 REPORT |
Mass Redistribution in Continental Magmatic-Hydrothermal Systems
Yellowstone National Park and the
Fairmont Hot Springs resort near Butte, Montana
21-26 April 2005
Conveners:
- John H. Dilles
- Department of Geosciences, Oregon State University, Corvallis, Oregon, USA
- Greg B. Arehart
- Department of Geological Sciences, University of Nevada, Reno, Nevada, USA
- Peter I. Nabelek
- Department of Geological Sciences, University of Missouri, Columbia, Missouri, USA
- Todd C. Feeley
- Department of Earth Sciences, Montana State University, Bozeman, Montana, USA
Continental porphyry Cu-Mo mine pit, Butte. Click on photo for larger image. |
The conference served as a forum for discussion of how large-scale magmatic-hydrothermal systems that operate in the continental crust produce enormous geochemical redistributions of elements that sometimes result in economic mineral deposits. The goal was to identify the key parameters of these systems so that realistic models can be built for how they form in time and space. The venues served to focus discussion on active shallow parts of hydrothermal systems and deep parts of exhumed fossil systems. The conference brought together 60 researchers from academia, industry, and the U.S. Geological Survey, post-docs and students, many of whom came from distant parts of the world. The participants represented different subdisciplines of geology that are used to study magmatic-hydrothermal systems, including geochemistry, geophysics, ore-deposits research, and igneous and metamorphic petrology.
The Old Faithful geyser provided an ideal backdrop for the first part of the conference in Yellowstone. The conference began with an evening overview of the Yellowstone system by Robert Smith of the University of Utah. Smith has spent much of his career studying the relationship between magmatism and hydrothermal activity in Yellowstone using geophysical methods. He highlighted how earthquake activity relates to faults and fracture systems that serve to focus flow of hydrothermal fluids. He also discussed the history of volcanism in Yellowstone and the shape of the mantle plume that provides the ultimate heat source for this magmatic-hydrothermal system. The first full day of the conference was dedicated to field examination of the volcanic and hydrothermal features of Yellowstone. The field trip was led by Smith, Hank Heasler and Cheryl Jaworoski, both national park rangers and active researchers of the Yellowstone geology, and Jacob Lowenstern, director of the USGS Yellowstone Volcano Observatory. Participants were awed by the active hydrothermal system, which led to lively discussions around geysers and lava flows.
Norris Basin, Yellowstone National Park. Click on photo for larger image. |
The first day of technical sessions began with several talks discussing current research on the Yellowstone system. Anahita Tikku talked about gravity fluctuations that may be related to hydrothermal activity, Guillaume Girard presented analogue modeling of magma mixing, and Lowenstern discussed the mass balance of volatiles as deduced from measurements in the system. David Hill's presentation on the current state of knowledge of the Long Valley caldera gave a comparison with the Yellowstone system. Before lunch, Lang Farmer covered the use of radiogenic isotopes in dating fluid flow through rocks and determining fluid sources, Steve Ingebritsen gave an overview of the crustal permeability structure, and Jeffrey Mauk discussed the use of remote sensing methods to determine the extent and type of alteration related to hydrothermal activity in mineral deposits. In the afternoon, Jim Webster presented a talk on experimental studies of melt-fluid-brine partitioning of CO2 and halogens, Martin Streck discussed the use of apatite as a monitor of the behavior of sulfur in magmas, Wendy Bohrson presented an energy conservation approach to quantification of open-system processes in magma chambers, and Werner Halter summarized the use of laser ablation inductively coupled plasma-mass spectrometry (ICP-MS) of fluid and melt inclusions in porphyry systems to evaluate mass-transfer processes. After the coffee break, Todd Feeley covered oxygen isotope evidence for modification of Lassen volcano magmas by the crust and vice-versa, Alan Matthews discussed potential isotopic exchange between pegmatitic dikes and host rocks on Naxos, Greece, Alan Whittington presented results of experiments on the influence of volatiles on viscosity of magmas and their eruptibility, and Margaret Baker presented a reconnaissance study of pyroxene chemistry to establish the source and modification of platinum reef pyroxenites in the Bushveld intrusion.
In the evening, John Dilles and Mark Reed gave presentations on the Butte porphyry Cu-Mo deposit. Dilles gave a regional overview of Butte and its tectonic context, and he summarized ages of the various stages in the evolution of the Butte magmatic-hydrothermal system. Reed gave a detailed accounting of its petrologic structure, including its stages of mineralization, veining, and the extent and sequence of alteration zones. These overview talks were followed the next day by relocation to the area of Butte. In the afternoon, participants looked at veining in the porphyry system and examined the various lithologies that make up the mining district. In the evening, participants were lodged in the Fairmont Hot Springs resort, where many enjoyed the hot springs during the rest of the conference.
