JOURNAL 0! GEOCHEMICAL EXPLORATION

ELSEV! ER Journal of Geochemical Exploration 51 (1994) 93-108

Exploration history and geology of disseminated gold deposit at Maoling in Proterozoic sedimentary

rocks in North China Platform

Qiuming Cheng a, Keiko Hattori a, Jizhang Fan b, Shichen Wang b aOttawa-Carleton Geoscience Centre, and Department of Geology, University of Ottawa,

Ottawa, Ontario, KIN 6N5, Canada bChangchun Univ. of Earth Sciences, Changchun, People's Republic of China

(Received 21 July 1993; accepted after revision 11 January 1994)

Abstract

The Maoling deposit, with average grade of 2 g/t Au and reserves of over 45 m.t., is a disseminated Au deposit hosted in Proterozoic sedimentary rocks in the Southern Liaoning Province of the North China Platform. The hidden ore bodies at Maoling were discovered in 1985 through a combination of geophysical and geochemical studies in the area which showed anomalies during the reconnaissance soil geochemical survey. The ore bodies slaow high magnetic susceptibility, low resistivity and high chargeability compared to unaltered host rocks and they are enriched in As, Ag, Pb, Co, Ni and Sb. The soils overlying the deposit show secondary halos of Au, As, Ag, Pb and Sb.

All Au deposits in the area are in and around Mesozoic granitic intrusions and the Maoling deposit is underlain by the Wo-Longquan batholith. The occurrences of Au deposits and cross-cutting rela- tionships between alteration zones and dykes suggest that the Au mineralization took place after the peak intrusive event during the Jurassic tectonic activity. The ore bodies at Maoling are composed mostly of fine dissemination of arsenopyrite and pyrrhotite and they are accompanied by intense sericitization and.silicification with minor biotitization. The characteristics of the deposit are akin to those of Carlin-type Au deposits in North America, including intense regional silicification, high tonnage/grade ratio, high As and Ag, low base metal contents, and dissemination of fine-grained metallic minerals in sedimentary rocks. Whereas the host rocks of North American Carlin-type deposits are unmetamorphosed, those of the Maoling deposit have undergone greenschist- to amphi- bolite-facies regional metamorphism and some ores occur within the andalusite-grade contact meta- morphic rocks of the intrusion. The lack of cristobalite, opaline silica, clays, and cinnabar which are common in Carlin-type deposits together with the evidence of brittle-ductile deformation in the Maoling deposit is consistent with the mineralization at Maoling at higher temperatures and deeper crustal level than Carlin-type deposits.

0375-6742/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDIO375-6742(94)OOOO3-T

94 Q. Cheng et al. / Journal ~ Geochemical Exploration 51 (1994) 93 108

1. Introduction

The Maoling deposit is a buried, disseminated Au deposit with no surface exposures and the hidden ore bodies were discovered in 1985 after extensive exploration eftbrts using geological, geophysical and geochemical techniques. The deposit occurs in metamorphosed sedimentary rocks of Proterozoic age in the northeastern part of the North China Platform (Fig. 1 ). More than 10 economic Au deposits have been discovered in the area in the past 30 years, which makes it an important Au producing district in China. The occurrences of metallic minerals and alteration in the Maoling deposit are different from those of previously known Au deposits in the North China Platform. Gold in the North China Platform occurs in discrete quartz veins or in "brecciated altered rock" (Wong et al., 1984; Tian, 1988). The latter has high base metal contents, ~ several vol%, and high grade, av. 9.3 g / t Au in 27 deposits.

This paper describes the geology and occurrence of the Maoling deposit and the explo- ration method which led to the discovery of the deposit. The information may be useful in designing the exploration program for disseminated Au deposits in other sedimentary terrains.

7 J Telin Qinyuan Shenyang

Yinlmo,

Luid~y~ 0, 150kin i ,

Fig. 1. Location of the Maoling deposit in the Southern Liaoning Province. The area shown in the map is indicated with an arrow in the inset map. FZ = Fuzhao Syncline, NCP = North China Platform. YK = Yinkao-Kuandian Subprovince, open squares = major cities.

