Making Metal Super-Charged Ore Fluids: The Key to Hydrothermal Base-Metal Ore Formation?

Jamie J. Wilkinson, David R. Cooke CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 126, Hobart, TAS 7001, Australia

Dominik J. Weiss, Helen L. Crowther, Barry J. Coles Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK

Teresa E. Jeffries Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 5BD, UK

Chris G. Ryan, Jamie Laird CSIRO Exploration and Mining, School of Physics, University of Melbourne, VIC 3010, Australia

Abstract. The formation of large hydrothermal ore 2 Mineralogical Hosts for Metals in the deposits is optimized by several factors: efficient Source Rocks extraction of metals from the source region, elevated metal concentrations in hydrothermal fluids, high fluid Pb isotope studies in the Irish orefield have convincingly flux, effective focusing of fluids into a limited rock volume and efficient precipitation. Of these parameters, metal demonstrated that the main source for metals is the solubility displays the widest variability (up to 4 orders of Lower Paleozoic, very weakly metamorphosed volcano- magnitude) and is potentially the most important. A case sedimentary succession that is unconformably overlain study tracing metal transport in the Irish orefield suggests by a thin clastic wedge and the mineralized ramp and that efficient extraction from the source rocks is critical for nearshore carbonate facies of Carboniferous age the formation of economic deposits. The observation that (Caulfield et al. 1986; O'Keeffe 1986, 1987; LeHuray et ore fluids trapped in sphalerite contain orders of al. 1987; Dixon 1990; Dixon et al. 1990; Everett 1999; magnitude more metal than those in quartz implies that Everett et al. 2003). There may also be a contribution ore deposition is related to the influx of metal super- charged brines produced by this efficient leaching from the underlying Precambrian high grade basement process. Large parts of the hydrothermal system in space (Walshaw et al. 2006). Although it is inferred that K- and time may be barren due to a failure to attain efficient feldspar is likely to have been the principal mineral host extraction and transport conditions. for Pb in these rocks (Everett et al., 2003), no detailed studies have been carried out on trace metal speciation. Keywords. Hydrothermal, ore, metal, solubility, source, Proton-induced X-ray emission (PIXE) analyses of leaching greywackes, the dominant rock type in the Lower Paleozoic source rock sequence (~60 ppm Zn, ~19 ppm 1 Introduction Pb), were carried out at the CSIRO-GEMOC Nuclear Microprobe facility, Melbourne University, to identify Hydrothermal ore deposits are products of flow regimes principal host minerals for ore-forming metals and their that extract metals from source rocks, transport them as concentrations in these phases (Ryan et al. 1990; Ryan complexes in solution and precipitate them in a limited 2000). In these rocks the predominant clastic grains are rock volume. Of the parameters that govern the total quartz with lesser plagioclase and K-feldspar, in a metal deposited, metal solubility varies over the widest matrix dominated by Fe-chlorite and subordinate range (up to 4 orders of magnitude) and it is suggested muscovite (Fig. 1). Fe and Zn are principally hosted by that this is the most important variable in the formation chlorite as shown by the spatial association between Fe of large deposits. This hypothesis is supported by results and Zn in rock scans (Fig. 1), and illustrated by the from a recent laser ablation ICPMS study comparing strong covariance of Fe and Zn in individual pixel data fluid inclusions trapped in sphalerite with those trapped (Fig. 2). The linear correlation is produced by in gangue from two major Zn-Pb systems (Wilkinson et covariation in X-ray yield as a function of mineral grain al. 2009). The fluids responsible for ore mineral thickness in polished thin section. The average precipitation were anomalously enriched in metals concentration of Zn in chlorite is ~215 ppm (Table 1). compared to those forming gangue and had chemical fingerprints of a distinct origin. This led to the Mn 0.30 % As <45 ppm conclusion that episodic influxes of specialized fluids Fe 18.03 % Rb 42 ppm were critical for ore formation. Here, we present results Ni 225 ppm Sr 53 ppm of a case study tracing the Zn pathway from source to trap in the Irish carbonate-hosted Zn-Pb-Ba orefield. The Cu <39 ppm Ba 45 ppm aim was to test the idea that processes occurring in the Zn 215 ppm Pb 27 ppm extraction zone are key controls on the formation of large base-metal ore deposits. Table 1. Average L. Paleozoic chlorite composition (sample no. 01-0108)

Figure 2. Left: element covariation plot showing strong Fe-Zn association in chlorite. Right: Threshold image, using data contained by ellipse on left, maps out chlorite grains. Zn-rich grain (white, top right) corresponds to circle in left image. Ni, a minor metal in the deposits, also occurs principally in chlorite. Arsenic appears to coexist with Pb and also with Ba (Fig. 1) in minor phases. Cu and Mo principally occur in rare, accessory phases.

