GREENSTONE/CHLORITE-SCHIST/GREENSCHIST

These rocks are schists which have the mineral chlorite as their chief determinant mineral. It occurs as fine, scaly aggregates, sometimes too fine for the individual scales to be seen by the eye; more rarely in foliated to coarse foliated aggregates. It is sometimes thinly, sometimes thickly, schistose, and in some cases almost massive; and although the rock is very soft and may be readily cut, it is very tough in the more massive varieties. Greenschist is a generally used field petrologic term applied to metamorphic or altered basic volcanic rocks like basalt, but also includes dolerite (diabase) as well. Temperatures of approximately 400 to 500 C and depths of about 8 to 50 kilometers are the typical of the metamorphic environments these rocks have been subject to. The term greenstone is sometimes used to refer to a greenschist especially a non-fissile form of that rock - but at times has been used to refer to other rock types as well. So for this webpage we are limiting ourselves to a discussion of greenstone and green schist as equivalent rocks. I will refer to greenschist as the fissile or schistose texture variety, and greenstone as the compact non-schistose variety.

The color varies through different shades of green, yellow-green, to dark green. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the rock. Different minerals are apt to accompany the chlorite, some of which may be in megascopic sizes; of these may be mentioned magnetite, often in fine crystals; hornblende in slender needles or prisms; corundum and kyanite in some cases; quartz, which is generally in veins and lenses; epidote in grains and crystals; in some instances graphite, calcite, dolomite, etc. The chemical composition of these rocks is very variable, so far as is known, for not many have been investigated; it indicates that they have resulted from several different sources.

 

 

Some specimens have a composition very similar to that of the group of igneous rocks known as gabbros, as may be seen by reference to their analyses, and to which also the dolerites and basalts belong, these being merely textural varieties of magmas similar to gabbros. This is the dominant variety and most greenstones are the metamorphosed form of basalts that were formed at spreading centers on the ocean floor. Textures of pillow basalt lavas, indicating rapid cooling in water, can sometimes be seen in greenstones, a mark of their original character.  On the other hand, some greenstone samples are very different, and do not correspond to any igneous rock; it suggests rather a very ferrugineous clay these are thought to correspond to sediments deposited on top of the basalts on the sea floor. The chlorite-schists are of wide distribution, forming subordinate layers or masses in the midst of gneisses, mica-schists and other such rocks, characteristic of metamorphic areas. Thus they occur in Canada, New England, New York, Pennsylvania, etc. They are also common in Europe, in the Alps, Germany, Sweden and other places.

Greenstone: Transitions of chlorite-schist into mica schist, into slates, into schistose serpentine, and into hornblende-schist occur in places. Under the description of gabbro and of dolerite it was mentioned that these rocks by alteration, through processes of regional metamorphism, passed into hornblende-schists and into so called "greenstone" or "greenstone-schist." In such cases the original ferromagnesian minerals, or the hornblende produced from them, have been largely changed by alteration into chlorite which gives the rock its green color. Such greenstones (if massive), or greenstone schists (if schistose), which thus represent altered dolerites, basalts, and gabbros, form transition types to chlorite schist. The alteration of hornblende in diorites to chlorite also produces greenstones. It is conceivable that a dolerite might pass directly by alteration into a chlorite rock, or greenstone, and thus be of massive character, or it might be first changed into a hornblende-schist and then secondarily altered into a chlorite-schist. But since hornblende-schists are produced, not only from igneous but also from sedimentary beds, as described in the account of these rocks, the mere fact that transitions from hornblende- schists into chlorite-schists occur does not alone prove these latter have been derived from igneous rocks in any given case. Transitions from dolerite into chlorite rocks, or greenstone-schists, have been observed in many regions; in Michigan, Maryland, Connecticut, in the south of England, in the Alps, Germany, etc. A greenstone-schist is composed of chlorite, with some feldspar, quartz and calcite. This should be compared with an analysis of gabbro which represents the gabbro-dolerite group.

The greenstones vary in color from pale gray green through yellow-green to dark green. The color depends on the proportion of chlorite to other minerals. These rocks are generally too compact for the megascopic determination of the individual mineral particles. They are generally rather soft rocks. Sometimes, if the original dolerite or basalt was an amygdaloid, this structure is retained and the rock is filled with little voids of calcite or quartz. In other cases, the rock has been strongly sheared and these have disappeared, but are still represented in the schist by ovoid spots of different colors and mineral compositions from the main mass. In some rare cases they have been replaced by ores. The amygdaloidal structure is a good proof of the original igneous character of the rock.

 

 

The reason I have especially included chlorite greenschist in this list of common rocks is its important association with gold and other mineral deposits. The large preponderance of greenstones are thought to have been formed from basalts and other volcanic or sedimentary rocks that have undergone metamorphic alteration related to subduction zones at the collision of ocanic and  continental tectonic plates . Normally greenstones will occur in a belt or zone that may be 20 miles to hundreds of miles long. They normally occur associated with larger bodies of granite and gneiss within a craton, but can occur with other rocks as in the Sierra Nevada of California. Though many examples of greenstone belts are Achaean in age, greenstone belts are found distributed throughout geologic history, from ancient to comparatively recent.  Greenstone belts often contain important ore deposits of gold, silver, copper ore, zinc and lead. Well known examples of economically valuable greenstone belts have been mined in Canada, Australia, Africa and Brazil- and to a lesser extent in the United States.

Greenstone belt deposits are related to mountain building events and are important sources of gold deposits. Greenstone-hosted quartz-carbonate vein deposits are a very productive sub-type of lode gold deposits. They are also known as mesothermal, Mother lode type, orogenic or gold-only deposits.  The quartz-carbonate veins in these deposits typically exhibit both veins in moderately to steeply dipping reverse shear zones as well as arrays of shallow-dipping extensional veins. The reverse character of the shear-zone-hosted veins and their typically shallow dips attest to their formation during crustal collision and subduction events. Many geologists feel that the gold found in greenstone belts is leached from the greenstone by supercritical water released during the metamorphic process and later deposited in the surrounding rocks in quartz veins or as disseminated gold.

Gold is largely confined to the quartz-carbon ate vein network but may also be present in significant amounts within iron-rich sulphidized wall rock selvages or silicified and arsenopyrite rich replacement zones. They are distributed along major regional scale fault zones. Common rocks associated with these gold deposits include iron rich rocks such as basalts, differentiated dolerite sills and banded iron formations.

Gold deposits occur in greenstone terranes of all ages, but are more abundant, in terms of total gold content, in Archean terranes. Examples include Campbell-Red Lake and Hollinger-McIntyre in Canada as well as Kalgoorlie in Western Australia. However a significant number of world-class deposits are also found in Proterozoic and Paleozoic terranes. International examples of this sub-type of gold-deposits include Mother Lode-GrassValley (U.S.A.), Mt. Charlotte, Norseman and Victory (Australia). The best Canadian examples are Sigma-Lamaque (Quebec); Dome and Kerr Addison (Ontario); Giant and Con (Northwest Territories); San Antonio (Manitoba); and Hammer Down (Newfoundland).

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