Putu - Jideh, Montroh
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The Putu iron ore deposit is located in the NNE-SSW striking Putu ranges, ~320 km ESE of Monrovia, and ~130 km NE of the Atlantic coast port of Greenville, within Grand Gedeh County of southeastern Liberia (#Location: 5° 38' 33"N, 8° 11' 4"W).
The Putu ranges comprise two prominent NNE-trending ridges known as Ghi Mountain to the west, and Jideh and Montroh Mountain to the east.
The overall structural grain of Liberia is dominated by a series of NE-SW trending Palaeo- to Mesoarchaean granite-greenstone terranes of the Archaean Man Shield, part of the West African craton. These terranes are truncated to the SW by the regional, NW-SE striking, composite, Todi Shear Zone, which separates them from a NW-SE trending, 20 to 40 km wide strip of high-temperature Pan African metamorphic rocks that occupy, and parallel the immediate NW-SE Atlantic coastline.
The granite-greenstone terranes are divided into two deformational provinces, with a diffuse boundary centred on the major NE-SW trending Cestos Shear Zone. The "Liberian-age province" to the NW, is characterised by both metamorphism and intrusion of plutonic rocks between 3.20 and 2.70 Ga. The rocks to the SE belong to the "Eburnean age province", metamorphosed at ~2.15 Ga. Rocks of the "Pan-African province" along the coastline, were metamorphosed and intruded by granitoids at ~500 Ma, although most of these basement rocks are believed to have originally been part of the Liberian province. The Pan-African province rocks were uplifted relative to the Liberian province following the 500 Ma thermal event. During the Jurassic, a swarm of NW-SE trending dolerite dykes were intruded throughout all three provinces. By 120 Ma it is suggested that the Pan-African and Liberian provinces, on either side of the Todi shear zone, were in juxtaposition. The Todi shear zone is defined by a series of faults and wide zones of mylonite, steeply dipping to the SW below the Pan-African belt.
The granite-greenstone terranes are composed of felsic, mafic and composite gneisses, amphibolites, granites and ultramafic rocks, with zones of migmatite, and a series of NE-SW, steeply dipping shear zones which have major vertical and possibly lateral displacement. These shears are generally marked by broad zones of mylonite and differing rock types. Large diorite masses are found adjacent to the Cestos Shear Zone. Prominent, 10 to 100 km long ridges of iron formation parallel the grain of the granite-greenstone terranes and occur in all three provinces, and in mafic, felsic and composite gneiss belts. However, the iron formation is more prevalent in the Liberian-age province. The Putu deposits are one of the few known occurrences in the "Eburnean age province".
The district geology at Putu comprises an Archaean granodiorite gneiss to the NW, separated by a NE-SW trending fault contact from the main mass of leucocratic gneiss to the SE. A 3 km wide SW to WSW striking band of schist is found within the granodiorite gneiss, cut by the faulted contact to the north, but diverging from it to the SW.
Two lensoid, enclaves of iron formation, or itabirite, mainly silicate facies, with a general, but non-consistent, NNE strike, occur within the the leucocratic gneiss. The western of these is 1 km SE of the faulted contact with the granodiorite gneiss in the north, but diverges to the south, and is 22 km long and up to 4 km wide corresponding to the Ghi Mountain. The second, 3 to 5 km to the east, that is 18 km long by 1.5 km wide, follows the Jideh and Montroh Mountain ridge to the east. An oxide facies itabirite, the main economic focus of the Putu range, is exposed along a central strip of the Jideh Mountain section of the eastern iron formation enclave. Another itabirite occurs along the spine of Montroh Mountain, on the northern extremity of the eastern enclave, where the strike veers easterly strike with a shallower dip than at Jideh, but with a much shorter strike length of only ~1.5 km.
NW-SE striking faults and dykes are interpreted to crosscut and displace the iron formation, as well as the faulted contact between the granodioritic and leucocratic gneisses.
The granitic gneiss of the lowland areas, weather to light coloured lateritic soils, while biotite and amphibole bearing gneisses weather to red laterite and kanga.
In the Ghi Mountain enclave, there is only sparse outcrop, with iron cap boulders over a deeply weathered limonitic soil, with minor outcrop of a hard, blue-grey smokey quartzite with a sugary texture. Oxide facies itabirite has been mapped at Ghi Mountain, which is not reflected in aeromagnetic data. In contrast, the Jideh and Montroh Mountain enclave corresponds to a strong positive magnetic anomaly.
