GENERAL COMMENT ON QUARTZ "REEFS" (VEINS)
Just a general comment about what I have found to be a common misconception. I often see people refer to quartz "intrusions" and commonly it appears to be that they see quartz as some sort of plastic or "gooey" thick fluid that "Intrudes" the surrounding rocks. That is never the case.
What you see now as a quartz vein was typically some sort of fracture in the rock, through which hot water was pumping. It was as thin as tap water but a lot hotter (most commonly 200 to 350 degrees Celsius for the fluids that deposit gold). It could be salty or very dilute (eg over 30% dissolved salt in some Cloncurry and Gawler region deposits. more like 6-8% dissolved salt in much of Victoria and NSW and the eastern Goldfields region of WA). For comparison, sea water is 3% salt. Often just good old sodium chloride that you put on your fish and chips. And it was commonly under significant pressure. It would commonly stink of sulphur in many cases. It would be 5-10 km below surface in some cases where it deposited its dissolved silica as quartz, only a few hundred metres below surface in others. The geysers and hot springs we see at surface in north island NZ are a surface manifestation of one type of such fluids that reach surface, usually boiling as they reach the shallow (low pressure) environment close to surface. The "sinters" and silica terraces that we see around hot springs mostly consist of silica (fine-grained quartz or opal - coarse quartz crystals usually only crystallize at greater depth).
The fracture might start off very narrow - the hot water comes pumping in and quartz (with or without gold) gets deposited on the walls of the fracture. Sometimes its interior between the quartz on its walls could be an open space through which much fluid was pumping towards surface (often resulting in "blows"), but in many cases the first lot of quartz that crystallized would seal the fracture, and the fracture would split open again because of another pulse of hot water trying to find its way to shallow depth through the same fracture - and it is easier to break an existing fracture open again rather than to create a completely new fracture. .
So quartz veins are not solidified toothpaste - they are solid deposits that precipitate mostly on the walls of fractures from hot water. Why are they white when most single perfect quartz crystals we see are transparent? Usuaaly because of all the tiny fluid and gas bubbles trapped in the quartz as it solidified, and because of fracturing that occurred later. Why are some black? Usually radiation damage from tiny amounts of uranium trapped in the fluid, or oil.
In general all quartz veins in one area formed at much the same time (there are exceptions but they are rare in, say the central Victorian goldfields)). Looking for different "types" of quartz in an area is usually fruitless in such areas because they all have equal opportunity to contain gold. However iron-staining of quartz within an area may indicate more chance of gold than pure white quartz of the type seen in big quartz blows. Why? Because gold often occurs with other sulphide minerals (pyrite, arsenopyrite, stibnite and copper, lead and zinc sulphides). These other sulphides weather near surface to iron oxides that stain or encrust the quartz. This is commonly the only way in which looking for one "type" of quartz rather than another can be fruitful.
There are chemical reasons for all of these things, for anyone interested. Gold does not dissolve in pure water, it has to attach itself to sulphur or chlorine for the water to transport much gold, and then usually only at elevated temperatures. Gold is often deposited each time a fracture breaks open (sudden pressure release that causes the gold to detach from the chlorine or sulphur that is transporting it in the hot water). Gold rarely deposits in large quantities in wide open fractures through which the hot water can continue to pump continuously (instead only quartz will dump out to form quartz blow but without much gold). A slight drop in temperature is sufficient to cause quartz to crytallize out, and temperatures in such water-filled fractures tend to decrease upwards towards surface). Gold prefers to occur on the margins of veins where things like carbon-rich or iron-rich wallrocks help deposit the gold (most people know that graphitic slate or ironstone or magnetite-rich dykes are often a good wallrock for gold deposits). Also, refracturing and pressure drop will usually occur along the margins of a vein, not in its centre, the edge of a vein in contact with wallrock is a better zone of weakness than the centre of a quartz blow, so re-fracturing occurs there..
