31 October 2010

Ch 2 : "The Wrangell Formation," Pt 4 from "Legacy of the Chief"

The  Wrangell Formation, Pt 4
Wesley Dunkle  -- Chapter 2 of "Legacy of the Chief"

      “Where was I?  Oh yes. I first heard about the building of the Copper River and Northwestern Railway and of the fabulously rich Bonanza copper mine while I was working at an iron mine in Minnesota.  I wanted to become a part of it. It was the copper which first brought me to Alaska.   In 1914 I made the first detailed study of the geology of the ore occurrences at Kennecott while employed as a field exploration engineer.  It was my study which determined that the ore bodies ran primarily to the northeast as massive bodies of chalcocite ore.   This was important later because it meant that Kennecott had to consider the possibility that it was sharing the same orebody with an adjacent mine on the opposite ridge facing McCarthy Creek.  This was the Mother Lode Mine.  The Mother Lode camp was actually sitting on the Bonanza-Motherlode vein, though the owners of the company did not realize it at the time.  The Mother Lode ultimately extended the life of the Kennecott mine system by many years.
      “Back to the topic of the terranes.  Let me show you on this map.”
AK minerals
Mineral Deposits in Alaska
      Behind him was a large United States Geological Survey map showing mineral occurrences in Alaska.
       “The mountains of volcanic origin which concern us are those of the western part of the Wrangells, which  were all formed within the last four million years.  The Wrangells extend over an  area beginning a few miles east of the Copper River--here-- and continuing  past the  eastern Alaska border into an area of the Yukon Territory known as the Kluane Range--there.  The Wrangells are roughly  bounded to the north and west by the Copper and Slana Rivers and to the south by the Chitina and Nizina Rivers. This is all a part of the Wrangellia Terrane.  The terrane extends well past the Wrangells to include part of the south slopes of the Alaska Range in the area of my own Golden Zone mine,   but it is the Wrangell Mountain formation where we found the unique Kennicott formation which contained those high-grade copper deposits.
      “The south end of the Chitina River valley, which essentially runs east to west, marks the northern edge of  the Chugach coastal range, which extends all the way to the Gulf of Alaska.  Running east to west this takes in an area extending from Yakutat,  to Cordova, Valdez and Seward, and then inland to include Anchorage. 
      “The Copper River is the only river system of such great size to pass through this coastal range. This river is the greatest in south-central Alaska, and includes such tributaries as the Chitina, Nizina, Bremner, Tonsina, Klutina, Tazlina, Gulkana, Gakona, Chistochina, and Slana Rivers.  The Chitina River is easily the equal of the Copper.  It enters the Copper near Chitina.  Both rivers flow through extensive areas of geologically-recent glacial deposits,  as well as  volcanic ash deposition.  As a result these combined rivers  contain an enormous amount of silt which rivals, if not exceeds, that of the more famous Yukon and Tanana Rivers north of the Alaska Range.
      “As late as ten thousand years ago the present-day Copper River valley was largely under the waters of Ahtna Lake.  This extinct, but once enormous water body was blocked along the Chugach Range by massive ice fields.   My guess is that the Miles and Childs Glaciers which face each other at the Million Dollar railroad bridge about fifty miles east of Cordova were at one time locked together as one super ice mass which effectively blocked any drainage from this extensive lake north of the coastal range.  As the last ice age came to an end, these glaciers receded until the lake water was finally able to burst free.   This rapid flood of water probably carved out the Woods Canyon area and began the process of building the Copper River delta area. 
      “The Wrangellia terrane includes  Mounts Wrangell, Blackburn, Sanford, Drum and Regal.   This terrane is a  part of a more complex group which were formed in a tropical environment that was very close to the equator.   We know this because of  the vast amount of marine fossils we have found in the limestone formations which cap the base rocks of this region. 
      There are at least two other distinct terranes in this area.  The composite terrane immediately south of the Wrangellia terrane contains the northern part of the Chugach Range.  It is called a composite terrain because of its  complex origins which point toward the likelihood that it is probably two or more combined terranes. Immediately south of this is the  much younger Yakutat terrane which accreted itself to this part of Alaska in the last 26 million years,  initiating the Wrangell Range volcanic formations.
Geology of the Kennecott area
      “The Wrangellia terrane began as a volcanic arc thousands of miles to the south along the  North American continent  300 million years ago.   As the volcanic activity subsided, the resulting rift between the main part of the continent and the arc  caused a massive eruption of basalt lava flows. This included a  significant layer of basalt which has come to be known as the Nicolai greenstone. This name was borrowed  from the great chief himself since he was the one who first revealed the Nicolai Prospect--the discovery which set into motion the prospecting activity which resulted in the discovery of the great Bonanza outcropping.”

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