Few exploration programs are so purely reconnaissance in nature that genetic models play no role i the design of the program, or in the interpretation of data. Through continuing refinement, models of ore genesis achieve greater and greater clarity, and new types of mineralization challenge our abilities to assimilate information and to transcend tradition. Intelligent interpretations reflect the training, experience, and indeed the personal bias of the explorationist.
The more that is known of the genesis and character of mineral occurrences, the more meaningful the picture becomes. Mineralization often is found where we expect it to be, produced by processes that in part are understood, within lithotectonic settings that can be recognized.
Even with recognizable geologic settings, however, data offer considerable opportunity for misinterpretation. The distributions of anomalous samples must be accurately determined to ensure success and the best return for dollars spent on exploration. T he determination of threshold concentrations remains a worthy but elusive goal in geochemical exploration.
Multiple element data packages now can be rapidly manipulated and integrated with data on structure, rock and mineral compositions, alteration, fluid characteristics, etc. The accompanying figure presents one example in which several data planes have be en overlain to provide clues to concealed mineralization.
The target is the Bonanza caldera in the San Juan volcanic field, Colorado, U.S.A. Several million dollars in lead, zinc, copper, silver, and gold were produced from veins in the area during the period 1880-1930, and it is suspected that the mineralizat ion may be the surface expression of one or more large igneous-hydrothermal systems concealed along the eastern margin of the caldera.
A regional study of fluid inclusions has shown that the exposed mineralization formed from fluids that were between 160o and 330oC, at salinities of less than five percent. In addition to several 1000's ppm combined Lead-zinc-copper, seventy-five sample s of mineralization average 207 ppm silver (max. 1870), 514 ppb gold (max. 8000 ppb), and 16 ppm molybdenum (max. 140 ppm). Tin was found in a small number of soil samples from the region.
The data base for the example includes geology, topography, and soil geochemistry for iron, silver, and molybdenum (16 samples per square mile). The occurrence of felsic (ring-type) intrusions is shown with respect to the margin of the caldera. Within caldera settings, these intrusions -- and/or the heat associated with them -- can make significant contributions to mineralization in the overlying rocks, and soil data for iron, silver, and molybdenum are examined with respect to the intrusions.
The topography of the region has been smoothed to facilitate comparisons between the chemical data and the geology. Interestingly, the topographic high for the entire Bonanza region marks a gap in the distribution of felsic intrusions along the eastern margin of the caldera, and as such may highlight the presence of concealed intrusive activity plus associated hydrothermal alteration.
Iron was included in the data base because anomalous concentrations in soils of the region may reflect the presence of epigenetic pyrite in the underlying rocks. The highest concentrations coincide with the regional topographic high. The regional plots for silver and molybdenum represent the distributions of positive residuals that lie above the threshold surfaces for those elements -- i.e., samples for which silver and molybdenum are above the trends in element concentrations for the region as a whole . The residuals for both elements appear to be related spatially to felsic intrusions, particularly if the gap conceals intrusive activity.
Always the goal in geochemical exploration, the isolation of authentic metal anomalies is accomplished most efficiently when various forms of data can be integrated. In the example, the most fertile area for follow-up study lies along the northeast marg in, and the area east of the caldera margin also should be evaluated.