Apatites from Trachydolerites of the Gorbiachin River

T. I. Poluektova

(Presented by Professor A. M. Kuzmin)

In studying the accessory minerals of trachydolerite (trapp) intrusive bodies of the northwestern part of the Siberian Platform, special attention was given to apatite, titanomagnetite, and ilmenite. The material for the present work was collected during practical production work in the summer of 1963 under the direction of Candidate of Geological-Mineralogical Sciences B. V. Oleinikov. In the course of its processing, great assistance was rendered by Professor A. M. Kuzmin, to whom the author takes this opportunity to express sincere gratitude.

The trachydolerites from the middle part of the Gorbiachin River occur in the form of a sill 30–50 m thick, locally up to 80 m, gently cutting or almost conformable with the enclosing coal-bearing strata of Permian age. In outcrops they are characterized by blocky and thin-platy jointing. The trachydolerites are medium-crystalline, spotty massive rocks with a poikilo-ophitic, ophitic structure, and with rare porphyritic segregations of plagioclase. The texture is taxitic. The rocks described consist of 30–55% plagioclase No. 45–60, titaniferous augite (15–30%), biotite (5–10%), ilmenite and titanomagnetite (5–8%), and apatite (2.5–5%). Secondary minerals—chlorite, bowlingite, serpentine—make up 5%.

Table 1

In chemical composition, the trachydolerite, in comparison with widely distributed dolerites, is characterized by undersaturation in silica—45.42%, as against 48.58% typical of normal dolerites—by a reduced magnesium content, and by a noticeably increased content of alkalis, titanium, and phosphorus (Table 1).

Apatite

Apatite in the trachydolerites constitutes 2.5–3%, sometimes 5%. Its average content fluctuates around 1.2%. Apatite, as a rule, is represented in the rock by well-formed prismatic, long-pris-

matic crystals. Relatively large prismatic crystals of calcium phosphate, ending in acute-bipyramidal forms that represent a combination of acute and normal bipyramids, occur no less often.

In section, apatite crystals have hexagonal outlines. However, sections of trigonal appearance are not rare, when the hexagon in a section perpendicular to the optical axis is an alternation of relatively well-developed sides of a triangle whose angles are truncated by a narrow prism face. The sizes of apatite grains vary within comparatively broad limits, even in one and the same thin section. The dimensions of the smallest of them within the section are \(0.03 \times 1.0\) mm, while the largest thick-prismatic, barrel-shaped ones reach \(3.4 \times 6\) mm. The largest apatite crystals have dimensions of \(3.5 \times 6\text{—}8\) mm; it is characteristic that the latter are concentrated in the upper parts of the sill. With depth, the sizes of apatite crystals decrease.

Small crystals, \(0.03 \times 0.9\) mm in size, have flat faces, their edges are straight, and traces of deformation are noticeable on them. Large apatite crystals, beginning with a size of \(0.08 \times 1.2\) mm, have uneven prism faces, which are often stepped; they are broken by numerous cracks parallel to \(\{0001\}\), and bear clear traces of displacement of individual parts of the crystal along the plane \(\{0001\}\).

In this case two types of cracks are clearly distinguished: one of them follows parallel to the plane \(\{0001\}\), parallel to which the displacement of individual parts of the fractured crystal also occurs; the others run along the crystal parallel to \(\{10\overline{1}0\}\). With the development of deformation, individual apatite grains break up into blocks, which are often displaced relative to one another. In addition to the indicated system of cracks, apatite is sometimes broken by frequent cracks having no crystallographic significance. Along crushing cracks, chlorite and bowlingite generally penetrate into apatite. Apatite is distributed unevenly in the rock and readily occurs together with ilmenite, magnetite, and titanomagnetite.

