APPENDIX B SAMPLE ADEQUACY EVALUATION FOR ROSEMONT GEOLOGIC MATERIALS

Transcription

1 APPENDIX B SAMPLE ADEQUACY EVALUATION FOR ROSEMONT GEOLOGIC MATERIALS

2 B1. INTRODUCTION An open pit copper mine and ore processing operation are planned for the Rosemont Copper Project (Rosemont) site, located approximately 3 miles southeast of Tucson, Arizona. Processing of approximately 546 million tons (Mt) of sulfide ore and 69 Mt of oxide ore is expected to generate up to 1,232 Mt of waste rock during the anticipated 2-25 year mine life. Consequently, a baseline geochemical characterization was prepared which focused on the potential water quality impacts from the various mine facilities (e.g., waste rock and dry stack tailings storage areas). One of the primary goals of the baseline characterization study was to test a representative number of samples in order to adequately characterize the geochemical behavior of the rock that would be developed from mining (Tetra Tech, 27). The results from this geochemical testing can also be used to estimate the geochemistry of non-ore rocks in the final walls of the pit. The exposed pit wall lithology will be dominated by arkosic rocks of the Willow Canyon Formation, the Horquilla Limestone, and Bolsa Quartzite, with less exposure of additional limestone, quartz monzonite porphyry, and andesite (Table B1). Most of the primary sulfide mineralization is hosted by the Horquilla, Colina, and Epitaph Limestones, although the total sulfide content of these Paleozoics is generally low compared to other southwest porphyry copper systems (Tetra Tech, 27). In fact, the total sulfur content of the overlying arkosic and andesitic lithologies is generally higher than the remainder of the deposit. A total of 226 applicable samples were subsequently submitted for standard static testing procedures. The most commonly-used static test is known as acid-base accounting (ABA), which measures the balance between the acid-producing potential (AP) and the acid-neutralizing potential (NP) (White and others, 1999). The AP is determined by sulfur analysis and determines the sulfur content associated with pyritic sulfur. The NP is determined by acid-titration and generally represents the carbonate content of the sample. The net-neutralizing potential () is the difference between these values ( = NP - AP) and is typically expressed in units of kilograms of calcium carbonate (CaCO 3 ) per ton of rock (kg CaCO 3 /t rock, or kg/t). The, together with the NP ratio (NP/AP), is an important parameter used to classify a material as either potentially-acid generating (PAG) or inert with respect to acid generation. Because the ABA characteristics for a given sample reflect the dominant mineralogic properties of the material (i.e., carbonate and sulfur mineral content), ABA results can be used to evaluate if a material has been adequately characterized with respect to its potential effects on water quality. Tetra Tech February 21 B-1

3 B2. TECHNICAL OBJECTIVE The objective of this sample adequacy evaluation is to assess the degree to which results from geochemical testing represent the overall geochemical tendencies of various rock types at Rosemont. Tetra Tech February 21 B-2

4 B3. TECHNICAL EVALUATION Numerous criteria for determining an adequate sample population have been suggested as a means of obtaining representative samples of waste rock (USEPA, 1983; USEPA, 1994; Maest and Kuipers, 25; Runnells and others, 1997). However, because it is impossible to confidently predict the degree of heterogeneity of a material, it is impossible to predict in advance how many samples will be required to representatively characterize it. In concept, a perfectly homogeneous material requires only one sample. Because the degree of variability in geochemical properties of rock is unique to each site, a reasonable approach is to determine the sample requirement based upon site-specific variability. Samples may be taken from over a reasonable volume of the rock unit under consideration and continuously characterized until no further significant variability is observed. Such a process explicitly determines the heterogeneity of geochemical characteristics and demonstrates an adequate level of characterization. The evaluation of sample adequacy presented herein was conducted using the approach outlined by Runnells and others (1997), which utilizes statistical measures of central tendency (mean) and dispersion (standard deviation) (USEPA, 2) to evaluate sample representation. The method uses a stepwise evaluation to evaluate the degree to which additional sample analysis improves the level of confidence for a given parameter. Once the naturally-occurring variability of a rock unit is established, specific samples may be selected for detailed characterization of water quality that results from contact with that material. During the baseline geochemical characterization (Tetra Tech, 27), samples of geologic materials were submitted for ABA testing in proportion to their expected occurrence in the waste rock. The previous baseline testing results can be found in Appendix A of the Geochemical Characterization Addendum 1 Report (Tetra Tech, 27). A subsequent evaluation of these data using the approaches described above indicated that insufficient information existed for several of the rock types, and therefore additional geochemical analysis was conducted in 28. This additional data was composited with data from Tetra Tech (27) for subsequent analysis. Illustration B1 shows the spatial extent of sampling of rock within the projected pit. In this illustration, the traces of the boreholes are shown as lines and the individual sample locations are shown as a colored segment of the line. Samples were collected from a relatively large volume in the area most proximal to mineralization, where variability of the unit would be expected to be greatest. Samples were also collected, although fewer in number, at more distal positions, where less variability was anticipated. The final composite ABA data for the rock types analyzed (Table B2) were first listed in random order, and then a moving average and standard deviation were calculated for. The resulting data were graphed. This analysis evaluates potential increasing convergence toward the population mean (average), and decreasing variability about the population mean, with increasing sample size. A given rock type was considered to be adequately characterized when increasing the sample size produces a change of <1% in the average, and if the slope of the standard deviation approaches zero. Tetra Tech February 21 B-3

