Diagenesis and Mineralization of the Neoarchean Bushy Park Lead-Zinc Deposit, Northern Cape Province, South Africa

Abstract

The Bushy Park Pb-Zn deposit, hosted in unmetamorphosed carbonates of Neoarchean age, displays similarities to Phanerozoic Mississippi Valley-type (MVT) and Irish-type deposits. Mineralization is dated, by radiogenic methods, to Paleoproterozoic time. As such, Bushy Park is one of the oldest mineral deposits of this type in the world. Synsedimentary silicification and dolomitization preserve sedimentary fabrics, including microbial laminates, stromatolites, and oolites. Dolomitization likely was by evaporated seawater, as in Phanerozoic analogs. Structural control on mineralization, particularly solution collapse breccias, is similar to many Phanerozoic MVT and Irish-type deposits. Fluid inclusion data indicate three fluid endmembers involved in mineralization: a high-temperature, moderate-to-high salinity fluid; a low-temperature, moderate-to-high salinity fluid; and a moderate-to-low temperature, low salinity fluid. Saline fluids may have been sourced by evolved, evaporated seawater, and dilute fluids by meteoric and/or normal seawater. The fluids repeatedly mixed during ore and gangue mineral formation. Compositional zoning in gangue dolomite cement indicates that mineralizing fluid chemistry fluctuated over time. Petroleum inclusions and solid bitumen indicate that petroleum (oil) was an important fluid component at Bushy Park. Petroleum may have played a critical role in sulfur availability, addressing the issue of limited oceanic sulfate prior to and during the Great Oxidation Event.

1. Introduction

The Pb-Zn deposit at Bushy Park, hosted in unmetamorphosed platform carbonate rocks [1], and the nearby Pering deposit (Figure 1) may be the oldest deposits of this type in the world [2]. The formation of carbonate-hosted Mississippi Valley-type (MVT) and the closely related Irish-type carbonate-hosted metal sulfide deposits is considered a typical aspect of sedimentary basin life cycles, involving processes such as tectonically driven fluid flow and hydrocarbon migration [3,4,5,6]. Frequently these two closely related deposit types are lumped together as “carbonate-hosted Pb-Zn deposits”. These types of deposits are a significant source of zinc and lead as well as other critical metals. They frequently consist of large districts scattered over wide regions comprised of numerous individual deposits of several million tons of ore and typically are hosted by dolomitized platform carbonate rocks [6,7,8]. Carbonate-hosted Pb-Zn mineral deposition spans most of Earth’s history [5,9]. Although argument continues as to the precise origin of MVT and Irish-type metal sulfide deposits, it is generally accepted that they were precipitated by hydrothermal fluids, consisting of relatively low-temperature (50 °C to 250 °C) sedimentary brines from multiple sources, including sulfate-rich evaporated sea water and meteoric water, both possibly interacting with basement and basement derived sedimentary rocks [3,5,8,10,11,12]. Ore precipitation occurred through various mechanisms, with the dominant process likely being the mixing of high-salinity, metal-rich basinal brines and lower salinity fluids [5,6,13].

While Pb-Zn deposits are predominantly a feature of Mesoproterozoic and later rocks [7], the Bushy Park lead-zinc deposit in Northern Cape Province, South Africa (Figure 1), stands out as it is hosted by Neoarchaean-age platform carbonates of the Transvaal Supergroup [1,2,14,15] (Figure 2). It has been proposed that the carbonate-hosted Pb-Zn mineralization in the Transvaal formed through similar processes as its geologically younger counterparts [16,17,18].

Despite substantial progress over the last three decades in understanding the mechanisms of base metal fluid genesis, migration pathways, and conditions that resulted in precipitation metals in the Transvaal section, a detailed description of the petrography and discussion of the diagenetic history of the Bushy Park deposit has not been published; however, see [17,19]. The objective of this report is to describe petrographic observations of host rocks, ore, and gangue mineralization at Bushy Park and to compare them with Pb-Zn ore-forming stages elsewhere [3,4,5,6], emphasizing the sequential relationship between mineralizing events. Petrographic observations are crucial for understanding the mineralogical and textural changes associated with ore formation, as these observations provide insights into the conditions and processes that facilitated mineralization.

2. Geological Setting

2.1. Tectonic Setting and Stratigraphy

The Transvaal Supergroup in the Griqualand West Basin (Figure 2) lies unconformably on the Neoarchaean Ventersdorp Supergroup, which was deposited in a rift-type setting [20]. During the final thermal sagging stage, the Ghaap epeiric sea began to form on the Kaapvaal Craton, leading to the deposition of the Transvaal carbonate sequence [21]. Beukes [14] divided the Ghaap Group into the Schidsdrift, Campbellrand, Asbesheuwels, and Koegas Subgroups (Figure 2). The Campbellrand Subgroup, consisting of dolomite, limestone, and chert successions, is the primary host to Pb-Zn deposits within these rocks. The sedimentary facies of the Campbellrand Subgroup were described in detail by Beukes [14], Altermann and Siegfried [22], and Baugaard et al. [1].

The lithologies under investigation at Bushy Park were cored by Doe Run Exploration, South Africa. From the base upward, the lithologies intersected at Bushy Park include the Papkuil, Klippan, and Kogelbeen Formations (Figure 2), which are interpreted to have been deposited in a shallow platform environment [1]. These units consist of meter-scale cycles or parasequences (likely Milankovitch cycles) [1]. Sedimentary slump breccias typically occur near the top of cycles in the upper Kogelbeen Formation [1], and prominent sedimentary breccias occur in the Klippan Formation, following a major sea-level regression, suggesting it to be part of a third-order depositional cycle [1,14,23]. Previous studies [15,18,24] found that sulfide mineralization is mainly hosted by solution collapse breccias in the Kogelbeen Formation (Figure 2 and Figure 3). Baugaard et al. [1] observed that the orthogonal geometry of the mineralized envelope suggests that not all breccias at Bushy Park are controlled by cyclic sedimentation and that structural influences play an important role. Mineralization occurs within sub-vertical rubble breccias that cut across all three formations and may continue at depth (Figure 3).

2.2. Previous Work on Mineral Deposits in the Griqualand West Basin

Baugaard [19] onducted an in-depth petrographic analysis of the dolomitic host rock at Bushy Park to elucidate its diagenetic history within the sedimentary sequence and the role of ore mineralization in this process. Using transmitted light, reflected light, and cathodoluminescence (CL) microscopy, the study detailed the alteration of host rock textures and developed a CL stratigraphy of dolomite cements within breccia bodies, which serve as the primary ore hosts. Schaefer [17] also conducted a CL study of the major dolomite cement and gangue mineral generations that occur at the Bushy Park deposit, identifying their relationships with distinct ore generations. Additionally, Schaefer [15,17] carried out a trace element geochemical analysis of lead-zinc ore minerals from multiple deposits in the Griqualand West Basin, including Bushy Park, using SEM. This study linked the trace element geochemistry of ore minerals to individual ore deposits and the physicochemical conditions of the aquifers through which the mineralizing fluids passed, offering valuable insights into these processes.

