The Alemao copper-gold deposit is located within the Carajas Mineral Province of Northern Brazil and was discovered in 1996 by DOCEGEO using geophysical and geological techniques. Alemao is hosted by the Igarape Bahia Group, which comprises two lithological and stratigraphic domains: a lower metavolcanic unit composed of metavolcanic rocks and acid to intermediate volcanoclastics; and an upper clastic-chemical metasedimentary unit with volcanoclastic rocks. The Alemao ore body is covered by a 250 metres thick unconfonnable siliciclastic unit referred as the Aguas Claras Formation. The ore body, which is 500 metres in length and 50 to 200 metres wide, strikes NE-SW and dips steeply to the NW, being emplaced along the contact between the two stratigraphic domains of the Igarape Bahia Group. In the ore zone, the hydrothermal paragenesis is marked by ferric minerals (magnetite-hematite), sulphides (chalcopyrite, pyrite), chlorite, carbonate (siderite, calcite, ankerite) and biotite, with minor quartz, tourmaline, fluorite, apatite, uraninite, gold and silver. Sericite and albite are rare. The mineralisation is represented by hydrothermal breccias and "hydrothermalites" classified into two types: (1) the BMS type, composed of massive bands of magnetite and chalcopyrite and by polymitic breccias with a matrix comprising magnetite, chalcopyrite, siderite, chlorite, biotite and amphiboles; (2) the BCLS type breccia which comprises brecciated hydrothermalised volcanic rocks with chalcopyrite, bornite, pyrite, chlorite, siderite, ankerite, tourmaline and molybdenite in the matrix, as well as dissemination in the rock. The geochemical association of Fe-Cu-Au-U-REE in iron rich, heterolithic, hydrothermal breccias at the Alemao Cu-Au Deposit, as well as its possible association with an extensional tectonic setting, suggests a correlation with Olympic Dam type mineralization. The total estimated ore resources based on a krigging method is 170 Mt @ 1.5% Cu and 0.8g/tAu.

The Silurian is a geologic period and system that extends from the end of the Ordovician Period, about 443.7 ± 1.5 Mya (million years ago), to the beginning of the Devonian Period, about 416.0 ± 2.8 Mya (ICS, 2004, chart). As with other geologic periods, the rock beds that define the period's start and end are well identified, but the exact dates are uncertain by several million years. The base of the Silurian is set at a major extinction event when 60% of marine species were wiped out. <...>

The Bulletin lode-gold deposit is within the northernmost part of the Norseman-Wiluna greenstone belt in me Archaean Yilgarn Block, Western Australia. It is located within a brittle-ductile shear zone and hosted by tholeiitic metavolcanic rocks. Syn-metamorphic wallrock alteration envelops the gold mineralisation and is pervasive throughout the entire shear zone and extends up to 150 m into the undeformed wallrocks. Alteration is characterised by the sequence of distal chlorite-calcite, intermediate calcite-dolomite, outer proximal sericite and inner proximal dolomite-sericite zones. The thickness of the alteration envelope, and the occurrence of dolomite in the alteration sequence, can be used as a rough guide to the width, extent and grade of gold mineralisation, because a positive correlation exists between these variables. Mass transfer evaluations indicate that chemical changes related to the wallrock alteration are similar in all host rocks: in general, Ag, As, Au, Ba, C02, K, Rb, S, Sb, Те and W are enriched, Na and Y are depleted, and Al, Cr, Cu, Fe, Mg, Mn, Nb, Ni, P, Se, V, Zn and Zr are immobile, while Ca, Si and Sr show only minor or negligible relative changes. The degree of mobility of each component increases with proximity to gold mineralisation. The largest potential exploration targets are possibly defined by regional As (>6 ppm) and Sb (>0.6 ppm) anomalies. These anomalies, if real, extend laterally for > 150 m from the mineralised shear zone into areas of apparently unaltered rocks. Anomalies defined by Те (> 10 ppb), W (>0.6 ppm), carbonation indices, local enrichment of Sb (>2.0 ppm) and As (>28 ppm), and potassic alteration indices also form significant exploration targets extending beyond the HJB shear zone and the Au anomaly (>6 ppb) and, locally, into apparently unaltered rock. Gold, itself, has a restricted dispersion, with an anomaly extending for 1-35 m from ore, and being restricted to within the shear zone itself. Amongst individual geochemical parameters, only As and Sb define significant, consistent and smooth trends (vectors) when laterally approaching the ore. However, the respective dimensions of individual geochenucal anomalies can be used as an extensive, though stepwise, vector towards ore

No single model satisfactorily accounts for the diverse characteristic of Fe-oxide-rich hydrothennal systems. Consideration of a spectrum of geologically reasonable models gives insight into the origins of variability among these deposits. Key features that need to be rationalized by any model are the abundance of hydrothermal magnetite and/or hematite, the chemically distinct suite of elements (REE-Cu-Co-Au-Ag-U), the variability of associated magmas, the distributions and volumes of associated hydrothennal alteration, and the broader geologic setting(s).

Geologic and geochemical evidence show that the ore-fonning fluids are brines, but the source of the brines is controversial. Multiple sources are possible, indeed likely. The identity and consequences of alternative sources - magmatic and non-magmatic are considered: First, by a review of plausible fluids and general consequences, second, by examination of the system characteristics, third, by specific consideration of the consequences of alternative models, and fourth, by consideration of selected systems where non-magmatic brines must play a major role.

We review some of the key characteristics of different types of hydrothermal Fe-oxide-rich(-Cu-Au) systems. Some are economic; many are only geochemically anomalous. Two end-members and several variants on these end members are proposed. One group is typified by relatively high-temperature mineralization, and relatively high K/Na and Si/Fe in the alteration. We suggest that these features (and others) are distinctive of magmatic fluid sources and that this group overlaps with porphyry Cu-Au and related deposit types. A second, broad group is typified by more oxide-rich, sulfide-poor mineralization, low Si/Fe ratios, and voluminous alkali-rich alteration where sodic types commonly exceed K-rich varieties. We suggest that the key features of this group reflect involvement of non-magmatic brines and that ore grades are less common as the metals are less easily trapped. Hybrid examples, where fluids of both types are involved, are expected (and known).

Conceptual and quantitative models of magmatic and non-magmatic fluid sources yield insight into the expected differences, the source and distribution of metals within these systems, and possible controls on ore deposition. These models highlight the difference between magma-sourced fluids and non-magmatic fluids. The former tend to be focused at the tops of magma chambers and have a built-in depositional mechanism -cooling. The latter require a different type of focusing mechanism - structural or stratigraphic, and different traps - mixing, specialized host rocks, and/or boiling. These models predict consistent differences in mineral assemblages, metal contents, alteration volumes, zoning, paragenesis, and geochemistry. For magmatic fluid sources, the models reproduce the key characteristics of that group, notably the porphyry-related systems. For non-magmatic brine sources, predicted characteristics match well with observations of Fe-oxide-rich systems in environments where these fluids are known to dominate, including mafic igneous systems and modem analogs such as the Salton Sea. These cases show that Fe-oxide (-Cu-Au-REE-Co) enrichments can result from non-magmatic sources. In other environments (e.g., with intermediate to felsic igneous rocks; settings deeper than ~5 km) the relative importance of various fluid sources and their consequences for mineralization remain to be fully explored. Young systems in the American Cordillera and elsewhere can help unravel the many threads that relate these enigmatic mineral deposits.