Petrogenesis of the late Tonian arc-related Um Balad gabbro-diorite complex (Egypt) and insight into its spatially related orogenic gold mineralization

ABSTRACT

The gabbro-diorite complex of the Um Balad prospect hosts lode gold mineralization. The complex is dated at 723 ± 4 Ma using the LA-ICP-MS U-Pb zircon method and is correlated with the late Tonian-early Cryogenian subduction-related magmatic stage during the evolution of the Arabian-Nubian Shield. The gabbro-diorite complex evolved through the crystallization of a calc-alkaline magma and the subduction signature of this magma is verified by primitive mantle-normalized trace element patterns that show enrichment in large ion lithophile elements, U, and Th relative to high field strength elements as well as negative Nb and Ti anomalies. The high La/Yb_cn (3.1–9.4) and Ta/Yb (0.12–0.41) ratios are consistent with a continental arc rather than an oceanic arc system. Amphibole chemistry indicates that this complex might have crystallized under a moderately oxidizing condition from a hydrous magma (>6 wt% water content) at temperature and pressure estimates of about 800°C and 3 kbar, respectively. Gold mineralization in the Um Balad prospect is confined to structurally controlled massive quartz±carbonate veins and surrounding alteration halos. Alteration in the prospect is represented by localized sericitization and carbonation as well as pervasive chlorite-sericite alteration. The alteration halos are characterized by enrichment in K and Rb and depletion in Ca and Sr compared to their host rocks. The veins of the prospect are related to lower order extensional fractures associated with the regional first order transpressional Najd Fault System. The high Fe contents of the gabbro-diorite complex represent a suitable chemical trap for gold through sulfidation of the host rocks. Supergene alteration resulted in the formation of goethite in association with atacamite and chrysocolla. Free mill gold is associated with these supergene phases, which were deposited in near neutral to slightly alkaline conditions.

KEYWORDS:

• Arabian-nubian shield
• continental arc
• gabbro-diorite complex
• orogenic lode gold
• supergene alteration

Research highlights

• The gabbro-diorite complex (723 Ma) of the Um Balad prospect hosts lode gold mineralization.

• Geochemistry of the complex is consistent with a continental arc tectonic setting.

• Gold lodes are structurally controlled and related to the Najd Fault System.

• Alteration halos are characterized by enrichment in K and Rb and depletion in Ca and Sr.

• Visible gold in the prospect formed during supergene alteration of sulfide.

Acknowledgments

This research was supported by the Alexander von Humboldt postdoctoral fellowship to the first author. This research would have been not possible without the support of Jens Gutzmer. Appreciation goes to Sabine Haser, Kai Bachmann, and Matthias Bauer for their help during SEM-EDX analyses. Thanks are extended to Ramadan Abdelaziz and Peter Segler for their help during zircon separation. Special thanks are given to the Nuclear Material Authority, especially Hossam Khamees for organizing our accommodation in the Hurgada Camp during the field work. Editorial comments from Robert Stern and considerations raised by two anonymous reviewers are appreciated. Thorough and constructive comments from Mohamed El-Bialy and Scott A. Whattam are gratefully acknowledged.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed here.

Additional information

Funding

This work was supported by the Alexander von Humboldt postdoctoral fellowship [postdoctoral fellowship].