Zinc

Zinc – the building block of summer fun

Many common minerals and metals are taken for granted. The National Mining Association reminds us of their common and vital uses. Below is an info-graphic on the common uses of zinc.

NMA reminds us, “A versatile element, zinc can be found in products ranging from swing sets and swimming pools to electric vehicles and airplanes. Beyond its everyday applications, zinc production is an economic stimulant critical to U.S. growth. In 2012 alone, zinc was produced in three states at 13 mines and employed nearly 2,500 Americans in mines, mills and smelters. However, despite a number of zinc deposits in the U.S., we are 74 percent import reliant for zinc, importing it from Canada, Mexico, and Peru. A more efficient U.S. minerals permitting process would help facilitate the production of an estimated 11 million metric tons of zinc in the U.S. and support the industries that rely on them for electronics, green technology and transportation.”

See the original version of the info-graphic here.

Zinc_Infographic_v1

 

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Saginaw Hill, another old mine in a Tucson area neighborhood

Saginaw Hill is located about 10 miles southwest of downtown Tucson.  See general location on the map below.

Saginaw-Hill-broad-location

The hill was home to several mines worked intermittently from the late 1800s to the mid 1900s.  Mindat.org describes three of the major workings:

The Saginaw Mine:

A former small surface and underground Cu-Zn-Pb-Ag-Au-Mo (copper, zinc, lead, silver, gold, molybdenum) mine located in the NE ¼ sec. 11 & NW sec. 12, T.15S., R.12E. Owned at times, or in part, by the Saginaw Mining Co.; and the Tucson Arizona Copper Co.

Mineralization is sparse, spotty, partly oxidized base metal sulfides along fissure zones in limestone lenses in silicified Cretaceous Amole Arkose. Ore control was quartz veins as siliceous replacements along fractures. Alteration included limonite, quartz, sericite, clay, pyrite, and garnetization-epidotization. Associated rock units include the Saginaw Hill Porphyry, latite. Weakly mineralized. Chalcopyrite & pyrite on the W side of the Saginaw property, copper oxides on the E side of the property (Papago Queen). Cerussite & galena on the S end of the property along rhyolite-limestone contact. The identified ore zone is 330 meters long.

Local structures include NE-trending fractures, brecciation. On the SE side of the fracture and extending 300 meters away is propylitized quartz monzonite. Local limonite and partly oxidized pyrite with minor copper silicates along the fracture zone.

Workings include shafts, pits, and minor underground workings. Worked prior to 1900 with a possible 100 tons of low-grade ore produced. Production was sporadic. Production of smelter flux from 1956-1959.  Has been prospected for porphyry copper. Prospect workings concentrated along N60E trend but no good fracture zone is exposed.

Palo Verde Mine:

 Worked intermittently from 1918 through 1954. Total production amounted to some 2,300 tons of ore averaging about 13% Zn, 2.2% Pb, 0.7% Cu, 2.5 oz. Ag/T and 0.06 oz. Au/T.

Papago Queen Mine:

 Mineralization is disseminated copper oxides and carbonates with minor molybdenum oxides in quartz veins and along fractures in a weakly altered, brecciated, and mineralized Laramide (?) porphyry stock.

Workings include tunnel and open cut operations. Sporadic production of copper ore occurred from 1917 to 1934, and of smelter flux in 1956 through 1959. Total output was some 3,700 tons averaging about 1% Cu and 0.5 oz. Ag/T.

This area had long been a favorite of mineral collectors.  It is one of the few places in the world  where peacock-blue cornetite is found.  Cornetite is a copper phosphate with the formula  Cu3(PO4)(OH)3.

cornetite

The Saginaw Hill mineralization is intimately related to the big copper mines west of Green Valley to the south.  That will be the subject of a future post.

Because subdivisions were encroaching on the area, Pima County investigated Saginaw Hill in about 1988 with the view of turning it into a park.  At the time, the mines were abandoned, there were mineralized dumps on the property.  An assessment of the property by Pima County officials found (surprisingly to them apparently) that the area contained concentrations of toxic heavy metals.

The County made several more assessments and by 2003, the Bureau of Land Management (BLM) got involved.  See the BLM report here, it provides many photographs.  In 2005, the BLM was deciding what to do and there was much furor in the press.  See examples from the Tucson Citizen here and here.

