Rare Earth Element Analyses in the Exploration for Volcanogenic Massive Sulfide Type Deposits

ABSTRACT
Rare earth element (REE) data for host rocks from the
alteration zones of massive sulfide deposits in
Paleoproterozoic volcanic rocks of the Flin Flon – Snow Lake
area, Manitoba have chondrite-normalized REE profiles that
are depleted in REE relative to unaltered host rocks. The
intensely altered portions of the alteration zones show greater
Eu depletion than less altered parts of the same zone. Layered
sulfide ores and exhalites proximal to the deposits typically
have positive Eu anomalies and are enriched in light REE. In
contrast, barren sulfide facies iron formations show chondrite
normalized patterns that have negative or ÔflatÕ Eu values relative
to Sm and Gd. Eu data for these deposits reflect the
hydrothermal pathways of the mineralizing fluids. Eu is
selectively leached by hot, low pH fluids in zones of intense
alteration, and redeposited together with sulfides and/or silicates
in: (1) the uppermost parts of the alteration zones; (2)
the sulfide lenses at the hydrothermal vent sites; and, (3) precipitates
derived from the hydrothermal fluids within approximately
a kilometer of vent sites. REE data can be used by
explorationists to determine the presence of Ôoff-holeÕ metalbearing
sulfide zones, to distinguish metal-poor sulphidic
layers related to ÔeconomicÕ hydrothermal vent activity from
barren sulfide facies iron formation, and to determine the
presence of Ôore-equivalentÕ strata in some volcanic rocks.
Positive Eu anomalies are common in volcanogenic massive
sulfide (VMS) deposits and associated exhalite material
from the Paleoproterozoic Flin Flon – Snow Lake district
(Trans-Hudson Orogen) of Manitoba and Saskatchewan,
Canada (Gale et al., 1997). The positive Eu signatures extend
well beyond the limits of the known sulfide ores. In addition,
REE patterns for rocks from stratigraphically underlying
alteration zones and Ôbarren sulfidesÕ are distinctively different
than the patterns for proximal exhalite-bearing material.
We suggest that these differences in REE contents can be utilized
in the understanding of, and the exploration for, VMS
deposits.