Mineral Preparation and Separation Methods at LSP-CPGeo

The Laboratory of mineral separation and sample preparation is of fundamental importance in subsequent isotopic analyses, since ensures that a material (whole rock and minerals) has a high degree of purity and is free of contamination. The CPGeo (Research Center on Geochronology and Isotopic Geochemistry)-IGc-USP respects norms and quality standards developed for use in the following analytical methods: Ar/Ar, Pb/Pb, U/Pb, Sm/Nd and Rb/Sr.

For whole-rock powder systems of Sm/Nd, Rb/Sr, and Pb/Pb analyses, the rock of adequate size (10 times larger than the largest mineral) must be cleaned, excluding any types of alteration, and fragmented into sizes of approximately 5 cm. The best fragments are selected and reduced in a steel pestle until they reach dimensions of about 0.5 cm.

The Tungsten Carbide Ring Mill is used for pulverizing and homogenizing, where the material previously quartered and reduced in the pestle is pulverized according to the volume and hardness of the rock.

For the Ar/Ar method, minerals such as biotite, muscovite, and amphiboles with fractions between 25-40 Tyler or 40-60 Tyler are ideal for hand-picking and analysis. The reduction of these minerals for sieving is carried out by a pestle and their purification is carried out by the electromagnetic separator (Frantz®) with variations in magnetic intensity according to the characteristics of each mineral.

In the Pb/Pb method applied to sulfides (pyrite, chalcopyrite, galena, magnetite, sphalerite) and gold, the mineral concentration process involves the use of a pestle for particle size reduction and a set of sieves that can vary from 40 to 100 Tyler and subsequent concentration on the Elutriator or Wilfley Table. As a last possibility, dense liquids (Bromoform and Methylene Iodide) are used.

The initial process for concentrating heavy minerals such as zircon, monazite, and baddeleyite for the U-Pb method involves crushing, milling, and sieving. In this process, mesh sizes of 100-250 Tyler are usually used, as well as the powder fraction < 250 Tyler.

The material is processed on a Wilfley Table that works with adjusted water flow and inclination. The material distributed on the table will be collected in jars or containers called mug 1 (denser minerals) and mug 2 (lower-density minerals).

For baddeleyite concentration, the sample is suspended in water together with a detergent agent to break the surface tension of the water and prevent the baddeleyite from floating and/or being carried by the water flow.

Both mugs with collected minerals are bathed in alcohol and placed to dry in infrared lamps. The reference mug for purification will be mug 1 which, with the use of a hand magnet, eliminates metal residues from the mill and minerals such as magnetite.