Igneous Rocks in the Bingham Mining District, Utah GEOLOGICAL SURVEY PROFESSIONAL PAPER 629-B Igneous Rocks in the Bingham Mining District, Utah By WILLIAM J. MOORE GEOLOGIC STUDIES OF THE BINGHAM MINING DISTRICT, UTAH GEOLOGICAL SURVEY PROFESSIONAL PAPER 629-B A petrologic study of intrusive and extrusive igneous rocks in a classic mining area UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1973 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600345 For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402 Price: Paper cover 85 cents domestic postpaid or 60 cents GPO Bookstore Stock Number 2401-00274 CONTENTS Page Pag-e Abstract Bl Extrusive igneous rocks B2" Introduction _ .________________ - 1 Summary of compositional trends for the igneous Acknowledgments _ .________________ _____ 3 rocks _ -- - 3^ Methods of study .________________ _____ 3 Modal variations 3^ Intrusive igneous rocks .________________ 5 Chemical variations - 3° Last Chance stock ___ .________________ ______ 5 Radiometric ages of igneous rocks in the Bingham Hybrid variants __-.________________ _ - 9 area _______-_-_______ ____ ___________ - 3~N Bingham stock ___________________ 13 History of emplacement and crystallization 3° Dike rocks _________________________ 18 References cited 41 ILLUSTRATIONS Pag" FIGURE 1. Map showing generalized distribution of igneous rocks in the Oquirrh Mountains and vicinity B? 2. Generalized geologic map of the Bingham area _ -- 4 3. Map showing underground workings examined in this study _ __ - 5 4. Photograph of angular siliceous limestone fragments border phase of Last Chance stock 6 5. Photograph showing block stoping of light-colored banded metalimestone by dark-colored dikelike apophysis of Last Chance stock __ _ _..___ - 6 6. Photograph and photomicrograph of typical sample from Last Chance stock 7 7. Diagrams showing modal mineral proportions, variations in specific gravity, and proportions of normative salic minerals in samples from the Last Chance stock 9 8-11. Photographs showing: 8. Contact between siliceous limestone and monzonite of Last Chance stock _ 9 9. Ribbons of aplitic hybrid phase in monzonite of the Last Chance stock at contact with lime­ stone_____________________________________________- 11 10. Dikelike segregation of aplitic hybrid phase ________________ _ 11 11. Silicated limestone breccia ________________________ _ _ 11 12. Maps showing specific gravity trends in monzonite of the Last Chance stock adjacent to limestone con­ tacts ____________________________________________________________ 11 13. Photomicrograph of micrographic intergrowth of K-feldspar and quartz in aplitic hybrid phase 12 14. Photomicrograph of quartz-calcite intergrowths interstitial to K-feldspar in aplitic hybrid phase ___ 12 15. Photomicrograph of pyroxene euhedra enclosed poikilitically in K-feldspar ___________ 12 16. Diagrams showing felsic mineral proportions and relations between specific gravity and modal min­ eral abundance in hybrid rocks from the Last Chance stock _____________________-- 13 17. Diagram showing chemical gains and losses of major oxides in hybrid rocks relative to uncontami- nated monzonite of the Last Chance stock ___________________________________ 14 18. Generalized geologic map of southeast margin of Bingham stock on Mascotte tunnel level of Lark mine _____________________________________________________ 15 19. Photograph of chilled augite monzonite border phase of Bingham stock _______ __ _ 16 20. Photomicrograph of chilled border phase of Bingham stock _________________________ 16 21. Photographs of subequigranular Bingham amphibole-augite monzonite ________________ 16 22. Photograph of the porphyritic amphibole quartz monzonite of the Bingham stock ____________ 17 23. Photomicrogaph of porphyritic amphibole quartz monzonite ______________________ 17 24. Diagrams showing modal mineral proportions, variations in specific gravity, and proportions of nor­ mative salic minerals in samples from the Bingham stock ___-_________________ 19 25. Generalized geologic map of Utah Metals tunnel __________________________________ 20 26. Photographs of pebble dikes ________________________________________________ 21 27. Photographs of latitic dike rocks _____________________________________________ 2? Ill IV CONTENTS Page FIGURE 28. Photomicrographs of latitic dike rocks __ B23 29. Diagrams showing modal phenocryst mineral proportions, variations in specific gravity, and prcr>or- tions of normative salic minerals in samples of latitic dike rocks - 25 30. Map of traverse across zoned latite porphyry dike in Utah Metals tunnel showing contrast in specific gravity and modal mineralogy between borders and center 26 