The Discovery of the Romero VMS Deposit and Its Bearing on the Metallogenic Evolution of Hispaniola during the Cretaceous

Abstract

The recently discovered Romero deposit, located in the Tres Palmas district, Cordillera Central of the Dominican Republic, has probable reserves of 840,000 oz gold, 980,000 oz silver and 136 Mlb copper. Mineralization is hosted by intermediate volcanic and volcaniclastic rocks of the lower stratigraphic sequence of the Cretaceous Tireo formation. The andesitic host rocks yield a U-Pb zircon concordia age of 116 ± 10 Ma. Au–Ag–Cu(–Zn) mineralization is divided into: (1) an upper domain with stacked massive sulfide lenses and sulfide dissemination within a 20-m-thick level of massive anhydrite-gypsum nodules, and (2) a lower domain with a high-grade stockwork mineralization in the form of cm-scale veins with open space fillings of fibrous silica and chalcopyrite, sphalerite, pyrite (+electrum ± Au–Ag tellurides). The δ³⁴S values of sulfides from the upper (−7.6 and +0.9‰) and lower (−2.4 and +5.6‰) domains are consistent with a heterogeneous sourcing of S, probably combining inorganically and organically induced reduction of Albian-Aptian seawater sulfate. Despite this, a magmatic source for sulfur cannot be discarded. The δ³⁴S (+19.2 and +20.0‰) and δ¹⁸O (+12.5 and +14.2‰) values of anhydrite-gypsum nodules are also consistent with a seawater sulfate source and suggest crystallization in equilibrium with aqueous sulfides at temperatures higher than 250 °C. These data point to a classification of Romero as a volcanogenic massive sulfide (VMS) deposit formed in an axial position of the Greater Antilles paleo-arc in connection with island arc tholeiitic magmatism during a steady-state subduction regime. Circulation of hydrothermal fluids could have been promoted by a local extensional tectonic regime expressed in the Tres Palmas district as a graben structure.

1. Introduction

Exploration for gold and copper is currently being carried out in Early Cretaceous-to-Eocene age volcanic belts on Hispaniola, an island divided between the Dominican Republic and Haiti [1,2]. Arc-related rocks from the Cordillera Central of the Dominican Republic and its extension in the Massif du Nord in Haiti have been, and continue to be, the most intensively prospected. Current exploration follows previous sustained efforts from the 1970s that led to the discovery of the Cerro de Maimón volcanogenic massive sulfide (VMS) and the world-class Pueblo Viejo epithermal deposits, both of which are currently in production (Figure 1) [2,3,4,5,6,7,8]. The value of Dominican export of mineral resources in 2017 was US$1766 M, of which gold was the main commodity with a value of US$1456.7 M [9]. Haiti currently has no metallic mineral exports.

Economic and political stability, as well as an enviable mineral endowment, encouraged GoldQuest Mining Corp. to commence regional exploration of the most prospective areas of the Dominican Republic in 2001. After a stream sediment program revealed geochemical anomalies and mineralized outcrops on the southern side of the Cordillera Central, GoldQuest followed up with geologic mapping and sampling that led to the 2010 discovery, by diamond drilling, of the Romero South deposit containing approximately 300,000 ounces of gold [10]. Following ground geophysics (induced polarization (IP)), detailed geology and alteration mapping defined the highly prospective N–S Las Tres Palmas trend. Testing of the best targets led the company to the discovery of the Romero deposit (100% owned by GoldQuest and located about 165 km west-northwest of Santo Domingo, in the Province of San Juan) in 2012. Current indicated plus inferred resources for the two deposits (Romero plus Romero South) contain 1.93 Moz Au, 2.86 Moz Ag, 14.8 Mlbs Cu and 7.7 Mlbs Zn or 2.265 Moz gold equivalent [10]. A probable (mineable) reserve for the Romero deposit is 7.031 Mt @ 3.72 g/t Au, 4.33 g/t Ag and 0.88% Cu or 1.126 Moz. of gold equivalent [10].

Here, we present new data on the mineralogy, ore mineral texture and geochemistry, sulfur and oxygen isotopes for the Romero deposit, and a new U–Pb zircon age for the andesite host rock. These data are used to discuss the genesis of the Romero deposit and its classification, as well as the relative position of the mineralizing system within the metallogenic evolution of the Greater Antilles paleo-arc. The findings of this study have wider implications for the understanding of the occurrence of both VMS and porphyry-epithermal mineralization in single arc-related igneous terranes and their juxtaposition over evolution of these systems.

2. Geologic Setting

2.1. Geodynamic Setting

It is widely accepted that the modern Caribbean Plate is an allochthonous, Pacific Farallon-derived fragment, of which progressive west-to-east insertion into its current position was enabled by the Jurassic breakup of Pangea and the separation of the North America and Gondwana blocks [11,12]. Synchronous sea-floor spreading along the so-called Proto-Caribbean Seaway developed an intervening ocean basin that was progressively consumed along a west-directed subduction beneath the Caribbean (initially Farallon) Plate. Such subduction began as early as ca. 135 Ma along a sinistral inter-American transform fault connecting mature, east-dipping Cordilleran subduction zones along the western margins of North and South America [12,13]. Arc-continent collision along the northern leading edge of the Caribbean Plate in the latest Cretaceous-earliest Tertiary caused obduction of ophiolitic complexes onto continental margins in Guatemala, Cuba, Hispaniola and Puerto Rico [12,14,15,16]. On Hispaniola, subduction ceased after collision with the Bahamas Platform in the Eocene time [17,18]. Post-Eocene motion along the northern Caribbean Plate margin continues to be dominated by left-lateral strike-slip tectonics [19,20].

