GY 111 Lecture Note Series Economic Geology

GY 111 Lecture NotesD. Haywick (2008-09)1GY 111 Lecture Note SeriesEconomic GeologyLecture Goals:A) Economic Minerals (resources versus reserves)B) Mineral formationC) Mineral extractionA) Economic MineralsBy now, you have been introduced to the minerals that you are going to be responsiblefor in the lab component of GY 111L. I hope that you have also begun to recognize theimportance of some of these materials. If not, consider this table that I extracted directlyfrom the GY 111L Lab manual:Table 1.5. Economic uses of some mineralsMineralEconomic Usealuminum orebauxitePortland cement, chalk, antacidscalcitevitamins, antacid, garden limedolomitechalcopyritecopper orelead oregalenagarnetgemstones, jewelry, sand papersheetrock, plaster, cosmeticsgypsumlubricants, pencil leadgraphitehalitetable saltjewelry (mineral peridot)olivineabrasives, girl's best friendsdiamondsulfurpharmaceuticals, asphalt, plasticsdental applications, steel fluxfluoriteclay, pottery, tile, Kaopectate, cosmeticskaoliniteiron orehematiteiron orelimonitemagnetiteiron oremalachitejewelry, copper orequartzelectronic applicationssphaleritezinc orelubricant, talcum powdertalcIt lists the main uses of some of the GY 111L minerals in the collection. Truth be told,there are many other uses of geological materials that are have not listed here and ofcourse many other minerals that are important to our society that are also not listed here.Definition time. Any mineral that has economic potential (i.e., it makes someone money),is defined as an economic mineral. All of the minerals listed in table 1.5 are economicminerals. Additional economic minerals include the native elements Gold, Silver andPlatinum, virtually every sulfide known (e.g., Molybdenite-MoS, cinnabar-HgS etc.),

GY 111 Lecture Notes1D. Haywick (2008-09)22345Figure 1: Examples of important economic minerals. 1-gold; 2-silver; 3-cinnabar; 4-molybdenite;5-uraninite. All images from http://webmineral.com/specimens/picshow.php?id 1478most of the oxides (e.g., uraninite-UO2), and lots of the minerals comprising theremaining mineral classes. You will not see these other economic minerals in GY111/GY111L, but if you are a geology major, you will see them in GY 302. Can’t wait?The minerals that are underlined are pictured above.Right now, you are probably thinking that on minerals factor in as economicallyimportant geological materials. As it turns out, neither, gold, nor diamonds, nor platinumare as economically important to the US economy as limestone, sand or gravel (crushedrock). These rocks are considered important building materials and are used soextensively in the United States that they dwarf the economic significance of theeconomic minerals; however, that’s not necessarily the case in other countries. In Canadaand Australia, where there are much lower populations (and therefore less demand forbuilding materials), and much more extensive economic mineral deposits, gold, diamond

GY 111 Lecture NotesD. Haywick (2008-09)3Gold mining data (USGS Commodities information for 2007)and sulfide mineral mining are major components of their economies. In fact., Canada1and Australia are considered to be primary resource economies which is why, in 2008,they are escaping the economic turndown that is really hurting the USA. At least for now.If the US economy does not improve soon, there will be a big decrease in the demand foreconomic minerals so even primary resource economies will start to contract.It’s time to now consider two other terms. Resources are defined as any substance vitalor necessary for a society. Obviously the term mineral resources refers to those mineralsthat we require. Building resources are materials that we need for construction likesand, gravel etc., energy resources are fossil fuels that we need for energy, and waterresources are supplies of drinking water. For your information, geologists areresponsible for locating and extracting all of these substances. If it sounds like you oweyou very existence to geology, you are absolutely correct. In fact, there is a saying Any resource that is not grown or raised, is ultimately geological in nature. and when you consider that you have to grow plants in soil and raise animals on plantsthat grow on soil, even food is somewhat geological in origin (soil is a geologicalmaterial too). Perhaps now you can appreciate why geologists are in such high demandeven despite the poor economy and why, should you ever pass a geologists, you shouldgive them a hug.The last definition that is required at this point in the lecture is reserves, as in mineralreserves. This is the total amount of an economic mineral that has been found and that islikely to be found in the future. For example, we can estimate the likely amount of gold inthe United States based upon the known amounts that have been found in all of the activemines in the country and in the new “plays” currently worked (see table at the start of thispage). Reserves are not necessarily the absolute total of a mineral that exists, just howmuch we’ve found to date. In some cases (e.g., uranium, aluminum), we will probably1This is especially true for Canada if you throw in the other major economic geology materials like oil,natural gas and fresh water.

