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Mining 101 - Rick Humphreys - Geologist

Special Guest Speaker and Geologist, Rick Humphreys, gives the community a backgrounder on mine waste and its effluents, which are some of the very real consequences of historical and potential future mining in our area. From the MineWatch Community Meeting September 2022.

Rick Humphreys, pursued his undergraduate and master’s degrees in geology at Cal State University, San Jose. Rick comes from a mining family and spent most of his career as a scientist with the State Water Resources Control Board where he worked on water assessments of active and historic mines. After retirement, Rick and was a Science and Policy Advisor to The Sierra Fund and worked on TSF's Toxic Mining Initiative.

Rick's talk is divided into three parts. The first part is an overview of mines and how they work with some terminology to help think and talk about what actually occurs underground in an operating mine. The second part will focus on mine waste, the different kinds there are and what is unique to each kind of waste. The third part is about mineral oxidization of earthen materials like mine rock and what are the results of these processes.


This expanded version of the presentation includes audience questions and answers.

00:00 - Introduction - Moderator, Greg Thrush introduces the session

04:57 – Project Update - CEA Foundation President Ralph Silberstein gives an update on project status.

12:00 – Mines – Rick Humphreys introduces the audience to key mining terminology and shows examples of how mines are typically laid out.

17:34 - Mine Wastes - Rick explains the differences between types of mine wastes, including mill tailings, heap leach waster, waste rock, and overburden. He also talks about the importance of characterizing the wastes and the complexity of sampling and testing in advance of mining and on an ongoing basis.

33:50 - Questions and Answers - Rick and Ralph answer audience questions

51:00 - Acid generation potential of earthen materials - Rick delves into the issues with acid mine drainage, gives an overview of the causes, and provides some discussion of approaches to mitigate the risks.

1:07:50 – Questions and Answers - Rick and Ralph answer audience questions

1:18:20 – Take Action - Greg describes current projects and ways to get involved in the fight against the mine.

Sept 2022 MineWatch Community Meeting
Download PDF • 7.42MB




Mine Waste

Class Acid Generation


Underground Mine Terminology

These are some terms used to describe underground mine features. Some are self descriptive while others, well, they’re head scratchers. Vein











Hanging wall




Roof bolt

Room and Pillar


Blast hole




Underground Mine Schematic #1

The schematic depicts a plane view of a generic underground mine. There should be an air shaft from the upper adit to the mountain top. Ground water draining into the mine working either flows out the portal or is pumped from the sumps.

Underground Mine Schematic #2

Appianing, E.J. et al, MOL Report Nine, 2018

The schematic depicts an underground mine in flat terrain. In this case ventilation is provided by compressors and ground water is pumped from the mine. You’ll get a better feeling of the three D nature of the mine workings under Grass Valley when we view the Empire Mine Model tomorrow.

Mine Wastes

Mill Tailings.

Heap leached waste.

Waste rock, overburden.

Waste rock and mill tailings common at abandoned mines.

The second presentation covers the types of mine waste produced by a modern mine, wastes at abandoned mines and dredge spoils. At new mines, waste behavior in its final resting place is unknown. Determining how a mine waste will behave (and what its effluent characteristics are) is the whole point of doing static and kinetic tests. In contrast, mine waste at old mines has had time to “behave” so the point of testing is to determine how it is behaving (is the mine waste or its effluent a problem).

For new mines, characterizing mine waste should be a continuous process that begins during exploration and continues as long as mining and processing ores continues.

In the end, mine waste produced falls in to three gross categories.

All three waste may be found at recent mines, waste rock and mill tailings are common at abandoned mines.

Mill tailings

Ore ground in a mill to sand size and finer.

Characterized chemically and mineralogically.

Often contain processing reagents.

Discharged to impoundments, lakes, the ocean.

Mill tailing are wastes from ores that have been mechanically ground and processed to recover the product of interest. Modern mines have data on tailings they generate.

Old gold mine mill tailings frequently contain mercury. Mill tailings often contain elevated levels of metals, metalloids, and constituents such as sulfate. There’s rarely any historical data for old mill tailings.

As with waste rock, old mill tailings are essentially an unmonitored kinetic test.

Heap leach waste

Ore ranges from 13mm to >30cm.

Often augmented with cement.

Not as well characterized as mill tailings.

Leached with cyanide on liners.

Closed in-place.

Heap leach waste is ore that has been leached with cyanide to recover gold and silver.

Heap leaching is a relatively new process (implemented in 1980).

In the early days of heap leaching, old mine waste (both mill tailings and waste rock) was “reprocessed”.

Waste rock (see overburden)

Particle size ranges greatly.

Most voluminous waste but poorly characterized.

Dumped as close to the mine as possible without (the company hopes) any need for containment.

Waste rock does not have enough of the product of interest to warrant “processing” at current market prices. Waste rock can become ore and ore can become waste rock over the course of days depending on the commodity market.

Old waste rock dumps are essentially unmonitored kinetic tests.


May be “soil” that is stockpiled for reclamation.

May be “non mineralized overburden” that is used for constructing impoundment berms, road beds, etc.

May be “mineralized overburden” which is really identical to “mineralized waste rock”.

Poorly characterized.

As you can see, there’s some ambiguity in terms.

Acid-generation mitigation measures for large masses of earthen waste are expensive so there’s incentive to minimize the mass of waste that must be mitigated at the beginning of a new mine.

However, it is much more expensive in the long run to mitigate unanticipated masses of problematic wastes.

Dredge spoils

Organic-rich, often described as “peaty”, iron sulfide rich sediment.

Fine grained.

Should be characterized before disturbed.

1. The literature of acid generating dredge spoils stresses that such material should be expected when working in estuaries. Mitigation measures stressed avoidance or placing materials under conditions that promote stability (reducing conditions)

Pre-mining sample sources

Surface grab and trench samples.

