Decrepitometry Application Note 1
Porphyry Type Deposits
Kingsley Burlinson 27/2/1981
The porphyry Cu and Mo type deposits have been the subject of much Fluid Inclusion (F1) study which is extensively reported in the literature. These deposits have quite characteristic FI types which show that they formed from highly saline, boiling fluids at temperatures of approximately 300 C to 450 C. The intensive fracturing of these deposits has resulted in FI's being particularly abundant in most rock types including the igneous stocks and the quartz veins of all stages. Most studies to date have been on fairly restricted sample sets but those covering both mineralised and barren regions almost always point out the spatial zoning of the FI characteristics.
No decrepitometry work has been done on these deposits in the Western world and only a little such data is reported in the Russian literature. Those cases reported from Russia indicate that the method can be expected to be applicable. Although salinity and accurate temperature measurements cannot be obtained, approximate temperatures, total inclusion abundances and data on the relative abundances of different inclusion types are obtained. Western world studios have already shown these parameters to be of importance in the exploration for porphyry deposits.
The FI data outlines palaeo-thermal and palaeo-salinity haloes around the mineralised centres of the deposits. Such haloes are comparable in size to the pyritic alteration halo. The use of such data in combination with geochemical and geological alteration zonation data would aid in any porphyry exploration programme. However in areas of poor outcrop or substantial supergene alteration the FI data (which is unaffected by weathering and usable on detrital grains) may well be the most important exploration tool available.
General features of the porphyry system and it's fluids
An extensive study of 37 Porphyry copper deposits has been carried out by Nash, 1976. He shows that the great majority of deposits have evolved through a hydrothermal stage characterized by very high salinities, generally 35 to 60 weight % NaCl equivalent. These saline fluids are not necessarily the earliest in the system but are always associated with the mineralisation stages. The fluid temperatures range from about 150 C in the peripheral areas to in excess or 700 C for some of the barren core pre-mineralisation stage fluids. The mineralisation stage fluids are generally in the range 300 C to 450 C. in most instances the fluids boiled, as shown by the complex assemblages of various types of inclusions present, which may homogenize to either the gas of liquid phase. Pressure measurements indicate that most deposits were emplaced under 1800 to 3000m of cover rocks and fluid pressures during mineralisation were generally less than 500 bars.
The porphyry systems occur in zones of intense fracturing which leads to the development of abundant FIs and a very complex assemblage in which it is difficult to differentiate the primary, pseudosecondary and secondary inclusions. These inclusions are ubiquitous in the quartz veins of all stages and in the igneous rocks present while being absent from unaffected country rocks.
Few deposits are simple and must show telescoping or reactivation or adjacent superimposed but temporally separated deposits. During the waning stages of the system the zones often collapse inwards which results in overprinting of the FI assemblages (as it does also for the alteration zoning). However in contrast to the collapsing alteration zoning which obliterates pre-existing zones, evidence of the previous temperature and salinity zones is preserved by the FIs. This contributes to the broad spread of FI measurements in such areas.
During unroofing of the deposits supergene alterations occur which overprint the hypogene alteration zoning. However the F1 data is preserved intact through these processes which gives such measurements a unique advantage either in determining the original conditions within the deposit or in exploration.
Most studies report the association of highly saline fluids with the ore mineralisation and Nash, 1976, considers this zone of high salinity to be the most important target for exploration programmes.
This zone may be areally not much larger than the mineralisation
itself as at Bingham (Moore & Nash 1974) or extend some 2Km
around the ore body as at Copper Canyon (Theodore & Nash
1973). This zone is generally also discernable above and below the
mineralisation and has been used as an exploration indicator for
still buried porphyry systems in the precious metal orefields of
Nevada and Colorado (Nash 1976) and at Red Mountain (Bodnar &
Beane 1980). Such salinity zoning is also commonly reported from
the Russian deposits (Khadzharan by Rekharsky at al 1973 and other
unnamed deposits by Sotnikov et al 1974)
Salinity studies often point out the existence of a salinity gap between 15% and 30% total salinity, there being a conspicuous absence of any such FIs in several deposits (Red Mountain, Bodnar & Beane 1980 and Santa Rita, Ahmad & Rose 1980). Hence the search for highly saline inclusions can be a quite specific exploration tool for the porphyry deposits.
