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Beachlands, New Zealand

Stanaway K.J.,31 Pohutukawa Road
Transactions of the Institutions of Mining and Metallurgy, Section B: Applied Earth Science | Year: 2012

Placers deposits are now known from five sedimentary environments; washout, river, aeolian, beach, and continental shelf. In each environment, the concentration of mineral grains, or sorting, takes place either by removal of gangue grains (denudation) or by addition of valuable grains (accumulation). Any given deposit will result from both processes but one will usually predominate. Denudation placers all sit on or just above erosive scour surfaces. They arise from a two-step process; initial particle deposition followed by selective removal of gangue particles. For example, deposits from a waning flood-stage river will include many different size, shape and density particles but a subsequent lower energy normal river flow might remove only the smaller, flatter or the less dense particles. The second fluid flow can be quite different from the first as, for example, when the wind selectively removes sand grains deposited by waves. Repeating these two steps, transportation from source and selective entrainment of grains results in a high placer mineral flux allowing denudation placers to achieve high concentrations of particular minerals. Denudation placers have a small thicknesses or vertical dimension, and so they are essentially condensed sections. To be economic, they must have a high value mineral, a large surface area, a long linear dimension, or exceptional grade, and preferably several of these features. Accumulation placer formation does not involve later partial rework and selective grain removal. Concentration grade depends on maximum availability of a valuable mineral and minimal availability of gangue grains capable of being carried with, and deposited from, a given fluid flow condition. Such placer deposits tend to be lower grade compared to denudation placers because the placer mineral flux does not focus on a single two-dimensional surface. Instead repeated favourable flow energy episodes superimpose placer grain enriched-sediment in situations of accumulation with minimal scour. Their large volume makes them economically valuable. © 2012 Institute of Materials, Minerals and Mining and The AusIMM. Source

Morisset C.-E.,University of British Columbia | Scoates J.S.,University of British Columbia | Weis D.,University of British Columbia | Sauve M.,Iron Ore | Stanaway K.J.,31 Pohutukawa Road
Canadian Mineralogist | Year: 2010

Deposits of concentrated Fe-Ti oxide minerals are a characteristic component of many Proterozoic anorthosite massifs. Ferrian ilmenite (ilmenite with exsolution lamellae of hematite) is the principal ore mineral of titanium in the largest deposits, Lac Tio - Lac Allard, Quebec, and Tellnes, Norway. Rutile, a rare oxide mineral in these deposits, occurs as discrete grains (<5.5 mm in length) with ferrian ilmenite, and as lenses (<200 μ.m across) included in the ferrian ilmenite, in both the Saint-Urbain and Big Island deposits, Quebec, which intrude the 1.05 Ga Saint-Urbain anorthosite and 1.06 Ga Lac Allard anorthosite suite, respectively, of the Grenville Province. At Saint-Urbain and Big Island, both rutile and sapphirine (Mg-Fe-Al silicate; XMg in the range 78-84) occur with ferrian ilmenite (Xbem between 11 and 30), plagioclase of intermediate composition (An 39-51), high-Al orthopyroxene (5.2-9.1 wt% Al 2O3; XMg between 70 and 75), and hercynite (XMg in the range 61-70), with trace amounts of apatite, corundum and sulfide minerals. Textural and mass-balance constraints suggest that sapphirine formed as a result of subsolidus reactions (e.g., hercynite + orthopyroxene + rutile ± corundum → sapphirine + ilmenite) that took place during slow regional cooling (3-4°C/million years) after crystallization. On the basis of mineral textures and a combination of bulk-rock and mineral geochemical variations, both ilmenite and the larger discrete grains of rutile in these deposits are interpreted as magmatic phases that segregated and accumulated by gravitational settling from Fe-Ti-enriched residual magmas (ferrobasaltic, jotunitic) following crystallization of the host anorthosites. The presence of the minor lenses of rutile 10 to 200 μm thick within ilmenite, typically surrounded by hematite, is related to late oxidation. The ore-forming magmas evolved under conditions essentially closed to oxygen, with preferential incorporation of Fe3+ into the early-crystallized ilmenite, which was stabilized by cocrystallization of hercynite rather than magnetite. As a result, the residual magmas were characterized by progressively increasing Fe2+/Fe3+ with a corresponding decrease in f(O 2), and they ultimately reached saturation in rutile. Source

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