Abstract Five important and ostensibly different types of carbonate-hosted ores, concordant lenses of massive sulfide, MVT deposits and chimney-manto ores, all in platformal limestone-dolostone, Zn-Pb-(Ag) skarns in similar strata and “polymetallic” skarns may all be related metallogenetically to one another. It is suggested that all five were initially formed from similar hydrothermal fluids of comparable physico-chemical composition and derivation. They differ, however, because of differences in their sites and mechanisms of sulfide emplacement, and also because of the different post-depositional processes that have, or have not, affected them. The massive sulfides formed syn-depositionally at, or just beneath, the sea floor to which growth faulting conducted the generative fluids. MVT ores were deposited in similar, but much older platformal strata, and in sealed stratigraphic traps. The chinmey-manto deposits, like the massive ones, were also syn-depositional but formed as stacked, concordant lenses of massive ore during prolonged periods of episodic carbonate sedimentation, thus are connected by discordant chimneys of stringer-disseminated ore that formed along the propagating growth fault-feeders. The Zn-Pb-(Ag) skarns formed initially as massive or chimney-manto deposits hut have been thermally metamorphosed, with development of calc-silicate minerals, by intrusions that invaded the earlier, controlling growth faults during post-depositional orogenic-tectonic re-activation. The intrusions may also have contributed additional metals such as Sn, W, Ag, Au, and their related magmatogenic fluids have overprinted the ores, all this resulting in new ore minerals and textures as well as new isotopic and geochemical signatures. The variably Fe-Cu-Zn-Ag-Au-rich polymetallic skarns have undergone similar complex post-depositional re-activation. However, like polymetallic VMS ores, they formed initially as syn-depositional deposits but not in platformal carbonate strata. Instead, they formed in reefal and related calcareous-tuffaceous-volcaniclastic strata that were deposited on volcanic edifices in island-arcs, back-arcs or on sea mounts. They were subsequently accreted to continental margins and, like the Pb-Zn-(Ag) skarns, also underwent consequent orogenic-tectonic-magmatic reworking.

Abstract Supergene silver sulfide enrichment has been widely accepted for the last 100 years, but has warranted little or no mention in descriptions of several silver-rich, bulk-tonnage orebodies defined over the last three decades. This dichotomy is addressed by reassessing the importance of enrichment in 40 of the world’s premier silver dominated and other silver-rich deposits, including several of historical significance. The deposits are of high-grade vein and low-grade, bulk-tonnage styles and varied genetic types, but are dominated by representatives of the intermediate-sulfidation epithermal and carbonate-replacement, chimney-manto classes. The results of this preliminary analysis show that only 12 (30%) of the deposits contain(ed) appreciable amounts of silver ore generated by silver sulfide enrichment, mainly in the form of acanthite and argentian chalcocite-group minerals in the cases where its mineralogic characteristics are recorded. Silver-rich oxidized zones are, however, well developed in 60 percent of the deposits and, locally, display silver enrichment of either residual or chemical origin. Irrespective of whether oxidative weathering takes place under acidic or alkaline conditions, a factor controlled mainly by hypogene iron sulfide and carbonate contents, silver tends to be retained in oxidized zones, with comparatively little remaining available in solution to generate underlying silver sulfide enrichment. The extreme insolubility of the silver halides (chlorargyrite, embolite, bromargyrite, iodargyrite) over broad pH and climatic ranges, besides efficient silver fixation as native silver, argentojarosite, or silver-bearing manganese oxides under the appropriate chemical conditions, explains the metal’s relative supergene immobility. The efficient dissolution and downward transport of copper under acidic supergene conditions, as exemplified by porphyry copper leached cappings and underlying multicyclic enrichment blankets, appears to have no counterpart in either silver-only or other silver-rich deposits. Nor are the silver equivalents of exotic oxide copper deposits, the products of lateral metal transport in the acidic supergene environment, considered likely to exist. Furthermore, the processing benefits accruing from supergene oxidation and enrichment of copper deposits are not as evident in the silver environment, in which the main supergene oxidation products, especially the silver-bearing manganese oxides and argentojarosite, commonly present metallurgical difficulties.

Abstract Most of the day will be spent underground and on the surface in the Fresnillo district. This consists of replacement chimney, manto and fissure-vein deposits, and disseminated sulfide ores, hosted mainly by Cretaceous marine sedimentary rocks. The base metal - silver deposits are zoned to the southeast, away from a quartz monzonite stock, with increasing silver, but decreasing base-metal content, distal to the stock. Wall rock alteration adjacent to the stock consists of silicification and calc-silicate formation, partially replaced by sulfide mineral. Wall rock alteration around veins exhibits potassic, phyllic, argiJlic and propyllitic alteration with increasing distance from the mineralized fractures. The ores contain recoverable amounts of lead, zinc, copper, silver and gold. The latter part of the day consists of a short journey from Fresnillo to Zacatecas, passing over a wide, cultivated valley, but climbing in elevation to about 2,250 m, where the colonial city of Zacatecas is situated, flanked by sierra in which the fissure-vein mineral deposits are located.

