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Massive sulfide deposition and trace element remobilization in the Middle Valley sediment-hosted hydrothermal system, northern Juan de Fuca Rdge1
Geochimica et Cosmochimica Acta, Vol. 68, No. 13, pp. 2863–2873, 2004 Copyright © 2004 Elsevier Ltd Printed in the USA. All rights reserved 0016-7037/04 $30.00 ⫹ .00
Pergamon
doi:10.1016/j.gca.2003.12.023
Massive sulfide deposition and trace element remobilization in the Middle Valley sediment-hosted hydrothermal system, northern Juan de Fuca Rdge J. L. HOUGHTON,1,* W. C. SHANKS, III,2 and W. E. SEYFRIED, JR1 1
Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 USA 2 U.S. Geological Survey, 973 Denver Federal Center, Denver, CO 80225-0046, USA (Received January 15, 2003; accepted in revised form December 19, 2003)
Abstract—The Bent Hill massive sulfide deposit and ODP Mound deposit in Middle Valley at the northernmost end of the Juan de Fuca Ridge are two of the largest modern seafloor hydrothermal deposits yet explored. Trace metal concentrations of sulfide minerals, determined by laser-ablation ICP-MS, were used in conjunction with mineral paragenetic studies and thermodynamic calculations to deduce the history of fluid-mineral reactions during sulfide deposition. Detailed analyses of the distribution of metals in sulfides indicate significant shifts in the physical and chemical conditions responsible for the trace element variability observed in these sulfide deposits. Trace elements (Mn, Co, Ni, As, Se, Ag, Cd, Sb, Pb, and Bi) analyzed in a representative suite of 10 thin sections from these deposits suggest differences in conditions and processes of hydrothermal alteration resulting in mass transfer of metals from the center of the deposits to the margins. Enrichments of some trace metals (Pb, Sb, Cd, Ag) in sphalerite at the margins of the deposits are best explained by dissolution/reprecipitation processes consistent with secondary remineralization. Results of reaction-path models clarify mechanisms of mass transfer during remineralization of sulfide deposits due to mixing of hydrothermal fluids with seawater. Model results are consistent with patterns of observed mineral paragenesis and help to identify conditions (pH, redox, temperature) that may be responsible for variations in trace metal concentrations in primary and secondary minerals. Differences in trace metal distributions throughout a single deposit and between nearby deposits at Middle Valley can be linked to the history of metal mobilization within this active hydrothermal system that may have broad implications for sulfide ore formation in other sedimented and unsedimented ridge systems. Copyright © 2004 Elsevier Ltd The sediment-hosted volcanic-associated massive sulfide deposits at Middle Valley on the northern Juan de Fuca Ridge with at least 8.8 million tons of massive sulfide mineralization, are the largest documented accumulations yet discovered on the seafloor (Zierenberg et al., 1998). Located 200 miles west of Vancouver Island, Middle Valley is a sedimented axial rift, extensively studied during the Ocean Drilling Program (ODP) Legs 139 and 169. Middle Valley sulfide deposits are characterized by hydrothermal sulfide mineral assemblages, and are broadly similar to other seafloor massive sulfide deposits such as Escanaba Trough, Guaymas Basin, 13°N EPR, and TAG (Peter and Scott, 1988; Ames et al., 1993; Humphris et al., 1995; Fouquet et al., 1996). The Bent Hill massive sulfide deposit (BHMS) at Middle Valley initially formed by high temperature reducing hydrothermal fluids venting at the seafloor, but also was strongly affected by an episode of later sulfide deposition and remobilization at lower temperatures due to mixing with seawater circulated through turbidite layers within the host sediment (Goodfellow and Franklin, 1993; Butterfield et al., 1994). Fossil hydrothermal systems, such as those in the Troodos ophiolite in Cyprus, reveal evidence of the structural control of fluid flow in paleohydrothermal systems that influenced the location and composition of metal-sulfides (Parmentier and Spooner, 1978; Gillis and Robinson, 1990). In addition, temporal and spatial observations of the chemistry of vent fluids at active hydrothermal systems such as TAG (Tivey et al., 1995) reveal the critical role of temperature and redox conditions on metal mobility and alteration of primary metal sulfides. Similarly, there is evidence of fault control of upflow at Middle
1. INTRODUCTION
The distribution of trace elements in sulfides from ancient and modern hydrothermal deposits has been used for decades to distinguish the source of metals and constrain the process of formation of ore deposits. Using trace elements in pyrite, Bralia et al. (1979) and Loftus-Hills and Solomon (1967) were able to distinguish between volcanogenic, sedimentary, and hydrothermal sources of metals in ore deposits from France and Australia. In addition, the abundance of trace elements in sulfide minerals from both natural and experimental systems has been used to indicate the processes of sulfide deposition (LoftusHills and Solomon, 1967; You et al., 1996). In modern midocean ridge hydrothermal systems, trace metal concentrations in massive sulfides have been correlated with temperature and bulk chemistry of coexisting vent fluids (Tivey et al., 1995; Mills et al., 1996). For example, in such deposits, high temperature hydrothermal fluids are responsible for the leaching of soluble trace elements during dissolution/reprecipitation processes, resulting in depleted regions at the base of deposits and corresponding enrichment at the margins (Bjerkgard et al., 2000; Petersen et al., 2000). However, the current database of trace element distribution in massive sulfides under hydrothermal conditions is limited to a few ancient deposits that are not directly comparable to the sediment-hosted Middle Valley system, which underscores the need for further studies of currently active areas of massive sulfide formation.
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Valley (Ames et al., 1993; Goodfellow and Franklin, 1993), but hydrothermal fluids pass through ⬃500 m of clastic sediments before venting at the seafloor, which strongly affects hydrothermal processes due to reaction with sediments and mixing with fluids circulating in sediments (Stein et al., 1998; Zierenberg et al., 1998). Thus, the chemistry of massive sulfides at Middle Valley provides critical information on the extent to which fluid/sediment reactions and processes such as mixing with seawater influence sulfide deposition and composition. Previous chemical and isotopic studies on the Bent Hill hydrothermal system indicate that the reaction of seawater with a mixture of basaltic and sedimentary sources controls the modern vent fluid chemistry (Goodfellow and Franklin, 1993; Butterfield et al., 1994). In addition, detailed studies of Pb isotopes and minor elements in sulfide minerals from Middle Valley indicate both basalt and sediment-sourced components (Stuart et al., 1999; Bjerkgard et al., 2000). Bulk chemical analyses of sulfide ore samples, however, do not provide sufficient data to detect differences in trace metal enrichment between multiple episodes of mineralization recorded at micrometer to millimeter scale by individual sulfide minerals or textural zones. Consequently, high spatial-resolution analytical techniques are needed. Here, we use laser-ablation ICP-MS analyses of sulfide samples from ODP Leg 169 to determine the trace elements in hydrothermal sulfides formed at Middle Valley. These data, together with petrographic analysis and fluidmineral equilibria calculations, help to constrain the origin and temporal evolution of trace element distributions caused by primary and secondary sulfide mineralization at Bent Hill and ODP Mound deposits. 1. GEOLOGIC SETTING
Middle Valley on the northern Juan de Fuca Ridge is overlain by thick deposits of continentally derived turbiditic sediments (Fouquet et al., 1998). Active hydrothermal venting of fluid at 265°C was observed at Middle Valley during ODP Leg 139 at Site 858, an area known as Dead Dog vent field (Butterfield et al., 1994). Venting of fluid at 264°C was also observed at ODP Mound (Fouquet et al., 1998). No active venting has been discovered at the largest sulfide mound, Bent Hill, although evidence of previous venting is abundant (Fouquet et al., 1998). The Bent Hill Massive Sulfide (BHMS) deposit is a very large, mature deposit with a 94 m thick massive sulfide zone, interpreted to have formed at the seafloor, overlying a substantial (⬃100 m thick) sulfide feeder zone (Fouquet et al., 1998; Zierenberg et al., 1998). Leg 169 made two transects in the region of the Bent Hill massive sulfide mound, one north-south (Hole 1035F) and one east-west (Holes 1035D, G) (Fig. 1a) and deepened Hole 856H from 93 to 520 m below seafloor (mbsf). One long hole was drilled into ODP Mound (Hole 1035H), 400 m to the south. Bent Hill is stratigraphically less complex than ODP Mound, having a single set of massive sulfide and associated feeder zones, as opposed to the three sets within the ODP Mound (Fig. 1b) (Fouquet et al., 1998; Zierenberg et al., 1998). Primary hexagonal pyrrhotite forms the bulk of the Bent Hill massive sulfide zone with minor concentrations of sphalerite and chalcopyrite (Goodfellow and Franklin, 1993; Fouquet et
al., 1998). Mineral paragenesis shows pyrrhotite replaced in the upper portions of the deposit by either secondary “vuggy” pyrite or a secondary pyrite-magnetite assemblage (Goodfellow and Blaise, 1988; Fouquet et al., 1998). Zinc is in greater abundance in the upper portion of the massive sulfide zone as shown by sphalerite inclusions within vuggy pyrite, whereas copper is concentrated in the lower sulfide feeder zone, consistent with zonations observed in other seafloor deposits (Duckworth et al., 1994; Fouquet et al., 1998). Similarly, the TAG hydrothermal field on the Mid-Atlantic Ridge and a possible counterpart in the geological record, massive sulfide in the Troodos ophiolite in Cyprus, exhibit zonation of high Cu/Zn ratios in the center of the deposit to low Cu/Zn ratios at the edges of the deposit (James and Elderfield, 1996; Petersen et al., 2000). The BHMS and ODP deposits are unique among seafloor deposits in having well developed feeder or stringer zones underlying massive zones. These zones are Cu-rich and Znpoor and consist of irregular crosscutting sulfide veins from submillimeter to ⬃3cm thickness. The upper portion of the BHMS feeder zone (down to 145 mbsf) consists of intensely mineralized veins of chalcopyrite solid-solution and pyrrhotite. The lower section of the feeder zone contains fewer and thinner veins, and more disseminated sulfides within the host sediment (Zierenberg et al., 1998). At the base of the BHMS feeder zone is a ⬃20-m-thick stratiform zone high in Cu that consists of chalcopyrite-cubanite solid-solution, pyrrhotite and pervasively chloritized and sericitized turbidites. A similar, but thicker, possibly correlative deposit occurs at the base of the ODP mound mineralization (Fig. 1). Hydrothermally altered basaltic sills and lava flows were encountered beneath the BHMS at a depth of ⬃500 m (Teagle and Alt, 2003, personal communication). 3. MATERIALS AND METHODS Mineral content and paragenesis were determined using reflected light microscopy on 22 thin sections taken from five of the holes drilled during Leg 169. Several sampling strategies were employed to determine trace elements within and between the deposits. A suite of 4 thin sections was chosen at approximately the same depth near the base of the massive sulfide zone from holes to the east, west, south, and in the center of the BHMS deposit. In addition, three samples were chosen at increasing depths in Hole 856H in the center of the BHMS deposit, allowing analysis of the upper portion of the feeder zone, and at a depth very near the deep copper zone. For comparison, four samples were taken at several depths in Hole 1035H, the only core drilled into the ODP Mound. These include representative samples from the upper and lower massive sulfide zone, the upper feeder zone, and the deep copper zone immediately underlying the lower massive sulfide zone. Analyses of Mn, Co, Ni, As, Se, Ag, Cd, Sb, Pb, and Bi in selected sulfide grains were performed with a CETAC LSX 200 laser ablation microprobe coupled to a Perkin Elmer ELAN 6000 ICP-MS. Measurements were made with a Q-switched Nd-YAG UV laser source, frequency quadrupled (wavelength of 266 nm), pulsed voltage of 1450 V, with He as the gas carrier. Single point laser shots were taken over a 7 s period, creating a crater 25 m in diameter, and approximately 10 m in depth. Most trace element concentrations (Mn, Co, Ni, As, Ag, Cd, Sb, Pb, Bi), as well as iron, were determined using an internal USGS glass standard prepared from a powder (Meyers et al., 1976; Ridley et al., 2000). Concentrations of the major elements Cu, Zn, and S, and the trace metal Se were determined using CANMET ccu-1b standard, a pressed powder pellet produced by a copper flotation concentrate from ore from the Ruttan Mine, Manitoba (see www.nrcan.gc.ca/mms/canmet-mtb/ccrmp/ccu-1c.htm). The major element concentrations for the ccu-1b standard are accurate within ⫾2% and the trace element con-
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Fig. 1. (a) Map of the Middle Valley area on the Juan de Fuca Ridge with enlarged schematic diagram of the Bent Hill site showing some of the cores drilled during ODP Leg 169. Both east-west and north-south transects were made across the BHMS deposit. (b) Cross section of the BHMS and ODP Mound deposits. The BHMS deposit consists of a single massive sulfide zone and underlying feeder zone. The ODP Mound deposit, in contrast, contains three massive sulfide and feeder zone sets. Both deposits have a thin deep copper zone at the base, depicted here as possibly a continuous stratigraphic unit.
