Crustal Geophysics and Geochemistry Science Center

Modeling Near-Surface Processes in Mineral Systems

Task 2: Source Characterization

Task Summary

The primary objectives of this task were to (1) better characterize and differentiate different deposit types by their mineralogy, host rock lithology, and trace element chemistry; (2) understand the variation and mode of occurrence of trace elements in primary minerals; and (3) understand the speciation (chemical form) of elements in solid primary minerals. The affects of climate on the above factors were considered.

The first objective was important because the primary minerals present in the deposit (and host rocks) determine the elements that are released to the secondary environment (Task 3). Understanding the trace elements that characterize different deposit types was critical for modeling or predicting the chemistry of the secondary environment surrounding mineral deposits. The second objective was important because the mobility of an element to the secondary environment depends, in part, on the mode of occurrence of an element in the primary mineral. For example, recent work on volcanogenic massive sulfide deposits shows that elements such as Cu, Ag, As, and Tl that are present as inclusions in pyrite are mobilized relatively easily. The third objective, understanding the speciation or chemical form of an element in primary minerals was important because this presumably affects the form of the element in the secondary environment; the form of the element relates to its toxicity, bioavailability, etc.

Task Highlights

Recent data for massive sulfide deposits in Alaska show that key geochemical and mineralogical parameters can be used to distinguish natural versus anthropogenic sources of metals. In another study, the focus is on the mineralogical transition from unweathered, pyrite-rich parent rocks to completely weathered and oxidized daughter material. Quantitative determination of the mineralogical transition from parent to daughter has identified the most important reactants and the nature and sites of reaction.

Photos (click image for full view, <350 k)

Below are selected photos from study areas.

Photo Description
North Alaska Photo Host rock lithology is important for controlling secondary dispersion signatures. Deposits in northern Alaska that have significant amounts of carbonate nearby have low metal mobility because carbonate buffers streams to near-neutral pH values. Alternatively, waters draining deposits hosted in black shale with no carbonate in the section are characterized by high metal contents.
North Alaska Photo - Lik Camp Studies are underway in northern Alaska to characterize the characteristics of massive sulfide deposits. Photo at left shows the Lik camp at the base of the mountain. The Lik deposit is a Zn-Pb-Ag massive sulfide deposit hosted in black shale. There are numerous other deposits that extend across the Brooks Range
Deposit Image Ongoing studies include in situ analyses of primary ore minerals using LA-ICP-MS method (Laser ablation inductively coupled mass spectrometry). Studies to date show that pyrite is the primary host to Tl, As, and Sb whereas sphalerite contains Ag and Hg. Correlations and patterns of results yield hints about how these trace elements reside in their host minerals.

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Mineral Resources Program
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