Fathom Geophysics Newsletter 22

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Ore Model News: Transvaporites get airplay

A GRAND unified theory of how and why supergiant deposits arise features transvaporization, a volatiles-exchange concept that was spurned decades ago but that is now being re-examined.

Transvaporization theory features prominently in a paper currently in press in Ore Geology Reviews. [1]

The paper appears to be one of the first — if not the first — time that transvaporization has been laid out in a peer-reviewed publication for an English-speaking audience. The paper's Russia-based authors, Boris Lebedev and Eduard Pinsky, have written quite extensively on the topic previously, but those earlier papers have been relatively inaccessible given they were written in Russian.

In presenting their ore-genesis theory, the two researchers discussed what they saw as transvaporization's indispensible role in producing supergiant ore accumulations, including those at the Norilsk nickel-copper-cobalt-platinum-palladium district, the El Teniente-Los Bronces copper-molybdenum district, the Almaden mercury deposit, the Ermakovskoe beryllium deposit, the Pebble copper-molybdenum-gold deposit, and the Olympic Dam copper-gold-uranium-rare-earth deposit.

"Deposits with such diverse and very different metal associations are rarely considered and compared with each other," the researchers said in their paper.

"However, they possess an important common feature — a clearly pronounced transvaporization," they said.

"The specifics of this process in different regions are distinguished by many [different] features, but the mixing of two contrasting fluids — gas-saturated brines and basaltic melts — is the main condition required for ore genesis in the transvaporite model," they said.

They attributed the origination of the transvaporization concept to Hungarian petrologist Elemer Szadeczky-Kardoss, who had material published on the topic in the 1960s.

They said that according to Szadeczky-Kardoss, "the necessary prerequisite for silicate melts and sedimentary fluids to mix is that ... the fluid pressure in the sedimentary rocks becomes higher than in the magmas."

"Consequently, the fluids migrate by a compressive mechanism, vaporize next to the melts, and are then absorbed by the magma."

Transvaporization was abandoned in favor of orthomagmatic-driven models of ore genesis because in about the 1970s and 1980s a consensus formed in support of the latter. Since then non-petroleum mineral exploration research has mainly been developing along lines investigating the relationships among fluid migration, ore genesis and temperature differentials. So it seems the extent of the role of pressure differentials has so far remained largely unexamined by workers in the 'hard rock' research arena.

General features

According to Lebedev and Pinsky, clues indicating transvaporization was likely to have played a role in creating a given giant ore system included:

  1. the presence of sedimentary host-rocks possessing high initial porosity and permeability, such as those found in former petroleum basins,
  2. the mixing of primitive mantle-sourced fluids and overpressured gas-saturated crustal brines, observable as fluctuating isotopic compositions,
  3. the presence of an open chemical system, observable as non-stoichiometric oxide compositions,
  4. a main stage of ore genesis that was geologically brief, usually shorter than 0.2 million years,
  5. explosive activity occurring either during the final ore-genesis stage or soon after it, and
  6. the presence of numerous, highly unusual combinations of rock-forming minerals in late-stage veins, breccia matrices, and altered wallrocks.

Transvaporization wasn't the only previously-unfashionable idea put forward for renewed consideration by the researchers.

They also drew upon the concept of overpressure — an uncontroversial matter in the petroleum industry, but one that may nevertheless put some economic geology thinkers' noses out of joint.

Overpressurization

Fluid migration can take place via convection or compression. Convection is controlled by relative temperature gradients, while compression is controlled by relative pressure gradients.

In rock formations, relative pressure is called overpressure. It's how great the formation pressure is relative to the hydrostatic pressure.

It means fluid flow through porous and permeable lithologies is (in the absence of any other forces) dictated by the difference in the overpressure existing between any two adjacent sites at any given time. Fluid moves from regions with higher overpressures toward regions with lower overpressures.

The main factor influencing compression-controlled fluid migration in a sedimentary basin is the creation of a major gas reservoir, the researchers said. This happens during the latest stages of a developing petroleum basin, when sustained regional-scale subsidence takes place.

The largest overpressures tend to form in the uppermost petroleum reservoirs in a given complex. The better the sealing properties of the lithology or structure that's capping the petroleum in place (such as clay-rich or salt-rich sedimentary sequences), the greater the overpressure that builds up. If overpressure is sufficient to breach the seal, hydrocarbons quickly migrate through a fracture network to seek less-overpressured sites.*

Another phenomenon that can trigger overpressure-controlled fluid migration is what the researchers call activation — breaches in a seal due to either tectonics-related structural activity (such as faulting and fissuring) or igneous activity.

