Lead and zinc in formation waters, Alberta Basin, Canada: Their relation to the Pine Point ore fluid

Abstract

The source of the ore fluid for the Pine Point Zn–Pb deposit has been controversial for decades. Both formation waters from deep in the basin and geothermal fluids from the Precambrian basement have been considered as sources of the ore in this carbonate-hosted deposit. A database of 694 analyses of Pb and Zn in formation waters from the Alberta Basin bears on this controversy. These waters have a mean content of 4.66 mg L −1 Pb and 6.88 mg L −1 Zn. For each of these metals, the maximum content is 360 mg L −1 and the median content is 0.9 mg L −1. In 71% of the samples Pb > Zn, with a mean ratio of 2.3:1. This is in contrast to the Pb:Zn ratio of formation waters in other sedimentary basins and geothermal waters but is consistent with that in brines from the Canadian Shield. Geochemical modelling using SOLMINEQ.88 shows that Pb and Zn occur mainly as the MeCl 4 2 - complex. It is not possible to produce a sulphide precipitate with Zn > Pb, such as the ore at Pine Point, from a formation water with Pb > Zn, even when the formation water carries the metals in the absence of H 2S, and the metals are precipitated by H 2S-rich brines. Therefore, the present formation waters are a poor candidate for the source of the metals at Pine Point. It is more likely that geothermal fluids rising along the Great Slave Lake Shear Zone were the source of the metals. Additional evidence for this alternate source includes: (1) the very homogeneous, non-radiogenic character of the Pb isotopes in galena; (2) the absence of a thermal anomaly along the Great Slave Lake Shear Zone; (3) a Famennian time of deposition (based on Rb–Sr dating of sphalerite), when overburden was thin and reservoir temperature low; and (4) the distribution of sphalerite and galena in the subsurface of the Alberta Basin in relation to major crustal discontinuities and normal faults cutting the crystalline basement. Exploration for other Zn–Pb deposits should therefore focus on the location of suitable shear zones and faults up which geothermal fluids might have migrated, rather than the flow and composition of formation waters.