Transport and deposition of selenium in felsic volcanic-hosted ...

Chapter 6-12

6-12

Transport and deposition of selenium in felsic volcanic-hosted massive sulfide deposits of the Finlayson Lake District, Yukon Territory, Canada D. Layton-Matthews, S.D. Scott Department of Geology, University of Toronto, 22 Russell Street, Toronto, Ontario, M5S 3B1 J.M. Peter Geological Survey of Canada, Central Canada Division, 601 Booth Street, Ottawa, Ontario, K1A 0E8 M.I. Leybourne Department of Geosciences, University of Texas at Dallas, 2601 N. Floyd Rd., Richardson, Texas, 75080

Abstract. The mobility and hydrothermal transport of selenium are a function of the fluid pH, fO2 and temperature. At high temperatures (>200 °C) and relatively acidic (pH 300°C), copper-rich sulfide assemblages at the base of VHMS and the selenium content of all sulfide minerals. Thermodynamic calculations for selenide and sulfide minerals indicate significantly higher temperatures and lower solubility indices for the formation of selenide versus sulfide pairs, which is consistent with petrographic observations and can explain the distribution of selenium in zone-refined VMS deposits. Keywords. Selenium, volcanic-hosted massive sulfide, Yukon, seafloor mineralization, ore deposit

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Introduction

In the mid 1990’s three polymetalic volcanic-hosted massive sulfide deposits of potential economic significance (Kudz Ze Kayah – KZK, Wolverine and GP4F) were discovered in the Finlayson Lake District (FLD) of the Yukon, Canada, these have a combined resource of 21.5 million tonnes grading 8.2% zinc, 0.97% copper, 1.7% lead, 203 g/ t silver and 1.6 g/t gold (see Figure 1). The Wolverine and KZK deposits have elevated selenium contents, whereas the GP4F deposit does not. Elevated selenium contents at

Wolverine and KZK massive sulfide ores were recognized during metallurgical testing and in the prefeasibility stages of exploration. Low base metal prices during and following these discoveries hindered further development of these resources, until recently. In this study we present field, petrographic, bulk geochemical and mineral chemical data on the mineralogical residence sites and bulk distributions of selenium within the KZK, Wolverine and GP4F VHMS deposits in the FLD. Whole-rock compositional data for host felsic volcanic and carbonaceous argillite host-rocks, sulfide mineralization, and altered host-rocks are presented together with thermodynamic models using the Geochemists Workbench® 4.0 computer program in order to quantitatively elucidate transport and depositional processes that resulted in elevated selenium contents of the Wol-

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verine and KZK deposits. Thermodynamic modeling results are then compared to actual selenium distributions in both the Wolverine and KZK deposits, and modern seafloor analogs.

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Selenium distribution

The study of the distribution of selenium in VHMS deposits provides important insights into the physiochemical nature of the fluids responsible for selenium-rich and selenium-poor mineralization. Recognition of selenium mineralogical associations within particular mineralization types may also be of relevance to metallurgical considerations and may contribute to the economic potential of a Se-rich deposit (i.e., selective mining or concentrate blending). Sulfide deposits in the FLD are variable, exhibit a wide range of mineralization styles and sulfide intersections have been broadly divided into six mineralization types based on mineralogy, mineral textures and spatial location relative to regards to proximal stratigraphic footwall alteration. Progressing upward in order from stratigraphic footwall to hanging wall, these are: 1) footwall, 2) chalcopyrite – pyrite – pyrrhotite predominant, 3) pyrite – sphalerite predominant, 4) sulfide breccia, 5) barite-rich, and 6) remobilized sulfides. In areas of thick sulfide intersections, well-defined systematic base-metal zoning is observed from stratigraphic footwall to hanging wall contacts is present at both the Wolverine and KZK deposits. At Wolverine, in both the Lynx and Wolverine zones, there is a strong positive correlation of selenium with Cu/Cu+Pb+Zn at the stratigraphic base of both sulfide lenses (r = 0.66). In diamond drill hole WV96-39 from the western edge of the Wolverine zone, a ~7 m massive sulfide interval was intersected. At the base a 1.6 m interval of Type 1 (footwall mineralization) was intersected with concentrations of up to 14.1 wt % Cu (0.54 Cu/Cu+Pb+Zn) and 2120 ppm Se. Conversely, Pb abundances and tenors are low at 0.48 wt % and 0.18 Pb/Cu+Pb+Zn, respectively. Immediately above this intersection occurs a narrow 0.6 m intersection of Type 2 chalcopyrite mineralization with up to 10.8 wt % Cu (0.41 Cu/Cu+Pb+Zn) and 3420 ppm Se. Selenium and Cu tenor drastically decrease in Type 3 mineralization stratigraphically up section, whereas Pb and Zn contents generally increase up section, reaching 2.97 wt % Pb (0.12 Pb/Cu+Pb+Zn) and 20.3 wt % Zn (0.83 Zn/Cu+Pb+Zn). At the KZK deposit, the thickest sulfide intersections are in the eastern down-dip portions of the deposit within the fold nose where meter-scale m-folds have structurally thickened the sulfide intersection. Within this area, diamond drill holes commonly intersect two stratigraphic hanging wall and footwall contacts with repeated Type 1, 2, 3 and 4 mineralization sequences and sulfide intersec-