Temperature zoning around porphyry deposits is considered to be a general feature and in cases where sampling is adequate such lateral zoning is always observed (Roodder 1977). Unfortunately few studies collect sufficient samples, particularly from locations outside the orebody.
Temperature decrease over a traverse 2.9Km. long from the centre of the Sierrita, Arizona deposit ranges from 380 C to 130 C (Haynes & Titley 1980). The only other study on a measured traverse is at Koloula, Guadalcanal (Chivas 1978) where a "substantial" temperature decrease occurs over a distance of 1000m from the centre and by 1500m there is virtually a complete absence of FIs, constituting an abundance anomaly also! A study of the mine area at Panguna (Eastoe 1978) maps a distinct temperature decrease away from the mine area. The temperature ranges from >580 C at the centre to <420 C just past the edge of the orebody where the sampling pattern stops.
Other studies do not state the distance over which the temperature effects were measured but uniformly agree on the presence of such temperature zonation. At Mineral Park measurements on quartz veins show a temperature decrease from 450 C to 230 C away from the centre. This zonation is concentric to the stock (Drake & Ypma 1969). it Bingham, temperatures are 640 C to 725 C in the core and 294 U to 330 C in the peripheral deposits (Roedder 1971). Dawson, 1973, shows that temperatures decrease away from the ore zone from >500 C in the potassic alteration zone to 400 C in the argillic zone. The Russian deposits at Khadzharan (Rekharsky et al 1973) and unnamed deposits from middle Asia (Sotnikov at al 1974) also show spatial temperature zoning. at Kounrad (Yudin 1968) several petrogenetic stages were identified and their temperatures ranged from 470 C to 150 C. Decrepitation and homogenization methods were both used in this work which noted the presence of horizontal temperature zoning around the ore centre. (Although this is one of the very few decrepitation studies reported, note that Roedder, 1977, recommends the use of decrepitation measurements in exploration because of their ease, rapidity and economy.)
Although temperature zonation is quite common it is not always simple. At Khadzharan (Karamyan & Madanyan 1968) and Kounrad (Piznyur & Poletaev 1973) reactivation of the system occurred, resulting in a second mineralisation phase before final cooling. Chivas & Wilkins, 1977, also show superimposed deposits at Koloula and similar multiple events are recorded at Panguna (Eastoe 1978). These effects would not be expected to interfere with temperature zonation studies for exploration purposes.
Moore & Nash, 1974, in their extensive study of the Bingham deposit did not report any significant spatial zoning of temperatures. The real reason for this was their admitted preoccupation with salinity measurements which could be done without using the heating stage microscope and were thus quicker and more convenient. This is a common problem as the measurement of homogenization temperatures is exceptionally tedious. Only by using decrepitation measurements, despite their inherent limitations, can sufficient data be obtained on F1 temperatures to enable the determination of spatial temperature patterns around the ore centres.
Although less commonly reported. probably because of the difficulty of obtaining samples over the vertical range of a deposit, several studies have shown the presence of such zoning. Several others have been unable to measure such zonation and it is clear that this feature varies considerably between deposits.
Gdovanov et al, 1974, used decrepitation measurements at the Almalyk deposits where they show that the temperature varies with depth in the mine. They also found that at low levels, near the maximum mineralisation grades, an anomalous decrease in temperature occurred. This is probably due to a change of inclusion type from gas to liquid phase homogenizing as these decrepitate more readily. At the Mamut deposit, Malaysia (Imai et el 1976) the ratio of the number of polyphase inclusions to the total number of inclusions increases lower in the orehody.
However, Nash, 1976, reports that although he has observed vertical zonation in the Park City district, he has been unable to detect any vertical changes at Bingham, Copper Canyon, Ely or Rey over the restricted interval examined (150m).
