Accepted: May 07, 2024 | Published Online: May 09, 2024

Citation: DeMatties TA (2024) Reassessment of the Big Mike Copper-Cobalt Volcanogenic Massive Sulfide Deposit (VMS): Implications for Further Exploration in the Late Paleozoic Havallah Ophiolite Sequence, north-central Nevada, USA. Int J Earth Sci Geophys 10:074

Copyright: © 2024 DeMatties TA. This This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

Abstract

The Big Mike copper-cobalt deposit is a small, high-grade, supergene-enriched mafic (Cyprus)-style volcanogenic massive sulfide (VMS) occurrence hosted in the Late Paleozoic (Early Pennsylvanian to Early Permian) - age Pumpernickel Formation. This host formation lies within the allochthonous Havallah sequence and is interpreted as representing the stratigraphically upper extrusive pillow flows component of a dismembered ophiolite complex.

This study re-examines the Big Mike deposit, its potential ore-equivalent horizon and host geology in light of new geologic and geophysical field evidence. From these data a working exploration model is proposed and new interpretations presented on ore genesis and the potential for additional VMS discoveries in the study area.

Further exploration in the study area is basically exploring “in the shadow of a headframe” or in this case an open pit where the odds of discovery are the best (Muessig's "Exploration Cannon #4"). Nevertheless, the proposed exploration model and mapped geologic framework that hosts the Big Mike deposit and several prospects coupled with new airborne geophysical data can serve as proxies and be applied to the rest of the greenstone (ophiolite) belt.

Several U.S. Geological Survey quantitative mineral assessments (1996 and 2007) of the belt suggest there is a high probability (50%-90% confidence level) for additional blind deposits to be discovered. These mafic (Cyprus)-type VMS deposits will fill out a tonnage distribution for this belt by following a natural geometric (log normal) progression consistent with established grade-tonnage models. Presently, the Havallah sequence remains one of least explored greenstone (ophiolite) belts in North America.

Keywords

Big mike copper-cobalt deposit, Mafic (Cyprus)-style volcanogenic massive sulfide (VMS), Big Mike ore-equivalent horizon, Havallah ophiolite sequence - Pumpernickel formation, Efficient cumulative supergene-enrichment

Introduction

The Late Paleozoic Havallah sequence is one of the least explored greenstone (ophiolite) belts in North America for volcanogenetic massive sulfide (VMS) deposits. Notwithstanding the discovery and successful commercial production of a modest (~576,000 tonnes), but high-grade (av. 3.41% Cu with grades > 10%), supergene-enriched ore reserve in the Big Mike deposit, this belt has been virtually ignored by companies operating in Nevada who are generally seeking more traditional gold-only deposits. At present this belt continues to be woefully underexplored and remains prime hunting ground.

The lack of a good geological model, failure to utilize modern geophysical surveying systems (borehole, ground and airborne electromagnetic-magnetic and gravity systems) as well as advanced soil geochemical surveying techniques (soil gas hydrocarbons [SGH]) coupled with the reluctance to diamond core drill likely contributed to the lack of new discoveries in the belt.

In the study area (Figure 1), the allochthonous Havallah sequence is recognized to host major components of a dismembered ophiolite complex [1-3] and Big Mike the only discovered mafic (Cyprus)-style VMS deposit within this complex [2,4,5]. Similar ophiolite-related mafic VMS deposits as well as its ultramafic companion Outokumpu-type copper-nickel deposits have been studied by Constantinou and Govett [6], Upadhyay and Strong [7], Constantinou [8], Papunan [9], Sherlock [10], Herzig and others [11], Parkkinen and others [12], Franklin and others [13], Peltonen and others [14], Adamides [15], Patten and others [16] and Martin and others [17]. This study is restricted to mafic VMS deposits generally located in the stratigraphically upper pillow basalt flow sequences (Extrusive basalt component) of the ophiolite complex (Figure 1A).

It is interestingly to note that during development of the Big Mike mine in 1970, the deposit was interpreted as a structurally-controlled massive sulfide replacement body [18]. Nevertheless, the ophiolite-related mafic VMS model was firmly established for the belt by 1984, but it was not until 2017 that the first modern airborne electromagnetic-magnetic VTEM (Versatile Time Domain Electromagnetic) survey designed to explore for these types of deposits was successfully completed by junior explorer Nevada Sunrise Metals (Nevada Sunrise Metals Corp. 2018 press releasessee https//nevadasunrise.ca).

Although these types of deposits are commonly small (worldwide 0.740 million tonnes; Mosier, et al. [19,20]), very high metal grades are possible as a result of the paleoweathering episodes as exemplified by the Big Mike mine that produced direct shipping copper ores. Such elusive but highly profitable and environmentally benign deposits are regarded as premier exploration targets. Generally considered politically and socially acceptable, such deposits have more than a reasonable chance of successful mine development under today’s strict permitting regulatory process even in Nevada which is considered a “friendly mining” jurisdiction. An excellent example of this type of deposit is Flambeau in Wisconsin. This profitable VMS deposit produced 1.73 million tonnes of direct shipping copper-gold supergene ores (grading 9.5% Cu and 5.46 g/t Au) from a modest size (~18 hectares) open pit operation [21-23]. As will be discussed, such metal grades and tonnages appear achievable in the study area as well as the Havallah greenstone belt generally.

This contribution was prepared from an exploration perspective. It will:

1) Re-examine in detail the Big Mike VMS deposit that is considered a reliable analog for other undiscovered deposits in the belt.

2) Analyze the geologic framework of the study area that includes newly released airborne geophysical data and how these data relate to the Big Mike deposit and its possible ore-equivalent horizon.

3) Discuss new interpretations on ore genesis and potential for new VMS discoveries in the study area.

4) Propose and discuss a working exploration model based on the results of this study that can help guide further VMS exploration in the study area and Havallah greenstone belt.