The Jideh Mountain enclave has been interpreted as a tight to isoclinal, steeply dipping antiform, with an overall flat plunge, such that the exposed iron formation corresponds to the fold nose over much of the length of the deposit, plunging shallowly to the north in the northern section of the deposit, and to the south in the southern portion.
A lateritic weathered cap occurs at the surface, comprising a ~30 m thick soft ore that overlays a transition zone to itabirite, which increases in competency and decreases in surficial weathering with depth. The bulk of the iron formation below the transition zone is a magnetite itabirite on both limbs of the antiform. Parts of the lower western limb are interpreted to be a haematite itabirite. A quartz rich itabirite is also present. Thin schist horizons have been interpreted within the main antiformal structure. These rocks exhibit amphibolite facies metamorphism.
The Jideh Mountain section of the deposit is ~10 km long, divided into three segments by minor fault offsets. The central section has a strike length of >3500 m, trending in a direction of ~30°. Iron mineralisation in the exposed nose of the antiform has a width of ~350 m, with the western and eastern limbs dipping at 75° and 65°SE respectively, and individual widths below the closure of ~120 m. The broad, shallowly plunging antiformal closure is evident to the south, with the two limbs gradually separating to the north. The western limb continues to depths of >750 m below the surface, while the eastern limb, which is thicker, terminates at a depth of ~550 m below surface, and has the appearance of an isoclinal synformal closure.
The northern part of the Jideh Mountain section, which is ~3500 m long, is separated from the central section by a probable cross-fault zone. The southern end of this section strikes at ~30°, and is folded, with a synformal structure adding an additional limb to the immediate east of the antiform. These structures plunge shallowly to the north, forming a broad, faulted, isoclinal antiform-synform pair, with each limb up to 200 m wide across strike. The limbs of these folds have similar dips to those of the central section. North of the closure of these shallowly north plunging structures, only the western limb is evident, striking at ~35°.
The southern part of the Jideh Mountain section, which is apparently fault offset from the central section, is only ~1100 m long, and strikes at ~30°. It occurs as a single, ~65°SE dipping limb, that is ~30 m thick.
The east-west striking Montroh deposit, immediately to the north of the Jideh Mountain section, extends over an interval of >1500 m with a strike of 10°. Mineralisation is present over a width of up to 150 m across strike in each of the limbs. The lower limb is thicker and larger than the upper limb, with average dips of 30° and 15° to the south in the lower and upper limbs respectively.
The oxide cap, which is 20 to 70 m thick, consists of a weathered itabirite, composed of a mixed assemblage of iron bearing minerals, principally hematite, limonite and goethite. Less intensely weathered magnetite itabirite represents freshly weathered BIF units, recently exposed, highly-enriched in magnetite and depleted in SiO2, but retaining the original banded textures.
The fresh haematite itabirite has little or no magnetic signature, and includes fresh material where no apparent weathering of the itabirite has apparently taken place. It has been suggested that this zone has been developed in localised zones of higher grade metamorphism and metasomatism and contains recrystallised hematite-martite bands which are deformed and altered by structurally-controlled events. This material does not appear to represent significant tonnages.
The magnetite itabirite consists of alternating layers of magnetite and quartz, from a few mm to a few cms thick, with a consistent dip direction, except in of areas of local folding. Zones of internal waste comprise a series of schists, amphibolites and gneisses.
SRK (2011) estimated:
a SAMREC compliant inferred mineral resource of - 2.37 Gt @ 34.1% Fe, 43.8% SiO2, 1.49% Al2O3, 0.07% P;
surface oxidised, dominantly hematite zone ore - 185 Mt @ 37.8% Fe;
fresh magnetite itabirite - 2.17 Gt @ 33.7% Fe;
fresh hematite itabirite - 17 Mt @ 48.25% Fe.
OAO Severstal released an updated resource in 2012, as follows:
total mineral resource - 3.25 Gt @ 34.3% Fe;
measured + indicated resource - 1.89 Gt @ 33.9% Fe, 44.5% SiO2, 0.98% Al2O3, 0.07% P,
inferred resource - 1.36 Gt @ 34.7% Fe, 43.3% SiO2, 1.37% Al2O3, 0.07% P;
surface oxidised, dominantly hematite zone ore - 224 Mt @ 36.8% Fe.
The most recent source geological information used to prepare this summary was dated: 2011.
This description is a summary from published sources, the chief of which are listed below.
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