Gold Potential of Intermediate Intrusive Rocks and Geochemical Conditions of Gold Redistribution
Abstract Generated abstract
This paper examines the relationship between gold mineralization, intermediate intrusive rocks, and postmagmatic metasomatic processes in the South Primorye and Selemdzha gold-bearing zones of the Pacific ore belt. Using spectrochemical determinations checked in part by radioactivation analysis, the authors compare gold contents in diorites, granodiorites, altered rocks, minerals, and contact zones. The study suggests that albitization and sodium-rich metasomatic environments can promote redistribution and local concentration of gold, whereas increased potassium and high carbon dioxide activity may be unfavorable. Results from both regions support the view that destruction of primary mineral concentrators during hydrothermal and metasomatic alteration is a key factor in gold mobility.
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Gold Potential of Intermediate Intrusive Rocks and Geochemical Conditions of Gold Redistribution
V. G. Moiseenko, G. S. Nechkin
(DVGI, Siberian Branch, Academy of Sciences of the USSR)
Genetic links between gold mineralization and magmatism, and the elements of the endogenous geochemistry of gold, are among the most important problems of minerageny. Gold mineralization is most often spatially associated with granitoids of increased basicity and with bodies of albitites [2, 4, 5, 6]. The authors of the present article studied basic and intermediate rocks of massifs and small intrusions of the South Primorye and Selemdzha (Priamurye) gold-bearing zones of the Pacific ore belt.
In one of the ore fields of the South Primorye zone, special attention was given to hornblende diorites and biotite granodiorites of a Paleozoic intrusive complex belonging to the gabbro-diorite–plagiogranite formation. The gabbros and diorites belong to the early phases of formation of this intrusive complex, and the granodiorites and plagiogranites that cut them belong to the late phase. The diorites form isolated massifs with areas of several square kilometers, located both in immediate proximity to the placer deposits being worked and at a considerable distance from them. In the diorites, near the contact with granodiorites of the late phase, ore bodies of the gold-quartz formation are situated.
To study the distribution of gold in intrusive rocks and their contact zones, the spectrochemical method for determining Clarke contents, proposed by N. T. Voskresenskaya and N. F. Zvereva (1965), was used. A number of the values obtained were checked by the radioactivation method.
Diorites from massifs that do not contain gold-bearing quartz veins consist of andesine and oligoclase-andesine, hornblende, biotite, and quartz, and have the following numerical characteristics: \(a = 8.5—12.9\); \(c = 8.5—2.9\); \(в = 23.8—14.1\); \(S = 59.2—70.1\); \(Q = -7.1—(+11.5)\), and to one degree or another have undergone propylitization. It has been established that relatively weakly propylitized diorites contain gold in amounts close to the Clarke value—\(2.7 \cdot 10^{-7}\%\) by weight, while granodiorites contain somewhat smaller amounts—\(0.4 \cdot 10^{-7}—1.1 \cdot 10^{-7}\%\) by weight. At the same time, gold was determined in other rocks and rock-forming minerals of the complex (Table 1).
Autometasomatic processes (propylitization of the chlorite-albite stage) are manifested unevenly in space in the rocks. The most characteristic change is albitization of plagioclase, often with the release of calcite, and chloritization of dark-colored minerals. Sericite, muscovite, and microcline also develop here, and metasomatic lenticular quartz-calcite veinlets arise as well.
Gold content in some rocks and minerals
Table 1
| Sample no. | Rock, constituent minerals | No. of determinations | Gold content, \(n \cdot 10^{-7}\), wt. % | \(Na_2O\), wt. % | \(K_2O\), wt. % | \(\dfrac{Na_2O}{Na_2O + K_2O} \cdot 100\) |
|---|---|---|---|---|---|---|
| K-21 | Hornblende gabbro | 2 | 3.6; 4.2 | — | — | — |
| K-21 | Hornblende \((2V N_p = 84^\circ;\ C N_g = 28^\circ;\ f = 10)\) | 1 | 25.0 | 1.23 | 1.23 | 79 |
| K-21 | Saussuritized plagioclase, marginal zones of composition \(Ab_{73}An_{27}\) \((\perp(010)B N_g = 8^\circ;\ B N_p = 86^\circ;\ B N_m = 84^\circ)\) | 2 | 580.0; 560.0 | 5.19 | 2.70 | 74 |
| K-211 | Aplite | 2 | 11.0; 12.0 | — | — | — |
| K-211 | Plagioclase \(Ab_{73}An_{27}\) \((\perp(001)B N_g = 80^\circ;\ B N_p = 78^\circ;\ B N_m = 14^\circ)\) | 2 | 13.0; 18.0 | 5.74 | 5.60 | 61 |
| K-211 | Orthoclase \((2V N_p = 64^\circ;\ B N_g = 89^\circ;\ B N_p = 89^\circ;\ B N_m = 2^\circ)\) | 1 | 4.4 | 2.76 | 13.9 | 23 |
| K-211 | Quartz | 2 | 7.0 | — | — | — |
| K-210 | Trondhjemite | 2 | 13.0; 15.0 | — | — | — |
| K-210 | Quartz | 2 | 27.0; 68.0 | — | — | — |
| K-216 | Trondhjemite | 2 | 14.0; 14.0 | — | — | — |
Some petrochemical features of the manifestation of propylitization of the lower stages in granodiorite and diorite (Table 2) indicate an increase in the gold content of the rock during its albitization, i.e., the possibility of redistribution and concentration of gold in an essentially sodium medium. With the replacement of sodium by potassium in the rock, the amount of gold may decrease. Thus, to characterize metasomatic changes of the potassium series, complete chemical analyses were used (Table 3, samples K—73^a, K—73^b), which showed a dependence of gold content on the amount of sodium in the sum of the alkalis. However, under these conditions there is apparently no correlation between gold and trivalent iron, as was established by other investigators [5].
