Lab 4 Porphyry and Bonanza Load Deposits PDF

LAB 4 Porphyry and Bonanza Load Deposits MGE 210 MINERALOGY AND PETROLOGY FOR ENGINEERS NAME: _______________________ Base metals are called such because they are not considered precious like gold, silver, and platinum. Base metals encompass a wide variety of metals with important indust...

LAB 4 Porphyry and Bonanza Load Deposits MGE 210 MINERALOGY AND PETROLOGY FOR ENGINEERS NAME: _______________________ Base metals are called such because they are not considered precious like gold, silver, and platinum. Base metals encompass a wide variety of metals with important industrial uses: copper, lead, zinc, tin, and molybdenum. Iron and ferroalloy minerals are used to make steel and include the minerals containing: iron, nickel, chromium, manganese, tungsten, vanadium. The light metals have high strength and light weight: aluminum, titanium, magnesium, and beryllium. The following are descriptions of the types of mineral deposits in which metals are usually found. COPPER PORPHYRY DEPOSITS The term Copper Porphyry has a very broad meaning: Porphyry deposits are large, low grade, epigenetic (ore emplacement after the deposition of the host rock) hypogene (primary) copper deposits that can be mined by bulk mining methods (large open pit). Copper porphyries in Arizona, one of the richest copper producing areas on Earth, were formed with very few exceptions, during the Laramide orogeny (75 - 55 million years ago). The porphyry deposits in Arizona are related to igneous intrusions. Heat from the intrusions produces hydrothermal processes that in turn cause alteration and mineralization. Different types of mineralization and alteration are found in very specific areas within the zone of influence of the hydrothermal fluids. Figure 1 shows zones of a typical hydrothermal deposit. Keep in mind that this figure is for the alteration-mineralization of a hydrothermal system that is only a part of the porphyry system. The porphyry system as a whole was seen in class. TYPICAL MINERALIZATION: Identify the following important minerals from a porphyry copper deposits. These minerals are found in different areas of the deposit depending on their chemical composition. Sample Number Class Formula Name KS70 KS13 KS68 1 AP1 AP2 AP3 AP4 AP7 AP5 AP6 KS23 2 AP13 KS42 AP15 KS22 KS7 KS50 AP8 3 Figure 2 shows a general cross-section for a supergene leach-cap on an Arizona porphyry copper deposit. The minerals that you examined occur at different depths in a deposit. Depending on how much of the deposit has eroded, you may not be able to find all these minerals in one deposit. Weathering in a deposit alters the minerals: changing some, leaching some, and redepositing some at depth. Weathering of a porphyry copper deposit produces four overlapping zones: 1. The top is the leached zone, in which the economic minerals are removed. The leached zone is characterized by the presence of iron hydroxides (goethite, limonite). Massive iron hydroxides are sometimes referred to as gossan. 2. The next deeper zone is the oxidized portion of the deposit where oxide and carbonate minerals are most commonly found. Minerals in this zone come from weathering the original or primary minerals, and from cations and anions leached from the gossan and deposited in the oxidized zone. This zone tends to lie just above the water table and can be very rich in interesting and economic minerals. New mines often count on the profits from this mineral-rich zone to help offset the startup costs of a new mine. 3. The third zone is the supergene enrichment zone. This zone contains both primary minerals and minerals deposited from cations and anions leached from the zones above. Thus we see a mixture of primary minerals plus additional sulfide and oxide minerals. 4. The deepest zone is the primary zone, containing unaltered "hypogene" minerals from the original hydrothermal system or deposited directly with the host rocks. Figure 2. Generalized cross-section of a leach cap on a porphyry copper deposit. ALTERATION Alteration is characterized by the cations added / leached and the resulting mineral associations. For instance, if lots of K has been added to a rock, we look for K-rich minerals, and refer to this zone as the Potassic alteration zone. In other areas, acid has leached major cations out of rock, leaving lots of fine white mica (sericite) behind: this area is referred to as the sericitic alteration zone. Alteration is very wide spread and zoned. Four major zones or types of alteration occur and are located in specific areas in the deposit. 4 Potassic zone: The alteration results from the leaching of calcium and sodium. The characteristic minerals are biotite, orthoclase and quartz. This zone is located in the center of the deposit. Sericitic zone or phyllic zone: Leaching of Mg, Na, Ca (most major cations) from aluminosilicate rocks. Minerals in this zone are quartz, sericite and pyrite. Quartz and sericite almost totally replace the original rock. Pyrite can be as high as 10-15 % by volume. Chalcopyrite is also present but at much lower levels, less than 1%. Argillic zone: This zone is characterized by the formation of clay minerals. This zone can be highly altered or weakly altered. The minerals in the moderately altered rocks are montmorillonite, chlorite, and kaolinite. The highly altered areas have quartz, amorphous silica, andalusite and at times corundum. The main sulfide is pyrite with some bornite and chalcopyrite. The pyrite is less abundant than in the sericitic zone. Propylitic zone: This zone is often very large. Minerals in this zone are chlorite, epidote, and calcite. This zone is the farthest from the center of the intrusion that formed the deposit. GEOLOGIC CONDITIONS The geologic conditions that exist to produce various copper porphyry systems are very similar. The following figure shows several simplified schematic cross sections of Arizona deposits. Describe the similarities that you find in the relationships of the cross sections. ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ ______________________________________________________________________________________ 5 key: T=Tertiary, M=Mesozoic, P=Paleozoic, v=volcanics, a=arkose, sh=shale, ls=limestone, dol=dolomite, ev=evaporite, s=schist, g=granite After becoming familiar with the basic ore and gangue minerals associated with many copper deposits, we are going to take a broader-scale view of alteration zoning, assemblages, and the processes associated with porphyry copper deposits. These magmatic-hydrothermal deposits are often simplified into a model that shows characteristic zoning of the main types of alteration, pictured below. These zones have been discussed in lecture. 6  Sericitic Alteration Sericitic alt. Figure 1: Schematic of alteration zones in a copper porphyry system Referring Figure 1, describe the samples provided in terms of the minerals present, and which alteration zone they would most likely be found in (Propylitic, Advanced Argillic, Sericitic, or Potassic). 7  Sample Minerals Present Alteration Zone Major cations Potential added or lost? engineering issues? GE224 - specifically look at vein JB35 BV10 If you were an exploration geologist looking for a new porphyry copper deposit and found a large area of quartz-kaolinite-pyrophyllite alteration, what is the name of the alteration zone you are standing in? Now you're an exploration geologist looking for porphyry copper deposits in Arizona, which has undergone a tremendous amount of extension, resulting in normal faults that cut and off-set rocks as seen in the schematic cross-section below. Cu Figure 2. Schematic cross-section of an extended terrain representing Basin & Range faulting. If you were mapping rocks on the surface at point A and found a very large copper deposit, where might you look to expand your resources? You are mapping at point B on Figure 2, and find a small amount of mineralized porphyry dike with some K-feldspar-biotite-chalcopyrite, but mostly find unmineralized, unaltered granite porphyry. What part of the porphyry copper deposit did you find this in? Why? 8  If you are at the bottom of a porphyry deposit, would you continue to explore for copper at point B? Why or why not? Bonanza Load Deposits Epithermal Gold-Silver Deposits Epithermal deposits are products of volcanism-related hydrothermal activity at very shallow depth (up to 1 km) and low temperatures (50 to 300 C). Many of the deposits known in the western US are very young. A few rare deposits are found in Mesozoic rocks (252 to 66 Ma) with the vast majority in Tertiary volcanic rocks (66 to 3 Ma). The most common forms of these deposits are hosted in quartz veins, irregular branching fissures, stockworks, breccia pipes, vesicle filling, and dissemination. High porosity and open channels are needed to allow mineralized fluids to transport metals in the country rock. These fluids will move over great distances and precipitate metals out in high to low transitional temperature gradients. Precipitation and metal solubility is a function of temperature and pressure gradients, biology, chemistry of country rock, and structural controls such as faults and fissures for the fluid to flow through. These mineralized veins tend to be drusy, or have comb structures or crustiform banding. The deposits are small in area and tonnage but have a very high grade. Grades up to 10,000 oz/ton gold and 130 lbs/ton silver have been recorded. Surface expressions of these deposits show up as hot springs or gossans. Hot springs deposits such as at Yellowstone National Park, are a characteristic source of mercury, arsenic, and antimony sulfides in addition to gold and silver. Why are most of these types of deposits so young? With these epithermal deposits having such a high grade, why aren’t they mined more often or looked at for exploration targets? 9  Identify the following four minerals that can be found in hot spring deposits Sample # Formula Diagnostic Feature Mineral Name KS24 GE45 KS69 GE453 Discuss the impacts that these minerals may have on your operation and how these impacts could be mitigated. Supergene Enrichment Supergene enrichment is a secondary process of concentrating metals at the surface of the earth. Commonly, a porphyry copper system will form ~4 km underground, and millions of years later it is brought to the surface of the earth due to erosion or faulting. Once the porphyry deposit is exposed at the surface, air and water begin to react with pyrite. Pyrite breaks down to form sulfuric acid, and this acid will break down copper-bearing minerals like chalcopyrite and bornite, freeing up copper to be concentrated in the acidic fluids. Eventually, higher grades of copper are concentrated in a much smaller area than before. In this case, we would refer to the original porphyry deposit at hypogene (meaning the initial deposit, here being high temperature and deep below the surface), and the secondary enrichment as supergene (generally lower T, at the surface, a younger process that modifies the original/older hypogene mineralization). Supergene enrichment tends to form different copper-bearing minerals than hypogene deposits (sulfides: chalcopyrite, bornite, etc.). Supergene processes form Cu-oxides, Cu-carbonates, and Cu- silicates. There are a few supergene sulfides, but they tend to have much higher Cu:S ratios that chalcopyrite or bornite, such as chalcocite (Cu2S). The supergene process also leaves an area known as a leach cap, where all the copper has been stripped or 'leached' out of the rock, and moved downwards into the areas of high copper concentration. The areas that have been leached of copper are typically composed of the minerals goethite, hematite, and jarosite. 10 Figure 3. Cross-section of supergene leach cap on a primary porphyry copper deposit. Take a look at the three groups of minerals and rocks (A, B, and C). Group A (Car 5, NC45). What minerals are present? Are these supergene or hypogene minerals? Why? Group B (CP1, and OTBL). What minerals are present? Are these supergene or hypogene minerals? Why? Group C (En2, and AP5). What minerals are present? Are these supergene or hypogene minerals? Why? On the column below, label the blank areas with either Group A, Group B, or Group C, based on where you would expect to find each assemblage of minerals in a porphyry / leach cap environment. 11 Thought question: Do you think the supergene process is good for mining companies? If so, why? What are some possible negatives associated with supergene enrichment of porphyry copper deposits (if any)? 12