5 B4. RESULTS AND DISCUSSION The statistical results (from the Geochemical Characterization Addendum 1 [Tetra Tech, 27] and subsequent additional analyses) for within each rock type evaluated are presented on Illustrations B2 through B14. Each illustration depicts the moving average and standard deviation of the for a given rock type. Based on these results, the rock types which will be exposed on the pit walls have been adequately characterized. Increasing the number of samples associated with these materials for analysis would yield limited or no increased definition of their chemical characteristics, and consequently no further sampling was deemed necessary. For example, Illustration B2 shows the variation in the moving average and standard deviation for the Willow Canyon Formation arkose values. For the first few samples, the running average changes by more than 1% as the sample number increases, but with increasing sample size, very little change is seen for both the moving average and the standard deviation (Illustration B2). Therefore, increasing the number of arkose samples will not change the level of confidence in the average value, indicating that an adequate number of arkose samples have been analyzed. Similar trends are apparent for the remaining rock types, which includes the rock types that are expected to dominate the final pit wall exposure. Tetra Tech February 21 B-4

6 B5. CONCLUSIONS Statistical evaluation of ABA data collected from the Rosemont geologic materials provides a method for evaluating sample adequacy using site-specific geochemical parameters, rather than relying on arbitrary literature criteria developed for mine materials in general. Application of a statistical technique to the Rosemont Project site indicates that an adequate number of samples have been analyzed to characterize their central geochemical tendency. Tetra Tech February 21 B-5

7 B6. REFERENCES Maest, A. S. and Kuipers, J. R. (25) Predicting Water Quality at Hardrock Mines: Methods and Models, Uncertainties and State-of-the-Art. Independent report prepared through support from EARTHWORKS, Washington, D.C. Runnells, D.D., M. Shields, and R.L. Jones (1997). Methodology for adequacy of sampling and mill tailings and mine waste rock. pp , Proceedings of the Fourth International Conference on Tailings and Mine Waste. Fort Collins, CO. January 13-17, AA. Balkema, Rotterdam, Netherlands. 788 pp. Tetra Tech (27). Geochemical Characterization Addendum 1. Prepared for Rosemont Copper Company. Report Dated November 27. U.S. Environmental Protection Agency (USEPA) (1983). Preparation of Soil Sampling Protocol: Techniques and Strategies. EPA-6/S4-83/2. Environmental Monitoring Systems Laboratory, Las Vegas, NV. USEPA (1994). Acid Mine Drainage Prediction. EPA53-R Office of Solid Waste, Special Waste Branch. Washington, DC. USEPA (2). Guidance for Data Quality Assessment: Practical Methods for Data Analysis. EPA QA/G-9, QA Update. EPA/6/R-96/84. Office of Environmental Information, Washington, DC. White, W.W., K.A. Lapakko, and R.L. Cox (1999). Static-test methods most commonly used to predict acid-mine drainage: Practical guidelines for use and interpretation. In G.S. Plumlee and M.S. Logsdon (eds.) The Environmental Geochemistry of Mineral Deposits, Part A: Processes, Techniques, and Health Issues. Vol. 6A. Society of Economic Geologists, Inc. Tetra Tech February 21 B-6

