SKARNS AND POST-SKARN METASOMATITES OF THE NORTHERN PART OF THE SARALINSK ORE FIELD

S. K. Knysh

(Presented by the scientific seminar of the Department of Geology and Exploration, MPI)

1. Geological Setting

The Saralinsk ore field, located in the axial part of the Kuznetsk Alatau, is one of the oldest gold-mining sites in Western Siberia.

Geologically, the ore field lies within a complex effusive-sedimentary sequence, composed of basic, intermediate, and acid effusive rocks, their tuffs, and various, predominantly clastic varieties of sedimentary rocks. From the east this sequence is adjoined by dark bituminous limestones, distinguished as the Glavstan Formation of Upper Proterozoic age.

In the northern part of the ore field, intrusive rocks are exposed that make up several massifs. Geological mapping has made it possible to subdivide the intrusive massifs and to distinguish within them two magmatic complexes: a gabbro-diorite–plagiogranite complex and a granite–granodiorite complex. The first of these is correlated with the Kundustyul intrusive complex, and the second with the Martaiga intrusive complex, described by M. P. Kortusov [1] for the Mariinsk taiga. Within the area under consideration, skarn bodies and post-skarn metasomatites are spatially drawn toward the contacts of the granite–granodiorite intrusion, which indicates their genetic relationship with the younger magmatic complex. Less commonly, these formations are confined to zones of rupture disturbances at some distance from the intrusive contacts.

Skarns and post-skarn metasomatites are formed through the skarnification of various types of enclosing rocks: contact hornfelses, hornfelsed plagiogranites of the gabbro-diorite–plagiogranite complex, and also carbonate rocks of the Glavstan Formation. The rocks described occur as fairly extended lenses or else compose deposits irregular in shape.

Below is given a brief petrographic and geochemical characterization of the above-mentioned formations.

2. Brief Petrographic Characterization

On the basis of mineralogical composition and the ratios of the principal rock-forming minerals, the following varieties are distinguished in limestone skarns and post-skarn metasomatites:

a) garnet, pyroxene-garnet, garnet-pyroxene, pyroxene;

b) pyroxene–tremolite and tremolite–calcite metasomatites.

As a result of variations in the quantitative ratios of the rock-forming minerals, gradual transitions are observed among the varieties distinguished within each group.

a. Calcareous skarns are characterized by a fine- to medium-grained structure. The color of the rocks varies from light green in pyroxene skarns to brownish-brown in garnet skarns. The texture of the rocks is massive, more rarely spotted, caused either by an uneven distribution of the principal rock-forming minerals or by the presence of areas of unreplaced rocks.

Under the microscope, a granoblastic structure is manifested in the rocks described. Varieties of pyroxene-garnet and pyroxene skarns with a typical porphyroblastic, nematoblastic structure are sometimes encountered, caused by the development of a tangled-fibrous aggregate of tremolite.

The main rock-forming minerals of the skarns are garnet and pyroxene. Amphibole, plagioclase, quartz, epidote, zoisite, calcite, tremolite, chlorite, and sericite are less widespread. Sphene and apatite are noted as rare accessory minerals.

Garnet of the grossular–andradite series in garnet skarns composes up to 80% of the rock; in garnet-pyroxene skarns its content falls to 20–50%. Macroscopically it has a brown, dark-brown to black color. Its distribution in the rock is rather uneven. It often forms almost monomineralic accumulations. Aggregates composed of garnet have a fine-grained structure with individual grains 0.2–0.8 mm in size. In some cases, however, porphyroblastic segregations up to 2–4 mm in size are noted.

In transmitted light, garnet has a brown and yellow-brown color. Colorless varieties of the mineral are occasionally noted. In crossed nicols it is isotropic. The refractive index varies from 1.773 to 1.855 depending on the type of skarn. For the most part it ranges within 1.827–1.834. Garnet grains are isometric, often with crystallographic bounding of the rhombododecahedron. The latter belong primarily to porphyroblasts of the mineral in a finer-grained mass. The noted porphyroblasts are often filled with more or less corroded inclusions of other minerals, imparting to such segregations a typical poikiloblastic structure. Garnet especially distinctly corrodes grains of pyroxene and plagioclase; and in areas where later hydrothermal processes were manifested, it is in turn intensely corroded and replaced by amphibole, epidote, and quartz.