Methodology and Sampling

Geologic data and interpretations presented in this paper were generated from field-based exploration programs conducted intermittently from 1995 to 2020. New geologic field mapping (1:10,000 and 1:1,500-scale), trench and outcrop sampling, an airborne VTEM-magnetic survey as well as limited diamond core drilling results have been integrated with legacy geologic mapping and geochemical - geophysical information for the study area. Standard petrographic analyses were completed previously on polished thin sections prepared from legacy drill core samples. Rock and core samples collected by Nevada Sunrise Metals, other former exploration companies (e.g., Utah International, Placer Dome, and U.S. Borax), the Nevada Bureau of Mines and Geology and U.S. Geological Survey were analyzed by conventional fire assay, X-ray fluorescence and ICP-MS analyses. A discussion of quality assurance (QA) and quality control (QC) is beyond the scope of this investigation and not needed for the generalized observations and conclusions for a field-based study of this nature.

Regional Geologic Framework

In northern and central Nevada, the informally named Late Paleozoic (Late Devonian - Early Mississippian to early Late Permian) Havallah sequence and other correlative strata make up volumetrically a significant portion of the regionally-extensive Golconda allochthon [1]. The sequence is exposed in the Sonoma, Tobin (Figure 1), East, Hot Springs, New Pass Ranges and Battle Mountain of central Nevada [1-3,24]. The lithology of the sequence is dominated by a suite of deep ocean basin sediments (Havallah basin) that includes radiolarian and ribbon cherts and argillites with subordinate siliciclastic, calcarenitic and volcanoclastic turbidites that stratigraphically overlie tholeiitic basalts [1,25].

The allochthonous Havallah sequence and superimposed structural regime have been investigated by Ferguson and others [26-28], Muller and others [29], Roberts and others [30], Silberling and Roberts [31], Gilluly [32], Nichols [33], Davis [34], Silberling [35], Erickson and Marsh [36], Snyder [5] Speed [37,38], Snyder and Bruckner [25], Miller and others [39], Brueckner and Snyder [1] Stewart and others [40,41], Jones [42], Jones and Jones [3], Cook and Taylor [43], Snyder and others [44] Ketnet [45], Crafford [46], Vetz [47], and Sturmer and Cashman [48]. Their work has generated various geologic and tectonic models, some controversial, concerning the timing, age, structural setting, and depositional environments. A thorough discussion of all these geologic models is well beyond the scope of this paper.

Although diverse views have been expressed, the present consensus is that an arc-continent collision resulted from tectonic emplacement by thrusting of the oceanic Golconda allochthon eastward onto the Paleozoic margin of western North America in Late Permian to Early Triassic or possibly later in the Early Jurassic [1-3,43,44,47]. This event has been recognized as the Sonoma orogeny.

Brueckner and Snyder [1] first recognized that in the Sonoma and Tobin Ranges, which includes the study area, the allochthonous Havallah sequence was structurally dismembered by a series of regional-scale thrust-bound lithotectonic units (structural panels) that could be defined by age difference and changes in lithology. Within these structural units, numerous thrust and high-angle reverse faults can occur; bedding plane thrust faults are particularly common [1]. Although displacements along the internal thrust faults are unknown, movement on these structures has produced complex polyphase folding within lithotectonic panels particularly in the chert-dominated sections of the Havallah sequence. Brueckner and Snyder [1] identified at least three pre and two post-Golconda thrusting deformational events that structurally overprint the sequence. However, major recumbent folds and overturning of lithologic units have not been delineated in the field. Deformation appears to occur within structural domains with some lithologic units totally unaffected by folding and shearing.

Paleogeographic settings or subterranes within these thrust-bound lithotectonic panels were later delineated by Jones and Jones [3]. At least five distinct structurally bound units were identified. Their Units 1 and 2 are the most relevant to this study. Unit 1 is characterized by an assemblage of interbedded, Upper Mississippian through mid-Permian manganifeous red and green radiolarian chert, argillite and basaltic tuff stratigraphically overlying pillow basalt and basalt breccia. Within this stratigraphic section are blocks of gabbro and lenses of altered serpentinite hosted in highly disrupted tuffaceous mélange [3,25,36]. Unit 2 contains thick sections of interbedded green and dark grey radiolarian-rich chert, argillite and basaltic tuffs. These lithologic assemblages are interpreted as major components of an ophiolite sequence that represent a deep ocean floor environment typical of a mid-ocean rise (spreading center) setting and the fragments of ultramafic rocks as silvers of deeper ocean crust and/or upper mantle.

Snyder [5] and Rye and others [2] were the first to recognize the ophiolitic nature of the Havallah sequence. Further recognition came in several U.S. Geological Survey mineral assessments (1996 and 2007) that included the study area (Cox et al. [67] and Mosier, et al. [19]). These were the last of any pertinent geological-VMS ore deposit studies of the Havallah sequence published in the literature.

Local Geologic Setting

Stratigraphy

This study is focused on the southern Sonoma and northern Tobin Ranges where most of the recent VMS exploration activity has occurred to date (Figure 1). As previously discussed, paleogeographic settings or subterranes Units 1 and 2 of Jones and Jones [3] are exposed in these areas.

In the study area these subterranes can be divided into two mappable units, the Havallah and Pumpernickel Formations that have been tentatively dated as Early Pennsylvanian to Early Permian based on fossil evidence (Figure 2; Silberling and Roberts [31]; Johnson [24]). The presence of a complex structural regime and similar lithologies, at least locally, can make distinguishing between these formations difficult in the field. Nevertheless, in the study area the Havallah Formation is characterized as thick, heterogeneous, and deformed sections of dominantly pelagic radiolarian chert that is commonly pyritic (Figure 3A and Figure 3B) and lesser argillite with subordinate limestone and sandstone as well as rare mafic (basaltic) volcanic units. This formation is generally expressed geophysically as magnetic neutral or low anomalies. In this study the Havallah Formation is interpreted as representing the flysch sediment component at the stratigraphic top of the ophiolite sequence (Figure 1A).