Table 2
Petrochemical characteristics of the propylitization process
| Sample No. | Intrusive and metasomatic rock formed from it | Gold content, n · 10^-7 % by weight | Na₂O, wt. % | K₂O, wt. % | $\dfrac{\mathrm{Na_2O}}{\mathrm{Na_2O}+\mathrm{K_2O}}\cdot 100$ |
|---|---|---|---|---|---|
| K—65 | Granodiorite (plagioclase No. 28) | 0.44 | 3.17 | 2.54 | 65 |
| K—66 | Quartz-albite rock (plagioclase No. 7) | 3.9 | 5.54 | 1.79 | 82 |
| K—69^a | Granodiorite (plagioclase No. 25) | 1.4 | 3.24 | 2.61 | 65 |
| K—69^b | Quartz-calcite-sericite-albite rock (plagioclase No. 5) | 6.2 | 4.86 | 2.13 | 78 |
| K—76^a | Albitized granodiorite | 4.7 | 4.99 | 1.06 | 87 |
| K—76^b | Quartz-calcite vein | 2.3 | — | — | — |
| K—80 | Diorite (plagioclase No. 45) | 2.7 | 3.78 | 1.93 | 75 |
| K—80^a | Albite with quartz and sericite (plagioclase No. 6) | 12.0 | 5.17 | 0.23 | 98 |
| K—74^a | Granodiorite (plagioclase No. 26) | 4.6 | 4.32 | 3.36 | 66 |
| K—74^b | Quartz-microcline-sericite-albite rock | 2.3 | 2.20 | 9.51 | 26 |
The diorites of massifs bearing mineralization contain gold in amounts of $10 \cdot 10^{-7}$—$40 \cdot 10^{-7}$ wt. %. The metasomatically altered diorites are very heterogeneous gneiss-like rocks, often consisting of separate alternating bands of normal and leucocratic composition. The bands of normal composition are made up of andesine (up to 58%), hornblende (up to 20%), biotite (3–5%), and quartz (up to 2%). In the leucocratic layers the plagioclase is represented by oligoclase, albite-oligoclase, and almost pure reaction albite. The amount of quartz increases sharply at the expense of hornblende, reaching 20% and more. The gneiss-like structure in the diorites and the phenomena of sodium metasomatism are associated with the intrusion of granodiorite bodies of a late phase.
The sequence of change in the chemical composition of the diorites, which proceeded under the influence of magmatic solutions of the granitoids and on the whole corresponded to the process of granitization, is shown in Table 3 (samples K—13^a, K—13^a/1, K—12). Here again a dependence of gold content on the character of alkali distribution in the rock is established, and the presence of a correlation between Au—Fe₂O₃ is also confirmed.
