What a mineral system is

The system, not the deposit

Geoscience Australia puts the relation plainly: a mineral deposit is one very small part of a much larger geosystem, and the signatures of that system extend well beyond the deposit. The approach exists because roughly 80 per cent of Australia is covered by younger basins and regolith that hide direct signatures, so a larger thing is easier to find than a smaller one. The consequence runs the other way as well, and matters more here: finding part of a system is not finding a deposit.^[1]

Lithoen therefore models the system, not the deposit. Six systems are recorded, each drawn from a published United States Geological Survey deposit model, and each one is stated as a chain: where its components came from, what carried them, what focused the flow, what caused them to stop, what modified the result, and what might be observed of it today.^[1,2,3]

The chain is different in every system

The porphyry copper system draws its metals and its fluid from the same magma. Fluid leaving a porphyry magma deeper than about five kilometres is a single supercritical phase of moderate salinity, and it separates into a high-salinity brine and a lower-salinity vapour as pressure falls below about 1.4 kilobars.^[1,2,3,4]

The volcanogenic massive sulfide system uses seawater. Chemical and isotopic studies of modern and ancient deposits indicate that seawater, or seawater evolved by reaction with the rocks it passes through, is the dominant mineralising fluid; the circulation is driven by magmatic heat and the fluid reaches up to about 400 degrees Celsius where it discharges.^[1,2,3]

The Mississippi Valley-type system has no igneous driver at all. Its metals and its sulfur come from crustal sources, its fluid is a basinal brine of 10 to 30 weight percent salts, and deposition happens between about 75 and 200 degrees Celsius — temperatures at which nothing glows and nothing is molten.^[1,2,3]

The magmatic nickel-copper system uses no aqueous fluid whatever. A mantle-derived magma becomes sulfide-saturated, usually after taking up sulfur or contamination from crustal rocks on the way up, and an immiscible sulfide liquid separates and settles.^[1,2]

The nickel-cobalt laterite system uses rainwater. The nickel is already in the rock at 0.2 to 0.4 percent, held inside olivine and serpentine; percolating water removes magnesium and silicon and leaves the nickel behind in hydrated alteration minerals.^[1,2,3,4]

Why the same architecture is used for all of them

The five chains above share no mechanism. Their fluids range from a supercritical magmatic phase to rainwater, their temperatures from 1,200 degrees to ambient, their drivers from a pluton to a passing weather system. What they share is a shape: something supplied the components, something mobilised them, something moved them, something focused the movement, something stopped it, and something either destroyed or preserved the result.^[1,2,3,4]

That shape is what Lithoen encodes. It is what lets a question be asked in the same form of any system — what evidence would we expect if this operated here? — without the question smuggling in an answer.^[1]

Commonly misunderstood