Foy.....thanks so much.....yea, not a day goes by that I don't think of home. When I had no home these past few years, there was no draw to anywhere special, but now my world has changed drastically. I seem to always have these battles within.....
One question for you Foy.....how in the world do people find these minerals ? How is it that someone looked at that cliff and said let's dig here ? All I see is different colors & sizes of rocks.....
Jerry, thanks for your interest. I realize you're not a big fan of mining and you have a lot of company on that subject. But having cut my professional teeth in the exploration side of the industry, my outlook differs from that of many. Personally, when I get around the West in a pickup made largely of steel, aluminum, and copper, with a lead-acid battery, and requiring diesel fuel or gasoline for power, I can't generate but so much animosity towards the mineral resources mining processes required to manufacture it. Can miners be better stewards--absolutely. Were 19th century miners particularly terrible stewards--absolutely. But I think to condemn all modern day mining and hydrocarbon production is unrealistic and unnecessary.
The question of "how in the world do people find these minerals" is at once very simple and highly complex. On the simple side, when we're talking about ore deposits of metallic minerals, to the greatest extent the base metals (copper, lead, and zinc) and precious metals (gold and silver), modern day exploration morphed from old style prospecting, which was in itself a search for "color" in outcrops of bedrock. And not just any bedrock. It was well known to 19th century prospectors that igneous and metamorphic rocks hosted the great majority of metallic ore deposits, so widespread areas where only sedimentary rocks were found typically received little attention. And reference to "color" is just that--the metallic ore minerals are mostly sulfides and oxides of the metal. Example: a common ore mineral of copper is chalcopyrite, or copper sulfide; lead's most common ore mineral is galena, or lead sulfide, and zinc's primary ore mineral is sphalerite, or zinc sulfide. As erosion of the rocks above the concentration of the ore minerals takes place, the orebodies are affected by groundwaters "as the surface comes down to meet them" so to speak. Exposure to percolating or circulating groundwater causes alteration (mostly oxidation) of the ore minerals and host rock into oxides of the ore minerals and host rocks. The oxides and other mineralogical children and grandchildren of the ore minerals are by and large bright in color, especially in the cases of copper alteration, which produces bright green and blue alteration minerals, termed secondary minerals. When the ore minerals themselves don't produce brightly colored secondary minerals, the fact that most sulfide deposits include high concentrations of iron sulfide (pyrite) and that pyrite chemically weathers to iron oxide (rust) at least leaves a distinctive patch of medium to light brown or light reddish brown behind. So, the old time prospectors could readily identify igneous and metamorphic rocks and would spend their time traipsing up and down canyons where bedrock outcrops were readily observable, and would literally find "color" in small to large patches on outcrop surfaces. In 1900, the vast copper deposits at Kennecott Alaska were discovered by a pair of prospectors working in a metamorphic rock area (a greenstone terrain--metamorphosed basalt) and noticed a huge area of bright green color high up on a canyon wall, well away from any grassy meadow areas. Prospectors also kept a weather eye peeled for outcrops of quartz occurring within the same igneous/metamorphic rock belts, as such veins would often be the result of fracturing of the bedrock creating pathways for hot metal rich fluids to emanate from a nearby igneous intrusion (such as a body of granitic rock) which provided both heat and mineralizing fluids which would enter the fractures and ultimately precipitate the metals in the form of sulfides.
