Program, Abstracts, and Guidebooks - University of Minnesota Duluth

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ROOT SEVERANCE AND TECTONIC TRANSPORT OF OREBODIES
IN METAVOLCANIC HOST ROCKS
WILLIAM F. JENKS
University of Cincinnati
ABSTRACT
Association of lenticular, concordant, semi—concordant, and
cross—cutting massive sulfide bodies with mafic and felsic volcanic
sequences is well known. Some are clearly of submarine exhalative
or replacement origin. Others may well be related to subaerial
volcanism, but near sea level in a zone of negative crustal movement,
since preservation of near surface phenomena in a eugeosynclinal
environment requires relatively rapid covering and burial. Meta—
volcanic sequences originating in active and subsiding tectonic belts
have been subjected to all postvolcanic events affecting the
enclosing metasedimentary rocks; they may have undergone deformation
by overthrust faulting, nappe folding, and refolding. Separation
of the volcanic pile (and associated ores) from its roots during
such deformation would be expected. These structures, in meta—
volcanic terranes, can go unrecognized because of originally complex
volcanic—stratigraphic relations, transposition by sliding, md
deep folding and metamorphism.
Tectonic severance appears to be the reason for the absence of
obvious plutonic source rocks in many metavolcanic sequences and
their ores. Separation of ores from roots may be more than 50 1cm
if we take Alpine deformation as a model. Certain types of
volcanic masses such as rhyolite domes would yield to tectonic
transport in a manner controlled by local contrasts in ductility,
shape, and size., The deformational style is quite unlike that
produced in regularly layered rocks. Orebodies, with their normal
envelopes of hydrothermal alteration, may 'be transported in an
environment especially susceptible to structural irregularity
because they are in ductile shells adjacent to irregular volcanic
masses. Resultant structural details would be expected to be
still more complicated by selective flowage of some sulfide minerals
and by migration in response to new chemical gradients.