Geology & GeoMetallurgy

Geology & GeoMetallurgy 2017-03-23T09:44:23+00:00
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Exploration Methods Porphyry Copper (22 replies)

Bill Fraser
1 year ago
Bill Fraser 1 year ago

What the best geophysical survey method for exploring and discovering a porphyry copper deposit.

After doing an extensive literature survey at the best University library I find these exploration methods:

  •  The radiometric survey its important to discover the alteration ( K U T).
  • IP survey can used for the sulfide mapping not for porphyry copper.
  • 3/ spectral analysis very important.

What more? Thanks

Alan Carter
1 year ago
Alan Carter 1 year ago

If you are trying to discover porphyries, grade is going to be irrelevant until you have half a dozen holes. That could be 3-5 years away.

Mohamed, to answer your question, the most useful geophysical techniques for discovery are radiometric and magnetics. Most porphyries display K metasomatism and even if it has been overprinted and covered in vegetation, this will commonly show up as a radiometric high. Spectral radiometric permit discrimination of K,U,T which facilitates analysis of intrusive phases. Magnetics help in identifying intrusive phases, magnetic destructive alteration and magnetite mineralization. Note that none of these are magic bullets, there is a lot of K rich granite out there which has no porphyry association whatsoever.

Which brings me to Deon's comment. Geology is the best tool in porphyry exploration. Besides the best University in your country, look through company reports, aid programmes. If you are checking out a virgin province, look for references to large areas of K silicate and sericite alteration, silicification, quartz veining and stockworks. Look for artisanal precious and base metal workings, particularly where these form a radial pattern around the margins of a 1-4 km diameter zone, which may be irregular in shape. Look for references to tourmaline and stockworks/brecciation.

Keep building up your regional and district scale pictures as you go and you will be able to target your radiometrics better. Or, better yet, send out teams of a junior geo and a field to drainage sample, and make sure they have a clear idea of what sort of float to pick up. If you can afford to send them to a porphyry mine to look at one of these systems in the flesh so much the better.

If you can, hire a consultant to give the field crews a short course (3-5 days) on what to look for. 

NOTE: there is very little correlation between profile and ability for these dudes; review their CV without reading the name. Look for someone who has ideally done a lot of porphyry exploration in similar geomorphology to your area, some time in porphyry mines and, ideally, a modest amount of time in academia.

Bob Mathias
1 year ago
Bob Mathias 1 year ago

I like and agree totally with your comments and approach. A combination of radiometric and magnetic is the best option. However, the geology has to be taken in to consideration when interpreting the magnetic survey because either a positive or negative anomalies may be equally indicative.

John Koenig
1 year ago
John Koenig 1 year ago

In porphyry copper exploration, topography of the region is very important. Mining analysis also helps to find the best method. Based on these two parameters may be combined with other parameters were good results. I quite agree that a good adviser to avoid wasting the costs. There are many parameters to choose the best method.

Victor Bergman
1 year ago
Victor Bergman 1 year ago

Induced polarization (IP) surveys are a valuable tool for base-metals and gold exploration. A junior exploration company can effectively narrow down drill targets on a vast exploration property with a well-planned IP Program, and an already operating miner can locate more metal deposits in the area by conducting an IP survey.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

A combined of magnetic + IP / resistivity (conventional or 24 bits) survey is a good option. If the system is in outcrop then radiometrics could be considered as well.

JohnnyD
1 year ago
JohnnyD 1 year ago

When exploring for porphyry systems in arid terrain I have always found geophysics to be costly and of limited effectiveness.

If doing grass roots exploration to identify porphyry systems, I start by defining my target region based on known tectonics and metallogeny. Then, to get targets of a size which can be ground checked in a day, I get a spectral study done using the imagery of choice (I use ASTER, but I have seen great results with Landsat and heard good things of SPOT) which will allow me to pick up quartz/sericite alteration and iron oxide anomalies. These can then be ranked for follow up by other geologic data. This is much cheaper than a mag/rad survey.

Although I have seen great radiometric and magnetic footprints over known porphyry deposits, I cannot recall a single case of a green-fields porphyry discovery being made by geophysical tools.

I am also very dubious about using IP in porphyry deposits. Again, I have seen great results over known targets, but I have also seen horrendous results when IP anomalies have been drilled with insufficient geological understanding and if you have that geological understanding why do you need the IP? Note that IP is not going to discover a porphyry system, it is a target evaluation technique.

