On Certain Patterns in the Distribution of Gold in Basic and Ultrabasic Rocks
Abstract Generated abstract
This study examines gold distribution in basic and ultrabasic intrusive rocks from the Far East and the Indian Ocean, including dunite, harzburgite, gabbro, peridotite, kimberlite, layered nickel-bearing intrusions, and related olivine inclusions. Using spectrochemical analysis, the authors compare gold contents across rock formations and alteration settings, and assess its behavior during magmatic differentiation, autometasomatism, serpentinization, hydrothermal overprinting, and granite-related reworking. The results suggest that hyperbasite formations contain gold above the Clarke value and higher than gabbroid formations, that gold is largely inert during differentiation and some alteration processes but accumulates with sulfide copper-nickel mineralization, and that granite-associated alteration can strongly remove gold from ultrabasic rocks.
Full Text
ON CERTAIN REGULARITIES IN THE DISTRIBUTION OF GOLD IN BASIC AND ULTRABASIC ROCKS
S. A. Shcheka, V. G. Moiseenko (Far Eastern Geological Institute, Far Eastern Branch, Siberian Division, Academy of Sciences of the USSR)
A study has been made of the distribution of gold in basic and ultrabasic intrusive rocks of various formations of the Far East and the Indian Ocean: dunite–harzburgite, gabbro–peridotite, kimberlite, nickel-bearing layered intrusions, and nickel-bearing dike cortlandite–norite. The first is considered to be derived from hyperbasitic magma, the others from basaltoid magma. Olivine inclusions in alkaline basalts and kimberlites were also analyzed. An attempt was made to establish the main regularities in the distribution of gold in primary rocks and its behavior during the differentiation of intrusions, during autometasomatic processes, and during superimposed hydrothermal processes. Gold was analyzed by a spectrochemical method with a sensitivity of \(1 \cdot 10^{-7}\) wt. %.
The rocks richest in gold (hyperbasites and gabbroids) of the hyperbasitic dunite–harzburgite formation contain \(9—40 \cdot 10^{-7}\%\), with an arithmetic mean of \(25 \cdot 10^{-7}\%\). In rocks of gabbroid formations these values are lower—\(1—9 \cdot 10^{-7}\%\), with an arithmetic mean of \(5 \cdot 10^{-7}\%\). Gold concentrations in olivine inclusions of alkaline basalts fall within a narrow interval—\(7—14 \cdot 10^{-7}\%\)—and are close to its contents in the enclosing effusives, \(5—16 \cdot 10^{-7}\%\).
Observations in the layered dunite–troctolite massif of Lukinda (Stanovoy Range) showed that gold concentrations in barren rocks along the vertical section of the intrusion practically do not change \((4—5 \cdot 10^{-7}\%)\); they are somewhat lower in vein formations \((1.5—2.5 \cdot 10^{-7}\%)\) and increase markedly in rocks with sulfide copper–nickel mineralization \((15—50 \cdot 10^{-7}\%)\).
During autometasomatic amphibolization of hyperbasites, gold is practically inert; during serpentinization not associated with granites, its concentrations decrease somewhat—by \(3—5 \cdot 10^{-7}\%\). The same processes caused by granite veins lead to removal of gold: unaltered hyperbasite—\(5—5 \cdot 10^{-7}\%\), serpentinized hyperbasite—\(1 < 1 \cdot 10^{-7}\%\), granite—\(3—< 1 \cdot 10^{-7}\%\).
The preliminary data obtained in this work show that the concentration of gold in rocks of hyperbasite formations (as exemplified by the Far East) significantly exceeds its Clarke value \((5 \cdot 10^{-7}\%)\) in basic and ultrabasic rocks and, in this respect, is close to hypersthene meteorites. In rocks of gabbroid formations, the conten-
gold contents are Clarke values. Gold is practically inert in the process of differentiation and autometasomatic alteration of basic–ultrabasic intrusions; it accumulates in the ore residue and is almost completely removed when ultrabasic rocks are reworked by granites. In terms of gold contents, olivine inclusions in basalts and kimberlites are closer to derivatives of basaltic magmas, which is unlikely to be characteristic of fragments of the Earth’s peridotite layer.