ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) ... ABSTRACT: The combination of high sensitivity ICP-MS and Nd-YAG UV laser ablation ...
The trace element zonation in vent chimneys from the Silurian YamanKasy VHMS deposit in the Southern Ural, Russia: insights from laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) V.V. Maslennikov & S.P. Maslennikova Institute of Mineralogy of RAS, Miass, Russia
R. Large & L.V. Danyushevsky Centre for Ore Deposit Research, University of Tasmania, Hobart, Tas., Australia
R.J. Herrington Natural History Museum, London, UK
Keywords: VHMS, Southern Urals, black smokers, paleohydrothermal chimneys ABSTRACT: The combination of high sensitivity ICP-MS and Nd-YAG UV laser ablation was utilised to determine the distribution of trace elements (Mn, Tl, As, Pb, Au, Ag, Bi) within the Silurian black smoker vent chimneys from Yaman-Kasy copper-zinc-massive sulphide deposit in the Southern Urals. The study has shown systematic distribution patterns within the chimneys for two groups of trace elements. Group 1 elements (Mn, As, Pb, Ag and Au) are enriched in colloform pyrite in the outer-most section of the chinmey wall. This enrichment probably results from rapid precipitation of colloform pyrite under low temperature conditions. Pyrite euhedra, which result from the recrystallization of colloform pyrite toward the inner wall, are depleted in the Group 1 elements. Group 2 elements (Bi, Ag and Au) are enriched in chalcopyrite along the boundary between the chalcopyrite inner wall and the sphalerite filled central conduit, where Bi, Ag, Au, Pb tellurides have been precipitated in a zone of strong temperature gradients. The main zone of chalcopyrite within the central inner wall is depleted in Group 2 elements, probably due to the high temperature of formation which is unsuitable for telluride precipitation. Generally, trace element concentrations of chimneys increase with the decrease in chalcopyrite content from pyrite-chalcopyrite- to marcasite-chalcopyritesphalerite- to marcasite-quartz-rich chimneys, due to decrease in temperature and increase in Eh of the black smoker fluids. 1
INTRODUCTION
The discoveries of ancient vent black smokers chimneys at VHMS deposits are very rare events (Shikazono-Naotatsu & Kusakabe-Minoru 1999; Oudin & Constantinou 1984; Maslennikov 1999). The Urals has yielded the most well-preserved sulphide vent chimneys from the Silurian YamanKasy VHMS deposit (Herrington et al. 1998). Trace elements precipitation and distribution in sulphide chimneys is sensitive to a variety of geochemical factors, and enables an interpretation of the chimney’s growth history and details of fluid interactions within chimney walls. Previous papers have provided information on trace element zonation of black smoker chimneys. For example the high grade of tellurium in pyrite in the chalcopyrite zone of Yaman-Kasy’s chimneys have been demonstrated by electron-X-ray spectral microprobe data (Herrington et al. 1998). However, the electron microprobe has severe limitations in the detection of most trace elements at low levels, of a few tens of ppm or less. Laser ablation inductively coupled plasma mass-spectrometry (LA-ICP-MS) offers enormous potential in advancing trace element
studies of sulphides through significantly improved detection limits for in situ analysis. The first semiquantative data illustrated the power of LA-ICPMS analysis to demonstrate the non-random distribution of V, Ag, In, Te, Ba, Au, Pd and U within the chalcopyrite wall of an immature black smoker chimney from the Broken Spur vent field (29q 10cN, MAR) (Butler & Nesbitt 1999). Many important innovations, including preparation of new glass standards which allow improved quantitative LA-ICP-MS analysis of sulphides, have been made over the last five years at the Centre for Ore Deposit Research, University of Tasmania (Norman et al. 1998). In this paper we focus on an LA-ICP-MS investigation of diverse Silurian vent chimneys recently collected by the co-authors at the Yaman-Kasy VHMS deposit. 2
GEOLOGICAL SETTING
The Yaman-Kasy deposit is situated in the western part of the Urals in Mednogorsk region, which is considered to be part of the Sakmara structuralformational zone, a fragment of the Ordovician-
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Silurian marginal sea developed at the periphery of the Urals paleoocean (Zaykov et al. 1995). The deposit is hosted by a dacite-rhyolite unit, which is located in a linear extensional zone developed on serpentinite-basalt basement (Maslennikov 1999). The deposit is currently exposed in an open pit. Yaman-Kasy is an example of one of the least metamorphosed VHMS deposits known (Shadlun 1991). The steep mound-form of the deposit, the presence of clastic ore, chimney fragments and vent fossils, indicate that the Yaman-Kasy ore deposit is an ancient analogue of a “black smoker” sulphide mound complex (Shadlun 1991; Zaykov et al. 1995; Herrington et al. 1998). Some of the vent chimney fragments were found in massive sphalerite and pyrite ore on the interpreted roof of the sulphide mound but other fragments have been collected from ore breccias in the footwall and the southern flank of the sulphide mound (Fig. 1).
