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.