| dc.description |
Porphyry copper systems may host a variety of alteration and mineralisation styles such as porphyry- or endoskarn-type within, and exoskarn- and/or manto-type outside the porphyry intrusion(s). However, why endoskarn- rather than porphyry-style mineralisation predominates in certain systems and regions is poorly understood. To address this, the Tonglushan porphyry system of the Daye Cu-Fe-Au ore district, eastern China, was studied as it hosts extensive but generally barren endoskarn and only minor porphyry-style mineralisation.
The Tonglushan endoskarn underwent: 1) muscovite-dominated alteration; 2) prograde and 3) retrograde skarnification; 4) potassic and 5) sodic alteration; and 6) carbonatisation. The quartz monzodiorites distal to the endoskarn underwent minor potassic alteration with weak Fe-Cu sulphide mineralisation, and then sodic alteration. The muscovite-dominated alteration was likely caused by hot and saline magmatic fluids which then mixed with Ca- (+/- CO₂), and possibly F-rich fluids, produced by the exoskarnification of carbonate xenoliths and wall rocks, to initiate skarnification in the variously altered quartz monzodiorites. A second pulse of moderately hot, weakly acidic-to-neutral magmatic fluids produced potassic alteration in the endoskarn; similar fluids caused weak potassic alteration and mineralisation in the quartz monzodiorites distal to the endoskarns and extensive sulphide precipitation in the exoskarns. Subsequent sodic alteration in both environments was due to ingress of Na-(CO₂)-rich fluids from the surrounding carbonate rocks.
Mineralisation was restricted in the endoskarns due to relatively oxidising conditions during potassic alteration which restricted the precipitation of sulphide. It was instead concentrated in the exoskarns which had a more reducing environment and into which fluid flow was focused. The latter was due to decarbonation of wall rock marbles and related upwards migration of CO₂ to produce a self-sustaining chimney effect which further transported fluids towards the marbles to form, alter and mineralise the exoskarns.
The higher porosity and permeability of the endoskarns compared with the surrounding quartz monzodiorites may have promoted the lateral flow of Cu-bearing fluids to mineralise the exoskarns, rather than upwards flow within the magmatic stock to produce porphyry-style alteration and mineralisation. In addition, certain pulses of magmatic-hydrothermal fluids being enriched in CO₂, due to exoskarnification and/or carbonate assimilation, may have reduced their capacity to transport Cu upwards within the system, and thus limited their potential to form porphyry-style ores.
Apatite crystallised during different episodes of skarnification and hydrothermal alteration shows distinct cathodoluminescence colours and chemical compositions. The correlation between apatite cathodoluminescence colours (e.g., yellow-green, green-blue and blue) and chemistry, particularly with regards to Mn, Na, Sr, Mg, Ca, F and Cl concentrations, was found to provide a potential indicator for the styles of alteration and mineralisation within the Tonglushan system. This could potentially aid in assessing the architecture of a particular porphyry-skarn system and therefore help in the discovery of new Cu deposits.
This thesis offers new insights into why certain porphyry systems host poorly mineralised endoskarns and mineralised exoskarns rather than porphyry-style Cu deposits. It is hoped that this will aid in the development of future exploration models for porphyry-skarn systems. |
|