SOME DATA ON THE CRUSHING OF IGNEOUS ROCK SAMPLES FOR THE EXTRACTION OF ACCESSORY MINERALS

E. V. Vladimirova

(Presented by the seminar of the departments of mineralogy, petrography, methods of prospecting, and mineral resources)

In the study of accessory minerals of igneous rocks, the method of artificial heavy-mineral concentrates has recently been used with increasing success. As yet, no single generally accepted and convenient working scheme has been developed for processing samples by this method. In particular, one of the most crucial stages of sampling—the process of crushing the original sample material—is treated differently by different authors. Some [2, 8], having established a sharp predominance among accessory minerals of the 0.05 mm fraction, crush granitoid samples to no coarser than 0.5–0.2 mm. Others, for complete extraction of accessory minerals, propose grinding rocks to 0.1 mm [4] and even finer [3, 5]. V. V. Lyakhovich and S. D. Turovskii [7] recommend crushing granite samples to particle sizes in the range of 0.5–0.25 mm. In the present work, an example is considered of crushing granite samples to a particle size of 1 mm, as well as separate variants of finer or coarser crushing for other rock varieties.

To determine the distribution of crushed material by grain-size classes, we carried out a series of observations on granites and aplites of the Belokamenka complex on the eastern slope of the Kuznetsk Alatau. The structure of the rocks considered is medium-grained (5–1 mm); the average sample weight is 6.5 kg; crushing was mechanical, on a disk grinder; washing was carried out in pans of the prospector type. Routine crushing of samples was adopted down to 1 mm. After crushing and washing, the material was divided into fractions: silts and sands. The sandy fractions (>0.07 mm) constitute 95–98% of the initial sample weight, and the silts, on average, 3.6%. When the “sands” are sieved into four groups by grain-size classes (Table 1), the sample material is distributed almost uniformly in each group. The latter also makes it possible to observe in samples accessory minerals of the most varied sizes, in most cases in their natural form (not crushed).

The principal losses of accessory minerals occur in intergrowths with rock-forming minerals. To determine the magnitude of losses of heavy (in these rocks, mainly accessory) minerals in intergrowths with light minerals (specific gravity <2.9), successive grinding of sample material and systematic separation of the concentrate from it were carried out. For this, a sample was processed in the usual manner (crushing to 1 mm), and the concentrate was washed from it—the routine operation “a.” The remaining part of the sample was dried and again crushed to particles of smaller size (crushing to 0.5 mm), with subsequent sepa-

Table 1

Sieve analysis of the sandy material of flow-through samples
after crushing (1.0 mm) and washing

…with concentration of the heavy-mineral concentrate—operation “b,” then operation “v” (crushing to 0.25 mm), and, finally, the last drying, crushing to particles 0.1 mm in size, and finishing of the concentrate—operation “g.” The silt material (fraction <0.07 mm) was analyzed as a separate weighed portion. The weighed portion was centrifuged in Thoulet’s solution; the heavy fraction was weighed.

Table 2 gives the results for the quantitative composition of the concentrates from each stage of processing. Since the concentrates of the rocks studied include much magnetite, the results for their mineralogical composition are given separately as magnetic and nonmagnetic heavy fractions. In all samples, a large loss of accessory minerals in intergrowths is confirmed, owing to their incomplete extraction from the rock after crushing and washing (on average 65.6%). Such losses must necessarily be taken into account. The mean correction coefficient in our case, calculated by the method of V. V. Lyakhovich and B. P. Zolotarev [6], is 2.96.

A complete mineralogical analysis of the concentrates showed that the species composition of the most common minerals in such samples does not change in each successively obtained concentrate, while the quantitative ratios of the minerals do not always remain constant during the transition from one processing stage to another. For example, in the bulk concentrate (processing stage “a”) the minerals are concentrated in the following sequence (in decreasing order): sphene, hematite, ilmenite, zircon; whereas in the concentrate of one and the same sample (at processing stage “b”) the most widespread are ilmenite and hematite. The obtained in-

Table 5

Yield of magnetic and nonmagnetic heavy fractions during sequential crushing of material (in g per 10 kg of rock, %)

interesting data on the distribution in samples of such rarely occurring accessory minerals as polycrase, gold, cinnabar, and galena. The minerals listed are more often detected in the heavy fraction from routine processing (operation “a”) even in cases where the size of their particles is much smaller than the grinding size of the sample. This is explained by the fact that most accessory minerals form in intergranular spaces, in microcracks, and in weakened zones, and at the very first mechanical disturbances in the rock they are loosened and enter the heavy concentrate. When sample material is ground excessively finely, fragile minerals (fluorite, barite, molybdenite, galena, cinnabar, tantalo-niobates, and many others) are rapidly overground and, during washing, pass into floury fractions, where identifying them is much more difficult.

