Facies analysis, sequence stratigraphy, and carbon isotope chemostratigraphy of a classic Zn-Pb host succession: the Proterozoic middle McArthur Group, McArthur Basin, Australia

The McArthur Basin is part of a Proterozoic basin system on the North Australian Craton that represents a world-class Zn-Pb province. Ore bodies are typically stratiform and hosted by pyritic, organic-rich, and dolomitic siltstones deposited in local depocenters and sub-basins. The mineralization is characterized by syngenetic and/or diagenetic textures. These characteristics highlight the need to understand the sedimentological and structural evolution of the basin for mineral exploration. Here we report a facies analysis of the middle McArthur Group (Tooganinie to Lynott formations) in the southern McArthur Basin, distinguishing four facies associations and 19 lithofacies. Depositional environments range from slope and deep subtidal settings to supratidal sabkhas. The middle McArthur Group records a sysPreprint submitted to Ore Geology Reviews January 18, 2019 tematic ∼3.5h shift in the carbon isotope ratio of carbonates (δCcarb) that can likely be used for basin-wide or even global correlation. The Barney Creek Formation, the main Zn-Pb host unit, was mostly deposited under deep subtidal to slope conditions, although shoaling to shallow subtidal environments locally occurred on paleohighs. Together with the overlying Reward Dolostone, it comprises two 3rd-order transgressive-regressive sequences, which distinguishes it from the younger and less prospective but lithologically similar Caranbirini Member, which only comprises one incomplete sequence. The HYC Pyritic Shale Member of the lower Barney Creek Formation, which hosts most of the known mineralization, is lithologically similar across the studied area, and reflects significant deepening of the entire basin. A maximum flooding surface in the HYC Pyritic Shale Member represents the most pyritic and organic-rich interval and can be developed as a black shale in sub-basin depocenters. It represents an ideal chemical trap for base metals in syngenetic models for mineralization; however, lithification and compaction would convert this black shale interval into a physical trap in diagenetic models. Regardless of the preferred model, sequence stratigraphy integrated with facies maps can be used to targeting.


Introduction
most prospective Zn-Pb-Ag provinces in the world (e.g., Leach et al., 2005, 26 2010; Huston et al., 2006). For example, dolomitic siltstones and shales of interpretations of the depositional environments can directly be applied to 45 new drill cores. We use our facies analysis to provide a lithostratigraphic 46 and sequence stratigraphic interpretation of this succession. Furthermore, 47 we tie a high-resolution carbon isotope record into our stratigraphic frame-48 work and test its applicability for future basin-wide and global stratigraphic     Member is only mineralized in the western portion of the Ridge II deposit. 204 The ore occurs ca. 300 m stratigraphically above the mineralization at the  Basin. All facies data are summarized in Table 1 and all carbon and oxygen 226 isotope ratios are provided in Supplementary Table 1. a presentation of all core data is beyond the scope of this paper. Therefore, 230 only three drill core logs are presented herein, which were chosen to represent 231 as much stratigraphy as possible, as well as sub-basin and paleohigh settings.  3.2. Sequence stratigraphy 237 We defined third-order transgressive-regressive (T-R) sequences following 238 the convention of Embry (1993Embry ( , 2009 and Embry and Johannessen (2017). carbonates. In addition to facies data and gamma logs, our sequence strati-256 graphic interpretation was also supported by carbon isotope chemostratigra-257 phy as we identified systematic shifts in the δ 13 C carb curve associated with 258 some sequence boundaries and MFS. The carbon isotopic composition of dissolved inorganic carbon (DIC) in 261 seawater varies secularly (e.g., Saltzman and Thomas, 2012). This feature 262 in the carbon isotope record is commonly used to correlate carbonate rocks 263 based on their isotopic composition because the precipitation of carbonate 264 involves little isotopic fractionation (e.g., Maslin and Swann, 2005). Photo-265 synthesis preferentially consumes the light isotope of C ( 12 C), which leads to 266 depletion of organic matter in the heavy isotope ( 13 C). The isotopic com-267 position of DIC thus reflects the partitioning of carbon between the organic 268 carbon and carbonate carbon reservoirs (e.g., Kump and Arthur, 1999).  sidering that the residence time of carbon in the modern ocean (ca. 100 kyr;  Apart from the composition of the global DIC, local processes can influence 274 the isotopic composition of local water masses and carbonate rocks that pre-275 cipitate from them. The biological pump produces a surface-to-depth isotope 276 gradient. Primary productivity leads to a 12 C-depleted surface ocean through 277 biological assimilation and a 12 C-enriched deep ocean through remineraliza-278 tion of organic matter (e.g., Sarmiento and Gruber, 2006). The magnitude of 279 this gradient depends on primary productivity in the surface ocean and the 280 export production of organic matter. It can reach 3 h in the modern ocean 281 (Sarmiento and Gruber, 2006 laminae and beds, cross-lamination (Fig. 3D), anhydrite nodules/veins, and      The common carbonate matrix, the compositional continuum and in-518 terbedding with bedded dolarenite (LF4) with floating quartz, and interbed-519 ding other facies from FA2 and FA1 suggests that marine sandstone was also 520 deposited in shallow subtidal to intertidal environments (Fig. 7). Roundness        to several mm in length. Some intervals occur as 'flake breccia', which is 748 matrix supported, and has pale grey dolostone clasts that are subangular to 749 rounded, mostly tabular, and subhorizontal. 750 We interpret this lithofacies to have been deposited in quiet subtidal en-751 vironments (Fig. 7). This is consistent with the interbedding with facies     slope environments (Fig. 7).  three drill cores (Fig. 8).

