Location
The Entiat mining district, the largest in the county in point of area, includes 790 square miles in the east central part of Chelan County· It is bounded on the west and south by the Entiat Mountains and the Chiwawa, Leavenworth, and Wenatchee districts, which lie on the ocher side of the mountains; on the east bv Columbia River and Chelan Butte district; on the northeast by Sawtooth Ridge separating Chelan and Okanogan counties; and on the north by Meadow Creek and Railroad Creek districts, separated from this district by the Chelan Mountains and some lesser ridges. The district includes all the area drained by Entiat River and its tributaries. The central part of Lake Chelan lies within the district. Topography in the northern part is rugged but southward it becomes somewhat less rough. The district is accessible by a good county road and its laterals in the Entiat valley and by Highway U.S. 97 and a branch line of the Great Northern Railway which follow Columbia River along the southeastern border of the district.
Geologic Section
The geology of the southern end of the district has been mapped in detail by Waters (1932), and the geology as here set forth for the whole district is based to a considerable degree on that work.
The oldest rocks in the district are grouped as Swakane gneiss, thought by Waters to be pre-Ordovician. A well-foliated biotite gneiss predominates, but included also are mica schist, marble, amphibolite, lime silicate rocks, and gneissoid rocks of intrusive origin. It is in these rocks :ind in the Chelan granodiorite which intrudes them that the gold deposits are found. Innumerable pegmatite and lamprophyre dikes of pre-Chelan granodiorite age cut the gneiss. In general, the Swakane gneiss occupies a wide northwestward-trending belt in the southwestern portion of the district, while the intrusive rock covers most of the remainder. However, in several very smalJ areas along both sides of Lake Chelan there are metamorphosed Paleozoic sedimentary rocks. Other rocks of small areal extent and of little economic importance are remnants of Columbia River basalt, lake sediments intercalated with the basalt, and late Tertiary lavas. In the lower regions are glacial drift and alluvium, and in the latter small gold and platinum placer operations have been attempted.
Ores and Ore Minerals
Little mining activity has been reported from this district; likewise, little information concerning its mineral deposits is avail- able. The only properties which have produced have been those with values in gold and silver, with free gold as the chief ore mineral.
Mercury in the form of cinnabar has been reported, as has also the titanium mineral ilmenite. Nickel prospects have nickeliferous pyrrhotite for their ore mineral, and at one property there possibly is enough of the sulphate moreoosite present to be an ore of nickel.
Ore Deposits
The gold ore occurs in quartz veins cutting Swakane gneiss. The veins may be related in origin to the intrusion of any or all of several magmatic bodies, among which are the pre-Chelan intrusion responsible for the dikes in the gneiss [Waters, 1932), the Chelan graoodiorite, and the younger diorite which is seen in the Chiwawa, Railroad Creek, and Stehekin mining districts. Neither the pre-Chelan intrusive body nor the younger diorite is known to outcrop in the Entiat district, but either may be buried at no great depth beneath surface rocks. The origin of the nickel, titanium, and mercury deposits is not known.
Timber and Water
Timber and water: Timber and water for mining and milling purposes are not abundant in the southern portion of the district but are much more abundant toward the north.
