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Sedimentation and Mineralization of a Sandstone Lead-Zinc Occurrence in the Helena Formation Belt Supergroup Lewis and Clark County Montana

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Sedimentation and Mineralization of a Sandstone Lead-Zinc Occurrence in the Helena Formation Belt Supergroup Lewis and Clark County Montana University of Montana ScholarWorks at University of Montana Graduate Student Theses, Dissertations, & Professional Papers Graduate School 1986 Sedimentation and mineralization of a sandstone lead-zinc occurrence in the Helena Formation Belt Supergroup Lewis and Clark County Montana Don Herberger The University of Montana Follow this and additional works at: https://scholarworks.umt.edu/etd Let us know how access to this document benefits ou.y Recommended Citation Herberger, Don, "Sedimentation and mineralization of a sandstone lead-zinc occurrence in the Helena Formation Belt Supergroup Lewis and Clark County Montana" (1986). Graduate Student Theses, Dissertations, & Professional Papers. 7511. https://scholarworks.umt.edu/etd/7511 This Thesis is brought to you for free and open access by the Graduate School at ScholarWorks at University of Montana. It has been accepted for inclusion in Graduate Student Theses, Dissertations, & Professional Papers by an authorized administrator of ScholarWorks at University of Montana. For more information, please contact [email protected]. COPYRIGHT ACT OF 1975 This is an unpublished manuscript in which copyright sub­ s is t s. Any further reprinting of its contents must be approved BY THE AUTHOR. Mansfield L ibrary University of Montana Date; 1 9 8 Q___ SEDIMENTATION AND MINERALIZATION OF A SANDSTONE LEAD-ZINC OCCURRENCE IN THE HELENA FORMATION, BELT SUPERGROUP LEWIS AND CLARK COUNTY, MONTANA By Don Herberger B.S., Colorado State University, 1981 Presented in partial fulfillment of the requirements for the degree of Master of science UNIVERSITY OF MONTANA 1986 Approved by: Dr. Ian M. Lange Chairman, Board of Examiners Graduate schDe Date UMI Number: EP38312 All rights reserved INFORMATION TO ALL USERS The quality of this reproduction is dependent upon the quality of the copy submitted. In the unlikely event that the author did not send a complete manuscript and there are missing pages, these will be noted. Also, if material had to be removed, a note will indicate the deletion. UMT Oissertaition PuWishing UMI EP38312 Published by ProQuest LLC (2013). Copyright in the Dissertation held by the Author. Microform Edition © ProQuest LLC. All rights reserved. This work is protected against unauthorized copying under Title 17, United States Code ProQuesf ProQuest LLC. 789 East Eisenhower Parkway P.O. Box 1346 Ann Arbor, Ml 48106 - 1346 Herberger, Don J., M.S., February 1986 Geology Sedimentation and Mineralization of a Sandstone Lead-zinc Occurrence in the Helena Formation, Belt Supergroup, Lewis and Clark County, Montana (82 ) Director: Dr. Ian M. Lange Disseminations of galena and sphalerite occur in the basal sandstone of the Helena Formation, Belt Supergroup near Wood creek, northwestern Montana. The basal sandstone is 1.5 to 2 meters thick and comprised of moderately to well-sorted, rounded, coarse-grained quartz. The coarse-grained sandstone is stratigraphieally encased in fine-grained sedimentary rocks. Siltstones and shales of the Spokane and Empire formations occur stratigraphically below the sandstone. calcareous siltstones, shales and dolomite beds of the Helena Formation overlie the sandstone. The basal sandstone of the Helena Formation was deposited as a nearshore sand along the eastern margin of the proterozoic Belt Basin. The presence of moderately to well sorted grains, oolites, low-angle bidirectional cross-stratification, intraclast-lag conglomerate and oscillation ripple marks in the sandstone indicate wave agitated sedimentation. Lithofacies in the Empire, Spokane, and Helena formations indicate deposition in a shallow water, tectonically stable environment. Mineralization of the sandstone consists of sphalerite, galena and pyrite. Sulfides occur primarily as interstitial growths between corroded quartz grains. Galena and sphalerite replace carbonate cement, pyrite, and quartz grains in the sandstone. Megascopically, the sulfides occur as spots distributed throughout the sandstone. Timing of mineralization can only be constrained to postdating partial quartz and carbonate cementation of the sand and predating Late cretaceous to Early Tertiary faulting. Mineralization was controlled by permeability, probably of secondary origin, and the presence of reduced sulfur in the sandstone. The data best support the concept of metal-bearing groundwater migrating from basement highs into the sandstone where sulfide precipitation occurred. 11 TABLE OF CONTENTS page INTRODUCTION................................................ 7 GENERAL GEOLOGY............................................. 