GRADE AND TONNAGE MODEL OF SEDIMENT-HOSTED Au

By Dan L. Mosier, Donald A. Singer, William C. Bagby, and W. David Menzie

COMMENTS: This model applies to the descriptive model for carbonate-hosted Au-Ag (Berger, 1986a) and supersedes the grade and tonnage model for that deposit type (Bagby and others, 1986). The change in the model name reflects the discovery of many deposits in siliceous shale and other noncarbonate host-rocks and the reassignment of some silver-rich deposits to the distal disseminated Ag-Au type (i.e., Hilltop, Candelaria, and Taylor); the few deposits remaining with reported silver grades are Alligator Ridge, Dee, and Standard. Other deposits in the original set were deemed atypical (Bald Mountain, Windfall, Giltedge, Tolman), or reclassified as other types (Atlanta and Florida Canyon, hot spring Au). This model represents considerable refinement of the data used by Bagby and others (1986). Deposits where mineralization is known to be within 500 meters of each other were combined. Most of the names listed below are property names that contain multiple zones or deposits. Well-known property names containing multiple deposits that are over 500 meters apart, such as Jerritt Canyon, are listed individually with corresponding deposit names in parentheses. For some property names with multiple deposits, such as Marigold, only the well explored deposits were included and are shown in parentheses. This model excludes deposits for which information on distances between discrete orebodies was not available at the time of the compilation (i.e., Big Springs, Northumberland, Tonkin Springs, and others). The distribution of tonnage is significantly skewed toward larger tonnages because of the two very large deposits. No geologic reason has been found to distinguish these large deposits from the other deposits, however these two deposits appear to be more throughly explored both laterally and vertically than most of the other deposits plotted. This suggests that many of the other deposits will eventually be found to be much larger than now estimated.

See figs. 134-135

Figure 134. Tonnages of carbonate-hosted Au-Ag deposits.

Figure 135. Precious metal grades of carbonate-hosted Au-Ag deposits. A, Gold. B, Silver.

DEPOSITS

TABLE X. —Tonnages and grades of sediment-hosted Au deposits

[Tonnages in million metric tons, silver (Ag) and gold (Au) grades in grams per metric ton.]