The views from the Mountain Village Ski Area just outside Telluride are beautiful especially when framed by aspens covered in their fall glory and crowned by brooding storm clouds.
Of course, it snowed on us as we were listening to an explanation of the hydrothermal alteration of these mountains and the emplacement of metals including gold and silver. The snow showers were brief and shafts of sunlight kept playing over the landscape and clouds making it difficult to pay full attention to the lecture.
I remember the main points, however, which were that there is a stratification to the mineral emplacement with gold being located highest in the hydrothermal system and in the ore bodies i.e. the gold mines are usually up high in the mountains.
In addition, the hydrothermal alteration and metal emplacement seems to be more recent than the main igneous pulses. In fact, dates as young as 5 mya are being found for these deposits. This collaborates the theory that a mantle plume is active under the San Juan mountains which are still being influenced by the plumes activity.
If you look at the mountains in this photograph carefully, you will see many horizontal lines which are the bedding planes of the San Juan Formation. The San Juan Formation was deposited during the huge caldera eruptions from the Mid-Tertiary Ignimbrite flare-up. Layers of ash and tuff are inter-bedded with pyroclastic and lava flows in a semi horizontal arrangement.
To give you some idea of how big these eruptions were, let us compare them to the Mount St. Helens eruption. Mt. St Helens produced about 1 cubic kilometer of ash and tuff in the 1980 eruption. Just one of the SW Colorado eruptions produced 5,000 cubic kilometers of ash, tuff, and lava.
Yellowstone makes all of the headlines right now but the Mid-Tertiary flare-up in SW Colorado was just as big and perhaps a little bigger.