Abstract

The New World district is located in the northern Rocky Mountains in south-central Montana and contains a Tertiary porphyry mineralizing system with five Au-Cu-Ag skarn and replacement deposits and one breccia-hosted Au-Cu-Ag deposit. Most of the stratabound deposits are hosted by gently dipping dolomitic limestone of the Cambrian Meagher Formation, and occur along the western contact zones of two dacitic intrusive complexes. The six deposits contain a total geologic reserve of 10.3 million metric tonnes averaging 7.54 g/t Au, 0.73% Cu, and 30.51 g/t Ag.

Hydrothermal alteration is concentrically zoned about the intrusive complexes. Early barren alteration in adjacent sedimentary rocks includes biotite-bearing hornfels and epidote and potassium feldspar-bearing calc-silicate hornfels. The metalliferous alteration assemblages in sedimentary rocks include quartz-pyrite rock, magnetite and / or specularite-rich replacements, and lesser epidote-rich retrograde skarn. Typical deposit characteristics include: 1) limestone-rich units as the dominant hosts, 2) mineralization occurring along the contact zone of two sericitized dacitic intrusive complexes, 3) abundant faulting, and 4) high sulfide (10-95% by volume) and specularite and/or magnetite content of the ore.

The proposed mining operations are currently being reviewed by the Federal, State, and county agencies. Mine construction is scheduled to begin in the summer of 1997 with a targeted annual Au production of 4,043 kg over a 12 to 15 year mine life.