# Presentation: 503082

## Late Cretaceous Tectonic Evolution and Metallogeny of Southwestern Alaska

## Marti L. Miller1Dwight C. Bradley1Thomas K. Bundtzen2Richard J. Goldfarb3

**U.S. Geological Survey, Anchorage**
**Pacific Rim Geological Consulting**
**U.S. Geological Survey, Denver**

**Notes:**
Today I will summarize what we understand about the tectonic setting of a significant and widespread metallogenic episode in southwestern Alaska.  This talk is based on a number of years worth of work by the four of us, and many others as well.  While we now have a toe hold on unraveling this story, it is still a work in progress.  

## Slide 3

**Notes:**
In Late Cretaceous time, a major episode of epigenetic ore deposit formation took place in what is now southwestern Alaska.  It appears that several large-scale tectonic factors came together here during a relatively short time frame.  This led to widespread 70 Ma magmatism and also formation of a number of 70 Ma  gold- and mercury-bearing deposits.  These deposits form part of what we call the Kuskokwim Mineral Belt, which occupies an ~190,000 sq km area.  Total production from this region includes 3.2 million oz (~100,000 kg) of gold, about half a million oz of silver, and 40,000 flasks of mercury.  In this talk we will:
 examine what we know about the tectonic setting
 review the deposit types involved
 and then step back to speculate on the potentially controlling factors.

## Tectonic setting of theKuskokwim Mineral Belt

**Lies within a broad zone of    dextral strike-slip faults**
**Occupies a backarc position ~400 km inboard of the present subduction zone**
**Occurs at the western end of a curved, continental-scale, strike-slip system**

**Notes:**
The Kuskokwim Mineral Belt lies within a broad zone of dextral strike-slip faults that have been active since at least Late Cretaceous time.  The belt occurs in a backarc position about 350-450 km inboard of the present-day Aleutian subduction zone.  The southwestern Alaska dextral faults are the western end of the continental-scale strike-slip system that curves around the Canadian-Alaskan continental margin.  The dextral strike-slip motion along this margin is a consequence of the relative motion between the Pacific and North American Plates. 

## SouthwesternAlaska—tectonostratigraphic terranes

**Southwestern****Alaska—tectonostratigraphic terranes**

- After Decker and others, 1984

**Notes:**
Southwestern Alaska is underlain by a variety of older tectonostratigraphic terranes including:
 Early Proterozoic metamorphic rocks (shown in brown here)
 Neoproterozoic to Jurassic largely carbonate rocks of the Farewell terrane—a microcontinental fragment (shown in blue)
 And Paleozoic to Mesozoic oceanic and arc assemblages (shown in purple, orange, and gray)
The older terranes were assembled by mid-Cretaceous time and then overlapped by the Upper Cretaceous Kuskokwim Group (shown in green).  The Kuskokwim Group, which is the dominant bedrock unit of the Kuskokwim Mineral Belt, was deposited into a basin that subsided between some of the amalgamated terranes.

## Geology--central Kuskokwim Mineral Belt

- Kuskokwim Group

**Notes:**
Now we will concentrate on the specific area of interest, the central part of the Kuskokwim Mineral Belt—where many of the 70 Ma mineral deposits lie.  The older terranes are shown in brown and blue.  But most of the deposits lie in the Kuskokwim Group (again shown in green), which is largely composed of sandstone and shale turbidites.  This unit was deposited beginning about 95 million years ago.  The waning stages of Kuskokwim Group deposition were accompanied by regional volcanism and intrusion beginning around 76 Ma.  A pulse of magmatism around 70 Ma is recorded by numerous igneous complexes shown in hot pink and subaerial volcanic rocks in light pink (in northwestern map area).

