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THE OSNACA PROJECT: NAVIGATING IN MAGMATO-HYDROTHERMAL SPACE

By Dr Carl Brauhart, Principal Geologist, CSA Global

Introduction Multi-element geochemistry is one of the most powerful tools available to an exploration geologist, but

it is also one that many skilled and seasoned practitioners struggle with. A new publicly available

resource, the OSNACA Project, may prove useful to exploration geologists wishing to learn more about

the geochemistry of the ore deposits we search so hard for. Carl Brauhart is a Principal Consultant

Geologist at CSA Global, but also takes time to lead the OSNACA (Ore Samples Normalised to Average

Crustal Abundance) Project in his role as an Adjunct Research Fellow at the Centre for Exploration

Targeting (University of Western Australia). This article gives a brief outline of the OSNACA Project,

focussing on its relevance to mineral exploration.

The OSNACA Project was deliberately established as on open-source project to maximise the exchange

of ideas between academia and industry. One key advantage of the approach described here is that

variations in ore-element signatures can be mapped for a single ore deposit, a whole ore deposit class,

or across all ore deposit classes.

Inception A dearth of publicly-available high-quality geochemical data for ore deposits has frustrated progress in

the exploration industry for many years. The OSNACA Project was conceived in 2011 as an attempt to fill

this gap. It is an open-source project that takes hypogene ore samples of any type from anywhere in the

world for geochemical analysis. All funds raised have been spent on analytical costs, with all labour

provided on a voluntary basis. The Project is deeply indebted to the initial donors who contributed

almost $70,000 which is administered under the Geoscience Foundation at the University of Western

Australia. The initial donors were Sipa Resources Limited, First Quantum Minerals Limited, Newmont

Australia-Pacific, Sandfire Resources NL, Antofagasta Minerals, CSA Global, Integra Mining Limited,

Wythenshawe Pty Ltd and Digirock. In-kind support continues to be provided by Bureau Veritas –

Ultratrace who do all of the analytical work with tremendous care at greatly reduced prices.

The starting samples were selected from the university collection by Professor Steffen Hagemann, and

comprise over 200 samples of Orogenic-Au, VHMS, Porphyry-Cu, Epithermal, Carlin, IOCG, Fe and other

mineralisation styles.

Eric Grunsky from the Canadian Geological Survey joined to OSNACA Project early on, providing valuable

assistance in statistical treatments of the data.

Current Status

The OSNACA collection now contains 683 samples not including standards and duplicates. Samples from

every major ore deposit class have been donated from all over the world (Fig. 1). There is a reference

collection of hand specimens and laboratory pulps stored at the University and the data is available

online at http://www.cet.edu.au/research-projects/special-projects/projects/osnaca-ore-samples-

normalised-to-average-crustal-abundance

Funds remain for a further 300-odd analyses after which further funds will be sought from industry. It is

anticipated that the OSNACA Project will continue for as long as funds can be raised.

Figure 1. Global distribution of OSNACA samples.

Data All samples are analysed for the same suite of 63 elements at Bureau Veritas – Ultratrace in Perth. Analytical

methods have been selected to provide the best detection limits for ore and pathfinder elements, but high quality

data are also generated for all major elements and a wide selection of lithogeochemical elements including a full REE

suite.

Metadata includes sample location, sample description, ore minerals, ore textures and deposit type

OSNACA Technique and Magmato-Hydrothermal Space Metal associations are recognized through quantitative assay data, but are normally described in a qualitative list

format; e.g., Mortenson et al. (2010) describe Au-As-W-Cu-Pb-Zn enrichment for the Macraes Orogenic-Au deposit in

New Zealand. Only subjective means of comparison are available such as “very similar”, “somewhat similar”,

“unrelated” or “completely different”. For example, a Zn-Pb-Ag-As-Tl-Sb enriched sample from a SHMS deposit has a

similar signature to a Pb-Zn-Ag-Cd rich sample of MVT ore, but quite different to a Zn-Cu-Ag-Au-Bi-Sn rich sample

from a VHMS deposit and very different to a sample of massive Ni-Cu-Pt-Pd-Au-Co ore from a magmatic Ni-sulfide

deposit. Clearly, a quantitative framework in which similarities between ore-element signatures can be measured is

an attractive proposition.

OSNACA stands for Ore Samples Normalised to Average Crustal Abundance and describes a new mathematical

transform that is applied to the 24 ore and pathfinder elements that define ore deposit signatures. These elements

are: Fe, Co, Ni, Re, Pd, Pt, Cu, Ag, Au, Zn, Cd, In, Pb, Tl, Hg, As, Sb, Bi, Te, Mo, W, Sn, La, U.

The OSNACA transform is described in full on the CET website, but the key aspects of the OSNACA transform are log-

normalisation of each element to average crustal abundance followed by scaling each sample to a fixed distance

from the origin.

The OSNACA-transformed dataspace is referred to here as Magmato-Hydrothermal Space. In Magmato-

Hydrothermal Space, a high-grade sample with the same signature (ore-element ratios) as a low-grade sample will

plot in the same place, and this is important because, for once, “Grade is NOT king”. Ore-element signatures are

independent of absolute ore grade. Therefore, any attempt to map ore-element signatures must account for this

independence of grade.

3 Dimensional Model of Magmato-Hydrothermal Space Data from the OSNACA database have been used to create a global map of Magmato-Hydrothermal Space (Fig. 2).

