Deciphering complex fluid-mineral interactions in the palm of your hand
Barbara L Dutrow
Department of Geology & Geophysics
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This exercise uses polished rock slabs containing veins sealed by copper-bearing minerals to understand the feedback effects in complex fluid-rock systems. By determining the minerals and their relative timing, the changes in fluid composition can be extracted as well as the fracture episodes needed to reopen veins. These palm-sized samples encapsulate complex feedback relations in fluid-rock systems.
This lab or in-class exercise can be used at levels ranging from the required undergraduate mineralogy and petrology courses through optional graduate courses in geochemistry, economic geology, sustainable environments - mining, fluid flow, fluid-rock interactions or modeling complex systems. The increasing difficulty derives from modifying the interpretations students are required to complete and/or asking students to develop the appropriate diagrams.
Skills and concepts that students must have mastered
At the beginning level, students must be able to: (1) determine cross-cutting relationships and (2) identify (primarily by color) a number of copper-containing minerals. Ideally they should know what compositional and environmental factors are required to precipitate these minerals. With increasing difficulty, students should be able to use activity-activity diagrams, and interpret geologic history based on a feedback loop.
How the activity is situated in the course
Currently this exercise is used at the culmination of my sophomore mineralogy course. I have also used it in an advanced fluid-rock modeling course.
Content/concepts goals for this activity
Content goals for this exercise include: determining the likely sequence of geologic events that formed the rock (e.g. initial conditions, fracture, mineral filling, etc.), mineral identification and determination of fluid composition from minerals. Concepts incorporated include deciphering the feedback mechanisms that are recorded by these slabs and how the output of one process alters the input of the next process, the relative timing of events, and the repetition of geologic processes when forming a rock.
Higher order thinking skills goals for this activity
Higher order thinking skills involve moving from the 'real' world to an analog model with the concomitant complexity of trying to model this 'real' world system. Students must be able to use detailed observations to determine larger scale processes to formulate an hypothesis about how the rock formed. Typically, the ambiguity of the real system requires critical evaluation.
Other skills goals for this activity
Students work in groups to determine the sequence of events. This typically results in debates as to who is "correct." Students must write up their results and interpretations.
Description of the activity/assignment
Copper is an element that is essential to our technology and to our standard of living. Commonly, the copper is extracted from a variety of copper-bearing minerals that occur in veins. These fossilized fluid pathways record a complex set of geologic processes with non-linear couplings that are the products of hydrothermal activity associated with igneous intrusions (e.g. heat transport, mechanical fracture, mineral precipitation, permeability changes). By carefully examining a rock slab and its mineralogy, one can decipher the series of interrelated processes and their resultant impact on the final product.
Students set about to determine the relative age of veins by visual examination of the rock slab provided. Several generations of veins are recorded by different colors representing different minerals. Using cross-cutting relationships, they list the veins from oldest to youngest. Based on their color, they determine the sequence of minerals that fill veins. This provides an opportunity to review why color can be used to identify some minerals but not others. Once minerals are identified, their ideal chemical formula allows the percent copper in the mineral to be determined as well as the additional elements that must be present to form the mineral. The consequent change in mineral chemistry can be linked to the alterations in fluids flowing through the fractures by analysis of fluid-mineral equilibria on activity-activity (a-a) diagrams. For the more advanced classes, relevant thermodynamic data can be provided and students can write hydrolysis reactions and calculate the (a-a) diagram themselves.
Interpretation of the geologic history begins with the matrix and initial conditions and follows through rock fracture, fluid flow, mineral precipitation, evolving fluid composition, fracture sealing, pore-fluid pressure buildup, fracture, precipitation, etc. in a series of feedbacks. A feedback diagram can be provided and used as a base-map for interpretation not only of the sequence but changes to each reservoir, or students can be asked to draw the series of events and their reservoirs with the mechanisms of change. In the end, students understand the complex series of geologic processes that must come together in space and time to produce an ore-deposit that can be mined for our use. They also wrestle with the complications of reading the rock record and with the ambiguity of interpreting the interaction of various mechanisms that control the final product.
Determining whether students have met the goals
Students write a report which I grade.
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