Crustal Geophysics and Geochemistry Science Center

Modeling Near-Surface Processes in Mineral Systems

Task 3: Source to Secondary Characterization


Task Summary

One of the most important environmental effects of mineral deposits is the migration of potentially toxic elements in surface and ground water. This migration can occur in natural, undisturbed deposits, as well as mined ones. The two principal objectives of this task were to develop reliable ways of discerning natural from anthropogenic inputs, and to understand the migration of chemical elements away from mineralized centers. To accomplish the first, we studied both mined and unmined deposits in a variety of geologic, hydrologic, and climatic settings in the US and abroad. To accomplish the second objective, we conducted integrated studies of geologic, hydrologic and geochemical processes in areas that included either mined or unmined mineral deposits.

Investigators under this task looked at three deposit types: porphyries, massive sulfides, and intrusion-related gold deposits. We built on our earlier results by sampling in new areas to add to our database and expanded the scope of our geochemical work to include isotope studies of mineral deposits and waters draining from them.

The primary objectives of this task was to (1) characterize the secondary weathering products associated with mineral deposits in different climates (i.e., extent and mineralogy of alteration associated with a given deposit type in different climates, and how alteration types differ between deposit types); (2) understand the speciation or chemical form of an element in soils and sediments in and around deposits of different types; and (3) characterize trace-element mobility in ground and surface waters (metal transport and fate).

The first objective was important because secondary weathering products represent potential sources or sinks for elements of concern. Each of these sources and sinks has a different affinity for various elements, with varying degrees of transience. Understanding the crystalline and amorphous solids present in weathering zones is critical to modeling the mobility of elements in the natural systems surrounding mineral deposits. The second objective was important because the mineralogic residences of trace elements in the rocks and sediments surrounding a mineral deposit exert a fundamental control on the availability of the elements to dissolution or other mobilizing processes. Depending on the nature of traceelement binding to secondary minerals, the trace elements might be dissolved in ground or surface water, desorbed from surfaces, removed by ion exchange, transported on suspended mineral grains or colloids, etc. Therefore, understanding the trace-element binding to soils and sediments yield important clues to the types of processes by which the elements move through the natural system. The third objective, to understand the transport processes, is also important because the mechanisms of transport will have a great effect on the possible spatial and temporal extent of transport, the possibilities for attenuation downstream, bioavailability of elements and effects on flora and fauna, etc.


Task Highlights

Red Mountain Study Photos

The unmined Red Mountain (Dry Creek) volcanogenic massive sulfide deposit in the Bonnifield district of Alaska is being investigated as part of Task 3. Photographs of the study area include brief descriptions of findings at Red Mountain.

Photo (click on image for full view, <350k) Description
Red Mtn Picture Field studies are ongoing at the Red Mountain (Dry Creek) volcanogenic massive sulfide deposit in the Bonnifield district, Alaska Range, south of Fairbanks, AK.
Red Mtn Picture Red Mountain was actively explored for Pb-Zn-Ag through the 1990s; exploration ceased in 1999. The site represents a spectacular laboratory for observing natural acid-rock drainage resulting in a naturally degraded ecosystem.
Red Mtn Spring Picture Acidic springs and surface waters are common in quartz-sericite-pyrite altered zones at Red Mountain . This spring, which flows about 8 gallons per minute on the west side of Red Mountain has a pH 2.7 and conductivity 2920 μS/cm. Filtered/acidified water from the site contains high concentrations of dissolved constituents (combined Al+Ca+ Fe+Mg = 590 mg/L; SO4 = 2,900 mg/L; combined As +Cd+Co+Cu+Ni+Pb = 7,100 μg/L; Mn = 14,000 μg/L; Zn = 120,000 μg/L; combined REE = 18,000 μg/L).
Red Mtn Stream Picture Salts and precipitates are common at Red Mountain. The bright orange-tan precipitate is dominantly the metastable Fe-sulfate mineral schwertmannite. Salts identified include halotrichite, pickeringite, kalinite, hexahydrite, melanterite, and alunogen. The black liverwort along the stream’s edge seems to thrive in the acidic waters and is under study by a botanist on the project.
Red Mtn Picture Pyrite and its alteration products are widespread in the quartz-sericite-pyrite zone. In partnership with Task 4 of the Tintina Metallogenic Province project (see http://minerals.usgs.gov/east/tintina/), Near Surface Modeling project studies underway at Red Mountain include aspects of ore genesis, environmental geochemistry, mineralogy, botany, and remote sensing.
Aerial Red Mtn Picture Results from the study at Red Mountain will provide a solid, unbiased pre-mining geochemical baseline should the deposit ever be developed, and these data will be useful in geologic/geochemical modeling of both mined and unmined VMS deposits. Further, these results will demonstrate that extreme acid and metals concentrations can be found locally in natural, unmined environments.

