Elevated Uranium and Arsenic Concentra4ons in Baseline Water Quality at the Coffee Gold Project: Implica4ons for Geochemical Predic4ons

Transcription

1 Elevated Uranium and Arsenic Concentra4ons in Baseline Water Quality at the Coffee Gold Project: Implica4ons for Geochemical Predic4ons John Dockrey 1, David Flather 1, Laura Findlater 1, ScoJ Jackson, 1 Jordi Helsen 1, James ScoJ 2 and Jennie Gjertsen 2 1 Lorax Environmental Services Ltd., Vancouver, Canada 2 Goldcorp Inc., Vancouver, Canada 24 TH Annual BC MEND ML/ARD Workshop November 29-3, 217

2 Overview Elevated metal(loid) concentra4ons are oren observed in surface and groundwater surrounding mineral deposits Inves4ga4ng elevated metal concentra4ons in the baseline environment has the poten4al to inform the geochemical characteriza4on of local bedrock This approach has been applied to the Coffee Gold Project, where elevated U and As observed in baseline water quality. The behaviour of these elements can be explained by using a combina4on of first principles and data collected as part of the geochemical characteriza4on program This understanding of the geochemical baseline environment has a number of implica4ons for source term predic4ons and mine waste management strategies.

3 Coffee Gold Project: LocaQon Alaska Yukon Livengood Yukon Fort Knox Alaska Uncle Sam Pogo Rackla Kaminak Brewery Creek LWM Eagle Dawson Keno Hill Operating Mines Mines White Gold Coffee Advanced Projects Road Casino Minto Carmacks Copper Revenue Nucleus Beaver Creek 2 km Wolverine Mine Whitehorse Source: Kaminak 216 Located 13 km south of Dawson in western Yukon The project is on the tradi4onal territory of the Tr ondëk Hwëch in and the asserted territory of the White River First Na4on.

4 Coffee Gold Project: MineralizaQon Gold mineraliza4on at the site is hydrothermal in origin and structurally controlled. Structural weaknesses associated with gold mineraliza4on have allowed for extensive in-situ weathering and oxida4on along near-ver4cal corridors. Weathering horizons include: Oxide zone Transi4on Zone Fresh (unweathered) zone Gold bearing structures transect three dis4nct lithologies: Gneiss Schist Granite

5 D-U µg/l Halfway Creek Surface Water Quality Monitoring: Uranium Yukon River D-U µg/l D-U µg/l LaJe Creek Proposed Mine Pits Coffee Creek Latte Creek (7 km 2 ) Coffee Creek (484 km 2 ) Naturally elevated uranium is observed in surface water Typical surface water in Canada is between 1 and 3 µg/l Site maxima range from ~3 to 1 µg/l Uranium concentra4ons are regionally elevated as shown by Coffee Creek Strong seasonality to U concentra4ons

6 Surface Water Quality Monitoring: Uranium D-U µg/l D-U µg/l D-U µg/l 1 Halfway Creek LaJe Creek Coffee Creek Flow L/s Latte Creek Yukon River Coffee Creek Inverse rela4onship exists between flow and U concentra4on. High flow condi4ons (freshet, high runoff); Surface water flow >>> groundwater discharge = low [U] Low flow condi4ons (winter, low runoff); Surface water flow <<< groundwater discharge = high [U] Indicates that groundwater is the dominant source of U in these catchments

7 Groundwater Quality Monitoring: Uranium Yukon River Proposed Mine Pits Gneiss Schist Latte Creek Granite Coffee Creek Groundwater Wells

8 6 5 Groundwater Uranium Granite Schist Gneiss Granite Schist Gneiss Groundwater Quality Monitoring: Uranium D-U µg/l D-U µg/l 2 1 Yukon River MW14-5A MW14-5B MW15-5WB MW14-3A MW14-3B MW14-2B MW15-2WB MW15-6WB MW15-4WB MW15-1WB Gneiss Uranium elevated in all lithologies, with highest concentra4ons in gneiss wells Peak U concentra4ons are not adjacent to mineraliza4on zones. Granite Schist Coffee Creek Results are consistent with surface water data, which indicated that source of uranium is groundwater seepage

9 2 Groundwater Arsenic Granite Schist Gneiss Granite Schist Gneiss Groundwater Quality Monitoring: Arsenic D-As µg/l D-As µg/l Yukon River MW14-5A MW14-5B MW15-5WB MW14-3A MW14-3B MW14-2B MW15-2WB MW15-6WB MW15-4WB MW15-1WB Gneiss Arsenic concentra4ons are also elevated in groundwater Highest As concentra4ons observed in granite wells In contrast to U, peak As concentra4ons tend to be adjacent to mineraliza4on zones. Schist Granite Coffee Creek

10 D-As µg/l D-As µg/l Halfway Creek LaJe Creek Arsenic concentraqon in relaqon to flow Yukon River D-As µg/l Coffee Creek Flow L/s Latte Creek Coffee Creek Elevated [As] in groundwater does not manifest as high [As] in surface water Seasonal signature of arsenic: Halfway and Coffee Creeks show lowest concentra4ons during low flow LaJe Creek shows peak concentra4ons during low flow Data shows that As groundwater load is being ajenuated.

