8/10/2019 Gemstone - Exploration Geology Poster

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GENESIS

GEMSTONE

Magma Crystalizationagma contains a variety of elements. As it cools, the elements combine to form minerals. Exactlyhat mineral is created varies with the available ingredients, temperature, and pressure. Each timene mineral forms, the available ingredients change. Different minerals form as it goes through thearious stages of changing temperature, pressure, and chemistry. Diamonds crystallize atmperatures higher than other minerals. Scientists now believe that they may form in the magma,

ear the earth's crust where it is the coolest. If this is true, it also means that conditions for diamond

ystallization are the most common in the earth.

Hydrothermals the name implies, hydrothermal involves water and heat. As water percolates through the earth,dissolves minerals, just as it did with the sugar in our rock candy. Deep inside the earth, it meetsth magma. Special fluids then escape from the magma that contain water, carbon dioxide and

olatiles (substances that give off gas). These hydrothermal fluids move through fractures in theust. Along the way, they may dissolve minerals or combine with other ground water. Mineral rich,ey begin to cool in veins. If combined with the right combination of temperature, pressure, time,

nd space, crystals form.

Environmental Changesreat stresses exist inside the earth. Under the right conditions, the temperature and pressure canse to the point where existing minerals are no longer stable. Under these conditions, minerals canange into different species without melting. This is known as metamorphism.

here are two types of metamorphism:

ontact metamorphism : occurs when magma forces its way into an existing rock formation.

nder the intense heat, existing rocks begin to melt and eventually recrystallize as new speciesat are stable at higher temperatures.

egional metamorphism : Enormous compression forces exist where land masses comegether, creating an area of intense heat and pressure. As the temperature approaches theelting point of rock, the minerals become unstable. Over time (possibly millions of years) theyange into new varieties.

METHODS The most widely-used gemstone exploration techniques today areground-penetrating radar, known as GPR; trace-element analysis, whichinvolves seeking signature elements as clues to where gems may lie; anduse of a device called a "terra thumper," which identifies differences in thestructure of the host rock through seismic analysis.

GPR has proven useful in providing subsurface mapping of potential

gem-bearing pockets, or "vugs," but the readings can be confused bymoisture in the ground, and they can't separate gem-bearing pockets fromthe non-gem-bearing ones.

Experts disagree on whether using devices like GPR can ever be cost-effective in exploration for gemstones, which normally occur in small lodesand do not fetch as much as diamonds or gold in the marketplace. Despiteresults like these, some believe that GPR is the wave of the future forgemstone exploration. The main drawback with GPR is the penetrationdepth.

GPR only works on ground level and is not applicable to airborneexploration. Making GPR airborne-friendly involves some complicatedphysics which presently lead to diminishing returns.

The use of GPR is not meant to replace trenching or drilling. but merelyto find the most likely places to dig About 99 percent of GPR usageworldwide is for civil engineering applications such as the locating ofburied pipes and other structures. Whether GPR manufacturers will worktoward improving mining applications will depend on the extent to whichminers embrace the technology.

Another popular technique for prospecting is trace element analysis ofareas suspected of containing gemstones. An extensive study has beenmade of an alluvial sapphire deposit in Montana by researchers from theUniversity of Toronto.

STUDY CASE

ccessful Application of Ground Penetrating Radar in the Exploration ofm Tourmaline Pegmatites of Southern Californiafrey E. Patterson and Frederick A. Cook

plication of ground penetrating radar has been successful in delineating gem-aring zones in the Himalaya pegmatite mine of the Mesa Grande district ofthern California. The high frequency of the electromagnetic signal allowstures as small as a few cm to be resolved within 1-2 meters of the surface of ae wall. Careful initial setup consisted of:

selection of antennas with sufficiently high central frequencies,ecording with a short time of scan to reduce end of scan noise levels, and

choosing appropriate color schemes to highlight extreme amplitude variations.

eration during data collection insisted of pre-painting marking points on the minee and air launching the signal to reduce false anomalies caused by rocking ofantenna on the rough surfaces. Data processing using the Hilbert Transformvided images of the cavity geometry that was then used by the blasting captainaccurate placement of explosives.

ta Acquisitionround penetrating radar, an electromagnetic pulse of several nanosecondsation (1 ns = 1 x 10-9 s) is transmitted via a tuned antenna into the subsurface

ere it may respond from rocks with contrasting electrical properties. Returnednals are sensed by a receiving antenna and recorded digitally in a mannerilar to seismic prospecting. This allows the signals to be displayed, processed

d enhanced using many standard signal-processing techniques.

Data ProcessingData processing of radar data can be similar to processing for seismic reflection data asthe returned signal provides a time series for each trace. Because these data wererecorded in continuous mode along a linear profile, we found that the most effective

processes were those that assisted in characterizing anomalies according to frequency,phase and amplitude. A series of deconvolution and migration tests (not shown here)provided little improvement in the data, and in most cases degraded them; hence, the datawere processed using complex trace analysis (Taner et al., 1979) that providesinformation about local variations in frequency, phase and amplitude and that can thus bevaluable in tracking characteristics of anomalies.

Conclusionhe development of high resolution geophysical methods, particularlyround penetrating radar, to produce detailed images of subsurfacetructures has led to the ability to identify gem tourmaline-bearing zones

within the Himalaya sheeted pegmatite in southern California. Whenppropriately recorded, processed and calibrated with geological

nformation, resolution of features (pockets and vugs) as small as a fewm within approximately 2 m, or 10's of cm within about 5 m of a wallurface may be obtained. This approach thus provides an opportunity tond gem-bearing zones that may not exhibit "typical" indicators and/or that

may be in areas that are no longer actively mined.

Referenceshttp://pegmatite.com/Gem-Star-GPR.pdf

http://www.gemsociety.org/article/gem-formation/