- Title
- Gold mineralization at the Blue Rock Deposit, Gadzema Greenstone Belt: Implications on genesis and exploration for orogenic gold mineralization within Archaean Greenstone Belts of Zimbabwe
- Creator
- Mavuwa, Tavashavira
- ThesisAdvisor
- Harmer, R.E.
- Subject
- Uncatalogued
- Date
- 2024-10-11
- Type
- Academic theses
- Type
- Master's theses
- Type
- text
- Identifier
- http://hdl.handle.net/10962/464944
- Identifier
- vital:76559
- Description
- The Blue Rock gold deposit was re-discovered by African Consolidated Resources (ACR) in 2007, over a defunct historical gold mine at Blue Rock, during a regional geochemical soil sampling program, within the Gadzema Greenstone Belt (GGB), in Central Zimbabwe. Most significant orogenic gold deposits within this belt occur as BIF- and quartz vein hosted orebodies. But unlike them, gold mineralization at Blue Rock is associated with felsite and quartz porphyry rocks. The GGB is a northern extension of the Midlands Greenstone Belt (MGB), where the common occurrence of mineralized felsites, in close association with major gold reefs within numerous gold mines is well documented. But no significant effort was directed towards their understanding or exploration in the past. They were never considered viable exploration targets for significant economic gold deposits, until recently. More attention was instead focused on high-grade BIF and quartz vein hosted gold, that dominate most orebodies exploited by numerous mines within the belt. At Blue Rock, ACR defined a significant JORC compliant felsite-hosted gold resource of close to a million ounces, which represents a brand new attractive open-pit mining opportunity. But the successful exploration for such type of mineralization, whose footprint is so different from the common ones previously mined within the GGB, no doubt, calls for a good understanding of this type of mineralization. Which makes felsite-hosted gold mineralization a prime candidate for research, based on a deposit whose discovery and development, I was fortunate to be part of during the past few years. In this contribution, the genesis, localization and economic significance of felsite-hosted gold mineralization is investigated, using the gold deposit at Blue Rock as a case study. The deposit could be understood best through the Mineral Systems Approach, used in this study to interrogate alternative ideas about its genesis using published information and deposit-scale exploratory data. Results from the synthesis of published information on the evolution of Archaean Greenstone Belts and genesis of their host orogenic gold deposits, are consistent with models that view orogenic terrains as having formed through horizontal accretion in modern-day like subduction-accretion systems, at continental margins, where orogenic gold deposition occurred via processes that could be explained quite simply, through a universal orogenic gold mineral systems model. According to this model, orogenic gold deposits are believed to have formed from near neutral fluids containing dissolved gold, generated directly from the devolatilization of a subducted oceanic slab together with its overlying gold-bearing sulphide-rich sedimentary package, or indirectly through fluid released from a mantle lithosphere that was originally metasomatized and fertilized during an earlier subduction event. The fluid migrated up-dip from the mantle to crustal levels, through advection or seismic pumping along lithosphere- to crustal-scale fault zones, to form orogenic gold deposits within lower order structures. If these models are all accurate, then the GGB formed through subduction-related east-directed horizontal accretion at the continental margins of the Sebakwe Proto-Craton (SPC), and the genesis of felsite-hosted gold mineralization at Blue Rock could be explained eloquently through a universal orogenic gold model, in which mantle derived auriferous fluids were localised within lower order structures associated with felsites during the late stages of terrain accretion. Evidence from surface mapping and 3D modelling of exploratory drilling data, conducted during this research, strongly support the argument that the felsite hosted gold mineralization at Blue Rock, is neither unique nor accidental. It is just but, a simple product of the conjunction of favourable geological factors, no different to those that birthed typical GGB orogenic gold mineralization hosted within sheared sulphidic BIFs and quartz vein reefs. They all share the same geodynamic setting, fertility, preservation and regional architectural factors reminiscent of accretionary orogenic settings, albeit with differences in local architecture, variably controlled by geochemical and rheological properties of the different local host rocks. At deposit scale, the felsites occur as small dykes and sills emplaced along pre-existing structural zones of weakness. Gold mineralization is structurally controlled and associated mostly with brittle-ductile shears. During deformation, rheological contrast played a significant role in the selective failure of the more competent felsite rocks, resulting in the creation of permeability channels that allowed fluid migration. The more brittle and competent felsites acted as rigid bodies, that localised strain along their contacts with the surrounding less competent ductile mafic schists which acted as a relatively less permeable fluid cap rock. The irregular felsite contact zones with surrounding mafic schist caused a significant variation in the orientation of local principal maximum stress relative to the internally imposed regional stress, causing anomalously low minimum stress zones at deposit scale. Gold deposition occurred within low minimum stress structural traps dominated by sheared felsite contacts and their fractured interiors as well as triple junctions formed by complex structural geometries created by multiple felsite intrusions. The felsite hosted gold at Blue Rock can therefore be recognized as an orogenic gold mineral system archetype, for which an occurrence model is proposed. Understanding this type of mineralization is key for developing a robust exploration strategy - one that could be applied in a predictive capacity in exploration, to locate new economic gold deposits especially within well-endowed mature orogenic terrains, where exploration risk could be minimized by leveraging on new forward-thinking initiatives like Artificial Intelligence (AI) to re-analyze data from previous mining and exploration, allowing for a faster route to a return on investment. In a world of diminishing natural resources, the potential for previously ignored gold mineralization like the one at Blue Rock, becomes very significant. The prophetic words of Foster (1984), writing in Gold ‘82, therefore remain true and relevant to our time, that; “…. the way ahead for successful gold exploration is to search for new deposits not commonly recognized – in auriferous muds, disseminations in carbonate rocks, porphyries, and in felsic intrusive and extrusive volcanics…”.
- Description
- Thesis (MSc) -- Faculty of Science, Geology, 2024
- Format
- computer, online resource, application/pdf, 1 online resource (104 pages), pdf
- Publisher
- Rhodes University, Faculty of Science, Geology
- Language
- English
- Rights
- Mavuwa, Tavashavira
- Rights
- Use of this resource is governed by the terms and conditions of the Creative Commons "Attribution-NonCommercial-ShareAlike" License (http://creativecommons.org/licenses/by-nc-sa/2.0/)
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details | SOURCE1 | MAVUWA-MSC-TR24-196.pdf | 5 MB | Adobe Acrobat PDF | View Details |