centrations of both standards are accurate to at least ⫾5%. Precision of analyses is determined using counting statistics on multiple laser shots and is ⬍1% for Cu, Zn, and Fe, and 2–5% for trace metals. Analyses of single sulfide minerals were obtained. When textural data suggested replacement reactions, both primary and secondary sulfides were analyzed. Examination of the grains by reflected light microscopy, however, often indicated the common occurrence of small inclusions within the grains and/or veins that could not always be discriminated during laser analysis. In addition to petrographic analysis, the major elements of the sulfides (Fe in pyrrhotite/pyrite, Cu in chalcopyrite, and Zn in sphalerite) determined by LA-ICP-MS were also used to distinguish the bulk phases. Pyrrhotite samples have an average Fe concentration of 62.3 ⫾7.8 wt%; pyrite samples have average Fe concentrations of 64.1 ⫾ 7.9 wt%; sphalerite samples have an average Zn concentration of 63.5 ⫾ 14.5 wt% and average Fe concentrations of 9.9 ⫾ 1.7 wt% due to variable Fe content; and chalcopyrite samples have average Cu concentrations of 27.8 ⫾ 7.3 wt% and average Fe concentrations of 29.8 ⫾ 7.9 wt% due to solid
solution with isocubanite. Analyses with major element concentrations outside these ranges were eliminated from consideration, which represent less than 6% of the total number of analyses. Geochemical reaction calculations were carried out using Geochemist’s WorkBench (GWB) software (Bethke, 1996) and thermodynamic data generated using SUPCRT92 with recent database updates (Shock et al., 1997). SUPCRT was run at 500 bars pressure and temperatures up to 400°C and the database was adapted to GWB format (W.E. Seyfried, unpublished data). Reaction calculations were carried out from 250 to 400°C to simulate incremental reaction of 1 kg of seawater with sulfide and oxide minerals. In these models, initial speciation calculations are computed for the fluid at the specified temperature. Then an increment of seawater is added to the system, and fluid speciation and mineral solubility calculations are adjusted and minerals are precipitated or dissolved until an equilibrium assemblage is attained. These calculations are repeated with successive increments of fluid mixing. No kinetic constraints are considered in these models and equilibrium is recomputed at each step.
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J. L. Houghton, W. C. Shanks, and W. E. Seyfried 4. RESULTS
4.1. Bent Hill Massive Sulfide Deposit The center of the BHMS deposit exhibits a much greater variety of sulfide minerals and textures than the flanks, while the trace element concentrations increase toward the edges of the deposit, both laterally and with depth. For example, samples at 100 mbsf in Hole 856H in the center of the deposit exhibit oxidized pyrrhotite groundmass coexisting with massive pyrite intergrown with magnetite, and irregular inclusions of chalcopyrite (Figs. 2a,b). Only the massive areas of pyrite were sufficiently large to sample with confidence with the laser. These grains are relatively free of inclusions of chalcopyrite or magnetite as indicated by chemical and textural data. Concentrations of trace elements in these grains are very low (Fig. 3). In comparison, a sample at 130 mbsf consists entirely of intergrown pyrrhotite/chalcopyrite (Fig. 2c), similar in texture to the groundmass at 100 mbsf. Most trace element concentrations in this sample are also low, with the exception of Co and Zn. Zn concentrations greater than 4000 ppm indicate the presence of sphalerite inclusions and are shown to correlate with high Co concentrations (Fig. 3). Concentrations of most trace elements remain uniform and low throughout the center of the BHMS deposit, despite large differences in sulfide mineral abundances (Fig. 3). The base of the massive sulfide zone in the distal portions of the BHMS deposit is dominated by pyrrhotite with colloform or lath-like textures, suggesting rapid precipitation (Roedder, 1968; Huston et al., 1995) (Fig. 4a). At a depth of 75 mbsf in Hole 1035D, at the east flank of the BHMS deposit, massive sphalerite coexists with massive pyrite (Fig. 4b). The massive sphalerite contains small inclusions of chalcopyrite. Moreover, sphalerite has a significantly larger range of concentrations of Pb, Sb, Cd, As than both fine-grained pyrrhotite and massive pyrite in the same sample (Fig. 5a). Such enrichments may be due to the coprecipitation of sphalerite and inferred trace inclusions of galena, which readily accepts metals available in solution, consistent with analogous phase relations in numerous other deposits (Huston et al., 1995). However, pyrrhotite groundmass contains higher concentrations and greater variability of virtually all trace elements (Pb, Sb, Cd, Ag, Mn) than massive pyrite (Fig. 5a). A depletion of trace element concentration in pyrite with pyrrhotite recrystallization to pyrite is consistent with observations in fossil VMS deposits (Gillis and Robinson, 1990; Huston et al., 1995). A sample from the south flank at 60 mbsf in Hole 1035F exhibits textures very similar to those of the east flank. Massive sphalerite containing small inclusions of pyrrhotite or pyrite coexists with massive pyrite (Fig. 4c), similar to previous observations in the BHMS deposit by Goodfellow and Franklin (1993). High concentrations of Pb, Sb, Cd, and Ag occur in the sphalerite grains relative to pyrite (Fig. 5b). These trace elements commonly reach high concentrations in sphalerite from lower temperature white smoker deposits on the seafloor (Hannington and Scott, 1988; Tivey et al., 1995). Samples from Hole 1035G on the west flank of the BHMS deposit at 60 mbsf contain dominantly highly porous vuggy pyrite with minor amounts of interstitial pyrite. Petrographic analysis indicates complex combinations of colloform and lath-
Fig. 2. Photomicrographs of mineral textures within samples analyzed from the center of the BHMS deposit. (a) A sample from 100 mbsf in Hole 856H in the center of the BHMS deposit shows a combination of pyrrhotite groundmass (upper right) and massive pyrite (lower left). The ablation crater (25 m diameter) is within the massive pyrite. (b) Another example of texture from the same thin section as in (a) with coexisting massive pyrite containing magnetite and chalcopyrite inclusions and irregular groundmass. (c) The sample from 130 mbsf in Hole 856H consists entirely of intergrown pyrrhotite and chalcopyrite within a sediment-hosted vein. Three ablation craters (25 m diameter) can be seen in the field of view.
like pseudomorphic textures becoming more massive near the edges of open vugs (Fig. 4d). Unlike other portions in the BHMS deposit, pyrite displaying both lath and massive textures in these samples contains relatively high concentrations of Pb, Mn, and Cu, and moderate Zn and As, likely from small inclusions of chalcopyrite and/or galena (Fig. 5c). Comparison of trace element concentrations between pyrite pseudomorphs
Trace element remobilization in Middle Valley sulfides
Fig. 3. Comparison of the ranges of trace element concentrations in veins from 100 and 130 mbsf in the BHMS deposit (Hole 856H). The number of data points (n) used is indicated in the legend. Trace element concentrations are generally depleted in both locations with the exception of Co and Zn, which are highly enriched at 130 mbsf.
