Geological Structure of the Zarkashan Skarn-Gold Ore Deposit (Afghanistan)

GEOLOGICAL STRUCTURE OF THE ZARKASHAN SKARN-GOLD DEPOSIT (Afghanistan)

V. P. Sayapin, A. F. Korobeynikov

(Presented by the scientific seminar of the Department of Geology and Exploration of Mineral Deposits)

The Zarkashan deposit is located in the high-mountain region (2350–3050 m) of Ghazni Province, Afghanistan, 24 km northwest of the city of Muqur, on the northwestern slope of the valley of the Tarnak River. The deposit belongs to a poorly studied gold-skarn formation of Mesozoic age, and therefore it seems advisable to briefly examine the features of its geological structure.

Brief features of the geological structure of the deposit

The oldest rocks developed in the central and northwestern parts of the area are light-gray marbled limestones and marbles of Middle–Upper Jurassic age (Fig. 1). They are represented by coarse- and medium-grained white and light-gray marbles, siliceous and pinkish-gray dolomitic marbles, and forsterite calciphyrs. In the upper part of the section, among the carbonate rocks, individual interbeds and lenses of clayey rocks appear; under contact metamorphism these have been transformed into hornfelses. The hornfelses are composed of a fine-grained plagioclase-pyroxene aggregate, in places with superimposed garnet, tremolite, vesuvianite, and wollastonite. The total thickness of the carbonate rocks in the area is about 100 m. Upon them, without visible angular unconformity, terrigenous-sedimentary formations of Upper Cretaceous age occur, with an abundant fauna of the Cenomanian stage. These rocks are developed in the southern part of the area. At the base lies a horizon of basal conglomerates with pebbles of limestones, 20–100 m thick. It is replaced by dark-gray, greenish-gray quartz and calcareous sandstones and limestone conglomerates. The thickness of individual interbeds does not exceed 2–10 m. These rocks are intensely hornfelsed and marbleized. The total thickness of the upper sequence is 150–200 m. Quaternary deposits are represented by alluvial-proluvial and deluvial deposits of valleys and slopes.

The features of the tectonic structure of the area are determined by the influence of the large Khennay fault, which controlled the emplacement of the Zarkashan granitoid intrusion. The principal plicative structure of the deposit is a second-order anticline of sublatitudinal trend, up to 1 km long, with a wing span of 300 m. The dip angles of its limbs range from 15–20 to 40–50°. In the central part of the an-

the anticlinal fold axis undulates owing to the superposition of a transverse synclinal fold of the third order. The folded structure of the deposit is complicated by disjunctives of meridional, latitudinal, and diagonal trends. Owing to the combination of transverse and longitudinal faults, the deposit has acquired a small-block structure. Displacements of individual blocks relative to one another reached 1–2 km vertically. The greatest influence on the localization of skarn-ore bodies was exerted by faults of northwestern strike, along with bedding-plane fractures along the boundaries of heterogeneous layers. Such interstratal fractures were widely manifested in the crest and limbs of the anticlinal fold in the areas of its bending.

Sedimentary formations in the central and southeastern parts of the deposit area are intruded by small stocks, sills, and dikes of acidic composition (Fig. 1). The rocks composing them, porphyritic in appearance, consist of plagioclase No. 32–46 (31–60.5%), K-feldspar (27–43.5%), hornblende and biotite (0.8–26.1%), quartz (1.2–18%), sphene, apatite, and magnetite (up to 0.7–3%). Among secondary minerals, chlorite occurs sporadically. The rock texture is monzonitic and poikilitic. On B. M. Kupletsky’s diagram they fall in the field of adamellites and quartz monzonites. In the near-contact parts of the intrusive bodies, adamellites gradually pass into quartz monzonites and monzonites. Dike bodies are composed of aplites, granosyenites, syenites, and more rarely syenite-porphyries and diorite-porphyrites. Their length is the first tens, more rarely hundreds, of meters, with thicknesses from 3 to 50 m. More massive dikes are composed of leucocratic granite-aplites. They are steeply dipping and have predominantly northeastern and northwestern strikes.