Figure 1. The plan of open pit of the Yaman-Kasy deposit. 1andesite-basalts and basalts; 2- sericite-quartz metasomatites after dacite dome; 3- lavas, lavoclastites, hyaloclastites of rhyolitic and dacitic composition; 4- lavas and volcanic sediments of basaltic composition with interlayers of aleurolites; 5- the copper-zink-massive sulphide ore body; 6position of vent chimney fragments; 7- horizons of ferruginous sediments; 8- faults; 9- open pit contours.
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METHODS
The analysis of major elements in sulphide minerals was carried out on an electron microprobe to assist calibration and recalculation of the LA-ICP-MS data. The choice of trace elements to be analysed by LAICPMS was dependant on the elements in the existing standard STDGL-1 of borate glass with sulphide powder, which contained Fe, Cu, Zn, Co, Ni, Au, Ag, Bi, Pb, Tl, Cd, As, Mo, Sn, Pt, La, V, Ti, Mn in relevant concentrations (Norman et al. 1998). The analytical system consisted of a laser ablation
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sampler UP-213 coupled to a plasma massspectrometer HP-4500 (Hewlett Packard) at the Centre for Ore Deposit Research, University of Tasmania (Hobart). Measurements were made with a Q-switched Nd –YAG UV source, frequency quadrupled (wavelength of 213 nm), using He and Ar as the carrier gas. Single point laser shots over a 100 second period, created a crater 80 Pm across and approximately 100 Pm in depth. Interlaboratory analysis of various types of sulphide standards developed by other LA-ICP-MS users is currently underway to check the ultimate accuracy of this analytical technique. 4
VENT CHIMNEYS STUDIED
The mineralogical zones, aligned from the exterior to the interior for the three types of the chimney found at Yaman-Kasy deposit are described below (Fig. 2). Type 1) Pyrite-chalcopyrite-rich vent chimney: The outer wall (A) of the chimney consists of colloform and botryoidal pyrite (zone A1). The next zone formed as pseudomorphous massive pyrite after tabular anhydrite (?) crystals (zone A2). This pyrite has recrystallized to pyrite euhedras towards the innermost part of this zone (zone A3). The inner wall (zone B) and conduit (zone C) of the chimneys are infilled by drusy chalcopyrite. A few sphalerite and pyrite euhedra can be found in the position of the axis of the conduit. Type 2) Marcasite-chalcopyrite-sphalerite vent chimney: The colloform pyrite of the outer wall zone A1 in type 1 has been replaced by chalcopyrite, sphalerite and sparry marcasite towards the inner part of the for type 2. The amount of euhedral pyrite and quartz increases close to the boundary with the drusy chalcopyrite inner wall (zone B). Tellurides and electrum in association with inclusions of marcasite, pyrite and/or galena occur in the boundaries of zones A/B and B/C. The middle part of chalcopyrite incrustation (zone B) is devoid of other minerals. An axial conduit (zone C) shows a cenripetal sequence from marcasite spheres to colloform or drusy sphalerite with disseminated tennantite. Type 3) Marcasite-quartz-rich chimney: The laminated colloform pyrite and massive finegrained marcasite zones in type 2 are replaced by quarz with minor sphalerite in the innermost part of zone A. Inner wall zone B is represented by a thin discontinuous layer of chalcopyrite with inclusion sphalerite and marcasite spheres. The conduit of the chimney (zone C) was infilled by quartz with minor marcasite spheres.
