This is an excellent paper. It contributes both valuable new data and solid interpretations of the origin of Nyakabingo, one of the Rwanda-Uganda “tungsten belt deposits”and its relations to rare-metal pegmatite formation.
The authors demonstrate by geochemical data the long-assumed connex between parental ca. 1000 Ma so-called G4 granites, tin and tantalum mineralized pegmatites (at Gatumba) and the wolframite-quartz veins at Nyakabingo. Considering the distance of 70 km between the two, a direct flow of liquids and fluids between the Sn and W districts is not insinuated. The assumption is that in both cases, differentiation and fractionation of G4 granite produced mineralizing residual liquids and/or fluids. The “text-book” model of the relation between pegmatites and hydrothermal mineralization predicts that the second should follow the first.
The authors used muscovite trace geochemistry to determine and compare the degree of differentiation/fractionation of pegmatites and W-ore veins. Curiously, these data imply that the tungsten mineralizing fluids were expelled at the same fractionation stage as early unmineralized pegmatites.
How is this to be explained? I wonder if the solution would not be the contemporaneous tectonic deformation!
My own mapping of the tungsten belt deposits Bugarama and Nyakabingo (with Günther & Ndutiye), and of the nearby large cassiterite vein fields of Rutongo, and of Wolfgang Frisch at Gifurwe all showed that the veins are tectonically controlled by thin-skinned folding and overthrusting. G4 melt bodies may have been disturbed by tectonic deformation, from a state of quiet fractionation and differentiation to sudden pressure changes and deformation of the whole melt body as the country rocks heaved.
“Typically in the Kibara belt, pathways and traps for pegmatite melts, or for metalliferous
hydrothermal fluids, were low-pressure hinge zones of tightening anticlinal folds with axial
planes or cleavage planes on the flanks of folds as feeder and break-through structures. The
compressive stress field was rotated compared to the Mesoproterozoic main deformation and produced fold axes and thrusts cutting earlier folds at sharp to orthogonal angles. Outcrop patterns created by fold interference are visible on geological maps; resulting highs often determined the location of tin granite cupolas or ridges and associated deposits” (citation from Pohl et al. 2014; for research use, I can provide a PDF).
The cause for both the compressive tectonics and the crustal melting that produced the G4 granite suite were the ca. 1 Ga global tectono-magmatic events of the final assembly of Supercontinent Rodinia (the “pan-Rodinian orogenic events” of Li et al. 2008).
Would you agree?
Tectonic processes govern the dynamic nature of Earth’s crust and shape the global distribution of continents, ocean basins, and landforms. Setting the template on which climate and erosion interact, tectonics elevates rocks above sea level where weathering prepares the ground for wind, rain, and rivers to erode and sculpt landscapes. It is no coincidence that many of our planet’s major surface features coincide with the boundaries of tectonic plates, where uplift, deformation, and erosion are focused. Tectonically active regions give rise to topography with spectacular vertical relief; tectonically quiescent areas often host broad lowlands with older, less steep highlands. The imprint of tectonics on geomorphology is apparent not only in the size, extent, and location of mountain ranges, but in the localized steepness of river profiles, the character of mountain slopes, and in the form of river networks that flow along regional joint patterns or are offset across faults. Tectonics sets the stage for, and sometimes directs, the work of erosion.
Tectonics influences geomorphology through both active and passive controls. Active tectonic controls involve landscape response to ongoing deformation of the surface, such as the response of a river profile to fault offset and regional uplift. Passive tectonic controls (commonly referred to as structural controls) are those that influence landforms and landscape dynamics indirectly through the variable erosional resistance of different rocks and the effects of geologic structure. In tectonically active regions, geologic structure and lithology often have only minor geomorphic expression during active mountain building as hillslopes driven to threshold slopes mask underlying geologic structures — if everything has about the same slope, structure is difficult to infer from hillslope morphology. But once active tectonic forcing ends, lithology and structure often become dominant controls on landforms. Extremely high rates of erosion in tectonically active areas, such as the Himalaya, can influence the growth and development of geologic structures by focusing both exhumation and isostatic rebound. This chapter explores the role of plate motion (tectonics), heat, and density contrasts in uplifting rock and considers the landforms that result from, and are indicative of crustal deformation and structural controls on landscape evolution.
This is a fascinating question you have asked. Although I am not familiar with your area of interest I see many similarities with other "tectonic controls" as regards orogenic gold deposits of the Archean that I am familiar with.
I see the emplacement of igneous intrusions (due to their volatiles they force themselves upwards) as a reason for the creation of structures where mineralizing fluids travel. In the end minerals get deposited according to many controlling parameters (salinity, temp., isotope, fluid composition and chemistry of wall-rocks among others) . I have followed Rob Kerrich's work on gold deposits, ever since he visited my gold mine (where I worked, it wasn't mine!) to study the nature of the gold deposits. He has explained those parameters that control deposition of metals in a satisfactory way.
The tectonic control of Sn-W mineralization in Cornwall, England that I am familiar with has many similarities to your mapping experience in Africa. As far as I know, the mineralization there has not been explained fully yet. Tectonics has been mentioned by several investigators to explain the distribution of metals around the granite intrusions.
To answer your question, I agree many factors play a role including the tectonic control.
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