40Ar/39Ar data from the Richardson mining district, Big Delta Quadrangle, Alaska

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Frequently anticipated questions:

What does this data set describe?

Title:
40Ar/39Ar data from the Richardson mining district, Big Delta Quadrangle, Alaska
Abstract:
During the 2017 and 2018 field seasons, geologists from the Alaska Division of Geological & Geophysical Surveys (DGGS) conducted geologic mapping and sampling in the Richardson mining district southeast of Fairbanks, including parts of the Big Delta B-4, B-5, B-6, and C-6 quadrangles. The project area has produced approximately 122,000 ounces of gold, mostly from placer mines, and it includes the Uncle Sam, Montecristo, Richardson, Tower, and Hilltop lode gold exploration properties. The goal of DGGS's work in this area is to conduct a mineral-resource assessment and to build an improved understanding of the area's geology and controls on gold mineralization to facilitate industry exploration targeting. The four 40Ar/39Ar ages reported here are all from samples at or near mid-Cretaceous intrusion-related gold prospects. Skarn hornblende from the Banner Creek area (18RN069) yielded an age of 104.0 +/- 1.6 Ma, which places its formation relatively early in the magmatic history of the district. Fine-grained, holocrystalline granodiorite in the immediate vicinity of the Democrat prospect yielded a biotite cooling age of 96.9 +/- 2.0 Ma (18RN568), slightly older than the white mica age reported for the nearby mineralized rhyolite dike (89.4 +/- 0.7 M). White mica from a sericite-pyrite altered, quartz-feldspar porphyry dike in the Rosa Pass area yielded an age of 87.0 +/- 1.5 Ma (17MBW526). Sericite-altered intrusive rock from drill core at the Lone Tree prospect yielded an age of 93.2 +/- 1.2 Ma (18ET057), somewhat younger than the 40Ar/39Ar age determined for presumably magmatic white mica from a dike at the nearby Naosi prospect on the Montecristo property (100.0 +/- 0.7 Ma). The complete report and digital data are available through the DGGS website: http://doi.org/10.14509/30530.
Supplemental_Information: 
data: Identification numbers, coordinates, sample descriptions, and summary ages of each sample; power output levels and resultant measurements from each sample run; and an index of the the contents, format, and structure of a data.
  1. How might this data set be cited?
    Twelker, Evan, Benowitz, J.A., and Naibert, T.J., 2020, 40Ar/39Ar data from the Richardson mining district, Big Delta Quadrangle, Alaska: Raw Data File RDF 2020-11, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 10 p.
  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -146.359570
    East_Bounding_Coordinate: -146.160105
    North_Bounding_Coordinate: 64.526155
    South_Bounding_Coordinate: 64.301195
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 2017
    Ending_Date: 2020
    Currentness_Reference:
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: report and digital data
  6. How does the data set represent geographic features?
    1. How are geographic features stored in the data set?
      This is a point data set.
    2. What coordinate system is used to represent geographic features?
      Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.000001. Longitudes are given to the nearest 0.000001. Latitude and longitude values are specified in decimal degrees. The horizontal datum used is World Geodetic System of 1984.
      The ellipsoid used is WGS 84.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257223563000025.
  7. How does the data set describe geographic features?
    rdf2020-11-summary-data-locations.csv, rdf2020-11-step-heating-data.csv, rdf2020-11-data-dictionary.csv
    Identification numbers, coordinates, sample descriptions, and summary ages of each sample; power output levels and resultant measurements from each sample run; and an index of the the contents, format, and structure of a data. (Source: Alaska Division of Geological & Geophysical Surveys (DGGS) and Geochronology Laboratory at the University of Alaska Fairbanks (UAF))

Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)
  2. Who also contributed to the data set?
    We thank Frazer Tabeart (PolarX Ltd.), Phil St. George and Jack Cote (Millrock Resources Inc.), Mike Stoltz, and Lynn Armstrong for access to the Lone Tree (LT) and Wolf (WFL) drill core on the Uncle Sam property. Rainer Newberry assisted with skarn petrography. The DGGS Richardson geologic mapping project was funded by the State of Alaska.
  3. To whom should users address questions about the data?
    Alaska Division of Geological & Geophysical Surveys
    Metadata Manager
    3354 College Road
    Fairbanks, AK
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov
    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays

Why was the data set created?

The goal of DGGS's work in the Richardson mining district is to conduct a mineral-resource assessment and to build an improved understanding of the area's geology and controls on gold mineralization to facilitate industry exploration targeting.

How was the data set created?