The Romero deposit is located in Central Hispaniola, on the southern side of the Cordillera Central of the Dominican Republic (Figure 1). The Cordillera Central is tectonically bounded by the left-lateral strike-slip, NNW–SEE to WNW–ESE trending Hispaniola and San Juan-Restauración fault zones (represented by HFZ and SJRFZ, respectively, Figure 1). Geologically, it is a complex composite of crustal and mantle units that formed within an intra-oceanic environment since the Late Jurassic time and that accreted in the aftermath of the collision between the Caribbean island-arc and the Bahamas Platform in the Eocene time [21,22,23]. Rock units cropping out along the Central Cordillera are representative of (1) ultramafic, mostly serpentinized peridotites of probable Late Jurassic or Early Cretaceous protolith (including the so-called Loma Caribe peridotites [15,24,25]), (2) MORB-type volcano-plutonic assemblages representative of the Proto-Caribbean oceanic lithosphere (Loma la Monja assemblage [26]), (3) basaltic volcanic rocks of the Caribbean Large Igneous Province (CLIP, which comprises the plume-related Caribbean–Colombian Oceanic Plateau—CCOP), of the Jurassic to the latest Cretaceous age (Duarte complex and Pelona-Pico Duarte, Peña Blanca and Siete Cabezas formations [23,27,28,29,30,31,32]), and (4) igneous and sedimentary rocks associated with island arc(s) of Cretaceous age [33] (the corresponding units are described below). These basement rocks were variably deformed and metamorphosed [28,34,35,36,37,38], and are unconformably overlain by late Eocene to Holocene sedimentary rocks, which include siliciclastic and carbonate deposits.

Arc-related intrusive, volcanic and sedimentary units are bountifully exposed along the Cordillera Central. The Maimón formation, located in a median belt in the Bonao tectonic block [23,35], extends to the eastern limit of the Cordillera Central. Mafic-felsic, bimodal metavolcanic rocks of the Maimón formation, with forearc basalt (FAB), boninite and low-Ti island-arc tholeiite (IAT) signatures, record a lithogeochemical progression that is characteristic of subduction initiation ophiolites [37,39,40,41]. The age of the Maimón formation has been assigned to the Early Cretaceous based on U–Pb zircon dating [42] and their geochemical similitude to other arc-related rock units of the Early Cretaceous age [6,33,38,40,41,43,44]. Thrust over the Maimón formation is the non-metamorphic Los Ranchos formation, which extends eastwards across the Dominican Cordillera Oriental and comprises mafic-felsic bimodal plutonic and volcanic series and volcaniclastic and terrigenous deposits. Igneous rocks from this geologic unit record archetypical geochemical progressions from subduction initiation to stable, mature subduction stages and embrace boninitic, low-Ti IAT, normal IAT and calc-alkaline signatures [7,8,22,45,46,47]. The age of the Los Ranchos formation has been constrained as Aptian-lower Albian, with U–Pb zircon ages determined between ca. 122 and 106 Ma [7,8,46,48,49]. The Maimón formation is separated from the Loma Caribe peridotite by the Peralvillo formation, a narrow sequence of undeformed and unmetamorphosed, arc-related volcanic and volcano-sedimentary rocks of the Late Cretaceous age [39,50] interpreted as evolved in a back-arc basin position [23]. To the south of the HFZ, on the southwest side of the median belt, crops out the Aptian-Albian Río Verde complex, composed of meta-igneous rocks with a weak subduction geochemical signature and proposed to be emplaced in a rifted arc or back-arc basin setting [51]. Finally, the arc-related Tireo formation, first mapped by Bowin [52] and hosting the Romero deposit, crops out along most of high rugged mountains of the western Cordillera Central in the Dominican Republic and Massif du Nord in Haiti (Figure 1).

2.2. The Tireo Formation

Rocks of the Tireo formation [47] or group [23,53] and related units compose a more than 3-km-thick pile of arc-related volcanic and volcano-sedimentary sequences cropping out along a 290-km-long and 40-km-wide belt. In this article, we will refer to this geologic unit as a “formation” (in line with [47]). The volcanic pile is divided into a lower, mafic-dominated and an upper, felsic-dominated sequence [47,53]. The lower Tireo volcanic sequence is largely composed of vitric–lithic tuffs and volcanic breccias of andesitic composition, which probably originated as pyroclastic deposits and were partially reworked as debris flows in a relatively deep submarine environment. However, accretionary lapilli levels deposited in shallow water transitional to a subaerial environment are described in SW Restauración (Figure 2) [47,53]. Interbedded with the dominant volcaniclastic levels are local massive basaltic flows and sedimentary rocks. The massive basalt flows are found towards the base of the lower volcanic sequence and are genetically associated with local microgabbros and dolerites [47]. Intercalation of discontinuous sedimentary strata occurs towards the base and the top of the lower volcanic sequence and includes cherts and limestones, of which the fossiliferous content allows bracketing the age of this volcanic sequence between the Aptian and Turonian times [53,54,55]. The upper volcanic sequence is composed mostly of lava flows, tuffs and tuff breccias of rhyolitic and dacitic composition deposited in a shallow submarine environment, the pyroclastic rocks being associated with phreatomagmatic activity [47]. The acidic deposits are interbedded with mafic flows and sediments, and the assemblage was intruded by dolerite dykes. Limestone lenses within the upper part of the upper volcanic sequence were dated at lower Senonian (i.e., broadly Coniacian–Santonian) in the Las Canitas area, and at early Maastrichtian to the east of Constanza [53]. In addition, radiolarians from a tuffaceous chert in the Pedro Brand section, northwest of Santo Domingo, indicate a Turonian–Coniacian age [32]. The coeval deposition of deep and shallow water pyroclastic rocks, the regional tectonics and the stratigraphic relationships with correlated units outside the main “Tireo basin” led Lewis et al. [53] to propose rapid changes in the depth of the main basin in the framework of caldera collapse associated with a graben structure.

