Significance of oscillatory and bell-shaped growth zoning in hydrothermal garnet: Evidence from the Navachab gold deposit, Namibia

Abstract

Garnet from hydrothermally altered calcsilicate rocks of the Navachab gold deposit, Namibia, records complex zoning patterns that reflect the alteration history during mineralization. In order to study the origin of the zoning patterns, the major and trace element geochemistry was analyzed by EMP- and LA-ICP-MS techniques. Garnet essentially forms spessartine–grossular–almandine solid solutions, and records a two-stage growth history. Stage I garnet usually forms the cores of porphyroblasts, and records a bell-shaped zoning profile. This garnet generation is highly enriched in spessartine, and documents a progressive depletion of manganese in the fluid. Stage II garnet overgrows the earlier garnet, and marks the onset of increased fluid–rock interaction and hydrothermal gold mineralization. In the semi-massive sulfide breccias, stage I garnets were dissolved, and the overgrowing stage II garnet displays gradational compositional variations. In the surrounding mylonites, stage II garnet is characterized by oscillatory zoning profiles, documenting the episodic supply of hydrothermal fluid in these rocks. All garnet grains display similar REE pattern, suggesting a formation under similar P–T–X conditions. The grains are enriched in HREE, depleted in LREE, and have a weak to pronounced negative Eu-anomaly. The sum of the REE increases with increasing Ca, pointing to a progressive increase of these elements in the ore fluid. In conjunction with considerations on the mechanisms of hydrothermal fluid flow, the data demonstrate that the type of zoning pattern developed in hydrothermal garnet is mainly controlled by the permeability of the rock: bell-shaped growth zoning may develop in hydrothermal systems during pervasive hydrothermal alteration. Oscillatory zoning requires that the fluid flow is episodic, and possibly accompanied by periods of reduced growth rates between individual pulses of chanellized hydrothermal fluid flow.