and massive pyrite grains surrounding the open vugs reveals little difference (Fig. 5c). A similar observation was made by Stuart et al. (1999), who reported identical Pb isotope ratios in primary pyrrhotite and secondary pyrite from the Bent Hill massive sulfide zone. These observations are consistent with recrystallization reactions that may have occurred in largely geochemically isolated environments, which were likely created by the extensional fault bounding the western margin of the BHMS deposit (Fouquet et al., 1998). This fault may be the primary conduit for seawater entering the deposit, and thus, may insulate this portion of the deposit from late-stage hydrothermal fluid alteration. 4.2. ODP Mound Deposit The ODP Mound deposit samples exhibit largely the same mineral abundance and textures as the BHMS deposit. Hole 1035H was drilled near the center of ODP Mound. Pyrrhotite is the dominant phase in this massive sulfide ore (Fig. 6a), in some cases exhibiting recrystallization textures deeper in the deposit (Fig. 6b), and is usually found coexisting with smaller patches of sphalerite. Commonly, massive sphalerite grains contain small inclusions of chalcopyrite (Fig. 6c), a texture commonly referred to as chalcopyrite disease (Barton and Bethke, 1987). Samples from 40 mbsf in this location contain massive chalcopyrite with inclusions of sphalerite, sphalerite with minor inclusions of pyrite or pyrrhotite, and massive pyrrhotite. Sphalerite is enriched in Mn and Cd, likely substituting for Fe (Smith and Huston, 1992), with Se replacing sulfur (Yamamoto, 1976). In contrast, chalcopyrite (Fig. 7a) is enriched only in Ag, which substitutes for Cu (Harris et al., 1984; Cabri et al., 1985), and As, which is typically incorporated as AsS3⫺ in the trivalent Fe site (Cook and Chryssoulis, 1990). Pyrrhotite veins are enriched in Co and Ni that substitute for Fe in the lattice, and Pb and Bi possibly present as inclusions of galena (Klemm, 1965; Huston et al., 1995) (Fig. 7a). Iron sulfides were also analyzed at 12 mbsf and 135 mbsf in ODP Mound. At 12 mbsf, pyrite exists as pseudomorphic laths after pyrrhotite, infilling between sulfide and oxide grains (Fig. 6d). Grains sufficiently large to sample contain inclusions of sphalerite and are enriched in As, Cd, Ag, and Sb as substitutions, with high concentrations of Pb inferred to be inclusions
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of galena, and depleted in Se, Ni, Bi, and Co. In comparison, at 135 mbsf in the lower massive sulfide zone the dominant Fe sulfide phase is pyrrhotite, existing as irregular aggregates intergrown with sphalerite (Fig. 6b). Massive pyrrhotite is depleted in Bi, Pb, and As and enriched only in Se relative to pyrrhotite groundmass (Fig. 7b). At depth in the core, the trace element distribution in Fe sulfides shifts from inclusion-rich secondary pyrite with high As, Ag, Cd, Sb, and Pb in the upper massive sulfide zone (12 mbsf), to high Co, Ni and Se incorporated in the lattice of pyrrhotite veins in the feeder zone (40 mbsf), to high Pb and Bi possibly from inclusions of galena in pyrrhotite groundmass at 135 mbsf in the lower massive sulfide zone (135 mbsf) (Fig. 7b). 5. DISCUSSION
Hydrothermal mineral deposits record the history of chemical and thermal effects governing primary metal precipitation reactions and secondary sulfide alteration processes responsible for metal mobilization and fractionation (Bjerkgard and Bjorlykke, 1996; Fouquet et al., 1998; Petersen et al., 2000). Previous analysis of mineral textures at the Bent Hill site suggest late-stage alteration by multiple generations of hydrothermal activity (Ames et al., 1993), and isotopic studies provide clear evidence in support of this hypothesis. For example, sulfur isotopes of the sulfides indicate the replacement of pyrrhotite to pyrite by more oxidizing fluids containing a significant seawater component (Goodfellow and Franklin, 1993; Zierenberg, 1994; Shanks, 2001). In addition, Sr and Pb isotopes in hydrothermal minerals indicate a significant sediment component in these secondary fluids (Goodfellow and Franklin, 1993). Trace element data from the present study, constrained by petrographic observations, provide information on episodic hydrothermal mineralization at Middle Valley. As the recrystallization of a hydrothermal sulfide mineral deposit generally leads to a coarsening of grain size, textural differences between sulfide grains can be used to distinguish primary from later-stage alteration phases. Comparison of remnant primary lath-like and colloform pyrrhotite groundmass with massive secondary pyrite veins suggests changes in hydrothermal fluid chemistry or conditions over time. For example, secondary sulfides precipitated at the margins of white smoker deposits such as in the TAG vent system are enriched in Zn and associated Cd, Pb, Ag, Sb, and As (Hannington and Scott, 1988; Tivey et al., 1995). Solubility of these metallic elements is controlled by highly mobile chloro-complexes that are very stable at the temperate and chloride contents of seawater-derived hydrothermal fluids (Hannington et al., 1995; Tivey et al., 1995; Mills et al., 1996; Metz and Trefry, 2000; Petersen et al., 2000). White smoker fluids (⬃275°C), with a significant seawater component, have comparatively low H2S (0.5 mM) and Cu (3 M) and high Zn (300 – 400 M) concentrations relative to their corresponding high temperature (360°C) end-member fluid (Edmond et al., 1995; James and Elderfield, 1996). Low temperature diffuse vent fluids have less sulfide and Zn (⬍0.04 mM and 62 M, respectively) with unchanged Cu (Edmond et al., 1995; James and Elderfield, 1996), indicating successive dissolution/reprecipitation first of chalcopyrite, then sphalerite, as the temperature decreases with increased mixing towards the edge of the deposit. Trace ele-
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Fig. 5. Ranges of trace element concentrations of each mineral population sampled from various locations in the BHMS deposit. The number of data points (n) used to construct the plot is indicated in the legend. (a) Pyrite and pyrrhotite grains from a sample at 75 mbsf in Hole 1035D (east flank) were divided by texture and indicate minor differences in trace metal distributions. Sphalerite grains have the highest concentrations of some elements as well as the largest ranges. (b) Sphalerite and pyrite grains from a sample taken 60 mbsf in Hole 1035F. Sphalerite grains are greatly enriched in most elements compared to pyrite grains. (c) Pyrite grains divided by textures sampled at 60 mbsf in Hole 1035G. No noticeable distinction can be made in trace metal distributions between massive pyrite lining vugs and interstitial pyrite.
ment distributions in Middle Valley sulfides reported in this study suggest the hydrothermal conditions that produce a fluid with a high Zn/Cu ratio may also enhance the transport capacity
Fig. 4. Photomicrographs of mineral textures within samples analyzed from the flanks of the BHMS deposit. (a) Partially oxidized colloform pyrrhotite from Hole 1035D on the east flank. (b) Sphalerite in the lower right corner coexisting with massive pyrite from Hole
1035D in the east flank. Inclusions of chalcopyrite grains were seen within the pyrite, but were avoided during sampling with the laser. (c) Massive sphalerite (dark grey) coexisting with massive pyrite in a sample from Hole 1035F on the south flank. Two ablation craters (25 m diameter), marked by arrows, can be seen in the field of view. (d) “Vuggy” and lacy textures in pyrite from a sample in Hole 1035G on the west flank. Black areas are open vugs. The pyrite texture becomes more massive and blocky along the edges of vugs.
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Fig. 7. Range of trace metal concentrations in minerals sampled in the ODP Mound. The number of data points (n) used in this figure is indicated in the legend. (a) Comparison of massive grains at 40 mbsf. Sphalerite contains the largest enrichments of some elements, except Co. (b) Comparison of trace metal concentrations with depth in pyrrhotite samples in Fe-sulfide minerals. Texture of the samples analyzed (fine-grained or massive) is also indicated.
of a wide range of trace elements owing to the effect of temperature and bulk fluid chemistry on phase equilibria. 5.1. Modeling of Hydrothermal Alteration Based on previous studies in the Bent Hill area and on geochemical evidence presented in this study, we infer the following sequence of events. During primary hydrothermal sulfide precipitation at this site, buoyant fluid derived from
Fig. 6. Photomicrographs of mineral textures in samples analyzed from Hole 1035H in the ODP Mound deposit. (a) An example of massive sulfide vein textures within host sediments at 40 mbsf. Pyrrhotite is the dominant mineral in this photo, with smaller grains of pyrite (in the upper left) and sphalerite (upper right). Three ablation craters (25 m diameter) can be seen in the pyrite grain. (b) Primary textures of intergrown pyrrhotite and sphalerite are more common at depth as shown here in a sample from 135 mbsf. The black area near the center of the photo is void space. (c) Massive sphalerite at 135 mbsf commonly contains small inclusions of chalcopyrite blebs. (d) A sample from the very top of the massive sulfide zone at 12 mbsf shows pyrite lath pseudomorphs of replaced pyrrhotite within the sphalerite groundmass. An ablation crater (25 m diameter) can be seen in the rectangular lath in the center of the photo. (e) A sample from 190 mbsf within the deep copper zone contains chalcopyrite veins within sediments. Three ablation craters (25 m diameter) can be seen in the field of view.
