Abstract The western Upper Peninsula of Michigan is well known for hosting significant concentrations of copper in copper-dominated deposits. Most of the copper is hosted by rocks of the Mesoproterozoic Midcontinent Rift. Copper deposits in the western Upper Peninsula can be subdivided into two overlapping world-class copper mining districts. The Keweenaw Peninsula native copper district produced 11 billion lbs of copper and a lesser unknown but significant quantity of silver. Native copper deposits in this district are stratiform and hosted by tops of rift-filling subaerial basaltic lava flows and interflow coarse clastic sedimentary rocks. These deposits are interpreted to be the result of mineralizing hydrothermal fluids derived from rift-filling basaltic volcanic rocks that migrated upwards, driven by late Grenvillian compression of the rift some 40–50 million years following cessation of active rifting. The Porcupine Mountains sediment-hosted copper district produced or potentially will produce 5.5 billion lbs of copper and 54 million ounces of silver. These stratiform/stratabound deposits are hosted in rift-related black to gray shale and siltstone and dominated by chalcocite rather than native copper. Chalcocite is interpreted to be the result of introduction of copper-bearing fluids during diagenesis and lithification of host sediments. At the now-closed White Pine Mine, the chalcocite mineralizing event was followed by a second stage of native copper deposition that demonstrates a spatial and temporal overlap of these two world-class mining districts. While these two districts have been dormant since 1996, favorable results from recent exploration at Copper-wood suggest a revival of the mining of copper-dominated deposits in the western Upper Peninsula of Michigan.

The crater lake at Santa Ana Volcano (El Salvador) was monitored during 1992–1993 and 2002–2007. Crater lake chemistry was generally similar until the 2005 eruption. Acidification of the hydrothermal system by condensing magmatic gases yielded fluids that sustained a cool acid sulfate-chloride lake roughly 200 m in diameter (temperature = 16–28 °C, pH = 0.7–2.0, SO 4 2− = 4500–14,000 mg/L, Cl − = 1100–9200 mg/L, total dissolved solids [TDS] = 7000–25,000 mg/L). The phreatomagmatic eruption Volcanic Explosivity Index (VEI) 3 of October 2005 modified the summit crater morphology, leading to physical, thermal, and chemical changes in the lake over the next few years. The lake became hotter and more acidic, with variable chemistry and color (temperature = 24–66 °C, pH = 0.4–1.3, SO 4 2− = 2500–9800 mg/L, Cl − = 3200–22,000 mg/L, TDS = 10,000–36,000 mg/L, turquoise-gray-yellow color). The SO 4 2− /Cl − ratio dropped below 1, indicating an increase in the rate of volcanic gas input and coincident S depletion by abundant precipitation of native sulfur and secondary S-bearing minerals (alunite, gypsum, iron sulfides, and anhydrite). An increase in rare earth element (REE) concentrations in lake waters indicated leaching of the newly intruded magma. The eruption likely enhanced permeability in the edifice, further increasing the amount of available fresh wall rock to react with acidic fluids, and the concentration of rock-forming elements in the lake increased fivefold to a maximum of 90 g rock dissolved per kg water. The magma continued to degas through the lake bottom at the drowned eruptive vent, providing a large, direct gas input into the lake. Direct gas discharge into the lake led to sulfur saturation and formation of hollow sulfur spherules by percolation of gas bubbles through the molten sulfur bottom layer. Increased heat input into the lake (8–830 MW, equivalent SO 2 flux of 16–1600 t/d) led to enhanced evaporation and highly variable lake mass. Consequently, on three occasions during 2006 and 2007, the lake area diminished to 70% of its former size.