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Preliminary comparison of the environmental signatures of porphyry deposits in the western U.S. and Alaska

At present, the economically most important sources of copper and molybdenum are found in porphyritic igneous intrusions, known as porphyry copper-molybdenum (Mo/Cu ≤0.1), porphyry molybdenum-copper (Mo/Cu 1–0.1), and porphyry molybdenum deposits (Mo/Cu >1). The intrusive rocks that host these porphyry deposits, as well as the surrounding alteration zones, have a geochemical weathering signature that is often distinct from that of the original country rocks. The table below summarizes the deposits we have sampled to date. Download short paper prepared on this topic [PDF file, 116 KB].

Table 1- deposits sampled for this study. Letters in brackets {} refer to locations in figure 1
Deposit name Location Characteristics
Redwell Basin {R} Western Colorado Climax-type porphyry Mo deposit- associated with high-silica rhyolite intrusion [2].
Mount Moly {MM} Southwest Colorado Porphyry Mo -Cu- associated with granodiorite and quartz monzonite [3].
Bond Creek {AK} Southern Alaska Porphyry Cu-Mo- associated with quartz-diorite to granodioritic rocks [4].
Orange Hill {AK} Southern Alaska Similar to Bond Creek [4].
Buckingham {B} Northern Nevada Porphyry Mo -Cu- associated with granodiorite and quartz monzonite.
Humboldt Canyon {H} Southern Arizona Porphyry Cu-Mo- associated with quartz-diorite to granodiorite.
Margaret {M} Southwest Washington Porphyry Cu-Mo- associated with quartz-diorite to granodiorite.
Cannivan Gulch {C} Southwest Montana Porphyry Mo -Cu- associated with granodiorite and granite.

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Investigating links between geologic structures and hydrologic processes

Our ongoing studies in the Redwell basin on Mount Emmons in SW Colorado have been directed at understanding the influence of geologic structures (faults, fractures, etc.) on local and regional hydrologic processes. We have used a variety of geochemical and hydrologic tools to show that major geologic structures probably are also major hydrologic features of the system.

Photo (click on image for full view, <350k) Description
Redwell Map Redwell Creek drains to the north on the north flank of Mount Emmons, near Crested Butte, Colorado. The fractures shown on the map are part of larger structural system and in particular, our geologic model predicts that the N - S trending fractures should be hydraulically conductive. Points labeled A-E on the map represent geochemical points of interest described in this paper [PDF file, 296 KB].
Redwell Picture The Red Well is an ancient (>2800 yrs.) natural ferricrete-depositing spring. It represents a major point source of some metals and acidity to Redwell Creek. The width of the pool is about 3-4 meters.
Daisy Mine Picture The Daisy mine is one of the main anthropogenic inputs of metals and acidity to Redwell Creek.
Redwell Basin Photo This photo is looking upstream at the headwaters of Redwell Creek. The white precipitate at the left side of the photo is caused by inputs of acidic, aluminum-rich waters from a prospect pit. The Red Well is in the right-center of the photo.
Standard Mine Picture The Standard Mine is on the SW side of Mount Emmons, about 3 km from the Daisy Mine. We are currently investigating whether a known geologic structural connection between Standard and Daisy might also be a hydrologic connection.

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Mineral Resources Program
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