  1. From what previous works were the data drawn?
  2. How were the data generated, processed, and modified?
    Date: 2018 (process 1 of 3)
    Fieldwork - Fresh, unweathered samples from surface outcrops or drill core were collected by DGGS field geologists and were selected for dating based on the presence of sufficiently large crystals appropriate for dating. Except where noted, sample location coordinates (in WGS84 datum) were obtained using GPS-enabled field tablets, with a typical reported accuracy of +/- 10 meters. Samples were examined under a binocular microscope and in thin section to select unaltered mineral phases before sample preparation.
    Date: 2018 (process 2 of 3)
    Sample preparation - Four rock samples were submitted to the Geochronology Laboratory at the University of Alaska Fairbanks (UAF) for 40Ar/39Ar analysis. The samples were crushed, sieved, washed, and hand-picked for muscovite, sericite, biotite, and hornblende mineral phases. The monitor mineral MMhb-1 (Samson and Alexander, 1987) with an age of 523.5 Ma (Renne and others, 1994) was used to monitor neutron flux (and calculate the irradiation parameter, J). The samples and standards were wrapped in aluminum foil, loaded into aluminum cans of 2.5-cm diameter and 6-cm height. The samples were irradiated in position 5c of the uranium-enriched research reactor at McMaster University in Hamilton, Ontario, Canada for 20 megawatt-hours.
    Date: 2018 (process 3 of 3)
    Geochronology analysis - Upon their return from the reactor, the samples and monitors were loaded into 2-mm-diameter holes in a copper tray, which was then loaded in an ultra-high-vacuum extraction line. The monitors were fused, and samples heated, using a 6-watt argon-ion laser following the technique described in York and others (1981), Layer and others (1987), and Benowitz and others (2014). Argon purification was achieved using a liquid nitrogen cold trap and a SAES Zr-Al getter at 400°C. The samples were analyzed in a VG-3600 mass spectrometer at the Geophysical Institute, University of Alaska Fairbanks. The argon isotopes measured were corrected for system blank and mass discrimination, as well as calcium, potassium, and chlorine interference reactions following procedures outlined in McDougall and Harrison (1999). Typical full-system 8-minute laser blank values (in moles) were generally 2 x 10^-16 mol 40Ar, 3 x 10^-18 mol 39Ar, 9 x 10^-18 mol 38Ar, and 2 x- 10^-18 mol 36Ar, which are 10-50 times smaller than the sample/standard volume fractions. Correction factors for nucleogenic interferences during irradiation were determined from irradiated CaF2 and K2SO4 as follows: (39Ar/37Ar)Ca = 7.06 x 10^-4, (36Ar/37Ar)Ca = 2.79 x- 10^-4 and (40Ar/39Ar)K = 0.0297. Mass discrimination was monitored by running calibrated air shots. The mass discrimination during these experiments was 1.3 percent per mass unit. During sample analysis, calibration measurements were made on a weekly to monthly basis to check for changes in mass discrimination with no significant variation observed during these intervals. Note: Complete list of references can be found in the accompanying report.
  3. What similar or related data should the user be aware of?
    Graham, G.E., and Jozwik, Diana, 2007, Data tables related to geology and gold mineralization in the Richardson district, east-central Alaska: Raw Data File RDF 2007-2, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 32 p
    Naibert, T.J., Benowitz, J.A., Wypych, Alicja, Sicard, K.R., and Twelker, Evan, 2018, 40Ar/39Ar data from the Tanacross D-1 and D-2, Big Delta B-4 and B-5, and Mount Hayes A-6 quadrangles, Alaska: Raw Data File RDF 2018-3, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 15 p
    Twelker, Evan, Werdon, M.B., Sicard, K.R., Wypych, Alicja, Naibert, T.J., Athey, J.E., Willingham, A.L., and Lockett, A.C., 2017, Geologic mapping in the Richardson-Uncle Sam area, interior Alaska (presentation): Alaska Miners Association Annual Convention, Anchorage, Alaska, November 7-12, 2017: Poster and Presentation Twelker, Evan and others, 2017, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 37 p
    Twelker, Evan, Wypych, Alicja, Sicard, K.R., Naibert, T.J., Montayne, Simone, Newberry, R.J., and Wyatt, W.C., 2018, Major-oxide and trace-element geochemical data from rocks collected in the Richardson mining district, Big Delta Quadrangle, Alaska: Raw Data File RDF 2018-6, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 4 p
    Twelker, Evan, Wypych, Alicja, Sicard, K.R., Naibert, T.J., Werdon, M.B., Athey, J.E., Willingham, A.L., and Lockett, A.C., 2017, Major-oxide and trace-element geochemical data from rocks collected in the Richardson mining district, Big Delta Quadrangle, Alaska: Raw Data File RDF 2017-11, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 4 p

How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?
    This data set is being released into the DGGS Raw Data File series. The information has been examined by several geologists familiar with the study area, but the data and accompanying report have not been formally reviewed for technical content or for conformity to the editorial standards of DGGS. Questions about the analytical values or procedures can be best answered by the originating lab.
  2. How accurate are the geographic locations?
    Sample location coordinates (in WGS84 datum) were obtained using GPS-enabled field tablets, with a typical reported accuracy of about 10 meters.
  3. How accurate are the heights or depths?
  4. Where are the gaps in the data? What is missing?
    This data release is complete.
  5. How consistent are the relationships among the observations, including topology?
    not applicable

How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?
Access_Constraints:
This report, map, and/or dataset is available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (see contact information below).
Use_Constraints:
Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way, the user is obligated to describe the types of modifications the user has made. The user specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys. The State of Alaska makes no express or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.
  1. Who distributes the data set? (Distributor 1 of 1)
    Alaska Division of Geological & Geophysical Surveys
    Metadata Manager
    3354 College Road
    Fairbanks, AK
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov
    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
  2. What's the catalog number I need to order this data set? RDF 2020-11
  3. What legal disclaimers am I supposed to read?
    The State of Alaska makes no expressed or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.
  4. How can I download or order the data?

Who wrote the metadata?

Dates:
Last modified: 11-Sep-2020
Metadata author:
Alaska Division of Geological & Geophysical Surveys
Attn: Simone Montayne
Metadata Manager
3354 College Road
Fairbanks, AK
USA

(907)451-5020 (voice)
(907)451-5050 (FAX)
dggspubs@alaska.gov
Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:
Generated by mp version 2.9.50 on Fri Sep 11 20:01:42 2020