VMS deposit (Tierra del Fuego, Argentina). Biel. C., Colás, V., SubÃas, I. Grupo Recursos Minerales, Dpto. Ciencias de la Tierra, Universidad de Zaragoza. c/ ...
Isotope constraints on the genesis of the Arroyo Rojo VMS deposit (Tierra del Fuego, Argentina) Biel. C., Colás, V., Subías, I. Grupo Recursos Minerales, Dpto. Ciencias de la Tierra, Universidad de Zaragoza. c/ Pedro Cerbuna 12 (Edificio Geológicas), 50009 Zaragoza. Spain Acevedo, R.D. Centro Austral de Investigaciones Científicas (CADIC). Houssay, 200, V9410CAB. Ushuaia, Tierra del Fuego. República Argentina Bilström, K. Laboratory for Isotope Geology. Swedish Museum of Natural History. Frescativagen 40 (Box 50007). SE-104 05 Stockholm, Sweden
Abstract. The Arroyo Rojo deposit is the most important polymetallic, volcanic-hosted massive sulphide close to the town of Ushuaia in the rhyolitic belt of the Andes of the Tierra del Fuego. This deposit is hosted by a volcanic and volcanoclastic sequence, Middle Jurassic in age. This ZnPb-Cu deposit display a lenticular morphology developed as a stacked lenses-style, with disseminated mineralization at both the footwall and hanging-wall. The ores and host-rocks have one penetrative tectonic foliation and have been metamorphosed to greenschist facies. Associated hydrothermal alteration follows a stratabound model. Stable isotopes geochemistry points towards the mineralizing fluids being derived from the interaction and re-equilibration with the siliciclastic portion of the host rocks, with sulphur being derived from a mixture of biogenic reduction of seawater sulphate in some restricted to closed ambient with probably magmatic influence, and 34 heavier δ SCDT values derived by leaching of H2S from the detrital rocks. Lead and strontium isotopes indicate that the metals were leached from the sedimentary rocks. This process allowed re-equilibration and homogenization of hydrothermal fluids. Keywords. Arroyo Rojo, stable isotopes, radiogenic isotopes, VMS, brine pool
1 Introduction Arroyo Rojo VMS deposit is the main prospect of the Sierra de Sorondo target belonging to the Fuegian Andes (Tierra de Fuego, Argentina) and is hosted by the volcanic and volcanoclastic portion of the Lemaire Formation (Middle Jurassic in age). The Lemaire Formation is underlain by a pre-Jurassic basement and overlain by the hyaloclastic andesites belonging to the Yahgán Formation (Late Jurassic to Early Cretaceous in age) (Biel et al., 2010). This polymetallic VMS mineralization displays a lenticular morphology with a stacked lens like, with disseminated mineralization at the footwall and the hanging-wall. The internal structure of the lenses is marked by massive, semimassive and laminated facies along with stringer, brecciated and minor intercalations of ore disseminations. The mineral assemblage consists of pyrite and sphalerite, with minor amounts of galena and chalcopyrite, and rare pyrrhotite, arsenopyrite,
tetrahedrite and bournonite (Biel et al., 2010). Ores and volcanic host-rocks were deformed and metamorphosed to prehnite-pumpellyite facies causing cataclasis in brittle sulphides, and remobilization, deformational, recrystallization and annealing textures in ductile sulphides (Biel et al., 2007). As a consequence of metamorphism and deformation, metarhyolites, mylonites and ultramylonites were developed in the host volcanoclastic sequence. Regional seafloor alteration is partially obliterated by hydrothermal alteration, which follows a stratabound model in which Mg-chlorite and phengitic white mica typically occurs in the vicinity of the ore lenses (Biel et al., 2012). The geological and mineralogical evidences point to a sulphide deposition in a brine pool at Arroyo Rojo deposit (Biel et al., 2010). In this contribution, isotopic geochemistry, both stable and radiogenic is used to constraint source rocks and geologic processes responsible for mineralization.
