"Look over here Dear. This is the stuff they mine at Chino!"
Every week or so, we have visitors looking at our collection of local minerals. Invariably, they end up looking straight at some native copper - that is, naturally occurring pure copper. They form the misconception that native copper is what the mines are extracting as copper ore. This is not true. So, let's set the record straight.
Native copper really isn't an important ore mineral at the mines. In fact, it is considered a deterrent to copper production. Why? The physical nature of native copper, especially it's ability to bend without breaking prevents it from being pulverized in the crushers and mills at the mine. In fact, native copper can clog the processing equipment. So it is an undesirable mineral for production.
What copper minerals are mined?
There are a number of copper minerals that are extracted. In general, the main types of minerals include those in which copper has combined with sulfur (i.e., sulfides), oxygen (oxides), and a combination of carbon and oxygen (carbonates). Common copper minerals include the copper sulfides bornite, chalcopyrite, chalcocite, and covellite; copper oxides cuprite and tenorite; and copper carbonates azurite and malachite. You may recognize the terms azurite and malachite, as these minerals are commonly used in jewelry.
The Chino, Tyrone, and Cobre mines belong to a class of deposits called porphyry (phonetically sounding something like "poor four ee") copper deposits. Copper in these types of deposits characteristically occur as fine particles in the rock, extending over a large area at a low concentration - less than one percent (often much lower). Most of the minerals are copper sulfides. As a result the only way to economically mine and extract copper is on a large scale. Hence the open pits rather than localized underground mining.
Chino is a classic example of a porphyry sulfide deposit. In general, mineral assemblages form a bull's eye pattern, with an iron core at the center, expanding horizontally (or laterally) outward to copper minerals, then lead and zinc minerals. Many of the mines, such as the Bullfrog, Blackhawk, and Groundhog mines surrounding the open pit a Chino were copper-lead-zinc mines.
If you connect the geographic dots between copper districts in the Southwest (that is between Chino-Tyrone to Morenci to Miami-San Manuel to Copper Basin to Bagdad and Ithica Peak) you will get a pretty straight northwest trending line. The same is true with the copper districts extending from Mexico into southern Arizona.
How did the copper deposits in this area form?
Several geologic processes were critical to the formation of our copper deposits. First, copper and other metals were transported towards the surface of the earth from deeper sources by molten rock. The copper combined with sulfur and formed the primary copper sulfides bornite and chalcopyrite. These particles of copper sulfide were finely concentrated in the rock matrix over a wide area. If left untouched by nature, these deposits would not be considered economical for extraction - that is, the cost of extraction would exceed the value of the copper.
Second, over a long period of time, copper and iron sulfides were exposed to oxygen-rich waters migrating downward from the surface of the earth. The interaction between sulfur and water formed acidic solutions. Copper was dissolved in the solution and continued to migrate downward with it. Some of the copper fell out of solution and formed copper oxides, copper carbonates and native copper. This is called the oxidized zone. At the same time the iron sulfides were converted to iron oxides and formed an ugly brown oxidized zone above the remainder of the copper deposit, which geologists call a gossan or leach cap. Actually, an exploration geologist would consider the gossan a beautiful sight, as it serves as an indication of potentially economic minerals below.
Third, some of the dissolved copper continued to migrate beyond the oxidized zone and combined with an altered existing copper sulfide minerals to form concentrated zones of copper sulfides. This is known as supergene enrichment, and it was this process that made the deposit economical for extraction. Copper minerals formed from supergene enrichment typically consist of chalcocite and covellite.
How do they extract copper?
Traditionally copper has been extracted by crushing and pulverizing the host rock, then separating (concentrating) the copper ore from uneconomical rock matrix and smelting (melting) the concentrated copper ore. It is shipped as solidified copper plates (called anodes) to a refinery for further purification. Concentrated copper is no longer smelted in New Mexico. Much of it is sent of Miami, Arizona.
A second method that is used extensively is known as hydro-metallurgical extraction. Copper ore is placed on large stockpiles and exposed to a mild acidic solution that extracts the copper. The solution is transported to a solution extraction/electro-winning (SX/EW) plant that concentrated the copper by exposing it to various organic and chemical solutions and then applying an electric current to plate it onto sheets. This copper is almost pure copper and does not require further refining.
We hope that this will give you a better idea of what is mined, why it is mined, and how it formed, so you will have an appreciation the next time you are looking at Grant County's biggest hole in the ground.