Pyroxene is most widely distributed in pyroxene skarns, but in one quantity or another it also occurs in other varieties. Macroscopically it has a light-green color. Under the microscope, in transmitted light, pyroxene is colorless; sometimes a very light greenish tint is observed; pleochroism is absent. The optical constants \((2V = +56—60^\circ,\ \mathrm{CN}_g = 38—40^\circ)\) correspond to diopside. Hedenbergite \((2V = +62—64^\circ,\ \mathrm{CN}_g = 48—49^\circ)\) is encountered more rarely in the skarns. In the rocks, pyroxene composes a granoblastic, uneven-grained aggregate with grain sizes from 0.1 mm to 2.0–2.5 mm. The grain shape is short-prismatic or isometric (Fig. 1). As one of the early minerals, pyroxene distinctly replaces plagioclase. In turn, it is corroded by garnet and sphene. Of the secondary minerals, chlorite is characteristic of pyroxenes.

Plagioclase is represented by albite-oligoclase. Its content

Fig. 1. Granoblastic, uneven-grained structure of a pyroxene aggregate (pyroxene skarn). Magnification 50. Nicols crossed.

rarely exceeds 10%. Macroscopically it is not encountered. Under the microscope, in transmitted light, it usually has a cloudy coloration because of intense secondary alteration. In individual cases a poorly expressed twinned structure is observed. The angle of symmetrical extinction is 10–13°. Plagioclase forms small grains ranging in size from 0.05 to 0.3 mm. Their shape is isometric, with extremely irregular, strongly corroded outlines. Among secondary minerals developing after plagioclase are sericite, zoisite, and epidote. It may be assumed that the plagioclase in the skarns is a relict mineral preserved during incomplete skarnification of the plagiogranites.

Amphibole is recorded as an admixture in garnet and pyroxene–garnet skarns. In them its amount does not exceed 10%. Its distribution in the rock is uneven. It often forms rounded or lens-shaped accumulations. In transmitted light the amphibole has a green color and distinct pleochroism according to the scheme: Ng—green, Nm—brownish-green, Np—light green, bluish-green. \(2v = -57–60^\circ\), \(CNg = 17–20^\circ\). On the basis of the given constants, the amphibole is identified as hastingsite. Amphibole distinctly corrodes pyroxene. Sometimes overgrowth of amphibole around garnet grains is observed (Fig. 2).

Fig. 2. Garnet skarn. Magnification 30. Nicols parallel.

Quartz, like the other minerals of the superimposed complex, occurs rather rarely in the skarns, and its amount does not exceed 2–3%. Under the microscope, in transmitted light, quartz forms irregular grains, often confined to the interstices of grains of other minerals, and it distinctly corrodes grains of plagioclase, pyroxene, garnet, and amphibole. In turn, the quartz grains are cut by veinlets of epidote.

Epidote and zoisite are observed in small amounts in garnet and garnet–pyroxene skarns. Their grain size varies from 0.01 to 1–2 mm. Epidote and zoisite more often develop after plagioclase. More rarely, veinlets filled with epidote are observed in the skarns; these cut segregations of plagioclase, garnet, pyroxene, and quartz.

Calcite occurs sporadically in the rocks described. In transmitted light the mineral is colorless. Refractive index \(N_o = 1.659\). In garnet–pyroxene skarns it forms accumulations up to 3–4 mm in size, consisting of grains up to 0.5–1.0 mm across. Such accumulations are often confined to the central parts of garnet aggregates. In this case the calcite grains distinctly fill the interstices of garnet crystals, often separating and corroding it. It is possible that such carbonate accumulations are the result of recrystallization of relicts of carbonate rocks in the skarns.