The problem is that IP is just mapping sulphides. There are many porphyry systems where the most sulphide rich parts contain little or no copper. Further, sericite is quite chargeable, so it is even possible to get an IP response where there is little in the way of sulphides.

If you have outcrop, spectral analysis followed by mapping should get you a drill target; if not you are really looking at spending a lot of money on mag followed with IP and even more money to drill on a slim chance of success.

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

From my experience, the best tool for Porphyry Copper systems exploration is a good and accurate geological surface mapping, that pick up and compile lithology, hydrothermal alteration, mineralization and structural tectonics. After this parameters have been understand you can choice any of the methods recommended above.

Helena Russell
1 year ago
Helena Russell 1 year ago

While I agree with all you say, the original question was about discovery, rather than evaluation. To get into a mappable area, you need to divide the prospective region into "areas we will map and areas we will not map" I would suggest that, whilst radiometrics will do that job, spectral analysis will do it better and cheaper.

David Kano
1 year ago
David Kano 1 year ago

It refers to the spectral analysis of satellite images. They provide indirect information on existing alterations. It must be very careful to use the mapping obtained from these images. We can make many observations, since the topic is too broad, questions should be improved.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

We use both IP and RS methods in Iran but it relate to the geological particulars. For example, if there are clay minerals or barren silisic veins then the results obtained by these methods are wrong. You can using the magnetic airborne data only for determination of shallow bodies and faults but geophysical data should be confirm with other data such as remote sensing and field observations. Furthermore, alteration zones with sub-volcanic data are more effective for discovery of a porphyry system in reconnaissance stage.

JohnnyD
1 year ago
JohnnyD 1 year ago

What are you thinking about porphyry target blind or cover by volcanic flows dacite and rhyodacite. The IPR and IP not work well because the volcanic are barren, is difficult crossing. In the other hand we should have than a good regional geological surface mapping,

Tony Verdeschi
1 year ago
Tony Verdeschi 1 year ago

IP method have good result. You can see other target Porphyre.

Bob Mathias
1 year ago
Bob Mathias 1 year ago

The deeper part of OT that plunges to the NE (Hugo Dummett deposit) was discovered and characterized by deep penetrating IP. IP is definitely useful for porphyry exploration but it is not a regional tool (could be, but would be staggeringly expensive). IP makes sense once you have a porphyry system mapped and you need to identify disseminated sulfide drill targets.

John Koenig
1 year ago
John Koenig 1 year ago

First of all we are assuming that you are in the right place related to the geological environment for the generation and occurrence of Cu-Au, Cu-Mo deposits. geophysics Mag + IP (chargeability and resistivity) in a regional scale is very usefu lin in order to outline targets, but at the end of the day the detailed regional / local mapping (hydrothermal alteration, lithology, structural control) + drilling are the best tools for an effective discovery of a porphyry deposit

Bill Fraser
1 year ago
Bill Fraser 1 year ago

For large-scale exploration: Magnetic (high or low) for intrusions, structure & alteration; EM mapping for alteration, low grade mineralisation and definitely supergene mineralisation. Sometimes you can use Gamma-spec for alteration? and Gravity or gradiometry for structure & intrusive? If terrain allows use fixed wing. IP has good depth but is very slow and expensive and sometimes too sensitive to low grade mineralisation, so you have to look at the rock characteristics in your area to pick the most appropriate method.

Helena Russell
1 year ago
Helena Russell 1 year ago

I am curious why people think mag is useful for alteration mapping. Yes, a mag low might be magnetite destruction, but a lot of geology has mag lows without alteration. Similarly gamma ray spectroscopy might map K alteration, but also maps high K unaltered rocks, e.g. rhyolite. EM can map silicification, but that is only one aspect of the bigger alteration patterns. In permissive environments (i.e. good exposure) it makes much more sense to directly map alteration mineralogy with optical spectroscopic methods. And at a cost similar to mag.

Seems to me to be a "use the same tools we have always used (since the 1960's!)" thinking.

Maya Rothman
1 year ago
Maya Rothman 1 year ago

Spectral mapping for the alteration mineralogy is critical: airborne hyperspectral if the exposure is good (e.g., Chile & Peru) and definitely a field/core shed spectrometer. It is also critical to quantify the spectroscopy by doing clay and chlorite feature analyses.

David Kano
1 year ago
David Kano 1 year ago

The original question was regarding geophysical tools so, I answered it. 

No, geophysical tool is diagnostic so I suggested a better approach was to use cheap spectral analysis followed by mapping.

