Download Regional alteration systems associated with Snow Lake VMS

Regional alteration systems associated with Snow Lake VMS deposits, Manitoba
Suzanne Paradis*(Geological Survey of Canada)
P.O. Box 6000, 9860 West Saanich Road, Sidney, BC, V8L 4B2, [email protected]
Alan H. Bailes
(Manitoba Industry, Trade and Mines)
A.G. Galley
(Geological Survey of Canada)
Volcanogenic massive sulphide (VMS)-hosting 1.89 Ga juvenile oceanic arc volcanic rocks of the Snow
Lake Assemblage are characterized by volumetrically extensive zones of anomalous 1.82 Ga
metamorphic minerals, such as porphyroblasts of biotite, chlorite, amphibole, garnet, kyanite, staurolite,
anthophyllite, cordierite, sillimanite, and sericite. These minerals and their assemblages are interpreted to
be the result of pre-metamorphic (1.89 Ga) syngenetic high- and low-temperature fluid-rock interactions
at the seafloor and within the subseafloor (intra-stratal) environments. More than 25 vol.% of the volcanic
rocks and subvolcanic intrusions in the Snow Lake area are affected by metamorphosed zones of
syngenetic alteration that occurs as discordant, and broad, semi-conformable zones that are often
associated with sulphide mineralization.
Several major hydrothermal events occurred within the primitive and mature volcanic arc succession of
the Snow Lake area: (1) Semi-concordant and discordant amphibole-chlorite, chlorite-biotite, and
aluminosilicate alteration zones associated with Cu-Zn-rich VMS mineralization of the primitive arc
volcanic sequence; (2) a concordant zone (>20 km long by 0.5 km wide) of silicified basalts capping
volcanic rocks of the primitive arc succession and underlying a regionally extensive base metal-poor
sulphidic sediment; and (3) semi-concordant and discordant amphibole-chlorite, chlorite-biotite,
aluminosilicate, and quartz-epidote alteration zones associated with Zn-Pb-Cu and Cu-Zn-Au VMS
mineralization of the overlying mature arc succession. Semi-concordant alteration zones can be over 20
km in strike length and up to 0.8 km wide. The surface exposure of the discordant alteration zones can
also be several km in strike length and up to 1 km wide.
The study of regional alteration systems within the Snow Lake district constitutes a major component of
the CAMIRO Project 94E07. Its objectives are to resolve some fundamental questions about alteration
zones and to identify features that could be used as exploration criteria in the search for VMS deposits.
Some of the most fundamental questions are: (1) what are the distribution, mineralogy, chemistry and
isotopic signatures of the discordant and semi-concordant alteration zones associated with the rhyolite
complexes? (2) Are there mineralogical, chemical or isotopic variations that can be used to ‘vector-in’ on
VMS-prospective domains along the semi-concordant zones? (3) What is the spatial relationship between
alteration, subvolcanic intrusions, and VMS deposits? (4) Are the VMS deposits and their footwall
alteration ‘pipes’ produced by the same hydrothermal event as that responsible for the semi-concordant
alteration zones or are they a product of a separate event? (5) Are there features in the semi-concordant
alteration zones that identify them as VMS-associated or VMS-poor?
To achieve these objectives and to answer some of the fundamental questions, geological and alteration
mapping of various rhyolite complexes within the Snow Lake area (i.e., Anderson, Daly, Sneath, Chisel,
Photo, and Cook) was done at the scale of 1:5000. In addition, regional-scale alteration patterns were
identified using chemical element abundance and mineral abundance maps. The latter maps showed the
relationship between chemical alteration and anomalous metamorphic mineral assemblages;
for example high concentrations of MgO, FeO(t), and AI (alteration index =
100[(MgO+K 2 O)/(MgO+K 2 O+CaO+Na 2 O)] and low concentrations of Na2O and δ 18 O values are present
Regional alteration systems associated with Snow Lake VMS deposits, Manitoba
in regions of known mineralization and correspond to anomalous concentrations of minerals such as
chlorite, amphiboles, and aluminosilicates.
In a terrane recrystallize to amphibolite-grade metamorphic mineral assemblages, such as that at Snow
Lake, mineral-directed prospecting and mapping (either visually or by XRD techniques) of anomalous
coarse-grained metamorphic mineral assemblages (such as, chlorite-garnet-kyanite-staurolite) is a useful
tool to identify zones of synvolcanic hydrothermal alteration that are potentially associated with VMS
deposits. The appearance in these zones of aluminosilicates accompanied by an increase in abundance of
chlorite and amphiboles has been shown to correspond to domains that were affected by high temperature
hydrothermal fluid-rock interactions.
Bibliographical Note:
Suzanne Paradis obtained a M.Sc. (1984) in geology at the Université de Montréal and Ph.D. (1990) in
geology at Carleton University. She practised as a project geologist with the Quebec provincial
government and as an exploration geologist with mining companies for more than 10 years before joining
the Geological Survey of Canada as a mineral deposit research scientist. Her research interests are in the
application of field and geochemical techniques to understand regional-scale hydrothermal systems and
the relationships between tectonism and hydrothermal activity that generates base metal deposits.