Drilling like an oiler is the compelling new model for lead-zinc discovery

A bold new exploration model for sediment-hosted lead and zinc in northern Australia suggests the right approach for mining companies is to act like they are searching for hydrocarbons.

This means giving more weight to exploration around inversion-related structural traps such as doubly-plunging antiforms into which the mineralising fluids may have migrated.

The new model, an adaptation of earlier ideas espoused by Graham Broadbent for the Century deposit on the Lawn Hill Platform, was presented by its main author, ANU’s Associate Professor George Gibson, on 11 May at a conference in Mt Isa, hosted by James Cook University’s Economic Geology Research Centre. The paper behind it was published last year in Solid Earth and can be found here.

The scientific study behind the new model is exhaustive and compelling and can’t be done justice in a few hundred word, but the key points are as follows:

  1. the lead-zinc deposits of the Carpentaria zinc belt were deposited at a time of crustal shortening and basin inversion, not extension as per the prevailing models and accepted thinking in the exploration industry;
  2. tectonic forces, possibly stemming from a collisional or far-field deformation event, pushed metal-rich fluids up conduits such as faults into an appropriate permeable layer where the fluid followed stratigraphy and travelled up-dip away from the fault conduit until it encountered a carbonaceous shale, black dolomitic siltstone or similar hydrocarbon-rich rock unit where the metals were precipitated as sulphides;
  3. the metal-rich fluids did not necessarily precipitate out in or around the fault zone when they hit shales or carbonaceous rocks (which is the current model, followed so unsuccessfully);
  4. Instead, the fluids were driven up-dip as far as they could go, following hydrocarbons into a sealed reservoir or similar trap, where they interacted with hydrocarbons or sour gas (hydrogen sulphide) which acted as a reductant;
  5. As in previous models, mineralisation occurred concurrently with sedimentation but under different conditions as the host rocks to mineralisation were already being folded and inverted before deposition concluded.

George told Precompetitive Review he had been trying to encourage industry to look at the problem from a different perspective. “We have had some success, but there are a lot of entrenched views in the literature and the geoscience community.”

He said the new model grew out of several years research at Mt Isa, working very closely with former colleagues from Geoscience Australia and others who knew about sequence stratigraphy.

“I had some familiarity with sequence stratigraphy, but I learned quite a lot from them. It was the combination of putting the sequence stratigraphy together with the structure that suddenly led me to made to certain questions. For example, how come if you look at sequence stratigraphy in a particular way, the mineralisation is concentrated in the post-rift if not the syn-inversion fraction of the sedimentary basins? If you ask that question then you have to wonder what is driving this.”

George’s thinking about the model drew inspiration from some unexpected sources, which highlights the importance of looking outside of your own domain for ideas.

“I used to be involved with UNESCO’s international geological correlation program. It gave me an opportunity to look across the full breadth of the earth sciences, particularly in the geodynamics sphere. And when I was going to various conferences, I made a point of sitting in on parts of the conference that were not directly related to what I did. So I used to sit on some of the talks by people from the petroleum world, including Petrobras and a lot of people who worked in the North Sea . They were talking about inversion structures, where a lot of the oil is found in the crests of antiforms, very similar to some structures we see in seismic data for northern Australia.”

He said petroleum companies had been aware for a long time that they found metals in the brines associated with their oil fields, but “because we have hard-rockers and soft-rockers, the twain never shall meet.”

George said the Century deposit is an important case study in the commonalities between lead-zinc deposits and petroleum systems. Carbonaceous host rocks at Century served as both a reductant and basin seal during the influx of more oxidized mineralizing fluids, forcing the latter to give up their lead and zinc metal. 

Another very interesting contributor to his thinking about the model is the correlation previously highlighted by colleagues at Geoscience Australia between “dated” bends in the apparent polar wander path for northern Australia and the ages of major lead-zinc mineral deposits at Mount Isa and elsewhere in the Carpentaria zinc belt. The ages of big deposits coincide with abrupt changes in the direction of polar wander, which logically coincides with a change in plate or plate margin forces.

George said the link between biogenic activity, hydrocarbons and mineral deposits was becoming more apparent.

“If you haven’t got the right biota, you will not get sulphate reduction. The presence of hydrocarbons and/or hydrogen sulphide gas is a prerequisite for the precipitation of some metals. There is clearly a link between biology and mineralisation.”

He said we don’t know if the proposed model will deliver fresh discoveries because the idea has not been fully tested “but the reaction I got at the Mt Isa conference is quite a few companies that are sympathetic to the idea. They have tried other models for mineralisation, which hasn’t given them any results, so they now they are looking at alternatives.

“This model is purely for lead-zinc, not copper-gold, although we think other fluids may migrate through the rocks in much the same way. Maybe the fluid source is different, with copper gold it more likely derives in large part from adjacent granites,” George said.

The EGRU conference at Mt Isa also featured a presentation with complementary research and ideas by Karen Connors of University of Queensland’s Sustainable Minerals Institute. This will be covered in the next upload.