Marma, John C., and Vance, Randall B., Newmont Mining Corporation, Midas Operations, 1001 Gold Street, HC 66, Box 125, Midas, Nevada
The Gold Circle district, also known as the Midas district, contains numerous well-developed examples of cymoid loop structures, which significantly affected the formation of veins, geotechnical aspects of safe mine development, and economic exploitation of ore.
The first use of the term cymoid loop to describe veins was suggested by Dr. Kirk Bryan in the early 20th Century as an analogy to the architectural form used to describe certain shapes of molding (McKinstry, 1948). Since then cymoid loops have been described in many well known gold and silver vein districts, such as Grass Valley, Calif., (Johnston, 1940), Pachuca, Mexico (Thornburg, 1945) and the Hauraki goldfield of New Zealand (Begbie, Spörli and Mauk, 2007). In general, a cymoid loop can be described as a vein or structure that branches and swerves from its course only to reunite and return to its general course whereby enclosing a doubly-convex lens of wall rock (McKinstry, 1948). Ultimately, by taking either branch of the structure one will return to the same general trend as one started, similar to “If you come to a fork in the road, take it.” – Yogi Berra. However, not all cymoids are the same.
In general there are three manifestations of cymoid loops at Midas, and in all cases the presence of vein material (qtz±cal±adul) does not imply gold or silver mineralization: 1) vein-structure cymoid loops, where one branch of the cymoid contains vein material and the other does not, 2) vein-vein cymoid loops, where both branches of the cymoid contain vein material, 3) structure-structure cymoid loops, where both branches lack vein, but are still important to understand along strike. All three types are the result of a left-lateral, oblique strike-slip, extensional structural regime.
In the production setting, mine geologists can make better development decisions by recognizing and understanding cymoid loops. Cymoid loops can impact ore/waste calls, sill development directions, stope design, dilution and in some cases offer economically significant opportunities in the ribs (such as missed cymoid branches). In addition, understanding cymoids greatly improves the safety of mining. By knowing the location of cymoid loops and the tendency of rock to fail to cymoid branches, cable bolting designs can be designed efficiently and effectively to mitigate failures in sills and stopes.
In district exploration, geologists can increase their margins of success. Recognizing and understanding cymoid loops can be useful in drill hole targeting, designing offset holes to significant drill hole intercepts, and structural interpretation at the local and district scale. Cymoid loops at Midas are fractal in nature, occurring at all scales, from inches to thousands of feet along strike. They are also similar in appearance whether they are viewed in plan or in cross-section.
As exploration continues in epithermal vein districts, familiarity with cymoid loops could be the difference between failure and the next big discovery.