Crushed said:
One more for Goldierocks,you have me understanding strike and dip.The other geological term I struggle with is an indicator.Near Amhurst in th GT is Knotts indicator . My wife and I climbed the hill and located an outcrop of stone just below the crest of the hill.This was not my understanding of an indicator . I was expecting a seam of Quartz and slate . Was the outcrop the indicator or did we not actually see the "indicator" ?
Most reference to indicators is mythology, many are no more useful than a road sign to Bakery Hill, Ballarat, but a minority have substance to them. They are simply geological features that old practical prospectors THOUGHT were spatially associated with gold, usually thin features of only mm width. Some were layers of pyrite in the rock, others layers of graphite, some were quartz veins (commonly parallel to rock bedding). To give an example that illustrates why their presence can be as unrelated as a road sign, the "pencilmark" indicator at Ballarat was probably graphite (carbon), probably simply a carbon-rich layer deposited in the sea when the rock was forming (commonly with slate, which forms from mud - if there is little oxygen on the sea floor, carbon-rich material will not oxidise away, but will stay as a carbon-rich layer). But as the author Mark Twain explained after a visit to Ballarat East field, there was no direct relationship between gold and the pencilmark indicator. The indicator simply occurred in the rock a certain distance from the system of gold-bearing quartz, so if you saw it you knew you had to walk a certain distance (I think it was about 7 m west?, normally up the hillside) to find the gold-bearing quartz - it "indicated" where to go to find it. This was because the gold-quartz veins had a geometric relationship to folds, and the rocks that make up a fold therefore also have a geometric relation to the gold-bearing quartz veins. For example, one important set of quartz veins at Ballarat East ("leatherjacket" structures), always cut through the east limb of folds. Prospecting along that pencilmark indicator itself would not find you the gold, it simply indicated how far you had to walk to one side of it (there is still a Pencilmark Lane in Ballarat City, although few know its derivation).
I notice that many people on this site thing the gold-bearing quartz veins themselves were called the indicator - that was rarely the case (but since the old prospectors had no committees to standardize terminology, a few used the term that way - ie in the sense that quartz was an indicator of gold, because most Victorian gold is found in quartz veins). More commonly, the indicator was a graphitic or pyritic layer (or bedded quartz vein without gold), or a combination of two or three of these, only mm thick. The prospectors would walk these out along their strike looking for where thicker quartz veins CROSSED (cut through) these indicators. They claimed that the intersection point commonly had nuggety gold (in the quartz vein that crossed the indicator). Probably in many cases it was coincidental, and in all cases the tonnage was so tiny that they were only of interest to a hand-to-mouth prospector (or a modern recreational detectorologist, who wants coarse gold specimens rather than a multi-million tonne low-grade mine).
Unfortunately no one seems to have ever photographed how such rich patches occurred in veins cutting indicators, and no very accurate sketches were made or quantitative evidence given, only generalised sketches and descriptions (we do have museum specimens of the indicators themselves, but not of gold-rich patches where they were crossed by quartz veins). However a few geologists (eg Dunn) gave good descriptions and illustrative cartoons, sadly in many cases from anecdotes from miners rather than through direct observation, but good enough to convince me that in some cases "indicators" were sites of gold enrichment in veins that cut them. They were not of much interest to big companies, which is probably a reason why government geologists did not describe them well (eg often just a few hundred ounces out of a couple of square metres area - or much less - surrounded by barren rock). However there is a chemical reason for supposing that some were real. If the hot water that deposited quartz veins intersected carbon, the gold bisulphide ion carried in that water would reduce to metallic gold, that would precipitate out at that point. The same thing can occur with gold dissolving at surface during weathering - the cold salty rain water could trickle down and precipitate gold where it hit carbon.
Personally I am dubious about using indicators on a regular basis in prospecting, beyond the fact that areas of carbon-rich slates (even thick beds of such rock) containing quartz veins are probably favourable for richer gold. I consider such rock preferable to say, clean quartz sandstone. The Ballarat West field is localised in a bed of black slate on multiple adjacent folds, despite most of the rock in those folds being sandstone (the reason could be chemical, or simply that a layer of slate between much thicker sandstone beds crumpled much more during folding, and allowed fluids to penetrate its faulted structures). However the slate is thick (many metres) and not what old miners typically called an indicator. The suburb of Magpie in Ballarat is named after the typical appearance of gold-rich quartz on the field ("Magpie quartz") so named for the alternating layers in it of carbon layers, interlayered with white quartz (black and white like the bird).
I hope that helps.
