4.3.1 Field observations.
Fresh rocks are not observed on the surface of Buru hill, which has an oval shape and measures about 500 m in length and 350 m in width (Figure 4-2). The surface is entirely composed of lateritized tuff with irregular veins of hematite and goethite (Figure 4-1). Some of the veins, now completely lateritized, are randomly distributed, but east-west and north-south trends appear to be the dominant directions. A number of hematite veins are observed along the north-west of the hill and a fluorite vein, purple in colour, is noted in the northern part of the hill trending in an east-west direction within the fenites. Fluorite is frequently associated with carbonatites and has been observed to form along fractures,
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Plate 4-1 The Buru mylonite shear zone with quartz porphyroclasts
Figure 4-2 The Buru hill carbonatite viewed westwards from
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particularly in the roof zones of carbonatite complexes or in fractures along which fluids passed (Mariano, 1989b). Barite was not observed in the surface samples, although it occurs abundantly in thin-sections throughout the length of the drill cores.
The gneisses on which laterite has developed acted as a rim of resistant country rocks around the hill and consequently prevented most of the weathering solutions from escaping, although there is an opening at the southern end of the laterite through which there has been loss of mobile elements such as sodium, potassium and calcium. The soils towards the southern end of the hill are extremely magnetic.
Another significant feature on the surface of the Buru carbonatite centre is the existence of an exploration adit at the southern foot of the hill which surprisingly is not mentioned anywhere by early workers, although the local community are aware of active mining and excavation of ore from the adit in the late 1950's. The adit is mentioned by the local community to be quite long, almost reaching towards the middle of the hill. The writer has no idea about the nature of the ore that was being mined, but it could have possibly been iron-ore (hematite), which probably contained high concentrations of niobium derived from pyrochlore during weathering processes.
4.3.2 Drill core observations.
Intensive supergene weathering processes witnessed in Buru hill have developed a thick lateritic cap overlying the Buru carbonatite with considerable economic enrichment in lanthanides. The Buru carbonatite, as revealed by drill cores at depth, is composed of a fine- to medium-grained calcite carbonatite, which is partly brecciated in some sections of the cores.
The carbonatites are cut by dykes of siderite carbonatite (confirmed by X- ray diffraction studies) and also by dykes of fine-grained ijolite.
Plate 4-2 gives a full pictorial summary of the different lithological units encountered in drill core BRL-1. The geological description of the rock units along drill core BRL-1 is summarized in Table 4-1. Details of the mineralogy and major and trace element geochemistry will be discussed in chapters 5 and 6.
Table 4-1. Summary of the geology along drill core BRL-1. Depth (m) Geological description
0-65 This section of the profile, which will be referred to as the 'Buru laterite', is composed mainly of iron hydroxides and oxides. The laterite exhibits different shades of colours; light grey to brown to reddish dark brown to brown to orange-brown towards the lower section of the laterite. Strongly weathered pseudomorphs of granitic gneiss
fragments are recognized.
65 - 80 Transitional zone consisting of weathered pale brown medium-grained carbonatite and some iron compounds. This section, also called saprolite, is typically grey to brown in colour.
80 - 130 Fresh medium-grained, pale grey to white
carbonatite. The carbonatite is sometimes banded and consists of disseminated magnetite and fluorite Dykes of fine-grained light brown siderite
7 4
Table 4-1 continued.
Depth (m) Geological description
130 - 142 Fine- to medium-grained brecciated carbonatite. The brecciated carbonatite contains minor
disseminated fluorite and magnetite and also dark green fragments of pyroxene.
142 - 165 Fine- to medium-grained white to grey carbonatite with disseminated magnetite and fluorite. The carbonatite is cut by later dykes of siderite carbonatite.
165 - 196 Greenish-grey fine-grained dyke of ijolite cross cutting the carbonatite. The ijolite is also cut by dykes of light brown siderite carbonatite.
196 - 200 Dark grey, medium-grained, magnetite-rich carbonatite.
Lithological observations along drill core BRL-1 (Table 4-1) reveal that three distinct zones (laterite, saprolite weathered carbonatite and fresh carbonatite) can be recognized. The other significant observation is the recognition of the brecciated nature of carbonatites and the cross-cutting relationships shown by both siderite carbonatite and ijolitic dykes.
0,00 - 9, 30m 9, 30 - 15, 00 m 15, 00 — 22, 80 m 22,80 - 29, 30m ■ i . ■ .i 29,30 - 41, 50m
Plate 4-2 Summary (pictorial) of the different lithological units
76 •'7 * ’ ip-»sns^j ktw?- »r j 41, 50 58,90 65,60 72,50 79, 20 — 58, 90m — 65, 60 m - 72,50m - 79, 20 m — 85, 90 m
85, 90 - 92, 40 m 92,40 - 102,00 m 102,00 - 109,80m 109, 80 - 116, 40 m 116, 40 - 123,00m 0 0.5 1 m I_______________________________ I--- 1
78 123,00 - 132,70 m 132, 70 - 142, 30 m 142,30 - 152,00m 152,00 - 161, 40m 161,40 - 170,90 m 0.5 L 1 m _____________
170, 90 - 180, 50m 180, 50 - 190, 10m 190,10 - 199, 70 m 199, 70 - 200, 10 m ___ ___ ___ __ 0 0.5 1 m I_______________________________ I________________________________I A — non __ ... Mk - i • -- • ■
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Plate 4-3 Laterite zone in the Buru carbonatite. Goethite (GOE)
and barite (BA) are characteristic. Magnification x 20
Plate 4-4 Drill core sample showing a brown and fine-grained siderite carbonatite dyke. Note the dominance of fluorite
observed replacing calcite in the Buru cores. The dark vein to the left comer is aegirine (AEG).
4.3.3 Thin-section studies.
Thin-section studies undertaken from samples selected from BRL-1 reveal a characteristic mineral zonation with depth. Thin-sections from the laterite are dominated by iron hydroxides/oxides (goethite and hematite), barite and fluorite (Plate 4-3). Fluorite (purple in colour; shown in Plate 4-4), appears to be reacting with and replacing calcite. The light brown fine-grained dyke of siderite carbonatite does not contain fluorite mineralization.
The carbonatites under the microscope show a typical fine- to medium grained texture in which calcite, siderite, barite and fluorite are the most common minerals (Plate 4-5). Pseudomorphs of calcite rhombs are still recognized where they have not been completely destroyed by recrystallization. Barite occurs as anhedral crystals and is commonly colourless in colour, whereas fluorite is usually pale purple and typically isotropic under the microscope. The siderite carbonatite shows chilled margins and a cross-cutting relationship against the earlier Buru carbonatite and is therefore clearly magmatic. Some of the siderite carbonatite is also seen along the existing cavities of the earlier calcite carbonatite (Plate 4-6).
Accompanying the calcite and siderite in most of the thin-sections studied is the dominant occurrence of fluorite and barite and the accessory minerals magnetite, apatite, biotite and aegirine. Fluorite is pervasive and can be observed in several thin-sections and on outcrop, reacting with and replacing calcite (Plate 4-4). The biotite and aegirine are thought to be xenocrysts incorporated from fenites and the surrounding basement gneisses.
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PLATE 4-5 Buru carbonatite. Rhombohedral pseudomorphs after calcite are recognized (CA) Fluorite ( F, dark rounded spheres) and barite (BA, white) are abundant within the section. Magnification xlO
PLATE 4-6. Siderite carbonatite (SID) dyke revealing sharp contacts with Buru carbonatite. The sharp contacts confirm its intrusive nature. Magnification xlO.