Paragenetic relations of apatite with the rock-forming minerals are as follows: apatite crystals, as a rule, form inclusions in titanaugite and in the products of its replacement. Long-prismatic apatite crystals often cut through pyroxene grains and adjacent plagioclase grains. Apatite inclusions in one and the same augite grain are often not solitary. Alongside large apatite crystals there are smaller ones. The chain-like character of the arrangement of small apatite crystals, recalling in aggregate a bead-like veinlet, should be especially noted.

In the relations of apatite with plagioclase, it can be clearly seen that apatite either occurs in close contact with plagioclase, following along the boundary of feldspar grains, or, in the form of two “whiskers” or long prismatic and needle-like crystals, it penetrates into plagioclase grains as it grows in. In two cases, subhexagonal inclusions of apatite in plagioclase were recorded. As inclusions, apatite is observed in orthoclase and biotite. Apatite crystals also occur as intergrowths in well-formed titanomagnetite.

Apatite is characterized by the presence of inclusions of foreign minerals; moreover, between the inclusions and the size of apatite grains there is a definite dependence: small crystals contain almost no inclusions; large ones are abundantly saturated with them and often have sheath-like—

... shape, the central part of which is completely filled with remnants of unsubstituted plagioclase, titaniferous augite, and flakes of biotite.

In individual thin sections the process of formation of sheath-like crystals can be traced very well. During its growth, apatite, approaching an already formed mineral, penetrates into it with two “whiskers,” continues to grow, and thus forms a sheath-like crystal. In all cases apatite tends to assume the crystallographic faceting characteristic of it.

All inclusions in apatite may be divided into two groups: 1) inclusions of foreign minerals and 2) gas-liquid inclusions.

Among the inclusions of foreign minerals it was possible to identify ore minerals, brown biotite, green chlorite, orthoclase, pyroxene, and plagioclase. Sometimes inclusions of several minerals are observed in a single apatite crystal: olivine, and an ore mineral with a biotite rim. Inclusions of biotite and orthoclase are oriented in such a way that their principal optical directions are spatially parallel, as is shown by the simultaneous extinction of the indicated individuals. In individual apatite crystals, inclusions of needles of finer apatite are noted. Apatites containing gas-liquid inclusions occur more rarely. The latter, as a rule, are concentrated in the central part of the grains.

In studying the behavior of apatite across the section of the intrusive body, it was possible to establish that, on approaching the contact, smaller sizes of apatite crystals predominate, the number of sheath-like crystals decreases, and acicular and hexagonal crystals increase.

Comparing the apatite studied with apatite from normal dolerites, described by E. D. Nadezhdina (1961), it may be noted that the apatites of trachydolerites are distinguished by a greater variety of crystallographic forms and by larger crystal sizes. The number of sheath-like crystals increases considerably in trachydolerites. Among the inclusions in apatites of normal traps, inclusions of olivine, pyroxene, and plagioclase were not noted; in apatites of trachydolerites the indicated mineral inclusions are also present. The number of apatite crystals containing gas-liquid inclusions in normal traps is significantly higher than in apatites of trachydolerites. In addition, E. D. Nadezhdina (1961) indicates that apatite in normal dolerites is present in two generations. In trachydolerites apatite is present in one, later generation.

As already noted earlier, the apatite content in the rock reaches 2.5–5%. Such a high percentage of this mineral in the rock indicates that it should be regarded not as an accessory mineral but as a secondary rock-forming mineral. The magma was enriched in phosphorus. The separation of apatite proceeded continuously over a long period; its formation began at early stages, after the beginning of crystallization of the principal rock-forming minerals—plagioclase, olivine, and pyroxene. The sheath-like crystals arose at the expense of residual solutions, i.e., they represent a product of autometasomatism. This is supported by such data as the confinement of the crystals to intergranular seams, to grain contacts, and, finally, by the sheath-like crystals themselves.

Thus, the trachydolerites of the Gorbiachin River, containing a high percentage of phosphorus, should be called apatite trachydolerite. Taking into account the high percentage of apatite content (up to 5%) in the rock, one may expect, under the most favorable geological conditions, an increased concentration of apatites up to industrial significance.