McElfish

Prospect, Inactive

Commodities and mineralogy

Main commodities Au
Other commodities Ag; Cu; Mo; Pb; Zn
Ore minerals chalcopyrite; galena; gold; molybdenite; pyrite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale TC
Quadrangle map, 1:63,360-scale C-1
Latitude 63.7368
Longitude -141.0321
Nearby scientific data Find additional scientific data near this location
Location and accuracy The McElfish prospect is located next to the Alaska-Canada border on a north-south trending ridge at the headwaters of a northeast branch of McElfish Creek that drains into the Ladue River. The site location is based on where strongly altered augen gneiss was mapped near the center of a 1.1 by 1.5 kilometer soil grid located in the northwest corner of section 31, T. 23 N., R. 23 E. of the Copper River Meridian.

Geologic setting

Geologic description

McElfish is located within the Yukon-Tanana Terrane between the Denali and Tintina faults. The basement rock is comprised of amphibolite- to greenschist-facies metamorphic rocks intruded by Jurassic, Cretaceous, and Tertiary mostly felsic units. The amphibolite-facies rocks consist of schist, gneiss including augen gneiss, amphibolite, quartzite, and minor marble. The greenschist-facies rocks consist of schist and minor phyllite and metavolcanic rocks. These two packages represent an autochthonous continental-margin of Late Devonian to Early Mississippian age (Dusel-Bacon and others, 2006).
The bedrock geology at McElfish, from north to south, consists of weakly foliated metamorphosed gabbro that is in an east-west trending thrust contact with a metamorphic assemblage of megacrystic augen gneiss and quartz-feldspar-sericite gneiss to schist. The augen gneiss contains very large potassium feldspar porphyroblasts that range up to 3 inches in diameter. A feldspar-quartz porphyry is found intruding the augen gneiss along a northwest trending fault on the south side of the main ridge. The porphyry and the augen gneiss along the northwest trending fault have strong quartz-sericite-pyrite alteration. To the south of the prospect area, biotite-garnet amphibolite and biotite schist overlie the augen gneiss and define a general east-west thrust fault contact (King and Tedeschi, 2012).
Mineralization at McElfish is mostly stockwork quartz veinlets up to 2 mm wide cross cutting foliation at high angles and the veinlets commonly exhibit strong jarosite staining and minor disseminated gossanous blebs after weathering of sulfide minerals along vein selvages (Siron and Grady, 2011).
Geologic map unit (, )
Mineral deposit model Low-sulfide Au-quartz veins? (Cox and Singer, 1986; model 36a).
Mineral deposit model number 36a?
Alteration of deposit The augen gneiss and feldspar-quartz porphyry have strong quartz-sericite-pyrite alteration along a northwest trending fault (King and Tedeschi, 2012).

Production and reserves

Workings or exploration
In the mid-1970s Cities Service Minerals Corporation (CSMC) conducted regional reconnaissance exploration in the Tanacross quadrangle area. Several ridge and spur soil samples and rock samples were collected over the McElfish prospect area at this time (Hedderly-Smith, 2009).
Full Metal Minerals (FMM), in a joint venture with BHP Billington, conducted a 31,000 line-kilometer high-resolution airborne magnetic survey covering 1.05 million hectares of the Tanacross quadrangle encompassing the McElfish prospect and collected rock and soil samples within the headwaters of McElfish Creek during 2008 (Hedderly-Smith, 2009). This work identified weakly anomalous gold-in-soils (up to 10 parts per billion (ppb) gold) and highly altered rocks consisting of an assemblage of clay and silica within quartz-sericite schist and augen gneiss. Samples of the stockwork quartz veining that contain disseminated trace pyrite returned values up to 185 ppb gold (Siron and Grady, 2011).
In 2010 FMM completed a 50 by 100 meter soil grid within a larger 1 by 1.3 kilometer area of intense and pervasive silicification and clay alteration of augen gneiss and quartz-sericite schist. The sampling showed a soil geochemical anomaly with strong gold, silver, molybdenum, lead, and zinc. Baseline gold-in-soil observations display weakly anomalous gold, around 13 ppb gold, but have shown values as high as 7,160 ppb gold. Silver distribution is much broader and ranges from 300 ppb and up to 1,900 ppb silver over the same area as anomalous gold mineralization. A strong molybdenum anomaly occurs to the east and downslope of anomalous gold-in-soils and ranges from 5 to 30 parts per million (ppm) molybdenum. Lead (50 to 180 ppm) and zinc (80 to 170 ppm) also occur with gold and silver (Siron and Grady, 2011).
The soil grid was expanded to the south by FMM in 2011 collecting 207 more soil samples. Results from the grid showed a silver and gold anomaly with gold showing potentially two northeast-trending zones of mineralization. There were no significant silver and gold values in the rock samples collected as most of the samples were gathered on the ridge top where the silver and gold anomaly was weakest. A copper and molybdenum soil anomaly trends northwest-southeast with up to 48 ppm molybdenum and up to 463 ppm copper. A rock sample of strongly oxidized augen gneiss with magnetite veins was collected in this same area and contained 551 ppm copper and 113 ppm molybdenum. This mineralized trend is parallel to foliaion of the strongly quartz-sericite-pyrite-altered augen gneiss observed on the ridge (King and Tedeschi, 2012).
Indication of production None

References

References

Reporters N.V. King (Alaska Earth Sciences)
Last report date 3/15/2016