Statement_of_work_CA-ID-TIMS.pdf

ATTACHMENTS

A STATEMENT OF WORK July 2021

I. GENERAL INFORMATION

A. Introduction

The USGS Geology, Minerals, Energy, and Geophysics (GMEG) Science Center requires U-Pb chemical abrasion isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) analyses of zircon and baddeleyite separates collected from the Stillwater Complex in Montana. These analyses will support on-going research being conducted for critical commodity studies of mineralized rocks in the complex.

B. Background

Recent advances in U-Pb CA-ID-TIMS analyses of zircon and baddeleyite minerals have allowed for age determinations precise enough to distinguish the crystallization sequence of individual layers within layered mafic and ultramafic intrusions (Mungall et al., 2016; Scoates et al., 2020; Wall et al., 2018). For this project, the precise age determination of individual layers of igneous rock within the Stillwater Complex help constrain the crystallization history and the origin of the igneous rocks that host the PGE-rich sulfide mineralization in the J-M Reef deposit. Further, we seek to test ages determined by other labs and workers at the University of British Columbia (Weis and Scoates) and Boise State University (Wall) for an interlab comparison of ages of zircons from the same layer within the Stillwater intrusions determined by this method in different laboratories.

C. Definitions/Applicable Documents

The primary use of the U-Pb CA-ID-TIMS method is to date age Zr-minerals in igneous rocks. Highly precise determinations of 207Pb/206Pb isotopic ratios with 2 sigma errors of ~0.01% can be achieved by analyzing multiple, concordant, and internally corrected for U and Pb fractionation, U-Pb data that correspond to 2 sigma age errors of <~200,000 years for rocks that are billions of years old. These age determinations are precise enough to distinguish the crystallization age of individual layers in layered mafic intrusions. Recent applications of the U-Pb CA-ID-TIMS method for dating zircon from cumulates hosted in layered intrusions can be found in the following papers:

Mungall, J. E., Kamo, S. L., & McQuade, S. (2016). U–Pb geochronology documents out-of-sequence emplacement of ultramafic layers in the Bushveld Igneous Complex of South Africa. Nature

Communications, 7, 13385. https://doi.org/10.1038/ncomms13385

Scoates, J. S., & Wall, C. J. (2015). Geochronology of layered intrusions. In B. Charlier, O. Namur, R.

Latypov, & C. Tegner (Eds.), Layered Intrusions (pp. 3–74). Springer Geology.

Scoates, J. S., Wall, C. J., Friedman, R. M., Mathez, E. A., & Vantongeren, J. A. (2021). Dating the Bushveld

Complex : Timing of Crystallization , Duration of Magmatism , and Cooling of the World ’ s

Largest Layered. Journal of Petrology, 62(2) 604. https://doi.org/10.1093/petrology/egaa107

Wall, C. J., Scoates, J. S., Friedman, M., Weis, D., Amini, M., & Meurer, W. P. (2018). Age of the Stillwater

Complex: Integrating zircon geochronologic and geochemical constraints on the emplacement history and crystallization of a large, open-system layered intrusion. Journal of Petrology, 59(1), 153–190. https://doi.org/10.1093/petrology/egy024/4913802

The following review papers cover the U-Pb CA-ID-TIMS dating method and application to mafic rocks:

Condon, D. J., Schoene, B., McLean, N. M., Bowring, S. A., & Parrish, R. R. (2015). Metrology and traceability of U-Pb isotope dilution geochronology (EARTHTIME Tracer Calibration Part I).

Geochimica et Cosmochimica Acta, 164, 464–480. https://doi.org/10.1016/j.gca.2015.05.026

Gerstenberger, H., & Haase, G. (1997). A highly effective emitter substance for mass spectrometric Pb isotope ratio determinations. Chemical Geology, 136(3–4), 309–312.

https://doi.org/10.1016/S0009-2541(96)00033-2

Jaffey, A. H., Flynn, K. F., Glendenin, L. E., Bentley, W. C., & Essling, A. M. (1971). Precision measurement of half-lives and specific activities of U235 and U238. Physical Review C, 4(5), 1889–1906. https://doi.org/10.1103/PhysRevC.4.1889

Mattinson, J. M. (2005). Zircon U-Pb chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages.