Table 3
Change in gold content in rocks during metasomatism
| Sample no. | “Formula” of the rock according to T. Barth, sequence of compositions | Gold content, n. \(10^{-7}\%\) wt. | ||
|---|---|---|---|---|
| K—73a | Southern Primorye zone Propylitization of granodiorites \(K_{33} Na_{84} Ca_{7} Mg_{15} Mn_{1} Fe_{25}^{2+} Fe_{5}^{3+} Al_{150} Ti_{5} Si_{613} (O_{1390}OH_{10})\) |
7.9 | 71 | 8.0 |
| K—73б | \(K_{52} Na_{71} Ca_{13} Mg_{87} Mn_{0} Fe_{11}^{2+} Fe_{4}^{3+} Al_{143} Ti_{11} Si_{643} (O_{1593}OH_{7})\) | 6.0 | 56 | 16.6 |
| K—13a | Granitization of diorites \(K_{16} Na_{54} Ca_{79} Mg_{98} Mn_{1} Fe_{74}^{2+} Fe_{13}^{3+} Al_{99} Ti_{12} Si_{432} (O_{578}OH_{22})\) |
10.0 | 71 | 7.9 |
| K—13a/1 | \(K_{18} Na_{99} Ca_{63} Mg_{84} Mn_{1} Fe_{45}^{2+} Fe_{11}^{3+} Al_{221} Ti_{11} Si_{501} (O_{580}OH_{20})\) | 20.0 | 83 | 10.4 |
| K—12 | \(K_{17} Na_{103} Ca_{92} Mg_{111} Mn_{1} Fe_{9}^{2+} Fe_{2}^{3+} Al_{231} Ti_{2} Si_{531} (O_{588}OH_{12})\) | 46.0 | 86 | 11.7 |
| I—484 | Selemdzha zone Propylitization of diabase porphyrites \(K_{16} Na_{45} Ca_{78} Mg_{133} Mn_{2} Fe_{39}^{2+} Fe_{21}^{3+} Al_{159} Ti_{2} Si_{492} (O_{1512}OH_{55})\) |
21.0 | 81 | 20.4 |
| I—495 | \(K_{20} Na_{47} Ca_{77} Mg_{128} Mn_{2} Fe_{59}^{2+} Fe_{27}^{3+} Al_{147} Ti_{3} Si_{480} (O_{1512}OH_{55})\) | 7.8 | 70 | 18.8 |
| H—120б | \(K_{29} Na_{16} Ca_{91} Mg_{127} Mn_{1} Fe_{62}^{2+} Fe_{10}^{3+} Al_{150} Ti_{4} Si_{470} (O_{1553}OH_{47})\) | 5.5 | 70 | 7.7 |
| I—242 | \(K_{33} Na_{32} Ca_{111} Mg_{85} Mn_{1} Fe_{57}^{2+} Fe_{7}^{3+} Al_{179} Ti_{6} Si_{488} (O_{1542}OH_{58})\) | 2.7 | 60 | 5.4 |
Gold-bearing quartz veins are noted in the field of development of the most intensely granitized diorites. In the leucocratic rocks surrounding the veins, with plagioclase \(Ab_{90}An_{10}\)—\(Ab_{70}An_{30}\), chloritized biotite, and a predominance of sodium in the alkalis,
\[ \frac{Na_2O}{Na_2O + K_2O} \times 100 = 81\text{—}83 \]
the gold content drops sharply, reaching \(4.0 \cdot 10^{-7}\)—\(5.0 \cdot 10^{-7}\%\) by weight. This is evidently connected with the leaching of gold from the diorites under the action of hydrothermal sodium solutions.
The emerging regularities of the distribution and redistribution (concentration and dispersion) of gold in massifs of intermediate rocks with superimposed metasomatic changes were confirmed in the study of massifs and small intrusions of the Selemdzha zone. The gold-ore bodies are located in metamorphic rocks intruded by quartz diorites of substantially sodic composition \((Na_2O = 2.23\%,\ K_2O = 1.17\%\),
\[ \frac{Na_2O}{Na_2O + K_2O} \cdot 100 = 74). \]
The diorites contain gold in amounts exceeding the Clarke value \((12 \cdot 10^{-7}\text{—}13 \cdot 10^{-7}\%\) by weight) and have not undergone substantial postmagmatic alteration.
It is interesting to note that in the same metallogenic zone, tin-tungsten mineralization is associated with substantially potassic granitoids of the Ezoisk complex \((Na_2O = 3.38\%;\ K_2O = 4.37\%;\)
\[ \frac{Na_2O}{Na_2O + K_2O} \cdot 100 = 54), \]
containing \(1.6 \cdot 10^{-7}\%\) gold.
In dikes of diabase porphyrites that underwent greenschist alteration, a different type of metasomatic metamorphism is observed than in the South Primorye zone. The porphyrites underwent chloritization and carbonatization with simultaneous removal of gold (Table 3, samples I—484, I—495, I—120б, I—242). This explains the increase in gold content not only in contact-metamorphosed rocks, but also in quartz veins where they are cut by small intrusions. A decrease in the concentration of gold in the rocks is accompanied by a reduction in the proportion of sodium in the total alkalis.
On the basis of the foregoing, it may be assumed that the destruction of primary mineral concentrators during one or another type of postmagmatic process leads to the redistribution of gold. In the general case, favorable geochemical conditions for its concentration are an increase in the sodium content in pore solutions, which agrees with existing ideas on the forms of migration of gold in hydrothermal systems [7]. The same influence is exerted by an increase in the ferrous character of the medium. High activity of carbon dioxide is unfavorable for the concentration of gold.
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