Activation

"During such activation, the regional fluid seals are opened, and hydrocarbon migration is eventually replaced by the migration of aqueous solutions, including ore [cation]-bearing fluids," the researchers said.

"The migrating melts and solutions may follow the hydrocarbon migration pathways, and [furthermore] residual decomposed oil and gas material can play a role in ore genesis."

Eventually, with continued fluid migration, overpressure drops in the region.

Encroachment by ascending magma may lead to a pronounced geothermal gradient, and a switch to mass transfer via convection.

However, this transition to a convective fluid-migration scheme is not necessarily permanent, the researchers said.

This is because a thick cloak of newly emplaced sill intrusions, volcanic extrusives, and volcanoclastics not only acts as a new fluid seal in its own right, it may also act in concert with the former petroleum basin's remaining intact fluid seals to cork up the region again.

Transvaporization

This re-established regional overpressure would in turn encourage a batch of gas-saturated basinal brines to be delivered through the permeable horizons in the basin, and causing those brines to mix with ascending silicate melts there — a reactive scenario that leaves behind obvious signs, such as the eruption of tuffs, the explosive brecciation of rocks, and the development of a wide array of unusual combinations of vein and matrix minerals.

At the same time, the pronounced geothermal gradient in the area would exaggerate this fluid-mixing transvaporization step when the products of contact metamorphism-induced carbonization of organic material (such as coal measures) and hydrolyzation of carbonate minerals (such as those in limestones and evaporites) are thrown into the actively mixing system.

This contact metamorphism generates a significant amount of volatile species (such as chlorine), which would heighten the degree of explosivity. Volatiles are also thought to drastically reduce the viscosity of melts by inhibiting their polymerization, permitting them to more readily reach the surface and produce a thick capping pile of flood lavas.

It also delivers excess sulfur to the system, which is handy for precipitating desirable metallic cations out of fluids in the blink of an eye, geologically speaking.

Implications for explorers

The above sequence of events was not uncommon, the researchers said.

If a former petroleum basin is subjected to structural and magmatic activity, then the remaining necessary ore genesis contributors — including the transvaporization fluid-mixing phenomenon — are likely to unfurl right on cue.

"A significant implication of the transvaporite model is that it should modify future strategies for ore-deposit exploration," the researchers said in their paper.

They said that explorers who are targeting giant ore deposits should focus their search efforts to "sites where former petroleum basins have been intersected by large igneous provinces".

"Basin analysis of enriched oil and gas provinces is already being conducted in detail [by the petroleum industry] by seismic transects ... followed by three-dimensional mapping of productive reservoirs," they said.

"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."

References

[1] B.A. Lebedev and E.M. Pinsky (in press) "Transvaporite model of ore genesis and an exploration strategy for new giant ore deposits", Ore Geology Reviews.

* Interestingly, there are signs that overpressure is starting to be increasingly understood and accepted within economic geology circles. For instance, a group of researchers looking at the Malbunka copper deposit in the Amadeus Basin of Australia's Northern Territory recently invoked an overpressure-caused failure of sealing sediments to explain the existence of a primary copper carbonate deposit that formed from the mixing of deep, metal-bearing oil-basin formation fluids with more shallow carbonate-rich fluids in a diagenetic setting. Details can be found in: E.B. Melchiorre, D. McLaughlin, R. Bottrill and J. Hight (April 2017) "Primary diagenetic copper carbonate at the Malbunka copper deposit, Amadeus Basin, Northern Territory, Australia", Ore Geology Reviews, 82, 170-180.

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In early 2008, Amanda Buckingham and Daniel Core teamed up to start Fathom Geophysics. With their complementary skills and experience, Buckingham and Core bring with them fresh ideas, a solid background in geophysics theory and programming, and a thorough understanding of the limitations of data and the practicalities of mineral exploration.

Fathom Geophysics provides geophysical and geoscience data processing and targeting services to the minerals and petroleum exploration industries, from the regional scale through to the near-mine deposit scale. Among the data types we work on are: potential field data (gravity and magnetics), electrical data (induced polarization and electromagnetics), topographic data, seismic data, geochemical data, precipitation and lake-level time-lapse environmental data, and remotely-sensed (satellite) data such as Landsat and ASTER.

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