PACROFI VI - Electronic Program

Fluid inclusion studies of the Metalore-Golden Highway gold deposits, Geraldton-Beardmore district, northwestern Ontario


*690 Dog Lake Road, Thunder Bay, Ontario P7B 5E5 Canada
**Department of Geology, Lakehead University, Thunder Bay, Ontario P7B 5E1 Canada

The Metalore and Golden Highway deposits are two adjoining gold deposits located in the Geraldton-Beardmore Greenstone Belt of northwestern Ontario, approximately 20km northeast of the town of Beardmore. The deposits lie within the Wabigoon Subprovince of the Canadian Shield and are hosted by rocks of Archean age. The Metalore deposit is located in a steeply dipping shear zone mostly within a thin, mafic metavolcanic unit overlain by polymictic metaconglomerate. The metavolcanic unit has been intruded by metadiorite. The east-west striking shear zone, which dips to the south roughly parallel to bedding, is a splay of the regionally extensive Paint Lake Fault. The Golden Highway deposit is more typically a fissure vein dominantly of quartz-carbonate gangue, which follows the contact between pillowed, mafic metavolcanics and metadiorite. The vein occurs in a fault interpreted as a subsidiary fault striking N30E and located immediately to the west of the Metalore deposit.

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:

Golden HighwayMetalore
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

These data suggest that a NaCl-H2O fluid evolved to a NaCl-CaCl2-H2O fluid or that the -22oC eutectic temperature is observed only because of the small amount of liquid formed at the eutectic for NaCl-CaCl2-H2O. The hot phase of crystallization observed only at Metalore suggests that an additional component, possibly FeCl3, has been incorporated in NaCl-CaCl2-H2O bearing inclusions.

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.

Golden HighwayMetalore
Tm(ice)-0.1-0.1 to -8.9

The stage 2 data suggest that a low salinity, MgCl2-H2O fluid accompanied gold deposition. A separate CO2 (L) phase was always observed at room temperature, although clathrate formation was generally missed, perhaps owing to the low salinity of the system.

Stage 3 fluid inclusions were of two populations at Golden Highway but only one at Metalore:

Golden HighwayMetalore
Tm(ice)-0.9-0.7 to -8.3
Th+30 to +56+33
Th+30 to +70

The -56oC eutectic temperatures are characteristic of the CO2-H2O system with negligible salinity. The homogenization temperatures above +31.1oC indicate the effect of low temperature, H2O-rich inclusions. The aqueous inclusions with Te clustering around -15oC at Golden Highway are difficult to explain, but Raman spectra of some of these reveal the presence of N2, which may account for the variable eutectic temperatures observed.

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