The chemical composition of the igneous rocks from the Tireo formation was addressed by Escuder-Viruete et al. [47], who identified contrasting geochemical affinities along its igneous stratigraphy and proposed a major change in its magma sources. Basaltic and andesitic rocks of the lower volcanic sequence yield IAT signatures and were sourced by a mantle wedge fluxed with hydrous fluids expelled from the downgoing slab. In contrast, the mafic volcanic rocks interbedded within the upper volcanic sequence are identified as low-Ti, high-Mg andesites and Nb-enriched basalts. With regards to the felsic volcanic (and related intrusive sub-volcanic) rocks, these authors identified transitional IAT to calc-alkaline as well as adakite affinities, with the former associated to deep partial melting of mafic rocks and the latter related to melts of the subducting slab.

Radiometric dating has been applied only to rocks of the upper volcanic sequence of the Tireo rocks and to rocks intruding the volcano-sedimentary pile. Published U–Pb zircon data include 91.3 ± 2.1 Ma [47] and 89.1 ± 0.9 Ma [23] ages for porphyric rhyolite flows of adakitic affinity from the lowermost levels of the upper volcanic sequence. On the other hand, a Nb-enriched andesitic dyke intruding the lower volcanic sequence was dated at 85.5 ± 2.6 Ma [47] and a gabbro intruding the Tireo formation in SW Villa Altagracia yields an age of 93.35 ± 0.23 Ma [23]. Whole-rock K/Ar dating of rhyolites (of unknown affinity) in the west of Valle Nuevo yield ages of 71.5 ± 3.6 and 85.1 ± 4.3 Ma, respectively [53]. Published hornblende ⁴⁰Ar/³⁹Ar plateau ages include 81.2 ± 8.2 Ma for a dacite sample [53], 91.8 ± 2.3 Ma for a rhyodacitic flow of adakitic affinity, 88.6 ± 1.8 Ma for a high-Mg andesitic dyke and 69.6 ± 0.7 Ma for a late biotite-bearing porphyritic andesite [47]. A plagioclase ⁴⁰Ar/³⁹Ar plateau age of 66.8 ± 0.47 Ma was obtained for a high-Mg porphyric andesite flow [47].

3. Materials and Methods

This study was based on 112 drill core and in situ field samples collected from the Romero deposit and surroundings. Studied samples are representative of the different host rock lithofacies as well as the different mineralization features.

The samples were prepared as polished thick (n = 26) and thin (n = 38) sections for their study under the optical microscope using reflected and transmitted light. A selection of these samples was examined on an environmental scanning electron microscope (SEM) Quanta 200 FEI, XTE 325/D8395 equipped with an INCA Energy 250 EDS microanalysis system at Centres Científics i Tecnològics of the Univeristy of Barcelona (CCiT-UB) and on an environmental SEM Quanta 650 FEI, equipped with an EDAX-Octane Pro EDS microanalysis system at Centro de Caracterización de Materiales of the Pontifical Catholic University of Peru (CAM-PUCP). Operating conditions were an accelerating voltage of 20 keV and 5 nA in backscattered electron (BSE) mode. Mineral chemistry analyses of sulfide minerals were performed using a five-channel JEOL JXA-8230 electron microprobe (EMP) at the CCiT-UB, operated at a 20-kV acceleration voltage, a 20-nA beam current and with a beam diameter of 5 µm. Analytical standards and lines used for analyses were: sphalerite (Zn and S, Kα), chalcopyrite (Cu, Kα), FeS₂ (Fe, Kα), Au (Au, Mα), Ag (Ag, Lα), PbTe (Te, Lα), Sb (Sb, Lα), Bi (Bi, Mα), CdS (Cd, Lα), HgS (Hg, Mβ), PbS (Pb, Mα), GaAs (As, Lα), and ZnSe (Se, Lα).

Mineralogical determinations were also carried out by means of X-ray Diffraction (XRD; n = 7). The samples were ground in an agate mortar and were manually pressed by means of a glass plate to get a flat surface in cylindrical standard sample holders with a 16-mm diameter and a 2.5-mm height. The diffractograms were obtained in a X’Pert PRO MPD Alpha1 powder diffractometer (PANalytical, Almelo, The Netherlands) in Bragg-Brentano θ/2θ geometry with a radius of 240 mm, nickel filtered Cu Kα radiation (k = 1.5418 Å), and 45 kV–40 mA at the CCiT-UB. The software X’Pert Highscore © (Version 2.0.1, PANalytical, Almelo, The Netherlands) was used to subtract the background of the patterns, to detect the peaks and to assign mineral phases to each peak.