Q. Cheng et al. / Journal of Geochemical Exploration 51 (1994) 93-108

2. Regional geology

95

Maoling is about 600 km northeast of Beijing in the Liaodong Peninsula (Fig. 1 ). It is located in the southwest part of the Yinkao-Kuandian Subprovince near the boundary with the Fuzhao Subprovince of the North China Platform (Fig. 1 ). The area is mostly underlain by Proterozoic sedimentary rocks and Mesozoic granites (Fig. 2). Prominent structures in the area are NE- and NW-trending conjugate faults and shear zones and they are considered to have been formed under a regional compression regime in the Proterozoic Luilang orogenic event (Fig. 2). Re-activation of the structures provided dilation zones for intrusions of Mesozoic granites (Dai and Fan, 1990). They are believed to be of early Jurassic age One of the conjugate systems, ~ 45 °, forms wide ductile shear zones and it shows prominent low magnetic anomalies on airborne- ( 1 : 200,000) and ground magnetic maps ( 1 : 10,000). Mesozoic granites occur at the intersections of the conjugate set and intrusions form a broadly NE-trending array, 300 km long and 30 to 60 km wide. (Fig. 2; Wang et al., 1988).

I N / Shengyang

iOlnyuanliiii:

Anxlan

t'inbo Kuandian

............... !i! ,..::~:ii~iiiii?ii::::~?!

...................... fill!i!

,%%%%%%%%~ 0 50km

I I

Fig. 2. Distribution of Mesozoic granites in the Eastern Liaoning Province (modified after Wang et al., 1988). Area with +pattern: granites. Shaded area: zones containing abundant granites. (O) Gold mines. Solid lines: major faults. (D) Cities.

96 Q. Cheng et al. / Jounzal ~'Geochemical Exploration 51 (1994) 93-108

The sedimentary rocks in the area are most ly arenites, calcareous arenites and l imestones

of early Proterozoic age. The rocks have undergone greenschist to amphibol i te facies

regional metamorphism and the fol iat ion general ly strikes 150 ° to 170 ° and dips 50 ° to 80 °

W.

3. Local geology of the Maoling deposit

The geology of the vicini ty of the Maol ing deposi t is shown in Fig. 3. The rocks in the

area most ly belong to the upper member of Gaixian format ion and the Yushulazi formation

of early Proterozoic age. The latter unconformably over l ies the former. The Gaixian for-

mation occurs in the central and southern parts o f the study area, and the latter occurs only

in the northern part o f the area. They are intensely deformed and pervas ively altered to

sericitic phyllite, ser ic i t ic -chlor i te -phyl l i te , and banded silicarenite.

Fig. 3. Geological map of the Maoling area (modified after unpublished map by Liaoning Geological Bureau). Area with + pattern: Mesozoic granite. Area with x pattern: Mesozoic monzonite. Vertical striped area: Mesozoic diorite. Left angle striped area: Lower Gixian formation. Right angle striped area: Upper Gixian formation. Cross dashed patterned area: the Yushulazi formation. Dotted area: the Middle Proterozoic Yongning formation. (O) gold deposits. Solid lines: faults. The Maoling granite stock is indicated by an arrow with "M" in the lower center of the diagram.

Q. Cheng et al. / Journal of Geochemical Exploration 51 (1994) 93-108 97

The sedimentary rocks are intruded by two Mesozoic granites: the Wo-Longquan bath- olith with surface exposure of 120 km 2 and the Maoling stock with 0.4 km 2 (Fig. 3). They are porphyritic biotite granite and they have similar mineralogy, texture and chemical compositions, suggesting that they are co-magmatic. Modal mineral abundances of the two are ~ 30 vol% quartz, 33 to 35 vol% plagioclase, 24 to 25 vol% K-feldspar, 5 to 10 vol% biotite. The contacts between the granites and the host rocks are gradational, and the sedimentary xenoliths are not abundant in the core of intrusions, but the amounts increase towards the margin of the intrusions.