The BLM filled in the many mine shafts which did pose a danger.  They also collected surface mineralization and dumps and buried the material on site to mitigate the imagined danger of heavy metal contamination.  The Google Earth photo below shows how the area looks now.  The orangish patch is were the material is buried.  Saginaw Hill is closed to mineral collectors.

saginaw-hill-close-satellite

See also:

Sierrita Mine is only U.S. source of Rhenium

The I-10 copper deposit

The Pontatoc mine in a north Tucson neighborhood

Florence Copper another mining controversy

Gold in Arizona

Gold of Cañada del Oro and rumors of treasure

Old mines of the Tucson Mountains

Oracle Ridge Mine on Mount Lemmon

Ancient Undersea Volcano in Arizona at Jerome

We recently were treated to videos of the eruption of an undersea volcano. (See Video ).

Similar volcanoes erupted during Precambrian time (about 1.7 billion years ago) in what is now Arizona. Some of them produced mineral deposits called volcanogenic massive sulfide deposits that contain copper, zinc, lead, silver, and gold. About a dozen such deposits occurring in northern Arizona have had some production, the largest known of which is the United Verde mine at Jerome.

blacksmoker1In such deposits, a shallow magmatic source under the sea bed provides heat which sets up a convection cell in the surrounding rocks. Sulfur and metals derived from the volcano and from leaching of the sea floor sediments are erupted into the sea and deposited as sulfide-rich sediments. The sulfides often form a chimney called a “black smoker”.

The sulfide-rich “clouds” are taken by ocean currents and eventually settle to the sea floor, around the vent, to form layers of sulfide material containing pyrite (FeS2), chalcopyrite (FeCuS2), sphalerite (ZnS), and galena (PbS), together with the sulfates anhydrite (CaSO4) and barite (BaSO4).

An idealized model of the volcanic edifice is shown in the cartoon below:

VMS2

The volcanogenic massive sulfide deposits at Jerome are zoned, with the copper-rich portion in the volcanic vent and on the ancient sea floor adjacent to the vent. Zinc and lead sulfides, with iron oxide occur as marginal banded sediments, and banded iron and silica deposits occur at the extremities.

The photo below is copper ore from the volcanic vent at Jerome. The brassy material is chalcopyrite. The darker material is chlorite, an alteration product.

OLYMPUS DIGITAL CAMERA

The next photo shows the banded zinc and iron mineralization. Notice that these bands are folded.

JeromeRocks05

The circular structures in the photo below are cross-sections of the black smoker chimneys formed during eruption and preserved by the erupted material.

JeromeRocks10

The deposit at Jerome is far from the idealized drawing above. In the approximately 1.7 billion years since the deposit was formed, it suffered several episodes of folding and faulting. The following cross-section by Paul Lindberg, shows the structure as currently understood. Lindberg hypothesizes that the volcanic vent was along the edge of a caldera (cauldron fracture zone). Besides the main United Verde body, shown on the left side, there are two smaller deposits which have been faulted off the main body, the Hermit orebody and the UVX orebody. The graphic below is a west to east section looking north. The town of Jerome sits just west of the fault above the Hermit orebody.

Jerome cross section

Production from the Jerome deposits totaled about 3.6 billion pounds of copper, 97 million pounds of zinc, 693,000 pounds of lead, 1.6 million ounces of gold, and 57.3 million ounces of silver. And there is much more remaining, just waiting for the right opportunity.

Reference:

Lindberg, P.A., 1989, Precambrian ore deposits of Arizona, in Geologic Evolution of Arizona, Jenney, J.P. and Reynolds, S.J. eds., Arizona Geological Society Digest 17.

Arizona Geologic History: Chapter 1, Precambrian Time When Arizona was at the South Pole

Approximately 650 million years ago, the place that would become Arizona was at the South Pole and the place that would become Antarctica was on the Equator.

Paleomap  650

 

 

Geologists classify that time as Precambrian, a time which extends from the beginning to about 542 million years ago. The Precambrian, therefore, represents about 80% of geologic time. The Precambrian is divided into two eons, the Archean (rocks older than 2.5 billion years) and the Proterozoic (rocks from 2.5 billion to 542 million years old). In the current classification, there are no Archean rocks in Arizona (although rocks at the bottom of the Grand Canyon were originally assigned to Archean time). See geologic time scale here.