Both fluid inclusion data and trace element geochemistry of galena and sphalerite suggest that Pb-Zn deposits at Bushy Park and elsewhere in the Transvaal Supergroup, Griqualand West Basin, exhibit thermal and geochemical similarities to Phanerozoic MVT and Irish-type deposits [17,18,25]. Kruger et al. [26] and Duane et al. [27,28] attribute mineralization to a gravity-driven flow system related to convergent tectonics during the Kheis Orogeny (1.9–2 Ga), which was part of a larger southern African magmatic arc that culminated just before the intrusion of the Bushveld igneous complex. Duane et al. [28] proposed that the source of the base-metal brines was the Maquasi Quartz Porphyry of the Ventersdorp Supergroup, which underlies the Transvaal Supergroup (Figure 2). This conclusion was based on the highly radiogenic and narrowly constrained ⁸⁷Sr/⁸⁶Sr isotope ratios in late-stage diagenetic calcite from several locations within the Ventersdorp Supergroup. Gleason et al. [29] suggested that regional hydrothermal fluid circulation operated through the Transvaal Supergroup just before, or during, the Great Oxidation Event (2.4–2 Ga). The thermal regime is marked by the deposition of liquid-vapor-rich fluid inclusions within sphalerite and fluorite, accompanied by high-salinity brines linked to the Bushveld complex, which manifest as liquid-vapor-daughter inclusions at Zeerust near the Bushveld complex in northeastern South Africa.

Sulfate availability in marine basins plays a critical role in the reduction processes that lead to the precipitation of sulfide minerals like galena and sphalerite. Elevated oceanic sulfate levels are closely linked to the formation of carbonate-hosted Pb-Zn deposits, as emphasized by Sverjensky [10] and further demonstrated by Kah et al. [30] and Kesler and Reich [31]. A significant challenge arises from the fact that the host sedimentary rocks predate the Great Oxidation Event, a period during which sulfate, a key sulfur source for base metal precipitation, would have been extremely limited or even absent in the world’s oceans according to the accepted model [2,32]. As a result, carbonate-hosted Pb-Zn deposits from this period should be rare in the geologic record.

3. Materials and Methods

Twenty-eight drill holes (Figure 3) were logged at the Bushy Park prospect. Stratigraphic analysis was conducted and documented by Baugaard [19] and Baugaard et al. [1]. Core samples were selected from Bushy Park and Koedoesdale Farm, located 200 m from the Bushy Park fence border, as well as a core from a polymictic breccia southeast of the orebody. These samples were chosen to broadly represent the lithologies of the Kogelbeen, Klippan, and Papkuil Formations. A total of 150 samples were collected for petrographic study. Core samples were stained with alizarin red S and potassium ferrocyanide (KCN) to visually differentiate chemically distinct dolomite and calcite cement types. Polished thin sections were prepared for petrographic study. Dolomite petrographic textures in the Bushy Park strata were classified according to Sibley and Gregg [33]. Dolomite and quartz crystal sizes were categorized according to Folk [34].

Transmitted and reflected light petrography was carried out using Nikon Optiphote petrographic microscopes at the University of Missouri-Rolla (UMR, now Missouri University of Science and Technology), Rolla, MO, U.S.A. CL microscopy was carried out using a Technosyn 8200 MkII cold cathode optical CL system mounted on a Nikon Labophote microscope with a film camera (at UMR) and a CITL MK5-1 cold cathode optical CL system mounted on an Olympus BX51 microscope equipped with a Q Color 3 cooled (low light) five-megapixel digital microscope camera at Oklahoma State University, Stillwater, OK, U.S.A. In both cases, operating conditions of an approximately 12 kV accelerating voltage, 200–400 mA beam current, and 0.05 Torr vacuum pressure were used.

Fluid inclusion microthermometry was conducted on four double-polished thick sections at the Kansas Geological Survey in Lawrence, KS, USA. The temperature and salinity of the fluid inclusions were analyzed using a Linkam THMSG 600 heating and cooling stage, which was mounted on an Olympus BX53/60-MTRF-S microscope equipped with 4X, 10X, and 40X long focal distance objective lenses. All four samples were analyzed for petroleum inclusions using an Olympus U-RFL-T and Olympus BX53 petrographic microscope, equipped with 4X, 10X, 40X, and 100X objectives and a UV epi-illumination assemblage. Homogenization (T_H) and last ice melting (T_Mice) temperatures have errors of ±1.0 °C and ±0.3 °C, respectively, based on synthetic fluid inclusions analyses of Shelton and Orville [35]. No pressure corrections were made to T_H data. First occurrence of ice melt (T_fm) was recorded when observed. The inclusions analyzed in this study were aqueous, two-phase, primary, secondary, and pseudosecondary inclusions, using the terminology of Roedder [36]. Fluid inclusion assemblages (FIAs) were analyzed to determine their origin, following the approach of Goldstein and Reynolds [37] (1994). FIAs are essential for determining fluid inclusion entrapment timing and thermal re-equilibration, while salinity is calculated from T_Mice measurements using the Bodnar [38] equation.

4. Results

The sulfide minerals at Bushy Park occur mainly in breccias and consist primarily of sphalerite, lesser amounts of galena, and minor amounts of pyrite, chalcopyrite, and a few other trace minerals. Fluorite crystals also were observed but are not common. Ore minerals also are associated with gangue minerals, including open-space-filling dolomite, calcite, and quartz cements, as well as bitumen (Figure 4). These are hosted by dolomitized platform carbonates composed of cyclic oolitic grainstones, mudstones, stromatolites, and microbial laminates [1].

4.1. Breccias

Breccias at Bushy Park can be divided into five main categories: (1) crackle breccia, formed by small-scale veining in the roof of karst systems due to stress; (2) mosaic breccia, composed of dislocated fragments retaining their relative positions; (3) rubble breccia, consisting of randomly distributed clasts of varying sizes; (4) rock matrix breccia, found on karst floors and made up of fine-grained dolomite, chert, and quartz residues left after dissolution; and (5) slump breccia, comprising clast-supported breccias filled with sub-granule material, likely formed during subaerial exposure [1]. The first four types are associated with collapse breccias, characterized by angular, poorly sorted clasts, while slump breccias are better sorted and subrounded, frequently found atop parasequences and correlatable between boreholes [1].

Sedimentary slump breccias occurring below the black shale of the Klippan Formation consist of angular to subangular clasts a few centimeters to about 20 cm in diameter that are cemented by a micritic matrix (Figure 5a). The micritic matrix is very rarely preserved as gangue mineralization and, in most cases, replaces it while also filling the interstitial spaces between dolomite breccia clasts (Figure 5b). Slump and crackle breccias within the Klippan Formation frequently are silicified. This silicification commonly extends to the upper Papkuil Formation (Figure 5c). Localized karst brecciation, including crackle, mosaic, rubble, and rock matrix breccias (Figure 5d), predominantly affects the Klippan and Papkuil Formations and, except in rare cases, are unmineralized.

Mineralized collapse breccias are influenced by northwest-southeast and northeast-southwest trending faults that crosscut formations [1]. Sulfide mineralization at Bushy Park occurs in the collapse breccia bodies following orthogonal patterns (Figure 3), with drilling to 450 m revealing a brecciated section spanning the Kogelbeen to Papkuil formations [1]. Collapse breccias associated with the crosscutting orebody exhibit a wide range of breccia types and sizes, with some clasts reaching several meters (Figure 5e), and are characterized by gangue minerals and ore filling the interstitial spaces. Ore mineralization primarily occurs within the voids between the larger clasts (Figure 5f). In general, the ore tends to be massive as it fills the spaces between the breccia voids (Figure 5e,f and Figure 6a). It also fills fractures and commonly replaces the host dolomite (Figure 6a). Rarely, sulfide mineralization forms stratabound zones of limited extent.