The Havallah Formation stratigraphically overlies and is locally interbedded with the Pumpernickel Formation. This formation contains an assemblage of siliciclastic, calcarenitic and volcanoclastic turbidites (Figure 3D and Figure 3E), cherts and argillites (commonly pyritic and carbonaceous [graphitic]) that stratigraphically overlay sections of massive tholeiitic basalt flows, pillow flows, flow breccias (Figure 3F) and associated pyroclastic rocks [1,5,24]. Locally the basalts are altered and mineralized by VMS-related hydrothermal systems (Figure 3G). The basalt flow sequences are generally expressed as linear magnetic high anomalies that can be easily mapped in the study area. This formation has been interpreted as the extrusive basalt component of the ophiolite sequence (Figure 1A).

Regional metamorphism that overprints these formations is generally lower greenschist facies, thus preserving all volcanic and sedimentary textures.

The Havallah and Pumpernickel Formations in the study area are unconformally overlain by the late Early Triassic Koipato Formation (249.59 to 248.32 Ma; Vetz [47]) that includes, in stratigraphic order, the Limerick Greenstone, Rochester Rhyolite and Weaver Rhyolite [24]. These members are not well defined in the Sonoma and Tobin Ranges and the Koipato in the study area is mapped as mostly undifferentiated (Figure 3C). These undeformed, subaerial volcanic flows, tuffs, tuffaceous sediments were deposited unconformably over a paleoweathered and deformed Havallah sequence surface [47]. Although there is some controversy over the nature of the Havallah-Koipato contact, the most plausible interpretation is that it is an angular unconformity along which block faulting occurred contemporaneously with deposition of the Koipato Formation resulting in uplift and partial erosion [24,31]. This would explain its discontinuous areal distribution and abrupt and erratic thickness changes [24]; see Stewart et al. [41] for an alternative interpretation). Older, stratigraphically lower Pennsylvanian to Early Permian unconformities identified by Sturmer and Cashman [48] elsewhere have not been recognized in the study area thus far.

Both the Havallah sequence and Koipato Formation are intruded locally by small to large Jurassic-aged granite-granodiorite plutons (Figure 1 and Figure 2) and locally Triassic felsic dikes and sills (Figure 4). Younger lithologic successions that unconformably overlie both the Havallah - Pumpernickel Formations and Koipato Group include Tertiary andesite flows, tuffs and sedimentary rocks as well as Quaternary alluvium and Quaternary-Tertiary gravels and massive basalt flows. However, most of these units are not present in the study area other than Quaternary alluvium and Quaternary-Tertiary gravels (Figure 2). Drilling indicates that the alluvium overburden can be as thick as 60+ meters and contain conductive red clay layers.

Structure

Extensive Tertiary high-angle faulting has dismembered the Havallah sequence into a number of structural blocks (Figure 2 and Figure 5). In the study area most of these structures are interpreted from topographic lineaments, outcrops, exposures in trenches and new VTEM survey magnetic data. Limited drilling indicates some of these fault blocks have been rotated to various degrees.

Folding has been interpreted within both the Pumpernickel and Havallah Formations. These structures are based mainly on geophysical interpretation of VTEM conductor dips (Figure 5). Intense folding particularly in the chert sequence of the Havallah Formation has been described by Brueckner and Snyder [1] outside the study area.

Thrust faults associated with the Golconda structural zone have been recognized in the study area. Snyder [5] mapped a series of imbricate thrust faults south of the Big Mike mine area that separate mafic flows from the stratigraphically overlying turbidite sequence (Figure 4). Similar structures are inferred to the northwest in the Coronado prospect (Figure 5 and Figure 6). Limited drilling (COR18-01) indicates these structures can be composed of tectonic breccias (fine red clay gouge supporting 70-90% fine-to block sized, subrounded to angular, crushed lithic fragments) that alternate with unconsolidated rubble zones. This type of structure is likely common in low-angle thrust or high-angle reverse fault zones throughout the study area. Additionally small-scale bedding plane thrust faults are common particularly along lithologic contacts.

Big Mike VMS Deposit

Mafic (Cyprus) - style VMS mineralization within the study area is known mainly from mine development at the Big Mike deposit and exploration of the nearby GEM prospect. The following descriptions of the mine geology and VMS mineralization are from Carithers and McLaren Forbes [18], Snyder [5], Hart [4], Rye and others [2], DeMatties and others [1] and unpublished sources.

Mining history

The first documented exploration in the study area was in 1930’s with the discovery of surface gossans capping the Big Mike copper deposit. In 1967, C.C. Chamberlain acquired the Big Mike property and formed the Big Mike Corporation. Drilling by the corporation delineated several thousand short tons of copper oxide ore. Small-scale surface mining began in 1967 and high-grade direct shipping ore produced. In 1968 a small mill and leaching plant were set up on the property to process lower-grade ore. In early 1968 Chamberlain sold the property to Cerro Corporation (Cerro). Exploration by Cerro successfully discovered a deeper (~60m) high-grade, supergene-enriched massive sulfide lens. A total of 59 mostly diamond core holes (7,039 m of drilling) were drilled to define a small but high-grade copper resource. Later in late 1969, Cerro sold the property to Ranchers Exploration and Development (Ranchers). By 1970 Ranchers had completed a positive feasibility study and commissioned Dravo Corporation to develop the identified reserve by open pit mining. The short-lived mine operated from January to August 1970 and produced nearly 100,000 short tons (grading 10.5% Cu) of direct shipping ore that was sent to a smelter in West Germany. Approximately 25 million pounds of copper were produced. A heap leaching facility was erected in November 1970 by Ranchers to treat stockpiled lower grade disseminated copper ore; approximately 300,000 short tons of mineralized rock were treated up to 1978 [2,4,18].

