SONORA PALEOCHANNEL
not appear to be of local derivation (George Wilson, personal communication). Matrix is sandy and varies from light brown to black to blotchy red in the exposed panels of pay gravel (Photo #2). Gravel thicknesses within the paleochannel are unknown but is at least as deep as George Wilson’ large trackhoe, and perhaps as much as 50 meters. Thickness of the capping swamp muds exceeds 30 meters in places.
Structure A northwest-striking splay of the Big
Creek fault, south of its juncture with the west-striking Hoocheekoo Fault, passes through the Sonora Au-Cu project and appears to form the southwest bound- ary of the Sonora paleochannel (Figures 4 and 5). The Big Creek splay is mapped with a steep southwest dip with Paleozoic metamorphics, ultramafics, and mineral- ized Cretaceous quartz monzonite in both hanging wall and footwall. The east-west Hoocheekoo Fault at this same juncture is mapped with a steep south dip and appar- ent normal movement.
The ultramafic lens has undergone sig-
nificant deflection (approx. 500 m) related to dextral motion (Schulz, 2009, page 27), indicating a large right lateral component to movement along the Big Creek Fault. The geometry and preservation of the Sonora paleochannel also suggests a reverse com- ponent (Figure 7).
Bedrock Alteration
Ultramafics are
associated with replacement and skarn mineraliza- tion along the we s t-northwest trending margin of the mid-Cretaceous, coarse grained, gra- nitic, Dawson Range batholith. The Late Cretaceous quartz monzonite stock has undergone moder- ate to strong phyl- lic, argillic, and silicic epithermal altera- tion along its west- ern extent towards Sonora Camp but has induced contact metasomatic and skarn alteration in its east- ern extent towards and across Hayes Creek (Figure 5). The metasomatic alteration in ultramafics may also have been induced by the quartz monzonite. The primary epither- mal alteration and later leaching has largely masked primary textures (Bennett, 2010).
Bedrock Mineralization Historic exploration identified two min-
eralized zones: 1) Cu-Au quartz veins near
Photo #3. George Wilson uncovering a mammoth tusk (and later a skull) high on the east bank of Sonora Gulch, in unconsolidated, black, organic swamp silt and mud 150 meters south of sample 100803.03 (Figures 5 and 6)
the western end of the quartz monzonite porphyry (Kqm) - the “Gold Vein System”, and 2) a northwest-striking Au-tetradymite quartz vein - the “Tetradymite Vein System” (Bennett, 2010). The Tetradymite Vein System is associated with the Big Creek fault for about 1.3 kilometers and is several hun- dred meters north of the Gold Vein System.
Mineralization in the ‘Gold Vein System’
consists of pyrite and minor chalcopyrite disseminations approaching 10% total sul- fide content. Au and Cu min- eralization is disseminated in the monzonite fabric and concentrated in northwest- erly shears as auriferous quartz-arsenopyrite-sphal- erite-galena-stibnite veins. Precious metal content within the ‘Gold Vein System’ appears to be directly asso- ciated with sulfide content and vein density.
Mineralization in the
‘Tetradymite Vein System’ is concentrated in several nar- row, high-grade auriferous quartz-arsenopyrite veins of limited strike length, includ- ing the Nightmusic (wall rock) and Amadeus (quartz monzonite stock) deposits and the Sonata, Jupiter, Wolfgang and Concerto anomalies. The Nightmusic zone is complex and consists of 1) Au-enriched Cu-Zn- Pb skarns, 2) auriferous replacements in calcareous metamorphics, 3) lode Au associated with listwanitic ultramafics, and 4) veins and
14 TPG •
Jul.Aug.Sep 2024
www.aipg.org
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