All known occurrences of Au deposits in this area are located in and around the two intrusions (Fig. 3 ). Deposits outside the intrusions are underlain by the intrusions. In local scale, they are along the faults and some of them are near the intersections of the two faults (Figs. 3 and 4).

The country rocks of the Maoling stock show contact metamorphic aureoles (Fig. 4); sillimanite zone, ~ 80 m wide from the contact, andalusite zone, 80 to 600 m from the contact, and biotite-phyllite zone, 600 to 800 m from the contact. Most mineralized zones occur farther away from the contact metamorphic aureoles but some auriferous zones are found within the andalusite and biotite-phyllite zones (Fig. 4). The alteration associated with the Au mineralization overprinted the contact metamorphic assemblages, suggesting that the mineralization post-dated the intrusion.

There are various dykes in the area (Figs. 4 and 5), including quartz-feldspar porphyries, dioritic porphyries and lamprophyres. They occur along the NE-SW, NNE-SSW and NW- SE faults. Dykes along the NNE-SSW direction are pre-ore dykes, as they are altered and mineralized. The dykes in other directions are mostly post-ore dykes which cut the ore bodies and alteration zones. The evidence suggests that the igneous activity in this area was contemporaneous with the Au mineralization, and that the two are probably genetically related.

4. Occurrence of the gold deposit

The Maoling deposit has reserves of over 95 million tonnes of ore (cut-off of 1 g/t) down to 400 m below the surface with average grade of 2 g/t Au (0.072 oz/t). Drilling confirmed the extension of auriferous zones at least down to 500 m and further exploration is expected to increase the reserves. Ore bodies have an average width of several meters sandwiched by alteration zones up to 500 m wide. Boundaries between ores and alteration zones and between the altered and unaltered rocks are gradational and there are only subtle differences in their appearances.

The emplacement of ore bodies is mainly controlled by two high angle, ductile shear zones at 30 ° and 140 ° (Fig. 3), in which NNE-trending zones predominantly control the ore strikes (Fig. 4). In the vicinity of ore zones the foliation of the host rocks strikes 30 ° and dips 500-80 ° W and the alteration zones and enclosed ore zones are generally parallel to the foliation. There are several EW and NE post-ore faults imposed on these structures.

98 Q. Chen g et al. / Journal ~)f Geo~ hemical Exploration 5l (1994) 93-108

Fig. 4. Geological map of the immediate vicinity of the Maoling deposit ( modified after unpublished map by Liaoning Geological Bureau). Area with + pattern with M: the Maoling stock. Cross-hatched area: siltimanite zone of the Gaixian formation. Horizontally striped area: andalusite zone of the Gaixian formation. Diagonally striped area on the right: biotite zone of the Gaixian formation. Diagonally striped area on the left: serici te- chlori te-phyll i te zone of the Gaixian formation. Zig-zag patterned area: sericite-phylli te and banded silicarenite

of the Gaixian formation. Area with inverted V pattern: silicarenite of the Yushulazi formation. Shaded area: alteration zone. Black area: auriferous zone, > I g / t Au. Heavy stippled area: fault zones. Solid line: inferred fault. Elongated zone with D: dykes. Open area with Q: Quaternary sediments. The rectangular areas indicated with dashed and thin solid lines are shown in Figs. 5 and 8, respectively. The vertical section along the line with W and E. striking 105 °, in the centre of the diagram is shown in Fig. 6.

Q. Cheng et al. / Journal of Geochemical Exploration 51 (1994) 93-108 99

Fig. 5. Geological map of the Maoling ore bodies. The area of the diagram is shown by a dashed rectangle in Fig. 4. Open stippled area: alteration zone. Open area with Q: Quaternary sediments. Dark shaded area: auriferous zone ( > g/t Au). Black area: auriferous zone ( > 3 g/t Au). Heavy stippled area: fault. Vertically striped area: dyke.

