountains, Antarctica (scale 1:250,000). (U.S. Geological Survey Map A - 9.) Wash-
ington, D.C.: U.S. Government Printing Office. Himmelberg, G. R., and A.B. Ford. 1976. Pyroxenes of the Dufek intrusion, Antarctica. Journal of Petrology, 17(2), 219 - 243. l-Iimmelberg, G.R., and A.B. Ford. 1977. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist, 62, 623 - 633. Himmelberg, G.R., andA.B. Ford. 1983. Composite inclusion of olivine gabbro and calc-silicate rock in the Dufek intrusion, a possible fragment of a concealed contact zone. Antarctic Journal of the U.S., 18(5), 1 4.
Parry, S.J. 1984. Abundance and distribution of palladium, platinum, iridium, and gold in some oxide minerals. Chemical Geology, 43, 115125. von Gruenewaldt, G. 1976. Sulfides in the Upper Zone of the eastern Bushveld Complex. Economic Geology, 71, 1324 - 1336. von Gruenewaldt, G. 1979. A review of some recent concepts of the Bushveld Complex, with particular reference to sulfide mineralization. Canadian Mineralogist, 17, 233 - 256. Wager, L.R., and G.M. Brown. 1968. Layered igneous rocks. Edinburgh: Oliver and Boyd. Walker, P.T. 1961. Study of some rocks and minerals from the Dufek Massif, Antarctica. International Geophysical Year World Data Center A, Glaciology Report, 4, 195 - 213.
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J.F. SPLETTSTOESSER Minnesota Geological Survey University of Minnesota St. Paul, Minnesota 55114
Much of the past year was engaged in continued coordination of the production of a volume on the geology and paleontology of the Ellsworth Mountains. Production of the volume has undergone a transition from data reduction and processing of rock and fossil material by senior investigators since the 1979 - 1980 field season, to preparation and submittal of final manuscripts, most of which had been reviewed by the end of May 1985. Final versions of all 25 chapters planned for the volume are expected to be completed by the authors by mid-1985, and submitted by the editors (C. Craddock, J. F. Splettstoesser, and G. F. Webers) to the Geological Society of America (GSA) shortly afterward. Printing is expected in 1986. The volume is proposed for the GSA Memoir series. A highlight of the volume will be the colored geologic map (scale 1:250,000) of the entire range, which will be printed in one sheet by Williams and Heintz Map Corporation.
Study of sulfide mineral distribution in the Dufek intrusion J.L. DRINKWATER, A.B. FORD, and G.K. CZAMANSKE U.S. Geological Survey Menlo Park, California 94025
We have investigated the occurrence and distribution of sulfide minerals in the layered gabbroic Dufek intrusion (82°30'S 50°W) in preparation for mineralogical studies by electron-microprobe analysis. Studies of cumulus pyroxenes and oxides (Himmelberg and Ford 1976, 1977) and plagioclase (Abel, Himmelberg, and Ford 1979) show compositional (fractionation) trends generally comparable to those of other differentiated layered mafic intrusions (Wager and Brown 1968). Presently little-studied sulfide minerals include pyrite, pyrrhotite, and chalcopyrite; bornite(?) is reported by Walker (1961). Our study was made by reflected-light examination of approximately 2,300 standard thin sections representing all lithologies from throughout the exposed layered sequence (samples were 50
Some of the rock and fossil material will continue to be studied by many of the investigators who participated in the 1979 1980 field season under the direction of G.E Webers, and also by other specialists. Several unresolved geologic problems remain enigmatic, and further field work is contemplated for a later time, especially to supplement existing faunal collections and to examine in detail structural relations of selected areas in the Heritage Range. A review of geologic studies in the Ellsworth Mountains was presented as a poster paper at the 97th Annual Meeting of the Geological Society of America, 5 - 8 November 1984, Reno, Nevada (Splettstoesser, Webers, and Craddock 1984). The annotated bibliography of the Ellsworth Mountains (Webers and Splettstoesser 1982), which is being compiled on a continuing basis, included about 150 citations as of May 1985. The bibliography is stored on a word processor disc at the Minnesota Geological Survey. A copy is available from the authors on request. This research was supported by National Science Foundation grants to Macalester College (G.F. Webers, principal investigator).