979
The W-Fold Shale is a transitional unit that is only developed as a one to more than 800 m in the sub-basin (Fig. 8). Furthermore, the Barney

998
Creek Formation in GRNT-79-7 is truncated by the sub-Cambrian unconfor- shallow subtidal to intertidal carbonate facies (Fig. 8). Therefore, Lamont  (Fig. 9). Correlation of these sequences as-1169 sumes that they formed synchronously, which means that the interplay of  Second-order sequences such as the River Supersequence are thought to 1199 have a duration between 3 and 50 million years (Vail et al., 1991) and are 1200 mostly controlled by regional tectonics (e.g., Nystuen, 1998;Embry, 2009). Basin. Following this assumption, we correlate previously described 3rd-1224 order sequences from the Lawn Hill Platform  with 1225 3rd-order sequences described in this contribution (Fig. 9).

1234
The sequence stratigraphic correlation proposed herein is only a first at-1235 tempt to reconstruct a regional 3rd-order sequence stratigraphic framework. Carbon and oxygen isotope ratios show no systematic relationship for La-1241 mont Pass 3 and Leila Yard 1 (Fig. 10), indicating a lack of strong secondary 1242 alteration. In contrast, carbon and oxygen isotopes in GRNT-79-7 show a 1243 weak positive correlation (Fig. 10), suggesting that some samples experi-   the RST shows slightly increasing δ 13 C carb values (Fig. 8). Importantly, the Group, within analytical uncertainty (Fig. 11).  demonstrates that this unit is not homogeneous. As expected, the formation 1349 is significantly thicker in sub-basins compared to paleohighs (Figs. 8, 12).

