“Alhambra’s mantle plumes”Fundamental to the exploration process is an understanding of the geological framework of ore deposit generation; which starts with the Earth’s internal structures and mantle dynamics. A primary focus must be on the theory of mantle plumes and the role that they have in generating (directly or indirectly) ore systems, - touching upon the Earth’s mantle, its composition, dynamics and evolution through geological time.
Plume theory offers an explanation for the vast amounts of melts that are generated in the mantle (in settings other than convergent plate margins); with important implications for both magmatic and hydrothermal ore deposits. For the Earth dissipates heat through various modes of magmatic activity, such as that which occurs at mid-ocean ridges, oceanic volcanic chains and in plate interiors; and, except in continental collision zones, the source of these magmas is in the mantle.
These phenomena are related to internal heat traceable to the early evolutionary stages of an accreting planet, during which segregation into distinct shells or layers took place; resulting in a layered internal structure, onset of convection, heat generation and dissipation, all of which have continued to play an important role. Eventually this is all destined to extinguish itself as the rate of internal heat production decreases with time and the planet loses most of its heat into space (someday ending up like the moon).
The Earth’s mantle, characterized by temperature and density differences, behaves plastically and flows at geological time scales; and the heat of the Earth’s core creates thermal instabilities at the core-mantle boundary, with plumes of hot mantle material rising from this boundary towards the surface. And, - it is thought that ore deposits have origins with either a direct or indirect link to mantle plumes.
Plume-related mafic-ultramafic magmas (and associated anatectic melts) constitute powerful heat sources; and these heat sources cause waves of high heat flow resulting in prograde high temperature/low pressure metamorphism. Thus, in addition to hydrothermal fluids that are exsolved from the differentiating magmas, dehydration (also called devolatilisation) reactions supply fluids that evolve through a wide range of temperatures and pressures. Then, connate and/or meteoric waters descending through fracture or fault systems nearer the surface are heated in these regions of high geothermal gradients and become ore solutions.
These ore solutions may mix with ascending metamorphic and/or magmatic fluids; and, in this way, the emplacement of large volumes of melts into the crust is capable of activating giant hydrothermal convection cells. Finally, these variably-sourced hydrothermal fluids carry metals in solution, which are then precipitated in structurally and/or stratigraphically-controlled locales, - to form giant ore deposits.
Geologically, deposits of each ore type tend to have a time-bound nature, and there are times in Earth history when particular deposit types are absent, times when these deposits are present but scarce, and other times when they are abundant. Naturally, understanding the details of such variation provides a critical tool for exploration targeting, - in that rocks that have formed or were deformed during a certain time slice may be very permissive for a given ore deposit type.
As fate would have it, orogenic deposits are the only important type of gold deposits for which many economically important examples are preserved throughout the geological record; and their existence correlates with the addition of new oceanic lithosphere onto older craton margins, - with these large-scale processes a major factor in determining gold endowment. Consequently, delineation of tectonomagmatic provinces of these ages provides a first-order control as to the most permissive ground for discovery of orogenic gold deposits.
Moreover, since ore-forming systems in different geodynamic environments differ in ore productivity, when searching for new giant ore deposits, by far, the most important targeting is done at the broad regional scale. Then (within this regional choice), the next essential ‘exploration challenge’ in targeting lies in discriminating between areas prospective for truly significant deposits and those likely to host only minor deposits.
No unique special features set apart ‘giant ore deposits’ from the ‘smaller deposits’, and in most respects they are similar, except for the fact that they are mass-concentrative systems that are (for fundamental reasons of mass-balance) operating over very large scales. And, in that these ‘giants’ become merely the final end-point of a much larger-scale system, (for predictive targeting) it is essential to focus on this much ‘larger entity’, - “the ore system” (a product of lithosphere-scale processes); rather than keying in primarily on the ‘deposit environment’ itself. In this light, the critical questions become “How large is the ore system?” and “What are its fundamental controls?”
In Central Asia (a productive metallogenic belt), gold deposits are generally localized in Early Paleozoic (Ordovician) and Hercinian (between Devonian and Carboniferous times) orogenic belts; with such Paleozoic deposits (occurring in rift structures or aulacogens) generally confined to continental rifts, being related genetically to the mantle plumes; denoting a mantle origin of the ore-forming systems, and highlighting the importance of plumes. The linkage involving dynamic earth processes is between mantle plumes, magmatic processes, rifting and ore deposition, - manifesting as mineralizing events in the crust.
Bottom line: gold deposits tend to have a time-bound nature, and circumstances in Earth history can conspire so that certain times and places (such as in Ordovician intrusive rocks at geosynclines in Kazakhstan) host abundant resources, - in fact, all of the multi-million ounce Orogenic deposits (such as Vasilkovskoe, Aksu, Stepnyak, etc.) are genetically related to this.