References Splettstoesser, J.F., G.F. Webers, and C. Craddock. 1984. Geologic studies in the Ellsworth Mountains, Antarctica—A 25-year journey from the unknown. Geological Society of America Abstracts With Programs,
16(6), 665. Webers, G.E, and J.E Splettstoesser. 1982. Geology, paleontology, and bibliography of the Ellsworth Mountains. Antarctic Journal of the U.S., 17(5), 36 - 38.
gathered during the 1965 - 1966, 1976 - 1977, and 1978 - 1979 austral summers). Sulfide amounts were estimated and ranked as "trace" (less than 0.1 percent), "minor" (0.1 to 0.5 percent), and "major" (greater than 0.5 percent). Samples containing largest amounts were point counted for greater accuracy. Of the intrusion's estimated 8 to 9 kilometers thickness (Ford 1976), about 1.8 kilometers of lower units are exposed in Dufek Massif (Ford, Schmidt, and Boyd 1978), and about 1.7 kilometers of uppermost units make up the Forrestal Range (Ford et al. 1978). Hidden parts inferred are a 1.8- to- 3.5-kilometer-thick basal section and a 2- to- 3-kilometer thick intermediate interval covered by an icefield between the two ranges. Exposed rocks are chiefly well-layered gabbroic cumulates that contain interlayers of pyroxenitic and anorthositic cumulates. Iron-titanium oxides (chiefly of the magnetite series) are scarce in Dufek Massif, but common (2 to 10 percent) and locally dominant (layers of magnetitite) in the Forrestal Range. Granophyre of a capping layer and felsic dikes are the magma's latest differentiates. General occurrence. Sulfides chiefly occur disseminated between cumulus silicates and oxides, and concentrations vary significantly along and across layering. They are much more common in upper than lower parts of the intrusion, in which occurrences are generally of a different type. Conspicuous ANTARCTIC JOURNAL
bright-green weathering efflorescences of atacamite typically make occurrences appear more significant than warranted by the few primary sulfide grains generally present. Dufek Massif (lower) section. Many rocks of Dufek Massif appear barren of sulfides, but trace or minor amounts occur locally. Of 1,200 samples examined, only about 12 percent were found to contain sulfides, commonly a few small grains and generally less than about 0.5 percent. Chief occurrences are within small autoliths (inclusions) of noncumulus fine-grained gabbro or metasedimentary xenoliths of hornfels and calc-silicate rock, or in cumulates adjacent to the inclusions. The inclusions are probably fragments carried in from a concealed contact zone and deposited by magma currents (Himmelberg and Ford 1983). A maximum of about 4 modal percent sulfides was found in one caic-silicate inclusion. An origin by contamination with sulfur of country rocks and minor redistribution of sulfides into cumulates seems to be indicated. Forrestal Range (upper) section. About 22 percent of the 1,100 samples examined from the Forrestal Range were found to contain sulfides, of which about 10 percent contain 0.1 to 3.0 modal percent sulfides (mostly in the lower half of the range). A maximum sulfide amount of 12 modal percent occurs in one sample from a layer of leucogabbroic cumulate. Sulfides generally occur in magnetite-rich gabbro or in and near thin layers of magnetitite. They seem to be mostly concentrated in an interval about 200 to 500 meters above the base of the section and in another about 100 to 300 meters below the capping layer of granophyre. Minor sulfide amounts occur in a layer of noncumulus mafic-rich diorite and quartz diorite, up to 50 meters thick, underlying the granophyre. Traces occur locally in the granophyre and in some aplite dikes. In the cumulates, sulfides commonly occur within interstitial space fillings; as inclusions within oxide grains; as fracture fillings; and, more locally, as (replacement?) stringers, lenses, and patches in altered postcumulus amphibole and biotite. Discussion. During fractional crystallization of Dufek Massif cumulates, the magma evidently was undersaturated with respect to sulfur, and an immiscible magmatic sulfide