## Geology--central Kuskokwim Mineral Belt

- Volcanic-plutonic complex

- Felsic porphyritic dike

**Notes:**
The intrusive rocks produced during this period are of two types, both of which are spatially associated with mineralization.  
 Rocks of the first type —volcanic-plutonic complexes—are shown in hot pink.  These consist of quartz monzonitic plutons and subaerial volcanic rocks that have a mantle signature.  The volcanic section and underlying sedimentary rock are intruded and baked by comagmatic plutons.  As illustrated in this photograph, the more resistant rim of hornfels surrounds a less resistant weathered intrusive core.
 The second type of Late Cretaceous intrusive rock includes structurally controlled, commonly peraluminous, felsic to intermediate hypabyssal dikes, sills, and stocks—represented in this photograph, but which are too small to show on the map.  These are the same age as the volcanic-plutonic complexes, but have a melted crust signature.

## Deposits ofthe central KuskokwimMineral Belt

**Epizonal Hg-Sb and Au**
**Precious metal-bearing intrusion related**

**Notes:**
Two important types of mineral deposit formed around 70 Ma:
Precious metal-bearing, boron-rich, high-temperature intrusion-related deposits shown in the yellow dots, such as Shotgun, Flat, and the Beaver Mountains.
Epizonal deposits, shown in the red dots, many of which are structurally associated with relatively competent sill and dike systems that make a favorable host.  These are primarily mercury-antimony deposits such as Red Devil and Cinnabar Creek, but also include gold deposits, such as Donlin Creek.
We will briefly visit a few examples of these deposit types, starting with one of the intrusion-related deposits,  Shotgun (located here). 

## Shotgun

**Mineralized qtz-feldspar porphyry**

**Kuskokwim Group**

**70 Ma qtz-feldspar  porphyry**
**Veins, breccias, stockworks**
**Au, As, B ± Cu, Mo, Bi, Te**

- Granodiorite stock

**Notes:**
We are currently studying this gold prospect in cooperation with John Mair (of UBC, Vancouver).  Typical of many of the intrusion-related deposit types, Shotgun is hosted by a quartz-feldspar porphyry (the hot pink color)—a phase thought to be related to the main granodiorite stock (the lighter pink).  The porphyry is cut by ~70 Ma gold-bearing quartz veins, breccias, and stockworks, which contain arsenopyrite, pyrite, pyrrhotite and chalcopyrite.  Gold-rich samples tend to contain 0.1-0.5 % arsenic, as well as anomalous copper, molybdenum, and tellurium.  Tourmaline veinlets are ubiquitous and indicate a very boron-rich magmatic fluid phase; albite alteration is widespread.  The volume of mineralized rock associated with the Shotgun area extends well south of the main property.

## Epizonal Hg-Sb deposits

- Red Devil

- Cinnabar Creek

**Notes:**
A prime example of the 70 Ma epizonal deposits is Red Devil, Alaska’s largest mercury mine—located here—which closed in the 1970’s after producing 36,000 flasks or about 2.8 million lbs of mercury.  The ore consisted of cinnabar in quartz, but abundant stibnite occurred deeper in the system.  Smaller deposits of this type are found throughout the region.  They are usually hosted in sedimentary rock at or near high-angle structures, and are sometimes spatially associated with dikes.  At Cinnabar Creek, the mercury ore is notable in that in addition to cinnabar, it also contains native mercury.

## Epizonal Au-bearing deposits

**Epizonal Au-bearing deposits**

- Donlin

**Notes:**
Some of the ~70 Ma epizonal deposits contain gold, the best example of this being the Donlin deposit, which is estimated to contain 25 M oz of gold.  The Donlin deposit consists of about 16 prospects within and adjacent to an 8-km-long dike/sill complex.  The ~70 Ma dikes, locally termed rhyolite and rhyodacite, are actually hypabyssal porphyritic intrusions of mainly granitic composition.  Metalliferous quartz veinlets fill brittle, extensional fractures in the dikes and were formed within a few million years of crystallization of the dikes.  The ore forming fluids were derived by either broad devolatization of flysch above rising mantle melts deep in the Kuskokwim basin or by exsolution from a magma derived largely from flysch melt.Other examples of epizonal mercury-antimony deposits that contain anomalous gold include Red Devil, Barometer, and Kolmakof (the last two lie near the red dot that is ~80 km north of Cinnabar Creek).  It is interesting to note that all of these Au-bearing deposits lie somewhat near Donlin, and more distal epizonal deposits do not show anomalous gold.

