Gross Reservoir

As the spring months roll around, there comes about this insatiable desire to get outside. After being bundled up all winter and working hard to finish my final exams, it was a great time to go and enjoy the warm weather.

I chose to go and spend a night on the shores of Gross Reservoir, it was an easy mile hike in, and I was hoping to gain access to some potential gold deposits in the gravels of South Boulder Creek, which have proven themselves historically and in my own pan.

The reservoir, approximately sixty feet below its full level, was amazingly desolate. Winds would blow the sand and mud from the exposed sides, whipping them into dirt devils and creating characteristic carvings on the sides of the banks. The bottom, covered with what would be a thick, oozing mud was dessicated into dramatic flats of mud cracks.

The river here was very interesting, because it flowed through a large basin of wind-blown sand with little to alter its flow. This last semester, in my Sedimentology and Stratrigraphy class, we learned how to identify the mechanics and formations of these river deposits. They were most certainly present and identifiable, I picked out parallel cross-laminations, some subaqueous dunes, and longitudinal point bars, all characteristic of a sandy braided river system.

Crayfish skeletons were abundant on the shores. As I stepped into the river to sample a long gravelly bar for gold, I almost crushed this very picture-compliant guy. 

I spent some time the next day sampling the area where the river meets the reservoir for gold. Most surface bars held some fine dust, and usually a decent amount of it. I didn't find anything that I could even call a flake, so i've concluded that the size of the gold depends on the size of the flood moving the sediments in. Finding a deposit from a large flood could contain some coarser gold.

Abandoned fluorite mine of Jamestown, CO

The Jamestown and Ward region, located up Lefthand Canyon endeared years of mining success during its booms, and like all other mining regions, misfortune and busts. The region produced gold, silver, lead, copper, and fluorite (then called fluorospar). Low grade fluorite is used as a flux in the melting of iron to increase fluidity, and so the region boomed during both world wars. Now, only a few year time residents and summer cabins remain in this area, but the aftermath of the mining is evident. Holes are dug into the side of the hills everywhere, and the yellow dirt can be seen pouring out onto the hills in many areas. These yellow rocks are sulfide minerals, which are frequently associated with veins of precious metals that have been deposited hydrothermally. These sulfides, when exposed to water with higher surface area react to form hydrosulfuric acid.

Chemically,

 2H20 + S ---- 2H2S +02

The additional oxygen molecule often proceeds to react to form carbon dioxide. The water that runs off  is then considered acid mine drainage. This region has a huge problem with this, and there are some mines designated as superfund sites that need to be cleaned up. One attempt to remedy this has been to put bags of limestone below the tailings piles, so that it will hopefully absorb the acid before it reaches the stream.

My mineralogy teacher, Joe Smyth, took us on a field trip up left hand canyon, and showed us some pegmatites, high grade metamorphic rocks, and then this fluorite mine. The few minutes we spent there wasn't enough for me, I have been back a few times exploring the area. The fluorite there is a pretty purple,  but doesn't occur in very solid crystals. The mine was originally a gold mine, but later was further developed for its fluorite.

The main pit. Fractures and fisures have been stained black by manganese bearing waters.

The pit was dug directly into the hillside, and the walls are about 60 to 80 feet tall. An old headframe sits on what used to be the side of the hill, and above the tailings pile. The tailings are a fine yellow dirt, that erodes easily into steep slopes, exposing the iron and wood fragments that make it so uninviting. The interior of the pit is also filled with this fine tan-yellow dirt, but no tetanus shot required.

Looking down at the side shaft. Pick axe for scale, ~3.5 ft long.

A small hole dug down connects through to the main pit. Its really quite tempting to climb down through it, but this is probably ill-advised with the unstable condition of the rocks. This shaft drops about fifteen feet before connecting horizontally with the main pit, still about forty or fifty feet above the bottom of the pit.

Quartz crystals in matrix.

                                                                                                                                                            










Some purple fluorite can be found in the walls and floor of the pit. It is quite rotted and is often found as a purple powder. Crystals here are rare, but I have found weathered cubes in some of the bigger boulders on the floor of the mine. Fluorite has a cubic crystal habit, and can form octahedrons, cubes, and plenty of like variations. On this trip, I wasn't looking for fluorite. I visited some outcrops above the mine and found some quartz crystals exposed from weathering on some fissures. They are a grey color, and embedded in matrix. I managed to chip away a crystal or two.

Two small, loose crystals.