3 Metal Extraction Efficiency

The region from which the hydrothermal fluids extracted metals in the Irish hydrothermal system is currently inaccessible, probably located at palaeodepths of >5 km (Wilkinson et al. 2005a). Consequently, it is only possible to infer the processes that occurred in this region from isotopic and experimental data. Zinc isotope data acquired from wholerock samples of Lower Palaeozoic greywacke gave 66Zn values of -0.01 to 0.26‰ (Lyon JMC Zn), slightly below average crustal values. This range is identical to the Zn isotope composition of sphalerite from the principal orebodies (average 0.13±0.28‰, 2; Wilkinson et al. 2005b In contrast, samples leached in sealed polystyrene test tubes using 6N HCl at 25°C for two hours produced 66 isotopically heavy leachates with  Zn values of 0.60 to 0.80‰ containing 11-32% of the total Zn budget (Fig. Figure 1. PIXE X-ray intensity maps showing metal distribut- 3). This result indicates that a minor, isotopically heavy, ions within Lower Paleozoic greywacke source rocks (sample cold acid-leachable fraction exists in the samples that no. 01-0110). Q: quartz, P: plagioclase, K: K-feldspar. Bright can be correlated with the non-silicate (?oxyhydroxide) grains on the Fe image are chlorite. Width of images: 4 mm. Zn component identified in the PIXE analyses. Because this component is likely to be extracted more easily Zn also occurs in higher concentrations (~500-1500 during hydrothermal interaction, fluids that only weakly ppm) in a minor secondary phase, possibly an leached the source rocks would probably acquire an oxyhydroxide (Fe ~11.5 wt%, Mn ~8.6 wt%). Pb occurs isotopically heavy Zn isotope signature. Conversely, to at ~40 ppm in K-feldspar (crustal average ~50 ppm; attain an isotopic composition consistent with the Patterson and Tatsumoto 1964) but the Pb budget dominant ore signature, >80% of the wholerock appears to be dominated by a minor unknown phase in (dominantly chlorite) Zn would need to be extracted. which Pb occurs at high concentrations (0.24-0.47 wt%), This assumes that negligible bulk fractionation occurred together with elevated Fe, As and Cu – possibly a during precipitation of sphalerite in the deposits which is sulphide. It is known that the dominant Pb-bearing probable based on current knowledge. This phase(s) are very easily leached or dissolved in low interpretation is consistent with the tendency for temperature acid-leach and high temperature subeconomic prospects to contain isotopically heavy hydrothermal experiments (Bischoff et al. 1981; Everett sphalerite (66Zn 0.03-0.64‰) that can be explained by et al. 2003; Crowther 2007). Ba is elevated in K-feldspar their formation from fluids that only interacted weakly (~900-1500 ppm) but also occurs at relatively high with the source rocks. concentrations in minor phases, possibly barite and/or WKH siltstone 01-0110 greywacke 01-0108 greywacke carbonates. Mn, substituted in hydrothermal dolomite or Leachates occurring with hematite in oxide facies mineralization in 0.80 Residues 0.78 the deposits, is enriched in chlorite (~0.3 wt%). 0.60

