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Assessment Techniques for Concealed Mineral Resources

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We will focus on methodologies to objectively define the possibility and/or probability of concealed mineral resources in an assessment area. We will pursue several parallel lines of research that all contribute to achieving the goal of a set of methods for assessing covered areas for various deposit types. We visualize the work as falling under four main tasks (larger headings) with associated sub tasks shown in bold:

Primary characteristics of concealed mineral deposits

The main objective of this task is to enhance assessment of concealed mineral resources through improved identification and characterization of the subtle effects of a mineralizing system. Such characterization requires thorough micro-scale documentation of chemical, isotopic, and physical properties of both unaltered and altered host rocks, and documentation of temporal relationships in systems comprised of multiple magmatic and hydrothermal events. Derivative objectives are to determine chemical and physical gradients within large hydrothermal systems, and to devise practical methods to recognize margins and gradients.

  • Sediment-hosted mineral deposits in the northern Cordillera - The objectives of this sub-task are to gain a better understanding on the controls that result in variability among sediment-hosted deposits. Some Pb-Zn districts and deposits have been well studied (i.e., Red Dog, Australian deposits). The goal will be to incorporate results from these studies with new data. Primary focus for new data acquition include 1) the Sheep Creek Cu-Co deposit, 2) the Ruby Creek (Cu-Co) and Baird Mountain (Cu-Pb-Zn) deposits in northern Alaska, and 3) Selwyn Basin deposits in Canada.
  • Porphyry deposits in southwestern Alaska - The sub-task objectives are:
    1. to evaluate the regional igneous geochemistry and ages of intrusion-related mineralization throughout the Iliamna mineral district
    2. precisely determine the lithogeochemical, isotopic and petrochemical features of the magmatic rocks associated with the large intrusion that generated the Pebble system
    3. determine chemistry of specific mineral phases within various alteration or ore zones
    4. test specific mineralogical, geochemical, and geophysical methods overlying the deposit to determine possible ways to detect the deposit below surface. These will include water, soil, and sediment sampling (along with mineral chemistry) and various geophysical methods that might include magnetotelluric, seismic, and potential field methods
    5. provide a set of geologic, geochemical, and geophysical characteristics upon which to construct exploration models to target similar concealed copper deposits in southern Alaska.
  • Characteristics of pre- and syn-mineralization rocks in major metal deposits, Basin and Range and northern Rocky Mountains- The main objective of this sub-task is margin-to-margin characterization of processes and products of hydrothermal systems that formed selected large and world class metallic deposits. Such characterization requires thorough microscale documentation of chemical, isotopic, and physical properties of both unaltered and altered host rocks, and documentation of temporal relationships in systems comprised of multiple magmatic and hydrothermal events. Derivative objectives are to determine chemical and physical gradients within large hydrothermal systems, and to devise practical methods to recognize margins and gradients. Advances in microanalysis portend recognition of micro-scale alteration, which will greatly expand the utility of limited samples for exploration vectoring.
  • Mineralizing systems in the evolution of the Upper Midwest region - The main objectives of this subtask are to characterize the geologic, physical and chemical signatures that relate to mineralizing processes in the Upper Midwest region, and to define the specific factors that influenced the localization of the major mineral deposits. The results will be incorporated into new mineral deposit models that can guide assessments and exploration for undiscovered mineral resources beneath the regions extensive Phanerozoic cover. Emphasis will initially be on the following deposit types.
    • Magmatic Ni-Cu-Co-PGE deposits
    • Sediment-hosted copper deposits
    • Volcanogenic massive sulfide deposits
    • Paleosupergene deposits
  • Base metal deposits in the Fortymile district, east-central Alaska - The objectives of this sub-task are:
    1. Characterize the nature and extent of the Zn-Pb-Ag-Cu prospects, their cover, and associated metasedimentary and
      metavolcanic host rocks and igneous intrusions through petrographic, geochemical, and radiometric and stable isotope
      analyses of drill core and surface samples.
    2. Utilize ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) scenes across the area,
      combined with field checking of remotely sensed data, to identify acid kill zones and gossans, and to identify and
      delineate surface expressions of known and undiscovered base-metal and silver prospects.
    3. Identify buried plutons and structural/ deformational/ lithologic controls on mineralization utilizing ground-based gravity,
      magnetics, and magnetotellurics.
    4. Determine the tectonic framework that controlled plutonism and mineralization in the Fortymile District through geologic
      and kinematic field investigations.

Characteristics of post-mineralization cover

The principal objective of this task is to define the time-space nature and distribution of post-mineralization processes, characterize the resulting products, and develop multidisciplinary methods to see through post-mineralization cover to determine the relative likelihood of a concealed deposit being present. Furthermore, modification of mineral deposits can produce environmentally sensitive byproducts, and an understanding of post-mineralization processes can improve the USGS’s geoenvironmental models.