Several studies have pointed out that the total abundance of inclusions. regardless of their salinities or temperatures, is also zoned around the porphyry centre. Chivas & Wilkins, 1977, show such a relation at Koloula, Guadalcanal and present a map of FI abundance. The resulting anomaly is of moderate size and good contrast. This map also shows an anomaly in the relative abundance of different types of F1, there being a relative increase of gas rich inclusions near the porphyry centre. Nash, 1971b, comments that the abundance of FIs commonly reflects the intensity of fracturing, hence implying abundance anomalies at porphyry centres, where intense fracturing is common. Nash, 1976, also points out that such measurements are useful in distinguishing the post ore intrusives at Ray, Ely and Park City as these intrusives are characterized by a paucity of FIs.
Use in Exploration
The main advantage of the use of FIs is that they survive weathering, supergene and even retrograde alterations unmodified (Chivas & Wilkins 1977, Nash 1971b) although subsequent tectonism can affect them. Because of their persistence they are of value in distinguishing between superficial and hydrothermal alteration (Poty & Weisbrod 1976) and can be used to determine the presence of an early brine stage despite later inward collapse of the porphyry system (Nash 1971a). They are unique in being able to provide information on the direction of fluid movement and thus infer mineralisation zonatian. This can be deduced from fluid densities or zonations (Nash 1971b). At the Sapo Alegre deposit, Puerto Rico, (Nash & Cox 1974) inclusions were studied in residual quartz grains from soil in a lateritized area.
In studies at Copper Canyon, Theodore & Nash, 1973, concluded that "chemical sampling for copper and many other metals would point toward the intrusion as the most likely host for ore rather than the metasedimentary wall rocks where the economic hypogene mineralisation occurs". However the FI data correctly identified the wall rocks as the exploration target.
In their work at Santa Rita, Ahmad & Rose, 1980, have pointed out the difficulty of distinguishing between primary and secondary inclusions. However their data shows that the predominantly liquid filled secondary inclusions homogenize at markedly lower temperatures than do the primary inclusions. Consequently the need to distinguish the two types is minimized. This is or particular relevance to decrepitometry where the inclusion types cannot be selectively measured. The lower temperature and liquid phase filling of these inclusions would result in a separate peak on the decrepigram and hence would not interfere with the information on the primary inclusions.
Temperature gradients from some 700 C at the mineralisation centre to approximately 200 C in the unaltered host rocks are common features of porphyry deposits. The radius of the resulting zone of anomalous temperature, as measured in FIs, is 2Km. or more. With rare exceptions, salinity gradients are also present around these deposits and the radius of the zone of highly saline FIs varies from hundreds to thousands of metres. Although vertical zonation of salinity and temperature of the deposits also occurs, such features are more difficult to observe and may be less reliably developed than areal gradients.
In addition to the use of Fls in determining zonation around mineralisation centres the unusual, highly saline fluids associated with the porphyry deposits are sufficiently unique that merely their presence can he a useful guide to the nearby occurence of such deposits. Because FIs are quite resistant to destruction from subsequent geological processes, their use results in an exploration method uniquely applicable in areas where conventional methods fail.
Previous Fl work on porphyry deposits has been hampered by the tedium of obtaining data by the conventional microscopic techniques, but adequately demonstrates the relevance of these results to exploration. By using decrepitometry it is possible to obtain much of this FI information rapidly and economically. The addition of this data in an exploration programme for porphyry deposits would represent a significant advance.
Much of the data used has been obtained from abstracts, particularly the less commonly available Russian papers. These are available in the volumes of Fluid Inclusion Research, Proceedings of COFFI, edited by E.Roedder. Reference is therefore made to these volumes rather than the original published paper. Such references are listed below simply as COFFI.