An unmineralized polymictic breccia, with clasts of banded iron formation (BIF), andesite, and dolomite (Figure 6b,c), occurs to the southwest of the ore body and appears completely disconnected from the other breccia types. Core samples of this breccia reveal multiple stages of alteration. Hematite laminae in BIF are replaced by pyrite and are accompanied by dolomite and BIF clasts that display silicification (Figure 6b). The clasts are cemented by a quartz matrix, with fractures also filled by dolomite, quartz, and bitumen. Clasts of plutonic, mafic rocks are observed in the breccia, consisting mainly of decimicron-sized crystals (200–700 μm) of altered feldspar embedded in a quartz matrix. Other breccia components include dolomite cements that crosscut or fill open spaces, along with smaller clasts of quartz, dolomite, and pyrite fragments.

4.2. Hand Specimen Petrology

In hand specimens (Figure 4), the host rock dolomite generation (designated as D₀) is dark gray and replaces mudstones, microbial laminates, LLH (laterally linked hemispheroids), and oolites, which are typically lighter gray in color (Figure 4c). Following brecciation, the host rock occasionally is crosscut by thin dolomite veins forming crackle breccias that are light gray in color. Larger voids are filled by open-space-filling dolomite cement. The initial dolomite cement (designated as D₁) is blocky with a slightly elongated crystal habit, matching the light gray color of the dolomite veins and the dolomite replacing the host limestone lithologies in fresh samples (Figure 4). Anhedral pyrite crystals typically mark the boundary between the first dolomite cement stage (D₁) and the subsequent stages of open-space-filling cement. The second open-space-filling dolomite cement stage (designated D₂) displays growth zones in hand specimens, particularly in weathered samples (Figure 4c). In fresh samples, it is white to very light gray in color, and the zonation is not as apparent but still is discernible. Dolomite crystals of the second and subsequent stages of dolomite cement display curved faces typical of saddle dolomite. The boundary between the second and third stages of saddle dolomite is sharp, as the third stage lacks zonation. The third stage of open-space-filling saddle dolomite (designated D₃) is generally dark to medium gray in color and has a gradational boundary with the fourth stage of open-space-filling saddle dolomite cement (designated D₄). The fourth stage of open-space-filling dolomite cement is pearly white in color and is frequently replaced by sulfide mineralization (Figure 4b).

The first stage (D₁) of open-space dolomite cement occurs throughout the Bushy Park area and across the entire stratigraphic column. It is particularly abundant in the Papkuil Formation, where it fills veins and karst breccias, and in the Klippan Formation, where it occupies spaces between clasts in slump breccias. It rarely occurs in association with sulfide mineralization, whereas the latter three dolomite stages almost always occur together and in association with sulfide mineralization. The second and third cement stages generally occur in mineralized, sub-vertically oriented breccias and in horizontally oriented breccias that emanate from them. However, occasional occurrences of all dolomite cement stages, along with sulfide mineralization, are observed in areas not associated with sub-vertical structures, though these are less intensely mineralized.

A KCN stain applied to a hand specimen obtained from a mine dump containing all the macroscopic dolomite stages reveals significant differences. The host rock, its replacement dolomite, the early veining, and the early blocky dolomite open-space cement do not take the stain (Figure 6c). The zoned appearance of the second stage of open-space cement is enhanced by the stain, with each zone staining to varying degrees. The boundary between the second and third stages of open-space-filling dolomite is sharply defined, as the third generation takes a more uniform stain compared to the zoning of the second. The fourth and final dolomite cement stage, observed in hand specimen, takes the stain similar to the third generation but is less intense (Figure 6c).

4.3. Petrography

4.3.1. Host Rock

Host dolomite (D₀) replaces almost all carbonate rocks at Bushy Park, with only scattered remnants of undolomitized limestone remaining [1]. Dolomite textures range from very fine to medium crystalline planar-e to planar-s dolomite (Figure 7a) grading into medium to coarsely crystalline planar-s to nonplanar dolomite (Figure 7b) near mineralization and brecciated zones. Microbial laminates, LLH, and domal columnar stromatolites (stacked hemispheroids or SH) [1] throughout the stratigraphic section typically are replaced by alternating laminae of fine- to medium-crystalline planar-e to planar-s dolomite and chert (Figure 7c,d). The primary fabric of microbial laminates and stromatolites frequently is preserved by this variation in crystal size, varying amounts of organic material, chert replacement, and fine- to medium-crystalline quartz filling intercrystal space in the dolomite. Oolitic grainstones typically are replaced by finely crystalline planar-e to planar-s dolomite (Figure 7e). The primary fabric of the oolitic grainstones is frequently preserved by replacement with chert or with interoolitic porosity filled by fine- to medium-crystalline quartz (Figure 7e,f).

Under CL petrography, the host rock (D₀) dolomites display a bright red to darker mottled CL with non-CL (organic?) residual material filling the intercrystal spaces, especially in SH columnar stromatolites and microbial laminates. Microbial laminates and LLH generally display brighter CL than domal columnar stromatolites, which contain more residual organic (?) matter in intercrystal spaces. Microbial laminates display inter-layering of dolomite lamina in various crystal sizes from finely crystalline planer-e to predominantly medium crystalline planer-s with non-CL quartz (chert) filling fenestral porosity (Figure 7c,d).

The D₁ stage dolomite cement is characterized by blocky or equant dolomite crystals lining open spaces in fractures and breccias that crosscut D₀ replacement dolomite. These cement crystals typically increase in size into the open space. Figure 8a,b displays a bright red CL early phase (z₁) of D₁ stage dolomite cement separated from a darker later phase (z₂) of D₁ dolomite by quartz. D₁ stage dolomite cement extends from the D₀ substrate toward open space, into which it grows. In transmitted polarized light, D₁ cement appears slightly cloudy (Figure 8a). D₁ cement infrequently displays a well-defined zonation in CL but more typically consists of three cloudy zones that are moderate to bright red (z₁), dull red (z₂), and moderate to dull red (z₃) under CL (Figure 8c,d). D₁-stage dolomite frequently is syntaxially overgrown by D₂-stage saddle dolomite cement (Figure 8c,d).

4.3.2. Gangue Minerals

The D₂ stage dolomite cement consists of coarsely to very coarsely crystalline saddle dolomite that lines open-space in fracture and breccia porosity (Figure 8c–f and Figure 9a–d). Using CL, D₂ stage dolomite cement typically exhibits five CL zones, labeled z₄ to z₈ (Figure 8d,f and Figure 9b,d). CL z₄, z₅ and z₆ occasionally display alternately bright-dark bands (Figure 8d,f and Figure 9b,d) with z₅ slightly darker than z₄, while z₆ is characteristically bright orange to red under CL. In hand specimen (Figure 4c) z₇ stands out as light colored. Using plane polarized, transmitted light it appears cloudy with inclusions and under CL z₇ appears as a dark red to nearly non-CL band (Figure 8d–f). This distinctive appearance of z₇ makes it a useful marker for recognizing D₂ stage dolomite cement in both hand specimen and thin section under CL. Using plane polarized light z₈ appears less cloudy with inclusions than z₇ and ranges from dull red to dark (but not as dark as z₇) under CL. Several poorly developed sub-bands occasionally are observed in z₈ under CL (Figure 8d).