phase was not present. Sulfide occurrences and concentrations in Forrestal Range cumulates are generally similar to those in the ironenriched upper zone of South Africa's Bushveld Complex (von Gruenewaldt 1976). A magma may reach sulfur saturation due to lowering of its iron content, for example by crystallization of magnetite or other iron-rich minerals (von Gruenewaldt 1976, 1979). As in the Bushveld magma's later crystallization stage (upper zone), the Dufek magma eventually crystallized large amounts of magnetite, and at times small quantities of immiscible sulfide droplets separated and settled to the floor. The largest Bushveld sulfide deposits (critical zone), however, are at a much lower stratigraphic position that is probably comparable in the Dufek intrusion to a height in the middle or upper part of its hidden basal section. Sulfides are collectors of platinumgroup elements (PGE) from magmas (Campbell, Naldrett, and
1985 REVIEW
Barnes 1983), and PGE may become concentrated in a magma and fractionate from late iron-enriched melts (Parry 1984). This process may account for the markedly greater, though low, PGE contents in Forrestal Range as compared to Dufek Massif cumulates (Ford et al. 1983). Our further study will focus on the mineralogy and compositional variations of the sulfide minerals and on relations between sulfide concentrations and magnetiterich layers of the intrusion. This work was supported in part by National Science Foundation grant DPP 80-20753. References Abel, K.D., G.R. Himmelberg, and A.B. Ford. 1979. Petrologic studies of the Dufek intrusion: Plagioclase variation. Antarctic Journal of the U.S., 14(5), 6 - 8. Campbell, I.H., A.J. Naldrett, and S.J. Barnes. 1983. A model for the origin of the platinum-rich sulfide horizons in the Bushveld and Stillwater Complexes. Journal of Petrology, 25(2), 133 - 165. Ford, A.B. 1976. Stratigraphy of the layered gabbroic Dufek intrusion, Antarctica. (U.S. Geological Survey Bulletin 1405-D.) Washington, D.C.:
U.S. Government Printing Office. Ford, A.B., R. E. Mays, J. Haffty, and B. P. Fabbi. 1983. Reconnaissance of minor metal abundances and possible resources of the Dufek intrusion, Pensacola Mountains. In Oliver, R.L., J.R. James, and J.B. Jago (Eds.), Antarctic Earth science. Canberra: Australian Academy of Science. Ford, A.B., D.L. Schmidt, and W.W. Boyd, Jr. 1978. Geologic map of the Davis Valley quadrangle and part of the Cordiner Peaks quadrangle, Pensacola Mountains, Antarctica (scale 1:250,000). (U.S. Geological Survey
Map A -. 10.) Washington, D.C.: U.S. Government Printing Office. Ford, A.B., D.L. Schmidt, W.W. Boyd, Jr., and W.H. Nelson. 1978. Geologic Map of the Saratoga Table quadrangle, Pensacola Mountains, Antarctica (scale 1:250,000). (U.S. Geological Survey Map A - 9.) Wash-
ington, D.C.: U.S. Government Printing Office. Himmelberg, G. R., and A.B. Ford. 1976. Pyroxenes of the Dufek intrusion, Antarctica. Journal of Petrology, 17(2), 219 - 243. l-Iimmelberg, G.R., and A.B. Ford. 1977. Iron-titanium oxides of the Dufek intrusion, Antarctica. American Mineralogist, 62, 623 - 633. Himmelberg, G.R., andA.B. Ford. 1983. Composite inclusion of olivine gabbro and calc-silicate rock in the Dufek intrusion, a possible fragment of a concealed contact zone. Antarctic Journal of the U.S., 18(5), 1 4.
Parry, S.J. 1984. Abundance and distribution of palladium, platinum, iridium, and gold in some oxide minerals. Chemical Geology, 43, 115125. von Gruenewaldt, G. 1976. Sulfides in the Upper Zone of the eastern Bushveld Complex. Economic Geology, 71, 1324 - 1336. von Gruenewaldt, G. 1979. A review of some recent concepts of the Bushveld Complex, with particular reference to sulfide mineralization. Canadian Mineralogist, 17, 233 - 256. Wager, L.R., and G.M. Brown. 1968. Layered igneous rocks. Edinburgh: Oliver and Boyd. Walker, P.T. 1961. Study of some rocks and minerals from the Dufek Massif, Antarctica. International Geophysical Year World Data Center A, Glaciology Report, 4, 195 - 213.
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