Archean rocks form the Craton (basement or earliest stable part) of the North American Continent. These rocks underlie Canada and the northern Rocky Mountains. Arizona formed at the southern edge of the craton by several methods.

As tectonic plates ground together and separated, they produced volcanos, island arcs, intrusions of magma, and sedimentation in shallow seas, all of which would eventually build the continent. Most of these rocks became metamorphosed due to the heat, pressure, and shearing of one plate grinding against another. Precambrian metamorphic rocks crop out in a belt extending from northwest Arizona near Lake Mead, through Kingman, Prescott, and Globe, to southeastern Arizona near Wilcox. There are scattered outcrops trending west from Tucson to Yuma.

In mid-Proterozoic time (1.7- to 1.1 billion years ago), things quieted down and there was deposition of sediments and volcanic rocks in shallow seas between land masses. These rocks remain relatively undeformed and only locally metamorphosed. They are exposed principally in the mountains north of the Salt River, near Globe and Miami, Mammoth, and in the Catalina Mountains. This series consist, from oldest to youngest, of the Pioneer Shale, the Dripping Springs Quartzite (a hard sandstone), The Mescal Limestone and a capping basalt flow (the volcanic rock that built Hawaii), which together are called the Apache Group. The Apache group is overlain by the Troy Quartzite which is intruded by Diabase along the sedimentary layers. Diabase is crystalline subvolcanic rock similar to basalt.

The Mescal limestone, which has been metamorphosed to dolomite (magnesium carbonate rather than calcium carbonate), contains some of the oldest fossils in Arizona: calcareous algal mats called stromatolites. Oxygen isotope data indicates that certain cherty parts of the limestone originated in ocean water with temperatures of 25- to 30 degrees Centigrade (77-86 degrees Fahrenheit). After deposition and consolidation, the Mescal was lifted above sea-level and intense weathering created caves and sink holes. This process continued even as river-borne (fluvial), and wind-blown (eolian) sands of the Troy Quartzite were being deposited.

In the Grand Canyon, the Vishnu Schist is the equivalent of the earlier metamorphic rocks, and these are overlain by equivalents of the Apache Group and Troy Quartzite, but have different formation names.

Precambrian rocks, principally those in Yavapai County, contain significant mineral deposits. The best known of these is the United Verde mine at Jerome. These deposits, about 1.7 billion years old, are called volcanogenic massive sulfide deposits. They formed from under-sea volcanic vents that erupted hot, sulfide-rich material into the ocean water which caused cooling and precipitation onto the ocean floor. Similar mineral deposits are forming today. These deposits are rich in copper and zinc with lesser lead, silver, and gold.

Jerome vent ore

 

 

 

 

 

 

 

 

 

 

 

 

Jerome distal ore

he Dripping Springs Quartzite contains uranium minerals along fractures and bedding planes. In other Precambrian rocks, there are vein deposits of gold, sometimes with base metals, tungsten deposits, iron deposits, and asbestos deposits (the mineral is chrysotile, the not so dangerous kind of asbestos).

The division of Precambrian time into two eons, the Archean and Proterozoic at 2.5 billion years ago marks a profound change in the atmosphere. In the Archean, bacteria were the only known life-form. Bacteria take in carbon dioxide and give off oxygen, and about 2.5 billion years ago, that caused an environmental crisis. By that time bacteria had used up much of the carbon dioxide in the atmosphere and the planet was thrown into a profound ice age that lasted for about 30 million years. The bacteria retreated to equatorial habitats and to warm volcanic vents. Populations became isolated and some changed; they became more organized into a new life form called Eukaryotic microbes. The Eukaryotes had discrete cell nuclei and chains of protein to organize internal structure. The Eukaryotes would eventually become animals, plants, and fungi. Also at that time, oxygen levels in the ocean reached some critical level which caused iron and manganese to precipitate. All of the world’s large iron deposits, called Banded Iron Formations, formed between 2.5 and 1.8 billion years ago, and none have formed since.

 

References:

Geologic Evolution of Arizona, Arizona Geological Society Digest 17

Proterozoic Geology and Ore Deposits of Arizona, Arizona Geological Society Digest 19