## Iditarod-Nixon Fork fault—at least 90 km dextral offset 

Denali fault—at least 134 km dextral offset 

**Denali fault—at least 134 km dextral offset **

**Notes:**
Now let’s look at the 70-Ma deposits in relation to the regional structures.  The Kuskokwim Mineral Belt lies in a broad zone of dextral strike-slip faults that have been active since at least the beginning of deposition of the Kuskokwim Group turbidites, about 95 million years ago.  Based on geologic map relations, accompanied by limited isotopic dating, we have defined the total amount of displacement on the regional master faults and bracketed time frames during which this motion occurred.
 The Iditarod-Nixon Fork fault shows at least 90 km dextral offset since the Late Cretaceous. (See offset pink unit on either side of “Iditarod Fault” label.)
 and the Denali fault in this part of Alaska shows dextral offset of at least 134 km. (See offset blue unit on either side of “Denali Fault” label.)But most significantly, our studies indicate that strike-slip motion on these faults was taking place at the same time as the ~70 Ma igneous activity and related hydrothermal mineral deposition.

## Along strike-slip faults: 
 Fortyseven Creek
 Nixon Fork

** ****Fortyseven Creek**
** ****Nixon Fork**

- Dike-bearing ridge
- west of Fortyseven Cr

**Notes:**
Some of the deposits are indeed spatially associated with strike-slip faults; others lie between the master faults.
Intrusion-related deposits that lie along the main first-order faults have structural features suggesting they were emplaced synkinematically during dextral strike-slip.  
For example at the Fortyseven Creek gold-tungsten deposit these structures are consistent with dextral strike-slip on the Denali fault.
And at the Nixon Fork gold skarn deposit, which lies along the Iditarod-Nixon Fork fault, the ~70 Ma intrusive bodies were emplaced parallel to the master fault and Bill McClelland showed that the mineralization occurs in pressure shadows that are consistent with dextral shear.

## Between master faults: 

- Between master faults:
-  Donlin
-  Red Devil

**Red Devil**

**Notes:**
In contrast, kinematic data for two ~70 Ma deposits that lie between the first-order faults (Donlin Creek and Red Devil) suggest a roughly north-south principal compression direction, which is perpendicular to what would be expected in a dextral regime on the main strike-slip faults.  
For example at Donlin Creek the earliest structural event was reported by Ebert et al. (2000) to be an approximately north-south compressional episode.
The story at the Red Devil mercury deposit is similar.  Here northwest-striking mineralized dextral faults are consistent with a roughly north-south principal compression direction, again perpendicular to what would be expected in a dextral regime on the main northeast-striking master faults.

## Dextral strike-slip motion was taking place at the time of ~70 Ma deposit formation
Faults focused the fluids and accompanying mineralization
Some of the deposits are spatially associated with the master faults and others lie between these faults

- Dextral strike-slip motion was taking place at the time of ~70 Ma deposit formation
- Faults focused the fluids and accompanying mineralization
- Some of the deposits are spatially associated with the master faults and others lie between these faults

**What we know**

**Notes:**
Now to summarize what we know about the tectonic setting of the 70-Ma deposits:
 The Kuskokwim Mineral Belt lies within a broad zone of dextral strike-slip faults that have been active since at least Late Cretaceous time.
 Dextral strike-slip motion was taking place at the time of ~70 Ma gold and mercury deposit formation (but was also occurring before and after this)
 Faults focused the fluids and accompanying mineralization
 Some of the gold and mercury deposits of the mineral belt are spatially associated with the master faults and others lie between these faults.

## Why was there voluminous ~70 Ma magmatism over a wide area?
Why was the regional thermal gradient elevated across a broad region?
Why are both mantle- and flysch-derived intrusive rocks present?
What got the fluids and melts moving?
Are mineralization and magmatism both products of the same tectonic event?
What changed in the tectonic regime at ~70 Ma?

