Sultan Basin

Region West, Northwest
Mineral systems
Deposit types
Critical minerals
Other minerals

Information leading to the delineation of this focus area

Basis for focus area This multipart focus area outlines Tertiary granitic rocks, porphyry copper deposits and prospects, and numerous copper and tungsten mineral sites from USMIN and MRDS that fall within 2 km of the mapped units (2 km buffer applied). Includes four porphyry Cu deposits and a northeast-trending belt of As-bearing, Au-Ag polymetallic vein deposits and occurrences in Snohomish County.
Identified resources Sunrise property: Identified resources for bismuth, copper, gold, lead, molybdenum, and zinc. Historical production of arsenic, gold, and silver.
Production Rainy mine (1913-1915): produced Au, Ag, and As (grade~20%) from undergrounds workings; Reiter mine (1922-1923): 22 tons red As.
Status Past mining; arsenic workings flooded.
Estimated resources The Sunrise breccia pipe contains 71 million tons of Measured, Indicated, and Inferred ore resources averaging 0.319% Cu and 0.071% MoS2 (1972 data). Underground drilling (1,322 ft) indicated an average grade of 0.062% W03; thirty chip samples ranged 0.04% to 0.07% WO3. Assays ranged from 0.002 to 0.006 oz/ton Au and from 0.049 to 0.088 oz/ton Ag. A copper-molybdenite concentrate assayed 0.4% Pb, 0.8% Zn, and 0.18% Bi. A 1983 estimate reports 45,000 t of ore with 15.8 Au g/t; 73.4 g/t Ag for the As-bearing veins. Porphyry Cu occurrences: Middle Fork property: 93 Mt at 0.41% Cu, 0.02% Mo; North Fork Snoqualmie River occurrence: 128 Mt at 0.68% Cu, 0.082% Mo, 0.1 g/t Au; Gold Mountain occurrence: 40 Mt at 0.5% Cu (Singer and others, 2008).
Geologic maps Carithers and Guard (1945, plate 1), scale 1:63:360; Tabor and others (1982), scale 1:100,000; Washington Division of Geology and Earth Resources (2016), scale 1:100,000; Dragovich and others (2013), scale 1:24,000.
Geophysical data Some adequate (50%) aeromagnetic coverage. Inadequate aeroradiometric coverage, poor gravity.
Favorable rocks and structures Veins in shear zones in quartz diorite.
Deposits Rainy mine (MRDS dep_id: 10036743), Reiter mine (MRDS dep_id: 10103536), Sunrise prospect (MRDS dep_id: 10095364), North Fork (MRDS dep_id: 10048440), Snoqualmie (MRDS dep_id: 10048623), Middle Fork property (MRDS dep_id: 10131061), Gold Mountain (MRDS dep_id: 10009065).
Evidence from mineral occurrences MRDS; Singer and others (2008).
Geochemical evidence Unknown.
Geophysical evidence None.
Evidence from other sources None.
Comments More than 60 MRDS records for As-bearing quartz veins southeast of the Sunrise porphyry deposit. Washington was the first state to produce As on a commercial scale as a byproduct of smelting Au ore from the Monte Cristo district. Focus area covers private, State (mineral leases), national forest, and wilderness lands. Future mineral development may be possible but anticipate significant opposition as it is in a watershed of a major population center.
Cover thickness and description Parts are exposed or buried under shallow cover.
Authors Daniel W. Eungard, Jane M. Hammarstrom.
New data needs High resolution mapping, geophysics, and some limited lidar to supplement existing datasets.
Geologic mapping and modeling needs For Washington <10% mapped at a scale of 1:24:000; only up to 25% at intermediate scales of 1:48,000 to 1:62,5000, predominately mapped at 1:100,000 scale. Need high resolution mapping for much of the focus area. Approximately 4.5 quadrangles need mapping at a scale of 1:24,000 or better resolution in the area around the Sunrise porphyry.
Geophysical survey and modeling needs Need high resolution aeromagnetic and aeroradiometric coverage, as it would key to identifying the system over this broad of area and delineating possible deposits at depth. Gravity would not be helpful with the similarity between the host/intrusive rock types.
Digital elevation data needs Lidar coverage adequate only over parts of focus area; need lidar for northern and southernmost parts.