4.1. Mineralogy o f ore

Most metallic minerals in the ores are fine grained and they are not easily identified with the naked eye. Ore bodies are grouped into three types: disseminated type, quartz-pyrite veinlet type, and coarse arsenopyrite type. The first type hosts the majority of Au, consists of fine native gold, arsenopyrite, pyrrhotite, galena, and minor pyrite, marcasite, and chal- copyrite and sphalerite. Grains of native gold occur together with marcasite, chalcopyrite, sphalerite and galena and along grain boundaries of arsenopyrite and pyrrhotite. In the second type ore, native gold forms irregular, dendritic, flaky and irregular shapes within and along grain boundaries of re-crystallized quartz. Some of the grains are coarse enough to be visible with the naked eye.

4.2. Veining and alteration

Multiple episodes of hydrothermal activity are recorded in the ore zones. Early quartz is highly deformed and re-crystallized during subsequent hydrothermal-deformation events.

100 Q. Cheng et a l . / Journal t~/'Geochemical Exploration 51 (1994) 93 108

The occurrences suggest that the introduction of auriferous fluids was synchronous with the regional deformation.

Alteration consists chiefly of silicification, sericitization, pyrrhotitization, and to a lesser extent, biotitization, chloritization and carbonatization. Silicification is pervasive and it extends farther away from the defined alteration zones. Secondary quartz replaces the primary and metamorphic minerals near the intrusion. The intensity of the silicification decreases away from the mineralization.

Two types of wallrock alteration are recognized: quartz-carbonate-pyrrhotite alteration and biotite-chlorite-sericite alteration. The former type occurs within and adjacent to the ore zones. The latter alteration occurs farther away from the ore zones and it surrounds the former alteration. The distribution of alteration zones was delineated by a combination of geophysical techniques (magnetic field, resistivity and chargeability) and geological sam- piing. One of the largest alteration zones extends 2,000 m long and 500 m wide, strikes 18°-45 °, and dips 50 ° N (Fig. 4).

4.3. Timing of mineralization

The exact timing of the auriferous mineralization in the district is uncertain, but the spatial association of the Au deposits with granitic intrusions in the area described above suggests that it was broadly synchronous with the regional deformation and Jurassic intrusive activity. In deposit scale, granodiorite and porphyry dykes which have similar compositions to granites are abundant in the auriferous zones and some are hydrothermally altered and some cut the auriferous zones. Contact metamorphic minerals adjacent to intrusions are replaced by hydrothermal alteration minerals. The evidence indicates that the mineralization post- dated the peak igneous activity, but they were broadly coeval. Furthermore, the occurrences of auriferous zones are controlled by the deformation of the area and auriferous zones show intense deformation, suggesting that the dilation zones for the auriferous fluids were prob- ably provided by Jurassic tectonic activity.

5. Geophysical and geochemical characteristics of the Maoling deposit

In the initial stage of exploration, only soil geochemical data were available and the data highlighted anomalous zones, in which the ore zones were later discovered using diverse geophysical and geochemical techniques. Employed geophysical techniques include ground magnetic field, resistivity and chargeability studies. Primary geochemical halos of selected elements were examined in rock samples collected from outcrops and trenches to charac- terize the type of mineralization and the extent of the mineralization.

5.1. Geophysical studies

As common in many sedimentary rocks, the rocks of the Gaixian formation are very weakly magnetic. In a 1 : 200,000 scale airborne magnetic map, they show relative magnetic values ranging from - 50 3' to - 250 3'. The rocks of the Yushulazi formation show their values ranging from - 20 3' to - 200 3'. Granitic rocks, such as the Wo-Longquan and

Q. Cheng et al. /Journal of Geochemical Exploration 51 (1994) 93-108 101

500r

300~m

W

400m

300m •

200m

1 0 0 m -

0

"x"''"--•. D5 D1 D A

;%

:i . i : : . : ! i i 1

0 lOOm I | I

E

Fig. 6. Geophysical signatures of the Maoling deposit (simplified after Wang and Wang, 1990 and Wang et al., 1990). The location of the vertical section is shown with a thin solid line in Fig. 4. Upper diagram: ground magnetic profile. Middle diagram: ground electrical profile. Lower diagram: vertical geological section. The numbers on y-axis in the lower diagram are elevations above sea level. Thin vertical arrowed lines show intersec- tions of diamond drill holes and labels beside the arrows are drill hole numbers. Black area: ore body with > 3 g/ t Au. Densely stippled area: auriferous zone with Au ranging from 1 to 3 g/t . Open elongated area: diorite dyke. Open stippled area: alteration zone. Note that the alteration zone is characterized by high magnetic values low resistivity and high chargeability.