- Why was there voluminous ~70 Ma magmatism over a wide area?
- Why was the regional thermal gradient elevated across a broad region?
- Why are both mantle- and flysch-derived intrusive rocks present?
- What got the fluids and melts moving?
- Are mineralization and magmatism both products of the same tectonic event?
- What changed in the tectonic regime at ~70 Ma?

**What we don’t know**

**Notes:**
But our list of what we don’t know is much longer —
 Why was there a flare up of igneous activity over a wide area at ~70 Ma?
 Why was the regional thermal gradient high during this time as evidenced by elevated mercury across much of southwestern Alaska?
 Why are both mantle- and flysch-derived intrusive rocks present?
 What got the fluids and melts moving?
 Are mineralization and magmatism both products of the same tectonic event?
 What changed in the tectonic regime to account for all of these effects specifically at 70 Ma?

## Present:  Dextral motion related to tectonic escape in collisional foreland, despite local sinistral sense of oblique subduction

- Escape

**Notes:**
In order to understand the regional tectonic framework during the critical latest Cretaceous time, it is helpful to work back through the history of interaction between southwestern Alaska and the plates of the Pacific. But even the current setting presents a potentially confusing problem. At present, the overall motion along the North American margin is dextral on the main fault segments.  However, the convergence angle in southwestern Alaska has a locally sinistral component (point to).  BUT we don’t see sinistral motion, so something else is operating.  The southwestern Alaska chunk is moving along dextrally because, in the big picture, it’s being driven by the overall Pacific-plate-with-respect-to-North America motion. This dextral motion could have two mechanisms if you will:--A push from the rear, causing the train cars to move along previously established tracks, which are the faults--Or perhaps tectonic “escape”, first proposed by Dave Scholl, where the pushed block would escape out the free face--Or maybe a combination of these

## ~55 Ma:  Dextral motion was possibly accentuated by “megakinking” during oroclinal bending

**~55 Ma:  Dextral motion was possibly accentuated by “megakinking” during oroclinal bending**

- Plate ?

**Notes:**
Now looking at ~55 million years ago, which is within the period of supposed Alaskan oroclinal rotation.  Dextral motion in southwestern Alaska could possibly have been accentuated by “megakinking” during oroclinal bending.  But what plate was subducting? 

## At ~60 Ma three possibilities for identity of subducting plate

- --Resurrection Plate preferred

- Engebretsenet al., 1985

- Bradleyet al., 1993

- Preferred:
- Miller et al.,2002
- Haeussler et al., 2003

**Notes:**
Uncertainties begin to mount as we go back further in time.  At about 60 Ma, one of the big questions is what plate was subducting? There are three alternative plate reconstructions for that time.  In the Engebretsen et al. interpretation, the Kula Plate was involved.  Based on recognition of the sweeping subduction of the Sanak-Baranof ridge, Bradley et al. suggested the Farallon Plate was subducted beneath the Kuskokwim region until about 61 Ma.  We prefer this third alternative, which involves a previously unrecognized oceanic plate that we call the Resurrection Plate.  In this scenario, prior to about 60 million years ago, and thus in our time of interest, it was not the Farallon Plate, not the Kula Plate, but the Resurrection Plate that was being subducted beneath southwestern Alaska.
An important consequence to note:  If this model is correct, it invalidates published Late Cretaceous through Eocene plate motion vectors calculated for Alaska.

## ~70 Ma:  Dextral motion driven by oblique convergence prior to ridge subduction

**~70 Ma:  Dextral motion driven by oblique convergence prior to ridge subduction**

**Notes:**
Now to the important 70 Ma period.  Given the presence of the Chugach accretionary complex and arc rocks in the Alaska Range, subduction was most certainly occurring along this margin at this time.  Dextral strike-slip was likely driven by oblique plate convergence, perhaps in concert with escape tectonics.  Late Cretaceous dextral displacement in the region enhanced crustal dilation and fluid migration.  
  But why was there such a wide zone of 70 Ma magmatism?
  Why was there both mantle and melted crust magmatism?
  Why was there a wide region of elevated heat flow?