The first three speakers on Friday morning presented metamorphic viewpoints on hydrodynamic systems in contact aureoles. John Bowman discussed the effects of reaction kinetics on distribution of mineral assemblages, Peter Nabelek presented results of 2-D numerical simulations of metamorphic reaction progress and oxygen isotope exchange, and Barb Dutrow showed 3-D simulations of the effects of heat and fluid flow on mineral growth. Mona-Liza Sirbescu examined the causes of very low crystallization conditions of Li-rich pegmatites and Larry Cathles discussed influences on the distribution of individual deposits within porphyry copper regions. Two talks previewed the afternoon trip back to Butte. Brian Rusk discussed fluid inclusion evidence for transition from lithostatic to hydrostatic conditions and Reed presented a model for the possibility that a single fluid could have produced observed mineralization and alteration patterns as it cooled and reacted along a flow-path. During the afternoon trip, participants examined a collection of drill core with Dilles and rocks along the rim of the Continental Pit with Steve Czehura of Montana Resources.
The evening was dedicated to presentation of posters. Greg Arehart presented a work on thermochemical profiling of fluids in Carlin-type deposits, Tucker Barrie showed finite element heat and fluid flow modeling of deep sill-driven hydrothermal systems, Byron Berger showed evidence for effects of external stresses on magma emplacement in the Boulder batholith, Clara Buckroyd showed results of laser ablation ICP-MS analysis of fluid inclusions from the pre-main stage at Butte, Dilles presented oxygen and hydrogen isotope evidence for magmatic-hydrothermal fluids at Butte, Abraham Escalante discussed distal alteration around the Antamina skarn deposit, Peru, Cy Field talked about implications of sulfur isotopes for the Butte hydrothermal system, Randy Griffin showed an edge convection model for magmatic emplacement process, Matthew Harper presented a poster on platinum group element mineralization in the Stillwater complex, and Emily Hartwick presented fluid inclusion evidence for fluid evolution and emplacement conditions of a pegmatite dike in Wisconsin. A poster by Shaul Hurwitz presented evidence for cycling of halogens through the Cascadia subduction zone and its volcanic arc, David Johnson discussed scales of mass transfer in igneous-related hydrothermal systems, Ellie Leavitt presented a study of the Midas epithermal gold deposit, Nori Tsuchiya discussed water-rock interaction in the Shuteen Complex, Mongolia, Sandy Underwood presented oxygen isotope evidence for subducted H2O in the source of primitive calc-alkaline mafic lavas in the southernmost Cascades, and Ian Warren presented implications of potassium metasomatism for Au-Ag mineralization.
On Saturday morning, there were talks on fluid-rock interactions and geochemistry of fluids, including a talk by John Ferry on the application of reaction-transport theory for modeling dolomitization of limestones, a talk by Jamie Wilkinson on the potential of Zn and Cu isotopes as tracers of metal transport in ore systems, and a talk by John Porter on the temporal and spatial variations of sulfur isotopes in the Ertsberg mining district, Indonesia. Kinetics of feldspar alteration were used by Peter Larson to estimate the duration of the hydrodynamic system at Rico, Colorado, and chemical and fluid inclusion arguments were made by Ed van Hees for a magmatic source of the Au mesothermal deposits at Yellowknife, Canada. There were several experimental talks in this session, including a summary by Scott Wood of experiments on rare earth element (REE) fractionation between minerals and liquids and solubilities of REE containing minerals, a presentation by John Kaszuba on chemical fractionation between CO2-H2O fluids and brines, a discussion by Craig Manning of the application of experimental results on mineral solubilities in fluids to crustal-scales mass-transfer processes, a talk by Phil Piccoli on application of halogen partitioning into apatite to determine their transfer from magmas to hydrothermal fluids and brines, and a presentation by Adam Simon of his work on Cu, Au, and As partitioning in sulfur-bearing systems.
In the final afternoon session, Stephen Kesler presented arguments that sulfide enclaves in Cu-rich felsic magmas may have been derived from source regions that contained copper sulfides. David John described hydrothermal alteration related to recent activities of several Cascade volcanoes. Additional speakers presented case studies of several mineral deposits. Shane Ebert talked about the Donlin Creek deposit in Alaska, Marcos Zentilli about the Chuquicamata porphyry in Chile, and Frank Dudas about the Lone Star porphyry in Arizona. The last talk of the meeting, by Stuart Simmons, was a summary of the characteristics and processes in epithermal ore deposits. A session in the evening was dedicated to presentation of ideas for future collaborative studies involving multiple subdisciplines that can be applied to studies of mass-redistribution in the continental crust.
On Sunday, 24 participants traveled to Butte, where Rob Cronoble of the Underground Miner Training Program led them to underground workings in the Lexington Mine, and visited the Geologic Research Lab rock sample collection of the Anaconda Company archived by the Montana Bureau of Mines and Geology.
Registration and logistics at the conference venues were handled by Dianna Gury of Quality Business Services. The conference costs were partially underwritten by the society's Penrose Fund as well as the National Science Foundation and the Society of Economic Geologists. The conveners thank all the attendees, the people and organizations noted above, for a successful meeting focused on scientific presentations, discussions, and planning.
Participants
Greg Arehart |
Lew Gustafson |
John Porter |