© 2007, NATI Research JSC

       Introduction       Mineralogy       Concluding remarks       References

Authors

ABSTRACT: Dating of geological processes and stages of ore formation based on individual grains of accessory minerals by isotope geochemistry methods yields more detailed information which can not be obtained by bulk rock or ore samples analysis. Extraction of minute accessory minerals-carriers of chemical elements suitable for isotope analysis, at their concentration in rocks less than n*0.01 - n*0.1 ppm represents a special problem, that predetermines the success of subsequent isotope analysis.

Go Top      Introduction

The contents of chemical elements used for studies based on U-Pb and Re-Os isotope systems, in rocks of various mafic-ultramafic complexes are usually quite low: less than n*0.01ppm in barren ultramafic rocks and less n*0.1ppm in pyroxenites, norites and gabbros. This note relates to the low contents of minerals-carriers of these elements (e.g. zircon, baddeleyite, uraninite, minerals of Os, U, Th-monazite and apatite (REE)). Usually these minerals are rare accessories whose grain size does not exceed 70-80µm. Meanwhile, extraction of these minerals from mafic and ultramafic rocks allows to apply  isotope measurement in individual grains in situ (SIMS, SHRIMP) or  high-sensitivity measurements of isotope ratios and element contents (REE, for example) in the solutions obtained by chemical decomposition of single grains.

Go Top   Mineralogy

The  "ppm-mineralogy" technique is an approach to high sensitive mineralogical works, based on  appropriate metrological maintenance (V. Knauf, 1996) allowing to extract and study the heavy minerals and especially the minerals-carriers of gold and platinum group elements (PGE) at theirs content in rock less than 1 ppm (see details section Technics,   "ppm-mineralogy”).

This technique was applied to gold grains extraction  during the study of till samples (<50µm) from Finland (V. Knauf et al., 2000) and Kola region of Russia with mineralogical sensitivity less than 0.2 ppb of Au. Detection of the complete mineral associations for all six PGE in Kraubath ultramafic massif is an another application of the "ppm-mineralogy" technique (K. Malitch et al., 2003). Mineralogical research of the PGE mineralization in various dunite-clinopyroxenite zonal complexes revealed the primary  and secondary PGM associations (O. Knauf, 2005). Investigation of the sulfide concentrates obtained by  "ppm-mineralogy" technique led to discovery of the new rhenium mineral - tarkianite  (K. Kojonen et al., 2004).

The"ppm-mineralogy" technique  has been adapted to extraction of fine grains of rare accessory minerals for subsequent isotope researches. As an example of one sample processing there are from 30 to 50 units of intermediate products, including mineral concentrates of various granulometric fractions (fig.1, a).

Fig. 1a.
For coarse fractions of concentrates (>50µm) it is possible to apply optical diagnostics of minerals, manual extraction of grains (fig. 1, b,c) and mounting of grains on a carbon stage (fig. 1, d) for subsequent checking under scanning electron microscope with ED spectrometer (SEM+EDS). Fig. 1  b, c, d
b	c	d
	Pl – plagioclase, Px-pyroxene,Slf-sulfides, ZR-zircon,  BD-baddeleyite.
For  grains less than 50µm and for larger mineral grains whose optical diagnostics are not reliable all the material of the concentrate (1-20mg) is mounted on a carbon stage for SEM+EDS examination (fig. 2, a, b, c).
Fig. 2  a, b, c.
a	b	c
In this case, the diagnostics of minerals runs under the control of a specially developed electronic device connected to an  SEM and computer program for image processing ImSca 14 (fig. 2d).
Fig. 2d
All the mineral grains mapped on a BSE-image of the scanned area are grouped according to brightness which is proportional to average atomic number of minerals and each group is given the certain brightness range (different colors). The program operates the electron beam and positions it on  grains of each brightness range, carries out diagnostics of the mineral by chemical composition (by X-ray spectrum),  codes the mineral and adds its coordinates to the table (fig. 2, d). The BSE images and coordinates of grains allow identification of grains on the SEM specimen (using optics), facilitating isotope analysis. In addition to extraction of single grains of accessory minerals it is possible to separate rock-forming minerals from intermediate products for Sm-Nd, Rb-Sr isotopic studies, sulfide concentrates (isotopy of S), Cr-spinel concentrates (isotopy of noble gases) and heavy concentrates for study of platinum group minerals (Fig. 3,  h-l). BSE images of grains, extracted after optical or SEM+EDS diagnostics are shown on fig. 3.
Fig. 3  a-l
a	b	c
d	e	f
g	h	i
k	l	ZR-zircon, AP-apatite, BD-baddeleyite, MN-monazite, SPR-sperrylite, CO-cooperite, VYS-vysotskite, PN-pentlandite, PY-pyrite, ILM-ilmenite.
For single grains of zircon, - there are many reasons to provide more detailed optical and cathodoluminescence study before isotope research starts (fig. 4, transmitted light). Many zircon grains are not homogenous and consist of an inner core and one or more zonal rims. Dating of core and rims separately inside one zircon grain may provide very different age: up to 100MA.
Fig. 4.

Go Top    Concluding remarks

 References

       Introduction       Mineralogy       Concluding remarks     References