Transvaporite model of ore genesis and an exploration strategy for new giant ore deposits

Abstract

In the current work we stand for the revival of the transvaporite ore genesis model, which was introduced by Hungarian academician E. Szadeczky-Kardoss more than half a century ago, but has now fallen out of favor and superseded by the orthomagmatic model. Our arguments in support of the transvaporite model are based on the contemporary overpressure theory developed for Mesozoic–Cenozoic petroleum basins. We propose a new variant of the transvaporization model whereby the formation of giant ore deposits occurs in the areas where former petroleum-rich basins are intersected by Large Igneous Provinces (LIPs). At such sites, structural–magmatic activity leads to hydrocarbon deposits being destroyed and many new ore deposits being created beneath a new regional volcanic fluid seal built by rapidly accumulating (up to 3–4km in 1Ma) lava flows. In such conditions, a zone of significant overpressure that is 1.4–1.8 times higher than hydrostatic pressure develops. Subsequently, under the influence of compressive mechanism of mass transfer, gas-saturated brines of sedimentary rocks migrate toward intruding melts, and then mix with them. As a result, unusual rocks (transvaporites) are formed and giant ore deposits created.Here, we review six supergiant ore deposits of various types and ages with different associations of metals and host rocks, including the Norilsk district in Russia (Cu–Ni–Co–Pt–Pd), El Teniente–Los Bronces district in Chile (Cu–Mo), Almaden deposit in Spain (Hg), Ermakovskoe deposit in Russia (Be), Pebble Copper deposit in southern Alaska (Cu–Mo–Au), and Olympic Dam deposit in southern Australia (U–Cu–Au–REE), noting that all listed metals in each and every deposit do reach giant reserves. In all these deposits transvaporization appears to play the key role in ore genesis given the: (1) short duration of the main ore genesis stage (<0.2Ma); (2) high original porosity and permeability of the sedimentary host rocks; (3) marked fluctuations in isotopic compositions indicative of mixing between mantle and crustal components; (4) non-stoichiometric relationships between oxides in “magmatic” transvaporites as revealed by chemical analysis; (5) clear evidence of explosive activity during or immediately after the final stage of ore genesis, as expressed by the presence of abundant breccias, pyroclastics, and lapilli; (6) presence of unusual original rock types that include pseudo-breccias, pseudo-skarns, and hybrid metasomatic rocks with rare mineral associations such as, for example, simultaneous crystallization of biotite and anhydrite in the El Teniente and Pebble Copper deposits. Moreover, a compilation and analysis of a database of 416 of the world’s largest ore deposits shows that transvaporization with the aforementioned features is evident in 167 of these deposits (i.e., 40%). Furthermore, within 52 polymetallic giant ore deposits in which two to five metals reach giant reserve levels, evidence of transvaporization is present in 33 deposits (i.e., 63%).A significant practical implication of the transvaporite model is that it should modify future strategies for ore deposits exploration. Instead of pursuing just any deposit, exploration efforts should focus specifically on giant ore deposits, concentrating search efforts at the sites where former petroleum basins have been intersected by LIPs. Basin analysis of enriched oil and gas provinces is already being conducted in detail by seismic transects, involving two-dimensional cross-sections and areal surveys, followed by three-dimensional mapping of productive reservoirs. The same strategy should be applied to ore-bearing regions, but at better precision and accuracy given the smaller velocity contrast in seismic wave propagation in ore rocks compared with oil- and gas-bearing sedimentary rocks. Important advances in the ore geology of LIPs such as geological mapping and gravity and magnetic surveys have already produced significant new data. These studies can be divided into two types, which first involve a general description of the LIP, and are then followed by an investigation of major intrusions likely to be directly linked to ore genesis. A combination of these investigations into hydrocarbon basins and LIPs will lay the foundation for transvaporite exploration, which will be the key criterion for the successful search for new giant ore deposits.