Abstract The stratiform and strata-bound ore deposits of the world include some of the most famous—for their size or richness, or both: the Rand, the Copperbelt of Zambia and Katanga, the Rio Tinto mines, the lead and zinc deposits of the middle United States, the Noranda field, the Sullivan mine, the iron ores, and the phosphates of the western United States. In Australia, ore deposits of this type include concentrations of Pb and Zn, Cu, Sn, W, Au, Ni, U, Fe, and phosphate. Most of the major Australian stratiform orebodies were found between 1870 and 1885. Significant gold discoveries had been made up to 25 years earlier, but an appreciation of the importance of gossans came later. Mount Morgan, Mount Lyell, Cobar, and Broken Hill all had prominent gossans, some of which had been well-known features of the landscape for up to 20 years before their metal content was realized. Mount Isa was found in 1923, 50 years after the development of the nearby Cloncurry copper field; no other discoveries of large, high-grade outcropping stratiform base-metal orebodies have been made since. All orebodies found later have been in fields which had been known and worked for many years.

ABSTRACT The California Coast Ranges mercury deposits are part of the western North America mercury belt, in which mercury occurs most commonly as red cinnabar (α-HgS), sometimes associated with its high-temperature polymorph, metacinnabar (β-HgS). In the Coast Ranges, ores were deposited from hydrothermal solutions and range in age from Miocene to Holocene. Ore deposition at Mount Diablo generally occurred along active faults and associated extension fractures in the Franciscan complex, most often in serpentinite that had been hydrothermally altered to silica-carbonate rock. The Mount Diablo mine lies ~48 km (~30 miles) northeast of San Francisco in Contra Costa County and is mineralogically unique in California, because metacinnabar, the higher-temperature polymorph of mercury sulfide, is a major primary ore mineral in the deposit, while at all other mercury mines in California, it is quite rare. In addition, hydrothermal activity is so recent that sulfurous gases and methane continued to be released into the mine at least into the 1940s. Historically, long before active large-scale mining began in the 1800s, the Mount Diablo mercury deposits were known to the Indigenous people of the Ohlone tribes, who used the cinnabar in rituals as well as for red pigment to decorate their bodies, and as a prized trade item. The deposit was later rediscovered in 1863 and mined intermittently until 1958. The Mount Diablo mine and adjacent Rhyne (also variously spelled Ryne or Rhine) mine were the sites of most of the mercury operations in the region, and at both mines, mercury ore occurs in structurally controlled lenticular bodies of silica-carbonate rock and serpentinite. The total district production probably exceeded 12,300 flasks (at 76 pounds or ~34.5 kg per flask) at an estimated grade of 2711 g per metric ton. Low-grade ore reserves are believed to still exist, with 17,000 short tons of indicated and inferred ore. Other minor deposits of copper, silver, and gold occur on Mount Diablo, principally in and around Eagle Peak, but mercury is not associated with these deposits. The most serious environmental problem associated with the mercury mines is the transport of mercury via mine drainage from the Mount Diablo mines to Dunn Creek, a tributary to Marsh Creek, which ultimately drains into Suisun and San Francisco Bay. The concern is that elemental mercury (Hg 0 ) and the mercury ion Hg(II) can be biologically converted to toxic monomethylmercury (MeHg). To combat this problem, a long-term process of mine remediation has been undertaken.