Zircons were separated from sample Romero-1 (a petrographic description for this rock is given below, and its whole rock composition is provided in Sheet-1 (Supplementary Materials)) at the University of Barcelona. The rock sample was crushed and milled using a tungsten carbide mill, and the resulting material was sieved (grain fractions: 125, 100, 75 and 40 µm) by means of disposable sieves in order to avoid contamination. Zircon crystals were separated using panning in water. Non-magnetic concentrates, after eliminating the magnetic fractions with a Frantz^® isodynamic LB-1 separator, were processed by applying the hydroseparation technique (HS) at the HS-11 laboratory to obtain high-density mineral concentrates. The resulting non-magnetic high-density concentrates went through acid digestion in open bombs with combinations of HF, HCl and HNO₃. Zircons were then handpicked under the binocular microscope and mounted in a 1-inch-diameter epoxy mount that was ground and polished to expose the grains. Photomicrograph maps and cathodoluminescence (CL) images were used to characterize the internal features of zircons such as growth zones and inclusions and to provide a base map for recording laser spot locations. For these images, a ZEISS EVO 15 SEM was used at the IACT-CSIC.

Zircon grains were analyzed for U–Th–Pb using a Cetac Photon Machines Analyte Excite 193 laser ablation system in conjunction with an Agilent 8800 QQQ inductively coupled plasma mass spectrometry (ICP-MS) in the IACT-CSIC. Fixed 20 μm-diameter spots were used with a laser frequency of 10 Hz and a fluence energy of 8 J/cm². The ablated material was delivered to the torch by He and Ar gas. Instrument details, operation and acquisition parameters are given in Sheet-2 (Supplementary Materials).

Sulfur isotope analyses were carried out on 42 single sulfide grains and on 9 sulfate nodules from the different mineralization domains identified from the Romero deposit. Sulfide and sulfate minerals were separated by hand-picking techniques. Purity of the samples was tested by examination with a binocular microscope. The sorted material was then weighed in a tin capsule, adding V₂O₅ as an oxidizing catalyst. Isotopic ratios were obtained using a Delta C Finnigan MAT Delta-S mass spectrometer with an elemental analyzer at the CCiT-UB. For calibration, the international IAEA S3, IAEA S1, NBS-123 and IAEA S2 standards were used at the start and the end of every run. Oxygen isotope analyses were carried out on 5 sulfate grains. About 0.1 g of gypsum and anhydrite were dissolved in 50 mL of MilliQ water using stirring to accelerate the dissolution process. When the solid phase was fully dissolved, sulfate was precipitated as BaSO₄ by the addition of excess BaCl₂ and acidifying the sample with HCl (pH < 2) and boiling it to prevent BaCO₃ precipitation. The precipitated BaSO₄ was then analyzed using the same equipment as for S isotope measurements. For calibration, the international NBS-127 and the internal YCEM (δ¹⁸O = 17.6‰) and BaSO₄ (δ¹⁸O = 13.2‰) standards were used. Sulfur and oxygen isotope compositions were expressed as the delta per mil (‰) values relative to the Canyon Diablo Troilite (CDT) and the Vienna-standard mean ocean water (V-SMOW) standards, respectively. Precision of the analyses was better than ±0.2‰. A table with the sulfur and oxygen isotope results is presented in Sheet-4 (Supplementary Materials).

4. Results

4.1. Petrography of the Hosting Rocks

Gold–silver–copper(–zinc) mineralization in the Romero prospect is hosted predominantly by volcanic and volcaniclastic rocks of intermediate and basic composition (Figure 3), which belong to the lower volcanic sequence of the Tireo formation [47]. The descriptions and interpretations of the volcanic structures and textures below follow the guidelines of McPhie et al. [56]. Lithofacies identified in the host rocks include minor coherent lavas (Figure 4A) and more abundant volcaniclastic (Figure 4B–E) and volcanogenic sedimentary and/or pyroclastic (Figure 4F) deposits.

Coherent, massive lavas are of andesitic and rare dacitic composition, and show aphanitic, feldspar-, amphibole- and quartz-phyric textures, with a predominantly cryptocrystalline to glassy groundmass (Figure 4A,G). The phenocrysts range in size between 500 µm and 4 mm (Figure 4H,I) and form local monomineralic and polymineralic glomerules (Figure 4J). Plagioclase phenocrysts are by far more abundant than K-feldspar; these phenocrysts show euhedral and subhedral tabular habits and frequent compositional zoning, often picked out by selective alteration (Figure 4H) or by strings of melt inclusions. Quartz phenocrysts range from euhedral to anhedral, with the latter characterized by abundant embayment and voids infilled with the groundmass material (Figure 4G). Amphibole crystals are mostly euhedral (Figure 4I).

A variety of volcanoclastic lithofacies of andesitic and rare dacitic composition have been identified. They show a massive (non-bedded) arrangement. Monomict clast, vitriclast and shard-bearing breccias with abundant matrix and dominant jigsaw-fit and clast-in-matrix textures, displaying glassy rims in vitriclasts (Figure 4B), are interpreted as hyaloclastite deposits formed by in-situ quench fragmentation. On the other hand, monomict breccias characterized by a low proportion of matrix and irregularly shaped clasts lacking evidence of quenching (Figure 4C,D) are interpreted either as autoclastic breccias or as proximal resedimented clastic deposits, as they regularly contain clasts with curviplanar outlines and sharp edges. A further lithofacies observed along the volcanic sequence hosting the Romero deposit is characterized by thinly bedded mud layers of grey color and of silicic composition cut by lobate inclusions of a green aphanitic, aphyric material, which is thought to represent chloritized glass; the contact between these two materials is lined with white quartz (Figure 4E). Such rocks are interpreted as fluidal peperites.

