MID

Prospect, Active

Commodities and mineralogy

Main commodities Ag; Au; Cu; Pb; Zn
Ore minerals arsenopyrite; chalcopyrite; galena; gold; magnetite; pyrite; pyrrhotite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale MH
Quadrangle map, 1:63,360-scale A-1
Latitude 63.2485
Longitude -144.0965
Nearby scientific data Find additional scientific data near this location
Location and accuracy The MID prospect is approximately 4 miles south-southeast of the junction of the Robertson River and Rumble Creek. It is at an elevation of about 5,700 feet at the toe of a remnant glacier on the section line between sections 16 and 17, T. 17 N., R. 7 E., Copper River Meridian. The deposit is concealed by glacial material. The location is accurate.

Geologic setting

Geologic description

The MID massive sulfide deposit is in metarhyolite of the upper Lagoon unit of Devonian age. The deposit is a sheet-like body as much as 47 feet thick in its longest drill intersection. It is the thickest block in what was once a single continuous massive sulfide layer that extends more than 2 miles along strike and is more than 40 feet thick. The layer is now offset by high-angle faults into six segments . The MID deposit is a blind deposit covered by glacial ice and moraine (Dashevsky and others, 2003).
The massive sulfide mineralization was intersected by drilling through the overlying Tiger unit into the metavolcanic rocks of the upper Lagoon unit which hosts the mineralization. The rocks surrounding the MID prospect are part of the Tiger unit; however, the massive sulfide mineralization was intersected by drilling into the underlying metavolcanic rocks of the upper Lagoon unit that host the mineralization. The Lagoon unit is a succession of dark gray, rusty, phyllitic metamorphosed mudstones interbedded with light-gray to white to pale green siliceous quartz-sericite(-chlorite) schist. Locally the rocks contain coarse, blue quartz eyes and rare fragmental volcanic textures preserved as chloritized lithic fragments. The upper part of the unit is dominated by white to pale green, massive to laminated quartz-eye-bearing quartz-sericite(-chlorite-pyrite) schist; finely laminated schist with minor metamorphosed black mudstone; and thin intercalations of quartzite and fine-grained metamorphosed grit. The protoliths of the volcanic rocks of the upper Lagoon unit are mainly rhyodacite and dacite, but they also include rare rhyolite and minor andesite and basalt (Dashevsky and others, 2003).
MID was discovered when massive sulfide float boulders were found during stream-sediment reconnaissance in 1976 (Rodney A. Blakestad and others, unpublished Resource Associates of Alaska Inc. report, 1976). The MID deposit is a blind target beneath glacial ice and moraine. Electromagnetic geophysical surveys were instrumental in recognizing this deposit. Extensive geological, geochemical, geophysical surveys and drilling were conducted by Resource Associates of Alaska from 1976 to 1981, by Phelps Dodge in 1990, and by American Copper and Nickel Company from 1994 to 1997. The alteration associated with the mineralization is characterized by the development of sericite, quartz, pyrite, and carbonate in the hanging wall and extensive chlorite and magnesium enrichment and sodium depletion in the footwall. Fifteen core holes totaling 11,655 feet have been drilled by 2002 (S.S. Dashevsky, unpublished data, 2002). The Drum unit, which is part of the metamorphic sequence that includes the rocks at this deposit is near the Devonian-Mississippian boundary on the basis of one SHRIMP U-Pb zircon age of 359 +/- 6 Ma at the nearby DD South prospect (MH325).
The MID deposit has an inferred resource of 7.2 million tons averaging 0.4 percent copper, 1.6 percent lead, 4.5 percent zinc, 62 parts per million (ppm) silver, and 1.6 ppm gold. The resource calculation includes only those contiguous bodies with a true thickness greater than 8 feet and a gross metal value of greater than $80.00 per ton at 1998 prices (S.S. Dashevsky and others, 2003).
In 2011, Heatherdale Resources Inc. (2011) drilled 3 holes at the MID prospect; two cut significant mineralization. Six intercepts were especially notable; they varied from 5.0 to 75.7 feet long with metal contents of from 0.12 to 0.56 percent copper, 0.01 to 5.21 percent zinc, 1.07 to 2.36 percent lead, 0.79 to 1.56 grams of gold per tonne, and 44 to 98 grams of silver per tonne. The best was 43.9 meters with 0.56 percent copper, 5.21 percent zinc, 2.36 percent lead, 1.56 grams of gold per tonne, and 85 grams of silver per tonne. Heatherdale also collected massive-sulfide boulders and outcrop samples. Thirty massive-sulfide boulders averaged 0.91 percent copper, 3.11 percent zinc, 3.57 percent lead, 14.86 grams of gold per tonne, and 477 grams of silver per tonne. Six outcrop samples contained 0.09 to 1.34 percent copper, 0.11 to 11.95 percent zinc, 0.59 to 11.54 percent lead, 0.25 to 11.25 grams of gold per tonne, and 28 to 280 grams of silver per tonne.
Geologic map unit (-144.098750411824, 63.2481242296867)
Mineral deposit model Kuroko massive sulfide (Cox and Singer, 1986; model 28a).
Mineral deposit model number 28a
Age of mineralization The Drum unit, which is part of the metamorphic sequence that includes the rocks at this deposit is near the Devonian-Mississippian boundary on the basis of one SHRIMP U-Pb zircon age of 359 +/- 6 Ma at the nearby DD South prospect (MH325).
Alteration of deposit The alteration associated with the mineralization is characterized by the development of sericite, quartz, pyrite, and carbonate in the hanging wall and extensive chlorite and magnesium enrichment and sodium depletion in the footwall.