The third stage of open-space-filling dolomite cement is termed the D₃ (Figure 8c,d). In hand specimens, it forms saddle-shaped crystals and is distinguished by its gray color (Figure 4). Petrographically, it appears cloudy gray with black (organic?) inclusions (Figure 9a,b,e,f). Under CL it is distinguished from the well-defined, zoned dolomite cement of the D₂ stage by its dull to dark red CL character (Figure 9b,d). In most cases, D₃ cement lacks zonation, but in rare instances, it may have very weak zonation observable with CL. For instance, occasionally D₃ dolomite cement may display a thin, dark CL band, termed z₉, following z₈ of D₂ stage dolomite (Figure 9b). Also under CL, D₃ stage cement typically contains numerous bright yellow, micrometer-sized calcite inclusions (dedolomite) (Figure 9d). The next dolomite cement stage, D₄, is similar to the D₃ dolomite cement, observable as gray saddle-shaped crystals in thin section, and is difficult to petrographically distinguish from D₃ using either transmitted light or CL. Under PPL, the D₄ dolomite cement tends to contain fewer inclusions (organic or otherwise), giving it a slightly lighter color. Under CL, D₄ typically displays as dark red to non-CL, but it is slightly brighter than D₃ stage cement and contains fewer calcite (dedolomite) inclusions. Occasionally, D₄ stage cement is terminated by a bright orange CL zone (Figure 8d and Figure 9b).

Open-space-filling calcite cement occurs throughout the Bushy Park area. It frequently is observed as large crystals following D₂ dolomite and paragenetically prior to quartz. A second generation of calcite cement appears to paragenetically follow ore-stage sulfide mineralization. Calcite cement can be difficult to distinguish from dolomite using transmitted light petrography but usually contains fewer inclusions and may display twinning lamella. Under CL, calcite displays as a bright yellow to yellow-orange color in contrast to dolomite, which typically displays as bright to dull red, red-orange, and non-CL (Figure 8f). Calcite also is observed as a replacement of dolomite cement, especially D₂ and D₃ stage cements. This “dedolomite” is observed under CL as bright yellow patches and inclusions in the dolomite cement.

Fine to medium crystalline authigenic quartz cement occasionally fills open space in host dolomite (Figure 7d–f) and as a coarsely to very coarsely crystalline open-space-filling, following early D₁ stage dolomite cement (Figure 8a,b). Authigenic quartz cement occurs with D₂ stage dolomite cement during z₈ precipitation (Figure 9c,d). Quartz cement forms as large, milky white crystals filling breccia porosity following D₄ stage dolomite cements and sulfide mineralization (Figure 8c,d) and fills veins that crosscut all other stages of dolomite cement (Figure 9a,b,e,f). Quartz occasionally replaces the host rock and dolomite cement (Figure 9g,h and Figure 10a,c,d). Large open-space-filling quartz crystals occasionally display compositional and sector zoning under CL (Figure 10d). Observing this phenonium typically requires long exposure times when taking CL photomicrographs and/or digital manipulation of the photomicrographs.

Bitumen appears to have been deposited sporadically throughout the pre-ore stage and ore-forming stage at Bushy Park. It fills open spaces within quartz (Figure 10c,d), voids between D₁ and D₂ stage dolomite cement (Figure 8c,d), spaces between sphalerite and D₃–D₄ stage dolomite cement, and between sphalerite and the host dolomite and sphalerite and quartz (Figure 10a–d). Sericite is the final gangue mineral phase observed at Bushy Park. It is uncommon and tends to form small veins that crosscut all earlier mineral relationships (Figure 9a).

4.3.3. Sulfides and Ore Minerals

Several stages of pyrite precipitation were observed at Bushy Park. The first stage is composed of anhedral to euhedral crystals, which are observed within the host rock (D₀). In D₁ stage dolomite cement, pyrite occurs between z₁ and z₂ as anhedral crystals, and in D₂ stage dolomite cement, after z₅ as euhedral crystals. The latter grows on the surface of z₅ and is overgrown by the bright CL z₆ dolomite cement (Figure 9c,d and Figure 10b). Lastly, pyrite occurs sporadically among other sulfide ore minerals. However, this occurrence is infrequent.

Sphalerite is primarily restricted to the collapse brecciated zone, where it fills open spaces following D₃ and D₄ dolomite cements (Figure 4b and Figure 6a). Masses of sphalerite crystals can accumulate up to several meters in size. It occasionally replaces D₂, D₃, and D₄ stage dolomite cement (Figure 9h) and, on mine faces, is observed to have remnants of the host rock and cements up to several meters across within the masses. Sphalerite also fills fractures that crosscut dolomite cements, as well as early stages of quartz and host rock. In thin section, corroded boundary relationships between quartz and sphalerite suggest replacement of quartz by sphalerite (Figure 9e,f,g,h). Two sphalerite varieties occur and appear intergrown with one another, brown and olive-green in hand specimen (Figure 6a). The olive-green sphalerite fills open spaces between the brown phase and contains exsolution lamellae of chalcopyrite or “chalcopyrite disease texture” (c.f. Eldridge et al. [39]), while the brown phase lacks these exsolution lamellae. Sphalerite is followed by the final stage of open space filling quartz (Figure 9g,h).

Galena typically appears as a sheet-like habit and follows sphalerite filling open space (Figure 10a). Galena also occurs as “islands” within and adjacent to sphalerite (Figure 10e), where the crystals frequently display a “rounded” habit. Rarely does galena fill fractures or intercrystal space in sphalerite (Figure 10a,f). Galena also fills vugs within sphalerite and, in rare cases, fills spaces along the D₃ and D₄ dolomite boundaries. Galena also replaces the host rock (D₀) dolomite, particularly along dolomite cement-D₀ boundaries, but forms compromise boundaries with quartz and bitumen (Figure 10a). Chalcopyrite, other than chalcopyrite disease texture in sphalerite (Figure 10e,f), is uncommon at Bushy Park. It occurs in cavities in quartz veins that crosscut D₃ and D₄ stage dolomite cement (Figure 4a). Fluorite, a non-sulfide economic mineral (Figure 10b) frequently associated with Pb-Zn deposits, is rare at Bushy Park. Where it occurs, this mineral is mostly in isolation, and it is difficult to determine its relationship to sphalerite and galena and pre- and post-ore gangue minerals.

4.4. Fluid Inclusion Microthermometry

One-phase and two-phase (liquid and vapor) fluid inclusions were identified in open-space-filling dolomite (D₂ and D₃–D₄ stage dolomite cement) and quartz samples (Figure 11). Homogenization temperature (T_H) and ice melting temperature (T_Mice) data from fluid inclusion assemblages (FIAs) are shown in Figure 12 and

Table 1. Fluid inclusion analyses for dolomite and quartz cements at Bushy Park (temperatures in °C).