V.V. Maslennikov, S.P. Maslennikova, R. Large, L.V. Danyushevsky & R.J. Herrington
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Figure 2. The types of studied paleohydrothermal chimneys of the Yaman-Kasy copper-zink-massive sulphide deposit: 1pyrite-chalcopyrite, 2-marcasite-chalcopyrite-sphalerite, 3marcasite-quartz. Scale bar 1cm.
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RESULTS OF TRACE ELEMENT ZONATION STUDY
Type 1) Pyrite-chalcopyrite-rich vent chimney: The colloform pyrite outer rim (Zone A1) is enriched in Mn (550-1650 ppm), Tl (20-160 ppm) and minor Ag (70-130 ppm), Au (8-20 ppm) and Pb (3000-5000 ppm). The maximum As values, up to 6100 ppm, are located in zone A2. The pyrite euhedra and secondary chalcopyrite in zone A3 are relatively depleted in these and other trace elements. The drusy chalcopyrite and pyrite euhedra in the conduits are depleted of all measured trace elements. Some erratic Ag (up to 253 ppm), Pb (up to 1600 ppm) and As (up to 3700 ppm) values are observed in the thin drusy sphalerite layers of this chalcopyrite zone (Fig. 3a). Type 2) Pyrite-chalcopyrite-sphalerite vent chimney. Colloform pyrite of zone A1 is characterized by high values of Mn (380 ppm), Pb (2200-5300 ppm), Ag (65-180 ppm) and Au (24-110 ppm) as in chimney type 1. However, the maximum As (4800-6400 ppm) and Tl (28 ppm) values are displaced toward the boundary with the inner chalcopyrite and besides enrich the sphalerite conduit. Bi (more than 2000 ppm) is the main trace element of the chalopyrite inner walls (zone B). Minimal values of other elements were observed in this zone. The highest values of Au (247 ppm), Ag (503 ppm), Pb (4000 ppm), As (3000 ppm) and Tl (24 ppm) appear in chalcopyrite along the boundary of zones B and C (Fig. 3b). Type 3) Marcasite-quartz-rich chimney. Mn, Tl, Pb and As are widespread due to the recurrent occurrence of colloform and botryoidal marcasite in different zones of the chimney. Elevated Au (67-123 ppm), Ag (1000-3000 ppm) and Bi (850-1960 ppm) values occur in chalcopyrite and marcasite of zones B and B/C. The Bi and Ag contents of this type of chimney are highest in comparison with other types of chimneys (Fig. 3c).
