Fluid inclusion magmatic gases are measured using quadrupole mass spectrometers and the data is compared to measurements of volcanic gas analyses. Data reported by Giggenbach from active volcanoes give N2/Ar ratios in the general range of 100 to 1000 whereas corresponding Ar/He ratios vary from 100 to 2. Plotted on a Ar/He vs. N2/Ar diagram the data defines a field that has a negative slope. Fluid inclusion gases from Mt. Erebus anorthoclase and analyses of magmatic gas-filled inclusions provided by Jake Lowenstern also plot within the same field. Thus we considered the Ar/He-N2/Ar field outline by volcanic gases and magmatic inclusion analyses to represent magmatic gases. The trend in the data is attributed to the stronger partitioning of Ar into the gas phase and as a magmatic system evolves; thus Ar becomes more rapidly depleted from the melt than N2 and He. The advantage of Ar/He vs. N2/Ar plots is that they discriminate between fluids that have magmatic N2/Ar ratios from admixed organic N2.

Hansonburg MVT (Mississippi Valley Type) deposit fluid inclusions have Ar/He ratios of 0.12 to 2.69 and N2/Ar ratios ranging from 69 to 182, and the ratios >100 might suggest a magmatic component. However, MVT deposits generally have some inclusions filled with hydrocarbon compounds, as does the Hansonberg deposit (ref), and MVT ore fluids are considered to be amagmatic and sedimentary in origin (ref). When plotted on a Ar/He-N2/Ar plot Hansonberg gas analyses plot outside the magmatic box. The same is true for Coso analyses that have some N2/Ar greater than 1000. One third of the gas data analyses from Broadlands plot within the magmatic box. The Geysers analyses plot both in and below the magmatic box and can be explained in terms of mixing between magmatic and a crustal component.

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