Any hypogene (unweathered) metal-enrichment produced by nature has a sensible position in this framework.

Coloured wireframe models enclose populations of data corresponding to each major ore deposit type.

The first global view of Magmato-Hydrothermal Space shows a transition from MVT samples, through SHMS and

VHMS samples to samples of Cu-Au rich mineralisation. A second “arm” of this continuum extends away from Cu-Au

rich mineralisation through Orogenic-Au samples to the Carlin-Au sample population. These are two of the major

trends identified in Magmato-Hydrothermal Space; Zn to Cu-Au and Cu-Au to Au only (Fig. 2). The majority of

Epithermal samples define a population that connects these two trends by extending from Carlin-Au and Orogenic-

Au across to the VHMS and SHMS sample populations. A smaller group of Cu-rich Epithermal samples overlaps with

the Porphyry-Cu population.

MVT

SHMS

VHMS

Sed Cu

IOCG

Orog Au

Epithermal Carlin Au A

Zn Au

Cu-Au Cu-Au

Porphyry Cu

Por. Mo

Sn-W

Skarn

a

b

c

d

e

Fig. 2.

A third major trend in MH-Space, from Ultramafic to Felsic, extends from the Magmatic Ni-Cu-PGE population

through IOCG and Orogenic-Au samples to Porphyry Cu and more felsic variants of Orogenic-Au and IOCG, to

Porphyry-Mo and Sn-W deposits (Fig. 2b). Most samples in this view lie along the main “hydrothermal plane”, with

ultramafic associated mineralization below and granite associated mineralization above. The main “hydrothermal

plane” contains the Zn to Cu-Au and Cu-Au to Au only trends shown in Fig. 2a, but in Fig. 2b the transition from

sediment-associated to igneous-associated deposit types is more obvious.

Ni-Cu

MVT

SHMS

Epithermal

Sed Cu

IOCG

Orog Au

Carlin Au

Ultramafic(Ni)

Sedimentary Igneous

Granite (Mo, W, Sn)

Por. Mo

Sn-W

B

a

b

c

d

e

Main Hydrothermal Plane

Deposit Scale Patterns in Magmato-Hydrothermal Space Only one ore deposit has been sampled in detail by the OSNACA Project. Nineteen samples from the Lake Cowal

Epithermal gold mine in NSW were collected within a 100 m RL slice across the deposit (Fig. 3). An OSNACA

Enrichment plot shows that the 19 samples have similar ore element enrichment patterns corresponding to their

elevated Au-Te-Ag-As-Sb-Zn assays (Fig. 3). However, they do not have the same ore-element signature.

Fig. 3. Map of Lake Cowal OSNACA samples, colour coded by Group. Inset shows OSNACA enrichment diagram for

same 19 samples.

A 3-dimensional view of Magmato-Hydrothermal Space shows that the Lake Cowal Samples occupy a volume in that

space (Fig. 4). Furthermore, cluster analysis can be used to break the Lake Cowal data into four sub-groups (A-D)

which occupy discrete parts of Magmato-Hydrothermal Space (Fig. 4) and discrete parts of real world space (Fig. 3).

Group A is a single sample of low grade mineralisation, whereas Group D has the strongest Zn-Cd-Pb-Hg enrichment.

Notably, Zn-Cd-Pb-Hg rich samples are also the highest grade gold samples with the three highest grade samples (26

– 370 g/t Au) all from Group D.

Fig. 4. Lake Cowal Subgroups A-D in Magmato-Hydrothermal Space. Epithermal, Orogenic Au, Carlin-Au, VHMS,

Skarn, Sn-W and Porphyry Mo wireframes removed for clarity.

B

A

C

D

Exploration Implications 1. The global view of Magmato-Hydrothermal Space is a very useful perspective to search for unexpected

relationships and thereby generate new exploration opportunities. For example, a group of Sediment-

Hosted Cu samples have an unusual Au-Te-Mo enrichment that places them away from other Sediment-

Hosted Cu samples on the other side of IOCG and Porphyry Cu wireframes. Is a different exploration model

warranted for these deposits?

2. The metal signature of attractive targets within a class can be defined more clearly than before. So, do the

largest or richest Porphyry-Cu deposits have a distinct signature? If so, is this signature developed deposit-

wide or just in certain parts? Is the Golden Mile Orogenic Au signature rare in nature or commonly repeated

at smaller scales?

3. The Lake Cowal example is a very small trial of how the OSNACA-transform might be used at the deposit

scale to map subtle zonation patterns that are obscured by absolute grade models. Better understanding of

ore deposits leads to better exploration outcomes at the mine and camp scales

If you are interested in a more detailed explanation of the OSNACA Project contact Carl Brauhart to arrange a

presentation.

Dr Carl Brauhart, Principal Geologist, CSA Global

PhD (Ore Deposit Geology), BSc (Geology), MAIG, MSEG

Carl is an exploration geologist with over 20 years' experience in gold and base-metal exploration, spending much of

that time working in frontier areas of Western Australia on grassroots projects. He has a particular interest in

geochemistry, both exploration geochemistry and ore deposit geochemistry, and a passion for field work,

particularly mapping. Carl has maintained a close association with the Centre for Exploration Targeting at the

University of Western Australia where he is the lead researcher on the OSNACA Project (an ore deposit geochemistry

research effort).