Abstract The world-class Pueblo Viejo Au deposit in the central Dominican Republic is one of the largest high-sulfidation epithermal Au deposits globally, with past production plus resources and reserves of 41.7 million ounces (Moz) in the Moore and Monte Negro deposits. Mineralization occurs within a 2- × 2-km Early Cretaceous volcano-sedimentary basin filled with felsic volcanic and volcaniclastic rocks, interlayered carbonaceous sedimentary units, and underlying andesitic flows and tuffs. The volcanic stratigraphy was developed during a period of tholeiitic magmatism that transitioned to calc-alkaline magmatism at the time of emplacement of the late- to postmineral Monte Negro dike (~109 Ma). Additional geologic controls to mineralization include high-angle, NE- and NW-faulting, phreatomagmatic breccias, and possible volcanic domes. Mineralization is present across the stratigraphic sequence, with mineralization at Moore dominantly hosted within quartz-bearing volcaniclastic rocks and overlying carbonaceous sedimentary units, whereas that at Monte Negro is in the andesitic sequence as well as overlying epiclastic and sedimentary units. Alteration at the shallowest level is dominated by quartz-pyrophyllite, whereas alunite alteration defines the deep roots to the ore-forming environment. Mineralization comprises early disseminated-type and late veins filled with pyrite ± sphalerite. Hypogene ore is refractory in nature, with Au in solid solution or as mineral inclusions within arsenian pyrite. Re-Os ages of 113.4 ± 2.6 Ma for auriferous pyrite along with new geologic observations appear to confirm an Early Cretaceous age for mineralization, although Re-Os enargite ages suggest the possibility of a second mineralization event in the Eocene.

Abstract Many archaeological sites with jadeitite artefacts are known in the Caribbean region, but defining the source of the raw material is a major problem because of great mineralogical heterogeneity both in potential sources and in artefacts. The archaeological settlement site of Playa Grande on the northern coast of the Dominican Republic is particularly significant because it yielded evidence of on-site axe manufacture, and lies only 20–30 km NE of a recently discovered potential source area of serpentinite mélanges in the nearby Río San Juan Complex (RSJC). A suite of nine artefacts was chosen from a collection of over 100 excavated woodworking tools rich in jadeite, as well as two blueschist artefacts. Permission to perform destructive analysis allowed data on petrography, mineral chemistry and bulk-rock chemistry to be obtained. Seven of the nine artefacts are jadeitite sensu stricto (>90 vol% jadeite), which are identical to material known from the RSJC. Two artefacts are jadeite–lawsonite rocks. These and the two blueschists show only minor differences from corresponding rocks of the RSJC source. With this direct linking of source and site material, it is now possible to better define source discriminators for the Caribbean and to assess sampling bias.

Marine low-magnesium calcite concretions are widespread in many siliciclastic and mixed carbonate–siliciclastic shelf and basinal settings. The process of concretion formation is generally well established and involves microbial influence (mostly sulfate reduction to oxidize organic material at or just below the seafloor). The microbes produce interstitial fluids that are conducive to abundant, and apparently rapid, precipitation of calcite cements. Pervasive cementation generates well-indurated beds or isolated flattened “pods” that are commonly confined to specific stratigraphic horizons. Stratabound concretions can be important as fluid-flow barriers during subsequent burial and compaction. Thin-section and scanning electron microscopy of Cenozoic and Mesozoic concretions has revealed a dense occurrence of small (mostly 2–10 μm), equant, mostly subhedral calcite crystals. The best resolution of both techniques is, however, unable to adequately characterize crystal boundaries, the distribution of clays or organic matter, or the nature of the pores within the calcite matrix. Here, we used scanning electron microscopy to examine ion-micromilled surfaces of concretions from Upper Miocene and Upper Jurassic strata. Results indicate that the dominant crystal size is 1 to 3 μm (mean 2.08 μm; standard deviation = 1.42 μm). Pores were formed at the intersections of calcite crystals by the constriction of the fluid-filled interstitial space, likely prior to dewatering and initial compaction. These (micro) pores are of the “type III, fitted fused” variety. Two-dimensional pore shapes analyzed on micromilled surfaces are near-equidimensional (length/width = ~1–1.5), oval (length/width = 1.5–5), and elongate (length/width = >5) forms. Equidimensional and oval pores occur at the intersections of calcite crystals (along with clay minerals and organic material). Elongate pores of uncertain origin are found at the boundaries between adjacent calcite crystals. The helium pycnometer porosity of the plugs associated with the Upper Jurassic micromilled sample is consistent with a relatively low total porosity, with values of 0.38, 0.58, and 0.82%. Micromilled surfaces improve our understanding of two-dimensional crystal structure and porosity within the matrix of marine concretions. The size and shape of cement crystals and pores suggest that relatively early, rapid, and pervasive precipitation produced a homogeneous mass of calcite and small isolated pores. The resultant low porosity and permeability formed a rock that was diagenetically stable and resistant to chemical and physical modification later during burial.

The measurement of porosity and permeability in shallow-water carbonates is often complicated by the nature and degree of diagenesis, as well as the issue of scale-dependence in the measurement technique. Vertical profiles of hydraulic conductivity were calculated from short-interval straddle-packer injection tests in a three-well transect across the Pleistocene reefal limestone of the southern Dominican Republic (DR). Combined with whole-core porosity estimates and small-diameter (2.54 cm) plug estimates of matrix porosity and permeability, these data provide a means of assessing the scale-dependent petrophysical variability within a complex carbonate pore system, as well as the primary factors that control flow within such a system. Interval permeability values (converted from hydraulic conductivity) based on in situ injection tests ranged between 5 and 25 Darcy (D) (12.2 D geometric mean), up to three orders of magnitude higher than associated plug permeability values (0.08 D geometric mean). Although plug permeability is related to depositional environment (backreef, reef crest, forereef), injection tests did not show a relation to environment. Furthermore, interval permeabilities showed no relation to “matrix” (plug-based) porosity or permeability values. Interval injection permeability was correlated to “total” (whole-core) porosity and, even more so to larger scale “vuggy” (>~5 mm) porosity, quantified by subtracting the plug-based “matrix” porosity from the whole-core “total” porosity. The differences in permeability between plug and interval injection tests for these vuggy carbonates becomes greater over time, since cementation occludes matrix porosity and dissolution opens up larger molds and vugs, especially corals and other large aragonitic grains. The in situ interval permeability values measured in the DR reefal carbonates provide better values (than plug or core) of the impact of early meteoric diagenesis. These results confirm early development of vuggy intervals that can have permeability that is orders of magnitude greater than measured plug permeabilities. A touching-vug pore system shifts the scale dependence of hydraulic conductivity from the plug scale to the packer (bed) scale and probably toward the regional scale.