  • Cenozoic history of intermontane basins and adjacent highlands in the Basin and Range region - These studies will focus on the sedimentology, volcanology, geochronology, geophysics (gravity, magnetics, magnetotelluric, seismic), and structure of the region; will identify subregional similarities and differences for each process; and will generate timespace maps showing the characteristics of basins in the region. These studies will also help to define the geometry, structure, and concealed bedrock lithology of Cenozoic basins in the Basin and Range in order to provide accurate mineral resource assessments of the Western United States. In addition, an understanding of post-Laramide basin forming structural events in time and space throughout the region is essential to identifying the sub regional processes necessary to develop more area-specific assessment methods.
  • Post-mineralization structural cover characteristics of Alaska - The primary objective is to construct balanced cross sections to project mineral deposits (and potential host strata) under structural cover in the foothills belt of the Brooks Range. Understanding the geometry and extent of the sedimentary basin in which Zn-Pb deposits formed is dependant on such cross sections.

Geophysical and geochemical techniques to identify concealed mineral resources

Find new geochemical and geophysical tools that might be useful for assessing the occurrence of concealed mineral resources, and to develop and test new innovative tools that might be used for assessing concealed deposits.

  • Synergy of regional stream sediment geochemistry, hydrology, and structure for assessments of concealed deposits - Three basin areas (see below) are good candidates for study. Initial objectives are to compile existing data in a digital platform mainly using well chemistry, core, and core logs from state and private institutions. Later phases will include field studies to look specifically at the hydrology of the basin, structural controls, and collection of new data in key areas. Three basin areas in southern Arizona are being considered for study: The Eloy-Stansfield, Douglas-Agua Prieta, and the central Gila River basin in the Superior-Safford, Az corridor.
  • Electrogeochemical techniques in detection of deposits - The objective of this task is to investigate the processes that produce electrochemical anomalies in overburden overlying massive sulfide or porphyry deposits. To meet these objectives, we propose a two phase approach. The first phase is to measure the electrochemical potentials over known deposits. The pH, oxidation-reduction potential (ORP) and self potential (SP) of the overburden will be measured. The second stage will be to conduct laboratory measurements on ore samples as a function of pH and ORP. Interpretation of geophysical and geochemical data will facilitate a better understanding of the processes responsible.
  • Evaluation of other geophysical and geochemical assessment methods not currently in use by the USGS - The purpose of this task is to research and test geophysical and geochemical methods that have not been applied, or have only marginally been applied, to mineral resource evaluation studies. The USGS is uniquely qualified to conduct this kind of investigation because of the wide range of in-house expertise in both established and frontier methods in geophysics and geochemistry with application to the full gamut of USGS programs and projects.
  • Textural analysis of geophysical data - The objective of this work is to find procedures for quantifying textural variations in gravity and aeromagnetic survey data to provide estimators of lithology types of the source rocks. The focus will be on new high precision aeromagnetic surveys that provide sufficient resolution to identify lithologies.

Methods of combining datasets to produce estimates of possibility/probability

Earlier studies have identified some techniques of combining disparate datasets such as geologic maps, aeromagnetic data, and geochemical point data into products that are useful in carrying out mineral resource assessments. However, recent advances both in theoretical studies (such as complex systems analysis, weights of evidence, fuzzy logic, etc.) and in computational ability suggest other approaches to enhance the accuracy and reliability of assessment of the possibility and probability of mineral resources concealed beneath cover. Because much of the prospective ground in the U.S. now is in areas of cover, tools are needed to produce maps or other coverages to assist in assessment of these areas. There is a need to both produce easily usable tools for known techniques, and to evaluate new and innovative techniques for assessment of mineral resources beneath cover.

  • Spatial/temporal data layer correlation - Evaluate generalized possibility/probability of occurrence using multiple spatial and temporal dataset correlations. We will investigate the application of techniques such as complex systems theory, fuzzy logic, weights of evidence and derivatives, and probability theory to combine disparate datasets, identify and rank correlations, and estimate the relative reliability of correlations.
  • Spatial scaling properties of geologic features - Quantitatively capture the spatial scaling properties of geologic features for the purpose of refining and extending classical geospatial analysis methods. This kind of analysis will help define terrane/geologic associations in new ways useful for extending geologic mapping beneath cover and enumerating properties conducive to the occurrence of mineralization.
  • Literature search and re-evaluation of known and documented discoveries of concealed deposits - Many older deposit discoveries were dependent upon available technology of the times. Reevaluation of some of these case studies in light of recent advances in mineral deposit science and, in particular, advances in data coverage and density and modern computer processing methods, may lead to new insights into deposit characteristics that could lead to their detection beneath cover.
  • Statistical methods for evaluating geophysical and geochemical anomaly coverage and developing most efficient data collection - The objective is to apply and evaluate several statistical methods to the coverage of geophysical and geochemical
    surveys to aid in: 1) survey design for most efficient data collection with given resources; and 2) to be able to evaluate
    uncertainties in assessments resulting from incomplete or uneven data coverage.

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