Exploration drilling samples (chips and core).

Metallurgical testing samples (e.g., pre- and post-processing samples for milling).

1. It’s always good to know what materials are being tested and when.

Mining sample sources

Ore control sampling during mining.

Head and tail samples during milling.

1. Sampling and testing should continue through mining. It should not stop when mining begins.


Testing programs result in lots of samples.

Predictions based on early testing may change if new wastes are identified as mining proceeds.

Predictions based on early testing may change if mineral recovery process change during mining.

1. Characterizing the amount and behavior of mine waste should run parallel to characterizing the amount and grade of the ore body. Data quality for both should be comparable.

Acid Generation Potential of Earthen Materials

We’re going to start with an overview of acid generation. Keep in mind that acid generation has been and still is the subject of research. As such, there are active discussion groups and a steady stream of reports for any of you who might want to pursue this subject in depth.

Why do we care?

The Bad:

Causes serious water quality problems.

Causes serious soil fertility problems.

Costly to clean up.

Clean ups usually require long-term maintenance.


Do we all agree these are bad consequences?

Does anyone have others we should list?

Why do we care?

The Good:

Enriches metal ore bodies.

Produces iron-rich soils which are fertile.

Makes cool crystals.

Remember, a “natural” process can’t be all bad, right?

What earthen materials display the problem?

Coal, base metal, and precious metal mine waste.

Iron sulfide-rich estuarine marine sediments.

Iron sulfide-rich metamorphic rocks (e.g., slates, phyllites).

Iron sulfide-rich sedimentary rocks (e.g., pyritic sandstones).

Hydrothermally altered rock.

Have I missed anything, does anyone have material they would like to add.

What is the primary cause?

Iron sulfide mineral oxidation catalyzed by bacteria.

Iron sulfide minerals responsible: acid volatile sulfide > marcasite > pyrrhotite > pyrite.

Other sulfide minerals: copper, nickel, mercury, etc. sulfides do not oxidize readily in air and water to produce acid. However, ferric iron from iron sulfide oxidation will oxidize them to produce mine waters rich in heavy metals, mercury, arsenic, etc.

I figured that you all have seen the usual reactions, and that you all know that acid generation is biologically characterized. So I didn’t reproduce them here but they are all included in the background reports on the web site. The reports also contain a lot of “research” reading for those of you whom want to delve into pyrite oxidation in depth.

Oxidation Reactions

FeS2(s) + (15/4)O2(aq) + (7/2)H2O => Fe(OH)3(s) +2SO42-(aq) + 4H+(aq) {general}

FeS2(s) + (7/2)O2(aq) + H2O => Fe+2(aq) +2SO42-(aq) + 2H+(aq)

4Fe+2(aq) +O2(aq) + 4H+(aq) => 4Fe+3 + 2H2O {bacteria catalyzed ferric iron production}

FeS2(s) + (14)Fe+3(aq) + (8)H2O => (15)Fe+2(aq) +(2)SO42-(aq) + 16H+(aq) {pyrite oxidation by ferric iron}

But I got a lot of requests to depict the reactions after the first class so here they are.

By D. Kirk Nordstrom

Bet reactions aside, the schematic convey the process much better as, once ARD gets going, all the reactions occur at the same time.

Iron sulfides

Well crystallized pyrite from Spain that’s similar in form to the example from a road cut near Carson Hill. The Carson Hill example is Cretaceous (>60 million years old) and is oxidizing slowly.

On the other hand, the mixed sulfide sample from Iron Mountain mine is older (Devonian) but is oxidizing rapidly.

What is the secondary cause?

Acid release from iron sulfate salt dissolution.

Rapid - Melanterite, rozenite, szomolnokite, romerite, copiapite, etc.

Slow - Alunite-jarosite

Dry mine waste may contain iron sulfate salts that are an easily released source of stored acid.

The Iron sulfate salts on the iron mountain sulfide hydrolyze in water to produce an acidic effluent rich in metals (Fe, Cu).

Iron sulfate salts

These are more examples from Iron Mountain Mine.

More Iron sulfate salts



Coquimbite, FeIII2(SO4)3·9H2O

These are more examples from Iron Mountain Mine.

For those of you who like chemistry, these are the ferrous and ferric iron salts that form in sequence through iron oxidation and dehydration.

Natural Buffers

Carbonate Minerals provide rapid buffering.

- Calcite > dolomite > magnesite > ankerite.

Silicate Minerals provide slow buffering (about 7 orders of magnitude slower than carbonates).

- Feldspars, olivine

Note that these are not biologically catalyzed reaction, and sulfate mineral armoring is not considered.

Acid Generation in the field

Copiapite-group minerals growing

on pyrite, Iron Mountain, CA

Iron sulfate salts on a road cut.

The same minerals that form in Iron Mountain mine may sometimes be found in iron sulfide bearing outcrops and road cuts. These minerals indicate that acid generation is occurring. These minerals also wash away easily.

Summary: All Iron sulfide minerals generate acid “Sooner” or “Later”

“Sooner” causes serious water quality problems because a lot of acid is generated over a short time. Natural neutralization or assimilation cannot keep up.

“Sooner” results from: low crystallinity, high surface area, impurities (e.g., arsenic in pyrite), lattice defects, strong oxidizing conditions.

“Later” causes no or limited to water quality problems because acid is produced over a time span sufficient for neutralization or assimilation by the environment.

“Later” results from: high crystallinity, low surface area, high purity, defect-free lattice, reducing conditions.

In a general sense, all iron sulfide generates acid. The trick is to find out if the generation rates will result in a low pH effluent, and if mineral oxidation in general will result in effluents that are problematic from a water quality and human health standpoint.


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