11 Key QuesQons: What processes lead to the mobiliza4on of arsenic and uranium in groundwater? Why does uranium remain elevated in surface water while arsenic appears to be a=enuated?

12 Uranium Enrichment in 1 Waste Rock Uranium Granite Schist Gneiss Bedrock U ppm 1 1 5xACA 1xACA Oxide Tran. Fresh Oxide Tran. Fresh Oxide Tran. Fresh.1 Average Con4nental Abundance (ACA) = 2.7 ppm Uranium Slightly elevated in gneiss and granite (1-5x ACA); not elevated in schist ( 1x ACA). Ore shows similarly degree of U enrichment as waste rock. The lack of significant U enrichment bedrock indicates that U must be present in a form that is readily soluble in groundwater and surface water.

13 Uranium Solubility Increasing Solubility U(4+) U(6+) UO 2(solid) Oxida4on UO 2 2+ Alkalinity U O 2 (C O 3 ) 3 2 Calcium Uranium Solubility Dependent on oxida4on state and availability of complexing ligands Ca 2 U O 2 (C O 3 ) 3

14 Uranium Solid-Phase SpeciaQon Results U(4+) UO 2(solid) Increasing Solubility U(6+) Oxida4on UO 2 2+ Alkalinity U O 2 (C O 3 ) 3 2 Calcium uxrf micrograph of Fresh Gneiss Waste Rock (67% U 6+, 33% U 4+ ) The two dominant oxida4on states of U in the environment are +4 and +6 XANES measured rela4ve abundance of U(6+) and U(4+) Oxide Zone: 9% - 1% U(6+) Transi4on Zone: 17% - 49% U(6+) Fresh Zone: 27% - 67% U(6+) Results show that even though the abundance is low in Ca 2 U O 2 (C O 3 bedrock, U is present in the rela4vely soluble U(6+) oxida4on ) 3 state throughout the deposit.

15 Increasing Solubility U(4+) U(6+) UO 2(solid) Oxida4on UO 2 2+ Alkalinity U O 2 (C O 3 ) 3 2 Calcium Carbonate NP (kgcaco 3 /t) Oxide Tran. ABA Results Paste ph vs. %CO3 Granite Schist Gneiss Fresh Oxide Tran. Uranium Solubility: Alkalinity Availability Fresh Alkalinity mg/l Groundwater Alkalinity MW14-5A Granite Schist Gneiss In neutral ph environments the solubility of U(6+) is dependent on CO 3 availability. In-situ weathering has lead to the deple4on of sulphur minerals and the oxida4on of reduced mineral phases, but has not removed carbonate content. Elevated alkalinity in ambient groundwater supports U(6+) Ca 2 U O 2 (C O 3 leaching and mobility. ) 3 Oxide Tran. Fresh MW14-5B MW15-5WB MW14-3A MW14-3B MW14-2B MW15-2WB MW15-6WB MW15-4WB MW15-1WB

16 Increasing Solubility U(4+) U(6+) UO 2(solid) Oxida4on UO 2 2+ Alkalinity U O 2 (C O 3 ) 3 2 Calcium Ca mg/l Ca 2 U O 2 (C O 3 ) % Uranium Specia4on 8% 6% 4% 2% % Uranium as Ca 2 UO 2 (CO 3 ) 3 Uranium Solubility: Modelled SpeciaQon % Ca mg/l Groundwater Calcium MW14-5A Granite Schist Gneiss Mobility of U O 2 (C O 3 ) 3 2 can s4ll be limited by sorp4on to mineral surfaces. Forma4on of Ca 2 U O 2 (C O 3 ) 3 ternary complex further enhances solubility. Calcium is the dominant ca4on in most groundwater Groundwater chemistry supports a majority of U as ternary Ca-U-CO 3 complex. Species should be equally soluble in surface water and groundwater environments MW14-5B MW15-5WB MW14-3A MW14-3B MW14-2B MW15-2WB MW15-6WB MW15-4WB MW15-1WB