At the contact with intrusive bodies, the host rocks have been transformed into hornfelses, marbles, forsterite calciphyres, magnesian and calcareous skarns.

Skarn-ore bodies are confined to bedding-plane fractures formed at the contacts of dense and plastic hornfelses with breccia-like marbles. Interbeds of hornfels served as screens during the formation of skarns and ores. The presence of several hornfels horizons in the stratigraphic section created favorable conditions for the tiered distribution of skarn-ore bodies. Exploration work has revealed four such horizons. The most persistent is the fourth horizon (Fig. 1), represented by a sheet-like skarn-ore body on the southern limb of the anticline, dipping to the southeast at an azimuth of 150–160° and an angle of 45–50°. Below it lies the so-called “pyrrhotite” ore body.

Gold-ore bodies are serpentinized, carbonatized, quartzized, and sulfidized skarns in the form of lenses and nests ranging in size from 1.5–3 to 25–40 m, more rarely 350 m along strike, with thicknesses from 0.5 to 2.4, more rarely 9.4 m. The boundaries of the ore bodies are indistinct and are determined only by sampling. Down dip, the ore deposits have been traced for more than 140 m along the dip of the skarn zones. Gold content in the ores ranges from 1 to 70, more rarely up to 245 g/t; its distribution is extremely irregular and depends mainly on the concentration of copper-sulfide minerals in the skarn zones.

Sequence of formation of skarns and ores and their mineral composition

In the history of formation of the deposit, two stages can be distinguished: I — magnesian and calcareous skarns and II — sulfide-gold ores. Mineral formation of the contact-metasomatic stage

(I) took place in two stages: the magnesian-skarn and the lime-skarn stages, and the hydrothermal stage (II) in three phases: pyrrhotite-serpentine, calcite–gold–chalcopyrite, and chalcedony–calcite. The sequence of mineral deposition is shown in the table.

During the formation of the granitoid intrusion, hornfelses, marbles, and skarns were formed. At the same time, at the contacts of the granitoids with dolomites, magnesian skarns and calciphyrs arose already during the magmatic stage. Bodies of these skarns have subtabular and lens-like forms, with a strike length of several hundred meters and thickness from the first meters to the first tens of meters. Magnesian skarns at the deposit are widely developed and are composed of forsterite, spinel, fassaite (10–90%), phlogopite (20–70%), vesuvianite, and magnetite in highly variable amounts. The skarns are vari-grained dark-green rocks of granoblastic structure, essentially of pyroxene and phlogopite composition. On the side of the carbonate rocks they are often surrounded by forsterite calciphyrs. The primary skarn minerals are fassaite, spinel, forsterite, and phlogopite-I; the superimposed minerals are phlogopite-II, vesuvianite, and magnetite. Phlogopite-I is usually coarse-scaly, whereas II is fine-scaly. The former replaces forsterite, spinel, and fassaite, and is itself, in turn, replaced by fine-scaly phlogopite-II and vesuvianite. According to the predominance of one or another mineral, fassaite, fassaite–phlogopite, fassaite–vesuvianite, phlogopite–fassaite–vesuvianite, and phlogopite–vesuvianite varieties of magnesian skarns can be distinguished.

Lime skarns formed in the postmagmatic stage at the contact of granitoids with marbles and magnesian skarns by infiltration. They have limited development. The forms of the lime-skarn bodies are nest-like, lenticular, and vein-like. Their length is usually 8–12 m, thickness—on the order of the first meters. They cut magnesian skarns or occur separately among marbles and are composed of garnet No. 20–80 (30–70%), wollastonite (20–40%), diopside, salite (10–50%), tremolite, sphene, apatite, axinite, and magnetite-II. The rock structure is vari-grained, metasomatic; the texture is disseminated, drusy, and more often massive.

