BARBARA S. KOWALSKI* and STEPHEN A. KISSIN**
*690 Dog Lake Road, Thunder Bay, Ontario P7B 5E5 Canada
**Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5E1 Canada
Three stages of hydrothermal mineralization are distinguishable on the basis of mineralogy and cross-cutting relationships in both deposits. Stage 1 (Pre-ore) precipitated quartz and ankerite accompanied by hematite, pyrite, and chalcopyrite in small amounts. Stage 2 (Ore stage) alteration is characterized by silica flooding accompanied by clinochlore, ankerite, potassium feldspar, and minor tourmaline and sericite. Native gold, argentite, pyrite, and chalcopyrite were deposited at this stage. Stage 3 (Post-ore) alteration is in the form of quartz-calcite and muscovite-sericite veinlets.
Fluid inclusions in quartz and carbonate are of five types:
type 1: aqueous (L + V)
type 2: aqueous (L + V + S)
type 3: H2O (L + V) + CO2 (L)
type 4: H2O (L + V) + CO2 (L) + S
type 5: CO2 (L + V)
Stage 1 mineralization at both deposits contains type 1 and type 3 fluid inclusions. All inclusions are of low salinity with freezing point depression of 1oC or less, consistent with observed clathrate melting temperatures of +10.4 to +10.6oC. The detection of clathrates in type 1 inclusions indicates the presence of dissolved CO2 or CO2(L) in quantities too small for visual detection. Homogenization and eutectic temperatures fall into three groups:
|Tm(ice)||-0.5||-6 to -0.1|
|Tm(ice)||-0.5||-0.3 to -0.8|
|Tm(ice)||observed||-0.2 to -1.2|
Stage 2 (Ore-stage) fluid inclusions are of type 3 and 4 at both deposits, where the solid (S) in type 4 is probably a poorly crystallized silicate and is unreactive.
|Tm(ice)||-0.1||-0.1 to -8.9|
Stage 3 fluid inclusions were of two populations at Golden Highway but only one at Metalore:
|Tm(ice)||-0.9||-0.7 to -8.3|
|Th||+30 to +56||+33|
|Th||+30 to +70|
The stage 2 (Ore-stage) fluid inclusions are particularly interesting in that their homogenization temperatures lie near the locus of critical end-points for the CO2-H2O system. Moreover, the majority of inclusions at Golden Highway homogenized to liquid whereas those at Metalore homogenized to vapor. Estimates of densities of fluid inclusions from the two deposits further indicate that they were trapped on alternate sides of the locus of critical end-points. Thus, the fluid inclusions record the onset of the breakdown of supercritical fluid in the CO2-H2O system, whereupon CO2-rich vapor effervesced from H2O-rich liquid.
The pressures as determined from P-V-T relations in the CO2-H2O system cluster around 1200 bars. The relatively low pressure together with change in style of mineralization from shear zone at Metalore to fissure vein at Golden Highway are suggestive of a transition from mesothermal to epithermal conditions. Moreover, the decomposition of supercritical fluid at the site of gold deposition may have been an important factor in its deposition as well as also marking the mesothermal-epithermal transition