Raymond A. Kukkee on the Association of Biotite and Gold

As the result of a question somebody asked that was engaged in diamond core drilling on a gold deposit in Canada about the association of biotite and gold prompted me to put the question up to a friend in Ontario.  Mr. Kukkee has been a professional prospector in Ontario for more then thirty years, this is his answer. . .

“As far as I have seen,  and from what I've stumbled across  putzing around in the bush,  I would have conclude that   biotite mica can certainly  associated  with gold,  ( or gold can be found with biotite mica).  I found some, so  the association is most certainly not impossible.   Any time you find thick bands of  biotite in situ  suggests there is high-grade shearing of the rock structures  at hand  --and those contact structures are the most likely targets for gold.  On the other hand you can see flecks  biotite in gneissic rocks at times which doesn't seem to mean much. 

Many people think biotite is only black, but  it can be  black, dark brown, and even green yet! )     It's got a lot of iron in 
it, the green cast  can be  from chromium,  and of course biotite mica  itself  is  a silicate.   Logically, iron and silicates  are often associated with gold.

In prospecting  field work,  and being a persistent type,  I always looked at it this way, gold is where you find it, and it's most likely to be found in contact areas.   Where there are contacts showing   slickensides and schistose rocks  which are often  associated with gold  ( and/or  perhaps sulphide mineralization with  shearing)  , there most certainly can be biotite  in those structures, so why not use the reverse association as well?  It's a good indicator.   

 I would guess that direct gold/biotite association is more likely  found where there is physical metamorphism caused by  shearing,  as opposed to gold solutions being injected into biotitic structures after-the-fact ?   Maybe in the long-term scheme of things-- D-2 and even D3 events ( is there such a thing as a D3?) ,  maybe  that  could happen too ?  
 
 One of my old  low-grade gold finds had a  band  or dike  of rotten biotite (maybe  a foot across)  between  silicate/quartz  gold-bearing rocks  and a structure, (I interpreted it as  foot rock)  that had sections full of garnet ( the garnet-bearing layers were metamorphic rock).  The adjacent  biotite-bearing rock did carry some gold. 
That's about all I know about gold and biotite, I've probably forgotten more than that I imagine..LOL


It's interesting that prospectors might  overlook biotite as a barren mineral , it's not really very  exciting stuff, but if you happened to  find a big enough  deposit,   the vermiculite industry would be interested in it.  China exports vermiculite for up to about  200-$400  USD/metric tonne,  at least they were  a couple of years ago.  The key is to find the stuff without any asbestos content.   I wonder if the Chinese worry about that?   the expanded vermiculite/perlite/agricultural/construction  market is huge if you can find just  the right stuff.”

That's about it,  how's the weather down there?  It quieted down here, about -10 C,,,,stopped snowing, not bad for Nov. 27th.

Biotite as an Indicator Mineral for Gold

Introduction:

There are many minerals used for gold, but until recently I was unaware of the use of biotite as an indicator mineral for gold.  It seems however that under some circumstances that is exactly what role biotite plays.  In a minerologically diverse set of amphibolite schists coming from altered metavolcanics both amphibole and biotite in the gold bears veins.  It is felt by many geologists that any gold present has been leached out of the surrounding greenstone making it younger then the rock it is contained in, this is true of the Abitibi Gold Belt and other greenstone belts that are found throughout the Canadian Shield. 

Biotite:

Biotite is a common rock forming mineral found in abundance in both igneous and metamorphic rock. The mineral is a member of the cyclosilicate family of minerals characterized by perfect cleavage parallel to the C-axis that produces flakes or thin layers.  Its fracture is indistinct and uneven. 

• Hardness = 2.5
• Specific Gravity = 2.9 – 3.4+ a bit above average.
• Streak = white
• Associated minerals: quartz, feldspars, apatite, calcite, hornblende, garnets and schorl, (a black variety of  tourmaline.)
• Other distinguishing features = its sheets can be bent and will flex back into shape, meaning its cleavage sheets are flexible.
• Occurrences = worldwide in igneous and metamorphic rocks.
• Color: = Black to brown shading into yellow with weathering.
• Luster: Vitreous to Pearly.
• Transparency = Transparent to translucent.
• Crystal System = monoclinic; 2/m.
• Crystal Habits = tabular to prismatic; many of its crystals are pinicoid termination forming books of mica like muscovite another member of the cyclosilicate family.  It is a lamellar to granular in appearance giving gneisses and schists their characteristic glitter.
• Cleavage = Perfect in one direction.
• Fracture = Not distinct and uneven due to its cleavage.

Gold Association:

Biotite is often found associated with lode gold deposits.  Because it contains iron it might be looked on where the biotite takes the place of granular magnetite where gold is often found.  It is well known that gold colors glass red so does it also affect biotite causing the presence of gold to increase the red hue found in some biotites. 

Telling gold from fools gold!

The problem of identifying real gold in the field to separate it from fools gold often arise. There are several minerals that mimic gold in appearance like iron pyrite and weathered biotite mica that both have a gold-like appearance.

The easiest field test for gold is simple its density is 19 grams per cubic centimeter that is far denser then any of its mimics. Some genius in antiquity developed the “touchstone” test for gold based on what color its streak is when left on a dark colored stone. In practice this is usually a piece of black slate. Gold will leave a gold-colored streak on this stone its mimics will not. For ages this was the definitive test for gold until it was replaced by the “Purple of Cassius” test during 1685 by Andreas Cassius.

Another ancient test used frequently was based on the fact gold is malleable. You can bend it, and it keeps its bend. You can also bend weathered biotite, but unlike gold it quickly snaps back into shape. The other method based on malleable gold is hammering it into a flat sheet all of those minerals that mmimic gold are brittle and will break-up into a powder if hammered.

The acid test works because gold is unaffected by all strong acids except aqua regia, all the other minerals that mimic gold are affected by acids and are dissolved. A possible exception to this is weathered biotite mica.

The Purple of Cassius’s history goes back to the fourteenth century where it was described as an artist’s pigment however Andreas Cassius gets the credit for its invention. The principle behind the purple of Cassius is the ability of Aqua Regia a two to one mixture of concentrated nitric and hydrochloric acids to dissolve gold.

Just by its dissolution in aqua regia gold produces a rough-and-ready test because the gold charged aqua regia assumes a golden-yellow color. This test is not accurate enough however to be definitive. The purple of Cassius is however. Until the invention of the modern science of spectrometry developed during the nineteenth and twentieth century’s the purple of Cassius was the recognized definitive test for gold.

The test works by adding a few drops of the gold charged aqua regia to 100 ml of clean water, then adding twice as many drops of a solution of tin chloride to the solution. The tin will cause the gold to precipitate from the solution forming a colloid that ranges in color from pink thru dark purple. The darker the color the more gold is present.

The science of “spectroscopy” has supplanted most of the chemical tests for gold in the laboratory, but not in the field where because the various manifestations of the spectroscope are just to bulky. Many of the old chemical tests for gold and other minerals are still being used.

References:

The creation of color in eighteenth century Europe, Gutenburg.org,
http://www.gutenberg-e.org/lowengard/C_Chap33.html

Acid test (Gold), Wikipedia, http://en.wikipedia.org/wiki/Acid_test_(gold)

Spectroscopy, Wikipedia, http://en.wikipedia.org/wiki/File:Light_dispersion_conceptual_waves.gif