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high-temperature (⬃400°C) basalt-seawater reaction ascends rapidly to the seafloor via large continuous faults that cut the sediments (Zierenberg et al., 1998; Goodfellow and Franklin, 1993). After a sizable massive sulfide deposit forms by this process, seawater penetrates laterally through the host sediment and massive sulfide, causing lower temperature (⬃280°C) alteration, recrystallization, and redistribution of metallic elements (Stein et al., 1998; Goodfellow and Franklin, 1993). To address the hypothesis of secondary alteration of the sulfide assemblage by seawater, a general thermodynamic model was used to compute the mass transfer reaction path, whereby primary high-temperature pyrrhotite and chalcopyrite coexisting with an end-member hydrothermal fluid were reacted with seawater, while temperature changed in accordance with the extent of mixing with seawater. Thus, the system is modeled by initially equilibrating an assemblage of pyrrhotite, magnetite, chalcopyrite, sphalerite, and quartz, with a hightemperature (400°C) NaCl (0.5M) fluid. This oxide/sulfide assemblage, consistent with alteration of basalt and/or deepseated sediments, produces a relatively reducing hydrothermal fluid. Subsequent reaction during seawater entrainment is predicted to cool and shift the redox conditions from reducing, H2-rich, to more oxidizing, SO4-rich fluids, allowing precipitation of anhydrite at temperatures below 350°C. Such a change in redox combined with the relatively unchanging total dissolved H2S(aq) concentration results in the system shifting from the pyrrhotite-magnetite-chalcopyrite stability field into the pyrite stability field. This model simulates well the observed mineral replacement textures of pyrrhotite oxidized to vuggy pyrite at Middle Valley (Fig. 8d), which is accompanied by a decrease in pH from ⬃5 to a minimum of 4.3 at a temperature of 300°C (Fig. 8c). Copper concentrations are predicted to decrease at a constant rate over the entire temperature range, owing to constraints imposed by chalcopyrite solubility (Figs. 8b,e). Dissolved iron is predicted to increase initially with both magnetite and pyrrhotite dissolution until below 300°C, where secondary pyrite and magnetite precipitation begins to remove Fe from solution (Figs. 8a,d). Zinc increases in solution below 350°C as sphalerite dissolves until below 275°C when sphalerite precipitation causes a net loss of dissolved Zn (Figs. 8b,e). This process is consistent with observations of hydrothermal fluids in the North Fiji Basin and TAG, which have less Cu, Mo, and Co and more Zn, As, Ag, Cd, Pb in white smoker fluids ⬍320°C (James and Elderfield, 1996; Metz and Trefry, 2000). Our simulation is largely consistent with petrographic and geochemical observations, suggesting very low trace metal concentrations in secondary pyrite that precipitates readily over a wide range of temperatures, and very high concentrations in Zn-bearing minerals that precipitate rapidly at lower temperatures and are potentially less discriminatory against high trace metal concentrations. By using the observed mineralogy and mineral replacement textures to interpret trace element distributions, the history of trace metal mobility in hydrothermal fluids at Middle Valley may be placed in the context of the overall geochemical model. Primary pyrrhotite in the Bent Hill deposit contains high concentrations of trace elements (Pb, Sb, Cd, Ag, Mn), likely in the form of galena inclusions, while secondary vuggy pyrite is depleted in trace elements, with the exception of the western
flank. This suggests that pyrrhotite replacement by pyrite, which can occur over a broad range of temperatures (Fig. 8), may be an effective method of excluding trace elements from the sulfide. The western flank sample is unique in that secondary pyrite retains high concentrations of trace elements (Pb, Mn, Cu, Zn, and As) owing to inferred chalcopyrite and galena inclusions, indicating a unique local environment isolated by faulting from secondary hydrothermal fluids, as previously discussed. Pyrite in the ODP Mound, which is less common than in the BHMS deposit, only shows high trace element concentrations when it contains significant inclusions of sphalerite that preferentially retains these elements. At the BHMS deposit, the removal of trace elements from the center of the sulfide mounds and subsequent reprecipitation in the distal portions reported in this study can be explained in terms of the seawater-mixing model. The model predicts remobilization of Zn at intermediate temperatures (275–325°C) due to the decrease in pH of the fluid (Figs. 8b,c), and reprecipitation of sphalerite at lower temperatures as continued seawater entrainment gradually raises the pH (Figs. 8e,c). In the BHMS deposit, sphalerite along the margins of massive sulfide lenses consistently displays very high concentrations of trace metals (Mn, Pb, Sb, Cd, Ag, As), consistent with the seawater mixing model predictions and observations at the TAG deposit (James and Elderfield, 1996). In addition, trace metal concentrations in sphalerite in the ODP Mound deposit, while elevated, are lower than in the BHMS deposit, suggesting these two deposits likely experienced episodes of remineralization characterized by very different conditions. 6. CONCLUSIONS
Observations of mineral textures and trace metal distributions of sulfides in both the BHMS and ODP Mound deposits at Middle Valley support previous interpretations that both deposits record a history of remineralization with decreasing temperature. Differences in replacement textures and trace metal distributions between the deposits may be explained by differences in the hydrothermal conditions (i.e., temperature or pH) during remineralization, as illustrated by a general model of the mixing of hydrothermal fluids with seawater. Within the BHMS deposit, the western flank at the base of the massive sulfide zone appears to have been isolated by faulting during the most recent alteration event, possibly acquiring an additional metal component from a local source. Sphalerite is more common at the outer edges of the massive sulfide zones in both deposits and is the most effective sink for trace metals, in particular Pb, Sb, and Cd, of all the sulfides, consistent with remineralization from the center to the perimeter during seawater entrainment. Likewise, secondary pyrite samples throughout the BHMS deposit are depleted in most trace metals relative to other phases. Differences in trace metal compositions between the feeder zones at BHMS and ODP Mound are evidence that both deposits were formed under different hydrothermal conditions, perhaps due to somewhat different primary fluid compositions or different seawater entrainment conditions. The selective enrichment of metals reported in this study suggests that secondary mineralization of sulfide mounds by mixing of hydrothermal fluids with seawater results in significant redistribution of metals in sulfide minerals.
Trace element remobilization in Middle Valley sulfides
Fig. 8. Predicted hydrothermal alteration pathway from a thermodynamic mixing model. Initial conditions for the model were defined by equilibration of a primary sulfide deposit consisting of a pyrrhotite-magnetite-chalcopyrite-quartz assemblage with a 400°C hydrothermal fluid. Temperature was controlled by the incremental mixing of 1 kg of the resultant high temperature fluid with 1 kg of cold (25°C) seawater (0.5 mol/L NaCl solution of pH 5.3, containing 30 mmolal SO4⫽, 55 mmolal Mg2⫹ and 10 mmolal Ca2⫹). (a) Predicted change in total dissolved iron reported as delta mmoles, illustrating initial enrichment of iron and subsequent depletion of iron as temperature decreases. (b) Change in dissolved copper and zinc with mixing. Metal speciation in this model is dominated by complexing with chloride, as expected at the low pH of these fluids. (c) Change in pH with mixing, reflecting lowering pH as pyrite precipitation proceeds, changing to increasing pH as the seawater component of the fluid begins buffering the solution. (d) Change in mineralogy during mixing, illustrating pyrite and magnetite precipitation at the expense of pyrrhotite, consistent with the dissolved iron trend in (a). (e) Change in chalcopyrite and sphalerite dissolution/reprecipitation with temperature.
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Acknowledgments—We thank T. Pulsipher for her time and expertise; I. Ridley, F. Lichte, and P. Aruscavage at the USGS, Denver for the use of their analytical facilities; I. Ridley and K. Kelley for their comments; the shipboard scientific party from ODP Leg 169; the Texas A&M Research Foundation and the National Science Foundation for support through grant 169-F000475-BA133; and M. Solomon, and several anonymous reviewers for their helpful comments. Associate editor: E. M. Ripley REFERENCES Ames D. E., Franklin J. M., and Hannington M. D. (1993) Mineralogy and geochemistry of active and inactive chimneys and massive sulfide, Middle Valley, Northern Juan de Fuca Ridge: An evolving hydrothermal system. Can. Mineral. 31, 997–1024. Barton P. B., Jr. and Bethke P. (1987) Chalcopyrite disease in sphalerite: Pathology and epidemiology. Am. Mineral. 72, 451– 467. Bethke C. M. (1996) Geochemical Reaction Modeling. Oxford University Press. Bjerkgard T. and Bjorlykke A. (1996) Sulfide deposits in Folldal, southern Trondheim region Caledonides, Norway: Source of metals and wall-rock alterations related to host rocks. Econ. Geol. 91, 676 – 696. Bjerkgard T., Cousens B. L., and Franklin J. M. (2000) The Middle Valley sulfide deposits, Northern Juan de Fuca Ridge: Radiogenic isotope systematics. Econ. Geol. 95, 1473–1488. Bralia A., Sabatini G., and Troja F. (1979) A revaluation of the Co/Ni ratio in pyrite as a geochemical tool in ore genesis problems. Mineral. Dep. 14, 353–374. Butterfield D. A., McDuff R. E., Franklin J., Wheat C. G. (1994) Geochemistry of hydrothermal vent fluids from Middle Valley, Juan de Fuca Ridge. In Proceedings of the Ocean Drilling Program, Scientific Results (eds. M. J. Mottl, E. E. Davis, A. T. Fisher and J. F. Slack), pp. 395– 410. Texas A & M University. Cabri L. J. J. L., Campbell J. H. G., Laflamme R. G., Leigh J. A., Maxwell J. D., and Scott J. D. (1985) Proton-microprobe analysis of trace elements in sulfides from some massive-sulfide deposits. Can Mineral. 23, 133–148. Cook N. J. and Chryssoulis S. L. (1990) Concentrations of “invisible gold” in common sulfides. Can. Mineral. 28, 1–16. Duckworth R. C., Fallick A. E., Rickard D. (1994) Mineralogy and sulfur isotopic composition of the Middle Valley massive sulfide deposit, Northern Juan de Fuca Ridge. In Proceedings of the Ocean Drilling Program, Scientific Results (eds. M. J. Mottl, E. E. Davis, A. T. Fisher and J. F. Slack), pp. 373–385. Texas A & M University. Edmond J. M., Campbell A. C., Palmer M. R., Klinkhammer G. P., German C. R., Edmonds H. N., Elderfield H., Thompson G., Rona P. (1995) Time series studies of vent fluids from the TAG and MARK sites (1986, 1990) Mid-Atlantic Ridge: A new solution chemistry model and a mechanism for Cu/Zn zonation in massive sulfide orebodies. In Hydrothermal Vents and Processes (eds. L. M. Parson, C. L. Walker and D. R. Dixon), pp. 77– 86. Geological Society, London. Fouquet Y., Knott R., Cambon P., Fallick A., Rickard D., and Desbruyeres D. (1996) Formation of large sulfide mineral deposits along fast spreading ridges: Example from off-axial deposits at 12°43⬘N on the East Pacific Rise. Earth Planet Sci Lett. 144 (1–2), 147–162. Fouquet Y., Zierenberg R. A., Miller D. J. and the Shipboard Scientific Party. (1998) Middle Valley: Bent Hill area (site 1035). In Proceedings of the Ocean Drilling Program, Initial Reports, pp. 35–152. Texas A & M University. Gillis K. M. and Robinson P. T. (1990) Patterns and processes of alteration in the lavas and dykes of the Troodos Ophiolite, Cyprus. J. Geophys. Res. 95, 21523–21548. Goodfellow W. D. and Blaise B. (1988) Sulfide formation and hydrothermal alteration of hemipelagic sediment in Middle Valley, Northern Juan de Fuca Ridge. Can. Mineral. 26, 675– 696. Goodfellow W. D. and Franklin J. M. (1993) Geology, mineralogy, and chemistry of sediment-hosted clastic massive sulfides in shallow cores, Middle Valley, Northern Juan de Fuca Ridge. Econ. Geol. 88, 2037–2068. Hannington M. D. and Scott S. D. (1988) Gold and silver potential of polymetallic sulfide deposits on the sea floor. Mar. Mining 7, 271–285.