Sample	Mineral	Assemblage	Th	Tfm	Tmice	NaCl eq	Description
376-FI-1	Dolomite	1	166.0	-	−3.3	5.4	Primary
chip 1	Stage 3/4	1	166.0	-	−3.3	5.4	Primary
		2	164.0	-	−3.3	5.4	Primary
		2	164.0	-	−3.3	5.4	Primary
		2	164.0	-	−3.3	5.4	Primary
		2	164.0	-	−3.3	5.4	Primary
		3	166.0	−18.3	−2.3	3.9	Primary
350-FI-2	Dolomite	1	140.0	-	−20.0	22.4	Primary
chip 1	Stage 2	1	140.0	-	−20.0	22.4	Primary
		1	142.0	-	−20.0	22.4	Primary
		2	110.0	-	−27.0	26.8	Primary
		2	110.0	-	−25.0	25.6	Primary
		2	110.0	-	−25.0	25.6	Primary
		3	194.0	-	−14.0	17.8	Primary
		3	194.0	-	−14.0	17.8	Primary
		3	194.0	-	−14.0	17.8	Primary
		3	194.0	-	−14.0	17.8	Primary
		4	194.0	-	−18.7	21.5	Primary
		4	194.0	-	−18.7	21.5	Primary
		4	194.0	-	−18.7	21.5	Primary
		4	194.0	-	−18.7	21.5	Primary
		4	194.0	-	−18.7	21.5	Primary
		4	194.0	-	−18.7	21.5	Primary
		5	124.0	-	−23.0	24.3	Primary
		5	124.0	-	−23.0	24.3	Primary
		5	124.0	-	−23.0	24.3	Primary
		6	198.0	−36.0	−23.0	24.3	Primary
		7	124.0	-	-	-	Secondary
		7	124.0	-	-	-	Secondary
350-FI-2	Dolomite	1	238.0	-	−8.8	12.6	Primary
chip 2	Stage 2	2	238.0	-	−11.5	15.5	Primary
		3	209.0	-	−11.5	15.5	Primary
		4	160.0	-	−11.5	15.5	Primary
		4	160.0	-	−11.5	15.5	Primary
		5	190.0	-	−12.2	16.1	Primary
		5	190.0	-	−12.2	16.1	Primary
		5	190.0	-	−12.2	16.1	Primary
		6	219.0	-	−11.5	15.5	Primary
		6	219.0	-	−11.5	15.5	Primary
		6	219.0	-	−11.5	15.5	Primary
		6	219.0	-	−11.5	15.5	Primary
		7	238.5	-	−22.0	23.7	Primary
515-FI-5	Dolomite	1	183.6	-	−24.5	25.3	Primary
chip 1	Stage 2	1	181.0	-	−20.2	22.5	Primary
		2	204.0	-	-	-	Pseudosecondary
		2	204.0	-	-	-	Pseudosecondary
		3	190.0	-	−9.7	13.6	Pseudosecondary
		3	190.0	-	−9.7	13.6	Pseudosecondary
		3	190.0	-	−9.7	13.6	Pseudosecondary
		4	183.6	-	−13.8	17.6	Secondary
		4	183.6	-	−13.8	17.6	Secondary
		4	183.6	-	−13.8	17.6	Secondary
		4	183.6	-	−13.8	17.6	Secondary
chip 2		1	98.0	-	−9.5	13.4	Secondary
		1	78.0	−13.0	−9.0	12.8	Secondary
		2	123.0	−13.4	−9.0	12.8	Secondary
		2	123.0	−13.4	−9.0	12.8	Secondary
		3	143.0	-	-	-	Secondary
		4	172.0	−28.4	−7.2	10.7	Secondary
		5	136.0	-	−6.2	9.5	Secondary
		5	136.0	-	−6.2	9.5	Secondary
		6	142.0	−13.8	−12.0	16.0	Secondary
		6	142.0	−13.8	−12.0	16.0	Secondary
		6	142.0	−13.8	−12.0	16.0	Secondary
		6	142.0	−13.8	−12.0	16.0	Secondary
		6	142.0	−13.8	−12.0	16.0	Secondary
		7	-	-	−4.4	7.0	Secondary
		7	-	-	−4.4	7.0	Secondary
		7	-	-	−4.4	7.0	Secondary
		7	-	-	−4.4	7.0	Secondary
		8	176.0	−13.0	−12.4	16.3	Secondary
		9	131.0	-	−9.5	13.4	Secondary
		10	105.0	-	−9.5	13.4	Secondary
		10	105.0	-	−9.5	13.4	Secondary
		10	94.0	-	−9.5	13.4	Secondary
		11	178.0	-	−12.5	16.4	Primary
		12	-	-	−9.4	13.3	Primary
		13	102.0	-	-	-	Secondary
515-FI-1	Dolomite	1	75.0	−17.1	-	-	Secondary-leaked
chip 1	Stage 3/4	2	130.0	−23.3	−4.7	7.4	Primary
		3	134.0	−20.0	−7.1	10.6	Primary
		3	130.0	-	-	-	Primary-leaked
		3	122.0	−27.0	−8.3	12.0	Primary
376-FI-1	Quartz	1	158.0	-	−4.2	6.7	Primary
chip 2		1	158.0	-	−4.2	6.7	Primary
		2	176.0	-	−4.2	6.7	Primary
		3	150.0	-	−5.8	8.9	Primary
		4	132.0	-	−5.8	8.9	Primary
		4	132.0	-	−5.8	8.9	Primary
		4	132.0	-	−5.8	8.9	Primary
		4	132.0	-	−5.8	8.9	Primary
		5	167.0	-	−4.2	6.7	Primary
		5	150.0	-	−5.8	8.9	Primary
515-FI-1	Quartz	1	128.0	−22.0	−16.2	19.6	Secondary
chip 1		1	128.0	−22.0	−16.2	19.6	Secondary
		1	128.0	-	−17.0	20.2	Secondary
		1	128.0	-	−17.0	20.2	Secondary
		2	154.0	-	−16.2	19.6	Secondary
		2	154.0	-	−20.0	22.4	Secondary
		3	141.0	-	−17.0	20.2	Secondary
		3	141.0	-	−17.0	20.2	Secondary
		3	141.0	-	−17.0	20.2	Secondary
		4	144.0	−23.0	−13.3	17.2	Secondary
		4	144.0	−23.0	−13.3	17.2	Secondary
		4	144.0	−23.0	−13.3	17.2	Secondary
		4	144.0	-	−16.4	19.8	Secondary
		5	123.0	-	−16.7	20.0	Secondary
		5	123.0	-	−16.7	20.0	Secondary
		5	144.0	-	−16.7	20.0	Secondary
		6	142.0	−29.0	−18.5	21.3	Primary
		6	142.0	-	−15.5	19.0	Primary
		6	142.0	-	−15.5	19.0	Primary
		6	142.0	-	−15.5	19.0	Primary
		7	133.0	−21.5	−15.1	18.7	Primary
		7	-	-	−15.1	18.7	Primary
		8	145.8	-	−19.3	21.9	Primary
		8	145.8	-	−15.5	19.0	Primary
		8	145.8	-	−15.5	19.0	Primary
		8	145.8	-	−15.5	19.0	Primary
		9	-	−22.5	−16.4	19.8	Primary

. Due to limited optical visibility, particularly on the fluid inclusion stage, and the small size of the inclusions observed (<5 mm), we were unable to measure inclusions in the cloudy darker zones of the dolomite cements. Additionally, some vapor bubbles within the fluid inclusion assemblages in the dolomite cements persisted beyond the freezing point. However, because they were large enough, we could observe the final melting point of these inclusions. First melting temperatures (T_fm) are reported where observed (Table 1), but due to the optics of working with mostly very small inclusions, we do not believe that these reliably indicate eutectic temperatures. However, T_fm values below −21.2 °C can be interpreted as indicating complex brines as opposed to a pure NaCl system.

