FEATURES OF METASOMATISM AND GOLD MINERALIZATION IN THE TAPSA-KAA-KHEM INTERFLUVE OF CENTRAL TUVA

A. F. KOROBEINIKOV, A. V. MATYUSHEVSKII, L. B. MATYUSHEVSKAYA (TPI)

Within the area, Lower Cambrian predominantly carbonate and effusive rocks are widespread, forming folds of latitudinal strike. The folded structures are complicated by sublatitudinal and submeridional faults, which determine the block structure of the area and control intrusions of the hyperbasite and Tannuola granodiorite complexes [3]. In the contact zones of the granodiorite massifs, gold-bearing magnesian and calcareous skarns are developed. The sequence of formation of contact-metasomatic and hydrothermal rocks is presented in Table 1.

The distribution of contact-metasomatic rocks was determined by the contact of the pluton, by faults, and by fracture zones. Skarns develop at the expense of carbonate and aluminosilicate rocks. Among them, two types are clearly distinguished: magnesian skarns, formed in the postmagmatic stage and cutting the magnesian rocks. In turn, the calcareous skarns are cut by thin veins of aplites and pegmatites. The processes of contact metasomatism ended with the formation of small magnetite deposits and amphibole-chlorite and serpentine rocks.

Magnesian skarns usually occur as thin selvages bordering outcrops of intrusive rocks among dolomitic and brucitic marbles. The form of the skarn bodies is lens-shaped, nest-like, or irregularly bedded. The dimensions of the bodies range from 2–3 to 10–20 meters in width, with a length of 5–30 meters. The boundaries with diorites are distinct, while on the side of the carbonate rocks they are more often gradual. Their internal structure is zonal: at the contact with diorites, essentially spinel rocks predominate; these are replaced by pyroxene-pargasite rocks, then by phlogopite rocks, and finally by fassaite-grossular skarns and skarnified marbles (fassaite-dolomite and forsterite-dolomite calciphyrs). The structure of individual skarn zones is very heterogeneous: very often spinel and pargasite appear within the pyroxene mass, giving the rock a spotted appearance. In other cases, the skarns are composed of spinel and pyroxene with an admixture of gehlenite.

Analysis of the material balance during metasomatism showed that the origin of magnesian skarns is due to processes of alumomagnesian metasomatism of carbonate rocks. In this case, into the metasomatism zones

Si, Al, Fe, and especially Mg were introduced, while Ca and CO₃ were removed; moreover, the introduction of material predominated over removal (Table 2). The increase in the concentrations of Si, Al, Mg, and Fe in the zones of metasomatism was apparently caused both by their mobilization from the adjoining effusive rocks and, especially, by introduction by solutions from the deep magmatic source that formed the granodiorite pluton. In general, skarn formation proceeded by an infiltration–bimetasomatic pathway. Among the paragenetic associations described, two groups are clearly distinguished: 1) spinel + fassaite + pargasite + phlogopite + xanthophyllite (hysterogenic) ± magnetite ± forsterite (as an impurity) ± dolomite and calcite (as a relic); 2) fassaite ± gelenite ± dolomite + calcite (as a relic) + serpentine (hysterogenic). Ultimately, an association arises corresponding to spinel–pyroxene–pargasite, spinel–pyroxene–gelenite skarns and dolomite–pyroxene calciphyres.

Calcareous skarns are formed at the expense of carbonate and aluminosilicate rocks in zones of infiltration of postmagmatic solutions of the near-contact aureole of the granodiorite intrusion. Among them, endoskarns and exoskarns are distinguished. The former are formed at the expense of diorites of the marginal facies of the intrusion and are composed of diopside-augite, grossular, scapolite, and relict plagioclase. As accessory minerals they contain sphene, apatite, zircon, orthite, hysterogenic tremolite-actinolite, magnetite, epidote, chlorite, and serpentine. Exoskarns arise in place of limestones, porphyrites, tuffs, and magnesian skarns and consist of diopside, salite, garnet, vesuvianite, wollastonite, hornblende, and relict carbonates. Among secondary minerals, actinolite, magnetite, hematite, epidote, chlorite, serpentine, talc, and prehnite are developed in them.

Calcareous skarns most often form lenticular, sheet-like, and vein-like bodies extending along contacts of intrusive bodies and zones of tectonic faults, and give discontinuous selvages along contacts of diorites with limestones and, less commonly, porphyrites. The dimensions of skarn bodies reach tens and the first hundreds of meters in strike length, with thicknesses from 3–5 to 30–60 meters. Skarnification of essentially carbonate rocks proceeds more intensely than that of effusive rocks. On the basis of quantitative ratios of the principal rock-forming minerals, it appears possible to distinguish pyroxene, garnet, and garnet–pyroxene skarns.

