Colluvial Placer Deposits
Most of us are small-scale placer miners. Although some of us may have ventured into the lode gold or hard rock arena (I have at various points in time), prospecting and mining for placer gold is where most of us cut our mining teeth and still remains our primary focus. In this post I'll be talking about a type of placer deposit that most of us aren't familiar with...the colluvial deposit.
Nearly all of you are familiar with the standard types of placer gold deposits. These include:
- alluvial (deposits found in rivers, streams, creeks, gullies, and washes)
- elluvial (deposits that remain close to their eroding source on slopes or hillsides and that haven't yet transitioned completely downward to various alluvial watercourses or drainages)
- eolian (wind-borne deposits of fine gold)
- beach (typically fine gold deposits mixed in with heavy beach sands and gravel)
- marine (offshore gold deposits directly adjacent to beach deposits or farther out at depth in seas and oceans)
( A good ol' "standard" alluvial placer.)
Colluvial (sometimes called "delluvial") placer deposits are a rarer breed, although they do exist in many locations throughout the world. These types of deposits were generally formed from old alluvial placers whose sources of gold and its deposition were "cut off" due to any number of factors, including uplift, being buried over time, or other significant geological forces that altered their basic alluvial nature. Additionally, colluvials may represent older or even ancient watercourses where the original gold-bearing streambed was left "high and dry" as streams or watercourses changed their paths. Now I know what you're thinking here. Colluvial placers sound a lot like the much-touted Tertiary gold channels (ancient rivers of gold) or even more recent bench gravels. In one respect you're right. But the truth of the matter is that colluvials don't have to be ancient and they aren't typically manifested as gold-bearing bench gravels.
(Imagine the left-hand stream in this diagram as a "colluvial" placer where there is no modern stream flow and that Ma Nature has covered up to a great degree. It could be "bonanza" time!)
Not Always Well-Versed
You see the key to understanding colluvial placers is the prefix "co." As in co-located. In other words colluvials are forgotten or hidden gold placers that are located near or adjacent to current streams or washes. The terms "forgotten" or "hidden" are worth addressing here so let me give it the good ol' college try. Forgotten means either deliberately bypassed or just simply missed or unrecognized by the old timers. Colluvials, whether in wet or dry placer environments, are usually bone dry. If the old timers were focused on getting good placer gold from existing running streams or readily accessible and known dry washes, they weren't likely to veer from that path. Additionally, it should be noted here that most of the would-be miners participating in early gold rushes throughout the world were not always well-versed in the finer aspects of geology and associated geological "quirks." In fact, most new arrivals to the goldfields of the past didn't know their asses from a hole in the ground when it came to prospecting or gold mining. The "hard cores" learned and learned well over time though.
Untouched or Unnoticed?
Anyway, colluvials can fall into this "quirk" category at times since they aren't like strobe lights announcing their presence in the middle of a pitch-black night. If colluvial placers were nearly or completely hidden from decades or centuries of stream shifting or buried underneath more recent topsoil layers and plant growth, then the likelihood is that even those flint-eyed old timers failed to realize they existed close by to the stream or washes they were currently working. Granted, some colluvials were discovered by the old timers either by happenstance or keen observation, but in general many were left untouched or went unnoticed.
(Most new arrivals to early gold fields throughout the world didn't know their asses from a hole in the ground when it came to what to look for and how to go about it. From a colluvial placer standpoint, this might work in your favor.)
There are some visual clues to spotting colluvials and here are three of those:
- Fluctuations in terrain such as barely discernible "ditch-like" dips or depressions that extend in a generalized parallel fashion to current stream or wash courses.
- Rounded or stream-worn boulders, rocks, or gravels just "peeking" above the surface of terrain near or adjoining current stream courses.
- Highly overgrown or nearly covered gravel "runs" that extend for some distance along or near the path of current gold-bearing streams or washes.
(Is there a colluvial nearby?)
Either Way, Good Sampling is Needed
If you think you've come across a colluvial placer, then you want to sample that streambed or wash in EXACTLY the same way you'd go at things in the existing stream or wash in the area. The hydrology characteristics and gold deposition physics that were once in play in a colluvial are no different than those in the most recent stream or wash you're familiar with. So go at things in similar fashion. As I recommend again and again, thorough sampling is a must when prospecting a stream or wash...modern or colluvial. Australian miner Jack laid out an excellent sampling pattern to employ in a previous post.
Best of luck out there!
(c) Jim Rocha 2018
Questions? E-mail me at firstname.lastname@example.org