Production and reserves

Workings or exploration MID was discovered when massive sulfide float boulders were found during stream-sediment reconnaissance in 1976 (Rodney A. Blakestad and others, unpublished Resource Associates of Alaska Inc. report, 1976). The MID deposit is a blind target beneath glacial ice and moraine. Electromagnetic geophysical surveys were instrumental in recognizing this deposit. Extensive geological, geochemical, geophysical surveys and drilling were conducted by Resource Associates of Alaska from 1976 to 1981, by Phelps Dodge in 1990, and by American Copper and Nickel Company from 1994 to 1997. Fifteen core holes totaling 11,655 feet had been drilled to 2002 (S.S. Dashevsky, unpublished data, 2002). Heatherdale Resources Inc. (2011) drilled 3 holes in 2011, sampled massive-sulfide boulders, and collected outcrop samples.
Indication of production None
Reserve estimates The MID deposit has an inferred resource of 7.2 million tons averaging 0.4 percent copper, 1.6 percent lead, 4.5 percent zinc, 62 parts per million (ppm) silver, and 1.6 ppm gold. The resource calculation includes only those contiguous bodies with a true thickness greater than 8 feet and a gross metal value of greater than $80.00 per ton at 1998 prices (S.S. Dashevsky and others, 2003).
Production notes None.

References

References

Heatherdale Resources Ltd., 2011, Drill hole 2011-154 intersects 43.9 feet of 0.56% Cu, 5.21% Zn, 2.36% Pb, 1.56 g/t/Au, and 85 g/t/Ag: http://www.heatherdaleresources.com/hdr/NewsReleases.asp?ReportID=490972&_Type=News-Releases&_Title=Heatherdale-Announces-Positive-Exploration-Results-From-The-Delta-VMS-Proje... (News release, November 16, 2011)
Lange, I. M., Nokleberg, W.J., Newkirk, S. R., Aleinikoff, J.N., Church, S.E., and Krouse, R.H., 1993, Devonian volcanogenic massive sulfide deposits and occurrences, southern Yukon-Tanana terrane, eastern Alaska Range, Alaska: Economic Geology, v. 88, p. 344-376.
Reporters W.T. Ellis (Alaska Earth Sciences); A.W. Wyatt and S.S. Dashevsky (Northern Associates, Inc.); W.J. Nokleberg (USGS): D.J. Grybeck (Contractor, USGS)
Last report date 4/1/2012