17 Uranium Solubility: Summary Elevated uranium concentra4ons in the baseline environment can be ajributed to: Enrichment of uranium in gneiss and granite host rock Occurrence of uranium in oxidized [U(6+)] state Alkaline and calcium rich groundwater and surface water which supports release and mobility of U(6+)

18 ImplicaQons for U PredicQon and Management Uranium release and mobility will be enhanced in mine waste environments with high alkalinity and calcium in pore water. In carbonate buffered waste rock dumps, alkalinity is in large part determined by the par4al pressure of CO 2 in pore gas. Uranium leaching will be accelerated in environments with limited gas exchange. Conversely, environments where pore gas CO 2 and Ca Rela4onship between alkalinity and pco 2 under calcite equilibrium are not able to accumulate 6 will limit U release (e.g., pit 5 Atmospheric pco2 wall rock). Alkalinity (mgcaco 3 /L) calcite equilibrium Low temp and TDS High temp and TDS.%.2%.4%.6%.8% 1.% pore gas CO 2 (%)

19 Arsenic Enrichment in Bedrock 1 Waste Rock Arsenic Granite Schist Gneiss 1 As ppm 1 1 1xACA Oxide Tran. Fresh Oxide Tran. Fresh Oxide Tran. Fresh 1 1xACA Average Con4nental Abundance (ACA) = 1.8 ppm Arsenic Elevated in all rock types, with waste rock typically equal to or exceeding 1xACA Ore is significantly enriched in As (>1, ppm) compared to waste rock

20 Arsenic Solid Phase SpeciaQon Aerobic As(5+) Dominant oxida4on state in host rock SEM micrograph showing occurrence of arsenian pyrite and arsenopyrite within illite matrix. Suboxic As(3+) In-situ Oxida4on XANES analysis found As primarily present primarily As(+5) and As(-1) Oxide Zone: 1% As(5+) Transi4on Zone: 91% - 1% As(5+) Fresh Zone: 2% - 59% As(5+) Anaerobic As(-1) sulphide minerals As(5+) is associated with Fe-oxide and Fe-arsenate minerals.

21 Arsenic Solubility: Redox Controls Aerobic As(5+) Dominant oxida4on state of As in mine rock. Reduc4on D-Fe µg/l Groundwater Iron Granite Schist Gneiss Suboxic As(3+) Anaerobic As(-1) Elevated D-Fe in groundwater indicates suboxic condi4ons MW14-5A MW14-5B MW15-5WB MW14-3A Groundwater chemistry shows that condi4ons are sufficiently reducing to support reduc4ve dissolu4on of Fe-oxide minerals Reduc4on of As(5+) to As(3+) occurs concomitantly with Fe reduc4on. Elevated As in groundwater is likely due to groundwater being sufficiently reducing to support As(3+) forma4on and stability MW14-3B MW14-2B MW15-2WB MW15-6WB MW15-4WB MW15-1WB

22 Arsenic Solubility: A]enuaQon Processes Why is arsenic absent from surface water? Aerobic As(5+) Aerobic surface water support As(5+) forma4on Suboxic As(3+) Anaerobic As(-1) Oxida4on Elevated D-Fe in groundwater indicates suboxic condi4ons Exposure to atmospheric oxygen when will result in the oxida4on of As(3+) and Fe(2+) As(5+) is readily sorbed by iron oxide mineral surfaces Aerobic metal leaching tests show that As solubility is related to Fe content and ph. Supports viability of sorp4on ajenua4on mechanism

23 Arsenic Solubility Summary Elevated arsenic concentra4ons in groundwater can be ajributed to: High degree of arsenic enrichment in ore and waste rock around the deposit. Occurrence of arsenic in an oxidized state which is prone to reduc4ve dissolu4on in suboxic groundwater environments. Low arsenic in surface water can be ajributed to: Oxida4on of D-As and D-Fe when groundwater exposed to atmospheric oxygen. AJenua4on of As(5+): sorp4on onto Fe-oxide surfaces.

24 ImplicaQons for Arsenic PredicQon and Management Placement of material with high As leaching poten4al in saturated environments should be avoided. Storage of As-rich mine waste in unsaturated environment is preferred.

25 Conclusions Analysis of background data can iden4fy geochemical processes occurring at the field scale Geochemical characteriza4on tes4ng can provide data to help explain site observa4ons, and provide data on how mining ac4vi4es will modify or amplify these processes. Overall, data gained from field observa4ons reduces uncertainty around predic4on of metal behaviour in a future mining environment and increases confidence in water quality predic4ons

26 QuesQons