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Viola, ID, United States

Hickey K.A.,University of British Columbia | Barker S.L.L.,University of British Columbia | Dipple G.M.,University of British Columbia | Arehart G.B.,University of Nevada, Reno | Donelick R.A.,Apatite to Zircon Inc.
Economic Geology | Year: 2014

The duration of hydrothermal activity required to form ore deposits is poorly constrained. We demonstrate that thermochronology data, coupled with thermal modeling, can be used to constrain the duration of hydrothermal fluid flow. Apatite fission-track (AFT) thermochronology data define a conductive halo around an Eocene hydrothermal system that formed the Betze-Post gold deposit on the northern Carlin trend in Nevada. The premineralization Goldstrike stock acted as an essentially impermeable side to the auriferous Carlin hydrothermal system. The hydrothermal fluid conductively heated the intrusion over the time that it flowed past it. To derive first-order estimates for the maximum duration of this flow we numerically modeled one-dimensional conductive heat flow into the intrusion and used the results to forward model ensuing AFT annealing. Modeled levels of annealing were compared to AFT dates and track length data measured across the intrusion. Our results indicate that the episode of main ore-stage hydrothermal fluid flow (mean temperature of 200°C) that formed the ∼1,150 metric ton (t) Betze-Post gold deposit had a maximum duration of <15 to 45 ka. The average gold flux over this period was ∼80 to 30 kg yr-1, comparable to that measured in the deep reservoirs of several modern geothermal fields. Conservative estimates of gold concentration in the main ore-stage fluids imply that fluid upflow rates and total advective heat flow were also comparable to modern geothermal systems. This suggests that the most important factors for generating the large gold deposits of the northern Carlin trend were a large and/or continuous source of gold, and a very efficient means of removing it from the fluid, rather than the hydrologic system itself. The short duration of main ore-stage fluid flow is unlikely to represent a steady-state convective system. Instead, it most likely reflects a transient period of flow following slip and permeability generation along the steeply dipping Post-Genesis fault system that hosts many of the deposits along the northern Carlin trend. A sudden increase in the permeability of a fault may have led to a transitory period of peak fluid temperature as the fault initially tapped meteoric fluid that had resided at depth and had thermally equilibrated with the host rocks. With continued convection the flow drew cooler, less rock-buffered meteoric water down from higher in the system. © 2013 Society of Economic Geologists, Inc.

Bermudez M.A.,Joseph Fourier University | Bermudez M.A.,Central University of Venezuela | Kohn B.P.,University of Melbourne | Van Der Beek P.A.,Joseph Fourier University | And 3 more authors.
Tectonics | Year: 2010

The Venezuelan Andes formed by complex geodynamic interaction between the Caribbean Plate, the Panam Arc, the South American Plate and the continental Maracaibo block. We study the spatial and temporal patterns of exhumation across the Venezuelan Andes using 47 new apatite fission track (AFT) ages as well as topographic analyses. This approach permits the identification of at least seven tectonic blocks (Escalante, Cerro Azul, Trujillo, Caparo, Sierra Nevada, Sierra La Culata and El Carmen blocks) with contrasting exhumation and cooling histories. The Sierra Nevada, Sierra La Culata and El Carmen blocks, located in the central part of the Venezuelan Andes and separated by the Bocon fault system, cooled rapidly but diachronously during the late Miocene-Pliocene. Major surface uplift and exhumation occurred in the Sierra Nevada block since before 8 Ma. A second phase of uplift and exhumation affected the El Carmen and Sierra La Culata blocks to the north of the Bocon fault during the late Miocene-Pliocene. The highest topography and steepest relief of the belt coincides with these blocks. The Caparo and Trujillo blocks, located at the northeastern and southwestern ends of the orogen, cooled more slowly from the Oligocene to the late Miocene. These blocks are characterized by significantly lower mean elevations and slightly lower mean slopes than the central blocks. Unraveling the cooling history of the individual blocks is important to better understand the control of preexisting faults and regional Caribbean geodynamics on the evolution of the Venezuelan Andes. Our data indicate a strong control of major preexisting fault zones on exhumation patterns and temporal correlation between phases of rapid exhumation in different blocks with major tectonic events (e.g., collision of the Panamá arc; rotation of the Maracaibo block). © 2010 by the American Geophysical Union.

Benowitz J.A.,University of Alaska Fairbanks | Haeussler P.J.,U.S. Geological Survey | Layer P.W.,University of Alaska Fairbanks | O'Sullivan P.B.,Apatite to Zircon Inc. | And 2 more authors.
Geochemistry, Geophysics, Geosystems | Year: 2012

Topographic development inboard of the continental margin is a predicted response to ridge subduction. New thermochronology results from the western Alaska Range document ridge subduction related orogenesis. K-feldspar thermochronology (KFAT) of bedrock samples from the Tordrillo Mountains in the western Alaska Range complement existing U-Pb, 40Ar/ 39Ar and AFT (apatite fission track) data to provide constraints on Paleocene pluton emplacement, and cooling as well as Late Eocene to Miocene vertical movements and exhumation along fault-bounded blocks. Based on the KFAT analysis we infer rapid exhumation-related cooling during the Eocene in the Tordrillo Mountains. Our KFAT cooling ages are coeval with deposition of clastic sediments in the Cook Inlet, Matanuska Valley and Tanana basins, which reflect high-energy depositional environments. The Tordrillo Mountains KFAT cooling ages are also the same as cooling ages in the Iliamna Lake region, the Kichatna Mountains of the western Alaska Range, and Mt. Logan in the Wrangell-St. Elias Mountains, thus rapid cooling at this time encompasses a broad region inboard of, and parallel to, the continental margin extending for several hundred kilometers. We infer these cooling events and deposition of clastic rocks are related to thermal effects that track the eastward passage of a slab window in Paleocene-Eocene time related to the subduction of the proposed Resurrection-Kula spreading ridge. In addition, we conclude that the reconstructed KFAT max negative age-elevation relationship is likely related to a long period of decreasing relief in the Tordrillo Mountains. Copyright 2012 by the American Geophysical Union.

A method of determining the concentration of an element of interest in a solid of interest based on the ratio of the measured relative abundances of two isotopes in the solid of interest, one isotope of the element of interest and the second isotope from an element represented in the chemical formula of the solid of interest, and comparing this ratio to the ratio of the measured relative abundances of the same two isotopes for a reference solid for which the concentration of the element of interest is known. A method of calculating the concentration of the element of interest in the solid of interest. A method of executing a computer software program with instructions for calculating the concentration of the element of interest in the solid of interest.

Apatite to Zircon Inc. | Date: 2013-05-01

A method for determining the position and its statistical uncertainty of a fission semi-track in a crystal based on detecting the tip and etch figure of a fission semi-track in a series of transmitted light images. A computer software program for: detecting the tip and etch figure of a fission semi-track in a series of transmitted light images and assessing the viability of the tip using a scoring equation; writing to and loading from a computer database of fission semi-tracks; modifying the scoring equation for assessing fission semi-track tip viability based on the contents of the computer database. A computer database consisting of transmitted light images of fission semi-tracks. A method for determining the statistical probability that a fission semi-track is a real fission semi-track.

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