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Mineral Systems Analysis – a workflow and example from the Halls Creek Orogen

Jun 22, 2016

S.A. Occhipinti, M. Lindsay, V. Metelka, *J. Hollis, *I.M. Tyler, A. Aitken, C. McCuaig, M.C. Dentith

* Geological Survey of Western Australia

Introduction

Opening up greenfields regions for minerals exploration can be helped through regional-scale mineral systems analysis, using government pre-competitive datasets. This may be completed on a commodity by commodity basis, or through multi-commodity minerals systems analysis, and used as the basis for prospectivity analyses and mapping. The Geological Survey of Western Australia, through the Exploration Incentive Scheme has sponsored mineral systems analysis of several regions within Western Australia in order to boost the State’s attractiveness to explorers.

One of these regions, the Halls Creek Orogen, in Western Australia contains mineral occurrences or deposits that formed from c. 1860–350 Ma. Ore deposition in this region took place during periods of supercontinent assembly and breakup that were manifested through the contractional 1865 – 1850 Ma Hooper and 1835–1805 Ma Halls Creek Orogenies, and to a lesser degree the Yampi 1400 – 800 Ma and c. 560 Ma King Leopold orogenies. These events were interspersed with periods of relaxation accompanied by extension. The CET completed mineral systems analysis in the Halls Creek Orogen in 2014 – 2015, followed by prospectivity modelling and mapping for seven commodity groups of various ages and ore genesis mechanisms within the region that include combinations of  Ni, Cu, PGEs, V, Ti, Au, Pb, Zn and diamonds.

Crustal-scale tectonic architecture was found to link disparate styles and ages of mineralisation in the region. This work clearly illustrates that different tectonic terranes or‘zones’ of the Halls Creek Orogen are prospective for diverse commodity groups due to the tectonic environment in which they developed through time, their potential to be preserved in the current day surface or subsurface, and favourable depositional sites that may be present in these zones (structural or lithological).

Mineral systems analysis

Mineral systems analysis involves understanding key processes that control the formation and preservation of ore deposits (Fig. 1). These can include large scale processes such as the secular evolution of Earth, lithospheric controls on mineral enrichment, and geodynamic drivers (Cawood and Hawkesworth, 2013; Groves and Bierlein, 2007; McCuaig and Hronsky, 2014; McCuaig et al., 2010; Wyborn et al., 1994). Supercontinent formation and breakup clearly controls lithospheric enrichment and geodynamic drivers. Deep crustal-scale structures can act as fluid pathways connecting the deeper lithosphere to the upper crust, and occasionally act as depositional sites for ore deposition. Thus for regional-scale mineral systems analysis a 4D understanding of geological processes is required. Other controls, such as the development, or presence of specific lithologies may also be important (Cline et al., 2005).

In order to implement a systematic approach to mineral systems analysis the CET has developed a workflow that maps proxies of tectonic triggers, fertility and depositional sites, preservation levels and deep crustal-scale structures. This was established from a review of ore deposit models and the understanding that key elements required for ore formation and deposition often overlap between specific ore deposit styles. The resultant key elements are summarised in a ‘Mineral Systems Diamond’ that is used as a summary of mappable proxies for mineral systems analysis (Fig. 1; Occhipinti et al., 2016).
Figure 1: Mineral Systems Diamond – a scale independent schematic diagram summarizing major components required to form and preserve mineral deposits (from (Occhipinti et al., 2016)).

Halls Creek Orogen – Key elements of mineral systems

The Hooper and Halls Creek Orogenies are thought to be results of accretion of the central and eastern zones of the Halls Creek Orogen during the diachronous assembly of the Paleoproterozoic supercontinent Columbia (Huston et al., 2012; Tyler et al., 2012). In addition to this the effects of the assembly of the younger supercontinent Rodinia (Li. et al., 2008) are noted in the region with the intrusion of c. 1200 Ma lamprophyres that host the giant Argyle diamond deposit (Jaques and Milligan, 2004). Phanerozoic basins in the Kimberley region, including the Devonian strata hosting Pb-Zn mineralization, developed during various stages of the amalgamation of Gondwana.

Fertility elements for the development of deposits of ore or gems could include hydrous magmas, zones of repeated magmatic fractionation, magmas with mantle input, metasomatism driven through magmatic processes, oxidised fluids, salinity or oxidised or reduced magmas (Groves and Bierlein, 2007). Although tectonic processes are a key element in ore deposit formation, specific triggers are required to form a mineral deposit (Wyborn et al., 1994), and these could include subduction initiation, changes in plate motion, compression along an arc section, topographic fluid flow via pressure gradients, magmatism, or a specific dynamic threshold barrier (e.g. resulting in the intrusion of sills as opposed to dykes; (Garwin et al., 2005; Groves and Bierlein, 2007; Gurney et al., 2005; Leach et al., 2010; Loucks, 2014; McCuaig and Hronsky, 2014; Occhipinti et al., 2016; Seat, 2007)).

Major crustal-scale faults or shear zones that intrinsically control the location of known ore deposits in the area, are implied to be sites of fluid migration and proximal to sites of ore deposition. Of these, orogen-perpendicular (northwest-trending) and orogen-oblique (north-trending) faults seem to be the most influential structures with respect to ore deposition in the Halls Creek Orogen, especially in regions where they intersect each other, or orogen parallel (northeast trending) major crustal-scale structures.

