U of M GEOL 2350 - Bringing a Mine into Production

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CHAPTER 6. Bringing a Mine into Production:Part I. From the Mining Resource Point of ViewINTRODUCTIONLOCATION AND GENERAL GEOLOGYGeology of the depositTable 1. Geochemistry of the volcanic rocks hosting the Largemouth depositMinor ElementsDETERMINING WHETHER THERE IS AN ECONOMIC DEPOSITTable 2. Price of MetalsMetal1997200120032007Cu$1.17/lb$0.63/lb$0.84/lb$2.58Zn$0.77/lb$0.35/lb$0.40/lb$1.49Pb$0.46/lb$0.21/lb$0.26/lb$0.84Ag$4.32/oz$4.28/oz$4.96/oz$13.9/ozAu$328/oz$282/oz$374/oz$665/ozYour instructor may prefer that you look up current prices on the Internet.Table 3. Composition of Concentrates from the Largemouth DepositWhat are the costs of developing a mine and those involved with mining?Effect of changing the price of metalsEstimation of waste rock producedKEY WORDSCHAPTER 6. Bringing a Mine into Production: Part I. From the Mining Resource Point of ViewNote: This mine is completely fictitious. It does not exist. However, the place names, topography and geography are all real. We have used data (with permission) from a Mining Simulation Project by E.K Lehmann & Associates Inc., Minnesota Department of Natural Resources, Minnesota Pollution Control Agency (MPCA), and the Project Environment Foundation.INTRODUCTIONBass Lakes Inc., a mining company particularly interested in base metals, was lucky enough to intersect Copper (Cu) -Zinc (Zn) mineralization in five out of sixty drill holes drilled into Archean age (older than 2.5 billion years) volcanic rocks of Northern Minnesota. After three more years of drilling, they outlined a Copper-Lead (Pb)-Zinc-Silver (Ag)- Gold (Au) deposit (called the Largemouth Deposit) that contains approximately 20.5 million tons of ore with a grade of 1.9% Cu,8.4 % Zn, 1.8 % Pb, 4.2 oz/ton Ag, and 0.025 oz/ton Au. During this period of exploration they leased lands from state, federal, and private owners as well as obtained permits for diamond drilling.The exploration geologists have now given these preliminary data to the mine geologists and environmental personnel to see if this mineral deposit can be brought into production. You, the students, are going to act as the mine geologists and the environmental personnel.Before development of a mine, many questions concerning economics, viability, and environmental impact must be answered. The questions are answered with results from the general geology, froma pre-feasibility study to see if the ore can be mined and processed at a reasonable cost, and then, from a feasibility study that must include an environmental impact statement (EIS). It must be Computer tips: This lab is greatly facilitated by use of a spreadsheet program. It is much easier and faster to do it this way. Daily metal prices can be found at: http://metalprices.com/ http://cgi.ino.com/exchanges/nymex_java.htmlCh. Pg. 1realized that the cost of baseline studies for an EIS and permitting are a major 'front-end' expense that can range from 50 to 200% of the exploration costs. As well, environmental issues during operation and post-closure will affect the cost of mine operation. We will deal with each of these issues in the order that a mining company would. You must realize that this will vary from area to area within the country but all states (and countries) have some system of rules and regulations that must be met before mining can occur. To answer the necessary questions, we must know something of the geology (both surficial and bedrock), the composition of the ore and the host rocks, general access and transportation, the surface characteristics such as drainage (surface and ground water), as well as the composition of the surface and groundwaters. This chapter specifically deals with the decisions leading to mining of the ore; the following chapter deals with the environmental questions that will be raised and must be answered.LOCATION AND GENERAL GEOLOGYThe Largemouth massive sulfide deposit (Lehmann et al., 1989) is located in northwestern Itasca County in northern Minnesota within the Chippewa National Forest, 37 miles northeast of the city of Grand Rapids, section 24,T149N, R26W (Wirt, MN quadrangle). It is hosted by volcanic tuffsof Archean age that have been folded and faulted such that the rocks strike northeast and dip approximately 85 degrees to the northwest (Figures 1, 2 & 3). Those rocks termed footwall occurbelow the ore and those called hanging wall occur above it. Bedrock is covered by a blanket of glacial sediments, called overburden, up to 100 feet thick that were deposited 20,000 to 12,000 years before present. They consist of a lower sequence of red sand- and gravel- rich till overlain bya grey-brown calcareous (lime-rich) till. These tills host several perched1 groundwater reservoirs.Geology of the depositThe deposit is hosted by mafic volcanic tuffs and flows (Figure 1). In the southeast the rocks are dark green basalts that are composed of fine-grained (less than 1 mm) chlorite, epidote, plagioclase, and sericite (fine-grained muscovite). Locally these rocks are fractured and filled with quartz, calcite, and pyrite. Above the basalt and directly beneath the ore deposit is a felsic tuff, 100to 250 feet thick. It consists of quartz, sericite, and chlorite with minor amounts of pyrite (up to 2%). Where this rock is in contact with ore, it has been altered causing an increase in SiO2, S, and Cu content (Table 1)2. Above the ore itself, is a series of tuffs with minor interbedded chert. The 1 A body of groundwater that sits above the water table within the glacial overburden.2 You will need this information to make informed decisions about where to put Ch. Pg. 2tuffs are fine-to medium-grained and consist of quartz, plagioclase, chlorite, and sericite with less than 1% pyrite.The orebody consists of up to 75% sulfide minerals with gangue silicates being chlorite and quartz.Sulfides, in order of abundance, are pyrite (FeS2), pyrrhotite (FeS), sphalerite (ZnS), chalcopyrite (CuFeS2), galena (PbS), and trace amounts of arsenopyrite (FeAsS). The upper two hundred feet of the orebody (just beneath the glacial till or overburden) has been enriched by supergene processes and now consists of mostly chalcocite (Cu2S) with minor bornite (Cu5FeS4). Supergene enrichment occurs when descending groundwater transports metals short distances forming a blanket with enhanced metal values. The total mineralized zone is 3000 feet long, up to 90 feet thick, and extends down dip 1600 feet


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