1350
The undifferentiated Barney Creek Formation shows lateral facies variation.  Here mass-flow breccias are common (Fig. 8). Although speculative, they     Thickly laminated; planar-parallel to wavy lamination; discontinuous or continuous laminae, beds, or channels of LF4 with uni-or bidirectional cross-lamination; starved ripples; rare mudcracks; synaeresis cracks; occasional ball-and-pillow structures Shallow subtidal to upper intertidal; less hydrodynamic energy than LF5; tidal current-and storm-influenced Tooganinie Fm LF7: Microbialaminite Light to medium grey (rare dark grey) doloboundstone; commonly silicified; floating quartz possible; interbedded with FA2, follows LF11 (FA3) in shoaling-up cycles Centimeter-to meter-thick microbialaminite; mm-scale flat, crinkly, and undulating lamination; irregular, < 5mm high domal structures; occasional fenestrae, tepees, and mudcracks; discrete intervals of intraclast breccias; sometimes vuggy or brecciated Low-energy inter-to lower supratidal environments (e.g., intertidal flats, levee crests) All units LF8: Dololutite Light grey to pink, rarely medium grey dololutite/ micrite; interbedded with Fa2 and LF11 of FA3 Thinly laminated to massive; common silicified fenestrae; common acicular, radiating, mm-to cmscale pseudomorphs in pink dololutite; silicified karst surfaces may occur; rare mudcracks filled with LF4; sometimes channels and continuous or discontinuous laminae (scoured bases) of LF4 (crosslamination, starved ripples, finingupward); LF4 intraclasts (often silicified) Deposition in complex mosaic of shallow subto lower supratidal environments such as protected lagoons and platforms; on the lee side of banks, shoals, and stromatolite build-ups; on levee crests; ponds; and low-energy parts of tidal channels; storm and tidal activity Flat, crinkly, and undulating lamination; occasional disrupted and buckled laminae; common molar tooth structure; rare fenestrae (laminoid, calcite-filled, 1 mm high and 5-10 mm long) Quiet shallow subtidal environments with low wave and tidal energy such as bights, lagoons, and embayments Reward Dolostone; Lynott Fm LF13: Dolomudstone Dark grey to black (minor medium grey) homogeneous dololutite to dolosiltite; clayrich; sometimes silty (quartz); pyrite and base metals may occur disseminated, stratiform, in streaks, spots, or along fractures; interbeds of muddy microbialaminite (LF12) or back shale (LF17) possible; can transition into dololutite (LF8) Thinly laminated to massive; nodular bedding and pale grey nodules (can be plastically deformed) may occur; flakes of organic matter (< 1 cm, subhorizontal); slumping, loading, ball-and-pillow structures may occur; molar tooth structures possible; rare dolarenite laminae with cross-lamination or starved ripples; rip-up clasts (dolarenite) possible; discontinuous or wavy shale laminae may occur; partly flake breccia (pale grey clasts, subangular to rounded; mostly tabular, matrix supported; clasts subhorizontal) Quiet subtidal environments above storm wave base; similar to LF12 by algae growth prevented by higher sedimentation rates or greater water depth Mostly medium grey dolosiltite to dolarenite (mostly very fine-to fine-grained arenite); may be silty or contain rounded quartz grains; disseminated or accumulations of pyrite may occur; interbedded with LF15, LF16, LF18; poorly to well sorted Thinly laminated to medium bedded; may have cross-lamination, starved ripples, HCS; fining-or coarsening-upward possible; often sharp and scouring base but transitional top; slumping possible; loading and ball-and-pillow structures into underlying facies common; may have mm to cm scale intraclasts (LF4, LF11, LF16, rounded); submm to mm scale organic matter flakes are common, may be disseminated, concentrated in certain beds or form discontinuous laminae Silty dolarenite/dolomitic siltstone Continuum between medium to dark grey silty dolostone and dark grey to black dolomitic siltstone/very fine sandstone (both brown or white/rusty weathering); siltstone often pyritic and/or bituminous; occasional dolarenite laminae and beds (LF14, LF15); interbeds of other FA4 lithofacies common; Thickly laminated to thinly bedded; generally parallel-planar lamination, occasional wavy lamination; occasional carbonate nodules, slumping, growth faults, fining upward; common loading structures associated with dolarenite laminae; often fissile breaking when interbedded with black shale (LF17); silty dolarenite very rare crosslamination (tangential or straight foresets), starved ripples, HCS Dolomitic siltstone subfacies below storm wave base, silty dolarenite in shallower environments close to storm wave base; both deposition from hemipelagic settling and/or low density turbidity currents Medium to dark grey (rare black), matrix-or clast-supported grainstones, conglomerates, breccias; mono-or polymict; no fitting; tabular and equant clasts; sand to cobble-sized; well rounded to very angular carbonate clasts; minor up to granule-sized dolomitic siltstone clasts and sand-sized quartz grains; moderately to very poorly sorted; interstitial sulfides; background facies is LF13 and LF16 Very thinly to medium bedded, sometimes massive; mostly ungraded but sometimes fining-or coarsening upward Deposition from sediment gravity flows in slope environments; sand-sized subfacies from fine-grained turbidity currents and grainflows; > sandsized subfacies from coarse-grained turbidity currents and debris flows

Barney
Creek Fm, Reward Dolostone LF19: Rhythmite Dark (rare medium) grey, very fine to fine dolarenite Thickly laminated to thinly bedded; beds are massive or have an internal, mm-scale planar lamination with fining-upward; slump folds common; cm-scale growth faults; scouring Deposition below storm wave base in slope environments; pelagic and hemipelagic fallout and deposition from dilute turbidity currents

Barney
Creek Fm