Magnetite forms fine-grained (1–2 mm) disseminations in skarns or, sometimes, massive bodies up to 60 × 4 m in size. In the latter case it is represented by a coarser-grained aggregate (up to 5–8 mm). Typical skarn minerals are garnets, pyroxenes, and wollastonite, whereas tremolite-actinolite, epidote, and serpentine formed together with magnetite in the final period of metasomatic mineral formation. The latter minerals cut or replace the skarn minerals.

The hydrothermal stage of mineral formation began after deformations that led to the appearance in the skarns of series of fractures and zones of crushing. The most favorable were renewed bedding fractures between heterogeneous horizons and beds, which, in the initial stage of the postmagmatic process, served as pathways for the penetration of skarn-forming solutions.

At this stage, in the skarn zones, pyrrhotite-serpentine, calcite–gold–chalcopyrite, and chalcedony–calcite associations were formed, corresponding to definite phases of mineral formation (table).

The pyrrhotite-serpentine association is represented by predominant serpentine (50–80%), calcite, tourmaline, epidote, quartz, pyrrhotite, chalcopyrite-I, and pyrite-I. They form dense disseminations, nests, small lenses, and zones in magnesian and, more rarely, lime-

Table

Scheme of post-sequential mineral formation of the Zarkashan skarn–gold deposit

... in skarns. Serpentine forms veinlets and nests ranging from 2–5 cm to 0.5–4 m, or continuous disseminations in skarns or calciphyrs. In thin sections, relics of forsterite, fassaite, and diopside are constantly observed among the fine-fibrous aggregates of serpentine; i.e., serpentine develops by replacement of skarn minerals. Pyrrhotite forms massive accumulations and small lenses in serpentinized skarns. Here it is associated with quartz-I, calcite, chalcopyrite-I, and pyrite-I. Quartz-I and calcite with fine disseminations of pyrite-I form nests, veinlets, and small lenses several millimeters thick, while chalcopyrite-I grows around the edges of pyrrhotite grains and more rarely forms separate fine disseminations in altered skarns. Tourmaline-I, like epidote, together with calcite and rare chalcopyrite-I, forms thin veinlets (up to 8 mm) in the skarns. No industrial concentration of gold has been found with this mineral association. No visible gold has been encountered.

The calcite–gold–chalcopyrite association is predominant in development and determines the practical value of the deposit. It forms veinlets and disseminations or cements brecciated parts of the skarns. Quartz–calcite–gold–chalcopyrite veinlets not infrequently cut serpentine–pyrrhotite accumulations and small lenses of quartz-I. The principal minerals of this association are calcite, more rarely tourmaline-II, quartz-II, chlorite-I, talc, pyrite-II, chalcopyrite-II, bornite, and gold. Fine- and coarse-grained calcite, together with quartz, tourmaline, and sulfides, cements angular fragments of skarns in brecciation zones or composes quartz–calcite veinlets up to 1–2 cm thick. Ore minerals in the calcite aggregate are situated at the contacts of calcite grains. Chlorite-II forms chain-like accumulations along the selvages of quartz–carbonate veinlets, while talc is found along the boundaries of grains of calcite and ore minerals in the form of small scales. Gold appears as irregular, hook-shaped forms ranging in size from 0.01 to 2 mm. The gold grains are located near veinlets of quartz-II with pyrite-II, or at the contacts of crystals of calcite, talc, serpentine, tourmaline, and chalcopyrite-II. The gold is bright yellow in color and of high fineness (assays 910–920). More rarely, gold grains are found as the finest segregations in chalcopyrite aggregates. Their distribution in the ores is extremely uneven. Finely dispersed gold associated with chalcopyrite is also widely distributed. For example, massive chalcopyrite ores not infrequently show gold contents from 50 to 245 g/t and copper up to 16.6%, although no visible gold is detected in them even under the microscope. It is characteristic that bodies of massive chalcopyrite ores are confined mainly to altered phlogopite skarns and silicified marbles. In other cases, gold grains are associated with areas of development of goethite, hydrogoethite, and malachite. Apparently, the formation of some part of the gold is connected with the supergene stage of mineral formation.