Local bedded deposits are found incidentally within the volcaniclastic sequence described above. They are characterized by thin beds, up to 1.5 cm thick, of fine grained (mud-sized) sediments interspersed with layers rich in sandstone-sized particles (Figure 4F). Coarse elements within the latter include black glassy, chloritized components of chiefly cuspate (shard-like) and blocky shapes. The conspicuous size selection and the occurrence of mudstones strongly suggest deposition from suspension, and hence either a pyroclastic fall-out or suspension associated with mass flows of volcanogenic origin is invoked. In the rock sample shown in Figure 4F, the bedding deforms around a blocky andesite clast, describing a sag structure, which is typically described in surge and fallout deposits; a detailed observation of the bright mudstone layer around the clast allows for the identification of asymmetrical folding probably developed by deformation of a wet sediment during the ballistic impact of the clast.

A ~25-m-thick discontinuous level containing abundant white and bright pink gypsum ± anhydrite nodules is present in the upper sectors of the andesitic volcano-sedimentary sequence, topographically on top of stockwork mineralization (Figure 3). Such nodules are rounded and show intricate irregular outlines (Figure 5); they are compositionally very homogeneous, lacking mineral inclusions, and are cemented by an altered and mineralized volcanogenic sedimentary matrix containing glassy andesitic fragments.

Most of the studied host rock samples have pervasive quartz, chlorite ± epidote (Figure 4G,L), or sericite (Figure 4J) alterations and dissemination of sulfides. The chlorite alteration is mostly developed on the recrystallized groundmass of lava flows or volcaniclastic deposits of andesitic composition, and is also observed pseudomorphically replacing crystals of mafic minerals. In contract, sericite is observed as a replacement of plagioclase crystals, often along growth zones evidencing a primary compositional zoning of the feldspars. Disruption by faulting in the area precludes a clear identification of the distribution of the alteration types, but in general, widespread outer chlorite alteration passes inward to dominant sericite and finally to massive silicification in the stockwork zone (Figure 3).

4.2. U–Pb Geochronology

Zircons for U–Pb analyses were separated from an andesite autobreccia (sample Romero-1) composed of irregular clasts of aphanitic, highly plagioclase- and poorly amphibole- and quartz-phyric textures. This mineral composition and texture is shared by the scarce matrix surrounding the autoclasts to the point that their limits are often diffuse. The groundmass glassy material within the clasts and the matrix and the amphibole phenocrysts are pervasively altered to chlorite ± epidote; in addition, the sample also shows intense silicification and a pyrite dissemination. Based on its immobile element composition (see data and related figures in Sheet-1 (Supplementary Materials), the rock is classified as andesite or andesitic basalt. The strong positive Th and negative Nb anomalies in the NMORB-normalized multi-elemental pattern points to a subduction-related origin for the sample [57] and are comparable to IAT basalts and basaltic andesites described by [47], as are the chondrite-normalized REE patterns.

The zircons separated from this rock are small, with lengths reaching around 50 µm, and their aspect ratios are in the range between 1 and 3. The bulk of the separated zircons yielded pre-Cretaceous ages, mostly around 300 Ma (not reported here) and hence are interpreted as inherited, xenocrystic grains. The relative abundance of inherited zircons in rocks from the Tireo formation is also noted by Escuder-Viruete et al. [47]. For a state-of-the-art reading on the occurrence of inherited zircons in arc-related crust and mantle rocks in the Greater Antilles, the reader is referred to Torró et al. [42], Rojas-Agramonte et al. [58] and Proenza et al. [59]. Only a 15% of the analyzed zircons yield Cretaceous ages (n = 8; see Sheet-2 (Supplementary Materials)) giving a concordia age at 116 ± 10 Ma (Figure 6). This Aptian-Albian age is interpreted as the crystallization age.

4.3. Petrography of the Ore

High-grade mineralization of the Romero deposit occurs as vertically stacked, NE-dipping, sulfide lenses, some tens of meters thick, underlain by a dense vein stockwork (Figure 3). The main mineralization is located between 85 and 420 m below surface; its only expression at surface is in the form of very local, low-density, pyrite disseminations (further details on the geometry of the deposit are gathered in [10]).

In general terms, two domains of gold and associated base metal mineralization are identified. In the upper domain, mineralization occurs either as dissemination in the matrix around gypsum-anhydrite nodules or, more importantly, as massive sulfide bodies. The first one (up to 7.2 g/t Au, 23 g/t Ag, and 2% Zn) is composed of scattered sub-millimeter anhedral pyrite crystals, minor sphalerite and traces of chalcopyrite (Figure 5E). Pyrite grains show common anhedral morphologies including dendritic and branchy arrangements and is also observed as a replacement of micro-fauna shells, occasionally overgrowth by subhedral crystals depicting atoll-like structures.