-0.30 0.17 0.05

Wholerock 66Zn= -0.01‰ 66Zn=0.26‰ 66Zn=0.14‰

Figure 3. Pie charts showing proportions of total Zn extracted 12

10 Sphalerite (P) Quartz (P) y 8 c

n Sphalerite (S) e

u 6

q Quartz (S) e r 4 in cold acid leachates in three Lower Palaeozoic metasediment F samples and the Zn isotope composition of these fractions. 2 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 4 Metal Super-Charged Ore Fluids Log Pb concentration (ppm) alteration and chloride complexing on metal solubilization at Laser ablation ICPMS analyses of fluid inclusions have 200°C and 350°C. Econ Geol 76: 659-676 provided new insights into the chemistry of Irish ore Caulfield JBD, LeHuray AP, Rye DM (1986) A review of lead and fluids (Wilkinson et al. 2007, 2009). Results from the sulphur isotope investigations of Irish sediment-hosted base Silvermines district show that sphalerite-hosted inclus- metal deposits with new data from the Keel, Ballinalack, ions contain Pb (and by inference Zn) concentrations up Moyvoughly and Tatestown deposits, in Andrew CJ, Crowe RWA, Finlay S, Pennell WM, Pyne J, eds, Geology and to two orders of magnitude higher than in Lower genesis of mineral deposits in Ireland. Dublin, Irish Assoc Palaeozoic-hosted feeder veins distal to deposits (Fig. 4). Econ Geol, 591-615 Crowther HL (2007) A rare earth element and transition metal Figure 4. Frequency histogram showing Pb concentrations in isotope study of the Irish Zn-Pb orefield. Unpub PhD thesis, individual inclusions in sphalerite and quartz from the Irish University of London, 198 p orefield. P: primary inclusions, S: secondary inclusions. Dixon PR (1990) The role of basement circulated fluids in the origin of sediment-hosted Zn-Pb-Ba mineralization in Ireland. The relationship between metal-rich fluids trapped in Unpub PhD thesis, Yale University, 249 p sphalerite and metal-poor fluids in gangue phases has Dixon PR, LeHuray AP, Rye DM (1990) Basement geology and also been observed in MVT deposits (Stoffell et al. tectonic evolution of Ireland as deduced from Pb isotopes. Jour Geol Soc London 147: 121-132 2008; Wilkinson et al. 2009). This implies that influx of Everett CE (1999) Tracing ancient fluid flow pathways: A study of metal-enriched batches of fluid may be responsible for the Lower Carboniferous base metal orefield in Ireland. Unpub ore formation. Intervening periods of time, or different PhD thesis, Yale University, USA, 354 p parts of the system, such as feeder veins distal to Everett CE, Wilkinson JJ, Rye DM (1999) Fracture-controlled deposits (Everett et al. 1999), may be characterized fluid flow in the Lower Palaeozoic basement rocks of Ireland: largely by precipitation of gangue phases from metal- Implications for the genesis of Irish-type Zn-Pb deposits, in McCaffrey, KJW, Lonergan, L, Wilkinson JJ, eds, Fractures, poor fluids. Fluid Flow and Mineralization. Geol Soc London, Spec Pub 155: 247-276 5 Conclusions Everett CE, Rye DM, Ellam RM (2003) Source or sink? An assessment of the role of the Old Red Sandstone in the genesis of the Irish Zn-Pb deposits. Econ Geol 98: 31-50 Based on the results of the case study in the Irish LeHuray AP, Caulfield JBD, Rye DM, Dixon PR (1987) Basement orefield and data from other systems (e.g., Stoffell et al. controls on sediment-hosted Pb-Zn deposits: a lead isotopic 2008), we suggest that hydrothermal ore formation may study of Carboniferous mineralization in Central Ireland. Econ commonly be episodic, driven by influxes of Geol 82: 1695-1709 anomalously metal-rich fluids. For the Irish Pb-Zn O’Keeffe WG (1986) Age and postulated source rocks for deposits, we propose that, at certain times, specific parts mineralization in central Ireland, as indicated by lead isotopics, in Andrew CJ, Crowe RWA, Finlay S, Pennell WM, Pyne J, of the hydrothermal system reached a critical threshold eds, Geology and genesis of mineral deposits in Ireland. in the reaction zone that allowed substantial leaching of Dublin, Irish Assoc Econ Geol, 617-624 metals to take place, thereby producing significantly O’Keeffe WG (1987) A regional lead isotopic investigation of more metal-rich fluids for a limited period. For the mineralization in Ireland. Unpub PhD thesis, University principal ore metal Zn, together with Mn and Ni, this College Dublin, Ireland, 261 p Patterson C, Tatsumoto M (1964) The significance of lead isotopes process required breakdown of chlorite at assumed in detrital feldspar with respect to chemical differentiation temperatures of ~300 °C. This conclusion places a much within the Earth’s mantle. Geochim Cosmochim Acta 28: l-22 greater emphasis on the nature of the source region and Ryan CG (2000) Quantitative trace element imaging using PIXE metal extraction processes as primary controls on the and the nuclear microprobe. Internat Jour Imaging Syst and generation of fertile hydrothermal systems with ore- Technol 11: 219-230 forming potential. Ryan CG, Cousens DR, Sie SH, Griffin WL, Suter GF, Clayton E (1990) Quantitative PIXE microanalysis of geological materials using the CSIRO proton microprobe. Nucl Instr and Acknowledgements Meth B47: 55-72 Stoffell B, Appold MS, Wilkinson JJ, Mclean NA, Jeffries TE Some of the samples for this study were collected under (2008) Geochemistry and evolution of MVT mineralising brines from the Tri-State and Northern Arkansas districts NERC grant GR9/03047. We are grateful to the determined by LA-ICP-MS microanalysis of fluid inclusions. Department of Earth Science and Engineering, Imperial Econ Geol 103: 1411-1435 College London, Anglo American plc, the University of Walshaw R, Menuge J, Tyrrell S (2006) Metal sources of the London Central Research Fund and CODES for Navan carbonate-hosted base metal deposit, Ireland: Nd and Sr additional financial support. Isotope and laser ablation isotope evidence for deep hydrothermal convection. Miner Deposita 41: 803-819 analyses were made in the Joint Analytical Facility of Wilkinson JJ, Everett CE, Boyce AJ, Gleeson SA, Rye DM Imperial College and the Natural History Museum, (2005a) Intracratonic crustal seawater circulation and the London. genesis of sub-seafloor Zn-Pb mineralization in the Irish orefield. Geology 33: 805-808 References Wilkinson JJ, Weiss DJ, Mason TFD, Coles B (2005b) Zinc isotope variation in hydrothermal systems: Preliminary evidence from the Irish Midlands ore field. 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