tions of ~20 m. At the stratigraphic footwall contacts there is commonly a ~2 m interval of Type 1 mineralization with up to 4.2 wt % Cu (0.79 Cu/Cu+Pb+Zn) and 210 ppm Se. Conversely, Pb abundances and Pb tenor are low, 0.28 w t % and 0.05 Pb/Pb+Cu+Zn, respectively. Stratigraphically above this, a ~1.75m interval of Type 2 mineralization with concentrations of up to 10.2 wt % Cu (0.86 Cu/Cu + Pb + Zn) and 551 ppm Se occurs. Selenium and Cu tenor decrease stratigraphically upward, whereas Pb and Zn contents increase through Type 3 mineralization, reaching 5.3 wt % Pb (0.33 Pb/Pb + Cu + Zn) and 13.9 wt % Zn (0.87 Zn/Zn+Pb+Cu). At the GP4F deposit, sulfide intersections are comparatively thinner than those at the Wolverine and KZK deposits and have poorly developed base-metal zoning in areas of thick sulfide intersection. Within these intersections there is commonly a thin (300 to 200 °C. The coexistence of pyrite, pyrrhotite, chalcopyrite and the presence of relict high Fe-sphalerite near the stratigraphic bases of the FLD deposits indicates a predominantly low and narrow range of fO2 (-35 to –43) and fS2 (8 to –13) conditions within the sulfide mounds (Barton, 1978; Helgeson, 1969). The absence of pyrrhotite, chalcopyrite and the presence of Fe-poor sphalerite near the stratigraphic tops of the FLD deposits indicate a somewhat higher fO2 (-35 to –30) and decreased temperature (< 250 °C) of the hydrothermal fluid. In hydrothermal fluids above 200 °C the dominant aqueous Se and S species are H2Se and H2S, respectively (D’yachkova and Khodakovkiy, 1968; Yamamoto, 1976). Furthermore, at temperatures above 220 °C, melting of native selenium occurs. It has been also shown in metalrich fluids that native selenium is not stable even at very low temperatures (150 °C) regardless of thermodynamic predictions (Elrashidi et al., 1987). Therefore, at temperatures > 150 °C, mH2Se/mH2S approximates mΣSe/mΣS of the hydrothermal fluid, and native selenium is not stable within hydrothermal fluids. As such, the partitioning of

