DISTINGUISHING BARREN FROM PRODUCTIVE EXHALATIVE STRATA AND VECTORING TOWARD HYDROTHERMAL VENT SITES USING EU

Layered sulphide ores and exhalites proximal to massive sulphide deposits in Proterozoic volcanic rocks of Manitoba and elsewhere typically exhibit positive Eu anomalies and are commonly enriched in some rare earth elements (REE). The magnitude of the positive Eu anomaly decreases away from the hydrothermal vent and commonly extends beyond the haloes created by base metals and other pathfinder elements.The Eu data reflect the hydrothermal pathways of mineralizing fluids for metal deposits. Eu2+-bearing, hot, low pH metalliferous fluids, derived from crystallizing magma chambers, selectively leach additional Eu2+ from zones of intense alteration directly underlying massive sulphide deposits. Eu is then redeposited together with sulphides and/or silicates in: 1) the uppermost parts of the alteration zones; 2) the sulphide lenses at the hydrothermal vent sites; and 3) precipitates derived from the hydrothermal fluids within approximately a kilometer of vent sites.

Sulphidic sedimentary rocks unrelated to hydrothermal vents have negative to flat Eu anomalies. These rocks do not show Eu variations along strike, exhibit signatures that are different from those of pelagic shales and can be distinguished from sulphide facies iron formations related to vents sites of volcanic- and sediment- hosted deposits even when the barren sulphide formations contain anomalous Cu and Zn.

Our data indicate that stratiform rocks associated with a number of gold deposits also have positive Eud anomalies. It is our contention that these deposits are of exhalative origin and have formed as a result of hydrothermal vent activity similar to those responsible for the formation of VMS deposits.

REE data can be used, together with other pathfinder elements, to indicate the presence of ‘off-hole’ metal-bearing sulphide zones, to distinguish between metal-poor sulphidic layers related to ‘economic’ hydrothermal activity and barren sulphide facies iron formation, and to determine the presence of ‘ore-equivalent’ strata in some volcanic and sedimentary rocks.