Hannington M. D., Jonasson I. R., Herzig P. M. and Petersen S. (1995) Physical and chemical processes of seafloor mineralization at midocean ridges. In Seafloor Hydrothermal Systems: Physical, Chemical, Biological and Geological Interactions (eds. S. E. Humphris R. A. Zierenberg L. S. Mullineaux R. E. Thomson), pp. 115–157. Geophysical Monograph 91. American Geophysical Union. Harris D. C., Cabri J., and Nobiling R. (1984) Silver-bearing chalcopyrite, a principle source of silver in the Izok Lake massive sulfide deposit: Confirmation of electron- and proton-microprobe analyses. Can. Mineral. 22, 493– 498. Humphris S. E., Herzig P. M., Miller D. J., Alt J. C., Becker K., Brown D., Bruegmann G., Chiba H., Fouquet Y., Gemmell J. B., Guerin G., Hannington M. D., Holm N. G., Honnorez J. J., Iturrino G. J., Knott R., Ludwig R., Nakamura K., Petersen S., Reysenbach A. L., Rona P. A., Smith S., Sturz A. A., Tivey M. K., and Zhao X. (1995) The internal structure of an active sea-floor massive sulphide deposit. Nature 377 (6551), 713–716. Huston D. L., Sie S. H., Suter G. F., Cooke D. R., and Both R. A. (1995) Trace elements in sulfide minerals form Eastern Austalian volcanic-hosted massive sulfide deposits: Part I. Proton microprobe analyses of pyrite, chalcopyrite, and sphalerite, and Part II. Selenium levels in pyrite: Comparison with ⌬34S values and implications for the source of sulfide in volcanogenic hydrothermal systems. Econ. Geol. 90, 1167–1196. James R. H. and Elderfield H. (1996) Chemistry of ore-forming fluids and mineral formation rates in an active hydrothermal sulfide deposit on the Mid-Atlantic Ridge. Geology 24, 1147–1150. Klemm D. D. (1965) Synthesen und analysen in den dreiecksdiagrammen FeAsS-CoAsS-NiAsS und FeS2-CoS2-NiS2 (in German). Neues Jahrbuch Mineral. Abhand. 103, 205–255. Loftus-Hills G. and Solomon M. (1967) Cobalt, nickel and selenium in sulfides as indicators of ore genesis. Mineral. Dep. 2, 228 –242. Metz S. and Trefry J. H. (2000) Chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids. Geochim. Cosmochim. Acta 64, 2267–2279. Meyers A. T., Havens R. G., Connor J. J., Conklin N. M. and Rose H. J. Jr. (1976) Glass reference standards for the trace-element analysis of geological materials: Compilation of interlaboratory data. Professional Paper 1013, p. 29. USGS. Mills R. A., Clayton T., and Alt J. C. (1996) Low-temperature fluid flow through sulfidic sediments from TAG: Modification of fluid chemistry and alteration of mineral deposits. Geophys. Res. Lett. 23, 3495–3498. Parmentier E. M. and Spooner E. T. C. (1978) A theoretical study of hydrothermal convection and the origin of the ophiolitic sulphide ore deposits of Cyprus. Earth Planet. Sci. Lett. 40, 33– 44. Peter J. M. and Scott S. D. (1988) Mineralogy, composition, and fluid-inclusion microthermometry of seafloor hydrothermal deposits in the southern trough of Guaymas Basin, Gulf of California. Can. Mineral. 26, 567–587. Petersen S., Herzig P. M., and Hannington M. D. (2000) Third dimension of a presently forming VMS deposit: TAG hydrothermal mound, Mid-Atlantic Ridge, 26°N. Mineral. Dep. 35, 233–259. Ridley W. I., and Lichte F. E. (1998) Major, trace, and ultratrace element analysis by laser ablation ICP-MS. Rev. Econ. Geol. 7, 199 –215. Roedder E. (1968) The noncolloidal origin of “colloform” textures in sphalerite ores. Econ. Geol. 63, 451– 471. Shanks W. C. III. (2001) Stable isotopes in seafloor hydrothermal systems: Vent fluids, hydrothermal deposits, hydrothermal alteration and microbial processes. In Stable Isotope Geochemistry, Vol. 43 (ed. J. W. Valley and D. R. Cole), pp. 469 –526. Mineralogical Society of America. Shock E. L., Sassani D. C., Willis M., and Sverjensky D. A. (1997) Inorganic species in geologic fluids: Correlations among standard molal thermodynamic properties of aqueous ions and hydroxide complexes. Geochim. Cosmochim. Acta 61, 907–950. Smith R. N. and Huston D. L. (1992) Distribution and association of selected trace elements at the Rosebery deposit, Tasmania. Econ. Geol. 87, 706 –719. Stein J. S., Fisher A. T., Langseth M., Jin W., Iturrino G., and Davis E. (1998) Fine-scale heat flow, shallow heat sources, and decoupled
Trace element remobilization in Middle Valley sulfides circulation systems at two seafloor hydrothermal sites, Middle Valley, northern Juan de Fuca Ridge. Geology 26, 1115–1118. Stuart F. M., Ellam R. M., and Duckworth R. C. (1999) Metal sources in the Middle Valley massive sulphide deposit, northern Juan de Fuca Ridge: Pb isotope constraints. Chem. Geol. 153, 213–225. Tivey M. K., Humphris S. E., Thompson G., Hannington M. D., and Rona P. A. (1995) Deducing patterns of fluid flow and mixing within the TAG active hydrothermal mound using mineralogical and geochemical data. J. Geophys. Res. 100, 12527–12555. Yamamoto M. (1976) Relationship between Se/S and sulfur isotope ratios, hydrothermal sulfide minerals. Mineral. Dep. 11, 197–209. You C. F., Castillo P. R., Gieskes J. M., Chan L. H., and Spivack A. J. (1996) Trace element behavior in hydrothermal experiments; implications for fluid processes at shallow depths in subduction zones. Earth Planet. Sci. Lett. 140, 41–52. Zierenberg R. A., Fouquet Y., Miller D. J., Bahr J. M., Baker P. A., Bjerkgard T., Brunner C. A., Duckworth R. C., Gable R., Geiskes J.,
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Pergamon
doi:10.1016/j.gca.2003.12.023
Massive sulfide deposition and trace element remobilization in the Middle Valley sediment-hosted hydrothermal system, northern Juan de Fuca Rdge J. L. HOUGHTON,1,* W. C. SHANKS, III,2 and W. E. SEYFRIED, JR1 1
Department of Geology and Geophysics, University of Minnesota, Minneapolis, MN 55455 USA 2 U.S. Geological Survey, 973 Denver Federal Center, Denver, CO 80225-0046, USA (Received January 15, 2003; accepted in revised form December 19, 2003)
Abstract—The Bent Hill massive sulfide deposit and ODP Mound deposit in Middle Valley at the northernmost end of the Juan de Fuca Ridge are two of the largest modern seafloor hydrothermal deposits yet explored. Trace metal concentrations of sulfide minerals, determined by laser-ablation ICP-MS, were used in conjunction with mineral paragenetic studies and thermodynamic calculations to deduce the history of fluid-mineral reactions during sulfide deposition. Detailed analyses of the distribution of metals in sulfides indicate significant shifts in the physical and chemical conditions responsible for the trace element variability observed in these sulfide deposits. Trace elements (Mn, Co, Ni, As, Se, Ag, Cd, Sb, Pb, and Bi) analyzed in a representative suite of 10 thin sections from these deposits suggest differences in conditions and processes of hydrothermal alteration resulting in mass transfer of metals from the center of the deposits to the margins. Enrichments of some trace metals (Pb, Sb, Cd, Ag) in sphalerite at the margins of the deposits are best explained by dissolution/reprecipitation processes consistent with secondary remineralization. Results of reaction-path models clarify mechanisms of mass transfer during remineralization of sulfide deposits due to mixing of hydrothermal fluids with seawater. Model results are consistent with patterns of observed mineral paragenesis and help to identify conditions (pH, redox, temperature) that may be responsible for variations in trace metal concentrations in primary and secondary minerals. Differences in trace metal distributions throughout a single deposit and between nearby deposits at Middle Valley can be linked to the history of metal mobilization within this active hydrothermal system that may have broad implications for sulfide ore formation in other sedimented and unsedimented ridge systems. Copyright © 2004 Elsevier Ltd The sediment-hosted volcanic-associated massive sulfide deposits at Middle Valley on the northern Juan de Fuca Ridge with at least 8.8 million tons of massive sulfide mineralization, are the largest documented accumulations yet discovered on the seafloor (Zierenberg et al., 1998). Located 200 miles west of Vancouver Island, Middle Valley is a sedimented axial rift, extensively studied during the Ocean Drilling Program (ODP) Legs 139 and 169. Middle Valley sulfide deposits are characterized by hydrothermal sulfide mineral assemblages, and are broadly similar to other seafloor massive sulfide deposits such as Escanaba Trough, Guaymas Basin, 13°N EPR, and TAG (Peter and Scott, 1988; Ames et al., 1993; Humphris et al., 1995; Fouquet et al., 1996). The Bent Hill massive sulfide deposit (BHMS) at Middle Valley initially formed by high temperature reducing hydrothermal fluids venting at the seafloor, but also was strongly affected by an episode of later sulfide deposition and remobilization at lower temperatures due to mixing with seawater circulated through turbidite layers within the host sediment (Goodfellow and Franklin, 1993; Butterfield et al., 1994). Fossil hydrothermal systems, such as those in the Troodos ophiolite in Cyprus, reveal evidence of the structural control of fluid flow in paleohydrothermal systems that influenced the location and composition of metal-sulfides (Parmentier and Spooner, 1978; Gillis and Robinson, 1990). In addition, temporal and spatial observations of the chemistry of vent fluids at active hydrothermal systems such as TAG (Tivey et al., 1995) reveal the critical role of temperature and redox conditions on metal mobility and alteration of primary metal sulfides. Similarly, there is evidence of fault control of upflow at Middle
1. INTRODUCTION
The distribution of trace elements in sulfides from ancient and modern hydrothermal deposits has been used for decades to distinguish the source of metals and constrain the process of formation of ore deposits. Using trace elements in pyrite, Bralia et al. (1979) and Loftus-Hills and Solomon (1967) were able to distinguish between volcanogenic, sedimentary, and hydrothermal sources of metals in ore deposits from France and Australia. In addition, the abundance of trace elements in sulfide minerals from both natural and experimental systems has been used to indicate the processes of sulfide deposition (LoftusHills and Solomon, 1967; You et al., 1996). In modern midocean ridge hydrothermal systems, trace metal concentrations in massive sulfides have been correlated with temperature and bulk chemistry of coexisting vent fluids (Tivey et al., 1995; Mills et al., 1996). For example, in such deposits, high temperature hydrothermal fluids are responsible for the leaching of soluble trace elements during dissolution/reprecipitation processes, resulting in depleted regions at the base of deposits and corresponding enrichment at the margins (Bjerkgard et al., 2000; Petersen et al., 2000). However, the current database of trace element distribution in massive sulfides under hydrothermal conditions is limited to a few ancient deposits that are not directly comparable to the sediment-hosted Middle Valley system, which underscores the need for further studies of currently active areas of massive sulfide formation.