31. Generalized geologic map of Tickville Springs 7%-minute quadrangle 28 32. Photographs of volcanic rocks in Tickville Springs quadrangle _ _ 31 33-37. Diagrams showing: 33. Modal phenocryst mineral proportions, variations in specific gravity, and normative salic min­ eral proportions in samples of volcanic rocks from Tickville Springs quadrangle - 33 34. Variations between selected modal minerals in samples from Last Chance and Bing'ham stocks ___________1__________--- __ ________ ___ _ _ 35 35. Weight percentage of combined alkalis (K2O+Na2O) and CaO plotted against weight per­ centage of SiOa for samples of igneous rocks in the Bingham area ___ _ _ 36 36. Weight percentage of common oxides plotted against differentiation index _ _ __ 37 37. Proportions of modal and normative salic minerals in samples from the Bingham area ____ 37 TABLES Page TABLE 1. Modes of the Last Chance stock _ - _____ B8 2. Chemical data for the Last Chance stock -- _ _____ 10 3. Modes of hybrid rocks from margins of Last Chance stock __________________________ 13 4. Chemical data for hybrid rocks from margins of Last Chance stock ________________ 14 5. Modes of rocks from southeastern part of Bingham stock __________________________ 18 6. Chemical data for southeastern part of Bingham stock _ _______ 22 7. Modes of latitic dike rocks _ -- - - 24 8. Chemical data for latitic dike rocks ___ _ -- - -- 27 9. Modes of latitic and rhyolitic volcanic rocks --- - - 32 10. Chemical data for latitic and rhyolitic volcanic rocks ____ _______ 34 11. Potassium-argon radiometric ages for biotite from igneous rocks in the Bingham area 36 GEOLOGIC STUDIES OF THE BINGHAM MINING DISTRICT, UTAH IGNEOUS ROCKS IN THE BINGHAM MINING DISTRICT, UTAH By WILLIAM J. MOORE ABSTRACT cias cover the lower east slopes of the Oquirrh Moun­ Igneous rocks in the Bingham area include two small tains and make up about 60 percent of the bedrocl- monzonitic stocks, a series of younger latitic dikes, and, exposures in the east-trending Traverse Range (figs. east of the mining district, a sequence of latitic breccias and 1, 2). Although not as closely related to the ore flows. The igneous rocks were emplaced or erupted over a period of about 8 million years in latest Eocene and early deposits as units of the intrusive complex, the vol­ Oligocene time (39-31 million years ago). Progressive in­ canic rocks in the Bingham district display close creases in silica content and in proportions of alkali feldspar chemical affinities to the intrusions and are con­ to plagioclase suggest that the rocks were derived from a sidered here as part of a single extended magmatic single magma source. Stratigraphic reconstruction gives a episode. maximum depth of cover of 7,500 feet at the time of mon­ zonitic plutonism. The magma system was probably fluid- In general terms, the rocks may be described as undersaturated (Pjiuid <£> *o»d) early in the igneous his­ monzonitic or latitic; textural and mineralogical tory, but separation of a fluid phase occurred at least twice variations are apparently greater than differences in in the later stages of magmatism. This enrichment in bulk chemical composition. Several small rhyolitic hydrous fluids is considered to be a further consequence of the same differentiation process that led to increasingly plugs and related vitrophyric flows represent the silicic or potassic bulk compositions or both. Vertical dis­ greatest departure from intermediate compositions placement and concomitant erosion of fault-bounded blocks in the entire igneous suite. of roof rock may have facilitated emplacement of the in­ trusive complex at the present level of exposure. Limiting ages, based on radiometric dates, range from latest Eocene for the oldest intrusive body to middle Oligocene for the topographically highest INTRODUCTION unit in the volcanic sequence. Similarities in compo­ This report describes the intrusive igneous rocks sition and age between igneous rocks at Bingham and in the Bingham mining district and the extrusive those in the nearby Tintic and Park City mining dis­ igneous rocks in an area adjoining the district to the tricts define a regional epoch of middle Tertiary east and south. Because the unmineralized rocks are magmatism and metallization in north-central Utah, emphasized in this report, it should be regarded as Several reports concerned directly with the igne­ background for subsequent reports in this series de­ ous petrology of the Bingham district had already tailing the geology of the ore deposits at Bingham. been published by the time this study began (1965). The Bingham mining district is in the east-central The earliest report (Boutwell 1905), a brief anc1 Oquirrh Mountains,