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Formation of Sedex and Sediment-hosted Manganese, Iron, and Phosphate, and Rare Earth Element Deposits; Interplay Between Exhalative Mineralization, Ocean Chemistry, Marine Redox Cycles, and Planetary Evolution

Project Chief:

Poul Emsbo
Phone: (303) 236-1113
Email: pemsbo@usgs.gov

Project Objectives

Sedimentary exhalative deposits (sedex deposits) are ore deposits believed to have been formed by the release of ore-bearing hydrothermal fluids into a submarine sedimentary environments. The resulting ore deposits are an important source of lead and zinc. The project research builds upon previous collaborative work that developed and tested the hypothesis that sedex-induced global ocean chemistry changes correlate with the formation of sediment-hosted manganese, phosphate, and iron deposits. Objectives are to better understand the formation of sedex and sediment-hosted mineral deposits with a focus on (1) global ocean hydrothermal chemal tracers in relation to ore genesis; (2) formation of sediment-hosted and Miocene phosphate deposits and the relation to oceanic events, and (3) numerical modeling of brine fluid flow in sedimentary basins, integrating geochemical, geologic, and hydrologic constraints.

Tasks

Hydrothermal Chemical Tracers in the Global Oceans: Applications for Ore Genesis and Mineral Assessment

Contact: Poul Emsbo

Our previous collaborative work has shown that the influx of radiogenic strontium, trace elements (zinc, lead, arsenic, barium, etc.), and radiogenic lead from sedex systems can be measured in the global marine secular record. This conceptual framework represents an entirely new approach to ore genesis research. As such, we plan to conduct high-resolution sampling of using global ocean tracers (strontium-, lead-, neodymium-, carbon-, oxygen-, nitrogen-, sulfur-isotopes, rare earth element and trace metals) through time periods that are known to host large and well-dated sedex systems. The focus will be on already studied Lower and Upper Silurian and Middle/Upper Devonian stratigraphic sections. Increased sample resolution in already studied sections, combined with strategic sampling of new stratigraphic sections in Iowa, Illinois, Wisconsin, and Nevada (to infill missing time intervals and demonstrate regional/global correlations), will allow us to test the validity and implications of this new hypothesis.

Formation of Sediment-Hosted Manganese, Phosphate, Iron, and Phosphate-Rare Earth Element Ore Deposits

Contacts: Poul Emsbo and Edward du Bray

The discovery that sedex-induced global ocean chemistry changes correlate with the formation of sediment-hosted manganese, phosphate, and iron deposits, suggests these phenomena are genetically linked. Consequently, we can now show that transitions from an oxic to anoxic oceanic conditions triggered dramatic redox cycling and significant ocean chemistry (i.e. manganese, iron, phosphate, and rare earth element) changes; these processes caused ore mineral precipitation (forming some the world's great sediment-hosted deposits) at redox boundaries along continental margins around the globe (McLaughlin and others, 2012). These concepts challenge models, some developed almost a century ago, concerning the genesis of these deposits. Results from the hydrothermal chemical tracers in global oceans research (previous task) will be the primary data source for this research. Our preliminary work suggests that the formation of great Miocene phosphate deposits, that supply most of the world's phosphate, may have formed during a similar oceanic event. To test this possibility we will analyze a high-resolution sample suite through this time interval from the worldclass phosphate deposits of Florida and from the Monterey Formation in California. In addition, the discovery and sampling of a series of bentonites that span much of the Miocene, and importantly bracket the phosphate horizons in the Monterey, represent an extraordinary opportunity to constrain, using modern geochronology, the precise duration of an oceanic event. Tephrochronology (use of tephra layers as geochronological tool), combined with isotopic and geochemical study of igneous apatite and zircon, may allow us to track down the source of these tuffs in the Cordilleran volcanic arc and potentially open the door to significant volcanogenic science. More importantly, determining the exact age of these tuffs and thus the exact duration of sedimentation will provide mass balance constraints that will enhance our understanding of phosphates and organic black shales deposit formation, thus providing a fundamental constraint on the evolution of the world's ocean chemistry.

Numerical Modeling of Sedex Flow

Contacts: Poul Emsbo and Andrew Manning

USGS scientists have developed new analytical techniques to determine ancient seawater chemical compositions. Early results have fostered development of collaborative relationships with leading paleontologists, stratigraphers, and chemostratigraphers (using chemical fingerprints in stratigraphy). The resulting synergies allowed us to measure rapid (ca. 2 to 5 ky) changes to the chemical and isotopic composition of the paleoocean, including global ocean metal concentrations, radiogenic/stable isotopes, and redox conditions (Hammerlia and others, 2013). These state-of-the-art chemical tracer analyses allow us to track the chemical and hydrologic evolution of sedex hydrothermal deposits, as proxied by effects on the global ocean, and to directly measure the effects of each hydrothermal pulse on oceanic, chemical and biologic cycles. We plan to apply these sophisticated analytical tools and methodologies and collaborative expertise to advance our understanding of the formation of sediment-hosted ore deposits that, simultaneously, will spur a step-change in our understanding of the evolution of the Earth's litho-, hydro-, atmo-, and biosphere. Resulting data, once combined with already collected traditional ore genesis data (radiogenic/stable isotopes, fluid inclusion analyses, fluid flow modeling, etc.) and mass balance oceanic box-modeling, will allow us for the first time to precisely constrain the age, fluid flux, metal flux, depositional efficiency, and duration of the planet's largest hydrothermal systems. Understanding the crustal processes that drove the deposit-forming massive fluid flow and caused these oceanic events is foundational to this project. Numerical modeling will integrate these constraints and use them to model brine fluid flow in sedimentary basins and test previous models. These geochemical, geologic, and hydrologic constraints, sought after since the beginning of ore genesis studies, have the potential to revolutionize our understanding of an entire class of ore deposits.


References

Hammerlia, J., Rusk, B., Spandlera, C., Emsbo, P., and Oliver, N.H.S., 2013, In situ quantification of Br and Cl in minerals and fluid inclusions by LA-ICP-MS: A powerful tool to identify fluid sources: Chemical Geology, 337-338, p. 75-87. doi: 10.1016/j.chemgeo.2012.12.002

McLaughlin, P.I., Emsbo, P., and Brett, C.E., 2012, Beyond black shales: The sedimentary and stable isotope records of oceanic anoxic events in a dominantly oxic basin (Silurian; Appalachian Basin, USA): Palaeogeography, Palaeoclimatology, Palaeoecology, 367-368, p. 153-177. doi: 10.1016/j.palaeo.2012.10.002

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