Gabbro - A dark, coarse-grained igneous rock. Galena - Lead sulphide, the most common ore mineral of lead. Gamma - A unit of measurement of magnetic intensity. Gangue - The worthless minerals in an ore deposit. Geiger counter - An instrument used to measure the radioactivity that emanates from certain minerals by means of a Geiger-Mueller tube. Geochemistry - The study of the chemical properties of rocks. Geology - The science concerned with the study of the rocks which compose the Earth. Geophysics - The study of the physical properties of rocks and minerals. Geophysical survey - A scientific method of prospecting that measures the physical properties of rock formations. Common properties investigated include magnetism, specific gravity, electrical conductivity and radioactivity. Geothermal - Pertains to the heat of the Earth’s interior. Glacial drift - Sedimentary material that has been transported by glaciers. Glacial striations - Lines or scratches on a smooth rock surface caused by glacial abrasion. Glory hole - An open pit from which ore is extracted, especially where broken ore is passed to under ground workings before being hoisted. Gneiss- A layered or banded crystalline metamorphic rock, the grains of which are aligned or elongated into a roughly parallel Gold loan - A form of debt financing whereby a potential gold producer borrows gold from a lending institution, sells the gold on the open market, uses the cash for mine development, then pays back the gold from actual mine production. Gossan - The rust-colored capping or staining of a mineral deposit, generally formed by the oxidation or alteration of iron sulphides. Gouge - Fine, putty-like material composed of ground-up rock found along a fault. Grab sample - A sample from a rock outcrop that is assayed to determine if valuable elements are contained in the rock. A grab sample is not intended to be representative of the deposit, and usually the best- looking material is selected. Graben - A downfaulted block of rock. Granite - A coarse-grained intrusive igneous rock consisting of quartz, feldspar and mica. Gravity meter, gravimeter - An instrument for measuring the gravitational attraction of the earth; gravitational attraction varies with the density of the rocks in the vicinity. Greenstone belt- An area underlain by metamorphosed volcanic and sedimentary rocks, usually in a continental shield Grizzly (or mantle) - A grating, usually constructed of steel rails, placed over the top of a chute or ore pass for the purpose of stopping large pieces of rock or ore that may hang up in the pass. Gross value - The theoretical value of ore determined simply by applying the assay of metal or metals and the current market price. It must be used only with caution and severe qualification. Gross value royalty - A share of gross revenue from the sale of minerals from a mine. Grouting - The process of sealing off a water flow in rocks by forcing a thin slurry of cement or other chemicals into the crevices; usually done through a diamond drill hole. Grubstake - Finances or supplies of food, etc., furnished to a prospector in return for an interest in any discoveries made. Guides - The timber rails installed along the walls of a shaft for steadying, or guiding, the cage or conveyance. Gypsum - A sedimentary rock consisting of hydrated calcium sulphate. Gyratory crusher - A machine that crushes ore between an eccentrically mounted crushing cone and a fixed crushing throat. Typically has a higher capacity than a jaw crusher.   Prospecting &  Mining Basics