Explained by James J. Rytuba
On the choice of deposit models
Hot-spring Au-Ag deposits are closely associated with intermediate to felsic volcanic fields emplaced in zones of extensional tectonics or in dilational jogs within transpresssional tectonic zones (Berger, 1986). Host rocks include volcanic and subvolcanic intrusive rocks as well as adjacent country rocks with high permeability or hydrothermally altered rocks with fracture permeability. Hot-spring sinter is typically present at the paleo-surface and the disseminated or vein type gold ore bodies occur below the sinter and are localized along a through going structure. Associated mineral deposits include hot-spring mercury deposits and antimony deposits.
On the delineation of permissive tracts
Permissive terranes include intermediate to felsic volcanic fields and areas of emplacement of subvolcanic intrusive rocks within zones of extensional tectonism. Factors that would exclude areas from such permissive terranes would include location outside of known volcanic belts and the presence of deeply emplaced plutonic rocks, with no subsequent igneous activity. Favorable areas are delineated by the presence of known economic and subeconomic hot-spring gold deposits or deposits commonly associated with these deposits.
Hot-spring Au-Ag deposits in the Coast Ranges of California are closely associated with Miocene to Holocene volcanic fields. The intermediate to felsic volcanism resulted from passage of the Mendocino triple junction as it migrated northward along the coast of California. Magmatic activity above the slab window, the area of thin crust underlain by hot asthenosphere that replaces the subducting slab, is characterized by andesitic to rhyolitic dome and ash-flow fields and associated plutons and batholiths. The volcanic fields and associated intrusive rocks extend eastward from the Coast Ranges into the Great Valley of California and the youngest volcanic field, the Clear Lake volcanic field is, in part, time equivalent to the Sutter Buttes dome field in the Central Valley. Permissive tracts for undiscovered hot-spring Au-Ag deposits were delineated to include those areas with known Miocene to Holocene volcanic and intrusive rocks and areas above the slab window where intrusive rocks can be expected to occur.
Favorable areas for undiscovered hot-spring Au-Ag deposits are those where prospects and deposits of hot-spring gold and related deposits are present in and adjacent to Miocene to Holocene volcanic and intrusive rocks. The Clearlake and Sonoma volcanic fields and areas adjacent to them are included as favorable because of the presence of volcanic and associated intrusive rocks and the occurrence of three known gold-mercury districts. Other areas in the Coast Ranges with known occurrences of volcanic rocks and occurrence of mercury, gold, and or antimony are also favorable for hot-spring gold deposits (Peters, 1991).
Important examples of this type of deposit
There are two examples of these types of deposits within this permissive terrane. One is the McLaughlin gold deposit (Lehrman, 1986), the largest and only active gold deposit in the Coast Ranges of California, and the second is the Cherry Hill deposit, which is presently uneconomic.
On the numerical estimates made
Factors that contributed to the numerical estimate for undiscovered resources included the presence of one mine and one undeveloped deposit of this type, the presence of other prospects and deposits related to hot-spring gold, and a large area of the permissive terrane having small volcanic centers and subvolcanic intrusions or high heat flow. Based upon these factors, the team estimated, for the 90th, 50th, and 10th percentiles, 1, 1, and 2 or more hot-spring Au-Ag deposits consistent with the grade and tonnage model of Berger and Singer(1992).
Berger, B.R., 1986, Descriptive model of hot-spring Au-Ag, in Cox, D.P., and Singer, D.A., eds., Mineral deposit models: U.S. Geological Survey Bulletin 1693, p. 143-144.
Berger, B.R., and Singer, D.A., 1992, Grade and tonnage model of hot-spring Au-Ag, in Bliss, J.D., ed., Developments in mineral deposit modeling: U.S. Geological Survey Bulletin 2004, p. 23-25.
Lehrman, N.J., 1986, The McLaughlin mine, Napa and Yolo Counties, California, in Tingley, J.V., and Bonham, H.F., Jr., eds., Precious-metal mineralization in hot-spring systems, Nevada-California: Nevada Bureau of Mines and Geology Report 41, p. 90-92.
Peters, E.K., 1991, Gold-bearing hot spring systems of the northern Coast Ranges, California: Economic Geology, v. 86, no. 7, p. 1519-1528.