T_H values for dolomites (primary, secondary, and pseudosecondary) range from 75 to 238 °C, and T_Mice values range from −2.3 to −27 °C (indicating salinities of 3.9 to 26.8 NaCl equivalent, respectively). In addition, T_H values for quartz (primary) range from 128 to 176 °C, and T_Mice values range from −4.2 to −20 °C (indicating salinities of 6.7 to 22.4 NaCl equivalent, respectively) (Figure 12, Table 1). Primary liquid petroleum (oil) inclusion assemblages were observed in quartz, and primary and secondary liquid petroleum inclusion assemblages were observed in dolomites and verified using epi-fluorescence microscopy (Figure 11). Although oil inclusions were present, none of the measured aqueous inclusions showed signs of oil contamination.

Data fields as well as selected individual fluid inclusion values for dolomite cement, quartz, and sphalerite from Schaefer [17] and Kesler et al. [2] are plotted with our data on Figure 12. These data [2,17] are in general agreement with this study.

5. Discussion

5.1. Paragenesis

We present a hypothetical paragenesis for the relative timing of diagenetic events and mineralization, based on our petrographic observations, in Figure 13. These events are divided into four periods of paragenetic time of indeterminate length. These are (1) deposition and early diagenesis, which includes sedimentation and marine diagenesis, as well as synsedimentary brecciation. (2) Pre-ore stage mineralization and burial diagenesis. (3) Ore stage mineralization and accompanying diagenetic events. And (4) post-ore stage mineralization and diagenetic events. The order in which open space-filling cements were precipitated, as well as crosscutting relationships observed in breccias and fractures in hand specimens, cores, and thin sections, were used to establish this paragenesis.

In most respects our paragenesis is similar to that of Schaefer [17]; however, ours differs in several ways. For instance, we did not observe overlap of gangue dolomite cement (D₂, D₃, and D₄ stages) with sphalerite and galena, and we did not observe post-ore dolomite cement. We also place authigenic clay late in the paragenesis, after sulfide mineralization. Schaefer [17] reports minor amounts of several sulfide and oxide minerals that we did not observe. These include argentite (Ag₂S), freibergite ((Ag,Cu,Fe)₁₂(Sb,As)₄S₁₃), millerite (NiS), cassiterite (SnO₂), and rutile (TiO₂). However, even though we did not observe these minerals, we have no reason to suspect that they are not present in small quantities. Schaefer [17] observed bitumen but did not report it as part of his paragenesis.

When comparing the paragenesis at Bushy Park with several Phanerozoic MVT and Irish-type deposits, several interesting similarities and differences arise. The Viburnum Trend in southeastern Missouri, U.S.A., hosted by the Upper Cambrian Bonneterre Dolomite, is one of the largest MVT districts in the world [8]. In contrast to Bushy Park and nearby deposits, galena is the predominant sulfide mineral mined in the Viburnum Trend, followed by argentiferous sphalerite and chalcopyrite [40]. Although ores frequently are hosted by breccias, as at Bushy Park, the distribution of most ore mineralization in the Viburnum Trend is controlled by favorable sedimentary facies [41]. The Viburnum trend also hosts a large variety of non-economic metal sulfide minerals, including a number of cobalt-nickel-copper sulfides [40]. Gangue dolomite cements are important in southeastern Missouri and are precipitated during main-stage sulfide mineralization as opposed to prior to sphalerite and galena, as at Bushy Park. Other non-sulfide gangue minerals, in common at Bushy Park and the Viburnum Trend, include calcite and quartz, but in the Viburnum Trend, both occur late in the paragenesis [40]. Bitumen is commonly observed in the Viburnum Trend, but it also occurs late in the paragenetic sequence [40].

The Tri-State MVT district is located west of the Ozark Uplift at the juncture of the states of Missouri, Oklahoma, and Kansas, U.S.A., and is hosted by Carboniferous (Mississippian) limestones. As at Bushy Park, sphalerite is the dominant ore mineral at Tri-State, followed by galena and chalcopyrite [42,43]. Significant enargite (Cu₃AsS₄) has also been observed at Tri-State [43]. As at Bushy Park, ore mineralization at Tri-State is largely associated with structurally controlled solution collapse breccias [42]. Gangue calcite, dolomite, and quartz cements were precipitated prior to and during main sulfide mineralization at Tri-State [43,44], and bitumen and liquid petroleum appear late in the paragenesis [43,44,45]. Fluorite has not been reported at Tri-State or in the Viburnum Trend.

The Irish-type deposits of the Rathdowney Trend, Irish Midlands, are hosted by Carboniferous (Mississippian) dolomites. Like Bushy Park, the Irish deposits are also sphalerite dominant, followed by galena and chalcopyrite [46,47,48]. Hitzman et al. [46] observed bornite (Cu₅FeS₄), tennantite (Cu₆[Cu₄(Fe,Zn)₂]As₄S₁₃₎, and arsenopyrite (FeAsS) as well as chalcopyrite. Also, as at Bushy Park, ore mineralization in the Rathdowney Trend is associated with fault-controlled solution collapse brecciation [46,47]. Unlike at Bushy Park, the paragenetic timing of gangue dolomite, calcite, quartz, and fluorite all coincide with main-stage sulfide mineralization in the Irish deposits [46,47,48]. As at the other Pb-Zn deposits discussed here, except Bushy Park, bitumen occurs late in the paragenesis in the Irish deposits [49].

A similarity common among all the Pb-Zn deposits discussed here, including Bushy Park, and commonly observed in many other carbonate-hosted Pb-Zn deposits, is compositional zoning in gangue dolomite (and frequently calcite) cements, which is revealed by CL microscopy [40,44,49,50]. Such “CL microstratigraphies” result from fluctuations in the mineralizing fluid chemistry resulting in variation in the uptake of trace and minor amounts of Mn²⁺ and Fe²⁺, the main CL activator and quencher, respectively, as well as other trace elements, in the growing carbonate crystals [51]. Observing and correlating CL microstratigraphies have allowed the mineralizing fluids that precipitated carbonate cements to be traced over large regions as well as to aid in establishing paragenetic timing [44,52,53,54]. A regional study, such as this, is yet to be undertaken in the Transvaal region of South Africa.

5.2. Deposition and Early Diagenesis

Baugaard et al. [1] discuss the cyclic nature of the platform carbonates hosting the Bushy Park mineral deposit. Early diagenetic events include limestone deposition and synsedimentary brecciation, which are treated by Baugaard et al. [1]. The synsedimentary breccias typically are not mineralized [1]. Initial limestone stabilization and cementation [55] are speculated to have occurred similarly to Phanerozoic rocks, but direct evidence for this is largely obliterated by later dolomitization (Figure 13). There is some evidence that aragonite seas were prevalent during the Neoarchean [56], suggesting that aragonite cements were dominant over calcite in Bushy Park sediments.

Silicification in the form of chert replacement, accompanied by finely to very finely crystalline quartz cement and finely to very finely crystalline, mimetic dolomitization [33], preserves some original sedimentary fabrics at Bushy Park, such as oolites, stromatolites, and microbial laminates (Figure 7c–f) [1]. Similar early diagenetic episodes of silicification, preserving the original depositional fabrics in limestones, are observed in Phanerozoic dolomites [57,58]. Early diagenetic dolomitization of the host limestone (D₀ stage dolomite) likely followed early chert and quartz precipitation (Figure 13); otherwise, the original depositional fabrics of the limestones would not have been as well preserved. The finely to very finely crystalline planar textures observed here (Figure 7a,c,d–f) are typical of early diagenetic dolomite [33] and early diagenetic recrystallization and stabilization of dolomite [59,60]. Dolomitization in cyclic platform carbonates in Phanerozoic rocks has been attributed to seawater modified by evaporation, based on depositional association with evidence of evaporites, petrography, and C and O isotope geochemistry [61,62,63]. Additionally, dolomitization of peritidal sediments in Holocene environments frequently is associated with evaporites [64,65,66].

