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Bou Grine (Bougrine), Bou Jabeur (Bou Jaber) |
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Tunisia |
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Zn Pb
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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Bou Grine (or Bougrine) and Bou Jabeur (or Bou Jaber) are the larger members of a group of small Pb-Zn-Ba-F and siderite deposits hosted by Mesozoic limestones and shales in a 250 x 150 km belt in the Atlas Mountains of Tunisia and eastern Algeria, located ~150 km SW of Tunis, straddling the Algerian border. Bou Grine is ~170 km SW of Tunis, and Bou Jabeur a further ~75 km SW, close to the Algerian border.
These deposits fall within the east-west trending Atlassic folded foreland, which is in structural contact with the African stable foreland of the Saharan platform to the south, and overthrust by thrust sheets derived from the northern margin of Africa to the north, above the regional Tellian Thrust (Bouhlel et al., 2009).
Bou Jaber
Bou Jaber is located at the edge of the Bou Jaber Triassic salt diapir in the Tunisia Salt Diapir Province. The ores are controlled by unconformities and faults and take the form of subvertical columnar bodies formed in dissolution-collapse breccias and in cavities within Late Aptian platform carbonate rocks that are unconformably overlain by impermeable shales and marls of the Late Albian-Cenomanian Fahdene Formation (Bouhlel et al., 2016).
Proximal to and within the ore bodies, the host rock is hydrothermally altered to ankerite, accompanied by fine-grained bipyramidal quartz crystals that formed during hydrothermal alteration. The ore mineral assemblage comprises barite, fluorite, sphalerite, and galena in decreasing abundance. Within the ore zones, distinct depositional events are recognised, characterised by fluid inclusions which are commonly oil-rich, and have distinct fluid salinities and homogenisation temperatures (Th), as follows:
i). sphalerite-galena, with fluid inclusion salinities of 17 to 24 wt.% NaCl eq., and Th from 112 to 136°C;
ii). barite-ankerite, with fluid inclusion salinities of 11 to 17 wt.% NaCl eq., and Th from 100 to 130°C;
iii). fluorite, with fluid inclusion salinities of 19 to 21 wt.% NaCl eq., and Th from 140 to 165°C
(Bouhlel et al., 2016).
The mean temperature of the ore fluids decreased from sphalerite (125°C) to barite (115°C) and increased during fluorite deposition (152°C); then decreased to ~110°C during late calcite precipitation. Analyses of fluid inclusions in fluorite are metal rich (hundreds to thousands of ppm Pb, Zn, Cu andFe), although in contrast the inclusions in barite are deficient in the same elements. Inclusions in fluorite have Cl:Br and Na:Br ratios of several thousand, interpreted to be consistent with dissolution of halite, while the inclusions analysed in barite have values lower than seawater which are indicative of a Br-enriched brine derived from evaporation plus a component of halite dissolution (Bouhlel et al., 2016).
The ankerite and calcite carbonate gangue minerals have δ13C and δ18O values close to the carbonate host rock, indicating fluid equilibrium between carbonate host rocks and hydrothermal brines. The δ34S values for sphalerite and galena fall within a narrow range of 1 to 10‰ with a bulk value of 7.5‰, indicating a homogeneous sulphur source. The δ34S values for barite are also relatively homogeneous (22‰), with 6‰ higher than the δ34S of local and regional Triassic evaporites (15‰), which is interpreted to be the source of sulphate (Bouhlel et al., 2016).
Bouhlel et al. (2016) suggest the temperature of deposition, together with sulphur isotope data, indicate the reduced sulphur in sulphides was derived through thermochemical sulphate reduction of Triassic sulphate via hydrocarbons produced probably from Late Cretaceous source rocks. They also suggest 87Sr/86Sr ratios in the barite, together with the lead isotope values of Bou Jaber galena, indicate that metals were extracted from homogeneous crustal source(s). The tectonic setting of the Bou Jaber ore deposit, the carbonate nature of the host rocks, the epigenetic style of the mineralisation and the mineral associations, together with sulphur and oxygen isotope data and fluid inclusion data show that the Bou Jaber Pb-Zn mineralisation is characteristic of a salt diapir-related Mississippi Valley-type carbonate hosted deposit with superimposed fluorite and of barite depositional events (Bouhlel et al., 2016). Field relations are consistent with mineral deposition during the Eocene to Miocene Alpine orogeny from multiple hydrothermal events: i). Zn-Pb sulphides formed by mixing of two fluids, one of which was metal-rich, but was reduced sulphur-poor, and a second fluid which was reduced sulphur-rich; ii). barite precipitation involving the influx of a meteoric water component that mixed with a barium-rich fluid; andiii). fluorite precipitated from a highly saline fluid at higher temperatures (Bouhlel et al., 2016).
Original resources at Bou Jabeur - 3 Mt @ 3.25% Zn, 0.6% Pb
Bougrine
The Bougrine deposit is located on the edge of the Lorbeus salt dome in the Diapir Zone of the Tunisian Atlas Mountains, and like the other Pb-Zn deposits of the area is close to the intersection of regional NE-SW trending faults, lined by Triassic diapirs, and NW-SE trendingTertiary grabens (Rouvier et al., 1985; Bouhlel et al., 2007).