Every week or so, we have visitors looking at our collection of local minerals. Invariably, they end up looking straight at some native copper - that is, naturally occurring pure copper. They form the misconception that native copper is what the mines are extracting as copper ore. This is not true. So, let's set the record straight.
Native copper really isn't an important ore mineral at the mines. In fact, it is considered a deterrent to copper production. Why? The physical nature of native copper, especially it's ability to bend without breaking prevents it from being pulverized in the crushers and mills at the mine. In fact, native copper can clog the processing equipment. So it is an undesirable mineral for production.
What copper minerals are mined?
There are a number of copper minerals that are extracted. In general, the main types of minerals include those in which copper has combined with sulfur (i.e., sulfides), oxygen (oxides), and a combination of carbon and oxygen (carbonates). Common copper minerals include the copper sulfides bornite, chalcopyrite, chalcocite, and covellite; copper oxides cuprite and tenorite; and copper carbonates azurite and malachite. You may recognize the terms azurite and malachite, as these minerals are commonly used in jewelry.
The Chino, Tyrone, and Cobre mines belong to a class of deposits called porphyry (phonetically sounding something like "poor four ee") copper deposits. Copper in these types of deposits characteristically occur as fine particles in the rock, extending over a large area at a low concentration - less than one percent (often much lower). Most of the minerals are copper sulfides. As a result the only way to economically mine and extract copper is on a large scale. Hence the open pits rather than localized underground mining.
Chino is a classic example of a porphyry sulfide deposit. In general, mineral assemblages form a bull's eye pattern, with an iron core at the center, expanding horizontally (or laterally) outward to copper minerals, then lead and zinc minerals. Many of the mines, such as the Bullfrog, Blackhawk, and Groundhog mines surrounding the open pit a Chino were copper-lead-zinc mines.
If you connect the geographic dots between copper districts in the Southwest (that is between Chino-Tyrone to Morenci to Miami-San Manuel to Copper Basin to Bagdad and Ithica Peak) you will get a pretty straight northwest trending line. The same is true with the copper districts extending from Mexico into southern Arizona.
How did the copper deposits in this area form?
Several geologic processes were critical to the formation of our copper deposits. First, copper and other metals were transported towards the surface of the earth from deeper sources by molten rock. The copper combined with sulfur and formed the primary copper sulfides bornite and chalcopyrite. These particles of copper sulfide were finely concentrated in the rock matrix over a wide area. If left untouched by nature, these deposits would not be considered economical for extraction - that is, the cost of extraction would exceed the value of the copper.
Second, over a long period of time, copper and iron sulfides were exposed to oxygen-rich waters migrating downward from the surface of the earth. The interaction between sulfur and water formed acidic solutions. Copper was dissolved in the solution and continued to migrate downward with it. Some of the copper fell out of solution and formed copper oxides, copper carbonates and native copper. This is called the oxidized zone. At the same time the iron sulfides were converted to iron oxides and formed an ugly brown oxidized zone above the remainder of the copper deposit, which geologists call a gossan or leach cap. Actually, an exploration geologist would consider the gossan a beautiful sight, as it serves as an indication of potentially economic minerals below.
Third, some of the dissolved copper continued to migrate beyond the oxidized zone and combined with an altered existing copper sulfide minerals to form concentrated zones of copper sulfides. This is known as supergene enrichment, and it was this process that made the deposit economical for extraction. Copper minerals formed from supergene enrichment typically consist of chalcocite and covellite.
How do they extract copper?
Traditionally copper has been extracted by crushing and pulverizing the host rock, then separating (concentrating) the copper ore from uneconomical rock matrix and smelting (melting) the concentrated copper ore. It is shipped as solidified copper plates (called anodes) to a refinery for further purification. Concentrated copper is no longer smelted in New Mexico. Much of it is sent of Miami, Arizona.
A second method that is used extensively is known as hydro-metallurgical extraction. Copper ore is placed on large stockpiles and exposed to a mild acidic solution that extracts the copper. The solution is transported to a solution extraction/electro-winning (SX/EW) plant that concentrated the copper by exposing it to various organic and chemical solutions and then applying an electric current to plate it onto sheets. This copper is almost pure copper and does not require further refining.
We hope that this will give you a better idea of what is mined, why it is mined, and how it formed, so you will have an appreciation the next time you are looking at Grant County's biggest hole in the ground.