Schaefer [17] reports values of δ¹⁸O between −8‰ and −12‰ (VPDB) and δ¹³C between −1‰ and 1‰ (VPDB) for host dolomite at Bushy Park. These values fall within the range for Neoarchean marine carbonates given by Sheilds and Veizer [67], although their δ¹⁸0 range is poorly constrained between −7‰ and −16‰. Therefore, any shift toward heavier than marine values of the Bushy Park host dolomite, which would be expected if it was precipitated under evaporative conditions, cannot be detected. Although lacking in strong isotope evidence, the platform carbonates at Bushy Park display abundant sedimentological evidence of evaporation, such as pseudomorphs of halite and karst-related breccias near and at the tops of depositional cycles [1]. Based on these sedimentological observations, we speculate that early dolomitization of the Bushy Park platform carbonates was facilitated by marine water modified by evaporation.

An early generation of open space-filling dolomite cement (D₁ stage dolomite cement) (Figure 13) accompanied host rock dolomitization at Bushy Park (Figure 4c, Figure 8a–d and Figure 9a,b). It is interpreted to have formed during stabilization of the host dolomite in the marine environment and during shallow burial. Later recrystallization of very finely to finely crystalline planar dolomite to medium and coarsely crystalline planar and nonplanar dolomite (Figure 7b) occurred during pre-ore stage burial diagenesis (Figure 13).

Occasional finely crystalline anhedral and euhedral pyrite crystals are disseminated in the host dolomite along with pyrite crystals accompanying D₁ dolomite cement (Figure 13). Early diagenetic pyrite would be expected under the anoxic Neoarchean atmosphere [68] with abundant iron sourced from seawater and sulfur from decaying microbial matter.

5.3. Structurally Controlled Collapse Brecciation

The development of carbonate breccia-hosted Pb-Zn mineralization at Bushy Park is controlled by two major factors: structure and stratigraphy [1]. These controls dictate both the spatial distribution and the concentration of ore-bearing fluids, ultimately influencing the geometry of the mineralized zones, including the distribution of metal sulfides and associated gangue dolomite, calcite, and quartz cements, as well as the recrystallization of host dolomite. Mining geologists describe the geometry of the Bushy Park ore body as an “upside-down tea-cup” that becomes wider with depth. Structurally, mineralized collapse breccias at Bushy Park align with orthogonal fault and fracture systems (Figure 3) that follow major structural orientations in the area [1,24] and likely provided pathways for hydrothermal fluid migration and ore deposition. These breccias crosscut the Papkuil, Klippan, and Kogelbeen formations (Figure 2), indicating post-depositional structural deformation. The observation of clasts of banded iron formation and volcanics (Figure 6b) originating in the overlying Asbestos Hills Subgroup and Ongeluk Formation, respectively (Figure 2), indicates the post-depositional timing of the collapse breccia bodies. The orthogonal fracture pattern suggests pre-existing structural weaknesses played a key role in localizing breccia and mineralization. The mineralized collapse breccias also transect sedimentary slump breccias within the Papkuil, Klippan, and Kogelbeen formations, indicating that the latter predate mineralization. They likely acted as porous zones that enhanced fluid flow. This crosscutting relationship of collapse and sedimentary breccias indicates a multi-phase brecciation history where interactions between collapse and sedimentary slump breccias influenced permeability and ore precipitation. The crosscutting relationships indicate that mineralization occurred during or after significant tectonic activity, highlighting the role of structural deformation in ore formation. The crosscutting relationships (Figure 6c) of the mineralized breccia with large clasts of gangue cements containing D₂, D₃, and D₄ stage dolomite, some displaying zebra structure, infilled by sphalerite (Figure 5d–f), also indicate that brecciation persisted throughout the mineralization process, enhancing porosity and facilitating the infiltration of base metal-bearing fluids.

Sedimentary slump breccia at Bushy Park [1] (Figure 5a–c) is generally barren of sulfide mineralization (Figure 4a–d) but likely acted as a conduit along with lithological units, such as permeable microbial laminates, channeling basinal fluids, including petroleum, into the structurally controlled collapse breccia, which served as the primary ore host. This is evident from cementation of slump breccia by D₂ dolomite (Figure 5a–c) with minimal occurrence of later-stage D₃–D₄ dolomite cement. In turn, the D₂ dolomite precedes precipitation of the D₃–D₄ dolomite in the collapse breccia, which indicates that the fluids from which the D₃–D₄ stage dolomite precipitated flowed through both sedimentary and collapse breccias. Large clasts of mostly D₄ and occasionally D₃ dolomite cement within sphalerite (Figure 6a) indicate that sphalerite (and galena) postdate the D₃–D₄ dolomite stage and that brecciation continued through mineralization. Our petrographic studies confirm that within the mineralized breccia body, sphalerite, galena, and later-stage quartz all postdate and replace both the D₃ and D₄ dolomite generations and the D₀ host rock.

5.4. Mineralizing Fluids

Data from fluid inclusion microthermometry of dolomite and quartz cements, obtained for this study (Table 1) at Bushy Park, are in good agreement with earlier work by Schaefer [17] and Kesler et al. [2], which focused on sphalerite and quartz (Figure 12). For comparison with data obtained in this study, T_Mice values reported by Schaefer [17] and Kesler et al. [2] were converted to NaCl equivalents using the Bodnar [38] equation. Excluding low-salinity outliers, sphalerite fluid inclusions at Bushy Park have salinities that range between 8 and 28 weight percent NaCl equivalent and T_H values ranging from 90 to 200 °C. Gangue dolomite cements display a similar salinity range of 0 to 28 weight percent NaCl equivalent and T_H values ranging from 74 to 238 °C. Some low salinity values may be outliers resulting from metastable superheated ice [69]. Quartz inclusions display a more restricted range of 6 to 26 weight percent NaCl equivalent and T_H values ranging from 100 to 200 °C. Homogenization temperatures and salinities obtained for this and earlier studies [2,17] fall into the general range of Irish-type Pb-Zn deposits as defined by Wilkinson [70]. These tend to be higher temperature than MVT deposits. However, the higher salinities encountered at Bushy Park are more characteristic of MVT deposits.

There appear to be three fluid endmembers defined by fluid inclusion homogenization temperatures and salinities at Bushy Park (Figure 12): (1) a high-temperature fluid of moderate to high salinity. (2) A low-temperature fluid of moderate to high salinity. And (3) a moderate-to-low temperature fluid with low salinity. These endmembers are observable in the dolomite, quartz, and sphalerite data. An alternate interpretation is that fluids 1 and 2 are the same and display a cooling trend. However, the lack of paragenetic overlap between sphalerite and dolomite, with sphalerite precipitating after dolomite cements (Figure 13), suggests that the three fluids repeatedly mixed at Bushy Park. If the three-fluid interpretation is correct, primary fluid inclusion assemblages in D₂ stage dolomite indicate that this cement was precipitated during mixing of fluids 1 and 2. Low salinity inclusion assemblages in D_3–4 cements indicate the more dilute fluid becoming dominant. Quartz inclusion assemblages reported here and the data of Schaefer [17] appear to follow this trend. It is likely that high-salinity fluid 1 and fluid 2 inclusions are from quartz precipitated during the D₂ stage of dolomite cement precipitation (Figure 9c,d) and dilute fluid 3 inclusions from quartz precipitated during sulfide mineralization (Figure 8a,b, Figure 9e,f and Figure 10a,f). Sphalerite fluid inclusion data from the previous studies [2,17] indicate a later mixing of the low-temperature, saline fluid (2) and moderate-temperature dilute fluid (3). Similar mixing and fluctuations in dominance of fluids with differing salinities and temperatures have been observed in Phanerozoic Pb-Zn deposits [44,71,72,73,74].

