The identification of types of gold deposits is economically relevant for many reasons. There are two main types of gold deposits, namely, orogenic gold systems (OGS) and intrusion-related gold systems (IRGS), each requiring its own approach to prospecting. OGS appear to be related to regionally metamorphosed terranes and result from the processes of compression deformation in convergent margins (Groves et al., 1998). IRGS show close spatial and genetic relationship with felsic magmatic intrusions (Elshourbagi & Fan, 2021). It is necessary to evaluate some criteria in order to distinguish between these systems in the field.
The first field criterion is the regional tectonic regime and host rock lithology. OGS occur in accretionary and collisional orogens, with the host rocks being greenstone belts and turbidites (Konate et al., 2024). The OGS are formed over a wide range of depths during major tectonic events (Groves et al., 1998). In turn, IRGS occur in deformed shelf sequences, showing close spatial association with felsic batholiths. The presence of nearby tungsten or tin magmatic provinces strongly favors IRGS models.
Structure and vein morphology can be considered as the second important field criterion. OGS have strong control of regional-scale deep-seated ductile and brittle-ductile shear zones (Konate et al., 2024). Extensively laminated or massive quartz-carbonate veins are usually observed in such faults. In case of IRGS, mineralization occurs in the apical part of intrusions or around intrusion contact aureole. Veins in IRGS appear to be localized sheeted vein systems or magmatic-hydrothermal breccias.
Another line of evidence is the hydrothermal alteration mineral assemblages. The interaction of deep fluids and host rocks at OGS leads to the development of significant alteration halos characterized by abundant carbonate alterations, sericitizations, and disseminated pyrites that spread over a wide zone away from the primary shear zones. The alteration of IRGS, on the other hand, is always restricted and typical of magmatic-hydrothermal environments, involving potassic, albitic, or greisen-type mineral assemblages.
Fourthly, the geochemical characteristics of the ore deposit are another key factor used in distinguishing OGS from IRGS. OGS are “gold-only” ores containing high amounts of Au, Ag, As, Sb, but no base metals (Groves et al., 1998; Konate et al., 2024). Intrusion-related deposits, on the other hand, are characterized by magmatically enriched fluids with a distinctive gold-bismuth-tellurium-tungsten-tin metal association (Elshourbagi & Fan, 2021). The occurrence of bismuthinite or scheelite or zonational development of silver-lead-zinc vein deposits in the field is a strong indicator of the intrusion-related type of deposit.
To sum up, despite the commonality of gold bearing veins in deformed terrains of both types of deposits, it is possible to confidently distinguish OGS from IRGS based on field observation. A geologist should be aware of the regional shear zones and extensive carbonate alteration when searching for OGS. Instead, one should look for the sheeted vein deposits, a distinctive magmatic metal association, and felsic intrusions when dealing with IRGS.
References
Elshourbagi, H. M., & Fan, H.-R. (2021). An oxidised intrusion-related origin in the controversial Jiaodong gold province (China) for the Shicheng Au-Cu deposit. All Earth, 33, 5–29. https://doi.org/10.1080/09853111.2021.1883314
Groves, D. I., Goldfarb, R. J., Gebre-Mariam, M., Hagemann, S. G., & Robert, F. (1998). Orogenic gold deposits: A proposed classification in the context of their crustal distribution and relationship to other gold deposit types. Ore Geology Reviews, 13, 7–27. https://doi.org/10.1016/s0169-1368(97)00012-7
Konate, S. I. M., Bolarinwa, A. T., Kazapoe, R. W., Bouare, M. L., Traore, E. M., Ngiamte, G. L., & Kouagou N’Dah, N. D. (2024). A review of the current state of knowledge on gold mineralization in Mali, Western Africa. Geology, Ecology, and Landscapes, 1–25. https://doi.org/10.1080/24749508.2024.2441514