Maoling intrusions, have magnetic characteristics similar to their sedimentary country rocks. This is not common because biotite-bearing granitic rocks commonly display higher mag- netic susceptibilities than sedimentary rocks. The granitic rocks, however, have lower densities than metamorphosed sedimentary rocks, producing low gravity anomalies. Thus, shallowly hidden granites in the area were identified using gravity data. The gravity survey

102 Q. Cheng et al. /Jouotal (~'Geochemical Exploration 51 (1994) 93 108

i! , i i

I

Fig. 7. Distribution of air-borne magnetic anomalies and alteration zones. Stippled area: sedimentary rocks. Area with + marks: granite. Shaded area: alteration zones. Closely striped area: magnetic anomalies. Intensity of the anomalies are schematically expressed by the length of bars. Shaded area: existing and inferred alteration zones.

shows that one branch of the Wo-Longquan batholith dips southeast and extends beneath the Maoling deposit (Fig. 3).

Alteration zones of the Au deposits were easily identified because of their high magnetic susceptibility values (Figs. 6 and 7), due to high contents of pyrrhotite and arsenopyrite. High contents of pyrrhotite and arsenopyrite are good indicators for the enrichment of Au, but the contents of Au are not necessarily related to the contents of pyrrhotite and arseno- pyrite. Magnetic susceptibilities of rock samples and their Au contents are not correlated with a correlation index (R) of 0.25 (Dong, 1988), but a positive relationship is observed between the two within individual rock types (Figs. 6 and 8). For example, seven phyllite samples, the major ( > 80 vol.%) host rock type of the ore, show R = 0.72, which is larger than the threshold value for six samples, R = 0.70.

The distribution of alteration zones is estimated from asymmetrical magnetic curves observed on the surface assuming that the zones are tabular. The result indicates that the alteration zones dip ~ 50 ° (Fig. 6; Cheng, 1989b). The estimated size and distribution of the alteration zone based on geophysical data were later confirmed by diamond drillings.

Resistivity and chargeability data were used to estimate the distribution of the alteration zones and ore bodies. Quartz veinlet-type ores generally show high resistivity and charge- ability (Cheng, 1989a) because quartz produces high resistivity and because dissemination of pyrite in the veins and wall rocks yield high chargeability. The disseminated-type ores like the Maoling deposit contain less quartz and dissemination of micaceous minerals is

Q. Cheng et al. / Journal of Geochemical Exploration 51 (1994) 93-108 103

300r

,

~uunnnnnnnnunnnnn,,, ............................................................ ,,bHbLFIIIIINqlllqlll

V # ' / / / / / / / h B B

8a 8b

f 30%

8c

0 2 0 0 m I i

Fig. 8, Geophysical characteristics of the alteration zones of the Maoling deposit. The area covered in the diagram is shown as a solid rectangle in Fig. 4. Diagonally striped area = host rock phyllites, Stippled area = main alteration zone of the Maoling deposit. (a) Magnetic susceptibility map; (b) resistivity map; (c) chargeability map.

common in the wall rocks, which produces low resistivity (Fig. 8b) and high chargeability (Fig. 8c) compared to unaltered host rocks. The resistivity values of altered rocks at Maoling are negatively correlated with the contents of Au in alteration zones, R = - 0.25. In sum- mary, the alteration zones at the Maoling deposit were recognized by high magnetic field, low resistivity and high chargeability and the combination of the data was used to recognize the alteration zones.