In the massive sulfide bodies (up to 7.2 g/t Au, 63 g/t Ag, 12% Zn, and 5% Cu), macroscopic textures include massive, non-bedded homogeneous accumulations of sulfide minerals, as well as breccias composed in a large proportion of sub-angular massive sulfide clasts with sizes ranging from some tens of millimeters to some centimeters (Figure 7A). In the massive mineralization, the chalcopyrite and sphalerite are the most abundant minerals and appear cementing subhedral to anhedral pyrite grains with sizes mostly between 0.25 and 1 mm across (Figure 7B). Sphalerite hosts abundant micrometric blebs of chalcopyrite (chalcopyrite disease), and is replaced by chalcopyrite along its margins. The sulfide clasts show an equivalent mineralogy and texture (Figure 7C). Pyrite grains in the massive lenses show a wide variety of textures, including subhedral cubic crystals and complex textural features such as colloform overgrowths (Figure 7D,E), spongy cores rich in mineral inclusions (Figure 7F), spongy cellular cores (Figure 7G), micro-fauna shell replacement (Figure 7H), vermiculations, stalks, filaments, etc. Often, spongy cores are overgrowth by subhedral pyrite crystals drawing atoll-like textures. Framboidal (sensu stricto) textures in pyrite are not observed; however, spheroidal structures in the form of (putative) floriform textures are observed along growth bands in some pyrite grains (Figure 7E). The voids within pyrite grains are filled with gangue minerals (mostly quartz), chalcopyrite and lesser amounts of sphalerite and tennantite (Figure 7E).

The (lower) main Au–Ag–Cu(–Zn) zone (up to 288 g/t Au, 46 g/Ag, 13% Cu, and 10% Zn) comprises a 200-m-thick domain with a dense stockwork sulfide mineralization (Figure 8 and Figure 9). Chalcopyrite- and gold-rich, centimeter-scale, sulfide-silica veins are conspicuous. Successive narrow subparallel layers of quartz of different colors and of sulfides (chalcopyrite + pyrite ± sphalerite) describe crustiform arrangements often with colloform and cockade textures (Figure 8A–E). On the other hand, protrusions of chalcopyrite mineralization perpendicular to vein walls resemble comb textures (Figure 8B,E). Sphalerite, when present, is concentrated along the outer margins of the veins, and is overgrown by chalcopyrite and quartz bands towards the core of these structures (Figure 8D,E). In the veins, pyrite appears as subhedral grains up to 1 mm across, and shows fractures, engulfments and voids filled with chalcopyrite (Figure 8F,G) and lesser amounts of sphalerite. Sphalerite, showing local chalcopyrite disease, is crosscut by chalcopyrite veinlets (Figure 8G). Detailed observations under the microscope reveal a fibrous texture for the vein quartz, which on the other hand is free of fluid inclusions, suggesting recrystallization of chalcedony after silica gel deposition.

Electrum occurs largely in the stockwork mineralization in association with chalcopyrite (often lining vugs within this mineral) and concentrated along chalcopyrite–pyrite contacts (Figure 9A–F). Also abundant are the electrum particles included within pyrite grains, either as inclusion or lining vugs and fractures together with chalcopyrite (Figure 9C,H,I). Finally, some electrum is observed within quartz (Figure 9C) or sphalerite (Figure 9G). Electrum grains are some tens of microns across and very irregular in shape. A series of Bi–Au–Ag–tellurides of some tens of microns (mostly between 10 and 15 µm) across appear in the same textural positions as electrum, and include hessite (Ag₂Te), petzite (Ag₃AuTe₂), calaverite (AuTe₂) and tsumoite (BiTe; Figure 9F,I).

The textural observations described above allow for the construction of a relatively simple paragenetic sequence shared by the mineralization in the upper (massive) and lower (stockwork) domains (Figure 10). Such paragenetic sequence includes an early precipitation of pyrite (±Au rich electrum as inclusions; see mineral geochemistry below) followed by a stage rich in Zn (sphalerite) and a subsequent stage rich in Cu and Au (chalcopyrite, tennantite, electrum and tellurides).

4.4. Ore Mineral Geochemistry

Analyzed electrum grains yield Au contents between 0.76 and 0.97 atoms per formula unit (a.p.f.u) and Ag contents between 0.03 and 0.24 a.p.f.u (Sheet-3, Supplementary Materials). The concentrations of other elements in electrum are conspicuously low and include Bi (up to 0.90 wt %, which is equivalent to 0.01 a.p.f.u.), Hg (0.16 wt %) and Pb (up to 0.11 wt %, although largely below the detection limit for this element). Two populations of electrum with contrasting Ag and Au concentrations have been identified (Figure 11A). The first one, which is Ag-rich (0.76–0.81 Au p.f.u., 0.19–0.24 Ag p.f.u.), is more abundant and occurs within chalcopyrite, sphalerite and gangue and also lining vugs within pyrite, often along with chalcopyrite. The second one has contrastingly lower Ag contents (0.91–0.97 Au p.f.u., 0.03–0.08 Ag p.f.u.) and has been observed only within pyrite as monomineralic inclusions.

Analyzed sphalerite crystals yield a wide range of Cu contents from values below the detection limit to 6.36 wt %. Such enrichment in Cu strongly correlates with the concentrations of Fe (see Sheet-3, Supplementary Materials), indicating incidence of mixed sphalerite–chalcopyrite analyses due to the common occurrence of micro-inclusions of chalcopyrite within sphalerite, as described above. Those analyses not affected by the chalcopyrite disease (threshold at Cu < 0.3 wt %; [6,60,61]) and hence preserving the primary compositions, have notably low Fe contents (up to 1.49 wt % or 0.03 a.p.f.u.), low Fe/Zn (<0.03) and S concentrations between 31.27 and 31.93 wt %. The negative correlation between Zn and Fe (Figure 11B) points to a substitution of Fe for Zn (e.g., [62]). Cadmium and mercury concentrations are as high as 0.35 and 0.59 wt %, respectively.

