The NBMG Standard Reference Material Project

Spring 1991

The Nevada Bureau of Mines and Geology provides inexpensive standard reference materials (SRMs) for precious-metal assays. In addition to producing quality assurance standards for geochemical laboratories, this project is designed to assess the quality of data generated by specific laboratories and to determine the variation in analytical results among laboratories. The SRMs produced by this project are priced significantly lower than the typical prices for SRMs, encouraging greater use of SRMs for quality assurance in precious-metal assays.

At present, six mines have supplied bulk samples for this project: the Mesquite (metamorphic-hosted) mine in California; the Paradise Peak (andesite-hosted), the Jerritt Canyon (carbonaceous limestone- hosted), the Round Mountain (rhyolite-hosted), and the Boss (carbonate-hosted) mines in Nevada; and the Stillwater (mafic-hosted) mine in Montana. At most of these mines, three bulk samples are taken: high-grade ore, low-grade ore, and unmineralized host rock.

All bulk samples are pulverized, ball-milled for 24 hours, and blended in a cement mixer at the NBMG laboratory. The resulting samples are 98% -200 mesh and are homogeneous, as indicated by repeated fire assays at the NBMG laboratory. Fifteen geochemical laboratories are analyzing splits of each of these samples for gold and silver. Boss and Stillwater samples are also being analyzed for platinum-group elements.

Analyses have been completed for five SRMs from Paradise Peak and Jerritt Canyon; these are now ready to use. SRMs from the remaining four mines are expected to become available during 1991. Results of fire assays of the five currently available SRMs can be summarized as follows:

SRM          Gold content                 Silver content
No.      Mean    Std. dev. Median     Mean  Std. dev. Median 

        Paradise Peak (andesite-hosted)
NBM-la   9.8 ppb   --     5.0 ppb    575 ppb   --   400 ppb
NBM-lb   0.046    0.003   0.045        0.41   0.09   0.40
NBM-Ic   0.464    0.016   0.468        5.79   0.54   5.80

Jerritt Canyon (carbonaceous limestone-hosted) NBM-2a 9.6 ppb -- 5.0 ppb 270 ppb -- 200 ppb NBM-2b 0.228 0.010 0.228 0.02 -- 0.01

All values are in troy ounces per short ton except where noted. The gold and silver contents of the two unmineralized samples (NBM-la and NBM-2a) are below the detection limits of several participating laboratories. The silver content of the ore sample from Jerritt Canyon is also close to detection limits of most laboratories. Standard deviations are not given for the unmineralized samples and the silver content of NBM-2b because their distribution curves are rather badly skewed due to truncation at the detection limits, and standard deviation is meaningless as an estimator of confidence limits when the distribution is not approximately normal. In these three cases the median is a more useful statistic than the mean because the means were calculated from detectable values only.

The distributions of the gold and silver measurements by the participating laboratories for the ore-grade SRMs are shown in the histograms on the next page. Distributions of gold values appear more tightly constrained than the silver values. This is probably a reflection of the poorer analytical precision for silver than a description of the homogeneity of silver in the SRMs.

These SRMs can be purchased in 30-gram bags and 300-gram bottles at the sales office (room 310 in the Scrugham Engineering-Mines Building on the University of Nevada, Reno campus), by mail (Nevada Bureau of Mines and Geology/178, University of Nevada, Reno, NV 89557-0088), by FAX (702-784-1709), or by telephone (702-784-6691). Mail orders must either be prepaid by check or money order made out to "Board of Regents," or charged to a MasterCard or VISA credit card. Telephone and FAX orders must be charged to MasterCard or VISA. When ordering by mail or FAX with a credit card, please include card number and expiration date. Each 300-gram bottle is US$50.00 + $5.00 shipping ($8.00 international) and each 30-gram bag is US$7.00 + $1.00 shipping ($3.00 international).

This project would not have been possible without the help and cooperation of the mine operators that contributed bulk ore samples and the geochemical laboratories that analyzed the blended materials. We appreciate the assistance of the following: American Assay Laboratories, Barringer Laboratories, Bondar-Clegg Inc., Chemex Laboratories Ltd., Cone Geochemical Inc., FMC Gold Company, Geochemical Services Inc., Goldfields Operating Company, Independence Mining Company, Legend Metallurgical Laboratory Inc., Mineral Processing and Environmental Laboratories Inc., Rocky Mountain Geo- chemical of Nevada, Round Mountain Gold Corporation, Skyline Laboratories Inc., Stillwater Mining Company, U.S. Bureau of Mines, and U.S. Geological Survey.

---Paul J. Lechler, Chief Chemist/Geochemist, and Mario O. Desilets, Assistant Chemist