Early Prograde Research Articles

Patterns of mineralization and alteration below the porphyry copper orebody at El Salvador, Chile

Three diamond drill holes, angled below the lowest haulage level at El Salvador, have doubled the vertical exposure of the deposit and revealed very different features of alteration and mineralization below this major porphyry copper orebody. Sulfide assemblages persist with depth, but the total sulfide content diminishes. Magnetite becomes a part of all sulfide assemblages, except very late pyritic D veins. Residual traces of pyrrhotite-chalcopyrite found locked in quartz and as abundant and wide- spread inclusions in pyrite apparently represent the remains of an early prograde mineralization obliterated by intense sulfidation of subsequent events. Relicts of specularite veinlets may be a similar phenomenon. Vein types change. Newly recognized, early biotitic (EB) veinlets, with and without sulfides, quartz, albite, anhydrite, and actinolite, have varied alteration halos containing albite, K feldspar, biotite, green sericite, anhydrite, and andalusite. They appear to be deeper equivalents of A quartz veins. Veinlets descriptively similar to both EB and A quartz veins formed as a second generation within the young intramineral L porphyry complex which truncates similar veinlets in older and better mineralized rocks. Granular A quartz-K feldspar-sulfide-anhydrite veins diminish in abundance and in content of sulfide and K feldspar with depth, and are hard to distinguish from B quartz-anhydrite veins with characteristic molybdenite. The latter have much better developed K feldspar alteration halos than seen above. Younger G sulfide veins with green sericite, biotite, and anhydrite, and halos with green sericite, alkali feldspar, and andalusite, cut B veins. They are older than relatively sparse D pyrite- quartz veins with sericite-pyrite-calcite-anhydrite halos and occasional tourmaline. Pervasive sericite- chlorite in the pyritic fringe terminates downward and biotitic alteration of andesite diminishes, revealing more restricted and residual actinolite hornfels. Ilmenite and then sphene appear as residual accessory minerals and minor vein constituents. Minor andalusite with alkali feldspar extends to deepest exposures, mostly within halos of B and C veins. Traces of corundum and cordierite occur with andalusite...

Internal control of fluid compositions in amphibolite-facies scapolitic calc-silicates, Mary Kathleen, Australia

Unaltered metasediments of the Mary Kathleen Fold Belt are composed predominantly of layered amphibolite-facies scapolitic calc-silicate rocks in which minimal infiltration of externally derived fluids occurred during regional metamorphism. There were substantial differences in volatile activities between different layers in the layered sequences, in particular: a CO2/a H 2 O inferred from reaction progress estimates and analysis of biotite-clinopyroxene-fluid phase relations; a NaCl/a H 2 O inferred from scapolite compositions; and a HCl/a H 2 O inferred from biotite compositions. In one outcrop in which a clinopyroxene-producing reaction dominated, differences in approximate X CO 2of up to 0.25 occurred between several samples collected over 50 metres. Variations in a H 2 O/a HCl of up to one order of magnitude are inferred at 1 to 50 m scales from biotite-Cl contents, and variations in NaCl contents of scapolite from 0.0 to up to 0.6 Cl atoms in the Cl−CO3−SO4 site reflect a large variation of a NaCl in the coexisting fluid at similar scales. Most calcsilicate layers internally buffered fluid compositions in the H2O−CO2−NaCl−HCl system. Local occurrences of NaCl-rich scapolite suggest that some layers may have been in equilibrium with halite during early prograde metamorphism. At peak metamorphic temperatures, disolution of halite was complete but layers containing high-NaCl scapolite continued to buffer fluid at high values of a NaCl. Fluid immiscibility does not appear to have affected the progress of the devolatilization reactions. Although fluid was predominantly internally buffered, moderate quantities of fluid were released by prograde mineral reactions in many layers, up to 30 cm3 fluid per 100 cm3 rock. Numerous episodes of fluid escape were required, probably via microfractures, such that the released fluid did not obviously influence reaction progress in the layers through which it passed. The anomaly of beautifully preserved internal buffering signatures and the requirement for produced fluid locally to pass across layers in a deforming rock sequence suggest that the escaping fluid did not leave any readily observable tracks. This is explained by rapid rates of fracture propogation and fluid migration therein. This internally buffered system contrasts strongly with adjacent calc-silicate rocks that show evidence for infiltration of externally derived fluids at high fluid/rock ratios, and highlights the broad range of fluid behaviour that can be expected in deforming, heterogeneous rock sequences.

High‐temperature ‘clockwise’P‐T paths and melting in the development of regional migmatites: an example from southern Brittany, France

The Southern Brittany Migmatite Belt (SBMB), which evolved through the metamorphic peak between c. 400 Ma and c.. 370 Ma ago, consists of a heterogeneous suite of high‐grade gneisses and anatectic migmatites, both metatexites and diatexites. Rare garnet‐cordierite gneiss layers record evidence of an early prograde P‐T path. In these rocks, growth‐zoned garnet cores and a sequence of included mineral assemblages in garnet, from core to rim, of Qtz + Ilm + Ky, Pl + Ky + St + Rt + Bt and Pl + Sil + St + Rt + Bt constrain a prograde evolution during which the reactions Ilm + Ky + Qtz→ Aim + Rt, Ms + Chl→ St + Bt + Qtz + V and St + Qtz→ Grt + Sil + V were crossed. Parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types. In all rock types, garnet has reverse zoned rims, and garnet replacement by cordierite and/or biotite and plagioclase suggests the following reactions have occurred: Grt + Sil + Qtz→ Crd → Hc ± Ilm, Bt + Sil + Qtz → Crd ± Hc → Ilm → Kfs + V and (Na + Ca + K + Ti) + Grt → Bt + Pl + Qtz. Microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained ‘clockwise’P–T path for the SBMB. The high‐temperature part of the path has a steep dT/dP slope characteristic of near isothermal decompression. It is proposed that the P‐T path followed by the SBMB is the result of the inversion, by overthrusting, of a back‐arc basin and that such a tectonic setting may be applicable to other high‐temperature migmatite terranes. The near isothermal decompression is at least partly driven by the upward (diapiric) movement of the diatexite/anatectic granite core of the SBMB.

