Steve Ludington

Rapid natural acid weathering, physical erosion, and debris-flow hazards in scar areas developed on hydrothermally-altered rocks along the Red River Valley near Questa, New Mexico, USA

In southern Rocky Mountains, catchments characterized by acidic, metalliferous waters that are relatively unaffected by human activity usually occur within areas that have active or historical mining activity. The US Geological Survey has utilized these mineralized but unmined catchments to constrain geochemical processes that control the surfaceand ground-water chemistry associated with near surface acid weathering as well as to estimate premining conditions. Study areas include the upper Animas River watershed, Lake City, Mt. Emmons, and Montezuma in Colorado and Questa in New Mexico. Although host-rock lithologies range from Precambrian gneisses to Cretaceous sedimentary units to Tertiary volcanic complexes, mineralization is Tertiary in age and associated with intermediate to felsic composition, porphyritic plutons. Pyrite is ubiquitous. Variability of metal concentrations in water is caused by two main factors: mineralogy and hydrology. Parameters that potentially affect water chemistry include: host-rock lithology, intensity of hydrothermal alteration, sulfide mineralogy and chemistry, gangue mineralogy, length of flow path, precipitation, evaporation, and redox conditions. Springs and headwater streams have pH values as low as 2.5, sulfate up to 3700 mg/L and high dissolved metal concentrations (for example: A1 up to 170 rag/L; Fe up to 250 mg/L; Cu up to 3.5 mg/L and Zn up to 14 mg/L). With the exception of evaporative waters, the lowest pH values and highest Fe and A1 concentrations occur in water draining the most intense hydrothermally altered areas consisting of the mineral assemblage quartz-sericite-pyrite. Stream beds tend to be coated with iron floc, and some reaches are underlain by ferricrete. When iron-rich ground water interacts with oxygenated waters in the stream or hyporheic zone, ferrous iron is oxidized to ferric iron, which is less soluble, leading to the precipitation of iron oxyhydroxides. Ground-water wells have been drilled and sampled in two unmined, alpine catchments to characterize constituent concentrations, to identify hydrogeochemical processes controlling constituent concentrations, to determine rock hydraulic properties, and to delineate flow paths. By using an integrated approach to investigating surface and ground waters in these acidic catchments a more complete understanding of the hydrogeologic framework is gained.

Porphyry‐style Mineral Deposit Taxonomy—Lessons from the USGS Global Assessment

The completion of the USGS’s Global Mineral Resource Assessment for porphyry copper deposits (Hammarstrom and others, 2013; Johnson and others, 2014) provides a new worldwide database of porphyry copper deposits and prospects that allows examination of how the resource characteristics of porphyry-style deposits vary by tectonic setting and other spatial and temporal characteristics. The new database is distinctly larger than the one published by Singer and others (2008), which contained 422 deposits. The new database contains 452 deposits (and excludes Precambrian deposits). Most of the newly compiled deposits are in Australia, Turkey, Iran, and China. These new data allow us to explore how some of the traditional classifications of porphyry-style deposits present taxonomic challenges.

New method to integrate remotely sensed hydrothermal alteration mapping into quantitative mineral resource assessments

Hydrothermal alteration data mapped using the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) were compiled into hydrothermal alteration polygons for use in an assessment of porphyry copper mineral resource potential in the southwestern United States. Hydrothermal alteration polygons along with geochemistry, gravity and magnetic, lithologic, and deposit and prospects data were compiled in a GIS to produce a quantitative set of physical properties for each polygon that were effectively used in making estimates of undiscovered deposits for each permissive tract. Results show a higher estimate of potential undiscovered deposits (17 vs 14) for permissive tracts when ASTER alteration data were used in the assessment.

Undiscovered Phanerozoic Porphyry Copper Deposits—A Global Assessment

Porphyry copper deposits represent the principal source of global copper supply. To address the questions of where future copper supplies are likely to come from and how much copper could exist within the upper kilometer of the earth’s crust, the USGS led a cooperative international effort to assess the world’s undiscovered Phanerozoic porphyry copper deposits using a geologybased, probabilistic form of mineral resource assessment (Singer and Menzie, 2010). Globally, 175 tracts permissive for porphyry copper deposits were defined to include volcanic and intrusive rocks of specified ranges of age and composition. The rocks represent: (1) magmatic arcs that developed on continental crust above subducting oceanic plates, (2) island arcs that formed on oceanic crust, and(or) (3) postconvergent magmatic belts within continents. Quantitative assessments of undiscovered resources were done for 155 of those permissive tracts.

Relationship between Porphyry Copper Occurrences, Crustal Preservation Levels, and Amount of Exploration in Magmatic Belts of the Central Tethys Region

A probabilistic assessment of undiscovered resources in porphyry copper deposits in the Central Tethys region of Turkey, the Caucasus, Iran, western Pakistan, and southern Afghanistan was conducted as part of a U.S.G.S. global mineral resource assessment. The purpose was to delineate areas as permissive tracts for the occurrence of porphyry Cu-Mo and Cu-Au deposits, and to provide estimates of amounts of Cu, Mo, and Au likely to be contained in undiscovered porphyry deposits (Zürcher et al., 2013; Zürcher et al., in review). Tectonic, geologic, geochemical, geochronologic, and ore deposits data compiled and analyzed for this assessment show that magmatism in the region can be rationalized in terms of fundamental plate tectonic principles, including mantle-involved post-subduction processes. However, uplift, erosion, subsidence, and burial of porphyry copper deposits also played an important role in shaping the observed metallogenic patterns.

The Outlook for Volcanic-Hosted Gold Deposits in the Republic of Costa Rica

Gold deposits in Costa Rica are found in lavas and pyroclastic rocks in volcanic arcs ranging in age from Miocene to the present. Although the arcs and some individual volcanos vary widely in alkalinity and other compositional parameters, there seems to be no close relationship between composition and associated type of epithermal precious-metal deposits.