Local mine geology

The local geology that hosts the Big Mike deposit has been mapped as the Pumpernickel Formation and is consistent with regional structural trends previously discussed. Near the deposit, the Pumpernickel Formation is composed of moderate to steeply dipping (30°-45° NE), submarine mafic volcanic units (basalt-andesitic basalt flows and pyroclastic rocks), stratigraphically overlain by radiolarian chert, pebble marlstone and argillite strata (Figure 4; Rye, et al. [2]; Snyder [5]). A series of at least four internal thrust faults dismember the sequence and likely repeat the volcanic-sedimentary section. Regional metamorphism of the mine sequence is low, never exceeding lower greenschist facies; all primary sedimentary and volcanic textures are preserved.

Mine stratigraphy

Mine stratigraphy includes a thin (9m) pyrite-bearing carbonaceous (graphic) cherty and chloritic argillite designated the middle argillite-chert unit that is the host ore horizon (Figure 7, Figure 8, Figure 9B, Figure 9C and Figure 9D). This unit is bounded stratigraphically by footwall pillow basalt (lower greenstone unit) and hanging-wall sequence of pillowed basalt flows, mafic hyloclastics, hyaloclastic breccias (upper greenstone unit) and pebbly mudstones interlayered chert and argillite (upper argillite-chert-pebble mudstone unit; Figure 9A). Manganese enrichments occur in the hanging-wall sediments and limited assay data suggest the middle argillite-chert unit is geochemically anomalous in copper, zinc, and gold (Table 1).

Sulfides present in the middle argillite-chert unit include locally framboidal pyrite that occurs as fine-grained disseminations and thin (1-5 mm) massive laminations (locally folded and small-scale faulting). Sulfur isotope studies by Rye and others [2] indicate this pyritic stockwork mineralization is biogenic (low temperature) rather than hydrothermal in origin. Such biogenic pyrite commonly occurs in other carbonaceous argillites as well as interflow sediments within both the Pumpernickel and Havallah Formations. Other disseminated sulfides (pyrite ± chalcopyrite) present in this unit are believed to be hydrothermal in origin related to massive sulfide deposition (i.e., sulfide halo). Geophysically, this unit was expressed as a distinct induced polarization (IP) anomaly [18].

Big Mike mineralized zones

VMS-style mineralization, hosted by the middle argillite-chert unit, consists of two moderate to steeply dipping (30°-45° NE), stratabound massive cupriferous pyrite lenses that are stratigraphically underlain by manganese-enriched chert and overlain by hematitic chert (jasper). These thin chert units are considered exhalites related to massive sulfide deposition. The shallower “upper” lens is thoroughly oxidized forming a subaerial gossan while the deeper “lower” lens is partially oxidized but variably supergene- enriched. Paleoweathering and supergene oxidation of the massive sulfide lenses has produced a well-developed vertically zoned supergene geochemical stratigraphy (Figure 8).

Upper lens

The strongly oxidized “upper” lens cropped out at the premining surface. This discovery outcrop was a one-meter high siliceous hematitic (“live hematite”) gossan that extended down-dip approximately 33 meters (Figure 7 and Figure 8). The gossan exhibits a well-developed, delicate cellular hematitic (± goethite), siliceous boxwork texture (siliceous sponge; Figure 10A and Figure 10B). Residual copper values are in the 100-400 ppm range and mercury ≥ 3000 ppb (Table 1). Of note is the presence of significant gold values (0.66 g/t-3.92 g/t) in dump samples that were analyzed (Table 1). Ore-grade copper oxide mineralization (i.e., oxide copper zone) occurred near the base of the gossan (Figure 8) and was mined-out during the early stages of development. Besides the typical mineral assemblage of malachite, covellite, cuprite, chrysocolla and native copper, cobalt oxide [heterogenite-CoO (OH)] has been identified (Figure 10C) in the zone although its distribution and concentrations are unknown. Cobalt was not recovered from the oxide copper ores. Nevertheless, significant amounts of cobalt oxide would have likely occurred in the oxide zone given the high cobalt grades present in hypogene massive sulfide mineralization.

Lower massive sulfide lens

The deeper (~60 to 90 m) “lower” massive sulfide lens exhibited variable supergene enrichment (Figure 8). The lens, which is now mined-out, was reported to have had a strike length of 76 meters, a width of 49 meters and true thickness of up to 15 meters. Other smaller lenses were present as well. The lens was dismembered by normal faulting and believed to be cut off at depth by a thrust fault [18].

Hypogene massive (> 80%) sulfide mineralization in the lens consists of dominantly fine to coarse-grained, granular-textured, anhedral to euhedral (recrystallized) pyrite. Locally, fragmental massive sulfide occurs as subangular, fine to medium-sized, pyrite grain aggregates/fragments supported in a finer-grained pyrite matrix (Figure 11A). Other more complex sulfide mineral textures (e.g., collomorphic) were present and have been described by Rye and others [2]. A crude layering that probably represents relict bedding was observed throughout the lens.

Late-stage chalcopyrite (15%-20%) occurs as cross cutting veinlets, replacement blebs (after pyrite) fine to very fine-grained interstitial anhedral grains (intergrowths) and micro-size inclusions within pyrite (Figure 11B, Figure 12A and Figure 12B); Rye, et al. [2]; Carithers and McLaren Forbes [18]; Ypma [50]). Minor sphalerite is associated with chalcopyrite. However, the zinc content is too low to establish any meaningful metal (Cu/Zn) zoning pattern within the lens.

The massive sulfide mineralization is siliceous with up to 10 percent interstitial quartz as fibrous, cryptocrystalline, microcrystalline, and macrocrystalline forms. It becomes dominant in the upper and lower margins of the lens [2].