An introduction to remobilization: Information from ore-body geometry and experimental considerations

Mobilization involves the translocation and concentration of originally dispersed metallic constituents; remobilization is the translocation resulting in modified concentration and distribution of pre-existing metalliferous concentrations. Mobilization and remobilization proceed by “chemical” (liquid-state), “mechanical” (solid-state), and “mixed” processes. The paradox of remobilization is that the more complete the process, the more difficult becomes substantiation. Regional metamorphic remobilization is commonly invoked to explain the morphology of supposedly once-stratiform massive sulphides and their feeder and alteration systems. Analysis of the principal dimensions of stratiform massive sulphides in non-metamorphic and regionally metamorphosed deposits shows that, whereas comparative morphology may be a guide to remobilization, it does not constitute proof. Similarly, feeder and alteration morphology suggests that geometric analysis may be of value in establishing mechanical remobilization; but the more complex alteration distributions which exist could well lead to misinterpretation in highly deformed terranes. Ore composition, internal structure and zonation influence the ore's capacity to be remobilized. Mechanical remobilization requires rheological divergence between sulphides and their host rocks. Experimental deformation of dry sulphides shows that most common sulphides pass through the brittle-ductile transition before entering the field of regional metamorphism, but that the strength-order of sulphides, silicates and carbonates remains dynamic until all are undergoing steady-state flow. Within this context, experimental strength-orders support galena (weakest) < pyrrhotite < chalcopyrite < sphalerite < pyrite (strongest), with some variation in the relative positions of the middle three. They also suggest that the middle three are of similar strength to carbonate rocks and that some silicate rocks are weaker than pyrite. In mixed sulphide and sulphide-silicate assemblages, behavioural convergence results from strength-extremes being diluted by weaker/stronger phases; in effect, an aggregate-strength exists. The presence of an aqueous phase further modifies mechanical behaviour; massive sulphides probably deform by dominant dislocation flow, whereas intercalated sulphides and silicates express a complex interplay of solution-transfer and other flow mechanisms. Experimental considerations indicate that, during diagenesis and early prograde metamorphism, the brittle-ductile transition will result in widely distributed micro- and mesoscale mechanical remobilization. Nevertheless, there is usually insufficient gross behavioural contrast to engender discordant macroscale remobilization. Possible exceptions, particularly in relation to thrusting, are outlined, but we conclude that macroscale mechanical remobilization during the brittle-ductile transition is an uncommon occurrence. Experimental data were also examined relative to subsequent prograde (to peak) metamorphism, and strength-contrasts during decaying and retrogressive metamorphism. Because the largest strength-difference between sulphides and the host-sequence is in the lower part of the 350°–650°C metamorphic range, we contend that macroscale mechanical remobilization is more probable during prograde metamorphism in the 350°–500°C interval. We similarly believe that macroscale mechanical remobilization is more likely during deformation accompanying retrogressive cooling in the range 500°–350°C; and we note that it may also result from deformation exploiting the brittle/ductile contrasts between silicate and sulphide rocks during the water-deficient, non-retrogressive, metamorphic-decay cycle. Chemical mobilization and remobilization require the dissolution of base metals by metamorphic fluids. The compositions of hydrothermal (low-grade metamorphic) fluids and their precipitates in active geothermal systems, show that the fluids can dissolve, transport, and form concentrations of base and precious metals. Similarly, fluid-inclusion compositions from low- to medium-grade metamorphic rocks show that such fluids can dissolve a few ppm of base metals. In both cases, low salinities and low dissolved base-metal contents raise doubts over whether such metamorphic fluids can form ore-grade base-metal concentrations; we submit that they can. Metamorphogenesis is favoured by high initial base-metal concentrations, preferential dissolution of sulphides over silicates, and fluid-dominated metamorphism as found in a multiple-pass fluid system. Preferential dissolution of sulphides and the need for high water/rock ratios are supported by theoretical considerations, experimental work on mafic, felsic and sedimentary source-rocks, and field-based studies. In the context of mobilization, the work strongly implies that prograde metamorphic devolatilization, and a mobile fluid phase have substantial ore-forming capability; retrograde revolatilization does not. Remobilization is more complex. Although favoured by high initial metal concentrations, dissolution is inhibited by the low permeability of homogeneous, low-silicate, massive sulphides, and the consequent peripheral fluid-channeling. It is conversely enhanced by fine grainsize in discordant, fluid-focussing (retro-grade) shear zones. In all cases, for remobilization to form a new ore body, both fluid-flow and the precipitation process must be focussed and efficient.

Metamorphic imprints upon sulphide mineralisation at Boksputs, Northern Cape, South Africa

A small stratiform Fe-Cu-Zn mineralisation has been examined from Boksputs within the metavolcanic Areachap Group (1.5 Ga), along the eastern boundary of the Namaqua Province. Distinctly deformational and metamorphic features within the mineralisation can be separated into an early and a late fabric; the former is related to an early prograde metamorphism (M1) and also to a subsequent mild retrogression (M2) which can be regionally distinguished in the silicate mineralogy, whilst the latter fabric is derived during a severe retrogressive event (M3) that is concentrated especially in shearzones that cut the mineralisation. A release of secondary sulphur-bearing fluids during M3 is evidenced by new growth and redistribution of sulphides and by local replacement of pyrrhotite by pyrite.