Geochemistry of Formation Waters, Northern Alberta, Canada: Their Relation to the Pine Point Ore Deposit

The source of the ore fluid which gave rise to the Pine Point Zn-Pb deposit has been controversial for many years. Although classified as a Mississippi Valley type deposit, a source from geothermal fluids rising from the Precambrian basement was also considered. None of the authors of these various studies had the benefit of determinations of both Pb and Zn in at least one of the proposed fluid sources. A new data base of nearly 200 detailed analyses of formation waters from northern Alberta helps to resolve this controversy.

Formation waters from northern Alberta, and possibly throughout the western Canada sedimentary basin, are unusual, if not unique, in having dominantly Pb>Zn. This is the same proportion as is found in brines from the exposed Canadian Shield, but is in marked contrast to formation waters from other sedimentary basins close to Mississippi Valley type ore deposits and to geothermal fluids at Cheleken and Salton Sea. Although analyses of formation waters with Pb>Zn can be manipulated to produce Zn>Pb deposits by means of water-rock reactions, the low-pH conditions are seldom met with in sedimentary basins. It has proved impossible to derive a Zn>Pb deposit from a formation water with Pb>Zn using SOLMINEQ.88, at least under conditions comparable to the deposition of the Pine Point ore. Therefore, formation waters were not the source of the metals at Pine Point.

Because there are no data on the composition of the geothermal fluid which gave rise to the Pine Point ore deposit, circumstantial and indirect evidence must be amassed to support such an origin. This information includes:

1. the observations respecting Pb and Zn in formation waters from northern Alberta, 2. the very homogeneous and nonradiogenic character of the Pb isotopes in galena, 3. the past and present thermal anomaly along the Great Slave Lake Shear Zone, 4. the time of deposition of the ore (Famennian) with respect to possible overburden and reservoir temperatures at that time, and 5. the distribution of sphaleriteand galena in the subsurface in relation of major crustal discontinuities and normal faults cutting the crystalline basement.

Exploration for other Zn-Pb deposits should therefore focus attention on two features: 1. the location of suitable shear zones and faults up which geothermal fluids might have migrated; and 2. the location of appropriate brines or at least saline formation waters with H₂S, although there is no compelling reason why the ore fluid should be saline.

Finally, Pine Point differs in a fundamental way from other Mississippi Valley type ore deposits inasmuch as the ore fluid was of geothermal origin. This would suggest that Pine Point should be designated a new mineral deposit type.