Assay data indicate consistently high cobalt values (± 0.2%) in the massive sulfide mineralization (Table 1). However, no hypogene cobalt minerals (e.g., carrolite - CuCo ₂ S ₄ ; cobaltite - CoAsS) have been identified to date. The most likely source for the cobalt is as solid solution within pyrite (cobaltian pyrite). Table 1 suggests potential ore grades are associated with copper values greater than 5.0 percent.

The economically important lower massive sulfide lens was hosted entirely by the middle argillite - chert unit (ore horizon) and underlain by intense hydrothermally altered (sericite-chlorite-quartz ± carbonate) and mineralized (disseminated chalcopyrite + minor pyrite) pillow basalt flow (Figure 12C). These sulfides minerals can replace the matrix between pillows in the basalt flow [2].

The lens graded vertically and laterally into a disseminated sulfide (pyrite + chalcopyrite) halo within the host ore horizon. Although this halo appears restricted to the immediate vicinity of the massive sulfide lens, it grades outward into the enveloping pyritic (biogenetic) stockwork mineralization hosted by the ore horizon.

In terms of exploration, Carithers and McLaren Forbes [18] reported that the massive sulfide lens was highly conductive and produced a strong ground electromagnetic (EM) anomaly that geophysically expressed the deposit. It should be noted that the 2017 VTEM survey did not detect any anomaly over the deposit. This suggests that the mined-out massive sulfide zone was restricted in size and depth extent (i.e., to the maximum depth penetration of the VTEM survey).

Supergene oxidation and copper enrichment

Both pyrite and chalcopyrite in the “lower” massive sulfide lens are variably replaced by high Cu/S supergene minerals (i.e., the enriched copper zone) that include digenite (Cu ₉ S ₅ )/chalcocite (Cu ₂ S) and djurleite (Cu ₂ S ₆ ). These minerals occur throughout the lens commonly replacing and/or coating hypogene pyrite and chalcopyrite as well as occurring as fine intergranular replacements after chalcopyrite or as fracture fillings (Figure 12A and Figure 12B). It is unknown whether distinct vertical mineral zoning and variations in the Cu/S ratios occurred with depth. However, covellite (CuS) was reported restricted to the upper margin of the lens suggesting at least some lateral variation.

Supergene copper enrichment has also occurred within the host ore horizon (argillite-chert unit; Figure 9C and Figure 9D). This includes partial or total replacement of pyrite + chalcopyrite grains by digenite and covellite in the disseminated halo but rarely occurring interstitially between framboidal pyrite grains [2].

Quantitatively measured mineral ratios, particularly the enrichment-related (digenite/chalcocite + djurleite)/pyrite ratio were not completed to assess the maturity of supergene enrichment [51]. Nevertheless, limited petrographic work by Ypma [50], Rye and others [2] suggest a mature and robust supergene system.

The shallower portion of the “lower” lens is partially oxidized. This oxide copper zone was reported to contain tenorite, native copper, cuprite, malachite, azurite and chrysocolla that are commonly mixed with unoxidized hypogene pyrite and chalcopyrite (i.e., mixed ore; Table 2). No supergene cobalt oxide minerals (e.g., heterogenite (CoO(OH); erythrite [cobalt bloom], Co ₃ (AsO ₄ )28H ₂ O) or hypogene cobalt sulfide/arsenide minerals (e.g., carrolite, CuCo ₂ S ₄ ; cobaltite, CoAsS) were reported in the zone.

Stringer sulfide zone

Stringer mineralization consisting of veinlets of quartz with pyrite, carbonate, sericite and some chalcopyrite and sphalerite occurs in both the stratigraphic footwall and hanging-wall pillow basalt flows. The most prominent zone is in the footwall at the west end of the pit (Locations C and C-1; Figure 7). Where exposed in the pit, the zone consists of a stockwork of completely oxidized (hematite-goethite after pyrite ± chalcopyrite), siliceous (quartz ± supergene silica (?)± chlorite veins/veinlets that cross cut oxidized (goethite + jarosite?), altered and bleached (sericite-chlorite-quartz ± carbonate) basaltic flow (Figure 12C, Figure 13A, Figure 13B and Figure 13C).

This footwall stringer mineralization is up-dip but offset from the lower massive sulfide lens and not directly below it. Its offset position relative to both lenses indicates massive sulfide deposition was distal to this fossil seafloor discharge center, with sulfides possibly accumulating down-slope and along the flanks of the vent. Other more proximal massive sulfide lenses close to or directly over the vent may have been eroded away or possibly not identified yet.

The lateral and downward extent of the stringer zone is unknown due to lack of drilling. However, pyrite mineralization has been logged in drill core located 300 meters stratigraphically below the lower massive sulfide lens [2].

Stringer sulfide mineralization has also been intersected in drillholes (35 and 59) north of the lower lens in the hanging-wall pillow basalt (upper greenstone unit; Figure 7). This would indicate a possible second mineralizing event at a higher stratigraphic level in this area. Supergene oxidation and enrichment of chalcopyrite in the hanging-wall stringer zone has not been observed [2].

Big Mike mineral resource

Mineral resources for the deposit are presented in Table 2. Although the mined reserve was modest in size, a shallow depth (open pitiable) coupled with supergene-enriched high copper grades allowed a portion of the deposit to be economically viable. This may be true for other undiscovered VMS deposits in the belt.

Additional inferred resources in this deposit of approximately 1.1 million tonnes grading 1.05 percent copper may be present in the pit; pit walls leach pad and dumps [52]. This increase would bring the actual deposit size to approximately 1.7 million tonnes. Figure 13D compares the Big Mike resource tonnage and grades with other worldwide mafic (Cyprus)-style VMS deposits.