The adopted crushing size (to 1 mm) for granites prevents the overgrinding in them of fragile accessory minerals and thereby reduces their losses during washing and subsequent processing of samples; it makes it possible to obtain material over a broad range of grain sizes, does not require special equipment or additional labor-intensive operations for extracting accessory minerals from a fine fraction, and is convenient for mineralogical observations, since grains and crystals of minerals that have not been deformed (not crushed) by excessively fine abrasion are easier to diagnose and provide more extensive information on features of structure, zonal coloration, secondary alteration, the presence of inclusions and intergrowths, etc.

The crushing intervals for samples of igneous rocks should probably be determined not only by the structure and grain size of the rock [1], not only by the sizes of the accessory minerals themselves, but also by the composition of the rock-forming minerals. Thus, according to our observations, samples from amphibolized gabbro of the Izykh massif (the accessory minerals in them are located mainly in hornblende as inclusions and have sizes of 0.1–0.15 mm) must be ground to a particle size no coarser than 0.2 mm, whereas samples from coarse-grained nordmarkites (south of the settlement of Tuim), which include large crystals (2.5 mm) of brown zircon that are readily destroyed during crushing, should preferably be crushed no finer than to 1.5 mm. In the latter case, after the sample material has passed through a 1.5 mm sieve, the sample contains enough material of any finer classes.

Thus, when choosing the crushing interval for samples in the study of accessory minerals, the mineral composition of the rock itself is of great importance. Combining all igneous rocks under the collective name “granitoids,” without subdividing them into petrographic varieties, and processing them according to a single scheme may lead to undesirable sampling errors. If rocks contain a large quantity of dark-colored rock-forming minerals, with which accessory minerals are often (but not always) spatially closely associated, such intrusive formations (regardless of the grain size of the rocks) should be crushed much more finely than rocks of the leucocratic series. Igneous rocks altered by postmagmatic processes (albitization, potassium feldspathization, silicification) have their own particular features, which must be taken into account during the mechanical processing of samples.

LITERATURE

1. Azimov P. G., Smorodinova L. D. Selection and processing of massif samples for accessory minerals.—Uzbek Geological Journal, 1960, No. 6.

2. Amshinskii N. N. Zoning of gamma fields and accessory minerals in granitoid massifs of Altai. Geology and Geophysics. Publishing House of the Siberian Branch of the USSR Academy of Sciences, 1960, vol. 1.

1. Amshinskii N. N., Marich I. V., Gorb A. M., Orlova L. I. Fundamentals of the methodology of mineralogical-geochemical investigations of granitoids. — In: Accessory Minerals of Igneous Rocks. “Nauka,” Moscow, 1968.

2. Efimov I. E., Satrapinskaya I. I. A method for determining the weight contents of accessory minerals in granitoids. Proceedings of the Kaz. IMS, 1959, 1.

3. Kots G. A., Razumnaya E. G. Micro-concentration apparatus for the separation of mineralogical samples and the possibilities of its application. — In: Accessory Minerals of Igneous Rocks. “Nauka,” Moscow, 1968.

4. Lyakhovich V. V., Zolotarev B. P. Accessory minerals in the granitoids of Gorny Altai. Proceedings of the Institute of Mineralogy, Geochemistry, and Crystal Chemistry of Rare Elements, 1959, vol. 2.

5. Lyakhovich V. V., Turovskii S. D. Methodology for studying accessory minerals of igneous rocks. — In: Accessory Minerals of Igneous Rocks. “Nauka,” Moscow, 1968.

6. Semenova E. D. Methodology for sampling granites and the accessory minerals of the Verkhne-Isetsky massif. Proceedings of the Mining-Geological Institute of the Ural Branch of the USSR Academy of Sciences, 1953, issue 20.