Unusual nickel mineralisation at Nullagine, Western Australia

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Abstract
Nickel mineralisation occurs in shears in Archaean serpentinised peridotite near Nullagine, Western Australia. The principal nickel minerals are millerite, polydymite and pecoraite, which occur chiefly as concentrations of nodular grains, mostly between 1 and 5 mm in diameter. The polydymite represents a replacement of millerite, and is in turn partly replaced by gaspéite. The pecoraite grains generally contain several percent chromium, and are frequently rimmed by millerite‐containing magnetite. A similar type of nodule consists of nullaginite (Ni2(OH)2CO3). Other major minerals in the shears include various types of serpentine and magnesite, coarse chlorite, and apatite. The assemblage is cut by late veinlets of gaspéite, pecoraite, otwayite (Ni2(OH)2CO3.H2O), and nullaginite. Trace amounts of barite, cinnabar, parkerite, shandite, breithauptite, and nickeloan greenockite are also present. The bulk of the mineralisation is attributed to the effects of hydrothermal and/or metasomatic activity along shears in the serpentine. Concretionary magnesite in some of the shears is believed to be the result of Tertiary weathering. The late veinlets of nickel minerals and the replacement of millerite and polydymite are attributed to supergene effects.