5. 2. G e o c h e m i c a l s tud ies

To delineate the alteration zones, 2400 soil samples from the area covering 10 km 2 were analyzed for major and minor elements including Au, As, Sb, Ag, Pb, Hg, Bi, Zn and Cu. Each sample was collected from a grid of 200 m × 20 m in the area. The elemental concen-

104 Q. Cheng et al. /Journal q['Geochemical Exploration 51 (1994) 93 108

2.5

k

2 . S

o U /

iiiiiiiiiiiii

2.5

0 2 0 0 m i t

~ J . s

Fig. 9. Contour map of Au (ppb) in 2400 soil samples. Each sample was collected in a grid of 200 m x 20 m. The sampes were analyzed by using instrumental neutron activation analysis with dictation limit of 0.1 ppb. Stippled area = main alteration zones of Maoling deposit.

trations were determined using atomic absorption spectrometry after HF-HC14 digestion except for Au, which was analyzed by instrumental neutron activation method. The data of Au and As are plotted on the contour maps (Figs. 9 and 10). Highest Au content, 640 ppb, was found in the NW corner of the area (Fig. 9) and the zones of high values correspond to the underlying alteration zones (Fig. 9). A similar pattern was also obtained for As.

Four major factors are obtained by factor analysis: {Au, As, Sb, Pb, Ag}, {Cu, Zn, Ag, Pb }, { Bi } and { Hg, Pb, Ag }. Similar results are also obtained by using cluster analysis with correlation distance measurement (Fig. 11). Au, Ag, As, and Pb were taken as indicator

Q. Cheng et al. / Journal of Geochemical Exploration 51 (1994) 93-108 105

IN 0 200rn i i

0

0

Fig. 10. Contour map of As (ppm) in 2400 soil samples. The sampes were analyzed by using an atomic absorption spectrometer after digestion of samples with HF-HCIO4-HNO 3 method with dictation limit of 0.5 ppm. Stippled area = main alteration zones of Maoling deposit.

elements in the secondary halos. These elements correspond to native gold, arsenopyrite, and galena in the ore zones.

To define the chemical characteristics of the ore and alteration zones, total 300 rock samples were obtained mostly from trenches and they were analyzed for Au, Ag, As, Sb, Pb, Hg, Bi, Co and Ni. Statistical analyses similar to the soil data were applied to the rock geochemical data and the result shows positive correlation among Au, As, Sb, Ag and Pb. Samples near and within the main disseminated-type ores show positive correlations among Au, Co and Ni. These elements correspond to native gold, arsenopyrite, stibnite, galena and pyrrhotite and they are considered as indicator minerals.

106 Q. Cheng et al. / Journal of Geochemical Exploration 51 (1994) 93-108

Ag

As

[ An Pb

Sb

Cu

Zn

Hg

Bi

r I I I I I I

0 0.2 0.4 0.6 0.8 1

Fig. 11. Cluster analysis of 2400 soil samples. The result identified four groups: { Au, As, Ag, Pb, Sb }, { Zn, Cu }, { Hg }, { Bi }, r represents correlation index.

6. Comparison of the Maoling deposit with other sediment-hosted gold deposits

Disseminated Au deposits in sedimentary rocks in North America are typified by Carlin- type deposits (Bagby and Berger, 1985). Examples include the Carlin, Getchell, Jerritt Canyon, Pinson and Cortez deposits (Nevada). They vary in their sizes, grades of ores, textures, and degrees of structural control, but they share several common features. Key characteristics include occurrences of fine-grained Au grains and associated metallic min- erals, calcareous to clastic sedimentary host rocks, extensive silicification of host calcareous rocks, low abundances of base metals such as Cu, Pb and Zn, and significant concentrations of As, Sb, Hg and TI in the ores (Roberts et al., 1971 ; Romberger, 1985; Wells et al., 1985; Berger, 1985; Bakken et al., 1989). Most are considered to be epithermal deposits formed at temperatures ranging from 150 to 250°C.

The occurrences of Au in the Maoling deposit are similar to those of Carlin-type deposits with some notable differences. Host rocks at the Maoling deposit are Proterozoic and they are regionally and thermally metamorphosed, but Carlin-type deposits are mostly hosted in Phanerozoic unmetamorphosed rocks. Shear deformation is evident at Maoling, whereas Carlin-type deposits lack such ductile deformation. Alteration zones at Maoling contain biotite, whereas the predominant K-bearing alteration mineral in Carlin-type deposits is sericite. The differences appear to suggest that the Maoling deposit was likely formed at higher temperature and under higher pressure than many Carlin-type deposits.