DISCUSSION AND CONCLUSION
The enrichment in Mn, Tl, As, Pb, Ag and Au in colloform pyrite can be explained by rapid hydrothermal precipitation of pyrite at lower temperature during the early evolution of the chimneys. Enrichment of Mn may indicate hydroxide precipitation on surfaces of growing colloform pyrite under relatively oxidised conditions. Tl enrichment is typical of colloform pyrite formed at lower temperature in a range of sulphide deposits. Fine grained mineral phases of As, Au, Ag and Pb remain unresolved in the colloform pyrite. They are not tellurides because of the lack of significant Te in the outer wall of the chimneys. The other zone of trace element enrichment (Ag, Pb, Au, Bi) which occurs at the boundary between the high temperature chalcopyrite zone and middle temperature conduit are explained by the appearance of sylvanite (AgAuTe4), hessite-stutzite (Ag1,88Te), altaite (PbTe) and tellurobismuth (Bi2Te3). Anomalous As and Pb were identified as tennantite and galena inclusions, which probably developed during recrystallisation of colloform pyrite toward the inner part of chimneys. Pyrite euhedra were found to be depleted of most of the trace elements measured. This depleted euhedral pyrite zone is considered to be a result of hydrothermal alteration and recystallisation of the innermost part of the colloform pyrite zone. A second trace element depletion zone occurs in the middle part of the chalcopyrite inner-wall, where the highest fluid temperatures are expected, being unfavorable for telluride precipitation. The relative amounts of high temperature drusy chalcopyrite and pyrite euhedra decrease in relation to medium temperature drusy sphalerite and lower temperature colloform marcasite across the three chimney types. This means that the median temperature of evolution of the chimneys studied here decreases in the pattern; Type 1 (hottest) to Type 3 (coolest). The high temperature conditions of Type 1 chimney formation was unfavorable for telluride precipitation and Bi, Pb, Au, Ag enrichment. The general increase in Mn, Ag Au, Bi, Tl values in the Type 2 and Type 3 chimneys can be explained by their formation from lower temperature and relatively more oxidised fluids. This mineralogical and trace element data is the first of it’s kind to be produced for ancient black smoker vent chimneys.
The trace element zonation in vent chimneys from the Silurian Yaman-Kasy VHMS deposit
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Figure 3. Diagrams of trace element distribution (in ppm) in wall chimneys. a, b, c –types of paleohydrothermal chimneys (see Fig. 2). Py – pyrite, Cph- chalcopyrite, Sph- sphalerite.
ACKNOWLEDGEMENTS This research was supported by the European Commission under the 5th Framework Programme INCO-2, MinUrals project number ICA2-CT2000-10011, Cordis-RTD projects. LA-ICP-MS analyses were carried out during a visiting programme funded by the ARC Special Research Centre grant to CODES, University of Tasmania. REFERENCES Butler, I.B. & Nesbitt, R.W. 1999. Trace element distribution in the chalcopyrite wall of a black smoker chimney: insights from laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). Earth and Planetary Science Letters 167: 335-345. Herrington, R.J., Maslennikov, V.V., Spiro, B., Zaykov, V.V. & Little, S.T.S. Ancient vent chimney structures in the Silurian massive sulphides of the Urals. Modern Ocean Floor Processes and the Geological Record 148: 241– 257.
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Maslennikov, V.V. 1999. Sedimentogenesis, halmyrolysis and ecology of the massive sulphide-bearing fields. Miass: Geotur. (in Rusian). Norman, M.D., Griffin, W.L., Prearson, N.J., Garcia M.O. & O’Reilly, S.Y. 1998. Quantitative analysis of trace element abundances in glasses and minerals: a comparison of laser ablation inductively coupled plasma mass spectrometry, solution inductively coupled plasma mass spectrometry, proton microprobe and electron microprobe data. Journal of Analytical Atomic Spectrometry 13: 477-483. Oudin, E. & Constantinou, G. 1984. Black smoker chimney fragments in Cyprus sulfide deposits. Nature 308: 349353. Shadlun T.N. 1991. Some sulphide intergrowths typical for modern oceanic and ancient massive sulphide ores. Geol. Rudn. Mestor. 33: 110-118 (in Russian). Shikazono-Naotatsu & Kusakabe-Minoru 1999. Mineralogical characteristics and formation mechanism of sulfate-sulfide chimneys from Kuroko area, Mariana Trough and mid-ocean ridges. Resource Geology Special Issue 20:1-11. Zaykov, V.V., Shadlun, T.N., Maslennikov, V.V. & Bortnikov, N.S. 1995. Jaman-Kasy sulphide deposit – ancient “black smoker” of Urals paleoocean. Geol. Rudn. Mestor. 37: 511-529. (in Russian).
V.V. Maslennikov, S.P. Maslennikova, R. Large, L.V. Danyushevsky & R.J. Herrington