Abstract Skarn deposits are one of the most common deposit types in China. The 386 skarns summarized in this review contain ~8.9 million tonnes (Mt) Sn (87% of China’s Sn resources), 6.6 Mt W (71%), 42 Mt Cu (32%), 81 Mt Zn-Pb (25%), 5.4 Mt Mo (17%), 1,871 tonnes (t) Au (11%), 42,212 t Ag (10%), and ~8,500 Mt Fe ore (~9%; major source of high-grade Fe ore). Some of the largest Sn, W, Mo, and Zn-Pb skarns are world-class. The abundance of skarns in China is related to a unique tectonic evolution that resulted in extensive hydrous magmas and widespread belts of carbonate country rocks. The landmass of China is composed of multiple blocks, some with Archean basements, and oceanic terranes that have amalgamated and rifted apart several times. Subduction and collisional events generated abundant hydrous fertile magmas. The events include subduction along the Rodinian margins, closures of the Proto-Tethys, Paleo-Asian, Paleo-Tethys, and Neo-Tethys Oceans, and subduction of the Paleo-Pacific plate. Extensive carbonate platforms developed on the passive margins of the cratonic blocks during multiple periods from Neoarchean to Holocene also facilitated skarn formation. There are 231 Ca skarns replacing limestone, 15 Ca skarns replacing igneous rocks, siliciclastic sedimentary rocks, or metamorphic silicate rocks, 113 Ca-Mg skarns replacing dolomitic limestone or interlayered dolomite and limestone, and 28 Mg skarns replacing dolomite in China. The Ca and Ca-Mg skarns host all types of metals, as do Mg skarns, except for major Cu and W mineralization. Boron mineralization only occurs in Mg skarns. The skarns typically include a high-temperature prograde stage, iron oxide-rich higher-temperature retrograde stage, sulfide-rich lower-temperature retrograde stage, and a latest barren carbonate stage. The zoning of garnet/pyroxene ratios depends on the redox state of both the causative magma and the wall rocks. In an oxidized magma-reduced wall-rock skarn system, such as is typical of Cu skarns in China, the garnet/pyroxene ratio decreases, and garnet color becomes lighter away from the intrusion. In a reduced intrusion-reduced wall-rock skarn system, such as a cassiterite- and sulfide-rich Sn skarn, the skarn is dominated by pyroxene with minor to no garnet. Manganese-rich skarn minerals may be abundant in distal skarns. Metal associations and endowment are largely controlled by the magma redox state and degree of fractionation and, in general, can be grouped into four categories. Within each category there is spatial zonation. The first category of deposits is associated with reduced and highly fractionated magma. They comprise (1) greisen with Sn ± W in intrusions, grading outward to (2) Sn ± Cu ± Fe at the contact zone, and farther out to (3) Sn (distal) and Zn-Pb (more distal) in veins, mantos, and chimneys. The second category is associated with oxidized and poorly to moderately fractionated magma. Ores include minor porphyry-style Mo and/or porphyry-style Cu mineralization ± Cu skarns replacing xenoliths or roof pendants inside intrusions, zoned outward to major zones of Cu and/or Fe ± Au ± Mo mineralization at the contact with and in adjacent country rocks, and farther out to local Cu (distal) + Zn-Pb (more distal) in veins, mantos, and chimneys. Oxidized and highly fractionated magma is associated with porphyry Mo or greisen W inside an intrusion, outward to Mo and/or W ± Fe ± Cu skarns at the contact zone, and farther to Mo or W ± Cu in distal veins, mantos, and chimneys. The final category is associated with reduced and poorly to moderately fractionated magma. No major skarns of this type have been recognized in China, but outside China there are many examples of such intrusions related to Au-only skarns at the contact zone. Reduced Zn-Au skarns in China are inferred to be distal parts of such systems. Tungsten and Sn do not occur together as commonly as was previously thought. The distal part of a skarn ore system may transition to carbonate replacement deposits. Distal stratabound mantos and crosscutting veins/chimneys may contain not only Zn-Pb but also major Sn, W, Cu, Mo, and Au mineralization. The Zn-Pb mineralization may be part of either an oxidized system (e.g., Cu, Mo, Fe) or a reduced system (e.g., Sn). In China, distal Zn-Pb is more commonly related to reduced magmas. Gold and W may also be related to both oxidized and reduced magmas, although in China they are more typically related to oxidized magma. There are numerous examples of distal mantos/chimneys that continuously transition to proximal skarns at intrusion-wall-rock contact zones, and this relationship strongly supports the magmatic affiliation of such deposits and suggests that distal skarns/carbonate replacement deposits systems should be explored to find more proximal mineralization. Carbonate xenoliths or roof pendants may host the majority of mineralization in some deposits. In contact zones, skarns are better developed where the intrusion shape is complicated. The above two skarn positions imply that there may be multiple skarn bodies below drill interceptions of intrusive rocks. Many of the largest skarns for all commodities in China are related to small or subsurface intrusions (except for Sn skarns), have multiple mineralization centers, are young (<~160 Ma), and have the