The origin of calcareous skarns is due to processes of calcareous–magnesian–ferruginous infiltration metasomatism, with a subordinate role of bimetasomatic phenomena. In this case, the formation of pyroxene and garnet skarns in place of magnesian metasomatites was caused by the introduction of Si, Fe, and Ca and the removal of the other petrogenic elements, and by their redistribution in the marginal parts of the zones of metasomatism (Table 2). Skarnification of diorites was caused by the introduction of Fe, Ca, and Mg and the removal of the other components by solutions. Conversely, the occurrence of apocarbonate skarns was accompanied by the introduction of Si, Al, Fe, and Mg and the removal of excess calcium.

Post-skarn metasomatites (magnetite-hematite ores) are most fully represented in zones of infiltration metasomatism in areas of skarn bodies that underwent intense crushing along tectonic zones. In many cases, magnesian and calcareous skarns are almost completely replaced by lower-temperature mineral formations. Among them one may distinguish (in order of age sequence): 1) magnetite ores, 2) tremolite-actinolite, 3) chlorite-serpentine, 4) quartz-cal—

Table 1

Classification scheme of contact-metasomatic and hydrothermal formations of gold-ore occurrences in Central Tuva

calcite-epidote and 5) quartz-hematite rocks (Table 1). Usually, small magnetite lenses are located in the center of the zones, bordered by tremolite-actinolite, chlorite, and serpentine rocks.

Serpentinization of skarns and skarnified limestones is widely developed. Serpentine replaces fassaite, dolomite, spinel, and tremolite, forming spotted disseminations and nest-like and lenticular segregations within a calcite aggregate. The process of serpentinization is accompanied by the introduction, by hydrothermal solutions, of magnesium into zones of skarns and near-skarn rocks and by the removal of the remaining petrogenic elements (Table 2).

Quartz–hematite formations among carbonate rocks and skarns are confined to rock fracturing. Possibly, they are the very latest products of the activity of solutions enriched in iron and silica during the period of skarn leaching. Along fractures, skarnized limestones are gradually replaced by quartz of hornfels-like structure and by hematite, up to the formation of bimineral quartz–hematite rocks. The thickness of such bodies does not exceed a few meters, and their extent is the first tens of meters. The bodies consist of quartz, hematite, relict calcite, dolomite, pyroxene, and serpentine. The texture of the bands is banded, massive, or reticulate, owing to the alternation of essentially hematite and quartz bands.

Carbonate–quartz–gold–sulfide ores

The hydrothermal gold-ore process was superimposed on the skarns after the latter had been crushed. During the hydrothermal stage several new mineral associations arose: 1) carbonate–quartz–gold–sulfide, 2) gold–quartz, 3) calcite–laumontite, 4) carbonate, and 5) beresite (Table 1). The last two are near-ore formations, the fourth being developed in gold-bearing skarns, and the fifth in the contacts of gold–quartz veins cutting granodiorites. In general, two types of ores are clearly distinguished: 1) calcite–gold–copper-sulfide veinlet-disseminated ores in skarns and magnetite lenses, and 2) quartz–gold-ore vein ores in granodiorites and, more rarely, in skarns.

Within the studied area, all known gold-ore occurrences are situated in the near-contact zone of the granodiorite intrusion and are confined mainly to skarn bodies, magnetite–hematite lenses, and, more rarely, quartz veins of the apical part of the pluton.

The gold–sulfide veinlet-disseminated type of mineralization occurs preferentially in garnet–pyroxene skarns and magnetite–actinolite–chlorite lenses. The ore bodies are zones of carbonatized and silicified skarns penetrated by thin veinlets and disseminated impregnation of carbonates, quartz, sulfides, and gold. The shape of the ore bodies is determined by the character of tectonic fracturing and porosity controlling the mineralization. In the general case it corresponds to a nest-like and lenticular form.

The gold–quartz type of ores is represented by individual quartz veins occurring in granodiorites. In the selvages of the quartz veins the rocks have undergone beresitization. The extent of the zones of near-ore alteration of the rocks usually corresponds to the dimensions of the quartz veins and amounts to the first tens of meters. The beresites are composed of carbonates, muscovite, pyrite, quartz, and rare albite and chlorite. The formation of these rocks preceded vein formation.

Thus, the staged character of the postmagmatic processes, the nonuniform heating of the enclosing rocks, and the presence of a sharply expressed near-intrusive temperature gradient determined the appearance of different-temperature metasomatites and their observed distribution within the ore field. The high-temperature magnesian skarns of the magmatic stage are confined directly to the contacts of granodiorite bodies with carbonate rocks, whereas the lower-temperature calcareous skarns replace them toward the continental rocks and appear along tectonic zones.