Chemical Geology, 220(1–2), 47–66. https://doi.org/10.1016/j.chemgeo.2005.03.011

T.E.Krogh. (1973). A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations. Geochimica et Cosmochimica Acta, 87, 485–494.

Scope

In collaboration with the Stillwater Mine, we have collected eight samples for whole-rock major, minor, and trace element geochemistry, polished thin section, and heavy mineral separation. Based on igneous rock texture, we have collected samples most prospective to contain Zr minerals for age determination. The samples were selected to test the hypothesis that the stratigraphic sequence of the layers does not obey the law of superposition. Sampling focuses on rocks in and around the layer that is being mined for platinum and palladium.

https://doi.org/10.1093/petrology/egaa107

II. WORK REQUIREMENTS

A. Technical Requirements

1. Work Requirements/Products

Technical requirements for U-Pb dating by chemical abrasion isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) include:

• Separation of datable minerals (in this case, zircon). Zircon can be extracted from the rock samples provided by the USGS using mineral separation techniques (such as rock crushing, magnetic mineral separation, Wilfley table procedures; heavy liquid separation, and hand picking).

• Selection of mineral grains by microscopic examination for chemical dissolution and isolation of U and Pb, and mass spectrometric analysis.

• Chemical abrasion of zircon. The zircons are treated using the chemical abrasion method (high temperature annealing and partial dissolution), to remove domains that have lost lead.

• Dissolution of the zircon and treatment of the samples with low-Pb contamination chemical procedures using ultraclean facilities (0.2-0.4 picogram Pb blanks). Purified Pb and U from single dissolved grains or grain fragments U and Pb are isolated from the zircon solutions using anion exchange chromatography, then dried in dilute phosphoric acid (H3PO4), and deposited onto outgassed rhenium filaments with silica gel.

• Mass spectrometry and data reduction. The samples are analyzed by ID-TIMS; data acquired are reduced to calculate an age for the zircon.

2. Criteria for acceptance of Product

The Geochronology Laboratory must be able to demonstrate that they:

• Have at least five (5) years of experience within the last 10 years performing high-precision U-Pb geochronology;

• Are able to perform contamination free mineral separation of samples;

• Are operating in accordance with the latest standards and calibrations, including the Earth-Time calibration of standard solutions;

• Are able to perform single zircon chemical abrasion analysis with blank Pb levels routinely below 0.5 picograms;

• Have the cumulative experience and state-of-the-art capabilities as demonstrated by a publication record of peer-reviewed papers on high-precision dating of geological samples and processes in the peer reviewed literature.

B. Deliverables

1. Characteristics for work product described in II.A.1.

Deliverables will include analyses of 4 to 6 zircon grains in each of 8 different samples from the Stillwater

Complex. A report will be submitted to the USGS upon completion of the work. This report will contain all relevant isotopic data, such as would routinely be included in scientific papers. This includes the following:

• A description of analytical techniques and methods of data reduction

• Sample descriptions

• Description of the U-Pb results

• Tabulation of the isotopic data

• Concordia diagrams displaying data

The report should also include full interpretations of the results that consider discordance, inheritance, and scattering of data, and the possible effects of metamorphism and alteration on the isotopic systems The contractor will communicate with the USGS on a regular basis, and in the case of any unforeseen complications with the rocks samples submitted.

2. Delivery dates Depending on lab schedules, the work for 8 samples will be completed within a year from the award of the contract. Digital data can be transferred to the USGS by email or other means of data delivery. Leftover mineral separates be returned to the USGS after work is complete.

III. SUPPORTING INFORMATION

A. Place of Performance To be determined.

B. Period of Performance Approximately 90 days of lab time, performed within a year from the award of the contract.

C. Government Furnished Property (GFP): None.

D. Special Considerations

1. Key personnel qualifications

The lead geochronologists in the laboratory should have Ph.D. degrees from leading universities and have ten

(10) or more years of experience. The person performing the analyses should also have experience collecting zircon and baddeleyite, chemically abrading the minerals, and successfully collecting the isotopic data from mafic and ultramafic rocks found in igneous intrusions—as shown by being author or coauthor on several peer- reviewed scientific reports or journal papers

2. Data Rights, Safety, Environment: None

3. Security/Privacy: None