The chalcedony–calcite association includes calcite, chalcedony (up to 90%), chlorite-II, and pyrite-III, forming veinlets, nests, and accumulations that cut all the previously considered mineral associations. No gold has been established with this association. On this basis it may be assigned to the post-ore stage.

During the supergene stage, aragonite, chalcosine, cuprite, tenorite, malachite, azurite, chrysocolla, goethite, limonite, and gold were formed (table). The principal non-ore mineral of the oxidation zone is aragonite, which fills cracks and “flows” covering areas up to 1–4 m². Most walls of ancient workings (10th–11th century A.D.) are coated with aragonite, up to the complete filling of some of them by its new formations. According to exploration data, the depth of development of the oxidation zone exceeds 70 m.

At the deposit, all ore bodies have a similar composition, but the individual ore-bearing skarn zones are characterized by unequal quantitative ratios of mineral assemblages of different ages. This is due to the fact that, in separate parts of the deposit, the hypogene stages of mineral formation were manifested with differing intensity.

Conclusion

The materials presented show that the formation of the Zarkashan deposit took place in a complex tectonic setting under hypabyssal conditions. The deposit formed in several successive stages of mineral formation (table). Each stage was preceded by an intensification of tectonic movements, after which the physicochemical conditions of ore formation changed and, accordingly, the composition of the mineral assemblages changed. The formation of magnesian skarns occurred at the contacts of granitoids with dolomites as early as the magmatic stage, by a contact-infiltration process during magmatic replacement of dolomites [3, 4, etc.]. Calcareous skarns arose during the postmagmatic stage of mineral formation in contact-infiltration zones among calcite marbles. The formation of skarn zones most often took place along the boundaries of zones of bedding-plane displacement and crushing of sedimentary strata, at the contacts of marbles with hornfels horizons.

In the hydrothermal stage of mineral formation, after repeated tectonic movements, gold-ore bodies arose in the same zones. The previously formed magnesian and calcareous skarns underwent hydrothermal metamorphism and veinlet-disseminated mineralization and are paragenetically associated with gold mineralization. The mineralized zones are characterized by the presence of accumulations of serpentine, calcite, and copper-sulfide minerals that replace and cement the skarn formations.

The ore bodies are characterized by breccia and veinlet textures. They usually form elongated interbed bodies, more rarely lenses and nests in skarn zones developed in zones of bedding-plane fracturing and brecciation.

Gold mineralization in skarns should be assigned to the superimposed type according to the classification of V. A. Zharikov [2]. The deposit is characterized by an oxidation zone more than 70 meters deep, which may have influenced the redistribution of gold in the skarn-ore zones.

In its geological features and mineral composition, the deposit described resembles the gold-skarn deposits of Central Asia [1, 2] and the Sayan-Altai region [4], and is a characteristic type of gold ore for these regions. Thus, in zones of Mesozoic activation of geological structures in the regions under consideration, the discovery of Mesozoic gold-skarn mineralization may be expected.

REFERENCES

1. Kh. N. Baimukhamedov, A. S. Kurbanov. Skarn-chalcopyrite-gold-ore formation (Rabindzhan ore field). In: Ore formations and the main features of the metallogeny of gold in Uzbekistan. FAN Publishing House, Tashkent, 1968.

2. B. O. Esimov. On the distribution of scheelite and gold in the skarn-magnetite zone of one of the deposits of the Kurama Range. In: Geology, mineralogy, and geochemistry of ore regions of Uzbekistan. FAN Publishing House, Tashkent, 1969.

3. V. A. Zharikov. Skarn deposits. In: Genesis of endogenous ore deposits. Moscow, Nedra, 1968.

4. A. F. Korobeinikov, A. V. Matsyushevsky, L. B. Matsyushevskaya. Features of metasomatism and gold mineralization of the Tapsa-Kaakhem interfluve, Central Tuva. In: Problems of the geology of gold deposits. Izv. TPI, vol. 239, 1970.