Over 1,892 tonnes (61 million ounces) of gold have been produced
from Archean vein deposits in the world class Porcupine Gold
Camp in the Abitibi greenstone belt at Timmins, Ontario. These
intrusion related gold deposits formed late in a sequence of localized
felsic volcanism, felsic intrusive activity, and fault-controlled
Timiskaming sedimentation between 2698 and 2673 Ma, which followed
earlier mafic and ultramafic volcanism. They were subsequently
deformed during Kenoran tectonothermal events that
affected the entire Superior Province.
Krist Formation heterolithic felsic volcaniclastic rocks (2698±4
Ma) were derived through local reworking of felsic volcanic material
and were deposited unconformably on Tisdale Group mafic and
ultramafic volcanic flows. Major deformation soon after Krist volcanism
resulted in east-striking, periclinal folds, which lack axial
planar fabrics, along with contemporaneous thrust faults.
Timiskaming sedimentation occurred in a basin along the north side
of the Dome and Destor Porcupine faults.
Epizonal porphyritic felsic stocks and dikes, and associated
phreato-magmatic breccias, were emplaced between 2688±2 and
2691±3 Ma into folded and faulted sites, including two major intrusive
centers that coincide with the most productive gold deposits in
the Hollinger-McIntyre-Coniaurum Complex, and in the Dome-
Paymaster-Preston Complex. These centers were likely feeder sites
for Krist volcanism some 8 Ma earlier.
Intrusion of 2673+6/-2 Ma old albitite dikes into older intrusive
centers initiated porphyry-style hydrothermal systems that culminated
in the formation of vein Au-Ag deposits and the McIntyre
stockwork Cu-Au-Ag-Mo deposit. The volume of fractured and
altered rock around mineralized structures is far greater than the
structures themselves, a result of catastrophic energy release during
early barren stages of the hydrothermal systems.
Gold was introduced during focused fluid flow along favorable
lithostructural sites late in the evolution of these hydrothermal systems
and is much more restricted in extent than early barren alteration.
Lithologic contacts, volcanic flow tops, competent units in
heterogeneous sequences, breccias, porphyry margins, faults, and
fold hinges are all loci of alteration, mineralization, and veining.
Mineralized zones are characterized by single or multiple quartzcarbonate
veins hosted in carbonatized, sericitized, albitized, and
pyritized wallrock. Albite, tourmaline, sericite, pyrite and other sulfides,
and native gold may also be present in the veins.
Tourmalinized breccia dikes, developed synchronously with early
(barren) tourmaline-rich vein phases.
Intrusions and hydrothermal features were modified, and early
folds refolded, during post-mineralization deformation. Veins are
deformed and flattened hydrothermal stockworks define LS fabrics
in penetratively deformed rocks. High strain zones, characterized
by strong foliation, developed preferentially in incompetent lithologies
and sericitic alteration zones.