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Campbell, K.B., 2006

The geology, alteration, and mineralization of the True North gold deposit, Fairbanks, Alaska

Bibliographic Reference

Campbell, K.B., 2006, The geology, alteration, and mineralization of the True North gold deposit, Fairbanks, Alaska: University of Nevada, Reno, Ph.D. dissertation, 295 p.


The True North gold deposit is a unique epithermal system, hosted in a klippe of eclogite-grade metasediments (Chatanika terrane) that rests on the Fairbanks schist about 28 km northeast of Fairbanks, Alaska. Host rocks are predominantly biotite to quartz-biotite schist, locally graphitic, in the Hindenberg Pit (northeastern part of the deposit) and graphitic slate-argillite in the Shepard Pit (southwestern part of the deposit). Mineralization associated with the intrusion of the Pedro Dome stock (Gilmore Dome intrusive complex) at ~92 Ma and is localized within two set of faults. A low-angle fault set is sub-parallel to and related to thrust faults that emplaced the Chatinika sub-Terrane over the Fairbanks Schist. Mineralized high-angle faults include faults sub-parallel to and related to district scale northeast-trending left lateral oblique-slip faults, and several north- to northeast-striking faults. All mineralized faults have tens of centimeters to meters of graphitic gouge with a well-developed planar, anastomosing fabric concentrated along slip planes with evidence of multiple stages of movement. The graphite is physically remobilized from graphitic host rocks. Mineralization consists of multiple generations of quartz-gold-arsenopyrite-pyrite veins and several generations of carbonate veins occurring within the fault zones. Carbonate veining locally extends out into relatively unaltered host rocks for several tens of meters. Veins are crushed and dismembered in the fault zones by repeated fault movement, and intact veins are only rarely preserved. Alteration consists of a halo of fuchsite-ankerite-arsenopyrite and pyrite around veins. Where veins are closely spaced, or veins intersect host rock foliation at high angles, large volumes of rock are altered. Quartz fluid inclusion mean homogenization temperatures, pressure corrected for an estimated 1 km formation depth, are 240°C for Hindenberg Pit veins (biotite schist host rocks) and 270°C for Shepard Pit veins (slate-argillite host rocks). Quartz inclusion fluids are dilute, ~4 weight percent NaCl equivalent, with as much as 3 mole percent CO2 . Carbon and oxygen isotopic data suggests that the fluid source was either igneous or groundwater isotopically equilibrated with the host rocks and that carbonate veins were precipitated in equilibrium with graphite in the faults that contain the veins.

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