In pyroxene skarns, calcite is closely associated with tremolite, cementing the acicular crystals of the latter.

Tremolite is noted only in pyroxene skarns, where its content reaches 10–15%. In transmitted light it is colorless. \(2v = -88^\circ\), \(CNg = 19^\circ\). Tremolite forms tangled fibrous aggregates with crystals up to 3 cm in size. Together with calcite, tremolite replaces pyroxene (Fig. 3).

Fig. 3. Replacement of pyroxene (dark gray, high relief) by an aggregate of tremolite. Magnification 20. Crossed nicols.

Chlorite is a secondary mineral developing after pyroxene and amphibole. Its content does not exceed 1%. In transmitted light it is pale green, with weak pleochroism. Chlorite forms small platy and scaly aggregates with grain sizes up to 0.03 mm.

Sphene is characteristic exclusively of skarns formed as a result of the skarnification of plagiogranites of the gabbro–diorite–plagiogranite complex, in which it is a typical accessory mineral. Thus, it may be assumed that in the skarns it is re-

a relic mineral, but one that has undergone recrystallization and, possibly, local redistribution.

In the rocks described, sphene forms wedge-shaped or irregular grains ranging in size from 0.05 to 1.5 (Fig. 4). Fine grains sometimes occur as poikiloblastic inclusions within large pyroxene grains. Its larger segregations penetrate along the boundaries of pyroxene grains, distinctly corroding their outlines. In turn, sphene grains are corroded by garnet and quartz.

Fig. 4. Wedge-shaped segregations of sphene. Magnification 10. Crossed nicols

Apatite apparently is also a relic mineral of the plagiogranites. In the form of elongate-prismatic crystals up to 0.1 mm in size, it occurs in grains of pyroxene, sphene, and other minerals.

Sericite develops as finely scaly aggregates replacing plagioclase. Its amount does not exceed fractions of a percent.

b. The post-skarn metasomatites are dense, light-gray rocks with an uneven-grained texture. Under the microscope they are characterized by a porphyroblastic, tangled-fibrous texture and are composed of diopside, tremolite, and calcite. In some cases the pyroxene proves to be completely replaced, and the rocks acquire a tremolite-calcite composition. Among the ore minerals in the rocks described there are pyrrhotite, marcasite, pyrite, sphalerite, and chalcopyrite.

Pyroxene (diopside). Its content rarely exceeds 60–65%. Under the microscope it forms long-prismatic grains up to 1.5 mm in size. The angle \(2v\) varies from \(+50^\circ\) to \(+58^\circ\), \(CN_g = 38^\circ\). Refractive index: \(N_g = 1.679\), \(N_p = 1.662\).

Tremolite forms tangled-fibrous aggregates of fine acicular crystals. Its content is 30–40%. The angle \(2v = -88^\circ\), \(CN_g = 16–19^\circ\). Refractive index \(N_g = 1.631\), \(N_p = 1.603\). The tremolite aggregate develops intensively on pyroxene and is cemented by later calcite.

Calcite. Its content does not exceed 5%. In transmitted light it is colorless. Refractive index \(N_o = 1.659\).

Pyrrhotite is the most widespread of the ore minerals. It forms accumulations irregular in shape, composed of a fine-grained aggregate.

Under the microscope in reflected light, pyrrhotite is characterized by

with moderate reflectance. In crossed nicols the anisotropy effect is distinctly manifested in pink and bluish-gray tones. Segregations of the mineral described cement the tremolite aggregate and in turn are replaced by later sphalerite and marcasite, which penetrates into the grains along fractures.

Marcasite forms irregularly shaped segregations. Under the microscope in reflected light its color is light yellow and somewhat darker than that of pyrrhotite. The form of its segregations is highly characteristic of marcasite. Unlike pyrite in the rocks described, it does not form regular shapes; its segregations cement the tangled-fibrous aggregate of tremolite, and along fractures they cut through sphalerite grains.

