Key to Finding Gemstones in Granite and Rhyolite
Feb 26, 2014 20:31:26 GMT -5
rockjunquie, Mattatya, and 1 more like this
Post by 1dave on Feb 26, 2014 20:31:26 GMT -5
An extension of Geology for Rockhounds.
The only difference between granite and rhyolite is rhyolite made it to the surface and cooled quicker.
Usually it's pretty boring stuff. Feldspar, quartz, Mica, amphobole, over and over again. Perhaps a change in crystal size from 1/4 inch to 1 inch.
BUT scattered at random between those crystals are many valuable atoms. Way to few to be worth trying to extract them - UNLESS Mother Nature did it for us!
She built Aquamarine and other Beryl crystals, quartz and smokey quartz, topaz, amazonite, and others right there inside the rocks.
Ever hear of Graphic Granite?
as granite crystallizes around vugs some feldspar crystals will grow into long needles that point straight back to the gem bearing vug!
Gas and fluids that flowed through the rocks were our friends.
They selectively gathered up specific atoms like beryllium, molybdenum, tin, silver, gold, whatever was available, tried to carry them to the surface, then, stopped by the impermeable cap where granite cooled and crystallized by contact with the above earth, gases like fluorine broke structures down into new minerals like mica called greisen.
This formed space - vugs - where large crystals could grow. That is where the good stuff is!
the cupola concept of mineralization.
From USGS PP 608-A by CC Howley page A-23
The term "cupola" was originally used by Daly (1911) for upward protrusions from the roofs of large igneous intrusives, and it has also been used, as here, for small outlying igneous bodies which probably connect at depth with larger masses. Many geologists, among them Butler (1915), have observed that ore deposits tend to be clustered around these upward projections rather than within the deeper granite. This has been found particularly true of the greisen deposits, as recognized many years ago by Ferguson and Bateman (1912). This type of ore occurrence has led to the concept that volatiles and metals tend to concentrate in the cupolas and form the clustered ore deposits (Kennedy, 1955, p. 496-498).
Page 9:
GREISEN
Although a few greisen bodies have been described as igneous rocks of exceptional composition, the great bulk of greisens are altered rocks formed from granitic hosts. They are characterized by distinctive mineralogy, trace-element content, and rock association. The most common variety is a granular rock composed dominantly of quartz and lithian muscovite or related mica.
Greisen also contains topaz and fluorite, and locally either of these minerals may predominate and form, for example, topaz greisen. The characteristic trace elements are lithium, molybdenum, tin, and tungsten, which occur with more common trace elements such as copper, lead, and zinc. Greisen is associated with granitic intrusives, particularly with late highly acid differentiates, many of which share the trace-element suite of the associated greisen. Evidently greisens form at temperatures above the stability range of the clay minerals characteristic of mesothermal or epithermal alteration
(Sainsbury, 1960).
GREISEN is a crystalline rock consisting of quartz , mica and clay found in pockets in porphyritic granite.
From the Encyclopaedia Britannica:
greisen, modification of granite, an intrusive igneous rock; it consists essentially of quartz and white mica (muscovite) and is characterized by the absence of feldspar and biotite. The rock usually has a silvery, glittering appearance from the abundance of layered muscovite crystals, but many greisens resemble a pale granite. The white mica mostly forms large plates with imperfect crystalline outlines. The quartz is rich in fluid enclosures. The most common accessory minerals are tourmaline, topaz, apatite, fluorite, and iron oxides; altered feldspar and brown mica also may be present.
Greisen occurs typically in belts or veins that intersect granite, and it passes into granite at the outer edges of these. The transition between the two rocks is gradual, indicating that the greisen has been produced through alteration of the granite by vapours or fluids rising through fissures. These vapours or fluids must contain fluorine, boron, and probably lithium, because these elements are contained in topaz, mica, and tourmaline, the new minerals of the granite. The change is induced by the vapours set free by the granite magma as it cools.
Greisen is closely connected with schorl, both in its mineralogical composition and in its mode of origin. Schorl is a pneumatolytic product consisting of quartz, tourmaline, and, often, white mica and thus passes into greisen. Both of these rocks frequently contain small percentages of cassiterite (tin oxide) and may be worked as ores of tin; the central filling of the fissure often contains much wolframite, the chief ore of tungsten.
So what does it all mean?
Imagine a blob of molten granite containing volatiles (water, carbon di-oxide, fluorine, chlorine, etc) rising through surface rocks. The lighter volatiles tend to float up to the top of the blob, taking special minerals with them and concentrating them.