4.5. Sulfur and O Isotopes

In Romero samples, δ³⁴S values for the analyzed sulfides range between −7.6 and +5.6‰, with most of the values preferentially distributed around 0‰ (Figure 12). The δ³⁴S values of sulfides from the lower domain (stockwork mineralization) range between −2.4 and +5.6‰ (+1.2 to +1.5‰ for sphalerite, −1.7 to +1.5‰ for chalcopyrite and −2.4 to +5.6‰ for pyrite). Analyzed sulfide minerals from the upper mineralization domain yielded δ³⁴S values between −7.6 and +0.9‰, which are, in general, lower than those yielded by the studied sulfides from the stockwork mineralization (Figure 12). Within this range, δ³⁴S values span between −1.2 and −0.9‰ for chalcopyrite, between 0.0 and +0.9‰ for euhedral/compact pyrite grains, between −7.6 and +0.4‰ for spongy/shell replacement anhedral pyrite grains and −1.2‰ for a single analyzed sphalerite crystal. It is noteworthy that shell replacement pyrite (Figure 7H) yielded the lowest δ³⁴S values. The δ³⁴S values for the analyzed sulfates (anhydrite + gypsum) range between +19.2 and +20.0‰.

The δ¹⁸O values for the analyzed sulfates are closely spaced between +12.5 and +14.2‰ (Figure 12).

5. Discussion

5.1. Classification of the Romero Deposit

In 2015, three years after the discovery of the Romero deposit, geotechnical drilling unexpectedly cut stacked lenses of massive sulfide. Prior to 2015, a lively debate over the origin and classification of this deposit existed among exploration geologists. This debate reflected a generalized uncertainty on the style of mineralization found and to be found in the Tireo formation and the still poorly understood metallogenic evolution of this unit [3,63,64]. Currently, mineral exploration companies active in the Tireo formation are preferentially targeting VMS (e.g., [65,66]).

The most prominent textural feature at the Romero deposit, and the first to be intersected by drilling is the high-grade stockwork mineralization (Figure 8). Open-space vein infillings with evidence of early amorphous and microcrystalline, fibrous silica precipitation could have misled early workers favoring an intermediate sulfidation epithermal mineralization style (e.g., [64]) despite the lack of clear mineralogical evidence for this assignation (e.g., the outstanding predominance of chalcopyrite over sphalerite and the absence of diagnostic tennantite-tetrahedrite in the veins; cf. [67]). The presence of colloform, microcrystalline textures would, in any case, suggest the existence of silica-supersaturated conditions, resulting in rapid nucleation and precipitation during the deposition of the silica bands, probably associated to boiling processes (see [68]). However, this last point could not be asserted for the Romero vein mineralization due to the lack of fluid inclusions in translucent minerals.

The upper levels of the Romero mineralization system offer the best evidence for its origin as a VMS deposit. Thick anhydrite-gypsum nodule levels spatially correlated with conspicuous metal values and the self-defining stacked massive sulfide lenses are diagnostic features. The anhydrite-gypsum nodules are mineralogically very homogeneous and lack either sulfide or gangue mineral inclusions ruling out a dominantly replacement origin; an exhalative genesis and later sedimentary reworking are, in contrast, very plausible and supported by isotopic data as discussed below. Significant amounts of exhalative anhydrite-gypsum are characteristic of many submarine hydrothermal deposits on mid-ocean ridges and back-arc basins in the modern seafloor and in ancient Kuroko-type VMS deposits [69]. The δ¹⁸O values for the analyzed sulfates agree with an Aptian-Albian seawater sulfate source according to the estimated uncertainty of the seawater isotopic variation of Claypool curve [70,71].

The δ³⁴S values for Romero sulfide minerals show a relatively wide distribution around a modal value of 0‰ with dissimilar values for sulfides from the lower (stockwork, biased towards more positive δ³⁴S) and upper (biased towards more negative δ³⁴S) mineralization domains (Figure 12). These observations suggest a heterogeneous source for sulfur, which in a VMS environment might typically embrace: (1) inorganic reduction of seawater sulfate; (2) magmatic sulfur, either from a direct contribution from a vapor-rich magmatic fluid or after leaching of the volcanic host rocks; and (3) sulfur from microbial activity in sediments by means of bacterial sulfate reduction [72,73,74,75,76]. For the Romero deposit, most δ³⁴S values (excluding the most negative ones, <6‰; Figure 12) can be explained either by an inorganic reduction of seawater sulfate or by a magmatic sulfur source. Even if an unambiguous discrimination between these two sources is not possible, it is noteworthy that an inorganic reduction of Aptian-Albian seawater sulfate (of which δ³⁴S was between ~15.3 and 16.6‰ [70,71]) would account for such a spectrum of δ³⁴S values and better explain the most positive values, as high as +5.6‰. The most negative δ³⁴S values, in sulfides from the upper mineralization zone, suggest a contribution of biogenically reduced sulfur, which is in good agreement with the biogenic textures described for sulfides in this domain (Figure 7). Finally, the δ³⁴S values of studied sulfate nodules are higher than those of Aptian-Albian seawater sulfate, which could point to seawater sulfate reduction at temperatures higher than 250 °C in equilibrium with aqueous sulfides (cf. [75]).

