The Chulitna region is located along Alaska's major rail-belt approximately 210 km to the north of Anchorage and hosts the Golden Zone deposit as well as numerous prospects with intriguing values of precious and base metals.
The Chulitna area is comprised of several northeast trending belts of rocks of upper Paleozoic to Cretaceous age (herein referred to as the "older assemblage") bounded to both the northwest and southeast by Jurassic to Cretaceous flysch of the Kahiltna assemblage (Reed and Nelson, 1980). Both the Kahiltna and older assemblages are sandwiched between Wrangellia terrane (Jones and others, 1981) to the south and the Denali fault and North America craton or Yukon-Tanana terrane to the north. Published literature describes much of the Chulitna region as allochthonous terranes most likely accreted to Wrangellia and rafted north to collide with North America in the Late Cretaceous (Csejtey and others, 1992). Based on our mapping and petrology, we suggest instead that Chulitna may be part of Wrangellia and can be correlated with upper Paleozoic to Mesozoic rocks of the east-central Alaska Range and western Yukon Territory, with a complex Cretaceous and younger structural history accompanied by two major intrusive episodes.
The dominant structures in the Chulitna district are high-angle faults, as indicated by linear patterns of shallow earthquake epicenters. Early workers (for example, Hawley and Clark, 1974) mapped these as high-angle faults because their nearly straight-line surface traces show no deflection by the greater than 2 km topographic relief. Subsequent workers (Jones and others, 1980; Csejtey and others, 1992) mapped these as [moderate-to high-angle] thrust faults to accommodate their geologic model of terrane collision. However, these faults, which can be traced both on the ground and by linear zones of high conductivity, show abrupt breaks on aeromagnetic maps, indicating they are very steeply dipping.
In an effort to better understand the geology and many known mineral deposits of this area along Alaska's main transportation corridor, a study was initiated by the Alaska Division of Geological & Geophysical Surveys (DGGS) in 1996 with a 364 mi2 airborne aeromagnetic and electromagnetic survey (magnetic and resistivity) (DGGS and others, 1997a,b,c,d; Burns, 1997). In 1997, a two year geologic mapping effort began as follow-up to the airborne geophysical surveys. Because many of the bedrock units in the region have distinctive geophysical signatures, the airborne surveys proved to be extremely useful mapping tools to delineate both map units and structure, particularly in areas of extensive surficial cover. The Bureau spent a total of six weeks in the field between 1997 and 1998.
The geologic setting of the southern part of the Chulitna district is very similar to that of the northern-Permian-Triassic rocks faulted against mid-late Mesozoic rocks on the east, and succeeded westward by Triassic, Jurassic, and Cretaceous and possibly Tertiary (Cantwell Formation) rocks. It differs in an unusual unit of basalt, mafic volcanic breccia, and interstratified limestone which overlies the red-bed limestone sequence of Permian-Triassic age and underlies the Mesozoic graywacke-argillite. The mafic volcanic limestone unit is important as the host of mineral deposits in Partin Creek and in Canyon Creek, part of the Ohio Creek area. Ultramafics tentatively considered to be ophiolitic are abundant along the eastern part of the belt.
Except for the granitic stock in upper Ohio Creek, and the cluster of intrusives near Coal Creek, intrusives are less abundant than in the northern part of the Chulitna district.
Types of mineral deposits include:
(1) segregations of chromite in ultramafic rocks;
(2) massive sulfide skarn in limy hosts; and
(3) veins.
Greisen and tourmalinized zones locally occur in granitic intrusives. Copper and locally native silver is disseminated in the basaltic volcanics.
