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Microfossil and Radioisotopic Geochronological Studies of the Greens Creek Host Rocks

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Microfossil and Radioisotopic Geochronological Studies of the Greens Creek Host Rocks Microfossil and Radioisotopic Geochronological Studies of the Greens Creek Host Rocks By Wayne R. Premo, Cliff D. Taylor, Lawrence W. Snee, and Anita G. Harris Chapter 11 of Geology, Geochemistry, and Genesis of the Greens Creek Massive Sulfide Deposit, Admiralty Island, Southeastern Alaska Edited by Cliff D. Taylor and Craig A. Johnson Professional Paper 1763 U.S. Department of the Interior U.S. Geological Survey Contents Abstract .......................................................................................................................................................287 Introduction.................................................................................................................................................287 Geology ........................................................................................................................................................291 Geochronology ...........................................................................................................................................293 Paleontology—Conodont Studies ..................................................................................................293 Conodonts from the Greens Creek Mine Area ....................................................................294 Underground Collections ...............................................................................................294 Outcrop Collections ........................................................................................................295 Color Alteration Index (CAI) ...................................................................................................299 Paleoenvironmental and Depositional Controls for Conodont Distribution ...................299 Conodonts from the Mansfield Peninsula and Northern Kupreanof Island ...................299 Radiogenic Isotopic Age Determinations .....................................................................................300 40Ar-39Ar Age Determinations .................................................................................................300 U-Th-Pb, Rb-Sr, and Sm-Nd Ages on Host-Rock Lithologies ...........................................307 Whole-Rock Compared to Mineral (Internal) Isochron Ages .................................307 Analytical Methods ........................................................................................................307 Results .........................................................................................................................................................310 Metabasalts and Phyllites ...............................................................................................................310 Metagabbros ...................................................................................................................311 Argillites............................................................................................................................320 Serpentinites ...................................................................................................................321 Discussion ...................................................................................................................................................324 Late Triassic Igneous Activity .........................................................................................................324 Age of Greens Creek Mineralization .............................................................................................324 Isotopic Resetting of the Older Stratigraphy ................................................................................324 Age of Cretaceous Overprinting on Greens Creek Mineralization ...........................................326 Acknowledgments .....................................................................................................................................327 References Cited........................................................................................................................................327 Appendix......................................................................................................................................................331 Figures 1. Generalized map of terrane boundaries and major volcanogenic massive sulfide prospects, southeast Alaska, showing the locations of the Late Triassic age volcanogenic massive sulfide (VMS) deposits, including Greens Creek, Pyrola, and Gambier Bay on Admiralty Island ...........................................288 2. Simplified geologic map of Admiralty Island, showing locations for Greens Creek and Pyrola deposits, Gambier Bay, Windfall Harbor, the Mansfield Peninsula, and Staunch Point (locations of some whole-rock samples) ...................................................289 3. Map of surface sampling sites at and near Greens Creek mine, Admiralty Island .......290 4. Photograph of Halobia in clast of siliceous argillite milled into massive sulfide ore ....292 5. Stratigraphy of the Greens Creek section, Admiralty Island, showing the orebody at the contact between footwall phyllites and hanging-wall argillites ...........................293 6. Lower and Upper primitius Zone conodonts (latest Carnian-earliest Norian) from the Hyd Group, northern Admiralty Island, Alaska ....................................................295 6. A-I, Pa elements from outcrop in cirque at head of upper Cliff Creek ............................295 6. A, B, Metapolygnathus primitius (Mosher), Upper views of two incomplete adult specimens with quadrate platform with five nodes on lateral margins and posterior margin without nodes ................................................................................................................295 6. C, D, Metapolygnathus primitius Upper and lower views of small adult with three lateral nodes on inner margin and four on outer margin ...................................................295 6. H, A Metapolygnathus primitius Pa element bent out of shape .......................................295 6. I, Metapolygnathus primitius Juvenile with three nodes on lateral margins and midplatform constriction .........................................................................................................295 6. E-G, Metapolygnathus primitius transitional to Epigondolella, well-preserved subadult, lateral, upper, and lower views .............................................................................295 6. J-O, Metapolygnathus primitius specimens from mine workings at 37 Ramp drift, no. 1 passing bay and within 20 meters of the orebody .....................................................295 6. J-M, Juvenile and subadult Pa elements of Metapolygnathus primitius (Mosher) ......295 6. N, O, Neogonodolella sp., Pa element, upper and lower views .......................................295 6. P-R, Specimens from mine workings in 620-2955 drift and within 15 meters of the orebody .......................................................................................................................................295 7. Carnian-Middle Norian conodont zonation showing age range of biostratigraphically useful and (or) geologically significant conodont collections from Hyd Group host rocks, Greens Creek mine and vicinity, and four collections from the Hyd in the vicinity of the Taylor Creek deposit, northern Kupreanof Island .......................................................................................................................298 8. Conodont zonation for the Lower and part of the Upper Permian showing age range of conodont collections from the uppermost exposed beds of the Cannery Formation in the vicinity of the Taylor Creek deposit and the Pybus Formation about 15 kilometers farther northwest, northern Kupreanof Island .................................301 9. 40Ar/39Ar step-release spectra for four samples: one from the Greens Creek mine, two from other locations on Admiralty Island, and one fuchsite, 92–GB–05, from altered ultramafic rocks on the north shore of Gambier Bay showing a two-step 40Ar/39Ar plateau date of 210.3 ± 0.3 Ma (1σ) ........................................................................306 10. 40Ar/39Ar spectra for Greens Creek fuchsite and sericite — Spectra that indicate complete resetting of argon in the alteration assemblage prior to 105 – 95 Ma ............................................................................................................................308 11. Uranium-lead isotopic correlation diagram of data from metabasalt rocks (Hyd Group and Gambier Bay Formation) and footwall phyllites of the Green Creek area ..................................................................................................................................311 12. Samarium-neodymium isotopic correlation diagram of data from metabasalt rocks and footwall phyllites — Greens Creek area ............................................................318 13. Rubidium-strontium isotopic correlation diagram of data from metabasalt rocks and footwall phyllites of the Greens Creek area ......................................................319 14. Uranium-lead isotopic correlation diagram of data from some of the Greens Creek metagabbros, yielding a whole-rock isochron age of 206 ± 35 Ma and initial 206Pb/204Pb value of 18.61 ± 0.19, similar to that of the Greens Creek ore ..............320 15. Samarium-neodymium isotopic correlation
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