Geological evolution of the Halls Creek Orogen

The western, central and eastern zones of the Lamboo Province make up the Halls Creek Orogen and contain distinct geological units that developed during the early Paleoproterozoic in diverse tectonic settings (Fig. 2; (Occhipinti et al., in press)). In summary, subduction, accretion and collisional processes are thought to have been interspersed with plate relaxation and rifting ((Occhipinti et al., in press; Tyler et al., 2012); Fig. 3) during the Paleoproterozoic. Rifting in the region culminated in the development of the Hart Dolerite and Carsten Volcanics, which are thought to be part of a large igneous province (Sheppard et al., 2012).



Figure 2: Top. Simplified geological map of the Eastern, Central and Western Zones, Halls Creek Orogen; Bottom. Time space plot for the Halls Creek Orogen; from Lindsay et al., (2015) and Occhipinti et al., (in press).


Figure 3: Tectonic development of the Halls Creek Orogen during the Paleoproterozoic (from (Occhipinti et al., 2016)).

Mapping deep crustal elements

Deep crustal-scale structures are important as they are thought to control fluid and magma movement through the lithosphere. For example, in the Gawler Craton in South Australia it is implied that the Olympic Dam Iron-Oxide-Copper-Gold (IOCG) deposit formed over the intersection of a northwest trending deep crustal-scale structure with a paleosubduction zone that dipped west under the Gawler Craton (Groves, 2010; Hayward and Skirrow, 2010). Following this, Lindsay et al., (2016) mapped deep crustal-scale structures, that trend parallel to the trend of the Halls Creek Orogen (northeast trending), perpendicular to it (northwest-trending), and oblique to it (north-trending) using a range of geophysical datasets as well as geological mapping in the region. However, for the most part the tectonic architecture was analysed by allying a 2D map view geological-geophysical interpretation with 2.5D magnetic and gravity joint inversions of selected profiles, and a 3D Moho gravity inversion (MoGGIE).

Mapping proxies for preservation

When applying mineral systems analysis to a region, the current day level of crustal exposure (i.e. upper, middle, or lower crust) should be considered as a factor that may influence whether or not a mineral deposit, if developed, was preserved. For the Halls Creek Orogen metamorphic maps were produced as a proxy for preservation. The metamorphic maps were constructed through a review of geological units and their mineral assemblages within the Halls Creek region and were produced for Hooper Orogeny metamorphism and Halls Creek Orogeny metamorphism (Fig. 4).


Figure 4: Metamorphic maps for the Halls Creek Orogen: a. Hooper Orogeny; b. Halls Creek Orogeny; c. Overview metamorphic map including effects of both the Hooper and Halls Creek Orogenies (modified from (Occhipinti et al., in press)).

Prospectivity modelling and mapping

The selected fuzzy logic-based inference network models require several predictor maps to be assigned membership values based on a combination of objective datasets and subjective model components (Occhipinti et al., in press). Each predictor map represents a component of a particular mineral system. We derived 70 predictor maps from geological maps, GSWA observations, geophysical interpretation layers and primary geophysical data. Seven prospectivity models (e.g. Figs. 5 and 6) partitioned by mineral/element associations were created: Au, Cu-Au-Mo, Pb-Zn-Cu-Ag, Ni-Cu-V-Ti-PGE, Sn-W-Mo, diamond and REE. The models combine predictor maps through a set of fuzzy operators to produce a final prospectivity map.

 


Figure 5: Prospectivity map for Diamonds

Figure 6: Prospectivity map for Ni, Cu, PGE, V, and Ti.

 

All of the models were constructed within the ESRI ArcToolbox ModelBuilder environment and can be easily rerun with different map weights or fuzzy operators (see Occhipinti et al., in press). The prospectivity toolbox is included in the final GSWA data package for distribution (Occhipinti et al., in press).

Results

The results of this work and a GIS are included in two GSWA reports and a digital data package to be released online soon (Occhipinti et al., in press; Lindsay et al., in press). The resulting prospectivity maps suggest that the search space for most commodity groups can be narrowed down to less than 0.04% of the Halls Creek Orogen (Occhipinti et al., 2016). Geographically this implies than the area of ground to be explored at the camp scale could be less than 620 km2 for all commodities reviewed during this project. Although different parts of the Halls Creek Orogen are prospective for different commodity groups (Occhipinti et al., in press), the eastern zone remains the most prospective for Au mineralization; whereas Ni-Cu-PGE’s are present across the western and central zones. Interestingly, results of the analyses illustrated that the most prospective regions for most minerals and diamonds is the central zone of the Halls Creek Orogen.

Conclusions

A workflow for regional to district scale mineral systems analysis has been developed at the CET. While this study analysed the east Kimberley region, our aim was to develop a generic and flexible workflow that can be easily applied to other regions. The results characterise areas of mineral prospectivity for several commodity groups in the Halls Creek Orogen and  illustrates the control that major structures have on the location of mineral deposits. A mineral systems toolbox has been created as part of this project so that anyone can complete their own mineral systems analysis on the Halls Creek Orogen and applying their own weights to key system elements that have been mapped that the user thinks are most appropriate.

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