This is a follow-up article to “Epithermal Ore Deposits...” published in the December 2000 issue; the figures included below are also from that issue.. —Editor
While there are numerous epithermal precious metal deposits within the Tertiary lavas throughout the North American Cordillera, most associated eluvial placers are too thin and scattered to be economically worked by conventional dry placer methods.
However, starting about 1980 with the development of relatively inexpensive, small, hand-held, VLF-type detectors, metal detectors are now a proven prospecting tool with the capability of covering large areas and locating sub-grain to multi-ounce nuggets within the scattered and easily overlooked residual and hillside placers in the North American Cordillera. More importantly, surfacial metal detecting does little to disturb the land surface; the small holes are quickly and easily backfilled.
Fig. 1 Map showing the surface distribution (areas of outcrops) of Tertiary and later volcanic rocks in North America. (Source: USGS)
Waldemar Lindgren, a former USGS geologist and late professor of economic geology at MIT, authored the first edition (1913) of Mineral Deposits. For over forty years it was one of the leading textbooks used by universities and colleges.
Professor Lindgren, in the preface to the fourth edition (1933) of Mineral Deposits made the following observation: “A comparison with the early years of this country accentuates the interesting fact that the epithermal gold and silver deposits are approaching exhaustion in the United States of America. The number of mining districts of this type which have [become] impoverished or abandoned is astonishingly large. It also brings out the fact that, on the whole, the ore-shoots of the mesothermal and hypothermal ore deposits are more persistent and reach much more greater depths than those of the epithermal class.”
Another opposing viewpoint was penned over 50 years later by John M. Guilbert, University of Arizona and author of The Geology of Ore Deposits. In the preface of the fourth edition (1986) Professor Guilbert wrote, “The book emphasizes that the days of ‘easy’ discovery are not over—tracts of land remain on every continent to be effectively examined as the world’s geologic perceptions and needs change. The productive explorationist of the twenty-first century will certainly be the one who understands and can most effectively utilize geochemical, geophysical, and geological knowledge of the Earth’s crust.”
Both Professors Lindgren and Guilbert are right. Starting with the building of the transcontinental railroad during the 1860s through more than a half-century of intensive prospecting by both “single-blanket burro prospectors” and large mining corporations, most of the more obvious ore-bearing out-croppings had been discovered. Today, however, prospecting tools such as the metal detector are being effectively used by knowledgeable prospectors to find and recover scattered and overlooked eluvial gold placers associated with epithermal class deposits.
Fig. 2 Relief model of the Cordillera region, showing the relationships of its major physiographic provinces. (Credit: Prof. William J. Miller)
To better understand the relative importance of Tertiary age epithermal class gold and silver deposits within the Cordillera, we must define “Cordillera.”
The term is derived from both the Latin chorda for “cord” and the Spanish cordillera referring to a “chain or range of mountains.”
Major Tectonic Activity: The tectonic history of the North American Cordillera is too complex to be summarized within a few paragraphs. However, during the Paleozoic, Mesozoic, and Cenozoic Eras, this vast mountainous region; from Canada southward into Mexico; from the eastern face of the Rockies to the Pacific Ocean; and including all of the associated basins, plateaus, rivers and lakes was essentially a shallow, vast, geosynclinal seaway, receiving thousands of feet of sediments.
During Late-Mesozoic time, the Cordillera experienced extensive plutonic north-south mountain building. It began with the Coast Ranges and extended northward into Alaska, creating the core of the Alaskan Range, and southward forming, the Peninsular Range. Then gradually this plutonic mountain building spread eastward, creating the Sierra Nevadas, the Wasach Range and then the Rocky Mountains.
Mesothermal-class Ore Deposits (a.k.a. Porphyry-system deposits) are associated with the placement of the plutonic, “granitic” rocks that formed these north-south trending mountainous chains within the Cordillera. Porphyry deposits include the ores of copper, molybdenum, manganese, lead, zinc, silver and gold. Non-economic traces of platinum, rhenium, uranium, tungsten, tin, arsenic and bismuth can be by-products during milling. Since most of these mesothermal (porphyry) deposits are associated with large granitic intrusives that expanded eastward, most have been well explored, developed and are beyond the scope of this article. However, by Eocene time, the intrusions reached the eastern margin of the Cordillera, causing extensive uplift and erosion.
Epithermal Ore Deposits: Uplift and erosion, known as the Laramide Orogeny, set the stage for extensive acidic Tertiary lava flows to blanket the Cordillera. Near the vents of volcanic lavas, such as rhyolite and andesites, hundreds of precious-metal (gold and silver) ore deposits were deposited throughout the region.
According to Guilbert (1986), Lindgren’s original classification of epithermal class ore deposits has been modified to include temperatures from 50 to 300°C. However, this is still consistent with Lindgren’s definition as he originally defined epithermal deposits related to volcanic activity at low temperatures (50 to 200°C), and at pressures existing about one km or less below the existing surface.
While it is true most of the deposits are small and shallow—with the noted exception of the Comstock Lode (Virginia City, Nevada)—epithermal type ore deposits have in the past produced most of the quicksilver, a considerable amount of the silver and gold, and subordinate amounts of antimony, arsenic, tungsten, lead, and zinc within the Cordillera.
