Minerals of the Sunset Mine, Snohomish County, Washington, USA

SUNSET MINE
Baring, Index District, Snohomish County, Washington, USA

LOCALITY IMAGES

GEOLOGY
Index Batholith (Oligocene) - Uplift and erosion of the Old Cascade Volcanic Arc exposed plutonic rock of the Index Batholith near Index, Washington. Intrusive rock types found within the Index Batholith range in composition from granodiorite to tonalite.

Ore at the Sunset Mine occurs as lenses of chalcopyrite and bornite and pyrite with quartz gangue in a shear zone in graodiorite (Derkey et al., 1990, p. 368). The shear zone occurs along a well-developed footwall and a diffuse hanging wall. Chalcopyrite and bornite are the primary ore minerals. The Sunset Mine was in production from 1902 to 1905, 1907 and 1916 to 1930 with the years of most prolific production between 1926 and 1929. The total production was 193,112 tons (1,500 oz gold, 156,000 oz silver and 12,912,000 lb copper from 263,500 tons of ore) (Huntting, 1956).

MINERALS

Bornite-	Cu₅FeS₄ - Occurs as disseminated crystals associated with other sulfides.
Chalcopyrite-	CuFeS₂ - Occurs as disseminated crystals associated with other sulfides
Malachite-	Cu₂(CO₃)(OH)₂ - Occurs as secondary fibrous coatings along some fracture surfaces.

BORNITE
with CHALCOPYRITE

MALACHITE
on BORNITE

BORNITE
(cut and polished)

REFERENCES
Derkey, R. E., Joseph, N. L. and Lasmanis, R., November 1990, Metal Mines of Washington - Preliminary Report, Washington Division of Geology and Earth Resources, Open File Report 90-18.
Huntting, M. T., 1956, Inventory of Washington Minerals - Part II, Metallic Minerals, Washington Division of Mines and Geology Bulletin 37, v.1 428 pages; v.2 67 pages.