Gold vein stockwork in limonite, from Gold Flake Vein, Farncomb Hill, Breckenridge Mining District, Summit County, Colorado, USA
Gold vein stockwork in limonite from Colorado, USA. (public display, Denver Museum of Nature & Science, Denver, Colorado) The Colorado rock shown above, nicknamed “Tom’s Baby”, is an extremely high-grade gold ore. It was found in 1887 at Farncomb Hill, Colorado by two miners, Tom Grove and Harry Lytton. The rock was carried from the mine by Tom Grove, who held it as if he was cradling a baby, which inspired the rock’s nickname. The specimen is derived from a “gold pocket” in a subsurface hydrothermal vein that produced about 243 ounces of gold rock. The original Tom’s Baby rock was a single specimen from the pocket that reportedly weighed 160 ounces. Portions of the original rock have since been removed. Tom’s Baby comes from the Gold Flake Vein, one of many native gold-bearing hydrothermal veins at Farncomb Hill. The bedrock at Farncomb Hill consists of tilted Pierre Shale (Upper Cretaceous) with early Tertiary intrusions of pophryritic quartz monzonite that form sills. Adjacent to the intrusions, the shales have been contact metamorphosed, brecciated, silicified, and mineralized with small amounts of finely disseminated (invisible) gold. Hydrothermal veins cut across both altered Pierre Shale and the quartz monzonite sills. In proximity to the veins, the shales have finely disseminated pyrite and chalcopyrite. Low-angle normal faults have offset the hydrothermal veins in places - observed offsets are usually less than 10 feet, but can reach up to 35 feet. These faults occur within the Pierre Shale & also occur at and parallel to shale-sill contacts. Native gold was mined from Farncomb Hill’s hydrothermal veins, which are usually about 0.5 inches wide. Unaltered, non-oxidized veins consist of calcite that hosts metallic minerals such as pyrite (FeS2 - iron sulfide), chalcopyrite (CuFeS2 - copper iron sulfide), sphalerite (ZnS - zinc sulfide), galena (PbS - lead sulfide), and native gold (Au). Most of the veins are partially to entirely oxidized, which has resulted in the formation of limonite (FeO·OH·nH2O - hydrous iron hydroxy-oxide), in which native gold occurs. Significant gold-bearing veins all occur within the Pierre Shale immediately above (adjacent to) quartz monzonite sills or above (adjacent to) within-shale fault zones. The famous “gold pockets” of Farncomb Hill are very rich, native gold-bearing hydrothermal veins that are two to three feet in diameter and up to one inch wide. Tom’s Baby is from such a gold pocket. Notice in the above photo that the gold forms abundant criss-crossing veinlets in orangish-brown limonite. Criss-crossing veins in matrix are often referred to as “stockwork” or “boxwork”. Tom’s Baby has been called a “nugget” by some, but it is not from a placer - it is from a hydrothermal vein. The best lithologic descriptor for the specimen appears to be “gold vein stockwork in limonite”. The occurrence of gold pockets directly above fault zones at Farncomb Hill suggests that the pockets contain secondary gold, and that gravity has played a role in their formation. Downward-percolating fluids have apparently leached the microscopic disseminated gold from the Pierre Shale and concentrated it just above faults, where the veins are offset. A gold deposit having this origin is said to have been formed by supergene enrichment. The native gold itself occurs as wires, leaves, and plates. Small, scale-like gold crystals often cover portions of leaf gold and plate gold. Locality: Gold Flake Vein, 90 feet below the surface in the Gold Flake Mine, Ware-Carpenter Claims, Farncomb Hill, ~4 miles east of the town of Breckenridge, Breckenridge Mining District, Summit County, Colorado, USA Geologic info. mostly synthesized from: Lovering & Goddard (1950) - Geology and ore deposits of the Front Range, Colorado. United States Geological Survey Professional Paper 223. 319 pp. 30 pls.