Big Mike Ore-Equivalent Horizon

Extensions of the Big Mike mineralized system and/or additional VMS deposits like Big Mike, if present, would likely occur at roughly the same time and therefore at the same stratigraphic level, the so called “ore-equivalent” horizon. Additionally, such deposits would tend to cluster along that horizon [53,54].

The ore-equivalent horizon at Big Mike has been defined as the strike extension of the host mineralized host middle argillite-chert interflow unit (i.e., ore horizon; Figure 4). The ore horizon was traced by previous surface mapping to the northwest and southeast a short distance beyond the mine pit before being covered by alluvial overburden [2,5]. Further mapping by Snyder [5] identified the broader contact between the flow and sediment sequences (Figure 2 and Figure 4); the middle-chert unit lies a short distant stratigraphically below this contact within the flow sequence (Figure 7).

VTEM conductor zones

Indirect evidence of the Big Mike ore-equivalent horizon occurs northwest along strike of the mine sequence (Figure 2 and Figure 5). Here the horizon may be geophysically expressed by the group of two parallel, weak to strongly conductive formational EM bedrock conductors identified in the 2017 airborne VTEM survey (Figure 14 and Figure 15). These extend along strike for over 4.5 kilometers but are disrupted by high angle normal faulting and possibly repeated by folding or more likely thrust faulting. Limited drilling at the Coronado prospect intersected the edge of one conductor (Coronado South zone) that consisted of weakly conductive black carbonaceous argillite-silty argillite (Figure 6 and Figure 19C). It is likely that the main source for these conductors is massive-semi massive pyrite (biogenic and/or hydrothermal) and graphite as well as clay that compose the argillites/mudstones. However, it should be noted that bedding plane thrust faults can develop within these structurally incompetent units. Thrusting would likely result in formation of clay fault gouges. Such clay gouges are conductive and when becoming wet and saturated with saline ground water that migrates along the fault planes, they can produce very strong EM conductors. Additionally, these clays if present could be at least a contributing and/or main source of the VTEM anomalies. Therefore, some of the formational conductors identified in the survey may also represent thrust fault zones.

A number of the identified formational VTEM conductors in the study area occur within or adjacent to interpreted mafic (basaltic) flow units that have also been delineated by the 2017 airborne VTEM survey. As previously noted, these flow units are expressed geophysically as linear magnetic high anomalies (Figure 14). Limited drilling has confirmed that these magnetic anomalies are sourced by mafic (basaltic) flows. The location of those conductors that are directly coincident with magnetic anomalies suggests they likely represent prospective interflow argillite sediment units that may be similar to the Big Mike mineralized middle argillite-chert unit (i.e., ore horizon). Conductivity along these formational units varies significantly with the highest priority targets in areas of strong EM conductance. Two high-conductance zones (Coronado North and South) have been identified and modeled on the Coronado prospect (Figure 15). These geophysical models of the conductor suggest a similar geometry to that of the Big Mike lower massive sulfide lens.

The formational conductors extend across this prospect and cover an area of approximately 1,400 meters by 700 meters. Within the formation, the Coronado South zone is interpreted as a northwest striking, cuboid-shaped body with estimated dimensions of 900 meters by 300 meters by 150 meters. Its width, dip, thickness and depth vary along strike suggesting the zone is dismembered by faulting (Figure 16). The Coronado North zone is slightly smaller but exhibits similar characteristics and amplitudes (Figure 17). Both zones have direct magnetic support.

VTEM conductor zones

A mineralized interflow horizon similar the Big Mike ore horizon has been identified along the regional trend at the GEM prospect approximately 8 kilometers northwest of the mine (Figure 2). Here, historical exploration trenches exposed a thin (≤ 9m) section of locally gossanous, interflow chert - cherty argillite exhalite (≤ 1m thick) within a fault-bounded silver of the Pumpernickel Formation (Figure 18). Assays indicate the cherty argillite contains anomalous copper (357 ppm), zinc (1,887 ppm), silver (570 ppb) and manganese (599 ppm) values (Table 3). The exhalite unit is in fault contact with a hydrothermally altered and mineralized (oxidized sulfide stringer zone) basalt flow (Figure 19A and Figure 19B). Gouge in the fault zone consists of conductive white clay. A legacy surface EM survey (MaxMin horizontal loop) was successful tracing this fault contact to the southeast under a juxtaposed structural block of Havallah Formation (Figure 18). The fault zone was expressed as several weak linear EM conductors.

This cherty argillite exhalite appears to mimic the geochemically anomalous Big Mike ore horizon. Diamond core drilling down-dip of the trenches and along strike of the fault trace, completed in 1976 by Utah International confirms the presence of multiple thin beds of high-grade cupriferous massive sulfide mineralization (1.84% Cu over 11 m; Table 3) on this property. Although much more exploration work is required, the lithologic and geochemical similarities of this mineralized unit to that of the middle argillite-chert unit at Big Mike suggests it could represent the northwestern most extension of the Big Mike ore-equivalent horizon.

Discussion

Available geologic and geophysical data generated from exploration in the study area has been interpreted to represent features common to ophiolite-related, mafic (Cyprus)-style VMS mineralization and geologic environments. To honor all the empirical data collected to date, a working exploration-geologic model is proposed that considers four components.

1) The temporal and spatial distribution of hypogene massive sulfides

2) Primary depositional environment

3) Hypogene enrichment - zone refining

4) Secondary paleoweathering and supergene oxidation effects

Temporal - spatial distribution of hypogene massive sulfides

The first component of the model considers the Havallah sequence a dismembered ophiolite complex where stratabound hypogene VMS-style mineralization within mafic volcanic sequences of the Pumpernickel Formation is localized along the contact with altered basaltic flow and stratigraphically overlying cherty (exhalative) carbonaceous and pyritic interflow argillite. The argillite unit hosts potentially ore-grade and tonnages of massive to semimassive sulfide mineralization. Unless the sulfides are silica encapsulated, zinc- rich and/or very deep, they should be highly conductive and thus produce a strong EM anomaly.

