On the Role of Basaltoid and Granitoid Magmatism in Gold Mineralization
Abstract Generated abstract
This paper examines the relative roles of basaltoid and granitoid magmatism in gold mineralization, using neutron activation data on average gold contents in igneous rocks from the Altai Sayan folded region. It compares gold concentrations in intrusive and effusive equivalents of basic and granitoid magmas, arguing that small differences in average content may indicate the amount of gold liberated during deep crystallization. The author further relates higher gold contents in basic rocks to mineral structural controls, especially the association of gold with iron and magnesium silicates, and suggests that granitoid related gold mineralization is mainly significant where intrusions interact with volcanogenic shale formations already enriched in gold.
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ON THE ROLE OF BASALTOID AND GRANITOID MAGMATISM IN GOLD MINERALIZATION
Yu. G. Shcherbakov (IGiG, Siberian Branch, Academy of Sciences of the USSR)
The potential gold-bearing capacity of magma is determined by the amount of gold that can be removed in the process of formation of an ore-bearing intrusion. The role of basic and granitoid magmatism can be judged from the regular, specific character—established by neutron-activation analysis—of the average gold contents in the corresponding types of igneous rocks. Using the example of the Altai–Sayan folded region, average gold contents are established in gabbroids on the order of 0.004 g/t, and in basic effusives about 0.006 g/t. The excess of gold contents in effusive rocks relative to their deep-seated equivalents by 0.002 g/t means the possibility of liberation of about 5 tons of gold from 1 km³ of magma that recrystallized at depth. The same order of ratios of gold contents is reached between deep-seated and erupted equivalents of granitoid magma. The higher gold contents in basic rocks compared with acidic ones are explained by the preferential association of gold with the maximally covalent structures of iron–magnesian silicates and by the more limited possibility of retaining gold in the more ionic structures of feldspars.
Granitoid massifs accompanied by gold mineralization in the Altai–Sayan folded region are predominantly confined to volcanogenic–shale deposits, characterized by an average gold content, elevated in comparison with the granitoids themselves, by approximately the same 0.002 g/t, sometimes more. Outside formations with an elevated gold clarke, batholith-type granitoid massifs, as a rule, are not gold-bearing. Upward along the section from the named formations, gold mineralization usually penetrates along deep faults for relatively short distances, accompanying fissure intrusions of granitoids and, more rarely, small intrusions of basic composition.