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Silver (Mesoproterozoic, 1.05-1.06 Ga; copper mine near Houghton, Michigan, USA) 3 | by James St. John
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Silver (Mesoproterozoic, 1.05-1.06 Ga; copper mine near Houghton, Michigan, USA) 3

Native silver from the Precambrian of Michigan, USA. (Cranbrook Institute of Science collection, Bloomfield Hills, Michigan, USA)

 

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 4900 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

 

Elements are fundamental substances of matter - matter that is composed of the same types of atoms. At present, 118 elements are known (four of them are still unnamed). Of these, 98 occur naturally on Earth (hydrogen to californium). Most of these occur in rocks & minerals, although some occur in very small, trace amounts. Only some elements occur in their native elemental state as minerals.

 

To find a native element in nature, it must be relatively non-reactive and there must be some concentration process. Metallic, semimetallic (metalloid), and nonmetallic elements are known in their native state as minerals.

 

Silver is part of the gold-group of metallic elements. Silver is a precious metal, but is far less valuable than gold or platinum. Silver usually occurs as a silver sulfide mineral, but it also occurs in nature in its native state, often in the form of twisted wires. Silver is moderately soft and has a silvery-white color on fresh surfaces that tarnishes to darker colors. Elemental silver in nature is often found alloyed with other metals. Naturally alloyed gold-silver is called electrum.

 

The spectacular silver mass shown above comes from northern Michigan's Portage Lake Volcanic Series, an extremely thick, Precambrian-aged, flood-basalt deposit that fills up an ancient continental rift valley. This rift valley, analogous to the present-day East African Rift Valley, extends from Kansas to Minnesota to the Lake Superior area to southern Michigan. Unlike many flood basalts (e.g., Deccan Traps, Siberian Traps, Columbia River), the Portage Lake only filled up the rift valley. The unit is exposed throughout Michigan’s Keweenaw Peninsula, in the vicinity of the towns of Houghton & Hancock.

 

The Portage Lake succession thickens northward through the Keweenaw, up to >5.5 km worth of section in places. The dominant rock type is basalt - vesicular basalts, for the most part. Most of the original vesicles (gas bubbles) have since been filled up with a wide variety of different minerals. A vesicular basalt that has had its vesicles filled up with minerals is called an amygdaloidal basalt (try saying that five times quickly). Keweenaw amygdaloidal basalts have long had significant economic importance because native copper (Cu) is one of the more common vesicle-filling and fracture-filling minerals. Native silver (Ag) is sometimes found closely associated with copper.

 

Silver and copper mineralization occurred during the late Mesoproterozoic, at 1.05 to 1.06 billion years ago. The Portage Lake host rocks are 1.093 to 1.097 billion years old.

 

Locality: unrecorded copper mine near Houghton, Keweenaw Peninsula, Upper Peninsula of Michigan, USA

 

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Taken on May 3, 2014