Red Lake Gold Ore (6.6 cm across at its widest) - highly concentrated network of gold-filled veinlets (gold veinlet stockwork) in biotite-carbonate altered, partially silicified metabasalt. Black = altered basalt; whitish-gray = quartz; gold = native Au. The crusts of gold are Au-filled fractures broken along the plane of the veinlets. Other Au-filled veinlets can be seen in cross-section view.

This rock leaves me speechless, in terms of its gold content and beauty. This is a sample of extremely high grade gold ore from the richest gold mine on Earth - northwestern Ontario’s Red Lake Mine.

The rocks and mineralization features in this area are fairly complex. The host rock here is a metamorphosed tholeiitic basalt dating to ~2.85 billion years (early Neoarchean). This basalt has been subjected to biotite-carbonate alteration and auriferous silicification. In addition to gold, the sample has a fair amount of biotite, some carbonate (it fizzes in acid), and many small, black rods of tourmaline. It also has patches of glassy gray quartz.

Geologic Provenance: extremely high-grade gold ore, possibly from the HWA Vein (Hanging Wall A Vein), upper High Grade Zone (HGZ; a.k.a. Goldcorp High Grade Zone), Balmer Assemblage, Red Lake Greenstone Belt, Uchi Subprovince, Superior Province.

Age: ~2.85 billion year host rock (altered basalt) with hydrothermal metamorphism and gold mineralization at 2.712-2.723 billion years (during collision-related emplacement of several nearby igneous intrusions - Uchian Phase of the Kenoran Orogeny) & at 2.63-2.66 or 2.699 billion years (during a regional thermal event or another igneous intrusive event).

Comment: the sample shown here is very similar to those shown in figures 18 and 19 of Dubé et al. (2001, The Goldcorp High Grade Zone, Red Lake Mine, Ontario: a photographic atlas of the main geological features. Geological Survey of Canada Open File 3890), which illustrate extreme high grade gold ores from Red Lake Mine's level 34. Rocks from the Goldcorp High Grade Zone average about 80 to 81 ounces of gold per ton of rock.