I have spent an inordinate part of the last 30 years researching the history of porphyry, epithermal and IOCG discoveries and, would define discovery at two levels:

One, the point at which the exploration team got the area to be explored down to something you could work on with day trips out of a central point;

Two, the tool which targeted the first hole to hit significant mineralisation.

I am interested in the comment confirming that Oyu Tolgoi was a discovery achieved by IP as I have heard that claim made on multiple deposits only to discover that IP proved very handy for interpolating between holes, or even extrapolating mineralisation out of holes, but I have never before confirmed that the discovery hole was actually targeted by IP. In most cases where this is claimed it is commonly found on closer analysis that hte IP proved useful for interpolating mineralisation between holes and projecting extensions below holes.

Painting the landscape with remote sensing tools of any sort is much in vogue, particularly if your GIS person has an artistic flair and can create lurid maps by overlaying them, but it is no substitute for a dirty fellow with a hammer running around mapping what he sees; it is usually much pricier. 

Maya Rothman
1 year ago
Maya Rothman 1 year ago

 I consider reflectance spectroscopy either with a portable instrument or from the air to be a geophysical method - I gave a paper to that effect at Exploration 2007. It is measurement of electromagnetic radiation and shares all of the data reduction and inversion procedures of any other geophysical technique.

A bit off topic.... I get a bit frustrated with the term "discovery". I think it is often used in the US Mining Law sense that there is a eureka moment. The reality is that it is more of an evolution - prospective geology -> indications of alteration -> alteration zoning -> indications of mineralization -> ore grade samples -> ore grade intercepts -> ore grade volume. All of throught the evolution the geologists involved are getting a better handle on the scale of the system. So where was the discovery? I argue that discoveries are only made in the heads of geologists; it is the growing "gut feeling" that something is different and big. End of off topic.

Using the classical definition Oyu Tolgoi was certainly not discovered by IP, it was discovered by Hugo Dummett. The only reason it became a mine was due to his dogged persistence - see my definition of discovery. Far NE (Hugo Dummett Mine) would meet your definition of what IP contributed - it extended mineralization. However the IP did turn a moderate size ore body into a world class deposit. The drill holes you commented on were drilled AFTER the IP was done and correlate because they were drilled BASED on the IP.

I wholly disagree with your last point on remote sensing. I think you would need to see some airborne hyperspectral alteration mapping over porphyry and epithermal systems to convince you that field mapping with a comprehensive and accurate alteration map is better than just using a hammer and a hand lens. 

David Kano
1 year ago
David Kano 1 year ago

I would have to disagree with you on how discoveries happen. My experience is that you can have hordes of well meaning people all subsuming their I to the team, all doing things right (and two cases of 16 years and 8 years come to mind). Then someone comes in and does the right 5 thing and something promising becomes economic, or nearly economic.

It is very rare that a mineral discovery is built up like a pyramid, although listening to the number of people who have claimed to find certain projects you would conclude that exploration transport should be organised using Mil 25s and double decker buses.

Bill Fraser
1 year ago
Bill Fraser 1 year ago

As the proverb tells it: "Success has many fathers but failure is an orphan". But no matter how many people claim a discovery, it seems there is no ideal geophysical method to locate porphyry coppers.

Perhaps infra red spectroscopy (using for example ASTER imagery) is one of the best regional tools but it has little use over dense vegetation (though I've used it to define vegetation anomalies on occasion).

A question though: is reflectance spectroscopy purely a geophysical tool, or is it best described as a hybrid, falling into the optical remote sensing camp which is taught now and degrees awarded as a separate subject? 

Maya Rothman
1 year ago
Maya Rothman 1 year ago

I think you hit on the problem - remote sensing generally lands in the geography department at many universities and is often inter-tangled with GIS. And for classic 4 band imagery (B,G,R,IR) that is probably appropriate. It is only when you get into ASTER, Worldview 3, and hyperspectral that things change and advanced tools like signal processing methods come into play.

There are few universities that grant degrees specifically in remote sensing (the only one that comes to mind is ITC). I think most of us that do remote sensing for mineral exploration ended up hand-rolling our geologic/geophysical graduate degrees by taking electives from a variety of departments (geography, EE, physics, mathematics) that filled in the gaps. The goal is to be an economic geologist with remote sensing expertise.

The reality is that in many geophysics departments, seismics is geophysics. The methods we routinely use in mining are often treated as a side issue.

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