Applications: Alteration Mapping


Alteration maps typically are based on macroscopic field observations, but in many deposits the alteration minerals are fine-grained and difficult to identify in the field. Many current exploration programs use field-portable VIS-NIR-SWIR spectrometers to facilitate field identification of important alteration minerals. The use of spectrometers at a field base allows the mineralogy to be mapped or placed on cross sections. The resultant interpretation can be applied in real time to guide drilling and exploration. Extensive mapping programs using field observations and spectral analysis should also be supported by limited petrographic or x-ray diffraction studies. The combined data can ultimately be integrated with other data to develop targets, models, and regional guides.
All alteration mapping first requires mineral identification, followed by grouping of the minerals based on their associations. The purpose of the mapping is to construct a model of the deposit in space and time. Field observations must be made in a careful and systematic manner. The recording of basic observations, such as color, texture, modes of occurrence, and weathering state, is important for the correct interpretation of the mineralogy. The relationship among minerals must be determined with care prior to assigning them to a single assemblage or interpreting their relationship to other minerals. In addition, sample descriptions need to include several characteristics that directly affect the quality of spectral data, including grain size, transparency, sulfide content, water content, heavy-element content, contaminants (such as oil and organic material), and orientation of minerals (such as micas).


A series of logical steps should be followed in order to make realistic interpretations of the observed hydrothermal alteration. These steps include (1) detailed field and sample descriptions, (2) mapping distribution of minerals at several scales of observations, (3) consistent use of SWIR analysis to supplement mapping, (4) selected use of petrography and x-ray diffraction to provide references, and (5) continual reevaluation of the interpretation and integration with other data sets.


Spectral analysis aids exploration from property to regional scales. Although useful for identifying minerals in individual samples, field spectroscopy is most effective when data are collected in a large-scale, systematic manner. Large-scale surveys carried out on grid or other systematic patterns provide valuable information. For example, in complex zoned intrusive systems the alteration mineralogy determined during routine mapping helps to define the vertical and horizontal zoning and related ore environments. Within each environment the alteration mineralogy can define local zoning, providing vectors to mineralization. Spectral analysis may also provide information on subtle mineralogical and chemical changes even where the dominant mineralogy is obvious to the field geologists. Data processed and evaluated concurrent with mapping can have a direct impact on an exploration program.


Source: A.J.B. Thompson


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