The Nicholson Gold-Silver Deposit, an Intrusion-Hosted, Tombstone-Like System in Southwestern Montana

The Nicholson deposit is situated within the late Cretaceous,
igneous-cored precious metal-rich Tobacco Root Mountains, which
are located in a basement uplift of the Rocky Mountain foreland.
The mostly felsic magmas that comprise the Tobacco Root batholith
intruded Archean age amphibolite and amphibolite-plagioclase
schist and gneiss of the Wyoming province.
The deposit is hosted by a calc-alkalic, weakly metaluminous
to weakly peraluminous, mafic-poor, porphyritic and sheared granite
pluton that was emplaced at shallow depth. Emplacement of the
pluton was controlled by the steeply dipping Bismark fault, one of
nine major northwest-trending intrusion- and/or deposit-controlling
structures in the region, which have been intermittently active since
middle Proterozoic time.
The deposit consists of at least five generations of quartz veins
and mineralized host rocks. The structurally oldest vein stages, one
and two, are one- to four-cm thick, of limited extent, barren of precious
metals and lack alteration envelopes. The northwest striking
and steeply dipping stage three veins contain Au and Ag, anomously
high but variable amounts of Mo, W, Bi, and/or Pb and low concentrations
of Cu, Zn, As and Sb. These tectonized veins do not have
wall rock alteration envelopes. Vein stage four consists of at least
eight prominent east-trending and steeply dipping Au- and Ag-bearing,
sheeted quartz veins including the Nicholson vein, which alone
has produced more than 10,000 ounces of gold. Anomalously high
amounts of Pb, Bi, and Te are also present in the veins. Gold- and
Ag-bearing sericite-quartz alteration envelopes halo both the stage
four and the much smaller mineralized, northwest-striking, stage
five veinlets.
The deposit has characteristics of intrusion-related, Tombstonetype
deposits found in magmatic belts in Alaska, Yukon and elsewhere
in the world. These characteristics include formation within a
tectonic and structural environment in-board of the proto North
America continental margin containing old cratonic rocks. On a
smaller scale, the district mineral deposit zoning, and the composition,
texture and depth of emplacement of the host pluton, vein types
and structures, and vein metal assemblage of the Nicholson deposit
are also similar to other intrusion-related deposits.