full system from causative intrusion(s) to distal skarns or carbonate replacement extensions discovered. Chinese skarn deposits fall in several age groups: ~830, ~480 to 420, ~383 to 371, ~324 to 314, ~263 to 210, ~200 to 83, ~80 to 72, and ~65 to 15 Ma. They are typically associated with convergent plate boundaries, mostly in subduction settings but also in collisional settings. Seven major skarn metallogenic belts are recognized based on skarn geographic location and geodynamic background. In subduction settings, skarns may form in a belt up to 4,000 km long and 1,000 km inland, with skarns continuously forming for up to 120 m.y., e.g., the eastern China belt. In most other belts, skarns form in 5- to 20-m.y. episodes similar to the situation in South America. In collisional settings, skarns may form up to 50 m.y. after an ocean closure, and the distance to the collisional/accretionary boundary may extend to ~150 km inland. The size of collision-related skarns may be as large as the largest skarns related to oceanic crust subduction. Older suture zones may be favorable sites for younger mineralization, for example, the Triassic Paleo-Tethys suture between the North and South China blocks for the younger and largest skarn cluster of the Middle-Lower Yangtze belt in the eastern China belt, and the Triassic sutures in southwestern China for Cretaceous to Tertiary mineralization.

Abstract Our studies of geology, karst and cave morphology, mineralogy, geochemistry, and isotopes of the carbonate-hosted ore deposits of central Colorado have compeled us to two principal conclusions. First, some manto orebodies typical of the area are mineralized modifications of preexisting paleokarst cave systems. Second, some of the ores are geologically, mineralogically, and geochemically distinct from ores of the main Leadville district (Sherman-type defined below), are substantially older (Late Mississippian) than, and unrelated to, mid-Tertiary mag-matism and main Leadville district ore formation, and formed by processes similar to those that formed some Mississippi Valley-type deposits in the midcontinent region of the United States. In response to Late Mississippian emergence, an extensive karst system developed on and in the Leadville Dolomite. The resultant integrated cave systems have a distinct and easily recognizable large-scale morphology in plan view: pipe-shaped conduits in the form of a distributary network are elongated downdip from a surface input zone and converge in a central conduit or room. The caves also include many small-scale features such as dolines, chimneys, and bypass tubes. On the eastern flank of the Sawatch uplift, strata dipped into a trough whose base level may have been sea level. To the west, strata dippedinto the incipieut Eagle basin. Long dimensions of the cave conduits parallel the paleodip while convergent zones are aligned along the paleostrike. Prior to deposition of the Molas Formation (a Late Mississippian-Early Pennsylvanian karst-derived paleosol), the base level intermittently rose, enabling alternating phreatic and vadose conditions in the caves. The caves filled with iron oxide, carbonate speleothems, stratified dolomitic sand, silt, and clay, with splatter marks, dessication cracks, and mud drapes, collapse breccia, huntite mud, and angular to rounded grains of silver-rich Sherman-type minerals. The Molas Formation forms the uppermost unit of this sedimentary sequence and restricts the age of formation of the cave fill Sherman-type mineral assemblage to Late Mississippian. Cave fill textures are best preserved away from the destructive effects of later mid-Tertiary replacement (manto) ores in the central part of the main Leadville district. The best documented example of Sherman-type ores occurs in paleocaves of the Sherman mine, east of the Leadville district. Characteristics of Sherman-type mineral assemblages include presulfide white barite, banded dolomite flowstone and dripstone, low iron sphalerite, a high silver content, a very low gold content, very low pyrite and chalcopyrite content, light stable isotope and lead isotope data indicating crustal sources, karst cave control on ore morphology, and occurrence of ore minerals as clasts within the cave sediments. The Pennsylvanian Belden Formation marks the beginuing of a brief marine transgression into central Colorado. Sherman-type mineralization had ceased by this time. Following transgression, the area was buried under 2,500 to 5,500 m of mostly continental sediments. Quartz-gold-pyrite and silver-base metal ores were emplaced in carbonate rocks in the Leadville and Gilman mining districts at depth iu the mid-Tertiary. The shapes of the mid-Tertiary orebodies are analogous to the shapes of fully integrated cave systems, though their mineralogic and isotopic data are significantly different from Sherman-type ores. Also preserved in these orebodies are several small-scale karst and cave fill features, including Molas Formation within the Leadville Dolomite and relict Sherman-type ore minerals. The mid-Tertiary orebodies therefore are controlled, at least in part, by the precursor paleokarst cave systems. We speculate that much of the silver in Leadville district orebodies is the result of redistribution of high amounts of silver from precursor Sherman-type ores.