(Bench gravels often provide visual clues.)
Getting a handle on the gold possibilities contained in bench gravels isn't always easy, but these visual clues and other assorted and sundry tips may help.
Although some "experts" will take issue with this, it's my contention that certain visual clues can help you find which bench gravels or sections of benches hold the best gold potential. Remember, for what it's worth, I'm speaking here from a small-scale mining and prospecting standpoint without the benefit of advanced degrees in mining engineering or geology.
Here are some potential visual clues that may lead you to better bench gold:
1) Gravels that are either moderately or strongly consolidated (or that display a mix of the two).
First off let's discuss what consolidated means in this context. Essentially I'm referring to how tightly packed the gravel and rock contained in a given bench or bench section are. Although each bench gravel location is unique and some are better than others in the placer gold they contain, I can tell you this for certain: I've never found much gold in benches that were loosely consolidated, especially if they contained little in the way of what I term good-looking "dirt" and instead held lots of blonde or "blow" sand. If a bench crumbles easily under the single blow of a miner's pick or you can shovel it without much problem, then I'd classify it as loosely consolidated as well. Now this doesn't mean loosely consolidated benches can't contain gold...they do. But in my experience the best bench gold recoveries I've made came from moderately or strongly consolidated gravels. (In other words, just like every other aspect of small-scale gold mining you're not going to get off easy working bench gravels!)
2) Gravels that display a range of sizes and shapes, as well as larger rocks or boulders.
This is a very important visual clue in all aspects of gold prospecting and mining and not just for benches. The best benches (like better gold ground everywhere) will contain a wide assortment or variety of gravel and rock sizes, including those that are well rounded as well as those that are coarser or angular. I can't think of one good bench I've worked that contained gravel and rock that were all close to the same size, shape, or level of wear or coarseness. (You "newbies" out there take note and file this info away for future reference.)
3) Gravels that indicate the presence of natural or artificial oxidation.
Most of you already understand the relative importance of iron sulphides and other forms of oxidation in the gold mineralization process. In a similar manner, the presence of rusty red or reddish-orange streaks, layers, or rocks in bench gravels can often be a signpost to gold (and good gold, for that matter). Laterite or other iron-rich soils are often a giveaway in fracture or contact zone regions, as well as in many other placer or lode mining contexts. What's artificial oxidation? The rusting out of man-made iron objects or pieces of those objects (nails, hinges, nuts, bolts, old cans, whatever...) and subsequent iron staining of the localized area(s) around those objects. Keep a sharp eye out also for the fusing or clumping together of these sorts of objects in benches or bench sections because in some instances there'll be gold within that "clump."
Gold Panning Kits
Yep, I hear some of you right now. "Well hell, if there's man-made iron pieces in those bench gravels then they've probably been worked already and aren't going to be worth a shit." Reasonable conclusion, by the way. But worked already or not, I'm here to emphatically state that if you walk away from visual clues like this one in benches (or in any other small-scale mining context) you're going to end up crying the blues and wishing you'd listened to me. Some of the best gold I've found over the course of nearly 36 years was found by following this oxidation visual clue, man-made or not. In benches or elsewhere, Ignore it a your own risk. (I can't be any clearer than that.)
(Slight oxidation is apparent in this bench.)
There are more visual clues to come, so stay tuned.
(c) Jim Rocha (J.R.) 2014
Questions? E-mail me at firstname.lastname@example.org