The purpose of this excerpt is to explain how the common types of mineral deposits form and describe the types and Availability of minerals that may be of interest to the recreational collector, Particular emphasis is on gold and gem minerals. Gold, in both lode and placer sources, is abundant in Idaho. Numerous gemstones such as sapphire, topaz, garnet, zircon, opal, jasper, aquamarine and man others have been found in both lode and placer deposits.
Mining began in Idaho about 1852 and continued on a small scale using mostly hand methods until the 1880s. Later, development of the lead-silver lodes began on a large scale. The initiation of large-scale placer mining using hydraulic mining and dredges resulted in a huge increase of gold production. Placer mines outnumbered lode mines until World War 11 put a temporary stop to gold mining.
A hydrothermal deposit is one precipitated from a high temperature solution. As hot water with minerals in solution rises towards the earth's surface, the lower temperature and pressure near the surface cause the minerals to precipitate out of solution. Molten rock or magma below the surface supplies the hot fluids which travel upwards along the pressure gradient. Magma is 3 to 8 percent water by weight and lavas contain about 4 percent water.
of Rocks for Mineralization
Several processes must affect the rock in order to make it more receptive to mineralization. The rock must become more permeable and brittle. Rocks are hardened by silica, then shattered by faulting so as to increase permeability. Broken silica causes clean fractures with little or no powder so that fluids may move easily through the rock. Typically rocks with a high porosity such as sandstones and conglomerates also have a high permeability. Shales, on the other hand, have a high porosity but a low permeability. Consequently, shale beds may confine and trap a mineral deposit rather than allow it to pass through. joints and contraction cracks in igneous rocks make excellent channelways for fluids. Vesicular layers and interbeds between lava flows also provide very good permeability.
veins and faults
Faults are fractures along which displacement has occurred. A shear zone is a highly-fractured zone with closely-spaced, subparallel fault planes. It is normally a very permeable zone; however, the presence of clay zones called gouge (finely-ground rock) greatly reduces permeability. The greater the displacement, the more gouge forms. Therefore, small faults with slight displacement are the most favorable locations for ore deposits. Brittle quartzites make either clean breaks or shattered zones, whereas shales and many igneous rocks make tight fractures with much gouge, so they have a low permeability. Faults formed near the surface are generally more open and consequently have higher permeability. Thrust faults are caused by compression and typically have a fault plane that dips 30 degrees to horizontal. Thrust faults have tight fractures containing much gouge, low permeability and are poor for mineralization. Gravity or normal faults are caused by extension; they tend to be open, permeable and excellent for mineralization. The fault planes of normal faults tend to dip 40 to 70 degrees. Mineralized faults generally occur where more than one fault is involved. Typical configurations include (1) subparallel groups of faults, (2) one fault intersected by another fault (the zone of intersection is very commonly mineralized), (3) faults that branch like the limbs of a tree, and (4) a zone of intersecting faults called stockworks. Stockworks generally have a cylindrical or pipelike shape and are caused by shattering of igneous rocks.
As hot fluids are discharged from magma, they circulate through huge volumes of shattered rock dissolving a variety of minerals. After taking minerals in solution at high temperatures and pressures, the fluids move towards the surface along permeable channels such as fracture zones. When the temperature and pressure drops sufficiently, minerals will begin to precipitate along the walls of the fractures.
of Hydrothermal Deposits
A widely used classification of hydrothermal mineral deposits is based on the temperature of formation:
Hypothermal: 300' to 500'
C (deep deposits)
Mesothermal: 200' to 300' C (medium deposits)
Epithermal: 0 to 200' C (shallow deposits)
Epithermal deposits are an important source of lode gold deposits in Idaho, They are formed at less than 3000 feet from the surface and at low temperatures ranging between 50 to 200 degrees centigrade. Mineralization occurs by open-space filling with such textures as drusy (crystal lined) cavities, symmetrical banding and comb structures. The fissures may open at the surface as hot springs. Epithermal veins are typically related to 'Tertiary plutons and volcanism.
of Lode Gold Deposits
Lode gold deposits are formed by hydrothermal solutions precipitating such minerals as quartz, barite, carbonate minerals, flourite, gold, gold tellurides and silver. Many of these deposits have yielded much more silver than gold.
Host rocks are typically found in altered volcanic rocks of Tertiary age, and to a much lesser extent they occur in granitic rocks of Late Cretaceous to Early Tertiary age.
The epithermal gold-quartz lodes have been referred to as "bonanza" lodes because they tend to be much richer than the other types of lodes. Although the ore grade commonly ranges to one ounce of gold per ton, ore can carry up to 20 ounces of gold per ton.
Veins and Gossans
Only a small percentage of vein quartz will contain gold. "Bull quartz" is a term for a glassy quart that is generally barren of gold. Gold below the oxide zone is generally associated with sulfides. Sulfide gold includes pyrite, chalcopyrite, arsenopyrite and galena- however gold may also exist in a free state be low the oxide zone. Iron streaks and vugs lined with rusty crystals in quartz veins are promising for got If gold is present in such veins, it may be possible t see it with a hand lens or the naked eye. Commonly gold is the only valuable mineral left in a gossan. Gossan (iron hat) is a porous, rusty capping on a sulfide deposit. Any outcrop or float of iron-stained light-colored igneous rock, fractured and recemented with silica, should be carefully examined.