According to Emmons nearly all of the epithermal deposits of sulphide ores, such as chalcopyrite, cinnabar, galena, pyrite, sphalerite and stibnite, are structurally related to simple fissures. Copper is rarely present in large enough quantities to be economically important within the shallow sulphide zone at the time of deposition.
The veins of epithermal silver and gold values characteristically “bottom” at shallow depths; while the sulphides (chalcopyrite, galena and sphalerite) and valueless fissure fillings such as quartz, carbonates and other gangue minerals continue on at depth. The quartz may be clear, or white, or greenish; or may occur as chalcedony, a fibrous variety of quartz, or as amethyst. The carbonates can include ankerite, calcite, dolomite, or rhodochrosite.
Many epithermal class deposits are associated with simple fissures and have a direct connection to the surface, allowing mineralized solutions an easy access to the surface. Non-economic epithermal mineral deposits are currently being formed at the orifice of hot springs. As an example, over a century ago it was noted at Sulphur Bank, California, in a region associated with late volcanic activity, the hot spring waters contained dissolved metals of stibnite, cinnabar, chalcedonic quartz, and other epithermal minerals.
with a Metal Detector
While it is true that high grade ores such as tellurides and sulphides do not respond to a metal detector; free-milling gold ores can produce good responses. Wherever gold outcrops, it is reasonable to find hillside placers. However, by secondary enrichment, gold can dissolve and leach downward.
If the primary ore-shoot hasn’t yet reached the surface, subsequent placers cannot form. Also, any shallow veins within the higher mountainous elevations can be completely eroded away by Pleistocene “little ice age” glaciation; therefore, prospect (detect) the slopes, slope-washes, and gulches within the lower foothills as “hit or miss” placer gold can concentrate from the erosion of many veinlets.
Prospecting Tips: The Cordillera is a broad area containing hundreds of eluvial dryplacers that were worked by the primitive dryplacer methods available to yesteryear prospectors and miners. Today, many of these paystreaks, once too thin to be drywashed, can now be successfully detected.
Research: In your research, seek USFS and BLM ground that is not under claim. Contact both state and federal agencies for a free list of available publications. Ask questions. Remember you are mostly interested in epithermal class precious mineral deposits associated with Tertiary acidic volcanics such as rhyolite and andesite. Also, epithermal silver and gold ores are often found as “electrum,” a gold alloy containing at least 20 percent silver.
Prospect: Seek old worked out drywash areas. Small areas are best because they haven’t been worked as much. Also seek old, worked out, idle mines, preferably ones that had a history of free-milling gold values. (However, a mining district primarily known for its lead or zinc values can also be worth researching for gold values.) Stay out of old epithermal mines—over time they have become hazardous! Instead, detect the dump itself. Look for spills along old haulage roads, especially any sharp switch-backs. Most importantly, detect the dry slopes, arroyos, and gullies for at least a quarter-mile within the watershed below the mine.
Alterations: The “country rock” near epithermal veins is often altered even though the vein is fresh looking. Mineral alterations such as clay, sericite, chlorite, zeolites, silica, and pyrite are often fine grained. Clays (kaolin) can be white or various colors of yellow to reddish. Sericite, a whitish mica (muscovite) can give a silky cast. Chlorite (an hydrous iron-aluminum-magnesium-silicate) can give a micaceous, soapy feel and greenish cast to andesite.
Gossans: The most common iron mineral found in veins is pyrite and, to a lesser extent, marcasite. Both oxidize to limonite and can give a leached, cellular and rusty appearance to the outcrops of a vein. Insoluble, free-milling gold can be concentrated within the limonitic, cellular box-work.
Quartz: Commonly, quartz is found within a vein as “gangue” (a valueless associated mineral). However, quartz can also carry “values.” Therefore, look for both irregular chunks of high-grade “float ore” displaced from a vein or chunks of “quartz float.” Run your search coil over any out-of-place, isolated, iron- stained rock. Be especially alert for concentrations of translucent crystals or white, massive “bull quartz.” Also, epithermal vein quartz can occur variously colored reddish and greenish (chalcedony); or with inclusions (rutilated) and “banded” with other ore minerals; or consist of calcite with quartz blades that fill thin cracks.
Stay away from Indian reservations, National Monuments, and all “archeologically sensitive” areas. Always keep a clean camp; do not contaminate any local water supply, destroy any vegetation, or harm any critters; and be sure to backfill any dig holes.
Do not trespass on posted, patented, or any other privately owned property. Ask for permission; sometimes you can work out a percentage deal with the owner of record. If you are given permission, always be completely honest and square in your dealings. If you cannot get permission, go on to your next researched area.
Dodge, Natt N., (1986) Poisonous Dwellers of the Desert, Southwest Parks and Monuments Association Tucson, AZ.
Emmons, W.H., (1940) The Principles of Economic Geology, McGraw- Hill.
Guilbert, John, (1986) The Geology of Ore Deposits, Freeman and Co.
Lindgren, W., (1933) Mineral Deposits, McGraw-Hill.