Old time prospectors also recognized that some sedimentary rocks were worth a look, primarily carbonates called limestone and dolostone (or dolomite). There is a long identified affinity of ore mineral formation when a carbonate is intruded by hot metalliferous fluids such as arise from an intruding body of granitic rock. The chemical reactions are beyond the scope of this simplified summary, but suffice it to say that many of the world's great metal ore mineral deposits were formed along the contact zone of older limestones and younger intrusive igneous rocks. The Madison Limestone which we have had fun talking about as an aquifer has been a great host of orebodies when in contact with much younger intrusives throughout the West, and in particular in Montana. The stable continental shelf on which the Madison its lateral equivalents formed covered vast swaths of the West, so there are widespread areas in which the much younger granitic intrusions, generally emplaced during the so-called Laramide Orogeny, emerged from deep in the crust as a result of the subduction of Pacific oceanic crust beneath the lighter continental crust as plumes of molten rock. There are hundreds of small to large intrusions (termed stocks, plutons, and batholiths as the areal size increases) throughout the West. In Montana, the largest is the Boulder Batholith along the continental divide just east of Butte. The large outcrop areas of spheroidal weathered granites along I-90 at Homestake Pass and for several miles east of it are parts of the Boulder Batholith, and the highly mineralized area around Butte received vast quantities of metalliferous fluids emanating from the batholith. As widespread as the Madison was, it and other older and younger carbonates were there for the cooking by the hot fluids. With gold occurring in small quantities within sulfide orebodies, it was common for prospectors to find gold in stream gravels then keep working upstream to find the quartz veins or sulfide outcrops from which it had weathered out of. Long story short, the old prospectors would look for widespread areas of the Madison and other carbonates (which you at least now realize are pretty much everywhere) in contact with granitic rock and would prospect for color along the contacts.
Now to the present question about how the prospectors came to discover silver at the Toquerville mine area now within the Red Cliffs preserve. It turns out the secondary minerals formed from the chemical weathering of silver sulfide (argentite) are varying in color, often gray, and sometimes colorless, and the argentite is not always found with pyrite which weathers to such a distinctive rust color. But the soft secondary silver minerals can also be yellow and found with other yellow minerals such as certain ore minerals of uranium. So prospectors in the mid-late 1860s had their normal "colors" to search for, at least to a degree. The occurrence of silver minerals in sandstone is also well outside of the norm, where prospectors were geared to finding primary silver minerals and their secondary products in either igneous/metamorphic or contact zones between intrusives and sedimentary carbonates--not in sandstone at some distances from either carbonates or intrusives. Most researchers came to believe the primary silver minerals were formed by leaching of silver from volcanic ash beds within the sedimentary pile and redeposit within porous sandstone beds along the groundwater/surface interface. Long after their deposition, the sandstones at Silver Reef, UT were folded and faulted by tectonic activity and the more resistant beds were exposed by erosion to resemble ribs or reefs, hence the term Silver Reef. It is thought that either before or after the tectonic deformation that groundwater conditions within some identifiable horizons of certain sandstone beds were conducive to precipitation of the oxides and chlorides of silver formed from the leaching of silver sulfides from the volcanics. And in the case of Silver Reef, the "rush" took over a decade to begin inasmuch as nobody would believe the prospectors had found the silver minerals in sandstone. In a way, it was similar to the belated realization that much of the "soft blue-gray clay" which greatly hindered gold mining was discovered to be silver chloride in fantastically rich concentration over in Nevada, at a little place which became known soon thereafter as the Comstock Lode.
Today's prospecting, more professionally termed exploration, actually uses former producing areas as one of the major guides. The best places to find orebodies is a place at which orebodies are known to have occurred, right? But with most surface outcrops in likely terrains well explored over the last 150 years, today's explorationists can use a variety of geophysical testing to locate subsurface or blind bodies of conductive minerals (sulfides are great conductors of electricity), by magnetic expression (higher than background degrees of magnetic attraction from iron oxides like magnetite), and varying degrees of inflection of the Earth's gravity field (dense orebodies have measurably higher gravitational pull). So remote sensing employing ground instruments or instruments towed over the surface in helicopters or fixed wing aircraft can be followed up by geochemical sampling of soils and stream sediments and by breaking rocks at outcrops, the latter requiring modern day prospectors employed by mining companies spirited across the landscape in F250 pickups.
So that, in way too many words, is how it was done back in the day and how it is done in the 21st century.
Foy