Schaefer [17] applied Raman microspectroscopy to characterize carbonic (CH₄ and CO₂) inclusions. He collected data for two quartz inclusions, identifying carbon residuals as amorphous graphite, and interpreted this data to indicate a lack of post-mineralization metamorphism on the Bushy Park deposit.

Data from fluid inclusion leachates and decrepitation compositions indicate that Na/Br–Cl/Br compositions of Bushy Park ore fluids are consistent with evolved evaporated seawater that has interacted with basement rocks or basement-derived sedimentary rocks, assuming Ca-rich Paleoproterozoic seas [2,17]. This is typical of fluid chemistries of Phanerozoic MVT and Irish-type deposits, which indicate that evaporated seawater is a major component of mineralizing basinal fluids [6,71,72,75]. Dilute fluid 3 is possibly sourced by meteoric and/or normal seawater. Mixing of the dilute fluid with evolved evaporated seawater may have contributed to ore mineralization [3,72,76]. A mixing model for sulfide mineralization, involving evolved evaporated seawater mixing with dilute water, is supported by stable C and O isotope analyses of gangue calcite and dolomite cements [17] and is frequently invoked for Phanerozoic carbonate-hosted Pb–Zn deposits, based on fluid inclusion and isotope data [44,48,76]. Schaefer [17] suggested that the primary source of metals for Bushy Park ores was volcanics in the underlying Vintersdorp Supergroup (Figure 2), although black shales in the Ventersdorp Group are a possible source of metals [77], as is the Maquasi Quartz Porphyry, also in the Ventersdorp Supergroup [28].

Both primary and secondary petroleum (oil) inclusions were observed in quartz and D₂ to D₄ stage dolomite cements (Figure 11c,d). Solid bitumen was also observed with early zones of D₂ stage dolomite cement (Figure 8c,d and Figure 10b,c), later zones of D₂ stage cement (Figure 10c,d), and as patches in sphalerite (Figure 10f). Schaefer [17] did not report observing petroleum inclusions, although he does report observing abundant bitumen (pyrobitumen), which he attributes to locally generated hydrocarbons. Our observations indicate that liquid petroleum (oil) was a component of the mineralizing fluids at Bushy Park throughout the pre-ore and ore-forming paragenesis (Figure 13). This contrasts with the Phanerozoic Pb-Zn deposits discussed above, where evidence for petroleum typically does not appear until late in the paragenesis. The likely source for the hydrocarbons observed in the Pb-Zn deposits in Griqualand West and adjacent basins are black shales in the Ghaap Group [77] (Figure 2).

A major problem with Bushy Park and other Neoarchean carbonate-hosted ore deposits is the source of sulfur for metal sulfides. Kesler et al. [2] correctly points out that oceanic sulfate was uncommon due to a lack of free oxygen prior to the Great Oxidation Event during the Paleoproterozoic [32]. Sulfate in seawater did not begin to increase to Phanerozoic levels until the Neoproterozoic (1.0 Ga) [30,31]. Further, sulfur isotope analyses conducted by Schaefer [17] indicate that it was unlikely sourced by seawater sulfate. He suggested that sulfur could have been sourced by the same rocks that sourced the metals (the Ventersdorp Supergroup) or from the host rock dolomite. We suggest that ubiquitous petroleum, during Bushy Park mineralization, may have been a source of sulfur for sulfide mineralization.

5.5. Timing of Mineralization at Bushy Park

The timing of deposition of the hosting carbonates at Bushy Park has been determined to be between 2.588 ± 0.006 Ga and 2.521 ± 0.003 Ga using zircon age data from the top of the Monteville Formation and the upper Gamohaan Formation, respectively [23] (Figure 2). This dates sedimentation toward the end of the Neoarchean Era. The age of sulfide mineralization relies on age dating of clay minerals presumed to have been precipitated during the mineralization event. Schaefer [17] obtained a date of 2.15 Ga based on K-Ar dating of hydrothermal illite, and Gleason et al. [29], using Ar-Ar dating of hydrothermal illite, obtained a date of 2.05 Ga. Roberts et al. [78] bracket petroleum migration in the Griqualand West Basin between 2.22 and 2.06 Ga. This is based on Rb-Sr dating of andesites immediately overlying the Ghaap Group (Figure 2) at 2.224 Ga [79] and the timing of the emplacement of the Bushveld complex at 2.06 Ga [80], which resulted in metamorphism of the hydrocarbons at the Zeerust carbonate-hosted Pb-Zn-F deposit in the Transvaal Basin northeast of the Griqualand West Basin. These dates for petroleum migration are similar to the radiogenic dates obtained for hydrothermal clays associated with mineralization [17,29] and are consistent with a middle Paleoproterozoic timing for ore mineralization. However, orogenic activity that may have activated mineralizing fluid migration in the Griqualand West Basin has been dated at between 1.7 and 1.8 Ga [81], which would time mineralization to later in the Paleoproterozoic Era.

6. Conclusions

Syngenetic and early diagenetic dolomitization and silicification played an important role in preserving sedimentary fabrics in the unmetamorphosed Neoarchean platform carbonates hosting Bushy Park sulfide mineralization. It is speculated that fine crystalline planar dolomite textures were precipitated by evaporated seawater during cyclic sedimentation, as occurred in Phanerozoic dolomite analogs. These diagenetic modifications significantly influenced later mineralization. Structural control on mineralization, particularly solution collapse breccias, displays similarity with Phanerozoic MVT and Irish-type deposits.

Transmitted light and CL petrography indicate a complex relationship between precipitation of open space-filling gangue cements. These include dolomite, calcite, and quartz, as well as both early and late diagenetic pyrite. Sulfide ore mineralization follows dolomite cement precipitation. Ore minerals include sphalerite, which is dominant, followed by galena and minor chalcopyrite.

Fluid inclusion data indicate three distinct fluid endmembers involved in mineralization: a high-temperature, moderate-to-high salinity fluid; a low-temperature, moderate-to-high salinity fluid; and a moderate-to-low temperature, low salinity fluid. Previous studies indicate that the saline fluid may have been sourced by evolved, evaporated seawater. The fluids mixed repeatedly during ore formation, resulting in dolomite, quartz, calcite, and sulfide mineral precipitation. Compositional zoning in gangue dolomite cement indicates that mineralizing fluid chemistry fluctuated over time, a pattern observed in many Phanerozoic carbonate-hosted Pb-Zn deposits.

Petroleum inclusions in quartz and dolomite cements and solid bitumen, observed throughout the mineral paragenesis, indicate that petroleum (oil) was an important fluid component at Bushy Park. This contrasts with Phanerozoic Pb-Zn deposits, where oil typically appears late in paragenesis. The presence of petroleum may have played an important role in sulfur availability, addressing the issue of limited oceanic sulfate prior to and during the Great Oxidation Event. Previous studies, applying radiogenic methods, date mineralization at Bushy Park to middle Paleoproterozoic time.