In this model the host argillite unit represents a period of quiescence during which massive sulfide deposition can occur between mafic volcanic cycles. This favorable time interval is identified stratigraphically as the ore horizon. Stratigraphic extensions of the ore horizon, or the so called ore-equivalent horizon, mark the ore-forming event. These productive intervals generally occur on a district-scale and are commonly cyclic or repeated by folding within the stratigraphic sequence.

In the study area the ore-equivalent horizon related to the Big Mike VMS deposit may have been identified. As previously noted, support for an ore-equivalent horizon comes from the group of parallel, weak to strong conductive formational EM conductors that extend from the Big Mike mine northwest into the Coronado prospect area. Limited drilling confirmed that weakly conductive black argillite was a contributing source of the formational conductors. Additionally, geophysical modeling suggests several strong conductors within the formations have geometries very similar to the Big Mike lower massive sulfide lens.

Although most of these formational conductors remain untested in the study area, typically such formations in other VMS districts are sourced by sulfidic (often with anomalous concentrations of Cu-Zn) - carbonaceous (graphitic) argillite/mudstone sediments very similar to the Big Mike mineralized middle argillite-chert unit. Their distinct linear electromagnetic patterns allow them to be used as mappable marker horizons as was done in the study area. Recognized stratigraphically as hiatuses in volcanic activity, these key units represent isolated (restricted) second-order rift sub-basins/depressions where exhalative VMS-type deposition is commonly localized.

The long (kilometers) formational electromagnetic conductors have several sources. They include highly conductive graphite and pyrite and/or conductive - magnetic pyrrhotite (which is not likely in the study area). These minerals commonly form disseminated and semimassive (≥ 30%) to massive (≥ 50%) textures that are commonly hosted in black to greenish gray, weakly to strongly schistose, chlorite-rich argillite - cherty argillite and associated interbedded tuffs and tuffaceous sediments. Individual units are generally less than 30 to as thick as 100 meters and commonly exhibit well developed internal lamination or bedding as well as other sedimentary features (e.g. graded bedding, load casts etc).

Textural evidence suggests that sulfide minerals in many of these formations are formed by hydrothermal (exhalative) rather than diagenetic/biogenic processes. Textural criteria include 1) Tenancy of sulfide to form in laminations or beds and small grains (spherical or cubic), 2) A lack of correlation between sulfide and graphite (carbon tends not to exceed the sulfide sulfur content), 3) Presence of bedded chert (chemical sediment - exhalite), 4) Lack of framboidal structures and 5) The presences of anomalous base metal concentrations [55,56].

Although this style of sulfide mineralization hosted by these argillite formations is not economic, it can be associated spatially and genetically with VMS deposits and is considered a precursor or successor to potentially economic VMS mineralization, i.e., occurring before, during and after the ore-forming event (e.g. Penokean volcanic belt; DeMatties [22,23]).

Depositional environment

The Big Mike middle argillite-chert host unit is thin (~9m thick) and has limited areal extent. This suggests that sulfide deposition was relatively short lived and occurred in a relatively small restricted basin before burial and preservation by mafic flows. A second period of quiescence is represented by the stratigraphically upper argillite unit. Possible sulfide deposition during this period may have been longer. However, reworking and erosion likely occurred as evidenced by sulfide and chert clasts in these sediments [2].

The depositional basin that hosts the Big Mike deposit is interpreted to have developed at very deep paleo-water depths. This interpretation is supported by the presence of abundant hypogene copper (> 1 wt% Cu) in chalcopyrite present in this deposit. Experimental evidence indicates chalcopyrite forms at ≥ 300 °C [57]. The combination of a minimum temperature of 300 °C combined with average salinity of measured high-temperature sea-floor hydrothermal fluids suggest this copper-rich deposit formed at water depths of > 850 meters [58]. At shallower depths VMS hydrothermal fluids would boil in the subsurface and not precipitate massive sulfide at or near the paleo-seafloor at this high temperature and salinity [59].

The EM formational group is interpreted as representing deep (?), second order, probably fault bounded, marginal basins or sea floor depressions. If this interpretation is correct, these basins appear more extensive in the Coronado prospect area. A larger basin implies thicker argillite units, longer periods of quiescence and sulfide deposition. Such prospective units have the potential for hosting thick, aerially extensive, blanket-type massive sulfide accumulations that can form large (> 50 Mt; e.g. Crandon), very large (> 100 Mt; e.g. Kid Creek) and in rare situations giant (> 200 Mt; e.g. Neves Corvo) VMS deposits or deposit clusters regardless of their geologic setting. This assumes a robust and protracted hydrothermal system has developed. These longer deposition periods subject massive sulfide accumulations at the paleosurface to potential submarine weathering and erosion but also oxidization and supergene enrichment [60,61].

Hypogene zone refining

An important primary effect considered in the model is hypogene metal grade and distribution during prolonged sulfide accumulation. This phenomenon, known as zone refining, would have a greater influence on copper which is dominant in these types of deposits. Upward refining would result from multiple hydrothermal pulses that dissolve, vertically move and re-precipitate copper at higher stratigraphic levels in the developing sulfide mound. Hypogene enrichment can result in high copper grades at or near the stratigraphic top of the mineralized horizon. New sampling and previous petrographic analyses of Big Mike massive sulfide ore samples suggests at least some zone refining likely occurred in the lower lens. The average hypogene grade in the massive sulfide ore zones was probably well above the 1.7% average for mafic (Cyprus) VMS deposits [20].