Gold Value with Depth
Gold is an inert, insoluable mineral and is not susceptible to teaching. As a result, the gold content o the rock may decrease with depth. Another depth problem occurs if the gold is contained in sulfides. In such a case, free gold is only available in the zone of weathering above the water table.
Major linear features may be used to find mineral deposits. Linear features are topographic features such as ridges and canyons that follow a straight line and are probably the surface expression of a fault. Satellite imagery and high altitude aerial photography are useful for this purpose. Mineral deposits tend to be aligned along linear features. The intersection of linear features are an excellent place to prospect. Lineaments may represent deep fractures which could provide access to ore fluids.
Formed from Secondary Enrichment
Some mineralized vein deposits are enriched at or below the water table by a process called supergene enrichment. Surface water moving along the fractures above the water table in the zone of oxidation dissolves minerals and carries them in solution down to the water table. At the water table, secondary minerals are deposited which are generally much richer than primary minerals originally deposited in veins. For example, a typical primary sulfide mineral is calcopyrite with 34.5 percent copper. If this mineral is taken into solution and carried down to the water table, the copper may again be deposited in the form of bornite (63 percent copper), covellite (66 percent copper) or chalcocite (80 percent copper). The following minerals are commonly found in gossans or oxidized upper portions of veins:
Iron minerals - rusty brown,
Copper minerals - blue, green
Nickel ores - pale green
Cobalt - pink, red color
Molybdenum - pale yellow
Manganese - sooty black
Uranium - bright orange, yellow, green
After intrusion, a magma gives off heat and fluids. These hot fluids migrate upwards towards low temperature and pressure. New minerals and textures form along the contact of the pluton and the country rock. Minerals grow larger and grain size increases. If the country rock is a limestone, it is recrystallized into a marble. The intruded magma supplies valuable metals and silica. Silica precipitates in the pores of sedimentary rock as a quartz cement. Silica also reacts with chemicals in the country rock to form silicate minerals. Hot solutions leach out portions of the country rock and in its place silica and other minerals are deposited. Deposition occurs in permeable beds along bedding planes, cavities and fractures. Metals are very mobile and tend to be driven out of the magma and localized in the roof of the magma chamber.
Skarn minerals are formed at the contact between a granitic pluton and a carbonate-rich rock such as a limestone. Skarn minerals desirable for collecting include garnet, mica, corundum, quartz, diopside, tremolite, spinel, epidote, wollastonite, flourite, tourmaline and topaz.
Pegmatites are very coarse-grained igneous or metamorphic rocks. Igneous pegmatites form from residual volatile-rich fractions of the magma whereas; metamorphic pegmatites are formed by mobile constituents that concentrate during metamorphic differentiation. Pegmatites have a tubular or dikelike shape or may be lensoid masses. They are generally small with a thickness from several feet to more than 100 feet and may have a length measured in tens or hundreds of feet.
Most pegmatites in Idaho have a silicic to intermediate composition- however, some mafic pegmatites are known. Pegmatites are generally found in and near the roofs of large plutons. Most pegmatites in Idaho have a very simple mineralogy. Typical minerals include quartz, orthoclase feldspar and mica. Small red garnets and black tourmaline are also common as small disseminated crystals. Many valuable economic minerals as well as crystal specimens are recovered from pegmatites. These minerals include quartz, feldspar, micas, chalcopyrite, molybdenite, sphalerite, beryl, apatite, tourmaline, monazite, topaz, garnet, spodumene, cassiterite and lepidolite. Rare earth minerals found in pegmatites include tantalum, niobium, beryllium, lithium, cesium, uranium, cerium and thorium. Most pegmatites are characterized by a crude zoning. This happens because a pegmatite crystallizes somewhat like a geode, from the outside towards the center. Pegmatites typically have a quartz core because quartz is generally one of the last minerals to crystallize.
Some pegmatites have a gas cavity at the center of the pegmatite. These cavities range from several inches to more than a foot in length and often contain large crystals with fully developed crystal faces. Gem minerals such as amazonite (green microcline), topaz, beryl (aquamarine in Idaho plutons) and smoky quartz are common in Idaho pegmatites.
One of the best ways to find pegmatites with pockets or cavities in which crystal specimens may be found is to carefully examine the float. Float is a term used to describe fragments of the pegmatite deposit that might be detached and moved downslope. Look for large pieces of quartz with attached crystals of amazonite and tourmaline. Also large pieces of feldspar and mica indicate a pegmatite. Crystals with faces are especially diagnostic because they indicate a pocket exists in a pegmatite where other crystals may be found. Pegmatites form low areas because they tend to weather relatively quickly, as a result, vegetation may thrive over pegmatites. However, the quartz core is more resistant that the surrounding minerals and will stand out in high relief. This quartz may be rose, gray, smoky or amethyst capped. Pegmatites generally do not occur as a single dike but rather as a group of dikes. So if you find one, there 'II most likely be more within 50 to 100 feet.
Books of white mica called muscovite are very common in pegmatites. These books tend to increase in size towards the centerline of the pegmatite body. Pink, lithium-rich mica is called lepidolite. Orthoclase feldspar is very commonly found as large fleshcolored crystals. Beryl crystals are generally found imbedded in quartz; they tend to range from pale green to blue in color. Tourmaline occurs as long, black rod-like crystals which generally point towards the center. Translucent white quartz typically forms in the core of pegmatites.