Pyrite is observed as fine, uneven disseminations of crystals of cubic habit. Their size does not exceed 1 mm. In the rocks described, pyrite is replaced by pyrrhotite and marcasite.

Sphalerite forms comparatively large segregations. The texture of its aggregates is fine-grained, with grain size up to 1–2 mm. Most grains are characterized by the presence of emulsion inclusions of chalcopyrite. Comparatively rarely, rounded grains of pyrrhotite are observed in sphalerite.

Chalcopyrite is rarely observed macroscopically. It forms fine emulsion disseminations in sphalerite, and also occurs as separate grains no more than 0.08 mm in size. The shape of its segregations is mostly irregular.

3. Geochemical features of skarns and post-skarn metasomatites

In the rocks described, spectral analysis established the presence of Ge, Pb, Ni, Co, U, Cr, Ti, Zr, Mo, Ba, Sr, P. Admixtures of As, Cd, Be, Bi, and rare earths are not detected at all.

The calcareous skarns are distinguished by a noticeable admixture of elements of the Fe family, especially Mn and Ti (up to 0.5%).

The presence of Ni and Co is established almost everywhere, but their content does not exceed thousandths of a percent. Cr is observed sporadically, and its content does not exceed 0.003%. Of the chalcophile elements, the most widely distributed is Zn, with a content from 0.01% to 0.1%; Cu—(0.001%), Pb—(0.001–0.005%). Other chalcophile elements, Ge and Mo, occur in single samples in amounts considerably below clarke values. Lithophile elements are also weakly distributed. Among them, an admixture of Zr—(0.005–0.01%) and P—(up to 0.1%) is noted more often. Less often, an admixture of Ba—(0.05%) and Sr (0.02–0.03%) is established. In one sample an admixture of Sn (up to 0.001%) is noted.

In general, the assemblage of impurity elements in the skarns is simple and in its composition is close to that of plagiogranites, at whose expense these skarns were formed.

In the post-skarn metasomatites the assemblage of impurity elements differs somewhat from that described above. First of all, these differences concern the distribution and role of elements of the Fe group. These elements, with the exception of Mn, occur sporadically (Ni up to 0.001%), while an admixture of Cr is not detected at all. Chalcophile elements play a more noticeable role. The Zn content reaches 0.3%. An admixture of Ge and Ag is established, with contents up to 0.0001%. An admixture of Mo is not detected at all.

Among the lithophile elements, only sporadic occurrences are noted of P (up to 0.1%), Zr (up to 0.005%), and Ga (up to 0.001%).

Conclusion

The similarity of the geological structure of the area described to the geological structure of gold-ore deposits of the skarn type in the Sayano-Altai folded region served as a prerequisite for prospecting for gold-bearing skarns within the Saralinsk district as well.

Attention is drawn to the similarity of the mineralogical features of the Sarala skarns with the gold-bearing skarns of other deposits: Natalyevskoye, Mayskoye, and Kaliostrovskoye.

However, the chip sampling carried out on skarns and post-skarn metasomatites distributed in the area described yielded negative results. The gold content usually does not exceed 1–2 g/t. Heavy-mineral concentrates collected in the area of skarn development likewise showed an absence of gold. This fact may be explained by various causes.

The gold mineralization of the Saralinsk ore field is genetically associated with an ancient intrusion of a gabbro-diorite–plagiogranite magmatic complex. In favor of such a genetic connection are the spatial proximity of the gold-bearing quartz veins to this intrusion, as well as certain geochemical features of the igneous rocks of the district.

At the same time, as noted above, the skarns and post-skarn metasomatites of the northern part of the Saralinsk ore field are genetically associated with a younger intrusion of granite–granodiorite composition.

REFERENCES

1. M. P. Kortusov. Paleozoic intrusive complexes of the Mariinsk taiga (Kuznetsk Alatau). Vol. 1, Tomsk, Tomsk State University Press, 1967.