2 Sampling and analytical methods Ore samples were collected from outcrops and drill hole cores attending different mineralogy and textural types. Stable isotope ratios were measured at the Servicio General de Análisis de Isótopos Estables (Universidad de Salamanca, Spain). 39 δ34S determinations on all textural types of ore sulphides were made by Nd-YAG laser ablation (Fallick et al. 1992). S from underlying host rocks were recovered as Ag2S by reaction with HCl + CrCl2, using a method modified after Canfield et al. (1986) and Hall et al. (1988). Replicate analyses of reference standards gave an average reproducibility of ±0.3. Results are reported in the familiar delta per mil notation relative to CDT (Canyon Diablo Troilite). H and O isotope analyses were carried out on chlorite hand-picked from altered wall rocks by a micro-drill. Hydrogen was extracted by induction heating (1200ºC) under vacuum of a platinum crucible containing the sample. The Pt crucible had been degassed prior to hydrogen extraction. Water produced during dehydroxylation was converted to H2 over U at 800ºC in a multiple pass system and yield was measured by manometer employing a Toepler pump for transport of H2. O isotope compositions were determined using the
fluorination laser technique (Sharp et al. 1990). Analytical precision of isotopic analyses is estimated at ±1‰ for δD and better than ±0.2 for δ18O. Values are reported relative to the SMOW (Standard Mean Ocean Water). Radiogenic isotope analyses were carried out at the Laboratory of Isotope Geology (LIG), of the Swedish Museum of Natural History. Pb isotope compositions have been determined for 19 ore sulphides, occurring in different assemblages, 12 volcanic and detrital whole rock and 10 chlorites. Isotopic compositions of Sr of whole rocks and chlorites were also carried out. Chemical separation of Pb from sulphides involved dissolution-evaporation in HNO3 at 100ºC. Dissolution of whole rock and chlorite samples requires about 100 mg of sample in HCl at 100º C and undissolved material was taken up in a HF/HNO3 mixture following the Teflon bomb method of Krogh (1973) at 205 ºC. Purified Pb and Sr, was obtained using anion columns and anodic electrodeposition. Pb isotopic ratio was determined by a Micromass ISOPROBE mass spectrometer and the reproducibility of analytical method is about 0.1% (2σ). Sr isotopic ratios were measured using a Finnigan MAT261 (TIMS) mass spectrometer. The 87Sr/86Sr isotope ratios were normalized to BCR-1 standard values.
lower than the starting sulphate. Assuming that Jurassic seawater sulphate had δ34S ca 17‰ (Claypool et al., 1980), its biogenic reduction could lead to sulphide with a δ34S value in the range of measured sulphides. Nevertheless, from inspection of figure 2, a possible magmatic influence and/or leaching of sulphide from the detrital portion of the Lemaire Formation cannot be ruled out. Similar sources of S have been described in other brine pool deposits as Tharsis (Tornos et al. 2008) or Feitais (Inverno et al. 2008) in the Iberian Pyrite Belt.
2 Isotopic geochemistry
Figure 2. Distribution of Arroyo Rojo δ34SCDT values and range of igneous (Ohmoto and Goldhaber 1997) and sedimentary rocks of Lemaire Formation.
2.1 Stable isotopes The δ34SCDT values of Arroyo Rojo ore display a wide range from -27.5 ‰ to 2.6 ‰ pointing to a high variability in sources of sulphur or mechanism of deposition. Footwall and hanging-wall disseminated ores have negative δ34SCDT values (-21.7‰ a -2.0 ‰) and massive ores show heavier values (-12.1‰ a 2.6 ‰). Arroyo Rojo δ34SCDT values display a shift to heavier values with metamorphic and ore textural evolution (Figure 1).
Temperatures of formation for pyrite-sphalerite and chalcopirite-sphalerite pairs using equations of Kajiwra and Krouse (1971), Friedman and O'Neil (1977) and Ohmoto and Rye (1979) range between 135º and 339ºC. Massive sulphides showing growing, deformation and cataclasis textures have higher equilibration temperatures (249-339ºC) than those showing deformation, recrystallization and annealing textures (135-249ºC). Sample AR6-202-C1 AR6-262-C1 AR6-262-C2 AR6-308 AR2-57-C1a AR2-57-C1b AR2-57-C4 AR2-105-C2a AR2-105-C2b AR2-75-C1 AR2-75-C2 AR6-139-C1 AR6-139-C3 AR7-133-C2 AR7-134-C3 AR7-134-C2 AR3-3-C1 AR7-94.8
Depth (m) 202 262.5 262.5 308 57 57 57 105.8 105.8 75.3 75.3 139.2 139.2 133.4 134.7 134.7 3 94.8
Ore Proximity Distal Distal Distal Distal Proximal Proximal Proximal Proximal Proximal Proximal Proximal Proximal Proximal Proximal Semimas Semimas Massive Massive
δ18OSMOW 11.3 12.3 12.1 13.9 9.1 9.6 9.8 9 9.6 13.4 11 11.3 13.1 8.3 9.8 9 11.7 7.7
δ18OSMOW fluido 11.68 12.68 12.48 13.3 8.5 9 9.2 7.56 8.16 11.96 9.56 9.86 11.66 6.86 8.15 7.35 10.05 6.05
δDSMOW -82.6 -86.8 -85.9 -85.6 -66 -68.9 -62.9 -68.7 -67 -89.7 -70.9 -54 -53.5 -53.9 -42.4 -59.9
Figure 1. Mean, maximum, minimum and standard deviation of δ34SCDT values in different ore textures.