* Author to whom ([email protected]).
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Valley (Ames et al., 1993; Goodfellow and Franklin, 1993), but hydrothermal fluids pass through ⬃500 m of clastic sediments before venting at the seafloor, which strongly affects hydrothermal processes due to reaction with sediments and mixing with fluids circulating in sediments (Stein et al., 1998; Zierenberg et al., 1998). Thus, the chemistry of massive sulfides at Middle Valley provides critical information on the extent to which fluid/sediment reactions and processes such as mixing with seawater influence sulfide deposition and composition. Previous chemical and isotopic studies on the Bent Hill hydrothermal system indicate that the reaction of seawater with a mixture of basaltic and sedimentary sources controls the modern vent fluid chemistry (Goodfellow and Franklin, 1993; Butterfield et al., 1994). In addition, detailed studies of Pb isotopes and minor elements in sulfide minerals from Middle Valley indicate both basalt and sediment-sourced components (Stuart et al., 1999; Bjerkgard et al., 2000). Bulk chemical analyses of sulfide ore samples, however, do not provide sufficient data to detect differences in trace metal enrichment between multiple episodes of mineralization recorded at micrometer to millimeter scale by individual sulfide minerals or textural zones. Consequently, high spatial-resolution analytical techniques are needed. Here, we use laser-ablation ICP-MS analyses of sulfide samples from ODP Leg 169 to determine the trace elements in hydrothermal sulfides formed at Middle Valley. These data, together with petrographic analysis and fluidmineral equilibria calculations, help to constrain the origin and temporal evolution of trace element distributions caused by primary and secondary sulfide mineralization at Bent Hill and ODP Mound deposits. 1. GEOLOGIC SETTING
Middle Valley on the northern Juan de Fuca Ridge is overlain by thick deposits of continentally derived turbiditic sediments (Fouquet et al., 1998). Active hydrothermal venting of fluid at 265°C was observed at Middle Valley during ODP Leg 139 at Site 858, an area known as Dead Dog vent field (Butterfield et al., 1994). Venting of fluid at 264°C was also observed at ODP Mound (Fouquet et al., 1998). No active venting has been discovered at the largest sulfide mound, Bent Hill, although evidence of previous venting is abundant (Fouquet et al., 1998). The Bent Hill Massive Sulfide (BHMS) deposit is a very large, mature deposit with a 94 m thick massive sulfide zone, interpreted to have formed at the seafloor, overlying a substantial (⬃100 m thick) sulfide feeder zone (Fouquet et al., 1998; Zierenberg et al., 1998). Leg 169 made two transects in the region of the Bent Hill massive sulfide mound, one north-south (Hole 1035F) and one east-west (Holes 1035D, G) (Fig. 1a) and deepened Hole 856H from 93 to 520 m below seafloor (mbsf). One long hole was drilled into ODP Mound (Hole 1035H), 400 m to the south. Bent Hill is stratigraphically less complex than ODP Mound, having a single set of massive sulfide and associated feeder zones, as opposed to the three sets within the ODP Mound (Fig. 1b) (Fouquet et al., 1998; Zierenberg et al., 1998). Primary hexagonal pyrrhotite forms the bulk of the Bent Hill massive sulfide zone with minor concentrations of sphalerite and chalcopyrite (Goodfellow and Franklin, 1993; Fouquet et
al., 1998). Mineral paragenesis shows pyrrhotite replaced in the upper portions of the deposit by either secondary “vuggy” pyrite or a secondary pyrite-magnetite assemblage (Goodfellow and Blaise, 1988; Fouquet et al., 1998). Zinc is in greater abundance in the upper portion of the massive sulfide zone as shown by sphalerite inclusions within vuggy pyrite, whereas copper is concentrated in the lower sulfide feeder zone, consistent with zonations observed in other seafloor deposits (Duckworth et al., 1994; Fouquet et al., 1998). Similarly, the TAG hydrothermal field on the Mid-Atlantic Ridge and a possible counterpart in the geological record, massive sulfide in the Troodos ophiolite in Cyprus, exhibit zonation of high Cu/Zn ratios in the center of the deposit to low Cu/Zn ratios at the edges of the deposit (James and Elderfield, 1996; Petersen et al., 2000). The BHMS and ODP deposits are unique among seafloor deposits in having well developed feeder or stringer zones underlying massive zones. These zones are Cu-rich and Znpoor and consist of irregular crosscutting sulfide veins from submillimeter to ⬃3cm thickness. The upper portion of the BHMS feeder zone (down to 145 mbsf) consists of intensely mineralized veins of chalcopyrite solid-solution and pyrrhotite. The lower section of the feeder zone contains fewer and thinner veins, and more disseminated sulfides within the host sediment (Zierenberg et al., 1998). At the base of the BHMS feeder zone is a ⬃20-m-thick stratiform zone high in Cu that consists of chalcopyrite-cubanite solid-solution, pyrrhotite and pervasively chloritized and sericitized turbidites. A similar, but thicker, possibly correlative deposit occurs at the base of the ODP mound mineralization (Fig. 1). Hydrothermally altered basaltic sills and lava flows were encountered beneath the BHMS at a depth of ⬃500 m (Teagle and Alt, 2003, personal communication). 3. MATERIALS AND METHODS Mineral content and paragenesis were determined using reflected light microscopy on 22 thin sections taken from five of the holes drilled during Leg 169. Several sampling strategies were employed to determine trace elements within and between the deposits. A suite of 4 thin sections was chosen at approximately the same depth near the base of the massive sulfide zone from holes to the east, west, south, and in the center of the BHMS deposit. In addition, three samples were chosen at increasing depths in Hole 856H in the center of the BHMS deposit, allowing analysis of the upper portion of the feeder zone, and at a depth very near the deep copper zone. For comparison, four samples were taken at several depths in Hole 1035H, the only core drilled into the ODP Mound. These include representative samples from the upper and lower massive sulfide zone, the upper feeder zone, and the deep copper zone immediately underlying the lower massive sulfide zone. Analyses of Mn, Co, Ni, As, Se, Ag, Cd, Sb, Pb, and Bi in selected sulfide grains were performed with a CETAC LSX 200 laser ablation microprobe coupled to a Perkin Elmer ELAN 6000 ICP-MS. Measurements were made with a Q-switched Nd-YAG UV laser source, frequency quadrupled (wavelength of 266 nm), pulsed voltage of 1450 V, with He as the gas carrier. Single point laser shots were taken over a 7 s period, creating a crater 25 m in diameter, and approximately 10 m in depth. Most trace element concentrations (Mn, Co, Ni, As, Ag, Cd, Sb, Pb, Bi), as well as iron, were determined using an internal USGS glass standard prepared from a powder (Meyers et al., 1976; Ridley et al., 2000). Concentrations of the major elements Cu, Zn, and S, and the trace metal Se were determined using CANMET ccu-1b standard, a pressed powder pellet produced by a copper flotation concentrate from ore from the Ruttan Mine, Manitoba (see www.nrcan.gc.ca/mms/canmet-mtb/ccrmp/ccu-1c.htm). The major element concentrations for the ccu-1b standard are accurate within ⫾2% and the trace element con-
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Fig. 1. (a) Map of the Middle Valley area on the Juan de Fuca Ridge with enlarged schematic diagram of the Bent Hill site showing some of the cores drilled during ODP Leg 169. Both east-west and north-south transects were made across the BHMS deposit. (b) Cross section of the BHMS and ODP Mound deposits. The BHMS deposit consists of a single massive sulfide zone and underlying feeder zone. The ODP Mound deposit, in contrast, contains three massive sulfide and feeder zone sets. Both deposits have a thin deep copper zone at the base, depicted here as possibly a continuous stratigraphic unit.
centrations of both standards are accurate to at least ⫾5%. Precision of analyses is determined using counting statistics on multiple laser shots and is ⬍1% for Cu, Zn, and Fe, and 2–5% for trace metals. Analyses of single sulfide minerals were obtained. When textural data suggested replacement reactions, both primary and secondary sulfides were analyzed. Examination of the grains by reflected light microscopy, however, often indicated the common occurrence of small inclusions within the grains and/or veins that could not always be discriminated during laser analysis. In addition to petrographic analysis, the major elements of the sulfides (Fe in pyrrhotite/pyrite, Cu in chalcopyrite, and Zn in sphalerite) determined by LA-ICP-MS were also used to distinguish the bulk phases. Pyrrhotite samples have an average Fe concentration of 62.3 ⫾7.8 wt%; pyrite samples have average Fe concentrations of 64.1 ⫾ 7.9 wt%; sphalerite samples have an average Zn concentration of 63.5 ⫾ 14.5 wt% and average Fe concentrations of 9.9 ⫾ 1.7 wt% due to variable Fe content; and chalcopyrite samples have average Cu concentrations of 27.8 ⫾ 7.3 wt% and average Fe concentrations of 29.8 ⫾ 7.9 wt% due to solid
solution with isocubanite. Analyses with major element concentrations outside these ranges were eliminated from consideration, which represent less than 6% of the total number of analyses. Geochemical reaction calculations were carried out using Geochemist’s WorkBench (GWB) software (Bethke, 1996) and thermodynamic data generated using SUPCRT92 with recent database updates (Shock et al., 1997). SUPCRT was run at 500 bars pressure and temperatures up to 400°C and the database was adapted to GWB format (W.E. Seyfried, unpublished data). Reaction calculations were carried out from 250 to 400°C to simulate incremental reaction of 1 kg of seawater with sulfide and oxide minerals. In these models, initial speciation calculations are computed for the fluid at the specified temperature. Then an increment of seawater is added to the system, and fluid speciation and mineral solubility calculations are adjusted and minerals are precipitated or dissolved until an equilibrium assemblage is attained. These calculations are repeated with successive increments of fluid mixing. No kinetic constraints are considered in these models and equilibrium is recomputed at each step.
