ABSTRACT    (abbreviated)
Zonation patters are well developed in the mafic metavolcanic rocks that host epigenetic quartz-carbonate-gold vein mineralisation at the Hollinger-McIntyre deposit.

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Lithogeochemical zoning is equally well developed as variations in whole-rock abundances of As, Au, Ba, Rb and Sb.
As and Ba form the most useful zonation patterns, outlining individual ore zones as well as broader trends of mineralisation.

Gas chromatographic analyses of fluid inclusions in the quartz-carbonate veins indicate that the mineralising fluid was an H₂O-CO₂ mixture. Locally, CH₄-rich fluid inclusions are associated with graphitic and/or carbonaceous wallrocks. Fluid inclusions in ore-bearing veins have higher CO₂ contents than those in non-ore veins. Extensive sampling and analyses of the veins from the area surrounding the deposit reveals a well developed zonation of CO₂ contents about the main zone of mineralisation. Low CO₂ levels (1-2 mole%) in veins are peripheral to the deposit and increase to over 12 mole% in mineralised zones.

The observed zonation of the carbonate alteration assemblages is the result of CO₂ addition to the wallrock, which is consistent with the zoning of CO₂ in the fluid inclusions.

The zoning patterns delineated in this study are useful in exploration both on a deposit scale and on the scale of individual ore zones.

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FLUID INCLUSION ZONATION

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The amount of CO₂ in the samples effectively distinguishes between ore and non-ore samples. In general, ore samples have group 1 (CO₂ rich) compositions, whereas non-ore samples have group 2 (low CO2) compositions (Fig. 20)(below). With a cutoff value of 4% CO₂ approximately 83% of the ore samples would be identified as anomalous. The distinction is not as accurate for the non-ore sample group where 64% of the samples have CO₂ contents below 4 mole %.



CO₂ levels in the fluid inclusions show distinct zoning relationships to the mineralisation. Figure 21 (below) shows contours of mole % CO₂ for the surface vein-quartz samples. Low CO₂ values of 1% to 2% surround the mine area and increase to values of over 12 % in the mineralised zones on the Hollinger property. It is interesting to note that high CO₂ values show a good correlation with the distribution of the intensely carbonatised Assemblage IV rocks. Lower values tend to be found in veins in less altered rocks. Careful examination of the fluid inclusions shows that the high CO₂ values observed in our analyses reflect actual CO₂-rich compositions rather than contamination from carbonate minerals.



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Application of Zoning to Exploration

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CO₂ levels in fluid inclusions in quartz veins in areas peripheral to mineralised areas are low, less than 4 mole %, whereas values of 4 % and above are restricted to areas of ore-grade mineralisation. Samples with significant amounts of CH₄ (greater than 2 mole %) are associated with wallrocks of graphite or carbonaceous material.The distinction in terms of CO₂ content between ore and non-ore samples still holds for these graphite samples. Obviously this method can only be applied to rocks which contain quartz veins and is most useful in exploration on an ore-zone scale, in evaluating the gold potential of individual veins within a larger vein system.


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