Hydrothermal formations, on the contrary, are controlled not by the degree of heating of the enclosing medium, but by the presence of conducting channels (fractures, boundaries of heterogeneous beds, porosity, etc.) and are located—

occur both within the contact-metasomatic aureole and beyond its limits (quartz–gold-ore veins in the apical part of the intrusive body). The probable commonality of the source and pathways of movement of the postmagmatic solutions led to the superposition of late metasomatic and hydrothermal-vein associations upon already formed skarn bodies.

The features of the contact-metasomatic and hydrothermal formations of the area are as follows:

1. The presence of magnesian and calcareous skarns, which indicates a rather long duration of the contact-metasomatic processes. Thermodynamic calculations and thermometric studies show that they formed at temperatures of 830–600°C and 600–400°C, respectively.

2. The spatial combination in the skarn zones of specific associations: spinel + fassaite + pargasite, arising under conditions of considerable depths; spinel–gehlenite and brucite associations, forming under conditions of shallow depths [1]. Their high-temperature paragenesis, mode of occurrence, and distribution make it possible to suppose the formation of contact-metasomatic and hydrothermal complexes in the hypabyssal depth zone, transitional to the mesoabyssal.

3. Intense serpentinization of skarns and near-skarn rocks. According to thermodynamic calculations and experimental data [2, 4], the temperature of the process may be taken as 300–200°C, which corresponds to the minimum temperature of replacement of pyroxenes and quartz–dolomite rocks by serpentine.

4. The widespread development of replacements of essentially carbonate rocks by quartz–hematite rocks. As the upper temperature limit for silicification of dolomitized limestones, one may take the minimum temperature of tremolite formation from dolomite and quartz, which is 150°C [4].

5. The connection of gold mineralization with a new hydrothermal stage superimposed on the skarn stage, during which actinolite–chlorite–talc–quartz–carbonate rocks formed, carrying pyrite, pyrrhotite, chalcopyrite, bornite, and galena. Gold occurs intergrown with andradite, pyroxene, magnetite, actinolite, chlorite, and especially quartz, calcite, bornite, and chalcopyrite, which it encrusts or replaces.

The ore-hosting skarns and diorites have undergone intense carbonatization and beresitization. Judging from the character of the near-ore processes and the decrepitation temperatures of quartz, it may be assumed that the temperatures of the mineral-forming solutions were 320–180°C. The most promising for gold content are silicified, carbonatized, and sulfidized portions of skarns in tectonic zones with repeated intramineralization movements.

1. At the deposit, an oxidation zone is widely developed, especially in areas of faults and zones of increased fracturing of the rocks (to depths of 50–100 and more meters), which may have influenced the distribution of gold in the ore bodies.

The appearance of secondary copper minerals in the oxidation zones of skarns (chrysocolla, malachite, azurite) may serve as a reliable prospecting indicator for copper-sulfide gold-bearing mineralization.

Change in composition and balance of matter during contact metasomatism of carbonaceous and aluminosilicate rocks

755-a — unaltered diorite of the Tannu-Ola granitoid complex (hole No. 7); 417 — marble with dioptase, scheelite, carbonate, and garnets (Izvestkovy quarry, northeastern zone); 313 — dolomitized marble (southwestern zone); 51-a — spinel-pyroxene-humite-phlogopite skarn at the expense of marbles (hole No. 8); 51-b — talc-ferruginous altered skarn at the expense of magnetite skarns; 51-v — talc-ferruginous altered skarn at the expense of magnetite skarns (sample 51-a2); 51-g — talc-ferruginous altered skarn at the expense of magnetite skarns (sample 51-a1); 837 — diopside exoskarn, formed in place of marble; 403-3 — granitoid; 403-8 — granitoid; exoskarn at the site of marble (Berezovaya zone); 755-b — skarnized diorite (hole No. 7); 7125 — actinolite mineralized exoskarn of intensely altered diorite; 6-2 — serpentinizated skarn from pyroxene talc; 6-b — skarnized marble.

Chemical analyses were performed at the Central Laboratory of the Krasnoyarsk Geological Administration.

Table 2

[[unclear: beginning]] rocks of Tardan (in grams per 100 cu. cm of rock)

References

1. Z. A. Zharnikova. Geology and metasomatic phenomena of skarn–polymetallic deposits of Western Karamazar. Moscow, Publishing House of the Academy of Sciences of the USSR, 1959.

2. V. A. Nikolaev, V. V. Delivo-Dobrovolsky. Fundamentals of the theory of the processes of magmatism and metamorphism. Moscow, Gostoptekhizdat, 1961.

3. A. M. Khazagarov, A. I. Neimark, K. M. Kilichakov. Gold mineralization in the skarns of the Tapsa-Kaa-Khem interfluve. Materials of the geological conference, Krasnoyarsk, 1966.

4. I. P. Shcherban. On the temperatures of formation of quartz–dolomite and quartz–magnesite rocks. In: Mineralogical Thermometry and Barometry, vol. 1. Moscow, Nauka Publishing House, 1968.