Geologic Criteria for the Assessment of Sedimentary Exhalative (Sedex) Zn-Pb-Ag Deposits By Poul Emsbo Open-File Report 2009–1209 U.S. Department of the Interior U.S. Geological Survey U.S. Department of the Interior KEN SALAZAR, Secretary U.S. Geological Survey Suzette M. Kimball, Acting Director U.S. Geological Survey, Reston, Virginia 2009 For product and ordering information: World Wide Web: http://www.usgs.gov/pubprod Telephone: 1-888-ASK-USGS For more information on the USGS—the Federal source for science about the Earth, its natural and living resources, natural hazards, and the environment: World Wide Web: http://www.usgs.gov Telephone: 1-888-ASK-USGS Suggested citation: Emsbo, Poul, 2009, Geologic criteria for the assessment of sedimentary exhalative (sedex) Zn-Pb-Ag deposits: U.S. Geological Survey Open-File Report 2009−1209, 21 p. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government. 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Contents Introduction .................................................................................................................................................................... 1 Deposit Summary .......................................................................................................................................................... 2 Regional Geologic and Tectonic Environment ............................................................................................................... 3 Theory of Deposit Formation ......................................................................................................................................... 3 Source of Fluids Involved in Ore Component Transport ............................................................................................ 3 Deduced Geologic Assessment Criteria ..................................................................................................................... 5 Sources of Ligands and Chemical Transport and Transfer Processes Involved in Ore Component Transport ............. 5 Deduced Geologic Assessment Criteria ................................................................................................................. 7 Sources of Metals and Other Ore Components ......................................................................................................... 7 Deduced Geologic Assessment Criteria ................................................................................................................. 8 Fluid Drive, Including Thermal, Pressure, and Geodynamic Mechanisms ..................................................................... 8 Sedimentary Compaction Model ............................................................................................................................ 8 Free Convection Models ......................................................................................................................................... 9 Topographically Driven Flow Model...................................................................................................................... 10 Density-Driven Fluid Flow: A New Fluid-Flow Model ............................................................................................ 10 Deduced Geologic Assessment Criteria ............................................................................................................... 11 Conduits or Pathways that Focus Ore-Forming Fluids ................................................................................................ 11 Deduced Geologic Assessment Criteria ............................................................................................................... 12 Nature of Traps and Wallrock Interaction that Trigger Ore Precipitation .................................................................. 12 Deduced Geologic Assessment Criteria ............................................................................................................... 13 Structure and Composition of Residual Fluid Outflow Zones ................................................................................... 13 Deduced Geologic Assessment Criteria ...................................................................................................................... 14 Hierarchy of Geologic Exploration and Resource Assessment Criteria ....................................................................... 14 Geologic Criteria that Define Permissive Tracts ....................................................................................................... 15 Geologic Criteria Used to Evaluate the Prospectivity of Permissive Tracts .............................................................. 16 Geologic Criteria that Map Favorability within Permissive Tracts ............................................................................. 17 Potential Environmental Effects of Sedex Deposits ..................................................................................................... 17 References Cited ......................................................................................................................................................... 17 Figures 1. Map of six sedimentary basins that are known to contain a total endowment of more than 10 Mt tons Zn+Pb metal ................................................................................................................................................................. 2 2. Block diagram showing geologic setting and geologic assessment criteria for sedex Zn-Pb-Ag deposits that are deduced form this synthesis ..................................................................................................................... 15 iii Conversion Factors SI to Inch/Pound Multiply By To obtain Length millimeter (mm) 0.03937 inch (in.) kilometer (km) 0.6214 mile (mi) meter (m) 1.094 yard (yd) Area square meter (m2) 0.0002471 acre square kilometer (km2) 0.3861 square mile (mi2) Volume liter (L) 1.057 quart (qt) cubic centimeter (cm3) 0.06102 cubic inch (in3) cubic kilometer (km3) 0.2399 cubic mile (mi3) Mass gram (g) 0.03527 ounce, avoirdupois (oz) metric ton 1.1023 short ton Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows: °F=(1.8×°C)+32 Abbreviations Used in This Report °C degree Celsius µg/L microgram per liter C/km degree Celsius per kilometer g/cm3 gram per cubic centimeter mg/L milligram per liter Mt metric ton mW/m2 megawatt per square meter ppm part per million sedex sedimentary exhalative W/m°C watt per meter per degree Celsius BSR bacterial sulfate reduction MVT Mississippi Valley type S/C sulfur/carbon TDS total dissolved solids TOC total organic carbon TSR thermochemical sulfate reduction iv Geologic Criteria for the Assessment of Sedimentary Exhalative (Sedex) Zn-Pb-Ag Deposits By Poul Emsbo Introduction Sedex deposits account for more than 50 percent of the world's zinc (Zn) and lead (Pb) reserves (Tikkanen, 1986) and furnish more than 25 percent of the world’s production of these two metals (Goodfellow and Lydon, 2007). More than 129 deposits of this type have been recognized in sedimentary basins around the world (Leach and others, 2005b; Goodfellow and Lydon, 2007). A compilation by Sangster and Hillary (2000) shows that the largest 65 deposits occur in 25 sedimentary basins, 6 of which contain more than 10 metric tons (Mt) combined Pb+Zn (fig. 1). In order of decreasing endowment these are Mt. Isa–McArthur basins (7 deposits totaling 112 Mt of Zn+Pb metal), Selwyn basin (17 deposits totaling 55 Mt), Brooks Range (3 deposits totaling 40 Mt), Rajasthan (5 deposits totaling 20 Mt), Belt-Purcell (1 deposit totaling 19 Mt), and the Rhenish Basin (2 deposits totaling 11 Mt). The published literature contains several thorough reviews of descriptive information, data, and conceptual or genetic models as well as geoenvironmental assessments of sedimentary-exhalative (sedex) deposits (Kelley and others, 1995; Lydon, 1995; Leach and others, 2005b; Goodfellow and Lydon, 2007). This report draws on previous syntheses, as well as on topical studies of sedex deposits, to determine the critical descriptive and genetic criteria that define sedex-type deposits. Also utilized in this analysis are studies of the tectonic, sedimentary, and fluid evolution of modern and ancient sedimentary basins. The focus here is on geologic characteristics of the six sedex-deposit-hosting basins containing greater than 10 Mt Zn+Pb. The enormous size of sedex deposits strongly suggests that basin- scale geologic processes are involved in their formation. It follows that mass balance constraints can form a conceptual underpinning for the evaluation of potential mechanisms and the identification of geologic indicators of ore-forming processes in sedimentary basins. The objective of this report is to use both descriptive information and conceptual models to identify processes critical to the formation of sedex deposits. The framework of the resulting ore deposit model is built on a “mineral systems approach” (Kreuzer and others, 2008). The mineral system schemes put forward by various authors have differed somewhat, but critical elements of hydrothermal ore-forming processes can be reduced to (1) source of fluids, (2) source of ore-transporting ligands,