Groundwater Dynamics as an Essential Factor in the Precipitation of the Pine Point MVT Lead-Zinc Deposits

Abstract:

Hypotheses on the genesis of MVT lead zinc deposits place that genesis generally well into the geological past with elevated temperatures in the 100 °C range. In the case of the Pine Point lead zinc deposits, the time of genesis has been assumed to have happened from the Middle Devonian age to the Tertiary age. It is generally said that, based on isotope data, the ore forming fluid there must have been hydrothermal in a temperature range of 100 °C or more. The average homogenized temperature in fluid inclusions in dolomite in the Pine Point area has been reported to be 116 °C and the burial temperature at about 70 °C.

In the course of a former joint industry/governmental research project on regional and local groundwater flow, water chemistry, and water isotopes, all available regional and local geological and mineral data for exploration bore holes were collected. The massive body of these data indicated that in the Pine Point region, the present groundwater flow systems and their respective chemistry would support the continuous formation of ore bodies from glacial times to the present day. This body of data provides strong indications that the interplay of today’s groundwater flow systems, their chemistry, and the associated microbiological activity may currently be forming MVT ore bodies and mineral showings even at low non-hydrothermal temperatures in the range of 3 °C. Upon abandonment of Pine Point Mines this suspicion was supported by the occurrence of a ‘black smoker’ discharging from a flowing hole near one of the formerly mined ore bodies (Figure 1).

At Pine Point, MVT ore bodies are positioned within karstic rocks at the intersection of two active and very substantial groundwater flow systems. In one of these systems, groundwater carries sulphate, while the other, upwelling one, also carries NaCl and metals. At the ore bodies, microbiological populations of sulfur-reducing bacteria are present and participate in forming conditions for ore precipitation.

Figure 1. ‘Black smoker’ at a flowing borehole near an mined-out ore body at the abandoned Pine Point mines. Photograph by J. Sandlos, 2009.