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In terms of zinc, lead, and silver lnetal endowment, the Proterozoic sedimentary basins of northern Australia rank number one in the world. The Mt. Isa-McArthur basin system hosts five supergiant, stratiform, sedimentary rock-hosted Zn-Pb-Ag deposits (McArthur River, Century, Mt. Isa, Hilton, and George Fisher) and one supergiant strata-bound Ag-Pb-Zn deposit (Cannington). These superbasins consist of units deposited during tlwee nested cycles of deposition and ed~umation that occurred in the period from 1800 to 1580 Ma. The cycles took place in response to far-field extension and subsidence associated with a major northward-d~pping subduction zone in central Austl-alia. All major stratiform zinc-dominant deposits occur within rocks of the sag phase of the youngest Isa superbasin, which was deposited between 1670 and 1580 Ma. The strata-bound silver-and lead-rich Cannington deposit is hosted by high-grade metamorphosed clastic sedimentary rocks that are temporal correlatives of the basal extensional phase of the Isa superbasin. It exhibits distinct differences from the stratifo~m zinc-dominant deposits but sho\vs similarities wit11 Broken Hill-type deposits. The major stratifonn Zn-Pb-Ag deposits exhibit many similar geological and geochernical features that include: (1) location close to regionally extensive normal and strike-slip synsedirnentary faults, (2) organic-rich black shale and siltstone host rocks, (3) laminated, bedding-parallel synsedirnentary sulfide minerals, (4) stacked ore lenses separated by pyritic and Fe-Mn carbonate-bearing siltstones, (5) lateral zonation eshibiting an increasing Z n P b ratio away from the feeder fault, (6) vertical zonation exhibiting decreasing Z n P b ratio upstratigraphy, (7) an extensive strata-bound halo of iron-and manganese-rich alteration in the sedimentary rocks surrounding and along strike from ore, (8) a broad range of values for sulfide minerals, from about 0 to 20 per mil, uith pyrite exhibiting a greater spread than base metal sulfides, and (9) lead isotope ratios that indicate derivation of lead from intrabasinal sources with intelpreted lead model ages being similar to the measured zircon U-Pb ages of the host rocks. These common features demonstrate that the stratiform Zn-Pb-Ag ores formed approximately contemporaneously uith sedimentation and/or diagenesis. The exact timing of minerahation relative to these processes varies from deposit to deposit. However, metamorphic overprints in some deposits (e.g., Mt. Isa, Hilton, Dugald River, Lady Loretta) have lead to recrystallization of sulfide minerals, making it difficult to interpret primary paragenetic relationships and absolute timing of mineralization. Mount Isa is the only northern Australian stratiform Zn-Pb-Ag deposit that has spatially associated high-grade copper mineralization. Textural and isotopic data for the stratiform Zn-Pb-Ag deposits suggest there is a spread of ore depositional processes from synsedimentary exhalative to syndiagenetic replacement. At McArthur River, for example, the high-gra...

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Faced with ongoing depletion of near-surface ore deposits, geologists are increasingly required to explore for deep deposits or those lying beneath surface cover. The result is increased drilling costs and a need to maximize the value of the drill hole samples collected. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) analysis of pyrite is one tool that is showing promise in deep exploration. Since the trace element content of pyrite approximates the composition of the fluid from which it precipitated and the crystallization mechanism, the trace element characteristics can be used to predict the type of deposit with which a pyritic sample is associated. This possibility, however, is complicated by overlapping trace element abundances for many deposit types. The solution lies with simultaneous comparison of multiple trace elements through rigorous statistical analysis. Specifically, we used LA-ICP-MS pyrite trace element data and Random Forests, an ensemble machine learning supervised classifier, to distinguish barren sedimentary pyrite and five ore deposit categories: iron oxide copper-gold (IOCG), orogenic Au, porphyry Cu, sedimentary exhalative (SEDEX), and volcanic-hosted massive sulfide (VHMS) deposits. The preferred classifier utilizes in situ Co, Ni, Cu, Zn, As, Mo, Ag, Sb, Te, Tl, and Pb measurements to train the Random Forests. Testing of the Random Forests classifier using additional data from the same deposits and sedimentary basins (test data set) yielded an overall accuracy of 91.4% (94.9% for IOCG, 78.8% for orogenic Au, 81.1% for porphyry Cu, 93.6% for SEDEX, 97.2% for sedimentary pyrite, 91.8% for VHMS). Similarly, testing of the Random Forests classifier using data from deposits and sedimentary basins that did not have analyses in the training data set yielded an overall accuracy of 88.0% (81.4% for orogenic Au, 95.5% for SEDEX, 90.0% for sedimentary pyrite, 73.9% for VHMS; insufficient data was available to perform a blind test on porphyry Cu and IOCG). The performance of the classifier was further improved by instituting criteria (at least 40% of total votes from the Random Forests needed for a conclusive identification) to remove uncertain or inconclusive classifications, increasing the classifier's accuracy to 94.5% for the test data (94.6% for IOCG, 85.8% for orogenic Au, 87.8% for porphyry Cu, 95.4% for SEDEX, 98.5% for sedimentary pyrite, 94.6% for VHMS) and 93.9% for the blind test data (85.5% for orogenic Au, 96.9% for SEDEX, 96.7% for sedimentary pyrite, 84.6% for VHMS). The Random Forests classification models for pyrite trace element data can be used as a predictive modeling tool in greenfield terrains by providing an accurate indication of ore deposit type. This advance will assist mineral explorers by allowing early implementation of predictive ore deposit models when prospecting for ore deposits. Furthermore, the ability of the classifier to accurately identify pyrite of sedimentary origin will allow researchers interested in paleoenvironmental conditions of ...