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Saturday, 3 December 2011

Hydrocarbons and Metallogenesis

Hydrocarbons and Metallogenesis

   There are several evidences of hydrocarbon association to metal ores. Black shales, specially those of high carbon content, have long been known to be enriched with a variety of transition metals, especially Mo, Zn, Vi, Cu, Cr, V, Co, Pb, U and Ag. The Kupferschiefer is associated with black shales and near Zechstein Salt in Germany. Miners try to found the black leader to prospect gold deposits. Mississipi Valley Type - MVT deposits are frequently associated with hydrothermal dolomite HTD and bitumen. In Australia, Proterozoic shales hosted Pb-Zn-Ag deposits, such as Mt. Isa, Hilton, McArthur River and Lady Loretta According. 

   The scientist Thomas Gold, remind that in geology there's no understanding about the role of hydrocarbon compounds and their capacity to transport metals. This is due few people reasoning with the possibility that hydrocarbons come from great depths as oil and natural gas and that they are primordial materials. Then, most part of Economic geologists still reasoning that metals are syngenetic with shale deposition and diagenetic process would be responsible for metal concentration.

   The understanding of real hydrocarbon origin and processes of hydrothermal salt formation maybe will be key to comprehension of certain metal ore accumulations in the Earth.

Metal Ores and Hydrocarbons
Thomas Gold, 1994

   The association of various metal ore deposits with hydrocarbons is a vast subject, but as yet very few people have worked on it. Many such associations have been seen, but as people did not recognize the possibility that hydrocarbons could come up from great depth, they could not see any reason for these effects. And people do not write papers to say they do not understand what they see.
   The general problems about concentrated mineral deposits are the following:

1.) The Earth formed by the collection of solids, mostly small grains, that had the elements pretty much mixed up. There may have been some layers that had a little more of this or that, but except for iron and nickel, there were no "clean" substances in this infall. We judge this from the great array of meteorites which are samples of the various contributions the Earth received. Many detailed trace element and isotope ratios show that this is true.
   What processes would single out a particular element and cause a deposition in a location which represents a concentration by a factor of one million or more from the original mix? A fluid that moved through a large amount of the mix, and picked up in solution the particular substance, and then shed it from solution as a result of changing circumstances such as temperature, pressure, ph, or the picking up into the solution of another substance that decreased the solubility of the first. All attempts at explanation assume processes of this kind and this seems inevitable. Water is generally considered the basic fluid, usually with aggressive contaminants like salts. But when it comes to the arithmetic of these processes, there is frequently serious trouble. Many metals , especially the heavy metals, are just not sufficiently soluble in brines. or in any aqueous fluids. The excerpt from Krauskopf (appended here) refers to this difficulty. Many other authors have also noted it.
   In my view hydrocarbons come towards the surface from depths between 150 and 300 km. They therefore leach through a very large amount of rock as they are driven up by buoyancy forces. Effective leaching requires powerful pumping action to drive fluids though fine pores and for a large distance: fluids coming up from great depth have of course this advantage. By comparison surface waters running through some crustal rocks have an incomparably smaller driving force. The leaching has to be due to fluids that originate at depth, because only those have the pressure differentials that are required for effective leaching.

2.) Which fluids have the capability to take into solution such substances as heavy metals or metal compounds?

   At high pressures and temperatures many metals will form organometallics, that means molecules that combine metal atoms with such elements as carbon and hydrogen, possibly with some nitrogen and oxygen also. Most organometallic compounds are soluble in hydrocarbon oils. Such oils, being forced through the rocks, will have a chance to combine with metals in the rocks to make organometallic compounds. In turn those that are soluble in the oils can then be transported by that same flow. This will be so also for many metals that have very low solubilities in aqueous liquids.

3.) What process can be so selective that it will deposit one metal ore in one location and another often nearby? What liquid stream will just leach out copper from the rocks, while another nearby stream will leach out zinc? Or why platinum here and gold there?