8 TABLES Tetra Tech February 21 B-7

9 Table B1 Projected Exposed Areas and ABA Summary for Various Rock Types in the Rosemont Pit Rock Type % of Exposed Area No. Samples Analyzed for ABA Willow Canyon Formation, Arkose Horquilla Limestone Bolsa Quartzite Abrigo Formation Epitaph Formation Tertiary Gravel Colina Limestone Earp Formation Glance Conglomerate Escabrosa Limestone Concha Martin Formation Precambrian Granodiorite 1. Willow Canyon Formation, Andesite Scherrer.62 Quartz Monzonite Porphyry.53 9 Overburden.15 6 TOTAL Table B2 Summary of ABA Data Used to Evaluate Sampling Adequacy Sample ID Rock Type # Samples AP NP - Mean - Std. Dev Abrigo Abrigo Abrigo A818-1 Abrigo Abrigo A78-1 Abrigo AR219-2 Andesite AR221-1 Andesite AR23-6 Andesite AR21-3 Andesite AR217-6 Andesite A Andesite AR29-3 Andesite AR214-3 Andesite AR217-1 Andesite A Andesite AR228B-1 Andesite AR243-1 Andesite AR213-1 Andesite AR213-2 Andesite Tetra Tech February 21 B-8

10 Table B2 Summary of ABA Data Used to Evaluate Sampling Adequacy Sample ID Rock Type # Samples AP NP - Mean - Std. Dev. AR23-5 Andesite AR211-3 Andesite AR232-1 Andesite AR226-1 Andesite AR238-4 Andesite AR229-1 Andesite A Andesite AR22-2 Andesite AR238-1 Andesite AR23-3 Andesite Andesite AR222-1 Andesite AR214-2 Andesite A88-1 Andesite AR238-6 Andesite A817-1 Andesite A Andesite AR216-1 Andesite AR237-1 Andesite AR238-3 Andesite AR217-5 Andesite AR213-3 Andesite AR225-3 Andesite AR225-1 Andesite AR237-2 Arkose AR211-1 Arkose A873-1 Arkose Arkose AR235-1 Arkose VABH69-1 Arkose AR29-2 Arkose AR24-1 Arkose AR236-1 Arkose AR22-1 Arkose AR25-1 Arkose AR22-1 Arkose AR211-2 Arkose AR226-2 Arkose AR217-7 Arkose AR213-5 Arkose AR23-1 Arkose AR29-1 Arkose A857-1 Arkose AR24-1 Arkose AR225-1 Arkose Tetra Tech February 21 B-9

11 Table B2 Summary of ABA Data Used to Evaluate Sampling Adequacy Sample ID Rock Type # Samples AP NP - Mean - Std. Dev. AR21-2 Arkose AR25-2 Arkose AR241-1 Arkose AR23-4 Arkose AR214-1 Arkose AR239-3 Arkose AR27-1 Arkose AR217-3 Arkose AR23-3 Arkose AR238-5 Arkose AR219-1 Arkose AR213-4 Arkose AR236-3 Arkose AR23-2 Arkose AR21-1 Arkose AH4-1 Arkose AR242-2 Arkose AR23-2 Arkose Arkose AR239-6 Arkose AR21-2 Arkose AR243-2 Arkose AR238-2 Arkose A886-1 Arkose AR225-2 Arkose A83-3 Arkose Arkose (AR254) Arkose A814-1 Arkose AR21-1 Arkose AR225-4 Arkose AR232-2 Arkose A831-1 Arkose AR215-1 Arkose AR2-1 Arkose AR242-4 Arkose AR267-1 Bolsa AR233-1 Bolsa AR259-1 Bolsa VABH68-1 Bolsa AR223-1 Bolsa A78-2 Bolsa A78-3 Bolsa AR266-1 Bolsa Bolsa AR273-1 Bolsa Tetra Tech February 21 B-1

12 Table B2 Summary of ABA Data Used to Evaluate Sampling Adequacy Sample ID Rock Type # Samples AP NP - Mean - Std. Dev Bolsa AR272-1 Bolsa AR26-1 Bolsa A852-1 Colina A84-1 Colina A815-1 Colina A865-1 Colina AR211-4 Colina Colina Colina A86-1 Colina AR 21-4 Colina AR22-2 Colina AR241-2 Colina AR242-1 Concha AR242-5 Concha AH4-2 Concha AR26-1 Concha A88-2 Concha A84-1 Concha AR219-3 Earp AR23-1 Earp A849-1 Earp A83-4 Earp Earp Earp A845-1 Earp AR235-2 Earp AR217-2 Earp A834-2 Earp AR2-3 Earp AR2-2 Earp AR214-5 Earp AR228B-2 Earp AR29-4 Epitaph A847-1 Epitaph A828-1 Epitaph A86-2 Epitaph AR24-2 Epitaph A85-1 Epitaph A86-3 Epitaph Epitaph Tetra Tech February 21 B-11

