Old Smoky Prospect, Tolovana Mining District, Yukon-Koyukuk Census Area, Alaska, USAi

Regional Level Types
Old Smoky Prospect	Prospect
Tolovana Mining District	Mining District
Yukon-Koyukuk Census Area	Census Area
Alaska	State
USA	Country

Location: The Old Smoky prospect is 1/4 mile north of Money Knob, just north of the old town of Livengood; NW1/4 sec. 23, T. 8 N., R. 5 W., of the Fairbanks Meridian.
Geology: Trenching near the head of Olive Creek has exposed narrow, northwest-trending auriferous arsenopyrite-quartz veins in ferruginous quartzite near the intersection of an altered, porphyritic, biotite-monzonite dike, and a potassium feldspar-porphyry dike (Foster, 1968). Mineralization at the Old Smoky prospect is in Devonian sedimentary rocks composed of shale, argillite, fine-grained sandstone, and pebbly conglomerate (Allegro, 1984, p. 3). Narrow zones of thermal metamorphism occur along sheared contacts between these sedimentary rocks and hypabyssal igneous intrusive rocks (Allegro, 1984, p. 3). Similar felsic intrusive rocks throughout the Livengood Quadrangle have been potassium-argon dated at 58.0 to 88.8 m.y. (Turner and others, 1975). Most of the intrusive rocks and some of the sedimentary host rocks have experienced variable degrees of metasomatic hydrothermal alteration followed by lower temperature supergene alteration (Allegro, 1984, p. 4). Allegro (1984) described four types of hydrothermal alteration: 1) silicification, as partial to complete replacement of the host rock by a dense network of quartz veinlets generally localized along contacts between the intrusive and sedimentary rocks; 2) sericitization, as fine- to medium-grained white mica in selvages along quartz veins, as anastomosing sericite-opaque mineral veinlets, and as patchy to massive sericitic replacement of feldspar, ferromagnesium minerals and quartz; 3) deposition of trigonal nets of needle-like rutile often associated with secondary quartz and minor feldspar; and 4) epidote +/- sericite as a replacement of calcic plagioclase and ferromagnesian minerals resulting in massive aggregates, pseudomorphs, veins, and vug fillings of epidote commonly associated with sericite, opaque minerals, and quartz. Allegro's (1984) investigation and sample data reveal that the mineralization in the southern portion of the cut is localized along the contact zones between the biotite monzonite and the surrounding sedimentary rocks, and along a contact between biotite monzonite and feldspar porphyry. Channel and chip samples of arsenopyrite-stibnite quartz veins from these zones contained 1.0 to 29.8 ppm gold. Selected samples from the prospect contain 3 to 13 ppm gold as determined by atomic absorption, and 1.6 to 7.0 ppm gold as determined by fire assay-atomic absorption (Foster, 1968, p. 2). Adjacent to the sheared contact zone, the intrusive rocks are either highly silicified with abundant rutile and some epidote, sericite, arsenopyrite, and monor stibnite, or contain epidote with sericite, rutilated quartz and arsenopyrite (Allegro, 1984, p. 6). Other rocks from the contact zone show intense supergene effects such as clay alteration, covellite and iron-oxides. In some cases these zones contain gold. Green scorodite is present throughout the mineralized areas. In the northern section of the prospect, the most abundant mineralization is located along the contact area between the feldspar porphyry and a roof pendant of sandstone and shale (Allegro, 1984). A massive 1-meter-wide stibnite lens surrounded by a bleached sericite zone occurs along the northern contact of the roof pendant. Channel samples along this contact zone range from 0.5 to 4.3 ppm gold (Allegro, 1984, p. 6). Some gold is also associated with saprolitic zones in all the intrusive phases but these zones are not limited to shear zones or contacts (Allegro, 1984, p. 6).
Workings: Some trenching and prospect pits that have been channel and chip sampled.
Age: Felsic intrusive rocks throughout the Livengood Quadrangle, similar to those at the mineralized contact zone of the Old Smoky prospect, have been potassium-argon dated at 58.0 to 88.8 m.y. (Turner and others, 1975).
Alteration: Most of the intrusive rocks and some of the sedimentary host rocks have experienced variable degrees of metasomatic hydrothermal alteration followed by lower temperature supergene alteration (Allegro, 1984, p. 4). Allegro (1984) described four types of hydrothermal alteration: 1) silicification, as partial to complete replacement of the host rock by a dense network of quartz veinlets generally localized along contacts between the intrusive and sedimentary rocks; 2) sericitization, as fine- to medium-grained white mica in selvages along quartz veins, as anastomosing sericite-opaque mineral veinlets, and as patchy to massive sericitic replacement of feldspar, ferromagnesium minerals and quartz; 3) deposition of trigonal nets of needle-like rutile often associated with secondary quartz and minor feldspar; and 4) epidote

Commodities (Major) - Au; (Minor) - Sb
Development Status: None
Deposit Model: Gold-bearing shear zone at contact of hypabyssal intrusions into sediments.

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Link to USGS - Alaska:	LG014

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