Being "wet," they begin altering the granite around them, converting them into greisen and providing space for larger crystals to grow.
The only difference between granite and rhyolite is rhyolite made it to the surface and cooled quicker.
Usually it's pretty boring stuff. Feldspar, quartz, Mica, amphobole, over and over again. Perhaps a change in crystal size from 1/4 inch to 1 inch.
BUT scattered at random between those crystals are many valuable atoms. Way to few to be worth trying to extract them - UNLESS Mother Nature did it for us!
She built Aquamarine and other Beryl crystals, quartz and smokey quartz, topaz, amazonite, and others right there inside the rocks.
Ever hear of Graphic Granite?
as granite crystallizes around vugs some feldspar crystals will grow into long needles that point straight back to the gem bearing vug!
Gas and fluids that flowed through the rocks were our friends.
They selectively gathered up specific atoms like beryllium, molybdenum, tin, silver, gold, whatever was available, tried to carry them to the surface, then, stopped by the impermeable cap where granite cooled and crystallized by contact with the above earth, gases like fluorine broke structures down into new minerals like mica called greisen.
This formed space - vugs - where large crystals could grow. That is where the good stuff is!
the cupola concept of mineralization.
From USGS PP 608-A by CC Howley page A-23
The term "cupola" was originally used by Daly (1911) for upward protrusions from the roofs of large igneous intrusives, and it has also been used, as here, for small outlying igneous bodies which probably connect at depth with larger masses. Many geologists, among them Butler (1915), have observed that ore deposits tend to be clustered around these upward projections rather than within the deeper granite. This has been found particularly true of the greisen deposits, as recognized many years ago by Ferguson and Bateman (1912). This type of ore occurrence has led to the concept that volatiles and metals tend to concentrate in the cupolas and form the clustered ore deposits (Kennedy, 1955, p. 496-498).
Page 9:
GREISEN
Although a few greisen bodies have been described as igneous rocks of exceptional composition, the great bulk of greisens are altered rocks formed from granitic hosts. They are characterized by distinctive mineralogy, trace-element content, and rock association. The most common variety is a granular rock composed dominantly of quartz and lithian muscovite or related mica.
Greisen also contains topaz and fluorite, and locally either of these minerals may predominate and form, for example, topaz greisen. The characteristic trace elements are lithium, molybdenum, tin, and tungsten, which occur with more common trace elements such as copper, lead, and zinc. Greisen is associated with granitic intrusives, particularly with late highly acid differentiates, many of which share the trace-element suite of the associated greisen. Evidently greisens form at temperatures above the stability range of the clay minerals characteristic of mesothermal or epithermal alteration
(Sainsbury, 1960).
GREISEN is a crystalline rock consisting of quartz , mica and clay found in pockets in porphyritic granite.
From the Encyclopaedia Britannica:
greisen, modification of granite, an intrusive igneous rock; it consists essentially of quartz and white mica (muscovite) and is characterized by the absence of feldspar and biotite. The rock usually has a silvery, glittering appearance from the abundance of layered muscovite crystals, but many greisens resemble a pale granite. The white mica mostly forms large plates with imperfect crystalline outlines. The quartz is rich in fluid enclosures. The most common accessory minerals are tourmaline, topaz, apatite, fluorite, and iron oxides; altered feldspar and brown mica also may be present.
Greisen occurs typically in belts or veins that intersect granite, and it passes into granite at the outer edges of these. The transition between the two rocks is gradual, indicating that the greisen has been produced through alteration of the granite by vapours or fluids rising through fissures. These vapours or fluids must contain fluorine, boron, and probably lithium, because these elements are contained in topaz, mica, and tourmaline, the new minerals of the granite. The change is induced by the vapours set free by the granite magma as it cools.
Greisen is closely connected with schorl, both in its mineralogical composition and in its mode of origin. Schorl is a pneumatolytic product consisting of quartz, tourmaline, and, often, white mica and thus passes into greisen. Both of these rocks frequently contain small percentages of cassiterite (tin oxide) and may be worked as ores of tin; the central filling of the fissure often contains much wolframite, the chief ore of tungsten.
So what does it all mean?
Imagine a blob of molten granite containing volatiles (water, carbon di-oxide, fluorine, chlorine, etc) rising through surface rocks. The lighter volatiles tend to float up to the top of the blob, taking special minerals with them and concentrating them.
Being "wet," they begin altering the granite around them, converting them into greisen and providing space for larger crystals to grow.