GY 111 Lecture NotesD. Haywick (2008-09)4never run out of reserves. In most cases however (e.g., gold, platinum), we will run outsomeday.B) Mineral FormationSadly, in this course, it is not possible to spare the time to go over how all economicminerals form. We have to choose one or two. In this country, much of the economicwealth in geology is courtesy of base metal sulfides. These include the GY 111L mineralssphalerite, chalcopyrite, galena, and other minerals like molybdenite, cinnabar andcovellite (CuS). The list is a long one that you need not concern yourself about here. Forthis lecture, we’ll stick with copper minerals.Copper-bearingsulfideslikechalcopyrite and covellite aremostly formed through the actionof hot water. Deposits like this arecalled hydrothermal and form inareas where either groundwater2comes in contact with hot magmadeep below the surface of theEarth, or a particularly “wet”magma dewaters near the surfaceof the Earth (see cartoon to the leftfrom www.answersingenesis.org).Under these conditions, liquidwater can heat up to 400 or 500 C; however, it remains a fluidowing to high confining pressure1000’s of m below the surface.Water this hot becomes extremely“aggressive” and is capable ofstriping normally insoluble materials from the host rocks. This includes Cu2 as well asAu2 gold and other metallic cations. The thing to note about this process is thathydrothermal water is really effective at extracting ions that are normally in lowconcentration within rocks. Remember also that this stripping process may be going onfor 100’s or thousands of years. The trick is that you somehow have to reprecipitate andconcentrate the dissolved Cu2 somewhere else.2There will be an upcoming lecture on groundwater in GY 111. For now, just assume that all groundwatergets there from infiltrated rainwater.

GY 111 Lecture NotesD. Haywick (2008-09)5Hydrothermal water tends to rise upwardas it is heated because it become lessdense. In contrast, groundwater tends tosink and/or flow “downhill” over time. Ifhydrothermal fluids diffuse outwards asthey rise, the result will be an ore depositthat is finely disseminated over a largearea (see porphyry copper ore pictured tothe left). These are called after theigneous rock that they form in (more aboutthis in the igneous section of the course)and they consist of a wide variety ofcopper sulfides as well as pyrite (FeS2) and other non-economic minerals like quartz.Mining geologists call the non-economic stuff gangue and need to consider it’s presencewhen determining if an economic deposit is worth mining or not. Ultimately it all comesdown to how much money you can make. If it’s a lot (i.e., the ore deposit contains a lotof reserves), the extra cost of disposing of the gangue is not an issue. However, this mightnot be the case for a small deposit.Finely disseminated ore deposits are great tofind, but even more impressive are ore depositsthat are concentrated over a small area. A verycommon location for concentrated ore depositsis along faults and what were originally openfractures (vein ore deposits) (see image to rightfrom warrandyte.bizhat.com and image belowfrom www.lepantomining.com). Hydrothermalfluids pass easily along these planes ofweakness and as they get closer to the Earth’ssurface, they either cool and precipitate outminerals directly from water, or flash to steam as the confining pressure is reducedprecipitating minerals during the process. By the way, the latter process wherebyhydrothermal water flashes to steam near the surface of the Earth is how geysers like OldFaithful in Yellowstone National Parkwork. Analysis of the mineralsprecipitated at geysers (see image attop of next page) demonstrates thattheycontainrelativelyhighconcentrations of gold and othereconomic minerals which leadseconomic geologists to speculate thatmany ancient vein deposits (especiallygold) formed just below geysers.Not all metallic ore deposits wereformedthroughreallyhot

GY 111 Lecture NotesD. Haywick (2008-09)6hydrothermal waters. Some wereformed through distinctly tepid water.Low-temperaturehydrothermalfluids as low as 25 C are thought to beresponsible for extensive deposits ofsphalerite (ZnS) and galena (PbS) insedimentary rocks around the world.The mechanism of formation was firstextensively studied in outcrops alongthe Mississippi River in Missouri andfor this reason, they are commonlyreferred to as Mississippi Valley-TypeLead-Zinc Ore deposits (MVT ores). The mechanism of formation is remarkablysimilar to that of the other hydrothermal deposits. Groundwater seeps downward throughsedimentary rocks and gets a little bit warmer as it sinks. There is no hot magma source.This heating is due entirely to the geothermal gradient, which is a natural consequenceof living on a planet with a hot core: the deeper you go into the interior of the Earth, thehotter it gets. The normal geothermal gradient is about 30 C per 1000 m. Fluids that sink3 km are heated up to almost the boiling point of water. This fluid is capable ofdissolving and transporting Zn2 and Pb2 from the sedimentary rocks that they arepassing through. The fluid is also very good at dissolving Ca2 . Since the host rocks forMVT ores are usually limestone, a type of rock composed of calcite, this means that a lotof holes open up as the fluids pass through. These voids end up being the sites ofprecipitation of galena, sphalerite and gangue minerals like calcite, dolomite and fluorite.This combination of minerals frequently produces absolutely gorgeous mineralspecimens, some of which go for 1000.00 or more. I have a few cheaper ( 20.00)examples in my office should you wish to see them. If you would like to see the reallyexpensive ones, I accept donations toward their purchase. Leave a few dollars in the“Support My Professor’s Mineral Collection” cookie jar on your way out.C) Mineral ExtractionGeologists do not have the best environmental reputation in part because of the waymineral resources are extracted out of the Earth. This is unfortunate as most geologistshave more respect for the environment than does the average person. But there is nodoubt that extraction of minerals does leave a pretty significant footprint.Minerals are extracted via a number ofdifferent techniques depending upon thenature of the deposit. If the minerals aresuitably concentrated, say in a vein deposit oralong a fault or fracture, an undergroundmine like that one pictured to the left (GoldenGiant Mine) is the most cost effective andleast environmentally destructive method. If,however, the ore body is more disseminated(e.g., porphyry copper deposits), open pit