13 Table B2 Summary of ABA Data Used to Evaluate Sampling Adequacy Sample ID Rock Type # Samples AP NP - Mean - Std. Dev. AR22-3 Epitaph AR234-2 Epitaph A829-1 Epitaph A825-1 Epitaph A81-1 Epitaph A83-1 Epitaph AR21-3 Epitaph AR214-4 Epitaph A81-1 Epitaph Escabrosa Escabrosa A814-2 Escabrosa A872-1 Escabrosa AR24-5 Escabrosa Escabrosa A812-1 Escabrosa Escabrosa Escabrosa A871-1 Escabrosa AR24-2 Glance A85-1 Glance Glance A834-1 Glance A845-2 Horquilla A878-2 Horquilla Horquilla AR239-7 Horquilla A89-1 Horquilla A86-1 Horquilla Horquilla A842-1 Horquilla A866-2 Horquilla AR27-2 Horquilla Horquilla AR24-3 Horquilla AR243-3 Horquilla AR24-4 Horquilla AR242-3 Horquilla AR2-4 Horquilla AR217-8 Horquilla AR242-6 Horquilla AR 23-7 Horquilla AR Horquilla Tetra Tech February 21 B-12

14 Table B2 Summary of ABA Data Used to Evaluate Sampling Adequacy Sample ID Rock Type # Samples AP NP - Mean - Std. Dev. AR 2-5 Horquilla AR Horquilla AR Horquilla AR 26-2 Horquilla AR Horquilla AR 24-6 Horquilla A856-1 Martin Martin A866-1 Martin Martin A878-1 Martin Martin Martin AR239-2 Overburden AR239-5 Overburden AR239-4 Overburden A821-1 Overburden Overburden AR239-1 Overburden AR236-4 QMP AR234-1 QMP A855-1 QMP AR237-3 QMP QMP QMP QMP AR236-2 QMP A815-2 QMP Tetra Tech February 21 B-13

15 ILLUSTRATIONS Tetra Tech February 21 B-14

16 Illustration B1 Drill Holes and Samples Used to Characterize Non-Ore Rock Tetra Tech February 21 B-15

17 Willow Canyon Formation, Arkose N Mean - Std. Dev Illustration B2 Moving Average and Standard Deviation of Values for Rosemont Willow Canyon Formation Arkose Samples Horquilla Limestone Mean - Std. Dev Illustration B3 Moving Average and Standard Deviation of Values for Rosemont Horquilla Limestone Samples Tetra Tech February 21 B-16

18 Bolsa Quartzite Mean - Std. Dev Illustration B4 Moving Average and Standard Deviation of Values for Rosemont Bolsa Quartzite Samples Epitaph Formation Mean - Std. Dev Illustration B5 Moving Average and Standard Deviation of Values for Rosemont Epitaph Formation Samples Tetra Tech February 21 B-17

19 Abrigo Formation Mean - Std. Dev Illustration B6 Moving Average and Standard Deviation of Values for Rosemont Abrigo Formation Samples Glance Conglomerate Mean - Std. Dev Illustration B7 Moving Average and Standard Deviation of Values for Rosemont Glance Conglomerate Samples Tetra Tech February 21 B-18

20 Escabrosa Limestone Mean - Std. Dev Illustration B8 Moving Average and Standard Deviation of Values for Rosemont Escabrosa Limestone Samples Earp Formation Mean - Std. Dev Illustration B9 Moving Average and Standard Deviation of Values for Rosemont Earp Formation Samples Tetra Tech February 21 B-19

21 Colina Limestone Mean - Std. Dev Illustration B1 Moving Average and Standard Deviation of Values for Rosemont Colina Limestone Samples Martin Formation Mean - Std. Dev Illustration B11 Moving Average and Standard Deviation of Values for Rosemont Martin Formation Samples Tetra Tech February 21 B-2

22 Quartz Monzonite Porphyry Mean - Std. Dev Illustration B12 Moving Average and Standard Deviation of Values for Rosemont Quartz Monzonite Porphyry Samples Willow Canyon Formation, Andesite Mean - Std. Dev Illustration B13 Moving Average and Standard Deviation of Values for Rosemont Willow Canyon Formation Andesite Samples Tetra Tech February 21 B-21

23 Overburden Mean - Std. Dev Illustration B14 Moving Average and Standard Deviation of Values for Rosemont Overburden Samples Tetra Tech February 21 B-22