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4.1. Bent Hill Massive Sulfide Deposit The center of the BHMS deposit exhibits a much greater variety of sulfide minerals and textures than the flanks, while the trace element concentrations increase toward the edges of the deposit, both laterally and with depth. For example, samples at 100 mbsf in Hole 856H in the center of the deposit exhibit oxidized pyrrhotite groundmass coexisting with massive pyrite intergrown with magnetite, and irregular inclusions of chalcopyrite (Figs. 2a,b). Only the massive areas of pyrite were sufficiently large to sample with confidence with the laser. These grains are relatively free of inclusions of chalcopyrite or magnetite as indicated by chemical and textural data. Concentrations of trace elements in these grains are very low (Fig. 3). In comparison, a sample at 130 mbsf consists entirely of intergrown pyrrhotite/chalcopyrite (Fig. 2c), similar in texture to the groundmass at 100 mbsf. Most trace element concentrations in this sample are also low, with the exception of Co and Zn. Zn concentrations greater than 4000 ppm indicate the presence of sphalerite inclusions and are shown to correlate with high Co concentrations (Fig. 3). Concentrations of most trace elements remain uniform and low throughout the center of the BHMS deposit, despite large differences in sulfide mineral abundances (Fig. 3). The base of the massive sulfide zone in the distal portions of the BHMS deposit is dominated by pyrrhotite with colloform or lath-like textures, suggesting rapid precipitation (Roedder, 1968; Huston et al., 1995) (Fig. 4a). At a depth of 75 mbsf in Hole 1035D, at the east flank of the BHMS deposit, massive sphalerite coexists with massive pyrite (Fig. 4b). The massive sphalerite contains small inclusions of chalcopyrite. Moreover, sphalerite has a significantly larger range of concentrations of Pb, Sb, Cd, As than both fine-grained pyrrhotite and massive pyrite in the same sample (Fig. 5a). Such enrichments may be due to the coprecipitation of sphalerite and inferred trace inclusions of galena, which readily accepts metals available in solution, consistent with analogous phase relations in numerous other deposits (Huston et al., 1995). However, pyrrhotite groundmass contains higher concentrations and greater variability of virtually all trace elements (Pb, Sb, Cd, Ag, Mn) than massive pyrite (Fig. 5a). A depletion of trace element concentration in pyrite with pyrrhotite recrystallization to pyrite is consistent with observations in fossil VMS deposits (Gillis and Robinson, 1990; Huston et al., 1995). A sample from the south flank at 60 mbsf in Hole 1035F exhibits textures very similar to those of the east flank. Massive sphalerite containing small inclusions of pyrrhotite or pyrite coexists with massive pyrite (Fig. 4c), similar to previous observations in the BHMS deposit by Goodfellow and Franklin (1993). High concentrations of Pb, Sb, Cd, and Ag occur in the sphalerite grains relative to pyrite (Fig. 5b). These trace elements commonly reach high concentrations in sphalerite from lower temperature white smoker deposits on the seafloor (Hannington and Scott, 1988; Tivey et al., 1995). Samples from Hole 1035G on the west flank of the BHMS deposit at 60 mbsf contain dominantly highly porous vuggy pyrite with minor amounts of interstitial pyrite. Petrographic analysis indicates complex combinations of colloform and lath-
Fig. 2. Photomicrographs of mineral textures within samples analyzed from the center of the BHMS deposit. (a) A sample from 100 mbsf in Hole 856H in the center of the BHMS deposit shows a combination of pyrrhotite groundmass (upper right) and massive pyrite (lower left). The ablation crater (25 m diameter) is within the massive pyrite. (b) Another example of texture from the same thin section as in (a) with coexisting massive pyrite containing magnetite and chalcopyrite inclusions and irregular groundmass. (c) The sample from 130 mbsf in Hole 856H consists entirely of intergrown pyrrhotite and chalcopyrite within a sediment-hosted vein. Three ablation craters (25 m diameter) can be seen in the field of view.
like pseudomorphic textures becoming more massive near the edges of open vugs (Fig. 4d). Unlike other portions in the BHMS deposit, pyrite displaying both lath and massive textures in these samples contains relatively high concentrations of Pb, Mn, and Cu, and moderate Zn and As, likely from small inclusions of chalcopyrite and/or galena (Fig. 5c). Comparison of trace element concentrations between pyrite pseudomorphs
Trace element remobilization in Middle Valley sulfides
Fig. 3. Comparison of the ranges of trace element concentrations in veins from 100 and 130 mbsf in the BHMS deposit (Hole 856H). The number of data points (n) used is indicated in the legend. Trace element concentrations are generally depleted in both locations with the exception of Co and Zn, which are highly enriched at 130 mbsf.
and massive pyrite grains surrounding the open vugs reveals little difference (Fig. 5c). A similar observation was made by Stuart et al. (1999), who reported identical Pb isotope ratios in primary pyrrhotite and secondary pyrite from the Bent Hill massive sulfide zone. These observations are consistent with recrystallization reactions that may have occurred in largely geochemically isolated environments, which were likely created by the extensional fault bounding the western margin of the BHMS deposit (Fouquet et al., 1998). This fault may be the primary conduit for seawater entering the deposit, and thus, may insulate this portion of the deposit from late-stage hydrothermal fluid alteration. 4.2. ODP Mound Deposit The ODP Mound deposit samples exhibit largely the same mineral abundance and textures as the BHMS deposit. Hole 1035H was drilled near the center of ODP Mound. Pyrrhotite is the dominant phase in this massive sulfide ore (Fig. 6a), in some cases exhibiting recrystallization textures deeper in the deposit (Fig. 6b), and is usually found coexisting with smaller patches of sphalerite. Commonly, massive sphalerite grains contain small inclusions of chalcopyrite (Fig. 6c), a texture commonly referred to as chalcopyrite disease (Barton and Bethke, 1987). Samples from 40 mbsf in this location contain massive chalcopyrite with inclusions of sphalerite, sphalerite with minor inclusions of pyrite or pyrrhotite, and massive pyrrhotite. Sphalerite is enriched in Mn and Cd, likely substituting for Fe (Smith and Huston, 1992), with Se replacing sulfur (Yamamoto, 1976). In contrast, chalcopyrite (Fig. 7a) is enriched only in Ag, which substitutes for Cu (Harris et al., 1984; Cabri et al., 1985), and As, which is typically incorporated as AsS3⫺ in the trivalent Fe site (Cook and Chryssoulis, 1990). Pyrrhotite veins are enriched in Co and Ni that substitute for Fe in the lattice, and Pb and Bi possibly present as inclusions of galena (Klemm, 1965; Huston et al., 1995) (Fig. 7a). Iron sulfides were also analyzed at 12 mbsf and 135 mbsf in ODP Mound. At 12 mbsf, pyrite exists as pseudomorphic laths after pyrrhotite, infilling between sulfide and oxide grains (Fig. 6d). Grains sufficiently large to sample contain inclusions of sphalerite and are enriched in As, Cd, Ag, and Sb as substitutions, with high concentrations of Pb inferred to be inclusions
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of galena, and depleted in Se, Ni, Bi, and Co. In comparison, at 135 mbsf in the lower massive sulfide zone the dominant Fe sulfide phase is pyrrhotite, existing as irregular aggregates intergrown with sphalerite (Fig. 6b). Massive pyrrhotite is depleted in Bi, Pb, and As and enriched only in Se relative to pyrrhotite groundmass (Fig. 7b). At depth in the core, the trace element distribution in Fe sulfides shifts from inclusion-rich secondary pyrite with high As, Ag, Cd, Sb, and Pb in the upper massive sulfide zone (12 mbsf), to high Co, Ni and Se incorporated in the lattice of pyrrhotite veins in the feeder zone (40 mbsf), to high Pb and Bi possibly from inclusions of galena in pyrrhotite groundmass at 135 mbsf in the lower massive sulfide zone (135 mbsf) (Fig. 7b). 5. DISCUSSION
Hydrothermal mineral deposits record the history of chemical and thermal effects governing primary metal precipitation reactions and secondary sulfide alteration processes responsible for metal mobilization and fractionation (Bjerkgard and Bjorlykke, 1996; Fouquet et al., 1998; Petersen et al., 2000). Previous analysis of mineral textures at the Bent Hill site suggest late-stage alteration by multiple generations of hydrothermal activity (Ames et al., 1993), and isotopic studies provide clear evidence in support of this hypothesis. For example, sulfur isotopes of the sulfides indicate the replacement of pyrrhotite to pyrite by more oxidizing fluids containing a significant seawater component (Goodfellow and Franklin, 1993; Zierenberg, 1994; Shanks, 2001). In addition, Sr and Pb isotopes in hydrothermal minerals indicate a significant sediment component in these secondary fluids (Goodfellow and Franklin, 1993). Trace element data from the present study, constrained by petrographic observations, provide information on episodic hydrothermal mineralization at Middle Valley. As the recrystallization of a hydrothermal sulfide mineral deposit generally leads to a coarsening of grain size, textural differences between sulfide grains can be used to distinguish primary from later-stage alteration phases. Comparison of remnant primary lath-like and colloform pyrrhotite groundmass with massive secondary pyrite veins suggests changes in hydrothermal fluid chemistry or conditions over time. For example, secondary sulfides precipitated at the margins of white smoker deposits such as in the TAG vent system are enriched in Zn and associated Cd, Pb, Ag, Sb, and As (Hannington and Scott, 1988; Tivey et al., 1995). Solubility of these metallic elements is controlled by highly mobile chloro-complexes that are very stable at the temperate and chloride contents of seawater-derived hydrothermal fluids (Hannington et al., 1995; Tivey et al., 1995; Mills et al., 1996; Metz and Trefry, 2000; Petersen et al., 2000). White smoker fluids (⬃275°C), with a significant seawater component, have comparatively low H2S (0.5 mM) and Cu (3 M) and high Zn (300 – 400 M) concentrations relative to their corresponding high temperature (360°C) end-member fluid (Edmond et al., 1995; James and Elderfield, 1996). Low temperature diffuse vent fluids have less sulfide and Zn (⬍0.04 mM and 62 M, respectively) with unchanged Cu (Edmond et al., 1995; James and Elderfield, 1996), indicating successive dissolution/reprecipitation first of chalcopyrite, then sphalerite, as the temperature decreases with increased mixing towards the edge of the deposit. Trace ele-
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Fig. 5. Ranges of trace element concentrations of each mineral population sampled from various locations in the BHMS deposit. The number of data points (n) used to construct the plot is indicated in the legend. (a) Pyrite and pyrrhotite grains from a sample at 75 mbsf in Hole 1035D (east flank) were divided by texture and indicate minor differences in trace metal distributions. Sphalerite grains have the highest concentrations of some elements as well as the largest ranges. (b) Sphalerite and pyrite grains from a sample taken 60 mbsf in Hole 1035F. Sphalerite grains are greatly enriched in most elements compared to pyrite grains. (c) Pyrite grains divided by textures sampled at 60 mbsf in Hole 1035G. No noticeable distinction can be made in trace metal distributions between massive pyrite lining vugs and interstitial pyrite.
ment distributions in Middle Valley sulfides reported in this study suggest the hydrothermal conditions that produce a fluid with a high Zn/Cu ratio may also enhance the transport capacity
Fig. 4. Photomicrographs of mineral textures within samples analyzed from the flanks of the BHMS deposit. (a) Partially oxidized colloform pyrrhotite from Hole 1035D on the east flank. (b) Sphalerite in the lower right corner coexisting with massive pyrite from Hole
1035D in the east flank. Inclusions of chalcopyrite grains were seen within the pyrite, but were avoided during sampling with the laser. (c) Massive sphalerite (dark grey) coexisting with massive pyrite in a sample from Hole 1035F on the south flank. Two ablation craters (25 m diameter), marked by arrows, can be seen in the field of view. (d) “Vuggy” and lacy textures in pyrite from a sample in Hole 1035G on the west flank. Black areas are open vugs. The pyrite texture becomes more massive and blocky along the edges of vugs.