Supergene oxidation and efficient cumulative supergene-enrichment

Another component of the model deals with supergene oxidation and enrichment. Supergene copper (and Co?) enrichment that has occurred at Big Mike was significant and allowed this small deposit to be economically viable. To achieve these high grades (> 10% Cu) multiple cycles of protracted paleoweathering are required. This involves an initial uplift, exhumation, paleoweathering and burial of the deposit followed by at least one more similar cycle. The second cycle includes re-exposure and reworking of the deposit. To achieve sufficient cumulative metal enrichment, uninterrupted supergene activity during each cycle must outpace denudation rates [22,51]. This condition is most favorable within down-dropped fault blocks.

In the study area, at least two episodes of uplift and paleoweathering - oxidation have been identified [24]. Support for two episodes comes from analyses of the Big Mike deposit gossan where its composition and boxwork structure suggest a second re-oxidization and subsequent supergene enrichment event has occurred (Figure 10A and Figure 10B; Blanchard [62]; Chavez [51]).

The first episode would have been after obduction and emplacement of the Golconda allochthon. Differential uplift of the Havallah paleosurface was likely a direct result of this compressional (complex thrust faulting) tectonic event that occurred during the Permian-Triassic transition. Dismemberment by imbricate thrusting resulted in repetition and exposure of the sequence at different stratigraphic levels. The resulting uplift would have exposed the Havallah paleosurface to its first major weathering episode. Mineralized horizons near the paleosurface were exhumed and affected during this protracted period which ceased upon deposition (249.59 to 248.32 Ma) of and burial by the Triassic Koipato Formation. Vetz [47] has constrained the unconformity time gap of the Koipato Formation to approximately 6 to 15 million years. During this episode of paleoweathering, supergene metal-enriched profiles could develop and geochemically mature in protective terrains with favorable fault block movements. Profiles covered by Koipato volcanic units were preserved. However, because this cover may have been discontinuous in aerial distribution, exposed profiles in up-thrown fault blocks would have likely been lost to erosion during unroofing of the Koipato Formation.

Several other minor (≤ 1 million years) unconformities between various members of the Koipato Formation have also been recognized by Vetz [47]. These short time gaps may have had some effect on re-exposure and/or erosion of supergene profiles.

Further uplift and subsequent unroofing of most, but not all, of the Koipato Formation likely occurred during the Middle and/or Late Jurassic Luning-Fencemaker compressional overthrust faulting event and possibly later by younger deformation phases resulting from emplacement of mid- Cretaceous granitoids (dated 102 Ma) related to the Sevier orogeny [63].

Parts of the Havallah paleosurface could have been re-exposed to erosion after partial unroofing of the Koipato Formation but again reburied by Tertiary volcanic rocks during the Late Cretaceous to Paleocene Laramide orogeny (80-55 Ma). Subsequent uplift of Tertiary formations caused partial unroofing and re-exposure of the Havallah paleosurface. This second major episode of paleoweathering and supergene activity has extended to the present. Newly exhumed massive sulfide horizons could develop supergene metal-enriched profiles assuming favorable conditions. However, pre-existing profiles, if re-exposed and remained in protected structural down-dropped fault blocks, could geochemically mature further. These profiles may have achieved maximum enrichment (i.e., “super-enrichment”) if preserved. Massive sulfide horizons exposed in terrains where erosion rates exceeded supergene activity would be lost. Some pre-existing supergene profiles may still be buried under erosional outliers of the Koipato Formation as mapped in the study area and were not subjected to the second paleoweathering episode.

Conclusions

Results from this investigation clearly demonstrate that the Pumpernickel Formation of the Havallah greenstone belt is highly prospective for the occurrence of economically viable mafic (Cyprus) - style VMS deposits. Data collected to date are sufficient to establish a reasonable working geologic model to guide future exploration in the study area (Figure 20).

Basic assumptions and corollaries that support and augment this exploration model include the following;

1) The Havallah greenstone belt (Havallah sequence) is interpreted as a structurally dismembered ophiolite complex. The stratigraphically upper portions of the ophiolite sequence are best preserved in the study area and represented by the Havallah and Pumpernickel Formations.

2) Interflow sediments within flow sequences of the Pumpernickel Formation may be the locus for potentially economic VMS deposits.

3) VMS mineralization in the study area is likely to mimic that found at the Big Mike deposit (i.e., high hypogene Cu-Co-grades in massive sulfide mineralization that are enhanced by supergene metal-enrichment) and hosted in interflow, geochemically anomalous argillite-chert sediment units.

4) The interflow argillite-chert ore horizon and massive sulfide mineralization are geophysically expressed as IP anomalies and/or EM conductors.

5) Two periods of sulfide deposition at separate stratigraphic positions have been recognized in the study area. These multiple depositional periods improve the odds of successful deposit discovery.

6) At least two episodes of paleoweathering have been identified that are responsible for the efficient cumulative supergene copper-gold-cobalt (?) enrichment at the Big Mike deposit and possibly other undiscovered deposits in the study area.

Further exploration in the study area is basically exploring “in the shadow of a head frame” or in this case an open pit where the odds of discovery are the best (“Exploration Cannon #4”; Muessig [64]). Nevertheless, the proposed model is considered sufficient to help guide exploration in other parts of the Havallah ophiolite belt with the objective of discovering one or more near-surface (bulk-mineable), supergene-enriched massive copper-gold-silver-cobalt-bearing VMS deposits that can support a modest to large open pit mining operation and produces direct shipping ores.

Several U.S. Geological quantitative mineral assessments for the belt suggest there is a high probability (50%-90% confidence) for additional blind VMS deposits to be discovered (Cox et al. [67] and Mosier, et al. [19]). These deposits will fill out a tonnage distribution for this belt by following a natural geometric (log normal) progression consistent with established grade-tonnage models.

Acknowledgments

The author would like to thank Nevada Sunrise Metals Corporation for allowing release of exploration data from the Coronado exploration program. Special thanks to John Munroe who analyzed and interpreted the airborne geophysical data of the study area. Also, I acknowledge and thank several reviewers that greatly improved the manuscript. This study did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

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