Ahmad S.N. & Rose A.W. 1980 Fluid inclusions in porphyry and skarn ore at Santa Rita, New Mexico Econ.Geol. V75,p229
Bodnar R.J. & Beane R.E. 1980 Temporal and spatial variations in hydrothermal fluid characteristics during vein filling in preore cover overlying deeply buried porphyry copper type mineralisation at Red Mountain, Arizona Econ.Geol. V75 p876
Chivas A.R. 1978 Porphyry copper mineralisation at the Koloula igneous complex, Guadalcanal, Solomon Islands Econ.Geo1. V73 P645
Chivas A.R. & Wilkins R.W.T. 1977 Fluid inclusion studies in relation to hydrothermal alteration and mineralisation at the Koloula porphyry copper prospect, Guadalcanal Econ.Geol. V77 p153
Dawson K.M. 1973 Geology of the Endako mine,British Columbia. PhD. Dissertation COFFI V7 P43
Drake W.E. & Ypma P.J.M. 1969 Fluid Inclusion study of the Mineral Park porphyry copper deposit, Kingman, Arizona COFFI V2 p15
Eastoe C.J. 1978 A fluid inclusion study of the Penguna porphyry copper deposit, Bougainville, Papua New Guinea Econ.Geol. V73 P721
Gdovanov I.M., Tsoi A.V. & Rakhunbekov A.T. 1974 Quartz in the Almalyk copper porphyry deposits. COFFI V7 p62
Haynes F.M. & Titley S.R. 1980 The evolution of fracture-related permeability within the Ruby Star Granodiorite, Sierrita porphyry copper deposit, Pima County, Arizona. Econ.Geol. V75 p673
Imai H., Takenouchi S. & Nagano K. 1976 Fluid inclusion study of the Mamut porphyry copper deposit, Sabah, Malaysia. (abst.) COFFI V8 p77
Karamyan K.A. & Madanyan O.G. 1968 Thermometric studies of vein quartz and stage character in the formation of the Khadzharan copper-molybdenum deposit. COFFI V2 p49
Moore W.J. & Nash J.Thomas 1974 Alteration and fluid inclusion studies of the porphyry copper ore body at Bingham, Utah Econ.Geol. V69 p631
Nash J.Thomas 1971a Composition of fluids in porphyry-type deposits (abst.) COFFI V4 p50
Nash J.Thomas 1971b Fluid inclusions as a guide to porphyry-type mineralisation (abst.) COFFI v4 p51
Nash J.Thomas 1976 Fluid inclusion petrology -- Data from porphyry copper deposits and applications to exploration. U.S. Geol. Survey Prof. Paper 907D 16pp
Nash J.Thomas & Cox D.P. 1974 Fluid inclusion petrography of quartz from a porphyry copper zone in Puerto Rico (abst.) COFFI V7 p153
Piznyur A.U. & Poletaev A.i. 1973 On thermobaric conditions of explosive breccia formation exemplified by Cu-Mo Kounrad deposit (abst.) COFFI V6 p125
Poty B. & Weisbrod A. 1976 Fluid inclusions as a guide in metal deposit exploration COFFI V9 p108
Rekharsky V.l., Pashkov Y.N. & Avetisyan G.G. 1973 Fluid inclusion evidence on the environment of deposition of hydrothermal-metasomotic formations of the Khadzharan deposit (abst.) COFFI V7 p180
Roedder E. 1971 Fluid inclusion studies on the porphyry-type ore deposits at Bingham, Utah; Butte, Montana; and Climax, Colorado COFFI V4 p65
Roedder E. 1977 Fluid inclusions as tools in mineral exploration Econ.Geol. V72 p503
Sotniknv V.I., Berzina A.P., Nikitina E.I., Skuridin V.A. & Proskuryakov A.A. 1975 Relationship between Cu-Mo mineralisation and subvolcanic granitoids (ext. abst.) COFFI V7 p217
Theodore T.G. & Nash J.Thomas 1973 Geochemical and fluid zonation at Copper Canyon, Lander County, Nevada Econ.Geo1. V68 p565
Yudin I.H. 1968 Temperature conditions and the formation
sequence of endogenic ores of the Kounrad deposit COFF1 V2 p79