Extracting ore-deposit-controlling structures from aeromagnetic, gravimetric, topographic, and regional geologic data in western Yukon and eastern Alaska

Abstract

Aeromagnetic lineaments interpreted from reduced-to-pole (RTP) magnetic grids were compared with gravity, topography, and field-based geologic maps to infer regional structural controls on hydrothermal mineral occurrences in a poorly exposed portion of the North American Cordillera in western Yukon and eastern Alaska. High-frequency and variable-intensity aeromagnetic lineaments corresponding to discontinuities with an aeromagnetic domain change were interpreted as steep-dipping and either magnetite-destructive or magnetite-additive faults. These structures were interpreted to be predominantly Cretaceous in age and to have formed after the collision of the Intermontane terranes with the ancient Pacific margin of North America. To demonstrate the reliability of the aeromagnetic interpretation, we developed a multidata set stacking methodology that assigns numeric values to individual lineaments depending on whether they can be traced in residuals and first vertical derivative of RTP aeromagnetic grids, isostatic residual gravity grids, digital topography, and regional geologic maps. The sum of all numeric values was used to estimate the likelihood of the aeromagnetic lineament as a true geologic fault. Fault systems were interpreted from zones of lineaments with high spatial density. Using this procedure, 10 major northwest-trending fault systems were recognized. These were oriented subparallel to the regional Cordilleran deformation fabric, the mid-Cretaceous Dawson Range magmatic arc, and well-established crustal-scale dextral strike-slip fault systems in the area. These orogen-parallel fault systems were interpreted to play a structural role in the emplacement of known porphyry Cu-Au and epithermal Au systems of mid-Cretaceous (115–98 Ma) and Late Cretaceous (79–72 Ma) age. The procedure also identified seven northeast-trending, orogen-perpendicular fault-fracture systems that are prominent in eastern Alaska and exhibit sinistral-to-oblique extensional kinematics. These structures were interpreted to govern the emplacement of Late Cretaceous (72–67 Ma) porphyry Mo- and Ag-rich polymetallic vein and carbonate replacement systems in the region.

Geophysical investigation and reconstruction of lithospheric structure and its control on geology, structure, and mineralization in the Cordillera of northern Canada and eastern Alaska

“…However, apparent changes in the structural orientation of the Liard transfer zone may indicate a response to additional factors such as crustal structure and stress directions during rifting. Alternatively, the changes in orientation may indicate the influence of cross‐cutting (NW striking) regional structures, such as the Tintina and Denali faults, which may have been associated with a lower crustal ductile flow, with seismic interpretation inferring a broad zone of associated deformation in the lower crust [ Snyder et al ., ], in addition to upper crustal brittle deformation [e.g., Sánchez et al ., ]. Alternatively, structures aligned with an ESE extension of the Hess River and Macmillan faults (Figure ) may have modified the structural orientation.…”

“…In western Yukon and eastern Alaska, a system of Late Cretaceous NE striking faults [ Sánchez et al ., , , and references therein] including the Sixtymile‐Pika and Kechumstuk faults (SPF, KF, Figure ) define the boundaries of crustal blocks with internally consistent suites of mineral deposits and played a controlling role in the emplacement of carbonate replacement, porphyry, and polymetallic vein mineralization types [ Allan et al ., ]. For example, the White Gold and Klondike districts lie on an upthrown crustal block south of the Sixtymile‐Pika fault [ Allan et al ., ].…”

“…In the Yukon‐Tanana terrane and underlying parautochthonous continental margin in western Yukon and eastern Alaska, steeply dipping NE striking faults and grabens are spatially associated with Late Cretaceous intrusions and the Carmacks Group [e.g., Foster , , ; Page et al ., ; Werdon et al ., ; Gordey and Makepeace , ; Dusel‐Bacon et al ., ; Colpron et al ., ; Colpron , ; Allan et al ., ]. They are interpreted to be primarily left‐lateral oblique‐extensional fault systems of predominantly Cretaceous age, between the Tintina and Denali faults [ Sánchez et al ., , ], that may bound clockwise‐rotating crustal blocks [ Page et al ., ].…”

“…Northeast trending low Bouguer anomalies at the northwestern edge of the NW gravity high coincide with a break in the magnetic fabric and a long NE trending valley (Figures a and c). These features are associated with the Sixtymile‐Pika, Dennison Fork, and Cariboo Creek faults that are interpreted to be related to Cretaceous transtension [ Page et al ., ; Sánchez et al ., , ]. The stepped form of the northwestern edge of the NW gravity high may also be associated with additional gravity lineaments (Figures a and d) that follow east‐west trending gravity lows (lineaments E and F), but no clear relationship to the topography, magnetic anomalies, and surface geology can be drawn.…”

“…Ancient lithospheric lineaments, interpreted to be associated with long‐lived crustal structures, are important globally in their influence and control on subsequent tectonic deformation and faulting, crustal evolution, and the development of mineralization [e.g., Hildenbrand et al ., ; Chernicoff et al ., ; Hill et al ., ; Austin and Blenkinsop , , ; McMechan , ]. However, the use of geophysical and geological data in the investigation of regional geology and structure and in the exploration for mineral deposits in the northern Canadian Cordillera of Yukon‐eastern Alaska has primarily focused on the examination of upper crustal geology and structure [e.g., Hart et al ., ; Ryan et al ., , , ; Allan et al ., ; Nelson et al ., ; Sánchez et al ., , ].…”