Recent U.S.-Canada research along the Gorda Ridge in the NE Pacific led to a special Economic Geology issue on seafloor hydrothermal mineralization, edited by Rona and Scott [1993]. Zierenberg et al. [1993] described Besshi-type massive sulfide deposits forming on a sediment-covered spreading center, the axial Escanaba Trough on the Southern Gorda Ridge. The deposits form along the margins of uplifted sediment fault blocks generated by intrusion of MORB (mid-ocean ridge basalt) laccoliths. Because of hydrothermal fluid interactions with sediments, the deposits are enriched in group IV, V and VI elements, thermogenic hydrocarbons, and radiogenic Pb compared with those deposits forming in sediment-free spreading centers. Doe [1994] analyzed and discussed source rock control on the Zn, Cu and Pb contents of ocean-ridge hydrothermal fluids; in particular the relatively low Pb contents of mid-ocean ridge basalts lead to a predominance of Zn- and Cu-rich sulfide deposits in sediment-starved ridge systems.
Studies of Mississippi Valley-type carbonate-hosted lead-zinc deposits continued to reveal the geologic, hydrologic and geochemical complexities of these fascinating epigenetic ore deposits, which form the major U. S. resources of lead and zinc. There are still many unresolved apects of the origin of these deposits. Several papers presented various geochemical arguments for the influence of multiple fluid sources and/or aquifers in the genesis of some deposits or districts (for example, compare and contrast the databases and conclusions of Viets and Leach, [1990], Shelton et al., [1992], and Kesler et al., [1994]). Structural and tectonic controls of ore genesis in the Southeast Missouri lead belt were examined by several authors. Horrall et al. [1993] suggested that much of the Cu, Co, Ni and siderophile element enrichments in the southeast Missouri Pb-Zn district were derived by basinal brine leaching of alkali mafic and ultramafic plutons occurring along the margins of the Reelfoot rift (New Madrid seismic zone). Clendenin et al. [1994] argued that local and micro-structural controls on fluid flow were important in localizing ore, and that stratigraphic units do not behave as homogeneous aquifers as is commonly assumed in many numerical fluid-flow models. Nonetheless, Garven et al. [1993] developed numerical simulations for Late Paleozoic regional gravity-driven groundwater flow triggered by uplift after the Alleghanian orogeny in the midcontinental region of North America. They used temporal and geographic variations in uplift to explain variations in the timing and directions of regional fluid flow and discharge, and the resulting genesis of carbonate-hosted Pb-Zn mineral deposits.
Much like diamonds, gold continues to garner a level of interest that is out of proportion to its true relative importance in technology and industry. Nonetheless, important contributions to the U. S. literature on hydrothermal gold deposits were made during the review period. A volcano-tectonic framework for epithermal Au-Ag deposits in the western United States was presented by Berger and Bonham [1990]. Lipman [1992] reviewed how structures in calderas influence and localize ore deposition. The geology and origin of the high-grade acid sulfate Cu-Au vein deposit at El Indio, Chile was described by Jannas et al. [1990]; they found that Au deposition actually occurred from late, low-salinity geothermal fluids that were different from those of more magma-hydrothermal affinity that deposited the enargite and alunite. Cunningham et al. [1991] described a conceptual genetic model for the diversity of volcanic-dome hosted precious metals deposits in Bolivia, which should prove more generally applicable.
In a paper with important exploration significance, Nelson [1990] evaluated the geochemistry of jasperoids from Carlin-type sediment-hosted Au deposits in the western United States. He found that elements characteristic of metalliferous marine black shales can be used to distinguish ore-bearing from barren systems. Acid alteration and oxidation are frequently cited as evidence of boiling of epithermal fluids in such deposits, but Kuehn and Rose [1992] showed that Au deposition at Carlin, Nevada, was structurally and stratigraphically controlled and occurred well before widespread oxidation, the latter of which is supergene (shallow weathering) in origin and not caused by shallow boiling during ore formation.