Table 1. O and D/H isotopic composition of Arroyo Rojo Hydrothermal alteration chlorites.
The wide variability of Arroyo Rojo sulphur isotopic values is consistent with a bacteriogenic reduction of sulphate. Typically, bacterial reduction of seawater sulphate results in sulphides with δ34S around 40±20‰ (Ohmoto, 1986) to 35±15‰ (Boyce and Fallick, 1995)
The above temperatures are in agreement with temperatures reached during metamorphic peak of Lemaire Formation corresponding to prenhite-pumpellyte facies (Biel et al., 2007). Arroyo Rojo hydrothermal chlorites displays lighter
δ18OSMOW values (Table 1) within massive ore levels (712 ‰), intermediate values in proximal zones (8-14 ‰) and the heaviest values in distal zones (11-14%). δ18O values of the mineralizing fluids (6.05-13.3 ‰) were calculated employing Savin and Lee (1988) chlorite-water fractionation equation, assuming temperatures calculated by Cathalineau (1988) thermometer (169º to 381 ºC). Calculated δDSMOW values of hydrothermal chlorite vary from -90 ‰ in distal zones to -40 ‰ in mineralized areas. δD values of the mineralizing fluids calculated using the curve of Savin and Lee (1988) shows a wide variability (-62.7 to -15.4 ‰), unusual in VMS deposit (Table 1). δD and δ18O values of mineralized fluids are suited in the range of metamorphic water with some influence of marine, magmatic water or metasediments (Figure 3).
crust reservoir, pointing to a main orogenic source of Pb along with some contribution from the upper crust during orogenic formation (Figure 4). Mineralization, hydrothermal chlorite and detrital rocks of Lemaire Formation show very homogeneous Pb isotopic compositions. Mean µ (9.75-9.95) and ω (36.1836.80) values calculating by Stacey and Kramers (1975) model point to a common source of metals. The similarity in Pb isotopic composition between sedimentary rocks, mineralization and hydrothermal alteration chlorites suggests that the former are the source of metals indicating a possible homogenization of hydrothermal fluids during the leaching of the sedimentary sequence.
Figure 4. Lead isotope data of Arroyo Rojo deposit. Lower Crust, Mantle, Orogen and Upper crust of Zartman and Doe (1981) are shown. Figure 3. δ18OSMOW and δDSMOW values of Arroyo fluids on natural waters diagram of Sheppard (1986). Main VMS deposit values range in grey.
Distal chlorites show lower isotopic signatures of H suggesting a metamorphic origin for fluids responsible for the regional hydrothermal alteration. The rest of samples are in the expected range for VMS deposit and display higher δ18O values and lower δD values. These values are consistent with a fluid that interacted and re-equilibrated with the siliciclastic rocks of the underlying Lemaire Formation and the metamorphosed basement of the Lapataia Formation. This has been described in other VMS interpreted as formed in brine pools (Tornos and Heinrich 2008; Lentz 1999).
87
Sr/86Sr current values vary from 0.70621 and 0.74312, showing values between 0.70621 and 0.72559 for sedimentary rock, between 0.71106 and 0.72803 for igneous rocks, and between 0.70934 and 0.74312 for hydrothermal alteration chlorites. 87Sr/86Sr value for basement is 0.71301. 87 Sr/86Sr ratios have been computed at 145 Ma (age of formation of the host Lemaire Formation, after Calderón et al. 2007) and at present for the various source rocks (volcanic, sedimentary and basement) and for hydrothermal chlorite (Figure 5).
2.1 Radiogenic isotopes Arroyo Rojo Pb isotopic relations show a narrow scatter of 206/204Pb (< 0.9%) pointing to a similar formation age and source of Pb. Lesser variations are observed between sulphides, sedimentary rocks and hydrothermal alteration chlorites (