The Maoling contains arsenopyrite as the major As-mineral, while orpiment and realgar are dominant As-minerals in many Carlin-type deposits in North America (e.g., Radtke, 1985). The variation of As-minerals in a single deposit is also noted in some Carlin-type deposit (Rytuba, 1986). The Getchell deposit, Nevada, contains arsenopyrite in the main- stage auriferous mineralization and realgar in late cross-cutting veins. At constant temper- atures, orpiment is stable at highfs2 and arsenopyrite, S-deficient compared to orpiment, is stable under low fs2 (i.e., Rytuba, 1986). fs2 during the mineralization at Maoling was

Q. Cheng et al. / Journal of Geochemical Exploration 51 (1994) 93-108 107

probably low compared to many Carlin-type deposits, but sufficiently high to preclude the formation of loellingite. The interpretation is consistent with the occurrences of Fe-minerals. S-deficient pyrrhotite is the main phase at Maoling and pyrite in the Carlin-type deposits.

Other minerals common in Carlin-type deposits, but not present at Maoling include cristobalite, opaline silica, kaoline cinnabar and stibnite (e.g., Radtke, 1985). The lack of these minerals at Maoling appears to suggest that the Maoling deposit was formed at higher temperatures than most Carlin-type deposits. Alternatively, the deposit pre-dated regional metamorphism and low temperature hydrothermal minerals were destabilized during the regional metamorphic event. The latter possibility is rejected because regional metamor- phism is believed to have occurred in Proterozoic time, long before the mineralization in Mesozoic time. Higher temperature of mineralization at Maoling is consistent with close association of intrusive rocks. The comparison between the Maoling deposit and North American Carlin-type deposits suggests that the Maoling deposit is a Carlin-type deposit formed at deeper crustal level and probably higher temperature than the Carlin deposits.

The techniques used for the discovery of the Maoling deposit may be applied in the exploration program for disseminated Au deposits elsewhere in sedimentary terrains. The sedimentary host rocks usually provide low magnetic susceptibilities. The combination of magnetic, resistivity and chargeability data may be used to recognize alteration zones. Some mafic and intermediate dykes in these deposits may be used to examine the structure of the area. The lack of prominent quartz veining of this type of mineralization is a disadvantage in geological exploration, but relatively consistent Au grade and large size are advantageous for the applications of geophysical and geochemical techniques in the mineral exploration.

7. Summary

The Maoling deposit is hosted by deformed, metamorphosed sedimentary rocks of the upper member of Gaixan formation in the Proterozoic North China Platform. Like many other disseminated Au deposits, the high angle shear zones of NE and NW orientations in the Maoling district and their intersections might have provided the conduits for ore-forming fluids and depositional sites for minerals. Igneous rocks in the area appear to have played a significant role in the genesis of the mineralization. Deposits occur at short distance from Jurassic granitic intrusions and many including the Maoling deposit are underlain by the granite. Granitic dykes are abundant in the mine property and some are intensely altered due to the mineralization and some cut the mineralization. The close association of the mineralization with the granitic intrusions in time and space appears to suggest the genetic linkage between the two. Granites possibly generated hydrothermal activity which resulted in Au mineralization.

Alteration zones are characterized by high Au, Ag, As, Pb, Co and Ni in halos and high Au, As, Ag, Sb, Pb in soils and high magnetic field, low resistivity, high chargeability. These features were successfully used to illustrate the hidden auriferous alteration zones at Maoling and these may be applied to exploration programs in other sedimentary terrains.

108 Q. Cheng et al . /Journal of Geochemical Exploration 51 (1994) 93 108

Acknowledgements

The authors thank the Liaonin Geologica l Bureau for their support through 1983 to 1986

and their permiss ion to use their unpubl i shed data and maps.

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