10 STRATIGRAPHY AND SEDIMENTATION............................12 Empire-Spokane formations............................... 12 Spokane Formation.........................................14 Empire Formation.......................................... 15 Helena Formation.......................................... 16 Basal sandstone.........................................16 Helena above basal sandstone......................... 22 Snowslip Formation........................................23 Shepard Formation.........................................24 IGNEOUS ROCKS................................................ 25 STRUCTURE.....................................................2 6 PALEOGEOGRAPHIC SETTING.................................... 28 Environment of deposition............................... 28 Basement rocks............................................ 30 LEAD-ZINC MINERALIZATION...................................31 Distribution and areal extent...........................31 Mineralogy and grade..................................... 33 Mineralization textures and petrology................. 34 Structure and mineralization............................42 Metal zoning...............................................44 paragenesis................................................44 FLUID INCLUSIONS............................................ 46 ISOTOPIC COMPOSITIONS...................................... 47 Lead isotope ratios...................................... 47 Calcite carbon and oxygen isotopes.....................47 Sulfur isotope composition of galena and sphalerite.51 DISCUSSION....................................................53 Timing of mineralization.................................53 Mineralization controls..................................54 Genesis.....................................................55 Basin brine model...................................... 56 Groundwater model.......................................58 Review of other sandstone lead-zinc deposits.........62 CONCLUSIONS.................................................. 67 111 Page ACKNOWLEDGEMENTS............................................ 6 8 REFERENCES CITED............................................ 69 APPENDIX......................................................75 IV LIST OF TABLES Page Table 1. Lead isotope ratios of Wood Creek galena.... 48 Table 2. Carbon and oxygen isotope measurements from Wood Creek calcites....................... 49 Table 3. Sulfur isotope values of Wood Creek galena and sphalerite...........................52 V LIST OF FIGURES Page Figure 1. Location and regional geology ............ 8 Figure 2. Generalized stratigraphy ................... 13 Figure 3. Quartz overgrowths........................... 19 Figure 4a. Quartz cement................................. 20 Figure 4b. carbonate cement..............................20 Figure 5. Distribution of lead and zinc.............. 32 Figure 6. Spots of galena in sandstone............... 36 Figure 7. Sulfides along cross-beds................... 37 Figure 8. Fine disseminations in sandstone...........38 Figure 9. Band of sulfides in sandstone.............. 39 Figure 10. Interstial sphalerite........................40 Figure 11. Galena replacing rhombs of dolomite....... 41 Figure 12. Mineralization cut by fracture............. 43 Figure 13. Groundwater mineralizing model............. 59 Figure 14. spotted texture of sandstone lead- zinc deposits.................................. 65 PLATES Plate 1. Outcrop geologic map of the Wood Creek area............................In pocket Plate 2. Stratigraphy of the basal sand­ stone of the Helena Formation............In pocket VI INTRODUCTION Madge and others (1968) described disseminated galena, sphalerite and pyrite in a sandstone outcropping near Benchmark Airfield, Lewis and Clark County, Montana. The occurrence is located approximately 120 km. northeast of Missoula, Montana (Figure 1.). The mineralized sandstone is 1,5 to 2 meters thick and stratigraphically located at the base of the Helena Formation of the Belt Supergroup. Outcrops of the mineralized sandstone occur in the lower part of the Eldorado thrust sheet along Wood creek and a portion of the south fork of the Sun River (Plate 1.). Texturally, the mineralized sandstone at Wood Creek resembles ore rocks from the Laisvall sandstone-hosted lead-zinc deposit, Sweden described by Rickard and others ( 1979) . Aside from the report by Mudge and others (1968) and a brief mention of the Wood Creek prospect in a mineral resource report (Mudge and others, 1974) no other work is published on the mineralization. McGill and Sommers (1967), Eby (1977) and Mudge (1972) have described portions of the stratigraphy of the Eldorado thrust sheet in the Wood Creek area. parts of the geology in the Wood Creek area have been mapped by Mudge (1966) and Mudge and others (1974). Mudge (1972a) described the structural geology of a portion of the area. The purpose of my studies at Wood
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