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Fig. 7. Range of trace metal concentrations in minerals sampled in the ODP Mound. The number of data points (n) used in this figure is indicated in the legend. (a) Comparison of massive grains at 40 mbsf. Sphalerite contains the largest enrichments of some elements, except Co. (b) Comparison of trace metal concentrations with depth in pyrrhotite samples in Fe-sulfide minerals. Texture of the samples analyzed (fine-grained or massive) is also indicated.
of a wide range of trace elements owing to the effect of temperature and bulk fluid chemistry on phase equilibria. 5.1. Modeling of Hydrothermal Alteration Based on previous studies in the Bent Hill area and on geochemical evidence presented in this study, we infer the following sequence of events. During primary hydrothermal sulfide precipitation at this site, buoyant fluid derived from
Fig. 6. Photomicrographs of mineral textures in samples analyzed from Hole 1035H in the ODP Mound deposit. (a) An example of massive sulfide vein textures within host sediments at 40 mbsf. Pyrrhotite is the dominant mineral in this photo, with smaller grains of pyrite (in the upper left) and sphalerite (upper right). Three ablation craters (25 m diameter) can be seen in the pyrite grain. (b) Primary textures of intergrown pyrrhotite and sphalerite are more common at depth as shown here in a sample from 135 mbsf. The black area near the center of the photo is void space. (c) Massive sphalerite at 135 mbsf commonly contains small inclusions of chalcopyrite blebs. (d) A sample from the very top of the massive sulfide zone at 12 mbsf shows pyrite lath pseudomorphs of replaced pyrrhotite within the sphalerite groundmass. An ablation crater (25 m diameter) can be seen in the rectangular lath in the center of the photo. (e) A sample from 190 mbsf within the deep copper zone contains chalcopyrite veins within sediments. Three ablation craters (25 m diameter) can be seen in the field of view.
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high-temperature (⬃400°C) basalt-seawater reaction ascends rapidly to the seafloor via large continuous faults that cut the sediments (Zierenberg et al., 1998; Goodfellow and Franklin, 1993). After a sizable massive sulfide deposit forms by this process, seawater penetrates laterally through the host sediment and massive sulfide, causing lower temperature (⬃280°C) alteration, recrystallization, and redistribution of metallic elements (Stein et al., 1998; Goodfellow and Franklin, 1993). To address the hypothesis of secondary alteration of the sulfide assemblage by seawater, a general thermodynamic model was used to compute the mass transfer reaction path, whereby primary high-temperature pyrrhotite and chalcopyrite coexisting with an end-member hydrothermal fluid were reacted with seawater, while temperature changed in accordance with the extent of mixing with seawater. Thus, the system is modeled by initially equilibrating an assemblage of pyrrhotite, magnetite, chalcopyrite, sphalerite, and quartz, with a hightemperature (400°C) NaCl (0.5M) fluid. This oxide/sulfide assemblage, consistent with alteration of basalt and/or deepseated sediments, produces a relatively reducing hydrothermal fluid. Subsequent reaction during seawater entrainment is predicted to cool and shift the redox conditions from reducing, H2-rich, to more oxidizing, SO4-rich fluids, allowing precipitation of anhydrite at temperatures below 350°C. Such a change in redox combined with the relatively unchanging total dissolved H2S(aq) concentration results in the system shifting from the pyrrhotite-magnetite-chalcopyrite stability field into the pyrite stability field. This model simulates well the observed mineral replacement textures of pyrrhotite oxidized to vuggy pyrite at Middle Valley (Fig. 8d), which is accompanied by a decrease in pH from ⬃5 to a minimum of 4.3 at a temperature of 300°C (Fig. 8c). Copper concentrations are predicted to decrease at a constant rate over the entire temperature range, owing to constraints imposed by chalcopyrite solubility (Figs. 8b,e). Dissolved iron is predicted to increase initially with both magnetite and pyrrhotite dissolution until below 300°C, where secondary pyrite and magnetite precipitation begins to remove Fe from solution (Figs. 8a,d). Zinc increases in solution below 350°C as sphalerite dissolves until below 275°C when sphalerite precipitation causes a net loss of dissolved Zn (Figs. 8b,e). This process is consistent with observations of hydrothermal fluids in the North Fiji Basin and TAG, which have less Cu, Mo, and Co and more Zn, As, Ag, Cd, Pb in white smoker fluids ⬍320°C (James and Elderfield, 1996; Metz and Trefry, 2000). Our simulation is largely consistent with petrographic and geochemical observations, suggesting very low trace metal concentrations in secondary pyrite that precipitates readily over a wide range of temperatures, and very high concentrations in Zn-bearing minerals that precipitate rapidly at lower temperatures and are potentially less discriminatory against high trace metal concentrations. By using the observed mineralogy and mineral replacement textures to interpret trace element distributions, the history of trace metal mobility in hydrothermal fluids at Middle Valley may be placed in the context of the overall geochemical model. Primary pyrrhotite in the Bent Hill deposit contains high concentrations of trace elements (Pb, Sb, Cd, Ag, Mn), likely in the form of galena inclusions, while secondary vuggy pyrite is depleted in trace elements, with the exception of the western
flank. This suggests that pyrrhotite replacement by pyrite, which can occur over a broad range of temperatures (Fig. 8), may be an effective method of excluding trace elements from the sulfide. The western flank sample is unique in that secondary pyrite retains high concentrations of trace elements (Pb, Mn, Cu, Zn, and As) owing to inferred chalcopyrite and galena inclusions, indicating a unique local environment isolated by faulting from secondary hydrothermal fluids, as previously discussed. Pyrite in the ODP Mound, which is less common than in the BHMS deposit, only shows high trace element concentrations when it contains significant inclusions of sphalerite that preferentially retains these elements. At the BHMS deposit, the removal of trace elements from the center of the sulfide mounds and subsequent reprecipitation in the distal portions reported in this study can be explained in terms of the seawater-mixing model. The model predicts remobilization of Zn at intermediate temperatures (275–325°C) due to the decrease in pH of the fluid (Figs. 8b,c), and reprecipitation of sphalerite at lower temperatures as continued seawater entrainment gradually raises the pH (Figs. 8e,c). In the BHMS deposit, sphalerite along the margins of massive sulfide lenses consistently displays very high concentrations of trace metals (Mn, Pb, Sb, Cd, Ag, As), consistent with the seawater mixing model predictions and observations at the TAG deposit (James and Elderfield, 1996). In addition, trace metal concentrations in sphalerite in the ODP Mound deposit, while elevated, are lower than in the BHMS deposit, suggesting these two deposits likely experienced episodes of remineralization characterized by very different conditions. 6. CONCLUSIONS
Observations of mineral textures and trace metal distributions of sulfides in both the BHMS and ODP Mound deposits at Middle Valley support previous interpretations that both deposits record a history of remineralization with decreasing temperature. Differences in replacement textures and trace metal distributions between the deposits may be explained by differences in the hydrothermal conditions (i.e., temperature or pH) during remineralization, as illustrated by a general model of the mixing of hydrothermal fluids with seawater. Within the BHMS deposit, the western flank at the base of the massive sulfide zone appears to have been isolated by faulting during the most recent alteration event, possibly acquiring an additional metal component from a local source. Sphalerite is more common at the outer edges of the massive sulfide zones in both deposits and is the most effective sink for trace metals, in particular Pb, Sb, and Cd, of all the sulfides, consistent with remineralization from the center to the perimeter during seawater entrainment. Likewise, secondary pyrite samples throughout the BHMS deposit are depleted in most trace metals relative to other phases. Differences in trace metal compositions between the feeder zones at BHMS and ODP Mound are evidence that both deposits were formed under different hydrothermal conditions, perhaps due to somewhat different primary fluid compositions or different seawater entrainment conditions. The selective enrichment of metals reported in this study suggests that secondary mineralization of sulfide mounds by mixing of hydrothermal fluids with seawater results in significant redistribution of metals in sulfide minerals.
Trace element remobilization in Middle Valley sulfides
Fig. 8. Predicted hydrothermal alteration pathway from a thermodynamic mixing model. Initial conditions for the model were defined by equilibration of a primary sulfide deposit consisting of a pyrrhotite-magnetite-chalcopyrite-quartz assemblage with a 400°C hydrothermal fluid. Temperature was controlled by the incremental mixing of 1 kg of the resultant high temperature fluid with 1 kg of cold (25°C) seawater (0.5 mol/L NaCl solution of pH 5.3, containing 30 mmolal SO4⫽, 55 mmolal Mg2⫹ and 10 mmolal Ca2⫹). (a) Predicted change in total dissolved iron reported as delta mmoles, illustrating initial enrichment of iron and subsequent depletion of iron as temperature decreases. (b) Change in dissolved copper and zinc with mixing. Metal speciation in this model is dominated by complexing with chloride, as expected at the low pH of these fluids. (c) Change in pH with mixing, reflecting lowering pH as pyrite precipitation proceeds, changing to increasing pH as the seawater component of the fluid begins buffering the solution. (d) Change in mineralogy during mixing, illustrating pyrite and magnetite precipitation at the expense of pyrrhotite, consistent with the dissolved iron trend in (a). (e) Change in chalcopyrite and sphalerite dissolution/reprecipitation with temperature.
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J. L. Houghton, W. C. Shanks, and W. E. Seyfried
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