The Flat project occurs within the Iditarod Quadrangle where the dominant structural feature is the regionally extensive right-lateral, strike-slip Iditarod-Nixon Fork Fault, which extends from the southwest to northeast corners of the quadrangle. Substantial vertical movement is inferred across the fault with the north side up thrown1.
North of the fault zoned multiphase stocks of Chicken Mountain, Black Creek (68.3 to 73.2 Ma) and Swinging Dome (59.4 to 63.4 Ma) intrude Kuskokwim sediments and are genetically related to accumulations of intermediate and felsic volcanics. Hornfelsing occurs within both the sedimentary and volcanic rocks adjacent to the intrusion margins. The intrusive centers are aligned in a broad belt paralleling the fault.
For reference, south of the fault, hypabyssal bodies of rhyolitic porphyry intruding Kuskokwim sediments are predominant, with rhyolitic dike swarms hosting the Donlin Creek Deposit which contains a resource of +30 million ounces of gold. Age of intrusion and mineralization of the Donlin dykes ~71Ma is similar to the age of emplacement of the Chicken Mountain complex2.
Both the Chicken Mountain and Black Creek stocks are compositionally zoned. At Chicken Mountain three major compositional phases are present: pyroxene monzodiorite and gabbro comprise the outer margin and initial phase, biotite monzonite forms a relatively homogeneous central transitional phase and leucocratic quartz monzonite forms the central phase. The Black Creek stock is comprised of an olivine pyroxene biotite gabbro outer phase and a fine-grained leucocratic biotite monzonite core. Younger northeast trending leucocratic monzonite, quartz porphyry and quartzo-feldspathic dykes cut the earlier phases in the Black Creek Stock.
Gold mineralization at Flat has been recognized in three geological settings.
- Low grade intrusion hosted (with potential high-grade zones) within altered, fractured and veined late-stage phases in the Chicken Mountain and Black Creek stocks. Notably the most significant gold is hosted within the late-stage quartz monzonite at Chicken Mountain, which appears to have been the major source of placer gold in the Flat district. At Chicken Mountain gold occurs as widespread disseminated and fracture-controlled gold mineralization in pervasively altered, unroofed central phase of the intrusion. Mineralization occurs within quartz-clay-iron oxide-sulphide veinlets and veins, which contain free gold, arsenopyrite, pyrite, stibnite, cinnabar and rare chalcopyrite molybdenite. Mineralization appears to be controlled by a north trending structural zone termed the “Chicken Mountain Structure”.
- At Black Creek mineralization is contained within northeast striking steep dipping, sheeted sub parallel veins. Narrow veins contain quartz and limonite while wider veins (to 18 inches contain open space fills of massive stibnite with pyrite, arsenopyrite, cinnabar, scheelite and native gold.
- Contact related vein and disseminated bulk tonnage mineralization hosted within sedimentary and volcanic rocks. Most notable is the Golden Hornfels zone which occurs along the eastern contact of the Black Creek stock.
Bundtzen et al 1992 classify the lode deposits at Flat as intrusion-hosted gold-polymetallic that have characteristics of alkaline related precious metal systems and interpret the Flat lodes to represent the same Late Cretaceous zoned system which spanned mesothermal and epithermal temperature ranges, now exposed at several erosional levels3. Shallow epithermal mineralization in the more deeply eroded Black Creek stock has been largely removed, whereas the partially unroofed Chicken Mountain stock still has much of the shallow, low temperature, mercury-gold-antimony association intact.
- 2002 Annual Report, Flat Property, Alaska. Prepared for Doyon Limited on behalf of Ventures Resource Corporation, February 2003.
- MacNeil, K. D. (2009). The timing and structural evolution of the Donlin Creek gold deposit, southwest Alaska (T). University of British Columbia. Retrieved from https://open.library.ubc.ca/collections/ubctheses/24/items/1.0052896
- Bundtzen, T.K., Miller, M.L., Laird, G.M., and Bull, K.F., 1992, Geology and mineral resources of Iditarod mining district, Iditarod B-4 and eastern B-5 quadrangles, southwestern Alaska: Alaska Division of Geological & Geophysical Surveys Professional Report 97, 88 p., 2 sheets. https://doi.org/10.14509/2278