Summary:
The Moa deposits are considered to be the best-known example of the oxide type of nickel laterites.
MINERALISATION
Laterite Profile
The laterite profile overlying the bedrock consists of four principal horizons. From bottom to top these are: (1) serpentinised peridotite, (2) saprolite, (3) limonite and (4) ferricrete.
The lowest part of the profile is represented by tectonised, serpentinised peridotite in which the first stages of weathering are seen at the top. The saprolite zone (which is less represented relative to the overlying limonite at the Moa Project is characterised by the preservation of the primary fabric, a reduction in the quantity of primary minerals and the formation of alteration minerals in the most fractured zones. The boundary between the saprolite zone and the peridotite substrate (the “weathering front”) is extremely irregular.
The saprolite zone passes upwards in the profile to a limonite zone, which is dominated by goethite and hematite. Two subzones can be defined: a lower limonite with faint remnants of a primary structure (“ochre estructural” or structured limonite) and an upper limonite in which the structure is collapsed (“ochre inestructural” or massive limonite). Massive limonite is composed of massive red-brown earthy fine-grained soil with no visible structure. Structured limonite is the largest and most important zone in terms of nickel and cobalt content. Structured limonite is yellow/brown in colour and exhibits remnant structures suggestive of pyroxene, represented by colour changes from the deposition of minerals such as MnO and MgO. Nickel grades range from 1 Ni% to 1.5 Ni% in the limonite zone, with approximately 0.1 Co% to 0.15 Co%.
All zones of the profile are overlain by ferricrete which takes the form of unconsolidated pisolites in a fine-grained matrix or massive hematite comprising amalgamated or welded hematitic pisolites.
The nickel laterites developed from weathering of the ultramafic units of the Moa-Baracoa ophiolite. Olivine in photolytic harzburgites and dunites typically contains 0.2 Ni% to 0.3 Ni%. During prolonged leaching by weathering in a tropical environment, MgO and SiO2, the principal chemical components, are leached out leaving a fine-grained concentrate of goethite, (Fe, Al, Cr, Ni)O(OH), hematite Fe2O3, gibbsite Al(OH)3 and an insoluble residue of chrome spinel from the parent rock.
Magnesium is the first element to be weathered in the lateritic profile. The absence of magnesium is the genetic marker of the lateritisation in any bedrock that occurs at the Moa Project (harzburgite or gabbro). Magnesium does not reprecipitate. Silicon is the second element to be weathered in the lateritic profile. In certain redox conditions, silicon can reprecipitate in the form of amorphous silica or chalcedony.
Iron is a marker of the lateritisation within the harzburgite bedrock, and is negatively correlated with magnesium content. However, when there is a magma differentiation, the liquids become enriched in aluminium (gabbroic rocks), and sometimes in potassium (syenitic rocks). In the gabbroic rocks, the negative correlation between iron and magnesium does not exist, as aluminium takes the place of the iron.
Nickel, manganese and cobalt are leached from the limonite zone and re-precipitated in the intermediate, partially leached saprolite, equivalent to some of the “ochre estructural inicial” and “serpentina lixivada” in the local classification. Manganese is re-precipitated in the saprolite zone as complex black manganese, cobalt and nickel oxides (e.g., asbolane and lithiophorite). Nickel replaces magnesium in the leached serpentine (or more rarely chlorite), or is precipitated as a green-coloured garnierite, a variety of minerals usually dominated by nickel-rich talc, in veinlets in the same zone. Typically nickel grades are highest in the saprolite and progressively decrease above and below it.
Saprolite at the Moa Project is relatively rare in the northern deposits, but is more represented within the slopes of the Camariocas Sur and Norte. Typical saprolite consists of a zone of intercalated structural limonite and grey-green to yellow-green saprolitic clay displaying fairly well-preserved remnant mineral structures of the underlying ultramafic rocks. There is little nickel enrichment at the Moa Project.
The Moa-Baracoa peridotites contain variable amounts of gabbroic dykes and sills. Such bodies produce a markedly different soil profile, more akin to bauxite. They are red or orange in colour, and contain high Al2O3, and TiO2 and low nickel contents. The high Al2O3 content is an undesirable element in the Moa Project metallurgical process.
Rocky Limonites
A phenomenon with important implications for mining is limonite containing a significant amount of bedrock or saprolite boulders. This “rocky” or “boulder” limonite was not found at Moa Occidental, and first started to be encountered in Moa Oriental as mining progressed to the south, up slope and towards the neighbouring Camarioca Norte concession.
The abundance of rocks in rocky limonite appears to be greatest in the lower part of the limonite zone. These observations are consistent with the view that the rocky limonite originates from mass flow and mixing of bouldery saprolite and limonite due to the mid-Tertiary period of coastal and submarine erosion experienced by Moa Oriental and the Camarioca deposits to the south. There is some evidence that rocky limonite will not be as abundant in Camarioca Norte as in Moa Oriental. The large limonite deposits in Camarioca Norte have a better development of saprolite below their central parts than do the Moa Oriental deposits. This is consistent with in situ development of the laterite profile, and not with significant accumulations of transported lateritic material.
