Regional Metamorphic Remobilization: Upgrading and Formation of Ore Deposits
Brian Marshall, Frank M. Vokes, Adrienne C.L. Larocque, 1998. "Regional Metamorphic Remobilization: Upgrading and Formation of Ore Deposits", Metamorphic and Metamorphogenic Ore Deposits, Frank M. Vokes, Brian Marshall, Paul G. Spry
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Metamorphic remobilization involves the translocation, during deformation and metamorphism, of preexisting, massive, semimassive or disseminated mineralization by solid mechanical transfer, liquid-state chemical transfer, or mixed-state transfer. Mixed-state processes, in which deformation plays a critical role, have dominated in nature. The terminology and concepts involved in remobilization are discussed; internal and external processes, extent of remobilization, parent-daughter relationships, and upgrading of ore are particularly emphasized.
The series of textural and mineralogical changes brought about by prograde metamorphism generally have little impact on the overall grades of the major, minor, and trace elements present. However, effective upgrading by grain size coarsening and formation of discrete trace element (e.g., precious element) minerals enables more efficient recovery. Some prograde changes can be reversed during retrogression.
Consideration is given to the probable extent of selective external remobilization of components from sulfide orebodies. Large-scale selective removal of components from massive ore is difficult to substantiate on both observational and theoretical bases. It is nevertheless concluded that such remobilization cannot entirely be rejected.
Transfer processes in regional metamorphic remobilization include cataclasis and granular flow, intracrystalline plasticity or dislocation flow, and dry- and wet-state diffusion, with the latter commonly being grouped with advective transfer. Because several processes may be rate competing in a single phase, although the dominant mechanism may differ between phases, consideration is facilitated by addressing solid-state remobilization before conjointly dealing with mixed and liquid state processes.
Intergranular and intragranular solid-state transfer is fundamental to dry-state ductile deformation, and of great importance for the degree of internal remobilization. Dislocation flow has been invoked to explain healing of fractures in high competence sulfides by low competence species, but fluid facilitation should not be discounted. Internal solid-state remobilization can form ore shoots by hinge zone thickening and elongation processes, although fluid-assisted diffusive and advective mass transfer probably contribute significantly to these cases. External solid-state remobilization has been invoked to explain piercement cusps and veins, as well as discordant, shear zone-hosted bodies of enriched mineralization, yet a degree of fluid involvement is usually envisaged. In effect, there are few, if any, undoubted examples of solid-state external remobilization.
The bulk of extensive internal and external remobilization involves mixed- or liquid-state transfer, the liquid phase having a range of possible origins. Pertinent mass transfer paths include diffusion around grain boundaries through a static film, flow around grain boundaries and through dynamic microfracture networks, and channelized flow through macrofracture systems, now commonly seen as vein arrays related to ductile and brittle shear zones. A substantial extent of remobilization will only result from the advecting processes. Liquid-state involvement in mixed-state transfer ranges from negligible or subordinate to totally dominating, before passing into true liquid-state transfer. The spectrum of mixed- and liquid-state processes includes fluid-controlled cataclasis, fluid-induced dislocation flow, fluid-facilitated shear zone transfer, fluid-assisted diffusion, fluid-dominated transfer, and advective transfer; each is assessed.
Magmatic transfer in the remobilization of sulfide species potentially results from production of sulfide melts during high-grade metamorphism, dissolution of ore minerals by a migrating silicate melt, and direct incorporation of ore components during production of a silicate melt. Silicate melts perhaps have a role in remobilization leading to the formation of new deposits, whereas the contribution of sulfide melts is at best minor. Nevertheless, both can induce local, as well as effective, upgrading. Magma related transfer, involving fluids expelled during magmatic cooling, can modify a deposit by replacement or addition of phases and result in daughter mineralization up flow from the parent body.
Transfer by solid-state, aqueous-dominated, or magmatic flow is integral to the remobilization (modification or formation) of ore deposits. The consequences of internal and external flow, as observed at the site of emplacement, can be recognized by the geometry and spatial relationships of one or more of the following: deformed ore (textural and localized distributional changes), dilational vein systems, and replacement effects over a range of scales. Despite these effects and the alteration of the silicate-rich host rocks that commonly accompanies aqueous flow, the site of emplacement consequences of remobilization, syntectonic introduction, and even post-tectonic epigenesis involve the same principles. Thus, because the site of emplacement features are non-definitive, genetic interpretation can be difficult and contentious.
Remobilization reflects a complex interplay between effective upgrading, the remobilization processes and whether they cause concentration or dispersion, and the selected reference frame. Its potential to upgrade and form ore deposits is soundly based. Transport rates and distances involved in external remobilization are derived from theoretical considerations and published examples. External remobilization seems to be extremely common to a meter, very common to 10 meters, moderately common to a few tens of meters, uncommon to a few hundred meters, extremely uncommon to a thousand meters, and unproved beyond that. Despite the formation of daughter orebodies, the relative sizes of parent and daughter imply only low to moderate degrees of external remobilization. A high degree of selective external remobilization is unproved and highly improbable; comprehensive external remobilization (magmatic incorporation excepted) is unproved and even more improbable.
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The types of mainly metallic mineralization found in metamorphic terranes are reviewed and an attempt is made to define the genetic relations between the mineralization and the metamorphic events.The terms metamorphosed, metamorphic, and metamorphogenic as applied to ores are also considered.The development of thought and the history of investigations on ores in metamorphic terranes aretraced from the early work in the second half of the nineteenth century onward. Early conceptions ofmetamorphism as an ore-forming process (metamorphogenesis) were seemingly not followed up by theiroriginators, contemporaries, or immediate successors and were neglected until comparatively recentyears. The idea of metamorphism as a modifier of preexisting, mainly sulfidic, but also oxidic, mineralizationwon more immediate and general acceptance in the early decades of the present century. InNorth America, emphasis seems to have been mainly on the deformational aspects of the metamorphism,whereas elsewhere, especially in Europe, the textural and mineralogical results of the metamorphic recrystallizationalso received considerable attention and metamorphism as an ore-forming process hadwon a certain degree of acceptance. This difference in emphasis may perhaps be referred to the differentviews held regarding the initial genesis of the ores in the two regions.The late 1940s and the 1950s witnessed a considerable revision of ideas on ore genesis, especially regardingstrata-bound massive sulfide ores. A parallel revival of interest in the role of metamorphism,probably not unrelated to the foregoing, began in the early 1950s, to begin with concerning metamorphosedores. However, new thoughts concerning metamorphogenesis related to granitization or ultrametamorphismas an ore-forming process began to be published.The following decades witnessed an almost explosive increase in the number of publications dealingwith the effects of metamorphism on ore mineralization of practically all types, but with a definite emphasison sulfide ores of the strata-bound type. One of the most significant breakthroughs in this respectconcerned the world-famous Broken Hill deposit, New South Wales, although the metamorphosed natureof ores in the Scandinavian Caledonides, the North American Appalachians, the Lachlan fold beltof eastern Australia, the Sanbagawa terrane of Japan, the Urals, and Proterozoic fold belts in southernAfrica, have all been thoroughly documented.In recent years, however, the interpretation of many massive sulfidic ores in metamorphic terranes asmetamorphosed has been increasingly questioned, and syntectonic, metamorphogenic, origins havebeen advocated. There is obviously a great need to be able to distinguish more