Baker, T., Ebert, S., Rombach, C., and Ryan, C.G. (2006) Chemical compositions of fluid inclusions in intrusion related gold deposits, Alaska and Yukon, using PIXE microanalysis. Economic Geology, 101 (2). pp. 311-327.

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Abstract

Proton-induced X-ray emission (PIXE) has been used to characterize the multielement chemistry of the diverse fluid inclusions found in intrusion-related gold systems in the Tintina gold province, Yukon and Alaska. The studied samples are from shallow-level examples that contain coexisting brine (type 3) and carbon dioxide-bearing vapor (type 4) inclusions (e.g., Shotgun, Donlin Creek Dome area, Mike Lake, and Brewery Creek) and deeper level deposits (e.g., Pogo, Dublin Gulch, and Emerald Lake) that contain low-salinity carbon dioxide- (type 1) and/or methane-rich (type 5) inclusions, which locally are overprinted by late secondary type 3 inclusions (e.g., Pogo and Emerald Lake).

Major element ratios, K/Ca and Mn/Fe, of both synore high-salinity (type 3) and low-salinity (types 1, 4, and 5) inclusions are >1 and <0.24, respectively. The latter is consistent with the reduced conditions in which intrusion-related gold systems form. Late, secondary type 3 inclusions at Emerald Lake and Pogo, however, are chemically distinct, with higher Mn/Fe ratios (>0.24), and at Pogo low K/Ca ratios (<0.2). Chlorine and bromine data have been used to trace the source of salinity. Two distinct groups of Br/Cl mol ratio are recognized. Group 1 includes type 3 inclusions from the Pogo region, Mike Lake, Brewery Creek, and Emerald Lake, which have Br/Cl mol ratios consistent with typical magmatic values, mostly above 0.5 x 10–3 and below 1.54 x 10–3 (seawater). Group 2 comprises type 3 inclusions from Donlin Creek and Shotgun in southwestern Alaska, which have Br/Cl mol ratios from 2.34 x 10–3 to 6.37 x 10–3, potentially reflecting a halogen contribution from the local sedimentary crust (the Kuskokwim basin) considered to be the primary source of the granite melts.

The data also provide insights into important metal contents of the fluid inclusions, including copper, zinc, lead, tungsten, and arsenic; however, gold, bismuth and antimony were all below the detection limits for these elements by the PIXE technique. The results explain some of the distinct metal associations of shallow and deep intrusion-related gold systems. Fluid inclusions in deposits emplaced at shallow crustal levels are characterized by higher iron, manganese, zinc, and lead contents due to the greater abundance of chlorine. Tungsten is more elevated in the low-salinity, carbon dioxide-bearing fluid inclusions in deposits at deeper levels, consistent with high tungsten in the deposits and likely due to the formation of tungstate rather than chloride complexes. Copper and arsenic have similar concentrations in both low- and high-salinity inclusions, also suggesting that ligands other than chlorine were important for these elements. Experimental and microanalytical studies have shown that copper, arsenic, and gold can complex with sulfur and do not require chlorine, exclusively, for metal transport. This may explain why deposits at both shallow and deep levels contain gold despite the wide variation in salinity and different fluid types present.

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