New Mexico Mineral Symposium — Abstracts

New ocean crust made of basalt is constantly being formed at the mid-ocean ridges. The Atlantic Ocean is growing at a rate of one or two centimeters per year. This basalt contains a small amount of metal. The amount of copper is about 100 ppm or 0.001%, the amount of molybdenum is about 2 ppm or 0.00002%, and the amount of lead is about 6 ppm or 0.00006%. There is a similar amount of gold and silver, and sulfur is present at about 800 ppm or 0.008%. Various geological processes enrich these very small amounts of metal to form the minerals that we find in Arizona.

As the basalt cools and cracks, seawater that is heated by the hot rocks, selectively leaches the metals and the sulfur from the basalt. When this hot seawater hits the cold ocean water, metal sulfides are deposited. These areas of deposition are known as black smokers. Pyrite (90% of the sulfides deposited), chalcopyrite, sphalerite, and galena are some of the minerals that are deposited at the vents of the black smokers. These sulfide deposits on the ocean floor are potential ore deposits, but they are so deep that it is not practical at present to mine them. But this process has concentrated the original 0.001% copper up to concentrations as high as 10% copper, a 10,000 times enrichment.

Sediments or volcanic rocks bury the black smoker deposits. As new ocean floor is formed the old ocean floor keeps moving away from the ocean ridge, carrying the sulfides with it. Two things can happen: The ocean floor and sulfides can be pushed up in mountain ranges as part of plate collisions, or the ocean plate can be subducted under a continental plate. When the sulfides are incorporated into mountain ranges, they are associated with metamorphism, igneous activity, and deformation (folding and faulting). They form orebodies known as volcanogenic massive sulfide deposits. In Arizona ore deposits of this type formed in the Precambrian (1.7 to 1.8 b.y. ago). The most famous of these are the United Verde mine and United Verde Extension mine in Jerome although there are other such deposits in Arizona including: the Iron King mine near Humbolt and the Old Dick mine and Copper Queen mine near Bagdad.

If the sulfide deposits are not pushed up into mountain ranges they are subducted under the continental crust. As the sulfides are buried with sediments, seawater, and ocean crust, the temperature rises until the mixture starts to melt. This melt moves into the continental crust where additional reactions can take place. The result is a complex series of intrusive and volcanic igneous rocks with associated ore deposits. The sulfur, the metals, and seawater that have been subducted make the ore forming solutions. In Arizona there is a complex history of the subduction of the Pacific plate under the North American plate. It starts about 80 m.y. ago and continues until about 20 m.y. ago when the subduction stops and the San Andreas fault forms. During this time of subduction there are two major periods of mineralization in Arizona. The first is called the Laramide from about 50 to 80 m.y. ago, and the second is called the mid-tertiary from about 20 to 35 m.y. ago. During the Laramide the major copper deposits in Arizona were formed including Bagdad, Christmas, Inspiration, Morenci, Ajo, Ray, Sierrita, Silver Bell, and many others. These are referred to as the porphyry copper deposits, as they are associated with an igneous porphyry intrusive. They are usually large with millions of tons of low-grade ore. The average copper content is around 0.5% (about 500 times enrichment over basalt) with varying amounts of molybdenum, gold, silver, lead, and zinc. During the mid-Tertiary the ore deposited contained copper, silver, lead, and gold. Some of the deposits of this age include the Silver district (Red Cloud mine), the Planet mine, the Mammoth—St. Anthony mine, Grand Reef mine, and many others. The metals in all of these deposits, Precambrian, Laramide, or mid-Tertiary, were initially deposited as sulfides. Gold is an exception to this and was deposited as native gold. Pyrite, chalcopyrite, bornite, galena, sphalerite, and molybdenite were the common sulfide minerals formed (primary ore).

When the sulfur in the sulfides reacts with oxygen in the air and ground water, sulfuric acid is formed. The metals, especially copper and iron, are dissolved in this acid solution and will percolate downward. If these solutions are neutralized, such as in contact with limestone, they will form carbonate minerals. If they reach the water table where conditions are more reducing the copper in solution will replace the iron in pyrite and chalcopyrite to form chalcocite. Chalcocite contains about 80% copper and forms rich supergene zones. In most of the major mining districts in Arizona, the early miners went for the supergene zones. They could have up to 12% copper. The present-day large copper mines are generally mining lower-grade primary ore.

Above the supergene zone and the water table is the oxidized zone. Many different minerals all containing oxygen as part of the formula can be found in this zone. There are several hundred different minerals with combinations of copper, lead, zinc, molybdenum, and other elements that can be found in this zone. The iron from the pyrite forms iron oxides, the most common minerals in this zone. The early prospectors would search for these rusty areas as clues to rich ore below. In the oxide zone the copper will form many minerals such as malachite, azurite, chrysocolla, and cuprite. Lead minerals include cerussite, anglesite, and vanadinite. Zinc minerals include smithsonite and hemimorphite.

So wulfenite (lead molybdate), that is so common in Arizona, starts as minute quantities of lead and molybdenum in basalt. The lead and molybdenum go through a series of geological processes including the formation of new sea floor, black smokers, subduction, igneous intrusions, weathering and oxidation, and eventually form wulfenite.