The Bougrine mineralisation is largely, but not completely associated with the Upper Cretaceous, Cenomanian-Turonian Bahloul Formation, comprising a laminated limestone characterised by high quantities of organic matter (4 to 5 w,t% TOC), and high porosity (Orgeval et al., 1994). The succeeding Senonian formations are more argillaceous and less permeable. The mineralised zones are located at the intersection of the Bahloul Formation and a densely faulted corridor (Bouhlel et al., 2009).
Four economic orebodies are known, F1 to F4 designated in order of their discovery (from Bouhlel et al., 2009).
• The F1 ore comprises various small lenticular, strata- or fault-controlled orebodies, containing variable amounts of galena, sphalerite, Fe-sulphides, occurring as low-grade disseminations in sandstone and carbonate rocks.
• The F2 ore consists of stratabound disseminations within high porosity (~10%) laminated limestone of the Bahloul Formation. Sphalerite is very fine-grained (<5 to 50 µm), filling partly or totally formaniniferal structures, intergranular space or very fine fissures. Advanced sphalerite mineralisation resulted in complete replacement of carbonate material by massive sphalerite. Abundant cross-cutting, 1 to 10 cm wide veins are filled by colloform sphalerite, with minor iron sulphides and galena. The density of these veins, results in ore grades varying from 6 to >30% Zn within a few metres. The F2 orebody mineralisation accounts for ~65% of the total Bougrine resource.
• The F3 orebody has an irregular pipe-like morphology, which is ~130 m in length, 80 m high, 50 m wide, enclosing 600 000 m3 of semi-massive ore @ 25 to 45% Zn and 5 to 15% Pb. This ore is a complex and chaotic combination of replacement, cavity-, and fracture-filling sulphides. Enclosed internal sedimentary rocks comprise fine-grained banded layers of pure sphalerite, with intercalations of debris flow breccia layers composed of host rocks in a detrital matrix of very fine-grained sphalerite and calcite. This orebody accounts for ~33% of the total Bougrine resource.
• The F4 ore comprises limited fault-controlled disseminations cutting across both the F3 orebody and the Cretaceous strata, with grades up to 28% Zn and 9% Pb. It may comprise a feeder zone to the F2 and F3 orebodies, with a general paragenetic sequence of i). fine-grained sphalerite as monomineralic impregnations within the host rocks, accounting for 20% of the resource, and ii). cross-cutting banded sphalerite (schalenblende-type, i.e., a mix of interbanded sphalerite, wurtzite, pyrite and galena), with galena, ±pyrite, and variable calcite, comprising ~80% of the total Bougrine resource. The schalenblende textured ore has local incrustations of very fine yellow to light-brown sphalerite crystals. Trace element contents of the two main sphalerite variants is very low, with 300 to 3000 ppm Fe, <50 ppm to 500 ppm Mn. Cd contents are 800 to 1200 ppm Cd in the fine-grained sphalerite type, and slightly higher in schalenblende-type (~1300 ppm). As in fine-grained sphalerite is <50 ppm, but may be as high as 2000 ppm in schalenblende-type sphalerite.
Fluid inclusions in late sphalerite and late calcite gave salinities of 16 to 20 wt.% NaCl eq. and homogenisation temperatures in the 70 to 100°C range, representative of the late-stage fluids of the hydrothermal system at Bougrine (Bouhlel et al., 2009).
Original resources at Bougrine - 7.3 Mt @ 9.7% Zn, 2.4% Pb, -or- 5.5 Mt @ 12% Zn, 2.5% Pb (Orgeval, 1994).
Break water Resources annual report, 1997, quoted reserves and resources as follows:
Proved + probable reserves - 2.848 Mt @ 12.0% Zn, 2.3% Pb (included in resources);
Measured + indicated resources - 2.765 Mt @ 14.4% Zn, 2.8% Pb;
Inferred resources - 1.258 Mt @ 8.8% Zn, 1.6% Pb;
TOTAL resources - 4.023 Mt @ 12.7% Zn, 2.4% Pb.
The Bougrine summary is drawn from "Bouhlel, S., Leach, D.L., Craig, A.J., Lehman, B., 2009 - Ore Textures and Isotope Signatures of the Peridiapiric Carbonate-Hosted Pb-Zn deposit of Bougrine, Tunisia; Proceedings of the Tenth Biennial SGA Meeting, Townsville, "Smart Science for Exploration and Mining" P. J. Williams et al. (editors),v.1, pp. 409-412."
The most recent source geological information used to prepare this decription was dated: 2016.
Record last updated: 20/7/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Bouhlel, S., Leach, D.L., Johnson, C.A., Marsh, E., Salmi-Laouar, S. and Banks, D.A., 2016 - A salt diapir-related Mississippi Valley-type deposit: the Bou Jaber Pb-Zn-Ba-F deposit, Tunisia: fluid inclusion and isotope study: in Mineralium Deposita v.51, pp. 749-780.
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Rouvier H, Perthuisot V, Mansouri A 1985 - Pb-Zn deposits and salt-bearing diapirs in southern Europe and North Africa: in Econ. Geol. v80 pp 666-687
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