See also: Chelan County Washington Gold Production
Location
The Blewett mining district as mapped by Weaver (1911) was limited to nine square miles on Peshastio and Etienne creeks. This included most of the areas originally called the Etienne Creek and Peshastin mining districts, but it did not include a number of nearby mines on Ingalls Creek, Etienne Creek, and upper Peshastin Creek and some of its smaller tributaries. In order to include these mines which naturally belong with those within Weaver's Blewett district, and in conformity with the rules set forth on page 5, The district is here extended to include approximately 160 square miles in south central Chelan County. It embraces all of the area drained by Peshastin Creek and its tributaries in addition to a small area east of this drainage. The district is separated from the Leavenworth mining district on the north and northwest by Wenatchee River and by the ridges immediately west and north of Peshastin and Ingalls creeks. It is bounded on the south along the Wenatchee Mountains by Kittitas County, and on tlhe east by the Wenatchee mining district. The district is accessible by highway U. S. 97 up Peshastin Creek to Blewett Pass, and by short roads up some of the tributary creeks. The topography is moderately rugged with narrow valleys and steep mountain slopes.
Geologic Section
The rock formations of this district naturally fall into two age groups: the pre-Tertiary metamorphic and igneous rocks, and the Cenozoic sedimentary rocks, granitic intrusives, and lavas.
Of the pre-Tertiary formations, the Peshastin rocks probably are the oldest. They are prevailingly very well indurated black slates and quartzites, with grit and conglomerate bands, and lenses of light-gray limestone. G. 0. Smith, wlio originally described the formation [1903] was undecided whether it was older or younger than the closely associated Hawkins formation. However, in his columnar section (1904) he indicated it to be younger. Later work by Weaver [1911] indicates the Peshastin to be the older. Both Smith and Weaver provisionally assigned the formation to a Carboniferous or older age, and W. S. Smith (1916] concluded it to be Ordovician. A few of the gold deposits of the district are found in these rocks, as are some also in the possibly Carboniferous [Smith, 1904] Hawkins formation. The Hawkins rocks are greenstones comprising altered basalt flows and pyroclastics. Both of the above formations were intruded in Jurassic ( ?) time [Smith, 1904) by an ultrabasic magma which crystallized to a peridotite now largely altered to serpentine. A prominent feature of this formation is the abundance of so-called "nickel ledges" thought by Smith (1904) to be limestone lenses of the Peshastin formation which were surrounded and altered by the peridotite magma at the time of its intrusion. The "nickel ledges" make prominent reddish-brown outcrops which at one time were extensively prospected for gold and silver with little success. In some places they are nickel-bearing and in others they contain small amounts of mercury. The serpentinized peridotite with its associated "nickel ledges" forms a narrow, curved, east-west band extending across the central part of the district, and in this rock are the most important of the Blewett gold deposits. The Mount Stuart granodiorite batholith which underlies and is in intrusive contact with all of these pre-Tertiary rocks crops out along Ingalls Creek and along the western border of the district. It was intruded probably in the Mesozoic era and is probably closely related to if not identical with the Chelan granodiorite of this county.
Oldest of the Cenozoic formations in the district is the Swauk with its moderately- to well-consolidated sandstones, shales, and conglomerates of continental origin. This formation is gently to strongly folded, and it unconformably overlies pre-Tertiary rocks in the southern and eastern portions of the district. Its age was regarded as Eocene by F. H. Knowlton [Russell, 1900), [Smith, 1904) on the basis of admittedly insecure fossil leaf evidence. Later paleo-stratigraphic developments among the forma- tions with which the Swauk was tentatively correlated have made the Eocene age of the whole of the Swauk formation even less sure. Cutting the Swauk and pre-Tertiary rocks are swarms of generally northeastward-trending diabase dikes from a few inches to several hundred feet wide. Smith (1904) considered them to be feeders to the Eocene Teanaway basalt which overlies Swauk south of this district. Gabbro, outcropping as a narrow belt nearly in the middle of the peridotite area, is intrusive into the pre-Tertiary rocks but it is not cut by the diabase dikes. For this reason Smith (1904} considered the gabbro younger than the Teanaway basalt and its diabasic feeder dikes, but Weaver [1911) in his later report called the gabbro older than the dikes. Pleistocene glacial drift forms areally unimportant deposits along Ingalls Creek and lower Peshastin Creek. The terrace gravels occurring along Peshastin Creek and stream gravels covering most of the valleys have furnished the district's placer gold.
Ores and Ore Minerals
The ores roost extensively mined have been of gold, both lode and placer. Silver, as an associate of the gold, has been of some importance as an ore. Other ores are of iron, copper, nickel, mercury, and chromium. Ore minerals present are native gold, pyrite, arsenopyrite, native copper, chalcopyrite, malachite, galena, stibnite, cinnabar, magnetite, hematite, and chromite. Tin and vanadium have been reported from the district, but reliable data concerning their occurrences are lacking.
Ore Deposits
The gold-silver ores are in quartz-calcite veins cutting serpentine in most places and also cutting Hawkins and Peshastin formations but never Swauk or younger rocks. Where the veins cut Hawkins formation the gangue is chiefly quartz with less calcite, but where they cut serpentine the calcite-quartz ratio is increased. Some of the veins are large and well- defined, but most are small and although well-defined are irregularly shaped. In Culver Gulch, where the richest ore bodies in the district are found, the veins strike about N. 75° W. Most of the ore is low grade, but much of the production bas been from very rich small ore shoots. Near the surface in the oxidized portions of the veins the gold occurs as free-milling native metal, and at depth it occurs in quartz-calcite veins carrying pyrite, arsenopy- rite, in some places chalcopyrite, and more rarely other sulphides. The mineralizing solutions responsible for these ore deposits probably were derived from the Mount Stuart granodiorite magma [Weaver, 1911}.
The nickel and mercury ores occur in the "nickel ledges" generally sur- rounded by serpentinized peridotite. The mode of origin of these deposits is somewhat in doubt and the minerals in which the nickel occurs are not surely known, but the ore mineral may be the fairly abundant green nickel- bearing mineral tentatively identified as garnierite. The nickel may well have been derived from the peridotite (which is nickel-bearing) adjacent to the "nickel ledges" either when the peridotite was still molten or at a later stage [Smith, 1904).
Cinnabar occurs in some places in the "nickel ledges" in small quantities as stringers and disseminations. Its origin also is obscure.
Two kinds of chromium occurrences are known in the district. The first is float chromite, which undoubtedly came from a nearby lode in the serpentine. The second type is the chrome-nickel-bearing iron ore found strati- graphically above the serpentine and below the Swauk sandstones, shales, and conglomerates. This has been considered not only as a possible ore of chromium but also as an ore of nickel and as an ore to be used directly to make a chrome-nickel-steel. Preliminary studies by Division staff members indicate this iron ore to be in the form of sedimentary beds, which have been compacted, faulted, and folded. They outcrop as discontinuous bands between the Swauk and serpentine.
Timber and Water
Sufficient timber and water for mining and milling purposes are available on most of the properties, and in the past a small amount of power has been generated from some of the streams.
