Laterization of Derim Derim Dolerite

The Derim Derim Dolerite is a continental tholeiitic dolerite that varies from aphanitic basalt to holocrystalline microgabbro in texture and composition. It comprises of plagioclase feldspar (variably altered to sericite), clinopyroxene (pigeonite-variably altered to epidote and chlorite), and iron-titanium oxides (magnetite, titanomagnetite and ilmenite, approx. 3-5%).

Historical sampling and testing indicates that the dolerites have undergone intensive in-situ laterization and contains heavy minerals of magnetite, titanomagnetite and ilmenite. Pisolites were also formed during intensive insitu lateritisation. Angular sandstone fragments and thin horizons of angular sandstone clasts suggest that the laterites were reworked locally into poorly bedded, pisolitic, strongly ferruginous, clay-rich eluvial and colluvial regolith deposits of enriched heavy mineral content where the heavy mineral grains are mostly euhedral and non-braided as a result of limited transport. They overlie weathered dolerite and form the bulk of the immediate resource potential. “Black soils” containing smectite clays (responsible for desiccation cracks during the dry season) in contact with the pisolitic regolith and often overlying weathered dolerite also contain heavy minerals that have been trapped by the cyclic shrinking and swelling habit of the smectite. These heavy mineral grains are more strongly braided. Alluvium in drainage channels comprising unconsolidated sands and grits also contains heavy minerals confined to the coarser layers.

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Ilmenite-bearing regolith including Pisolitic Soil and Black Soil form the ore of Roper River Ilmenite Mine. Large-scale drilling program and surface sampling have confirmed that ilmenite-bearing regolith shows a correlation with underlying dolerite distribution, which presents a guide for exploration.