Technical Memorandum

Transcription

1 1383 :-2 o/c) it TETRA TECH Tel ? Fax Tu,!;fin 1;1 1;: Technical Memorandum To: Company: Re: CC: Kathy Arnold From: Mark A. Williamson Rosemont Copper Company Date: October 26, 21 Rosemont SPLP Usage for Pit Wall Runoff Doc #: 256/ David Krizek, P.E.; Amy Hudson (Tetra Tech) 1. Introduction Review comments from SRK Consulting (SRK, 21) regarding the Geochemical Pit Lake Predictive Model report (Tetra Tech, 21) indicate a hesitation to use Synthetic Precipitation Testing Procedure (SPLP) results to represent pit wall runoff chemistry. Rather, the use of other testing methods that have water:rock testing ratios lower than the SPLP ratio of 2:1 are preferred. Tests such as humidity cell testing (HCT) and the Meteoric Water Mobility Procedure (MWMP) are cited as more appropriate. Testing results compiled as part of the Rosemont geochemical characterization program suggest, however, that the SPLP is an adequate and acceptable method for representing pit wall chemical loading. 2. Results Attachment 1 provides graphical representations of average HCT results for the tested rock types, along with average SPLP and MWMP results, as available. Where analyzed, graphs are provided for key major and trace elements such as Ca, Mg, As, Cu, Se, and Mn. As shown on the graphs, overall the SPLP results are consistent with the long-term HCT results, if not higher in concentration. Although some exceptions are noted, these are not great enough to alter the overall assertion that SPLP results are suitable for predicting pit wall runoff chemistry at Rosemont, especially in light of the limited accuracy of such predictions. Under normal circumstances, long-term HCT test results are a suitable choice for modeling pit wall runoff, as this component of the model represents the repeated, seasonal rinsing of wall rock that does not provide for significant weathering time between rinses. The results illustrated in Attachment 1, however, indicate that despite the relatively high water:rock ratio of 2:1, SPLP provides a reasonable source term characterization for pit wall runoff that is consistent with HCT test protocols that utilize lower water:rock ratios (of about 1.5:1) and provide opportunity for material to weather. 3. Conclusion The suitability of using SPLP results over HCT results as illustrated on the graphs provided in Attachment 1 is not common. Review comments have expressed the concern that the high

2 TETRA TECH water:rock ratio used in the SPLP (2:1) result in solution concentrations that are too low compared to HCT and MWMP. Although this statement represents the normal rule, the Rosemont materials appear to be an exception. Taken as a whole, the HCT, MWMP, and SPLP results suggest that water:rock ratio does not exert as strong an influence on leachate chemistry as would normally be expected for mine rock characterization. Therefore, the use of SPLP results to represent pit wall runoff in the Rosemont geochemical pit lake predictive model would appear to be suitable. Other method results, such as early week HCT results, would also be appropriate to represent the flushing of fractures within the damaged (fractured) pit wall as a result of groundwater inflow and/or rinsing by a developing pit lake. In general, the damaged rock zone will experience greater weathering time, which may result in some buildup of soluble salts prior to being rinsed by either inflowing groundwater or contact with the developing lake, thus resulting in relatively higher rinse water chemical concentrations.

3 TETRA TECH References SRK Consulting, Ugorets V. and Day, S. (SRK, 21). Technical Review of (Tetra Tech, 21) Geochemical Pit Lake Predictive Model, Rosemont Copper Project. Technical Memorandum to Dale Ortman (SWCA). Technical Memorandum Dated May 3, 21. Tetra Tech (21). Geochemical Pit Lake Predictive Model. Prepared for Rosemont Copper Company. Report Dated February 21.

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5 Page 1 of 1 Calcium (mg/ L) Bolsa * na E z Bolsa 2-4 I Su lfate (mg/ L) Bolsa M a

6 Page 2 of 1 Arsen ic (mg/ L) t. Bolsa Bolsa a 6 1) cu ro. a. u Bolsa E.35 - a.3 - v.25 #

7 Page 3 of ) Magnesium (mg/ L) Sulfate (mg/ L) Manganese (mg/ L)

8 Attachment 1- Rosemont SPLP Usage for Pit Wall Runoff Page 4 of Arsenic (mg/l) Copper (mg/l) bg.4 E.35 a.).3 Ln

9 Page 5 of 1 Calc ium (mg/l) Magnesium (mg/l) FICT be Manganese (mg/l)

10 Page 6 of Arsenic (mg/l) Copper (mg/l) FICT Selenium (mg/ L)

11 Page 7 of 1 Calcium (mg/l) Arkose ea Arkose E SPLP Arkose a t Ma nganese (mg/l) Arkose i

12 Page 8 o f a Arkose Copper (mg/l) SPLP Arkose I ::1.2 ' MWMP Arkose E '

13 Attachment 1- Rosemont SPLP Usage for Pit Wall Runoff Page 9 of I 1 8 MWMP Andesite E E 3 - Andesite 7; 6-7 u Su lfate (mg/l) Andesite j Manganese ( mg/ L) Andesite o - o

14 Page 1 of :.25 ao I.2 - u ('-t' SPLP Andesite Andesite A SPLP MWMP Selen ium (mg/l) SPLP Andesite