## A witches brew:
Curved margin
Oblique subduction
Escape to free face

**A witches brew:**
**Curved margin**
**Oblique subduction**
**Escape to free face**

**Notes:**
Geometric complexities involving a curved margin, the consumption of the Kula and then Resurrection plate, and possible escape on the free western face led to a complicated set of forces that likely played a role in ore genesis of the region.
We know some of the ingredients of this witches brew, but we haven’t yet determined the recipe. 

## Possible tectonic scenarios

**Low angle subduction**

**Slab break off**

**Ridge subduction**

**Escape tectonics**

**Possible tectonic scenarios**

**Notes:**
What scenario would allow a broad region to experience a flare up of magmatism and widespread elevated thermal gradient?  Some possibilities are:
Low-angle subduction—this would seem to be a reasonable mechanism for a wide belt of magmatism, but it’s not obvious how this would explain the voluminous pulse just at 70 Ma.  And further, in the Andes, the flat slab segments don’t even have volcanoes.  So actually it is not a given that low-angle subduction causes widespread arc magmatism.
Slab break off (or a variant, delamination)—Could this work?  This is just a brainstorming idea at this point.  It would generate a slab window without subducting a ridge.  It would seem to account for the regionally extensive mercury belt and for the magmatic flare up.  But there is no independent evidence that it happened and it’s not clear how it could be tested.
Ridge subduction beneath Bering Sea margin—ridge subduction is the usual way to make a slab window.  On the Pacific margin ridge subduction didn’t get going until 61 Ma, but the Bering margin was a subduction zone at 70 Ma.  However, looking at the geometry, It seems that the Kuskokwim region is too far inland to be affected.
Escape tectonics—We have already mentioned Dave Scholl’s idea that southwestern Alaska has moved toward the free face in the Bering Sea.  Was this process also operating at 70 Ma? As pointed out by our colleague Jim Riehle, in the Neogene of Turkey, escape along active strike-slip faults produced a broad magmatic province not related to subduction.  This process may have operated in the Late Cretaceous in southwestern Alaska, but what would cause the specific 70 Ma pulse?

## Slide 23

**Notes:**
Well, as I started out saying today, this is a work in progress.  We don’t have all the answers yet.  We’re expanding our work to the south and hoping eventually for that nugget at the end of the rainbow.

## References cited

- Bradley, D.C., Haeussler, P.J., and Kusky, T.M., 1993, Timing of early Tertiary ridge subduction in southern Alaska:  U.S. Geological Survey Bulletin 2068, p. 163-177.
- Decker, J., Bergman, S.C., Blodgett, R.B., Box, S.E., Bundtzen, T.K., et al., 1994, The geology of southwestern Alaska, *in*  Plafker, G., and Berg, H.C., eds., The geology of Alaska:  Geological Society of America DNAG Series, v. G-1, p. 285-310.
- Ebert, S., Miller, L., Petsel, S., Dodd, S., and Kowalczyk, 2000, Geology, mineralization, and exploration at the Donlin Creek project, southwestern Alaska:  British Columbia and Yukon Chamber of Mines Special Volume 2, p. 99-114.
- Engebretsen, D.C., Cox, Allan, and Gordon, R.G., 1985, Relative motions between oceanic and continental plates in the Pacific Basin:  Geological Society of America Special Paper 206, 59 p.
- Haeussler, P.J., Bradley, D.C., Wells, R.E., and Miller, M.L., 2003, Life and death of the Resurrection plate:  Evidence for its existence and subduction in the northeastern Pacific in Paleocene-Eocene time:  Geological Society of America Bulletin, v. 115, p. 867-880.
- Miller, M.L., Bradley, D.C., Bundtzen, T.K., and McClelland, W., 2002, Late Cretaceous through Cenozoic strike-slip tectonics of southwestern Alaska:  Journal of Geology, v. 110, p. 247-270.