5.2. The Romero VMS Deposit within the Metallogenic Evolution of the Tireo Formation

Volcano-sedimentary facies of the lower Tireo formation, in which the Romero VMS deposit is hosted, indicate abrupt changes in the depth of deposition [53]. The referred workers contextualized such changes as the result of a locally extensional tectonic regime in the form of graben structure leading to caldera collapse and, back in 1991, already described it as “similar to Kuroko in Japan”. Certainly, extension might have played an important role during the circulation of hydrothermal fluids leading to the formation of VMS mineralization. Lithofacies and important petrogeochemistry of igneous rocks indicate, on the other hand, deposition of the sequence in an intra-oceanic island-arc scenario and, more specifically, in an axial arc position during Albian-Aptian times [47].

The Tireo formation in the Dominican Republic and its continuation to the Massif du Nord in Haiti is known for hosting a variety of deposit typologies that include porphyry and epithermal as well as VMS deposits [3]. Porphyry and epithermal mineralization in the Tireo formation are systematically hosted by rocks of the upper sequence and connected to calc-alkaline plutonic and volcanic rocks of the Late Cretaceous age. The high-sulfidation epithermal mineralization of the Au–Ag–Cu Restauración district is the most highlighted one in terms of reserves and include the Candelones deposit (5.27 Mt at 5.27 g/t Au and 0.35% Cu) and a number of smaller mineral occurrences [3,77,78,79]. Porphyry Cu–Au mineralization is described in the Massif du Nord and includes the Blondin and Douvray deposits along with other smaller occurrences [3,80]. Re–Os dating of molybdenite from the Douvray deposit (inferred resources of 624,834 tons Cu, 0.303 Moz Au, 6.805 Moz Ag and 4790 tons Mo) yielded an age of 93.3 ± 0.3 Ma [4], which is coeval, within analytical error, with rocks of adakitic affinity from the Tireo formation [47].

The Tireo formation, therefore, records not only a major change in the chemical affinity of arc magmas (cf. [47]) but also a major change in metallic mineralization styles from its lower to its upper volcano-sedimentary sequences. With new data in hand, we shall conclude that at least two different episodes of hypogene mineralization are recorded in this unit. A first mineralization event in the form of VMS deposition was associated to IAT magmas during steady-state subduction of the Proto-Caribbean Plate beneath the Caribbean Plate; such mineralization was apparently linked to local extensional tectonics in the form of caldera collapse that could promote the circulation of hydrothermal fluids during Albian-Aptian times. A second event is characterized by the occurrence of porphyry Cu (±Au ± Mo) porphyry and epithermal mineralization connected to calc-alkaline magmatism during mature stages of the subduction and adakite-like felsic rocks associated to slab-window formation during the Late Cretaceous time.

5.3. The Romero Deposit in the Metallogenic Evolution of the Greater Antilles

As previously discussed, the Romero VMS deposit (and likely other new discoveries in the Tireo formation) formed in an axial position of the Caribbean intra-oceanic paleo-arc in Albian-Aptian times in connection with IAT magmas. Keeping to the most recent review on the metallogenic evolution of the Greater Antilles [3], the Romero deposit could be broadly correlated in time with bimodal mafic VMS deposits in Cuba and the Dominican Republic. However, the correlation is not that straightforward with regards to the relative position (axial vs. back vs. fore arc) and geodynamic setting of the mineralizing systems, as these authors traced the formation of such bimodal VMS deposits in a forearc position in connection to early magmas emplaced during a subduction initiation regime (cf. [81,82]). More recent studies on such VMS deposits hosted in the Early Cretaceous Los Pasos (Cuba), Maimón and Los Ranchos (the Dominican Republic) formations have strengthened the connection of the VMS deposits they host to a subduction initiation regime during the emplacement of forearc basalts and boninitic lavas [6,8,40]. We should hence conclude that the new VMS mineralization found in the Tireo formation, of which Romero is the highlighted representative, might deserve a separate group within the metallogenic evolution of the Greater Antilles, in which its position in an axial arc position is emphasized as a contrasting feature regarding most coeval VMS described in the region.

6. Conclusions

The results of this study have led to the following conclusions:

• The newly discovered Romero deposit is hosted by volcanic and volcaniclastic rocks of the lower sequence of the Tireo formation, and a hosting andesite has been dated at 116 ± 10 Ma.
• The hypogene mineralization can be subdivided into: (a) an upper mineralization domain, in the form of stacked massive sulfide lenses and sulfides in the matrix of stratigraphic levels hosting massive anhydrite-gypsum nodules, and (b) a lower, high-grade stockwork domain.
• Ore mineralogy in both domains comprises an early crystallization of pyrite, followed by a stage rich in sphalerite and a further stage rich in Cu (chalcopyrite ± tennantite) and Au–Ag (electrum ±Au–Ag tellurides).
• Sulphur and oxygen isotopes ratios on sulfide and sulfate minerals suggest that these elements were sourced by coeval seawater sulfate and that sulfate reduction was dominated by inorganic processes along with a variable contribution of biogenically reduced sulfur; nevertheless, a magmatic source cannot be ruled out.
• The Romero VMS mineralizing system operated in an axial arc position during steady-state subduction, connected to IAT magmatism and local extensional tectonics (caldera collapse). This geologic setting contrasts with the forearc position of most broadly synchronous VMS deposits in the Greater Antilles, which formed in a forearc position linked to a subduction-initiation regime.