History of exploration at the Magma Mine, Superior, Arizona

The Magma mine at Superior, Arizona had a production history spanning 84
years, yielding some 24.5 million tonnes of ore grading nearly 5% Cu from
Cordilleran veins and massive carbonate replacement mantos. The recent discovery
of a large porphyry center 2 km south of the historic mine now indicates that the
Superior (Pioneer) mining district holds a much larger mineral endowment than the
first nine decades of mining ever managed to extract. The porphyry system was identified
in core holes drilled between 1992 and 1998; the culmination of a mine- and district-
scale exploration program originally aimed at delineating additional high-grade
reserves for the Magma mine. Results at that time indicated a mineralized deposit at
least 750 m long by 250 m wide by 300 m high with hypogene copper mineralization
grading >1% Cu. Recent published estimates suggest that the porphyry resource
exceeds 1 billion tons of 1.5% Cu (Robertson, 2004).
The porphyry discovery resulted from district-scale exploration that commenced
in 1970 near Arizona’s largest Cordilleran vein deposit, the Magma vein. Over the
course of most of the 20th century, the Magma Copper Company exploited this vein to
a depth of 1500 meters below surface and 2.5 km eastward from No. 1 Shaft under the
Apache Leap plateau. Mine development followed vein and replacement orebodies
eastward beneath Tertiary cover where in 1959 a large block of Mesozoic volcanic and
clastic sedimentary rocks was encountered to the south of the carbonate replacement
mantos. This block of Mesozoic rocks proved to be mineralized by small but highgrade
copper veins that stimulated interest in the possibility that other vein and/or
replacement lodes might lie undiscovered in the district. Evaluation of the structural
and stratigraphic implications of the Mesozoic block, hidden beneath the Apache Leap
plateau, has since been the key to locating additional copper resources in the Magma
mine and the adjacent district. Holes drilled from surface to test the postulated south
boundary of the Mesozoic block, between 1972 and 1981, failed to locate the structure.
When the mine reopened in 1990, after shutdown in 1982, exploration was continued
to the south. Surface drill hole MB-9, collared in 1991, was the first of several
that cut Mesozoic rocks altered to quartz, sericite, pyrite and cut by copper veins.
Underground drill hole 36108-S27E in 1994 intersected a wide zone of Mesozoic clastic
rocks exhibiting quartz-sericite-pyrite alteration and cut by thin chalcocite veins
surmised to lie at a high structural level over a concealed porphyry system. Underground
drill hole 36108-S27H inclined southerly into a zone exhibiting secondary
biotite (potassic alteration) with grades near 2% Cu indicated the potential for an
economically viable deposit. Four additional drill holes produced intercepts with
grades indicating a system capable of sustaining grades above 1% Cu.