The Buckhorn Mountain (Crown Jewel) gold skarn deposit, Okanogan County, Washington

Abstract

The gold skarn system at Buckhorn Mountain is related to the Cretaceous (?) Buckhorn Mountain pluton (diorite to granodiorite) and its associated dikes and sills. The host rocks are Pennsylvanian to Triassic (?) clastic and carbonate sedimentary rocks of the Thompson assemblage and the andesitic flows of the Brooklyn Formation. These have been altered to proximal garnet-magnetite-sulfide, intermediate garnet-pyroxene-epidote-sulfide, and distal pyroxene and hornfels assemblages. Typically, the pyroxene is iron rich (Hd (sub 20-94) ), averaging Hd 51 , and the garnet is intermediate granditc (Ad (sub 36-75) ). Retrograde epidote, calcite, amphibole, and zoisite are common. Skarn associations vary according to original lithologies; the carbonate-rich layers altered to a more garnet-magnetite rich assemblage, whereas the argillaceous and siliceous layers altered to a pyroxene dominant assemblage. Alteration in the intrusion takes many forms, including potassic, propylitic, argillic, albitization, and complete alteration to garnet-epidote-pyroxene skarn.Gold mineralization is spatially associated with a variety of skarn assemblages in locations ranging from proximal to distal. There are positive correlations of gold with epidote, magnetite, bismuthinite, native bismuth, pyrrhotite, and cobaltitc. Structure and original lithologies were important controls for fluid movement, and therefore, gold mineralization. The gold probably was carried in reduced fluids by bisulfide complexes and probably was deposited at temperatures less than 400 degrees C.Average pressure-corrected temperatures for skarn-forming fluids are 465 degrees C for prograde (garnet-pyroxene) skarn and 360 degrees C for retrograde (epidote) skarn. Fluids in skarn minerals, as measured from last ice-melting phenomena in fluid inclusions, are undersaturated, and fall within the 18 to 22 wt percent NaCl equiv range. No daughter minerals were found in any skarn mineral fluid inclusions. Liquid to vapor ratios are relatively constant, implying that there was no boiling. A comparison of prograde skarn and quartz vein fluid inclusion characteristics indicates that the two may have formed at roughly the same time from similar fluids. The skarn formed from pervasive fluid infiltration under dominantly lithostatic conditions, whereas quartz veins formed from fluids channeled along fault structures, largely under hydrostatic conditions.