Finding a mine is a science, and like all good science the search begins in the library. Mineral deposits usually are found in a group of similar deposits; rarely alone. Often they are also found in a particular rock type. If the library doesn't tell you where to look, it will usually tell you where not to look.
The old fashioned way of finding a mine was your prospector with a donkey, pick and shovel, a gold pan. and a lot of luck. This image prevails with the public to this day, and it's entirely false. Today's prospectors are armed with a plethora of hi-tech devices that the old timers could only dream about. These devices are deployed when all the library research is ended, and it's time to take to the field.
Some of the things you are apt to find in the library are several different highly useful; maps. The very first map you will need is a topo-map with one of these you will at least know where you are going. Most states in the US have done considerable amounts of map making. The bedrock maps and the surfical maps are the most useful.
Not all of the useful material is found in a library even more can be found in the local historical societies. This is especially useful to know what kind of rocks and minerals were found in the past. Its true that 150 years ago the head of a local government knew more about the mineral resources found in his town then the present government head.
Before you even take to the field you should prepare a plan built around what you have learned in the library, and other sources. The fieldwork itself is almost an anticlimax to the preparation that you have already undertaken. There is no substitute to being prepared.
There are certain indicator's of a mineral deposit the most obvious is a "gossen" which us a heavy iron staining on the exposed surface rock. This is the result of weathering of mineral deposits caused by iron sulfides that may appear in conjunction with more valuable minerals gold being one of them. Any heavy staining on surface rocks is well worth having a serious look at since there are other minerals beside iron sulfide that can leave a stain. These are the first targets that you will examine in your study area,
More often then not mineral deposits are buried out-of-sight to the casual observer, but by some hi-tech methods available to today's prospector it is possible to find these hidden treasures. Probably the commonest field method available is the geochemical anomaly map. All of the elements are present in all the soils and rocks at specified amounts. Whenever you find quantities of a specific element present that are higher then the norm it is well worth your time to see if you can find the source by geochemical means. Several important mines have been discovered by this means.
A mineral deposit always has indicator minerals that are associated with the deposit. A good example of this is finding diamond bearing kimberlites. Down grade from the actual deposit is a dispersion train of these minerals. In the case of kimberlites they are chrome diopside, a chrome rich pyrope garnet, and if the kimberlite is diamondiferous there are diamond crystals found in the dispersion train. This is also true of many mineral deposits. The dispersion train contains fragments of the desired minerals that are always down grade of the deposit. Sometimes these dispersion trains are quite short, other times they can be kilometers long. If you discover a dispersion train it always pays to trace it to its source. Many valuable mines have been found this way.
It is possible to buy kits for doing this from chemistry supply houses, or you can ship bulk samples off to a lab equipped for the analysis of minerals. In the field Zinc and Silver are the easiest minerals to trace so it's a good idea to at least have these kits available. There are kits on the market for virtually any element you looking for.
There are a whole plethora of geophysical methods used in prospecting, but these methods only come into play once the initial discovery has been made. These methods are used to further develop information about your initial find. The proof of the pudding however is when you start to pull diamond drill cores of the deposit. This is where you actually locate minerals you can go to the bank with. The diamond core drill also determines whether or not you have a viable mine, or just another "Duster."