   The hydrocarbon flow, on the way up, will make a large array of molecules, in detail depending on such things as the carbon-hydrogen ratio, the ratio to other elements like nitrogen and oxygen, the catalytic action of specific minerals in the rocks, and the pressure-temperature regime it finds on the way. Among those molecules may be a class that is particularly favourable for forming a particular organometallic compound with one metal, another class with another. The great diversity of hydrocarbon molecules is thus the reason for the selectivity in the metal deposits. Certain groups of metals occur in close association, presumably because there exists a hydrocarbon stream there, and similar hydrocarbons that were abundant in that location have selected that group because these respond similarly. Thus lead and zinc are found together, gold and silver, etc.
   When these metal-laden streams come nearer to the surface, and reach lower pressures and temperatures, many of the compounds become unstable (many carbon compounds are stable at a high pressure only, like diamond). Also bacterial action may destroy them, as the bacteria will preferentially remove the hydrocarbon components. In this way the naked metal atoms remain.
   The close association of gold with carbon is well recorded in the literature. Conventional wisdom gives no hint of an explanation either for the association with carbon, or even for the occurrence of metallic gold altogether. It seems that carbon is an essential component in the laying down of gold. The gold miners of olden days knew this very well, and followed the "black leader", a trail of carbon black that led frequently to a gold deposit.
   It is interesting that the other substance that is commonly associated with gold is silicon dioxide. Silicon is in the same column, two below carbon, in Mendeleev's table of the elements and it has very similar properties. It will form oils that are quite similar to hydrocarbon oils, but frequently with higher thermal stability. I do not know (and possibly no one knows) whether at high temperatures and pressures, it will form silicon-metallic compounds, analogous to organometallics. An argument in favour of this would be the occurrence of gold in quartz veins rather than in quartz deposits, suggesting a common migration path. Mercury, found as the sulfide cinnabar, is often together with oil and tar.
   Many metals will of course make sulfides, if sulfur is available. Thus mercury may come up in a gas stream as mercury vapor or as dimethyl-mercury, but have enough sulfur to be turned into cinnabar. It is the same for many other metals, they would not resist being turned into the sulfide. For mercury it is particularly clear that it has come from great depths, as it is strongly associated with helium, in particular with helium high in helium-3, which is the marker for primordial helium, caught in the formation process of the Earth, and not merely derived from the radioactivity of uranium and thorium.
   In the drilling in the Siljan Ring structure in Sweden, large quantities of magnetite were found. Some twelve tons of a mix of very fine grained magnetite and natural petroleum were pumped up from one wellbore, and some kilograms of a similar paste were pulled up on the drillstring in a second hole. At the deeper levels, below 5 km, the magnetite paste impeded the drilling operation in both holes. It appears that it was this same paste that prevented any substantial inflow into the wellbores, necessary for any commercial production. Investigations by laboratories including that of the Danish Geological Survey, showed the oil to be an ordinary type of crude, somewhat biodegraded. In the second hole no drilling fluids were introduced that could possibly have resulted in the oils seen.
   The origin of such clean, concentrated magnetite and its very small grain size, much of it in the micron size range, certainly present a puzzle. Moreover the entire Siljan Ring structure displayed a positive magnetic anomaly, quite accurately centered in the ring. It therefore seems very likely that this same magnetite paste was the source for the magnetic anomaly, and that it was present in sufficiently large amount to account for it. If this is considered a possibility, then one may well wonder whether the various other large magnetite deposits of Sweden have a similar origin.
   The only clues we have about the origin of the Siljan magnetite come from the detailed trace element and isotope observations of it. Neutron activation analysis (done by the Los Alamos National Laboratory) showed a substantially different admixture of trace elements from the local granite or the much larger magnetite grains in it. For example the paste magnetite contained only 1/30th of the amount of Mg-27 as the magnetic grains of the granite; 1/7th of the Na-24; but 100 times as much Zn-65 (there is a commercial zinc mine in the region); 10 times as much Ba-131 and Ba-139; less than 1/10th the amount of Nd-148. Several other equally large differences were found. It does not seem probable that any iron oxide in the local granite can be the origin of the magnetite paste: no processes are known that could have separated these elements so sharply. One may therefore consider the possibility that all this magnetite has been brought up as an organometallic from a totally different chemical domain such as the mantle. It would be most illuminating to analyze some of the other magnetite deposits of Sweden for similar anomalies.
   From Introduction to Geochemistry, Konrad B. Krauskopf, McGraw Hill, 1982, p. 395.
   This is similar to the question we tried to answer in the last section, as to the minimum concentration of metal in a magmatic gas that would be significant for the formation of ore deposits. We proceed in the same way, using rough numbers to establish a limit of reasonableness. Suppose, for example, that an ore solution carried 10-7 g/liter of zinc. To deposit 1 ton of metal would require a minimum of 1010 cubic meters of solution, approximately the volume of water carried to the sea each year by the Hudson River (average flow approximately 10,000 sec-ft). Such a solution traversing a vein at a rate of 10 ft3/sec could deposit 1 ton of zinc in a thousand years, provided that all the dissolved zinc precipitates. The amount of water and the amount of time seem excessive, by comparison with scanty data on the flow of hot springs and on the geologic times required for the formation of ore bodies. Thus 10-7 g/liter can be taken as an absolute minimum, below which the concentration of metal is too small to be of interest. For most purposes a somewhat larger figure, say 10-5 g/liter, is a more reasonable minimum.
By this criterion the solubility of ZnS is barely high enough to be of interest at a temperature of 200° and a pH as low as 5.  The calculated solubilities of the sulfides of some other common metals (Mn, Fe, Co, Pb) have a similar order of the amounts of metal that can be carried by hot sulfide solutions seem far too small, except for a few metals under the most favorable assumed conditions, to account for the origin of ore deposits. This is the long-standing difficulty with the classical hydrothermal hypothesis.

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