selenium and sulfur between a hydrothermal fluid and sulfide minerals should be recorded in the sulfide minerals and the ΣH2Se/ΣH2S of the volcanogenic fluid can be calculated given the appropriate Kreaction data and mineral compositions (Huston et al., 1995). The high selenium contents at the stratigraphic base of KZK are similar to those for modern Cu-rich chimneys at seafloor hydrothermal black smoker sites (e.g., up to 1000 ppm at East Pacific Rise 13°N; (Auclair et al., 1987). However, pyrite compositions within Type I and 2 from the Wolverine deposit are more variable and consistently higher (3200 to 1000 ppm) than those of KZK. Calculations indicate that a significantly higher selenium concentration in the hydrothermal fluid would be required (perhaps >1000 ppb). The textural coexistence of chalcopy rite and clausthalite within Type 1 and 2 mineralization provides further evidence for the high activity of H2–Se in the oreforming fluid at the Wolverine deposit. The solubility products of clausthalite from D’yachkova and Khodakokiy (1968) and Xiong (2003) indicate significantly higher temperatures of precipitation of selenides than the sulfide solid solution end members. A first-order estimate of the hydrothermal fluid was modelled for a conductively cooling system from 300 to 150°C using the Geochemists Workbench® 4.0 computer program (Bethke, 2001). Variation of the activity of H2Se from values equivalent to endmember fluids venting on the modern seafloor (7x10-8 to 8x10-8 m) (Von Damm et al., 1985) to estimates based on pyrite compositions from Type 1 and 2 mineralization at the Wolverine deposit demonstrate that at very high aH2Se (1 mg kg -1) the solubility minimum (log Q/K) for clausthalite and chalcopyrite will overlap and thus these minerals will co-precipitate (see Figure 2). This is consistent with our petrographic observations and mineral compositional data for Type 1 and 2 mineralization. Unlike in other deposits (i.e., Mount Lyell, Rosebery, Tazmania) in which bulk selenium contents sharply decreases to zero at the stratigraphic top of the sulfide lenses (Huston et al., 1995), at the edges of the Wolverine and KZK deposits the selenium contents of bulk sulfide and individual sulfide phases increase non-uniformly, as observed in Type 4 mineralization. This observed relationship is supported by the different stabilities of H2S and H2Se at higher fO2 (~-30), the absence of native Se in the presence of high metal concentrations (Elrashidi et al., 1987), and the likely ingression of seawater. The ingress and mixing of seawater with the mineralizing fluids near the stratigraphic tops and edges of these deposits created an environment where SO42- was the dominant sulfur species but H2Se was the stable selenium species; this created a unique assemblage of selenide and sulfate minerals, consistent with our petrographic and field observations.

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References Auclair G, Fouquet Y, Bohn M (1987) Distribution of selenium in high-temperature hydrothermal sulfide deposits at 13 degrees North, East Pacific Rise: The Canadian Mineralogist 25: 577-587 Barton PB Jr, (1978) ore textures involving sphalerite from the Furutobe mine, Akita Prefecture, Japan: Mining Geology 28:293-300 Bethke CM (2001) The Geochemist’s Workbench® Release 4.0, A Users Guide to Rxn, Act2, Tact, React, and Gtplot: Urbana, University of Illinois 184 D’yachkova IB, Khodakovkiy I L (1968) Thermodynamic equilibria in the systems S-H2O.Se-H2O and Te-H2O in the 25-300°C temperature range and their geochemical interpretations: Geochemistry International 5:1108-1125 Eldridge CS, Barton PB, Jr, Ohmoto H (1983) Mineral textures and their bearing on formation of the Kuroko orebodies: Econ. Geol. Mon. 5:241-281 Elrashidi MA, Adriano DC, Workman SM, Lindsay W L (1987) Chemical-Equilibria of Selenium in Soils - a Theoretical Development: Soil Science 144:141-152

Helgeson HC (1969) Thermodynamics of hydrothermal systems at elevated temperatures and pressures: American Journal of Science 267:729-804 Huston DL, Sie SH, Suter GF, Cooke DR, Both RA (1995) Trace elements in sulfide minerals from eastern Australian volcanichosted massive sulfide deposits.1. Proton microprobe analyses of pyrite, chalcopyrite, and sphalerite, and.2. Selenium levels in pyrite: Comparison with delta S-34 values and implications for the source of sulfur in volcanogenic hydrothermal systems: Economic Geology 90:1167-1196 Ohmoto H (1996) Formation of volcanogenic massive sulfide deposits: the Kuroko perspective: Ore Geology Reviews 10:135177 Von Damm KL, Edmond JM, Measures CI, Grant B (1985) Chemistry of submarine hydrothermal solutions at Guaymas Basin, Gulf of California: Geochimica et Cosmochimica Acta 49:22212237 Yamamoto M (1976) Relationship between Se/ S and sulfur isotope ratios, hydrothermal sulfide minerals: Mineralium Deposita 11:197-209